CE413 Highway Eng II[1]

KYAMBOGO UNIVERSITY

CE 413‐Highway Engineering II Lecture Notes F.E. Okello 8/23/2010

The notes deal with the design of flexible pavements based on the TRL Standard together with aspects of drainage design based on the Ministry of Works and Transport Guidelines

Preamble

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Preamble Global developments in highway engineering are evolving at an incredible speed. New materials, planning and design concepts together with radically distinctive construction practices are emerging as road users demand high quality roads. Following the recent restructuring of the Ministry of Works and Transport, Uganda is due to modernise its road transport system with subsequent construction of high speed highway facilities. Highway Engineering II is hereby presented as part of the comprehensive courses taught in Kyambogo University in partial fulfilment of the award of the ‘Bachelor of Engineering in Civil and Building Engineering of Kyambogo University’. This course is delivered in the second semester of fourth year and carries a credit unit of four (4) with a total of 60 contact hours. It introduces and examines the major principles and practices encountered during the ‘Flexible Pavement Design Process’ of highway systems. The main Objective of the course is to impart and equip the student with the values, knowledge and skills necessary to design a flexible pavement. By the end of the course the candidate should be able to: Survey possible routes, assess traffic flow, measure subgrade strength, select pavement materials and select the appropriate pavement structures required for flexible pavement construction. The authors hope is that this revised edition of the notes will be simpler to read, revise, comprehend and apply to the daily challenges that are faced by the student and the practicing Highway Engineer. May the Almighty God richly bless you! F.E. Okello

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Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

Table of Contents

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Table of Contents Preamble ....................................................................................................................................... i Table of Contents ......................................................................................................................... ii List of Tables .............................................................................................................................. iv List of Design Charts .................................................................................................................. vi Acronyms ................................................................................................................................... vii Chapter One: ................................................................................................................................ 1 Introduction to Pavement Design................................................................................................. 1 General ......................................................................................................................................... 1 1.1 Types of Pavements .......................................................................................................... 2 1.2 Elements of a Flexible Pavement ...................................................................................... 3 1.3 Overview of the Design Process ....................................................................................... 5 1.4 Design Approaches ........................................................................................................... 7 1.5 Design Standards .............................................................................................................. 7 1.6 Questions........................................................................................................................... 8 1.7 Bibliography ..................................................................................................................... 8 Chapter Two: ............................................................................................................................. 10 Traffic Assessment..................................................................................................................... 10 2.1 General ............................................................................................................................ 10 2.2 Estimation of Traffic Flows (F) ...................................................................................... 10 2.3 Determination of Cumulative Standard Axles (T) .......................................................... 18 2.4 Example 2.1: Traffic Assessment ................................................................................... 19 2.5 Questions......................................................................................................................... 21 2.6 Bibliography ................................................................................................................... 23 Chapter Three: ........................................................................................................................... 24 Subgrade Strength Assessment .................................................................................................. 24 4.1 General ............................................................................................................................ 24 4.2 Climatic Regime ............................................................................................................. 24 4.3 Testing Subgrade Soils ................................................................................................... 25 4.4 Example 3.1: DCP Test................................................................................................... 37 4.5 Defining Uniform Sections ............................................................................................. 39 4.6 Design of Earth Works.................................................................................................... 39 4.7 Questions......................................................................................................................... 45 4.8 Bibliography ................................................................................................................... 48 Chapter Four: ............................................................................................................................. 49 Selection of Pavement Materials ............................................................................................... 49 5.1 General ............................................................................................................................ 49 5.2 Unbound Pavement Materials ......................................................................................... 49 5.3 Bitumen Bound Pavement Materials .............................................................................. 57 5.4 Bituminous Surfacings .................................................................................................... 63 5.5 Bituminous Roadbases .................................................................................................... 70 5.6 Surface Dressing ............................................................................................................. 73 5.7 Example 4.1: Surface Dressing ....................................................................................... 86 5.8 Questions......................................................................................................................... 89 5.9 Bibliography ................................................................................................................... 92 Chapter Five: .............................................................................................................................. 93 The Structure Catalogue ............................................................................................................ 93 5.1 Basis for the Structure Catalogue ................................................................................... 93 5.2 How to use the Structure Catalogue................................................................................ 93 5.3 Key to Structural Catalogue ............................................................................................ 95 5.4 Pavement Design Charts ................................................................................................. 96 Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Table of Contents

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5.5 Questions....................................................................................................................... 104 5.6 Bibliography ................................................................................................................. 104 Chapter Six: ............................................................................................................................. 105 Highway Drainage ................................................................................................................... 105 6.1 General .......................................................................................................................... 105 6.2 Main functions of Drainage .......................................................................................... 105 6.3 Highway Drainage Terminologies ................................................................................ 105 6.4 Surface Drainage ........................................................................................................... 106 6.5 Sub-Surface Drainage ................................................................................................... 111 6.6 Cross Drainage .............................................................................................................. 112 6.7 Culverts ......................................................................................................................... 113 6.8 Questions....................................................................................................................... 119 6.9 Bibliograhy ................................................................................................................... 120 Chapter Seven: ......................................................................................................................... 122 Conclusion ............................................................................................................................... 122 7.1 Road Deterioration ........................................................................................................ 122 7.2 Economic Considerations ............................................................................................. 122 7.3 Effects of Climate ......................................................................................................... 123 5.7 Variability in Material Properties and Road Performance ............................................ 123

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List of Tables

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List of Tables Table 2.1: Selection of Design Life ........................................................................................... 17 Table 2.2: Axle Factor Multipliers............................................................................................. 19 Table 2.3: Traffic Classes .......................................................................................................... 19 Table 2.4: Two-Way Traffic Volume at the Project Road (in vehicles/day) ............................. 19 Table 2.5: Axle Weights (in tonnes) .......................................................................................... 19 Table 2.6: Axle Loads (in tons) ................................................................................................. 21 Table 2.7: Friday 24-hr count summary .................................................................................... 22 Table 2.8: Sunday 24-hr count summary ................................................................................... 22 Table 2.9: 16-hr counts on the remaining days .......................................................................... 22 Table 3.1: Water Table Correction Factors for Soil Type PI ..................................................... 26 Table 3.2: Subgrade Strength Classes ........................................................................................ 28 Table 3.3: Number of blows required in the compaction test .................................................... 29 Table 3.4: Typical CBR Values ................................................................................................. 29 Table 3.5: CBR Preparation Sheet ............................................................................................. 31 Table 3.6: CBR Penetration Sheet ............................................................................................. 32 Table 3.7: DCP Field Results..................................................................................................... 37 Table 4.1: Properties of Unbound Materials .............................................................................. 49 Table 4.2: Grading Limits for crushed stone base materials (GB1,A; GB1,B) ......................... 51 Table 4.3: Mechanical strength requirements for the aggregate fraction of crushed stone roadbases (GB1,A; GB1,B) as defined by the Ten Percent Fines Test ..................................... 51 Table 4.4: Typical Coarse aggregate gradings for Dry-bound (GB2,A) and Water-bound Macadam (GB2,B) ..................................................................................................................... 53 Table 4.5: Recommended Particle size distribution for mechanically stable natural gravels and weathered rocks for use as roadbases (GB3) ............................................................................. 53 Table 4.6: Recommended Plasticity characteristics for granular Sub-bases (GS)..................... 56 Table 4.7: Typical Particle Size distribution for sub-bases (GS) which meet strength requirements ............................................................................................................................... 56 Table 4.8: Coarse Aggregate for Bituminous mixes .................................................................. 61 Table 4.9: Fine Aggregate for Bituminous Mixes ..................................................................... 62 Table 4.10: Asphaltic Concrete Surfacings ............................................................................... 66 Table 4.11: Suggested Marshall Test Criteria............................................................................ 66 Table 4.12: Voids in Mineral Aggregate (VMA) ...................................................................... 67 Table 4.13: Bitumen Macadam Surfacings ................................................................................ 68 Table 4.14: Suggested Marshall Criteria for Close Graded Bitumen Macadams or DBMs ...... 68 Table 4.15: Hot Rolled Asphalt (HRA) Surfacings ................................................................... 69 Table 4.16: Bituminous Macadam Roadbase ............................................................................ 71 Table 4.17: Rolled Asphalt Roadbase........................................................................................ 71 Table 4.18: Job-mix Tolerances ................................................................................................. 73 Table 4.19: Manufacturing and rolling temperature (in degrees centigrade) ............................ 73 Table 4.20: Category of Road Surface Hardness ....................................................................... 78 Table 4.21: Traffic Categories for Surface Dressing ................................................................. 78 Table 4.22: Recommended maximum chipping size (mm) ....................................................... 78 Table 4.23: Condition Constants for determining the rate of application of Binder ................. 83 Table 4.24: Typical Bitumen Spray Rate Adjustment Factors .................................................. 85 Table 5.1: Summary of Material Requirements for the Design Charts ..................................... 94 Table 6.1: Run off coefficient for the rational method ............................................................ 108 Table 6.2: Run off coefficient for the rational method ............................................................ 110 Table 6.3: Maximum Permissible velocity in open channels .................................................. 110 Table 6.4: Manning’s n Values ................................................................................................ 111 Table 6.5: Entrance loss Coefficient (Outlet Control, Full or Partially full) ........................... 118 Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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List of Tables

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List of Figures Figure 1.1: Definition of Pavement layers ................................................................................... 3 Figure 1.2: The Pavement Design Process .................................................................................. 6 Figure 2.1: Form for Manual Classified Counts ........................................................................ 12 Figure 2.2: Axle Load Survey Form A for recording vehicle survey data ................................ 15 Figure 2.3: Axle Load Survey Form B for recording vehicle wheel loads ................................ 16 Figure 3.1: CBR Test Machine .................................................................................................. 30 Figure 3.2: Dynamic Cone Penetrometer in use ........................................................................ 34 Figure 3.3: The TRL Dynamic Cone Penetrometer ................................................................... 35 Figure 3.4: DCP-CBR relationships .......................................................................................... 36 Figure 3.5: The DCP Test Result ............................................................................................... 36 Figure 3.6: Defining Uniform Sections ..................................................................................... 39 Figure 3.7: Dry density – moisture content relationships for a gravel-sand-clay ...................... 43 Figure 4.1: Type of Surface Dressing ........................................................................................ 76 Figure 4.2: Surface temperature/choice of binder for surface dressings ................................... 81 Figure 4.3: Determination of average Least Dimension ............................................................ 82 Figure 4.4: Surface Dressing Design Chart ............................................................................... 84 Figure 6.1: Road Drainage features ......................................................................................... 106 Figure 6.2: Nomograph for the Calculation of Headwater Depth with Inlet Control .............. 116 Figure 6.3: Headwater Losses for Concrete Pipe Culverts Flowing Full ................................ 117 Figure 6.4: Headwater Losses for Concrete Pipe Culverts Flowing Full ................................ 118

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List of Design Charts

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List of Design Charts Chart 1: Granular Roadbase / Surface Dressing ........................................................................ 96 Chart 2: Composite Roadbase (Unbound & cemented) / Surface Dressing .............................. 97 Chart 3: Granular Roadbase / Semi-Structural Surface ............................................................. 98 Chart 4: Composite Roadbase / Semi-Structural Surface .......................................................... 99 Chart 5: Granular Roadbase / Structural Surface..................................................................... 100 Chart 6: Composite Roadbase / Structural Surface.................................................................. 101 Chart 7: Bituminous Roadbase / Semi-Structural Surface ....................................................... 102 Chart 8: Cemented Roadbase / Surface Dressing .................................................................... 103

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Acronyms

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Acronyms AADT

Annual Average Daily Traffic

AASHTO

American Association of State Highways and Transportation Officials

ADT

Number of average daily traffic

ALD

Average Least Dimension

CBR

California Bearing Ratio

ESA

Equivalent Standard Axle

GB3

Granular Base-material type 3

GIS

Graphical Information Systems

HW

Allowable Headwater depth

KUTIP

Kampala Urban Transportation plan

LL

Liquid Limit

LS

Linear Shrinkage

M.S.A

Millions of equivalent standard axle

MC

Moisture Content

MDD

Maximum Dry Density

OMC

Optimum Moisture Content

ORN

Overseas Road Note

PI

Plasticity Index

PL

Plastic Limit

TRRL

Transport Road Research Laboratory

TW

Tailwater depth

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Chapter One:

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Chapter One:

Introduction to Pavement Design General A road is a path established over land for the passage of vehicles, people, and animals. Roads provide dependable pathways for moving people and goods from one place to another. They range in quality from dirt paths to concrete-paved multilane highways. Roads are used by various forms of transportation, such as trucks, automobiles, buses, motorcycles, and bicycles. Roads allow trucks to move goods from points of production, such as fields and factories, directly to markets and shopping centres. Private individuals rely on roads for safe and efficient automobile, motorcycle, and bicycle travel. Fire departments, medical services, and other government agencies depend on an organized system of roads to provide emergency services to the public in times of need [Urbanik, 2007]. There are many different types of roads, from multilane freeways and expressways to twoway country roads. One important quality of a road is known as control of access. This term describes how vehicles are allowed to enter and exit a road. By controlling access to a road, the road can support more traffic at higher speeds. Roads can be classified into three broad categories: highways, urban or city streets, and rural roads. Each type of road controls access to different degrees. Each type also differs in location, the amount of traffic it can safely support, and the speed at which traffic can safely travel. To support heavy vehicles moving at high speeds, a modern road is made up of several layers. Each layer helps the layers above it support the weight and pressure of moving traffic. Roads that carry more traffic at higher speeds, like highways, are built to stronger standards than roads that carry less traffic, such as rural collector roads. The number of layers in a road often depends on the intended use of the road, but generally roads have three distinct layers. From bottom to top, the layers are the subgrade (or roadbed), the roadbase (or base course), and the Surfacing (or wearing course). These layers form part of what is known as the pavement structure of a road [Urbanik, 2007]. Road traffic is carried by the pavement, which in engineering terms is a horizontal structure consisting of superimposed layers of selected and processed material supported by in-situ natural material. The task before the pavement designer normally entails the development of the most economical combination of layers that will guarantee adequate dispersion of the incident wheel stresses so that each layer in the pavement does not become overstressed during the design life of the highway. The major variables in design of a highway pavement are: the type of vehicles in the traffic stream, the volume of traffic predicted to use the highway over its design life, the strength of the underlying subgrade, the material contained within each layer of the pavement and the thickness of each layer in the pavement [Rogers, 2003]. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Types of Pavements

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In order for the pavement to support the design year traffic without exceeding its support capacity existing records must be examined, possible routes surveyed, traffic flow assessed, subsurface explorations conducted and appropriate materials selected. Based on this information, a pavement structure can then be selected from a structure catalogue, the maximum slopes for embankments and cuttings established, the degree of compaction to be achieved during construction determined, and drainage needs specified before the construction process is undertaken. Before adopting any material for use within the pavement structure, the engineering properties of the local rock and soil are established, particularly with respect to strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time. The relevant properties are determined by either field tests, empirical estimates based on soil type, or by laboratory measurements. The material is tested in its weakest expected condition, usually at its highest moisture content. Probable performance under traffic is then determined, soils unsuitable for the final pavement are identified for removal and suitable replacement materials are reserved for use on the pavement [TRL, 1993].

1.1 Types of Pavements Pavements are called either flexible or rigid depending on their relative flexural stiffness. Two main types of pavements are used—bituminous, or flexible, pavement and concrete, or rigid, pavement. Generally, bituminous pavements are cheaper and easier to construct, but they require more maintenance. Concrete pavements, however, last for a very long time with minimal upkeep but are much more expensive and time-consuming to build [Urbanik, 2007]. a) Flexible Pavements These pavements are rather flexible in their structural action under loading. They are surfaced with bituminous or asphalt materials. Flexible pavements consist of several layers of materials and rely on the combination of layers to transmit load to the subgrade. As a result of this action, flexible pavements distribute load over a small area of subgrade. b) Rigid Pavements Rigid pavements are made of Portland Cement Concrete (PCC). The concrete slab ranges in thickness from 6 to 14 inches (or 152.4 - 355.6mm). These types of pavements are called rigid because they are substantially stiffer than flexible pavements due to the high stiffness of PCC. As a result of this stiffness, rigid pavements tend to distribute load over a relatively wide area of subgrade. The concrete slab that comprises a rigid pavement supplies most of its structural capacity. In deciding whether to use flexible or rigid pavements, engineers take into account the following factors: • • • • •

Traffic disruptions due to maintenance; Riding characteristics; Ease and cost of repair; Effect of climatic conditions. Lifetime costs.


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Elements of a Flexible Pavement

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1.2 Elements of a Flexible Pavement A flexible pavement is built up of layers namely; surfacing course, roadbase, sub-base, capping layer and subgrade [Kadiyali, 2006]. Wearing Course Base Course or Binder Course

Surfacing

Roadbase

Sub-base

Subgrade Figure 1.1: Definition of Pavement layers Source: TRL (1993)

a) Subgrade This is the top surface of a roadbed on which the pavement structure and shoulders including kerbs are constructed. Generally the top soil portion up to 500mm of the embankment or cut section is referred to as the subgrade [Bindra, 1999]. It may be undisturbed local material or may be soil excavated elsewhere and placed as fill. The loads on the pavement are ultimately received by the subgrade layer; it is therefore, essential that the layer should not be over-stressed. The top part of the layer requires preparation to receive the layers above either by stabilizing it adequately (which reduces the required pavement thickness) or designing and constructing a sufficiently thick pavement to suit subgrade strength [TRL, 1993]. The subgrade strength depends on: • The type of material; • Moisture content; • Dry density; • Internal structure of the soil particles; • Type and mode of stress applied. The major factors that influence pavement thickness are; design wheel load, strength of subgrade (and other pavement materials), climatic and environmental factors [Singh, 2001]. b) Capping Layer (Selected or Improved Subgrade) A capping layer may consist of better quality subgrade material brought in from somewhere else or from existing subgrade material improved by mechanical or chemical stabilisation. It is usually justified where weak soils are encountered [TRL, 1993]. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Elements of a Flexible Pavement

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c) Sub-base This is the secondary load-spreading layer underlying the roadbase. It will normally consist of a material of lower quality than that used in the roadbase (but of higher quality than the subgrade) such as unprocessed natural gravel, gravel-sand, or gravel-sand-clay. It may or may not be present as a separate layer since its presence is justified by the insufficiency of the subgrade or to enhance reliability of the pavement performance [TRL, 1993]. The functions of this layer are to: • Distribute stresses to the subgrade - the sub base material must therefore be stronger than the subgrade material; • Act as a drainage layer in case the subgrade is poor - A good drainage layer should be able to drain very fast if water is logged, but also must be able to retain some moisture in times of extreme drought; • Prevent capillary attraction effect; • Serve as a separating layer preventing contamination of the roadbase by the subgrade material; • Protect the subgrade from damage by construction traffic especially under wet conditions. The sub-base is omitted when the subgrade is a hard intact rock or if it is granular and has a CBR greater than 30% and without a high water table [TRL, 1993]. d) Roadbase The roadbase is the main load-spreading layer of the pavement. It is structurally the most important layer of a flexible pavement. It distributes the applied wheel load to the subgrade in such a way that the bearing capacity of the subgrade soil is not exceeded. This layer requires higher quality material often obtained by stabilizing sub-base materials. It will normally consist of crushed stone or gravel, or of gravely soils, decomposed rock, sands and sand-clays stabilised with cement, lime or bitumen [TRL, 1993]. e) Surfacing The surfacing forms the topmost solid layer of the pavement usually designed to be smooth and to withstand erosion from traffic and weather [Urbanik, 2007]. It usually consists of a bituminous surface dressing or a layer of premixed bituminous material. It is comparatively thin, but resists abrasion and the impacts caused by wheel loads and the effects of weather condition [Bindra, 1999]. The functions of this layer are to: • Provide a safe and comfortable riding surface to traffic; • Take up wear and tear stresses caused by traffic; • Provide a water tight surface against infiltration of water; • Provide a hard surface which can withstand tyre pressure. Where premixed materials are laid in two layers, these are known as the wearing course and the base course (or binder course) as shown in Figure 1.1 [TRL, 1993]. 4


Overview of the Design Process

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1.3 Overview of the Design Process ORN31 (TRL, 1993) sums up the overall process of pavement design as constituting the following major activities: 1. Surveying possible routes - which are part of the feasibility study process; 2. Assessing traffic; 3. Measuring subgrade strength; 4. Selecting pavement materials; 5. Selecting the type of pavement structure with subsequent design of the drainage system. This is then followed by the actual implementation of the project. However, the three major steps to be followed in designing a new road pavement are: • Traffic Assessment; which involves estimation of the amount of traffic and the cumulative number of equivalent standard axles that will use the road over the selected design life; • Subgrade strength assessment; which entails assessing the strength of the subgrade soil over which the road is to be built; • Material selection; which involves the selection of the most economical combination of pavement materials and layer thicknesses that will provide satisfactory service over the design life of the pavement. It is usually necessary to assume that an appropriate level of maintenance is also carried out throughout the design life of the road [TRL, 1993]. The following chapters will consider each of these steps in turn and put special emphasis on five aspects of design that are of major significance in designing roads in most tropical countries: • The influence of tropical climates on moisture conditions in road subgrades; • The severe conditions imposed on exposed bituminous surfacing materials by tropical climates and the implications of this for the design of such surfacing; • The interrelationship between design and maintenance. If an appropriate level of maintenance cannot be assumed, it is not possible to produce designs that will carry the anticipated traffic loading without high costs to vehicle operators through increased road deterioration; • The high axle loads and tyre pressures which are common in most countries; • The influence of tropical climates on the nature of the soils and rocks used in road building. The overall process of designing a road is illustrated in Figure 1.2. Some of the information necessary to carry out the tasks may be available from other sources e.g. from a feasibility study or from Ministry records, but all existing data will need to be checked carefully to ensure that it is both up-to-date and accurate. Likely problem areas are highlighted in the relevant chapters of these notes.

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Overview of the Design Process

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6 Figure 1.2: The Pavement Design Process Source: TRL (1993)


Design Approaches

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1.4 Design Approaches There are two basic approaches to design namely empirical and semi-empirical methods [Arora, 2000]. Empirical methods include: • Group index method; • CBR method (or thickness design method) Semi - empirical methods include: • AASHTO method; • Tri-axial test; • Nottingham method; • California Resistance Value Test; • McLeod method; and • Banister method. In Uganda, the AASHTO and Thickness design methods are most commonly applied. These methods will be looked at in more detail during the assessment of subgrade strength. The Group index method is limited as it considers only the particle distribution of the soil and its atterberg limits.

1.5 Design Standards a) General Design of flexible pavements in Uganda has been based on a number of design standards that include the TRL, Overseas Road Note 31 (1993), Uganda Road Design Manual (2005), the Kenya Road Design Manual and the American Association of State Highways and Transportation Officials (AASHTO) interim guides for design of pavement structures 1972-1986. The latest version of the AASHTO design guide was printed in 1993. The above design guides have been adopted to suit most materials and climatic conditions found in developing countries. The AASHTO design equation in the design guide 19721986 was also modified through research done by the World Bank to suit conditions in developing countries. b) Uganda Road Design Manual The Uganda Road Design Manual 2005 has incorporated the pavement design guide prepared for SATCC countries. The SATCC design guide was developed for Southern Africa Transport and Communication Commission for use in Tanzania, Zambia, Zimbabwe Mozambique, Malawi, Swaziland, Lesotho, Angola, and Botswana [Thagesen, 1996]. The method follows the AASHTO design concept as set forth in AASHTO interim guides for design of pavement structures 1972-1986 published by the American Association of State Highways and Transport Officials. The pavement strength required for a given combination of subgrade bearing capacity, traffic load, service level and climate is expressed by means of the subgrade structural number. Layer coefficients, according to the position in the structure, are given to determine the structural number of the pavement. For each type of pavement, the thickness of the base and sub base layers are determined so that the required structural number is satisfied [Uganda Road Design Manual, 1994].


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Questions c)

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Kenya Road Design Manual The materials and pavement design in the Kenya Road Design Manual sets forth the standards for structural design of new bitumen surfaced roads in Kenya. The Kenya Road Design Manual includes design of gravel wearing course on unpaved roads.

d) TRL Overseas Road Note (ORN) 31 The British Transportation and Road Research Laboratory (TRRL) published the first version of Road Note 31 in 1962 and subsequently revised it in 1976 and 1977. The Road Note 31 has in 1993 undergone a comprehensive revision by the transport research laboratory (TRL) and now includes the structural catalogue where a layer thickness can be selected for a whole range of common pavement combinations. The guidelines are based on an empirical method taking into account the organisation’s vast experience in understanding the behaviour of road building materials and their interactions in composite pavements. e)

Conclusive Remark It is important for engineers, especially those in developing countries like Uganda, to exercise judgement in the use of a given design standard to ensure that they come up with an economical solution for a pavement design. Use of local materials has to always be taken into consideration. Sometimes, more than one design standard is used for the purposes of comparing one pavement design with another so that the comparison guides the engineer in selecting the most economical option.

1.6 Questions a) b) c) d)

In less than 300 words, write an executive summary about the pavement design process. Discuss the elements that make up a flexible pavement structure and their significance. Discuss the three major activities that constitute the pavement design process. Mr. Juan Chávez Fernández is a Spanish Highway Consultant who wishes to open up a new Highway Engineering firm in Kampala. He is faced with a problem of deciding which highway design standards he should use in order to come up with the most economical pavement designs. Using the knowledge you have acquired from your recent training in highway engineering, advice Mr. Fernández.

1.7 Bibliography 1. Arora, K. R., 2000, Soil Mechanics and Foundation Engineering, 5th Edition. 2. Bindra, S.P, 1999, A Course in Highway Engineering, 4th Edition, Dhanpat Rai Publishers, New Delhi. 3. Gupta, B.L., 1995, Roads, railways Bridges and Tunnels engineering, 4th edition, Standard publishers Distributors, Nai sarak, Delhi. 4. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including Expressways and Airport Engineering), 4th Edition. Khanna Publishers, New Delhi. 5. Ministry of Works, and Transport, 1994. Road Design Manual, Republic of Uganda, Kampala. 6. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and Maintenance of Pavements. 4th Edition, Oxford, Butterworth Heinemann. 7. Rogers, M., 2003, Highway Engineering, Oxford, Blackwell Publishing Ltd. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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9 8. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil Engineering, Kyambogo University. 9. Singh, G., 2001, Highway Engineering, 3rd edition, Standard publishers and Distributors, Delhi. 10. Transport Research Laboratory, 1993, A Guide to Design of Bitumen Surfaced Roads in Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England. 11. Urbanik, T., 2007, “Road", Microsoft® Student 2008 [DVD]. Redmond, WA: Microsoft Corporation.

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Chapter Two:

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Chapter Two:

Traffic Assessment 2.1 General On completion of the route location process (not discussed in these notes), the designer is then expected to estimate the amount of traffic and the cumulative number of equivalent standard axles that will use the road over the selected design life. In this step, other subactivities include: measurement of traffic volume by class; measurement of axle loads; choosing the design life and the calculation of the total traffic in esa or msa. The final step at this stage is the assignment of a traffic class to the amount of traffic ascertained with guidance from the structure catalogue in chapter five. It should be noted that information on traffic flow of vehicles past a given point in a specified time period provides a key input to decisions on the planning, design and operation of transport systems. This data is used in highway planning and helps in the design of road pavements, establishment of control measures, carrying out of cost benefit analyses and studying accident patterns in relation to traffic volume. Accuracy of traffic flow data is therefore extremely critical as any inaccurate data is useless for any design purposes [O’Flaherty, 2002].

2.2 Estimation of Traffic Flows (F) a) Introduction Generally, heavier loads require thicker pavements provided other design factors remain constant. The structural design of a pavement largely depends on the traffic (or design wheel load) projected to use that pavement. In design of a pavement, knowledge of the maximum wheel load is more important than gross weight of vehicles [Gupta, 1999]. During design, emphasis is placed on commercial and heavy goods vehicles whose axle weight is greater than 1,500 kg. It is these classes of vehicle that are most damaging to the pavement making their volumes a critical parameter in design [TRL, 1993]. When designing a new road, the total flow of commercial vehicles in one direction per day at the roads opening are normally required in order to determine the cumulative design traffic over the design life. For purposes of pavement design, vehicles weighing less than 1500 kg may be ignored. If the traffic flow figures available are for two way flow, the directional split is assumed to be in ratio 1:2 (in favour of the heavily trafficked lane) unless traffic studies show otherwise [Kadiyali, 2006]. The distribution of commercial vehicle traffic can be expected to vary at particular points along the road e.g. where lanes leave or join a carriageway, or at traffic signals or at roundabouts. Nonetheless in the design of new roads the traffic distribution considered is that away from junctions. All lanes are designed to carry the heaviest traffic load assessed from the most trafficked lane.


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Estimation of Traffic Flows (F)

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b) Baseline Traffic Flows (Fo) In order to determine the total traffic over the design life of the road, the first step is to estimate baseline traffic flows. The estimate should be the (Annual) Average Daily Traffic (AADT) currently using the route, classified into the vehicle categories of cars, light goods vehicles, trucks (heavy goods vehicles) and buses. The AADT is defined as the total annual traffic summed for both directions and divided by 365. It is usually obtained by recording actual traffic flows over a shorter period from which the AADT is then estimated. For long projects, large differences in traffic along the road may make it necessary to estimate the flow at several locations. It should be noted that for structural design purposes the traffic loading in one direction is required and for this reason care is always required when interpreting AADT figures. Traffic counts carried out over a short period as a basis for estimating the traffic flow can produce estimates which are subject to large errors because traffic flows can have large daily, weekly, monthly and seasonal variations. The daily variability in traffic flow depends on the volume of traffic. It increases as traffic levels fall, with high variability on roads carrying less than 1000 vehicles per day. In order to reduce error, it is recommended that traffic counts to establish ADT at a specific site conform to the following practice: i) The counts are for seven consecutive days. ii) The counts on some of the days are for a full 24 hours, with preferably at least one 24-hour count on a weekday and one during a weekend. On the other days 16-hour counts should be sufficient. These should be grossed up to 24-hour values in the same proportion as the 16-hour/24 hour split on those days when full 24-hour counts have been undertaken. iii) Counts are avoided at times when travel activity is abnormal for short periods due to the payment of wages and salaries, public holidays, etc. If abnormal traffic flows persist for extended periods, for example during harvest times, additional counts need to be made to ensure this traffic is properly included. iv) If possible, the seven-day counts should be repeated several times throughout the year [TRL, 1993]. The following steps are generally taken when carrying out a traffic survey; i) Traffic count data sheets are made indicating the classification of vehicles, i.e. cars, pick-ups, minibuses, buses, trucks and trailers (See figure 2.1 for a sample of a classified count data sheet); ii) Traffic count stations along the road are then identified; iii) Enumerators who are trained to carry out the traffic survey are positioned at the identified station.

11



12

Figure 2.1: Form for Manual Classified Counts Source: TRL (2004)

c) Projected Traffic (Fp) Forecasting traffic growth is a difficult exercise and may involve uncertainty in growth predictions. Some factors considered include economic growth, vehicle growth, and land Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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13

use development. These factors are considered together with traffic modelling. To reduce this uncertainty, sensitivity and risk analyses are involved in the process [TRL, 1993]. Even with a developed economy and stable economic conditions, traffic forecasting is an uncertain process. In a developing economy the problem becomes more difficult because such economies are often very sensitive to the world prices of just one or two commodities. In order to forecast traffic growth it is necessary to separate traffic into the following three categories: a) Normal traffic; Traffic which would pass along the existing road or track even if no new pavement were provided. b) Diverted traffic; Traffic that changes from another route (or mode of transport) to the project road because of the improved pavement, but still travels between the same origin and destination. c) Generated traffic; Additional traffic which occurs in response to the provision or improvement of the road. For existing roads, the greatest traffic contribution is from the normal traffic. We shall therefore only examine the methods used to forecast normal traffic (The student is encouraged to find out how the other two types of traffic are forecast). The commonest method of forecasting normal traffic is to extrapolate time series data on traffic levels and assume that growth will either remain constant in absolute terms i.e. a fixed number of vehicles per year (a linear extrapolation), or constant in relative terms i.e. a fixed percentage increase [TRL, 1993]. A constant growth rate formula shown below is normally used to project the traffic to the design year. 1

… . 2.1

Where, Fp

=

Cumulative number of commercial vehicles after ‘n’ years

Fo

=

Present number of vehicles after the traffic survey;

r

=

Growth rate of commercial vehicles;

n

=

Number of years of projection.

d) Axle Loading (W) i) Axle Equivalency The damage that vehicles do to a road depends very strongly on the axle loads of the vehicles. For pavement design purposes the damaging power of axles is related to a 'standard' axle of 8.16 tonnes using equivalence factors which have been derived from empirical studies. In order to determine the cumulative axle load damage that a pavement will sustain during its design life, it is necessary to express the total number of heavy vehicles that will use the road over this period in terms of the cumulative number of equivalent standard axles (esa). Axle load surveys must be carried out to determine the axle load distribution of a sample of the heavy vehicles using the road. Data collected Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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from these surveys are used to calculate the mean number of equivalent standard axles for a typical vehicle in each class. The axle loading for each category of commercial vehicle is the sum of the front and rear axles. For commercial vehicles with more than one rear axle, the total equivalent standard axle for the vehicle will be the sum of the front and each of the rear equivalent standard axles. These values are then used in conjunction with traffic forecasts to determine the predicted cumulative equivalent standard axles that the road will carry over its design life. The wear factor can be calculated from the following equation;

, 8.16

.

. . … . 2.2

ii) Axle Load Surveys If no recent axle load data is available, it is recommended that axle load surveys of heavy vehicles are undertaken whenever a major road project is being designed. Ideally, several surveys at periods which will reflect seasonal changes in the magnitude of axle loads are recommended. It is also recommended that axle load surveys are carried out by weighing a sample of vehicles at the roadside. The sample should be chosen such that a maximum of about 60 vehicles per hour are weighed. The weighing site should be level and, if possible, constructed in such a way that vehicles are pulled clear of the road when being weighed. The portable weighbridge should be mounted in a small pit with its surface level with the surrounding area. This ensures that all of the wheels of the vehicle being weighed are level and eliminates the errors which can be introduced by even a small twist or tilt of the vehicle.

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Figure 2.2: Axle Load Survey Form A for recording vehicle survey data Source: TRL (2004)


15

15


Figure 2.3: Axle Load Survey Form B for recording vehicle wheel loads Source: TRL (2004)


16

16


17

e) Growth Factor (G) Growth is assumed to be compound over the design period. The Portland Cement Association developed a formula that applies traffic at the middle of the design period as the design traffic as shown below; .

1

… . 2.3

Where; G r n

= = =

the growth factor; the growth rate; and the design period

The ‘Asphalt Institute’ and the ‘AASHTO Design Guide’ recommend the use of traffic over the entire design period to determine the total growth factor as follows; 1

1 .

… . 2.4

Where; G, r and n are as previously defined.

f) Design Life (Y) The design period is the time during which the road will accommodate traffic at a satisfactory level of service without requiring capital intervention (or further funding) in the form of rehabilitation or strengthening. For most road projects an economic analysis period of between 10 and 20 years from the date of opening is appropriate, but for major projects this period should be tested as part of the appraisal process discussed in Overseas Road Note 5, TRL (1988). Below is a table used to guide the pavement designer in choosing the appropriate design life as recommended by the Ministry of Works and Transport, Pavement Design Manual (2005). Table 2.1: Selection of Design Life Design Data Relibility

Importance/Level of Service Low

High

Low

10 - 15 yrs

15 yrs

High

10 - 20 yrs

15 - 20 yrs

Source: Uganda Road Design Manual (2005)

A pavement design life of 15 years also reduces the problem of forecasting uncertain traffic trends for long periods into the future. It should be noted that design life does not mean that at the end of the period the pavement will be completely worn out and in need of reconstruction; it means that towards the end of the period the pavement will need to be strengthened so that it can continue to carry traffic satisfactorily for a further period.

17


Determination of Cumulative Standard Axles (T)

18

2.3 Determination of Cumulative Standard Axles (T) A successful outcome of the pavement design process in any given instance is dependent upon the accuracy with which the total number of standard axle loads, and their cumulative wear or damage effects, can be predicted for the design lane(s) over the period of the selected design life. Below is a summary of the steps involved in carrying out a full traffic assessment: a) Estimate the present one-way commercial vehicle flow or the traffic flow, F, at the opening of a new road. For each class of vehicles select the initial design period Y; b) Determine the appropriate average wear factor, W, to be used with each vehicle class; c) Determine the growth factor, G, for each category of vehicle; d) Calculate the cumulative design traffic in each vehicle class using the equation shown below: … . 2.5 Where; 365

10

… . 2.6

And; i

=

vehicle class

Note: In case of a two-way single carriage-way pavement, the total design traffic, T, is the summation of the cumulative design traffic in each category in a given direction. In case of a dual carriageway road the proportion of vehicles in the most heavily trafficked lane is normally obtained and applied to the total accumulation to derive the design traffic. g) Channelization Factor (Ch) In certain cases, the equation for the cumulative design traffic includes a channelization factor, thus Ti = 365 F.W.G.Y.Ch (10-6) msa. In the urban area it is relevant to consider the effects of vehicle channelization which may be caused by various factors. In 1983 the County Surveyors Society report entitled “Vehicle Damage Factors Present, Past and Future Values” indicated that where the normal tendency for transverse wander is constrained by, for example , traffic islands then the damaging effect can be at least twice that normally expected. In narrow urban streets where one street parking is permitted there is a tendency for buses and Heavy Goods Vehicles (HGV’s) to use the same wheel tracks when passing in both directions [Ruhweza, 2005]. The effect of bus stop areas is another location where an increased axle load factor may be relevant. The same applies to traffic signal junctions and roundabouts. In the case of long severe gradients where there are significant HGV flows, there is an indication that the normal damaging effect calculations may not be adequate. The following overall multipliers are suggested:


18

Example 2.1: Traffic Assessment

19

Table 2.2: Axle Factor Multipliers Effect

Multiplier

Traffic island

2.0

Parking

2.0

Traffic Signals

1.5

Roundabouts

1.5

Severe Bends

1.5

Steep Hill

2.0

Source: County Surveyors Society (1983)

N.B: It is usually advised that the total multiplier used should not exceed 3.0. In certain locations and circumstances, it may be appropriate to consider the multipliers to be cumulative. Table 2.3 below shows the various traffic classes and their corresponding equivalent standard axles, in msa. Table 2.3: Traffic Classes Traffic Classes Ranges (msa) T1 < 0.3 T2 0.3 - 0.7 T3 0.7 - 1.5 T4 1.5 - 3.0 T5 3.0 - 6.0 T6 6.0 - 10 T7 10 - 17 T8 17 - 30 Source: TRL (1993)

2.4 Example 2.1: Traffic Assessment The Kampala – Gayaza road is in a state of failure and is due for reconstruction. Tables 2.4 and 2.5 below show the results of a traffic survey at different stations on the above road. The survey was carried out on 1st January, 2008 and construction is to begin in December, 2009. The road is expected to be opened to traffic on 1st January, 2010. Table 2.4: Two-Way Traffic Volume at the Project Road (in vehicles/day) Vehicle Class Minibuses Buses Pick-Ups 2-Axle Trucks 3-Axle Trucks Total

A 4,013 220 850 182 23 5,288

Table 2.5: Axle Weights (in tonnes) Description Mini Buses Gross Weight (t) 3.00 Front Axle Load (t) 1.00 Rear Axle Load 1 (t) 2.00 Rear Axle Load 2 (t) -

Station B C 2,271 4,647 82 5 348 1,621 255 447 11 73 2,967 6,793

Buses 15.00 3.00 6.00 6.00

D 4,507 3 1,845 507 44 6,906

Growth Rate r% 6.0% 4.5% 6.0% 5.0% 4.0%

Pick-Ups Cars 2-AxleTrucks 3.00 12.00 1.00 4.00 2.00 8.00 -

3-AxleTrucks 20.00 4.00 8.00 8.00

Considering the design traffic loading at station “D” and assuming a 15 year design period, design the pavement using the TRL approach. (Assume that the traffic grows at the rates indicated for each vehicle class in Table 2.4 above). Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Example 2.1: Traffic Assessment

Solution 1.0 Design Information (a) Traffic growth rate, r (b) Design life, Y (c) Construction Period, n 2.0

= = =

20

6% 15 yrs 2 yrs (2010 – 2008)

Determination of cumulative design traffic, T Where; 365

2.1

10

Unidirectional traffic Flow, F Assuming a 1:2 directional traffic split, then; 2 of the traffic volume for each vehicle class 3 e.g. for minibuses at station D; 2 4508 3005 veh/day 3 3005 1

2.2

Wear factor, W From equation 2.2

0.06

, 8.16

e.g. for minibuses at station D; 2.00 . 1.00 . 8.16 8.16 2.3

3376 veh/day

.

0.0001

0.0018

0.0019

Growth Factor, G According to the Portland Cement Association (equation 2.3); the growth factor for minibuses at station D is given by; 1

0.06

.

1.5481

20


Questions 2.4

21

Table of results Wear Factor, W Rear 2 Vehicle Class Front Axle Rear 1 (esa) (esa) (esa) 0.0001 0.0018 0.0000 Minibuses 0.0111 0.2507 0.2507 Buses 0.0001 0.0018 0.0000 Pick-Ups 0.9147 0.0000 2-Axle Trucks 0.0404 0.9147 0.9147 3-Axle Trucks 0.0404 Cumulative Design Traffic, T (in msa)

Rate, r % 6.0% 4.5% 6.0% 5.0% 4.0%

F Fo Fp W (Veh/d) (Veh/d) (esa) 3,005 3,376 0.0019 2 2 0.5124 1,230 1,382 0.0019 338 373 0.9552 29 32 1.8699

G 1.5481 1.3911 1.5481 1.4418 1.3420

Y Ti (yrs) (msa) 15 0.053 15 0.008 15 0.022 15 2.813 15 0.440 3.335

From table 2.3 a cumulative design traffic of 3.335 msa corresponds to a traffic class of T5 i.e. 3.0 < T (in msa) < 6.0.

2.5 Questions a)

On 1st December 2006, Mr. Fernández was awarded a contract to upgrade Kansanga – Lukuli road, which is in an alarming state of failure, to a bituminous surfaced road. He now wishes to carry out a baseline traffic survey together with an axle load survey to ascertain the amount of traffic currently using the road and the amount of axle loading on the project road respectively. Describe in detail how you would expect him to carry out the above surveys in preparation for pavement design.

b) Upon completion of the surveys in part (b) above, Mr. Fernández summarised his findings in Table 2.6 to Table 2.9 below. The tables show the results of the traffic and axle load surveys at station ‘A’ on the project road. The survey was carried out in December, 2007 and construction is to begin in January, 2009. The road is expected to be opened to traffic on 31st December, 2012 with traffic projected to grow at a rate of 4%. Ascertain the design traffic loading for station ‘A’ assuming that the design data reliability is low yet the expected level of service is high. Assign a traffic class to this section and comment on your results. Table 2.6: Axle Loads (in tons) Axle Load Weights (in tons) Gross

Front

Rear

Rear

Weight

Axle

Axle 1

Axle 2

(tons)

(tons)

(tons)

(tons)

Vans, PickUps & 4WDs

3.00

1.00

2.00

Minibuses and Matatus

3.00

1.00

2.00

Coasters

3.00

1.00

2.00

Vehicle Class

9.00

3.00

6.00

Dynas and Tractors

12.00

4.00

8.00

2-Axle Trucks

12.00

4.00

8.00

3-Axle Trucks

20.00

4.00

8.00

Buses

8.00

21


Questions

22

Table 2.7: Friday 24-hr count summary Heavy Commercial Goods Vehicles Buses Dynas & Truck Truck Tractors 2-Axle 3-Axle 0 6 1 0 0 2 4 0 1 1 3 0 0 6 0 0 0 4 9 0 0 0 4 0 0 3 3 0 0 0 4 0 0 3 0 0 0 7 2 0 0 4 1 0 0 2 0 0 0 5 2 0 0 2 0 0 0 2 1 0 0 1 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0

Light Goods Vehcles

Duration of count 07:00-08:00 a.m. 08:00-09:00 09:00-10:00 10:00-11:00 11:00-12:00 p.m. 12:00-01:00 01:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00 07:00-08:00 08:00-09:00 09:00-10:00 10:00-11:00 11:00-12:00 a.m. 12:00-01:00 01:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00

Saloon Cars 136 111 50 69 40 41 25 58 98 50 34 41 52 46 38 55 27 19 9 7 7 4 11 18

Vans, Pick- Minibuses Ups & 4WD & Matatus 125 100 144 70 57 34 65 24 47 30 50 32 44 23 79 44 120 52 49 27 55 40 43 38 41 27 26 34 24 35 32 55 12 25 3 17 6 3 4 0 5 0 3 1 4 6 3 26

Coasters 3 0 0 1 1 5 1 2 3 5 3 1 0 0 0 0 0 0 0 0 0 0 0 0

Two Wheeled Motor Bicycles Cycles 68 19 109 14 0 12 63 10 60 5 67 11 53 5 38 3 63 5 61 5 62 8 60 9 84 8 124 3 102 6 84 2 65 2 74 1 50 0 26 0 29 0 41 0 43 2 36 8

Carts 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Table 2.8: Sunday 24-hr count summary Heavy Commercial Goods Vehicles Buses Dynas & Truck Truck Tractors 2-Axle 3-Axle 0 6 1 0 0 1 0 0 0 4 0 0 0 4 0 0 0 2 0 0 0 4 0 0 0 4 0 0 0 1 0 0 0 4 0 0 0 7 0 0 0 0 0 0 0 0 3 0 0 3 1 0 0 5 1 0 0 3 1 o 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 2 0 0

Light Goods Vehcles

Duration of count 07:00-08:00 a.m. 08:00-09:00 09:00-10:00 10:00-11:00 11:00-12:00 p.m 12:00-01:00 01:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00 07:00-08:00 08:00-09:00 09:00-10:00 10:00-11:00 11:00-12:00 a.m. 12:00-01:00 01:00-02:00 02:00-03:00 03:00-04:00 04:00-05:00 05:00-06:00 06:00-07:00

Saloon Cars 9 30 43 38 48 45 48 50 34 32 50 60 53 41 38 33 11 18 3 3 1 1 4 28

Vans, Pick- Minibuses Ups & 4WD & Matatus 19 38 25 23 58 32 50 32 57 21 58 23 62 32 74 33 41 27 48 28 42 34 36 37 52 49 36 51 16 45 10 34 12 30 4 21 3 9 0 2 1 1 1 1 3 12 18 38

Coasters 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 1

Two Wheeled Motor Bicycles Cycles 59 11 28 15 88 10 111 4 72 5 58 12 66 6 56 9 44 2 56 9 50 4 43 0 84 8 77 8 44 1 46 0 56 3 49 1 39 0 30 0 27 0 28 0 30 2 40 11

Carts 1 0 0 0 2 1 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0

Table 2.9: 16-hr counts on the remaining days Day

Saloon Cars

Vans, PickUps & 4WD

WED THU SAT MON TUE

921 1044 646 804 974

836 1236 576 3290 1075

Minibuses Coasters & Matatus

553 950 606 785 671

17 60 39 16 16

Buses

Dynas & Tractors

Truck 2-Axle

Truck 3-Axle

Motor Cycles

Bicycles

Carts

0 0 0 2 2

89 91 71 54 52

18 51 8 41 59

1 2 0 5 4

1080 1482 1183 1281 1259

142 240 119 130 152

10 8 2 9 1


22

Bibliography

23

2.6 Bibliography 1. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including Expressways and Airport Engineering), 4th Edition. Khanna Publishers, New Delhi. 2. Ministry of Works, and Transport, 2005. Road Design ManualVol.III, Pavement Design Manual, Republic of Uganda, Kampala. 3. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and Maintenance of Pavements. 4th Edition, Oxford, Butterworth Heinemann. 4. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil Engineering, Kyambogo University. 5. Transport Research Laboratory, 1993, A Guide to Design of Bitumen Surfaced Roads in Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England. 6. Transport Research Laboratory, 2004, A Guide to Axle Load Surveys and Traffic Counts for Determining Traffic Loading on Pavements, Overseas Road Note 40, Crowthorne, England.

23


Chapter Three:

24

Chapter Three:

Subgrade Strength Assessment 4.1 General This third stage of the pavement design process can be carried out concurrently with the traffic assessment immediately following the route location process. The sub-activities involved in assessing the subgrade strength of the soil are: Assignment of climatic a regime, testing of soils, definition of uniform sections, and designing of earth works [TRL, 1993]. Properties of the subgrade soil are important in designing the depth of the pavement. Weak subgrade material requires higher thickness to protect it from traffic loads. Pavement deformation mainly depends on the subgrade properties and drainage. During design and construction, proper drainage has to be maintained in order to control pavement deformation. Climatic factors are important here because rainfall affects the moisture of the subgrade and pavement layers. The daily and seasonal variations of rainfall are important in the design and performance of the pavement. Where the water table is close to the formation level of the roads, adjustments in the design of the pavement layer thicknesses are necessary. Embankment heights and the depth of water table below the embankment have an effect on the performance of an embankment and must be examined [Kadiyali, 2000, Arora, 2000]. The strength of road subgrades is commonly assessed in terms of the California Bearing Ratio (CBR) and this is dependent on the type of soil, its density, and its moisture content. For designing the thickness of a road pavement, the strength of the subgrade should be taken as that of the soil at a moisture content equal to the wettest moisture condition likely to occur in the subgrade after the road is opened to traffic. In the tropics, subgrade moisture conditions under impermeable road pavements can be classified into three main categories. Some of the key tests in the design of the subgrade include the Compaction test, the Dynamic Cone Penetrometer test and the California Bearing Ratio (CBR) test.

4.2 Climatic Regime a)

Category (1) Category (1) subgrades are those in which the water table is sufficiently close to the ground surface to control the subgrade moisture content. The type of subgrade soil governs the depth below the road surface at which a water table becomes the dominant influence on the subgrade moisture content. For example, in non-plastic soils the water table will dominate the subgrade moisture content when it rises to within 1 m of the road surface, in sandy clays (PI<20 per cent) the water table will dominate when it rises to within 3m of the road surface, and in heavy clays (PI>40 per cent) the water table will dominate when it rises to within 7m of the road surface. In addition to areas where the water table is maintained by rainfall, this category includes coastal strips and flood plains where the water table is maintained by the sea, by a lake or by a river.


24

Testing Subgrade Soils

25

b) Category (2) Category (2) subgrades are those with deep water tables and where rainfall is sufficient to produce significant changes in moisture conditions under the road. These conditions occur when rainfall exceeds evapotranspiration for at least two months of the year. The rainfall in such areas is usually greater than 250 mm per year and is often seasonal. c)

Category (3) Category (3) subgrades are those in areas with no permanent water table near the ground surface and where the climate is dry throughout most of the year with an annual rainfall of 250 mm or less. Direct assessment of the likely strength or CBR of the subgrade soil is often difficult to make but its value can be inferred from an estimate of the density and equilibrium (or ultimate) moisture content of the subgrade together with knowledge of the relationship between strength, density and moisture content for the soil in question. This relationship must be determined in the Laboratory. The density of the subgrade soil can be controlled within limits by compaction at suitable moisture content at the time of construction. The moisture content of the subgrade soil is governed by the local climate and the depth of the water table below the road surface. In most circumstances, the first task is therefore to estimate the equilibrium moisture content. A method of direct assessment of the subgrade strength, where this is possible, will be discussed later together with less precise methods of estimation which can be used if facilities for carrying out the full procedure are not available.

d) Equilibrium Moisture Content An impervious road surface isolates soil from rainfall, evaporation and plant transpiration. After the construction of an impervious pavement, the moisture content within the soil tends to settle to a set of more or less steady values. For each depth there is a particular set of values referred to as the equilibrium moisture content. It has a value between the wetter and drier values of moisture content in an unprotected subgrade during the wetter and drier months respectively. For economic reasons, a roadway should be designed to suit the subgrade when it has reached equilibrium moisture content conditions. For small works the equilibrium moisture content can be taken as being equal to the moisture content occurring in the natural soil at a depth of 1m, provided that this soil is the same as the soil that will be the formation level [Ruhweza, 2005].

4.3 Testing Subgrade Soils Before the subgrade strength can be determined the probable equilibrium moisture content of the pavement during its design life must be determined. a) Equilibrium Moisture Content Category (1); The easiest method of estimating the design subgrade moisture content is to measure the moisture content in subgrades below existing pavements in similar situations at the time of the year when the water table is at its highest level. These pavements should be greater than 3m wide and more than two years old and samples should preferably be taken from under the carriageway about 0.5m from the edge. Allowance can be made for different soil types by virtue of the fact that the ratio of subgrade moisture content to plastic limit is the same for different subgrade soils when the water table and climatic conditions are similar. If there is no suitable road in the vicinity, the moisture content in the subgrade under an impermeable pavement can be estimated from knowledge of the Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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26

depth of the water table and the relationship between suction and moisture content for the subgrade soil [TRL, 1993]. The test apparatus required for determining this relationship is straightforward and the method is described below: The subgrade moisture content under an impermeable road pavement can increase after construction where a water table exists close to the ground surface. This ultimate moisture content can be predicted from the measured relationship between soil suction and moisture content for the particular soil and knowledge of the depth of water table. Measuring the complete relationship between suction and moisture content is time consuming and a simpler, single measurement procedure can be used. A small sample of soil, compacted to field density and moisture content, is placed within suitable laboratory equipment that can apply a pressure equivalent to the 'effective depth' of the water table (e.g. a pressure plate extractor). The 'effective depth' of the water table for design purposes comprises the actual depth from the subgrade to the water table plus an apparent depression of the water table due to the pressure of the overlying pavement. This apparent depression varies with soil type and approximate correction factors used in calculating effective depth of water table are given in table 3.1 below: Table 3.1: Water Table Correction Factors for Soil Type PI PI

Correction Factor, SF

0

0.00

10

0.30

15

0.55

20

0.80

25

1.10

30

1.40

35

1.60

>35

2.00

Source: TRL, 1993

To calculate the effective depth D which is used to determine the applied suction in the pressure plate extractor, the following equation is used: … . 2.7 Where; WT = level), SF = t =

Depth of water table below subgrade (at its highest expected seasonal Correction factor from table 3.1, Pavement thickness, with consistent units for WT, t, D

When equilibrium is attained in the pressure plate extractor, the sample is removed and its moisture content measured. This moisture content is the value at which the CBR for design should be estimated following standard soil tests to be discussed later in this chapter. Category (2); When the water table is not near the ground surface, the subgrade moisture condition under an impermeable pavement will depend on the balance between the water entering the subgrade through the shoulders and at the edges of the pavement during wet weather and the moisture leaving the ground by evapotranspiration during dry periods. Where the average annual rainfall is greater than 250 mm a year, the moisture condition for design purposes can be taken as the optimum moisture content given by the British Standard (Light) Compaction Test, 2.5 kg rammer method. When deciding on the depth of Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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27

the water table in Category (1) or Category (2) subgrades, the possibility of the existence of local perched water tables should be borne in mind and the effects of seasonal flooding (where this occurs) should not be overlooked. Category (3); In regions where the climate is dry throughout most of the year (annual rainfall 250 mm or less), the moisture content of the subgrade under an impermeable pavement will be low. For design purposes a value of 80 per cent of the optimum moisture content obtained in the British Standard (Light) Compaction Test, 2.5 kg rammer method, should be used. The methods of estimating the subgrade moisture content for design outlined above are based on the assumption that the road pavement is virtually impermeable. Dense bitumenbound materials, stabilised soils with only very fine cracks, and crushed stone or gravel with more than 15 per cent of material finer than the 75 micron sieve are themselves impermeable (permeability less than 10-7 metres per second) and therefore subgrades under road pavements incorporating these materials are unlikely to be influenced by water infiltrating directly from above. However, if water, shed from the road surface or from elsewhere, is able to penetrate to the subgrade for any reason, the subgrade may become much wetter. In such cases the strength of subgrades with moisture conditions in Category (1) and Category (2) should be assessed on the basis of saturated CBR samples as described in the following section. Subgrades with moisture conditions in Category (3) are unlikely to wet up significantly and the subgrade moisture content for design in such situations can be taken as the optimum moisture content given by the British Standard (Light) Compaction Test, 2.5 kg rammer method. b) Determination of Subgrade Strength Having estimated the subgrade moisture content for design, it is then possible to determine the appropriate design CBR value at the specified density. As a first step, it is necessary to determine the compaction properties of the subgrade soil by carrying out standard laboratory compaction tests. Samples of the subgrade soil at the design subgrade moisture content can then be compacted in CBR moulds to the specified density and tested to determine the CBR values. With cohesionless sands, the rammer method tends to overestimate the optimum moisture content and underestimate the dry density achieved by normal field equipment. The vibrating hammer method is more appropriate for these materials. If samples of cohesive soils are compacted at moisture contents equal to or greater than the optimum moisture content, they should be left sealed for 24 hours before being tested so that excess pore water pressures induced during compaction are dissipated. If saturated subgrade conditions are anticipated, the compacted samples for the CBR test should be saturated by immersion in water for four days before being tested. In all other cases when CBR is determined by direct measurement, the CBR samples should not be immersed since this results in over design. In areas where existing roads have been built on the same subgrade, direct measurements of the subgrade strengths can be made using a dynamic cone penetrometer. Except for direct measurements of CBR under existing pavements, in situ CBR measurements of subgrade soils are not recommended because of the difficulty of ensuring that the moisture and density conditions at the time of test are representative of those expected under the completed pavement. If the characteristics of the subgrade change significantly over sections of the route, different subgrade strength values for design should be calculated for each nominally Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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28

uniform section. The structural catalogue requires that the subgrade strength for design is assigned to one of six strength classes reflecting the sensitivity of thickness design to subgrade strength. The classes are defined in table 2.7. For subgrades with CBR values less than 2, special treatment is required which is not covered in these notes. Table 3.2: Subgrade Strength Classes Class Range (CBR %) S1 2 S2 3-4 S3 5-7 S4 8 - 14 S5 15 - 29 S6 30+ Source: TRL, 1993

The design subgrade strength class together with the traffic class obtained in the traffic assessment is then used in juxtaposition with the catalogue of pavement structures to determine the pavement layer thicknesses. This will be demonstrated in chapter five. It is recommended that the top 250 mm of all subgrades should be compacted during construction to a relative density of at least 100 per cent of the maximum dry density achieved in the British Standard (Light) Compaction Test, 2.5 kg rammer method, or at least 93 per cent of the maximum dry density achieved in the British Standard (Heavy) Compaction Test using the 4.5 kg rammer. With modern compaction plant a relative density of 95 per cent of the density obtained in the heavier compaction test should be achieved without difficulty but tighter control of the moisture content will be necessary. Compaction will not only improve the subgrade bearing strength but will reduce permeability and subsequent compaction by traffic. c)

The Soil Compaction Test These are used whenever compacted earthworks are required especially on road construction and land reclamation projects. Many civil engineering projects require the use of soils as “fill” material. Whenever soil is placed as engineering fill, it is normally compacted to a dense state, so as to obtain satisfactory engineering properties. Compaction on site is usually effected by mechanical means such as rolling, ramming or vibrating. Control of the degree of compaction is necessary to achieve a satisfactory result at a reasonable cost. Laboratory compaction tests provide the basis for control procedures used on site [ELE, 2006]. The test produces two values; a soil density and moisture content. The density value represents a reasonable achievable density which will give a well compacted soil. It is used as a standard against which field densities may be judged, to assess whether earthworks have been adequately compacted. The moisture content value represents (to a good approximation) the best moisture content the soil should have during compaction in order to obtain the maximum benefit from the compactive effort used. The soil density achieved can be expressed in two ways; bulk density which includes the weight of both solids and water, and dry density which considers only soil solids. Dry density is a more appropriate measure of compaction since it represents the state of the soil solids. Testing Procedure A sample of air dried soil is passed through a 20 mm sieve and mixed with water – its amounts being judged by experience during the test. A layer of soil is placed in a standard mould, with a base plate and collar attached. It is compacted by repeatedly dropping the hammer onto the surface. Further layers of soil are added until the mould is slightly


28


29

overfull i.e. with compacted soil protruding slightly into the collar. The collar is then removed and the excess soil is struck off level with the top of the mould using a straight edge. The mould is now weighed to obtain the density of the soil and small specimens are taken for moisture content measurement. The test procedure is repeated for various moisture contents; typically 5-6 values are obtained. From the results, a graph can be drawn to obtain the maximum dry density (MDD) and the optimum moisture content (OMC) at which this is achieved. In addition to MDD and OMC it is useful to know the amount of air present in the compacted soil, since a low air content implies good compaction. The air voids present (i.e. the volume of air expressed as a percentage of the total volume of the soil) can be calculated for any value of dry density and moisture content provided the value of average specific gravity of the soil is accurately known. Compaction is either in three layers using a 2.5 kg rammer falling through 300mm or to give a heavier standard, in five layers using a 4.5 kg rammer falling through 450mm. Table 2.8 below shows the rammer blows per layer required to effect compaction. Usually the same soil is used repeatedly throughout the test such that 5 kg of test soil is sufficient. Table 3.3: Number of blows required in the compaction test Number of Blows

Standard

Light Compaction

Heavy Compaction

American

26

56

British

27

52

Soils containing friable material tend to break down during the test so a fresh sample must be used for each moisture content value. Sands and gravels do not compact well under the standard rammer and give unrealistically low values. To overcome this, a vibrating hammer is used to compact the material. Values obtained from the soil are not fundamental properties of the soil but depend on the compactive effort and the method of compaction. They are nevertheless useful as a guide to specify, monitor and control field compaction [MoWH&C, 1994].

a) The California Bearing Ratio (CBR) Test The CBR value of the subgrade is first obtained using the CBR test. The CBR test is a penetration test developed by the California division of highways as a method of evaluating the stability of the subgrade soil and further pavement moisture ratios. It is usually used in the design of pavements and runways. A cylindrical plunger is pushed at a specific rate into the soil sample and the force required at standard penetrations (typically 2.5mm and 5.0mm) is recorded. The values obtained are expressed as a percentage of the standard values (i.e. plunger load of standard high quality crushed rock of 13.28kN at 2.5mm and 20kN at 5.0mm) to give the CBR value of the soil. The test does not measure any fundamental property of the soil but is specified in many parts of the world as a basis for pavement design. Typical values for some materials are given in table 3.4 below. Table 3.4: Typical CBR Values Material Tilled Farmland Turf or Moist Clay

CBR Value 3% 4.75%

Moist Sand

10%

High quality Crushed Rock

100%


29


30

The CBR test is usually performed on samples prepared in the laboratory in a special mould but may also be carried out as a field test using equipment mounted on the back of a vehicle. The following factors influence the results of the test; the density and moisture content of the sample, whether the sample is soaked before testing, whether and how many surcharge weights are placed on the sample during the test, whether both ends of the sample are tested or just the base and the method of compaction used. The details of the method used must be consistent with what has been assumed in the particular pavement design method to be used. CBR tests require about 7 kg of soil and individual results can show quite a large scatter especially with weak clay soils so a number of tests should be carried out before a design value is adopted. In addition, a compaction test must first be performed so that the sample can be compacted to a suitable condition [MoWH&C, 2005].

Figure 3.1: CBR Test Machine Source: ELE, 2006

30



31

Table 3.5: CBR Preparation Sheet CBR PREPARATION SHEET

Project:

UPGRADING OF KANSANGA - LUKULI ROAD

Location/Source/Depth:

KIZUNGU BORROW PIT

Pit / Borehole No:

Soil Description:

REDDISH BROWN CLAY GRAVEL

Sample No:

1140

Depth:

1.7 m

Ref: BS 1377: Part 4:

Sample Reference:

A001

Sampling Date

Technician:

Geofrey

Testing Date

TP 1

4/13/2006

Volume of Mould (cm3)=

REG.No: 4.5 kg rammer

Bulk density Determination No. of Blows

10 Blows / 5Layers Before Soak

Mould No.

4.5 kg rammer 30 Blows / 5Layers

After Soak

Before Soak

9759

10068

65 Blows / 5Layers

After Soak

Before Soak

10190

11089

11297

After Soak

No.

XX

Weight of Mould + BasePlate + Soil

g

9536

Weight of Mould + BasePlate

g

4769

4769

4728

4728

5481

98% MDD

g

4767

4990

5340

5462

5608

5816

2305

2305

2305

2305

2305

95% MDD

kg/m

2068

2165

2317

2370

2433

No.

5C

1L

YY

5Y

8E

7Y

Weight of wet soil + Tin

g

456.1

369.0

499.0

296.0

326.2

338.0

Weight of Dry soil +Tin

g

424.2

342.0

466.0

276.0

300.0

319.0

Weight of Moisture

g

31.9

27.0

33.0

20.0

26.2

19.0

Weight of Tin

g

92.0

116.0

122.0

92.0

26.7

123.0

Weight of Dry soil

g

332.2

226.0

344.0

184.0

273.3

196.0

Moisture Content

%

9.6

11.9

9.6

10.9

9.6

9.7

DRY DENSITY

kg/m3

1887

1934

2114

2137

2220

2300

Weight of Soil Volume of Moulld

cm3 3

BULK DENSITY

1R

2305

4.5 kg rammer

27R 100% MDD

5481

2305

93% MDD

2523

Moisture Content Determination 90% MDD

Container Number

Date of Moulding Soaking Days Date of Penetration Swell Determination Initial gauge reading

mm

Final gauge reading

mm %

% SWELL Checked by:

Isooba John Bosco

Prepared by: Okello Francis Eugene

Laboratory Technician

Student

Source: Okello, 2006

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32

Table 3.6: CBR Penetration Sheet CBR PENETRATION SHEET

Project:

UPGRADING OF KANSANGA - LUKULI ROAD Ref: BS 1377: Part 4:

Location/Source/Depth:

KIZUNGU BORROW PIT

Pit / Borehole No:

Soil Description:

REDDISH BROWN CLAY GRAVEL

Sample No:

1140

Depth:

1.7 m

Sample Reference:

A001

Sampling Date

Technician:

Geofrey

Testing Date

Plunger Load @ 2.5mm =

13.28

kN

Plunger Load @ 5.0mm =

10 Blows / 5 Layers /4.5 kg Penetration of Dial Gauge Plunger (mm)

Penetration of

Dial Gauge

Force on

Penetration of

Dial Gauge

Force on

CBR

Plunger (mm)

Reading

Plunger (kN)

Plunger (mm)

Reading

Plunger (kN)

%

58

1.5

116

2.9

0.50

126

3.2

226

5.7

333

8.4

428

10.8

540

13.6

600

15.1

685

17.3

764

19.3

839

21.2

1110

28.0

1180

29.8

1143

28.8

1180

29.8

1270

32.0

1310

33.1

0.50

75

1.9

87

2.2

1.00

1.50

98

2.5

107

2.7

116

2.00

21

2.9

124

Checked by:

1.25 1.50 1.75 310

7.8

2.00 2.25

355

9.0

68

3.50

129

3.3

4.00

392

9.9

3.00

3.4

4.50

423

10.7

3.50 3.75

450

11.4

4.00 4.25

480

12.1

4.50

4.75 142

3.6

148

3.7

18

5.00

4.75 508

12.8

530

13.4

5.25 5.50

3.9

6.00

4.1

6.50

555

14.0

4.2

7.00

580

14.6

4.4

Isooba John Bosco Laboratory Technician

7.50

5.50 6.00 6.50 6.75

602

15.2

7.25 173

141

6.25

6.75 167

5.00

5.75

6.25 161

65

5.25

5.75 155

103

3.25

4.25 136

2.50 2.75

3.75

7.25 7.50

2.50 3.00

3.1

6.75 7.00

6.5

3.25

6.25 6.50

256

1.00

2.75

5.75 6.00

4.9

2.25

5.25 5.50

193

1.75

4.75

5.00

0.75

1.25

4.25 4.50

0.25

0.75

3.75 4.00

CBR

0.25

3.25 3.50

kN/mm

%

2.75 3.00

0.02523

CBR

2.25

2.50

=

65 Blows / 5 Layers /4.5 kg

Force on

1.75 2.00

Plunger Load factor

Plunger (kN)

1.25 1.50

kN

Reading

0.75 1.00

4/13/2006

30 Blows / 5 Layers /4.5 kg

0.25 0.50

20

TP 1

7.00 7.25

620

15.6

7.50

Prepared by: Okello Francis Eugene Student

Source: Okello, 2006

32



33

b) The Dynamic Cone Penetrometer (DCP) Test The TRL Dynamic Cone Penetrometer (DCP), shown in Figure 3.3, is an instrument designed for the rapid in situ measurement of the structural properties of existing road pavements with unbound granular materials. Continuous measurements can be made to a depth of 800 mm or to 1200 mm when an extension rod is fitted. The underlying principle of the DCP is that the rate of penetration of the cone, when driven by a standard force, is inversely related to the strength of the material as measured by, for example, the California Bearing Ratio (CBR) test (see DCP-CBR relationships in Figure 3.4). Where the pavement layers have different strengths, the boundaries between the layers can be identified and the thickness of the layers determined. A typical result is shown in Figure 3.5. The DCP needs three operators, one to hold the instrument, one to raise and drop the weight and a technician to record the results. The instrument is held vertical and the weight carefully raised to the handle. Care should be taken to ensure that the weight is touching the handle, but not lifting the instrument, before it is allowed to drop and that the operator lets it fall freely and does not lower it with his hands. If during the test the DCP tilts from the vertical, no attempt should be made to correct this as contact between the shaft and the sides of the hole will give rise to erroneous results. If the angle of the instrument becomes worse, causing the weight to slide on the hammer shaft and not fall freely, the test should be abandoned. It is recommended that a reading should be taken at increments of penetration of about 10 mm. However it is usually easier to take readings after a set number of blows. It is therefore necessary to change the number of blows between readings according to the strength of the layer being penetrated. For good quality granular roadbases readings every 5 or 10 blows are normally satisfactory but for weaker sub-base layers and subgrade readings every 1 or 2 blows may be appropriate. Little difficulty is normally experienced with the penetration of most types of granular weakly stabilised materials. It is more difficult to penetrate strongly stabilised layers, granular materials with large particles and very dense, high quality crushed stone. The TRL instrument has been designed for strong materials and therefore the operator should persevere with the test. Penetration rates as low as 0.5 mm/blow are acceptable but if there is no measurable penetration after 20 consecutive blows it can be assumed that the DCP will not penetrate the material. Under these circumstances a hole can be drilled through the layer using either an electric or pneumatic drill or by coring. The lower layers of the pavement can then be tested in the normal way. DCP results are conveniently processed by a computer and a program has been developed TRRL (1990) that is designed to assist with the interpretation and presentation of DCP data.

33



34

Figure 3.2: Dynamic Cone Penetrometer in use Source: ELE, 2006 and Okello, 2006

34



35

Figure 3.3: The TRL Dynamic Cone Penetrometer Source: TRL (1993)

35



36

Figure 3.4: DCP-CBR relationships Source: TRL (1993)

Figure 3.5: The DCP Test Result Source: TRL, 1993

36


Example 3.1: DCP Test

37

4.4 Example 3.1: DCP Test The table below shows results of a Dynamic Cone Penetrometer test carried out on Kansanga – Lukuli road in Makindye division at chainage 0 + 100 (LHS +RHS). Table 3.7: DCP Field Results Kyambogo University Faculty of Engineering Dept. of Civil & Building Engineering UPGRADING OF KANSANGA - LUKULI ROAD TO A PAVED SURFACE Dynamic Cone Penetrometer (D.C.P) test DCP FIELD SHEET Date: 02/03/06 On Left Hand Side Initial Reading On Right Hand Side at 3.0 m at 3.0 m 930 No. of Blows Reading Cummulative Penetration Chainage No. of Blows Reading Cummulative Penetration Blows mm mm Blows mm mm 0 930 0 0 0 930 0 0 0+100 10 830 10 100 10 870 10 60 10 785 20 145 10 825 20 105 10 735 30 195 10 785 30 145 10 695 40 235 10 730 40 200 10 645 50 285 10 680 50 250 10 605 60 325 10 620 60 310 10 560 70 370 10 570 70 360 10 515 80 415 10 515 80 415 10 475 90 455 10 430 90 500 10 420 100 510 10 370 100 560 10 365 110 565 10 295 110 635 5 330 115 600 10 200 120 730 5 290 120 640 10 100 130 830 5 255 125 675 5 230 130 700 5 200 135 730 5 165 140 765 5 125 145 805 Remarks

Supervisor ______________________

Signature ____________________

Source: Okello (2006) This point is found to be representative of the entire 500m stretch that will undergo repair. Assuming that the subgrade strength computed at a depth of 450-500mm is adequate for design, determine the subgrade strength and assign it a subgrade strength class. Solution 1.0 Plot the Graph and Determine the Gradient • Plot a graph of cumulative blows Vs penetration (or depth); • Mark and compute every change in gradient in mm per blow (i.e. DCP readings in mm/blow); • Assuming the DCP was carried out using the standard equipment recommended by TRL, (1993) (see figure 2.7), the DCP readings should be converted into in-situ CBR values by using the following relationship as recommended by TRL, (1993) below; 2.48

1.057

… . 2.8 37


Example 3.1: DCP Test

38

Cummulative Blows Vs Depth Cummulative Blows 0

20

40

60

80

100

120

140

160

0

100

200 DCP = 4.46 mm/blow CBR = 62%

Depth (in mm)

300

400 CH.0+100 LHS

500

600

DCP = 6.86 mm/blow CBR = 39%

700

800

900

Cummulative Blows Vs Depth Cummualtive Blows 0

20

40

60

80

100

120

140

0

100 DCP = 5.22 mm/blow CBR = 53%

200

Depth (in mm)

300

400 CH. 0+100 RHS

500

600 DCP = 9.00mm/blow CBR = 30% 700

800

900

2.0 Tabulate the CBR values at each road Cross-section and Select its Design CBR Chainage 0 + 100 0 + 600 1 + 1100

LHS 62% -

CBR CL -

Design CBR Subgrade Class RHS 30% -

30% -

S6 -

From the table above, the lowest CBR value obtained at a depth of 450-500mm for this road cross section, i.e. 30% is reported as the CBR of the cross-section. From table 3.2 the subgrade strength class of this subgrade would be designated as S6. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

38

Defining Uniform Sections

39

4.5 Defining Uniform Sections The laboratory CBR tests are carried out over a range of conditions of moisture content and density that are likely to be experienced during construction and in the finished pavement. From a construction point of view, it is not desirable for the subgrade design to often vary along the road and, consequently tests are carried out only where there are significant changes in subgrade properties. In practice design changes rarely occur at intervals of less than about 500m. CBR tests should be carried out on areas with uniform appearance (using visual inspection). However, the lowest CBR test result at a given uniform section of the road should be assumed to represent the design CBR. In all cases of CBR, the material used in a capping layer should have a CBR of at least 15%.

Figure 3.6: Defining Uniform Sections

4.6 Design of Earth Works a) Pavement Thicknesses This involves the selection of the most economical combination of pavement materials and layer thicknesses that will provide satisfactory service over the design life of the pavement. It will also involve the assessment and design of embankments and cuttings. Considering a traffic class of T5 and a subgrade strength class of S6, the structure catalogue provides a surface dressing and agranular roadbase of 225mm with no subbase required since the subgrade CBR is equal to 30%. The details of the pavement materials required will be discussed in the succeeding chapters. b) Embankments (i) Introduction and Survey Embankments and cuttings will be required to obtain a satisfactory alignment on all but the lowest standard of road. Embankments will be needed (i) to raise the road above flood water levels, (ii) in sidelong ground, (iii) across gullies and (iv) at the approaches to water crossings. High embankments impose a considerable load on the underlying soil and settlement should always be expected. Potentially compressible soils should be identified at the survey stage which precedes new construction. During the survey it is also essential to look for evidence of water flow across the line of the road, either on the surface or at shallow depth. Temporary, perched water tables are common within residual soils and may not be readily apparent in the dry season. Drains must be installed to intercept ground water, and culverts of suitable size must be provided to allow water to cross the road alignment where necessary. It is also important to identify any areas of potential ground instability which might affect embankments. Particular care is required in gullies, which themselves may Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

39

Design of Earth Works

40

be indicative of weakness in the geological structure, but steep side-sloping ground may also be suspect and evidence of past soil movement should be sought. Evidence of past (dormant) instability is revealed by a range of slope features. On the surface, springs or patches of reeds or sedges are a sign that the slope may become saturated during the rainy season. Trees leaning at different angles (especially upslope) are a sign of disturbance by ground movement. However, it should be noted that trees leaning outwards (down slope), all at a similar angle, are usually not so much a sign of instability as a sign that the trees have grown at an angle to seek light. The age of trees can indicate former movement if they are all of a similar youthful age and there are no old trees present, this suggests that regeneration has taken place following a recent slide. The shape of the ground itself is a good indicator of past movement. The classic features of hummocky ground (irregular, pocketed surface), cracks and small ponds are signs of a deep-seated landslide mass. Another sign is the presence on the slope of hollow bowlshaped depressions with a steep head, curved in plan, which may represent the head area of old slips. They can be of any size, from a few tens of metres across to several hundred metres. Gullies that are active can put embankments at risk by bringing down debris, blocking the culvert and damming up against the embankment. An active gully carries a heavy load of material, typically of mixed sizes including sand and fines. Stable gullies generally contain only boulder and cobble sized material (the smaller sizes wash away), and may bear vegetation more than one year old in the gully floor. If the gully sides are being undercut by the stream and loose sediment moving in the gully floor, fresh debris will be brought into the gully, making the situation downstream worse. The slope below the road should be examined to ensure that it is not being undercut by the stream at the base. If this is happening, the whole embankment and road are at risk from slope failure expanding upwards from below. Evidence of slope instability is not easy to detect in trial pits because soils on steep slopes are often disturbed by slow creep under gravity, resulting in a jumbled soil profile. However, former slope movement is sometimes indicated by ancient organic horizons (buried soils) lying parallel to the present surface, or by clayey horizons lying parallel to the surface, that represent old sliding surfaces. Water often travels along these. The bedrock, too, can indicate a danger of movement. Rocks whose bedding lies parallel to the hillside, or dips out of the hillside, are prone to failure along the bedding plane, as are rocks containing joint surfaces (parallel planes of weakness) oriented this way. Weak, weathered and highly fractured rocks all represent a hazard, especially if the fissures are open, showing that the rock mass is dilating under tension. In steep side-sloping ground where the slope exceeds 1 in 6, it is normal practice to cut horizontal benches into the slope to simplify construction and to help key the embankment to the slope. At the same time, internal drainage is usually installed to remove sub-surface water from within the structure. Problems with embankments are fortunately rare but when they occur the consequences can be serious. It is therefore important that all potential problems are identified during the survey and recommendations made for more detailed investigations where necessary. Such investigations are expensive and need to be planned systematically, with additional testing and expert advice being commissioned only as required. (ii)

Materials Almost all types of soil, ranging from sandy clays through to broken rock, can be used for embankment construction, the main limitation being the ease with which the material can be handled and compacted. The embankment material will usually be obtained from borrow areas adjacent to the road or hauled from nearby cuttings. Material of low plasticity is preferred because such material will create fewer problems in wet weather.


40


41

With more plastic soils, greater care is necessary to keep the surfaces shaped and compacted so that rain water is shed quickly. If the embankment is higher than about 6 metres, it is desirable to reserve material of low plasticity for the lower layers. (iii)

Design Side slopes for high embankments should normally be between 1 in 1.5 and 1 in 2 (vertical: horizontal) Variations from this slope for local soils and climates are more reliably derived from local experience than from theoretical calculations. Slacker slopes are sometimes desirable for silty and clayey soils, especially in wet climates. In all cases it is important to protect the side slopes from the erosive action of rain and wind. Usually this should be done by establishing a suitable cover of vegetation but granular materials will be needed in arid areas. Particular care is needed with expansive soils, especially those containing montmorillonite. If construction in such soils cannot be avoided, earthworks must be designed to minimize subsequent changes in moisture content and consequent volume changes. For example, the soil should be placed and compacted at a moisture content close to the estimated equilibrium value and it may also be advantageous to seal the road shoulders with a surface dressing. On low embankments in expansive soils, relatively shallow side slopes should be used i.e. 1 in 3, and these should be covered with well graded granular material. Nevertheless some volume changes must be expected with expansive soils and any cracks which develop, either in the side slopes or shoulders, should be sealed before water enters the structure. When the subgrade is a particularly expansive soil, it may be necessary to replace the expansive material with non-expansive impermeable soil to the depth affected by seasonal moisture changes [TRL, 1994].

(iv)

Construction over Compressible Soils Transported soils; In the design of embankments over compressible soils, it is necessary to determine the amount of settlement which will occur and ensure that the rate of loading is sufficiently slow to prevent pore water pressures from exceeding values at which slip failures are likely to occur. A reasonably accurate estimate of total settlement can be obtained from consolidation tests but the theory usually overestimates the time required for settlement to occur. This is because most deposits of unconsolidated silt or clay soils contain horizontal lenses of permeable sandy soil which allow water to escape. High pore water pressures can be detected using piezometers set at different depths This often provides a reliable method of estimating the time required for consolidation and also provides a means of checking that pore water pressures do not reach unacceptably high levels during construction. Further precautions can be taken by installing inclinometers to detect any movement of soil which might indicate that unstable conditions exist. If necessary, consolidation can be accelerated by installing some form of vertical drainage. Sand drains consisting of columns of sand of about 500 mm diameter set at regular intervals over the area below the embankment have been used successfully but nowadays wick drains are more common. If the embankment is sufficiently stable immediately after construction, the rate of consolidation can be increased by the addition of a surcharge of additional material which is subsequently removed before the pavement is constructed. Organic soils; Organic soils are difficult to consolidate to a level where further settlement will not occur, and they provide a weak foundation even when consolidated. It is therefore best to avoid such materials altogether. If this is not possible, they should be removed and replaced If neither of these options is feasible, and provided soil suitable for embankments is available, methods of construction similar to those adopted for unconsolidated silt-clays should be used.


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(v)

Compaction of Embankments Uniformity of compaction is of prime importance in preventing uneven settlement. Although some settlement can be tolerated it is important that it is minimised, especially on the approaches to bridges and culverts where adequate compaction is essential. It is essential that laboratory tests are carried out to determine the dry density/moisture content relationships for the soils to be used and to define the achievable densities. In the tropics the prevailing high temperatures promote the drying of soils. This can be beneficial with soils of high plasticity but, generally, greater care is necessary to keep the moisture content of the soil as close as possible to the optimum for compaction with the particular compaction plant in use. The upper 500 mm of soil immediately beneath the sub-base or capping layer i. e. the top of the embankment fill or the natural subgrade, should be well compacted In practice this means that a minimum level of 93-95 per cent of the maximum dry density obtained in the British Standard (Heavy) Compaction Test, 4 5 kg rammer should be specified (a level of 98 per cent is usually specified for roadbases and sub-bases). The same density should also be specified for fill behind abutments to bridges and for the backfill behind culverts. For the lower layers of an embankment, a compaction level of 90-93 per cent of the maximum dry density obtained in the British Standard (Heavy) Compaction Test, 4.5 kg rammer, is suitable, or a level of 95-100 per cent of the maximum density obtained in the lighter test using the 2.5 kg yammer. The British Standard Vibrating Hammer Test (BS 1377, Part 4 (1990)) should be used for non-cohesive soils and a level of 90-93 per cent of maximum density should be specified for the lower layers and 95 per cent for the upper layers. Compaction trials should always be carried out to determine the best way to achieve the specified density with the plant available. In areas where water is either unavailable or expensive to haul, the dry compaction techniques developed by O'Connell et al (1987) and Ellis (1980) should be considered. Figure 3.7 illustrates that high densities can be achieved at low moisture contents using conventional compaction plant, and field trials have shown that embankments can be successfully constructed using these methods.

(vi)

Site Control It is not easy to obtain an accurate measure of field density on site. The standard methods of measurement are tedious and not particularly reproducible. Furthermore, most soils are intrinsically variable in their properties and it is difficult to carry out sufficient tests to define the density distribution. An acceptable approach to this problem is to make use of nuclear density and moisture gauges. Such devices are quicker and the results are more reproducible than traditional methods, but the instruments will usually need calibration for use with the materials in question if accurate absolute densities are required. It may also be advisable to measure the moisture contents using traditional methods but improvements in nuclear techniques are always being made and trials should be carried out for each situation. Tests for bulk density and moisture content must be carried out at regular intervals in order to achieve proper control of compaction. The American practice is to take at least four density tests per 8-hour shift with a minimum of one test for each 400 cubic metres of work compacted. In-situ tests used to test for bulk density and/or moisture contents include; the sand replacement method, the core-cutter method, the moisture condition test and the nuclear density test [TRL, 1993].


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Figure 3.7: Dry density – moisture content relationships for a gravel-sand-clay Source: TRL (1993)

Sand Replacement Method A small round hole about 100mm diameter and 150mm deep is dug and the mass of the excavated material is carefully determined. The volume of the hole is obtained by pouring sand into it using a special graduated container. Knowing the weight of the sand in the container before and after the test, the weight of sand in the hole and hence the volume of the hole can be determined. A larger pouring graduated cylinder is used for coarse grained soils as opposed to a smaller cylinder for medium-fine grained soils. For coarse grained soils, the cylinder would have an internal diameter of 215mm and a height of 170mm to the valve. The estimated hole for coarse grained soil would be about 200mm in diameter and 50mm deep. Nuclear Density Test The instrument consists of an aluminium probe which is pushed into soil. The neutrons emitted from the source lose their energy by collision. Since the number of slow neutrons produced depend on the amount of water present, measurements of the water content, bulk density and dry density of the soil can be obtained. This apparatus gives rapid and dependable results than those from the traditional sand replacement method. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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The Moisture Condition Test This is an essential strength test in which the compactive effort necessary to achieve full compaction of the test sample is determined. The moisture condition value (MCV) is a measure of this compactive effort and is correlated with the un-drained shear strength or CBR value that the soil will attain when subjected to the same level of compaction.

c)

Cuttings Cuttings through sound rock can often stand at or near vertical, but in weathered rock or soil the conditions are more unstable. Instability is usually caused by an accumulation of water in the soil, and slips occur when this accumulation of water reduces the natural cohesion of the soil and increases its mass. Thus the design and construction of the road should always promote the rapid and safe movement of water from the area above the road to the area below, and under no circumstances should the road impede the flow of water or form a barrier to its movement.

(i) Slope Stability Methods of analysing slope stability are usually based on measurements of the density, moisture content and strength of the soil together with calculations of the stresses in the soil using classic slip-circle analysis [Bishop, 1955]. This type of analysis assumes that the soil mass is uniform. Sometimes failures do indeed follow the classic slip-circle pattern, but uniform conditions are rare, particularly in residual soils, and it is more common for slips to occur along planes of weakness in the vertical profile. Nevertheless, slope stability analysis remains an important tool in investigating the likely causes of slope failures and in determining remedial works, and such an analysis may be a necessary component of surveys to help design soil cuttings. (ii) Surveys The construction of cuttings invariably disturbs the natural stability of the ground by the removal of lateral support and a change in the natural ground water conditions. The degree of instability will depend on the dip and stratification of the soils relative to the road alignment, the angle of the slopes, the ground water regime, the type of material, the dimensions of the cut, and numerous other variables. A full investigation is therefore an expensive exercise but, fortunately, most cuttings are small and straightforward Investigations for the most difficult situations are best left to specialists. An integral component of a survey is to catalogue the performance of both natural and man-made slopes in the soils encountered along the length of the road and to identify the forms of failures to inform the design process and to make best use of the empirical evidence available in the area. Where well defined strata appear in the parent rock, it is best to locate the road over ground where the layers dip towards the hill and to avoid locating the road across hillsides where the strata are inclined in the same direction as the ground surface. During the survey, all water courses crossing the road line must be identified and the need for culverts and erosion control established. (iii) Design and Construction The angle of cutting faces will normally be defined at the survey stage. Benching of the cut faces can be a useful construction expedient enabling the cutting to be excavated in well defined stages and simplifying access for subsequent maintenance. The slope of the inclined face cannot usually be increased when benching is used and therefore the volume of earthworks is increased substantially. The bench itself can be inclined either Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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outwards to shed water down the face of the cutting or towards the inside. In the former, surface erosion may pose a problem. In the latter, a paved drain will be necessary to prevent the concentration of surface water causing instability in the cutting. A similar problem applies to the use of cut-off drains at the top of the cutting which are designed to prevent run-off water from the area above the cutting from adding to the run-off problems on the cut slope itself. Unless such drains are lined and properly maintained to prevent water from entering the slope, they can be a source of weakness. Control of ground water in the cutting slopes is sometimes necessary. Various methods are available but most are expensive and complex, and need to be designed with care. It is advisable to carry out a proper ground water survey to investigate the quantity and location of sources of water and specialist advice is recommended. As with embankments, it is essential that provision is made to disperse surface water from the formation at all stages of construction. Temporary formation levels should always be maintained at a slope to achieve this. Drainage is critically important because pore water pressures created by the available head of ground water in the side slopes can cause rapid distress in the pavement layers. Subsoil drains at the toe of the side slopes may be necessary to alleviate this problem. The subsequent performance, stability and maintenance of cuttings will depend on the measures introduced to alleviate the problems created by rainfall and ground water. Invariably it is much more cost effective to install all the necessary elements at construction rather than to rely on remedial treatment later [TRL, 1993].

4.7 Questions a) After he had obtained the traffic class, Mr. Fernández then went ahead to ascertain the climatic characteristics of his project area. He found out that the water table was sufficiently close to the ground surface along the entire project road and that there was no existing pavement under similar conditions within the vicinity. Advice Mr. Fernández on the best method of determining the equilibrium moisture content for this category of subgrade. b) After ascertaining the traffic loading at station ‘A’ in as 0.635msa, Mr. Fernández, the Spanish Consultant, went ahead to examine the subgrade strength of the pavement from Station 0 + 000.000 (LHS & RHS) off Ggaba road to Station 1 + 320.000 (LHS & RHS) in Lukuli. Tables A.11 to A14. show the results of the dynamic cone penetrometer test carried out on the project road. The test points were found to be representative of the entire 1.32 km stretch that is due for upgrading. The material to be used for the roadbase is “lime-stabilised roadbase of category CB2”. Using the TRL approach design the different pavement layers in terms of their thicknesses clearly naming each layer and indicate the designed thicknesses. For the surfacing layer, only indicate the type of surface. Assume that the subgrade strength calculated at a depth of about 450–500mm is adequate for design. c) While penetrating a point at chainage 0 + 825 LHS using the DCP Machine, Mr. Fernández noticed that the penetration rate was as low as 0.5mm/blow after 20 consecutive blows. He considered this penetration rate as abnormal. Explain the possible causes of this phenomenon and propose a viable remedy.


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d) Stirling Contractors procured for the above road and would prefer to use a surface dressing option as a surfacing course. As a consultant engineer, how many layers of surface dressing would you expect the contractor to lay and why? e) Briefly describe how you would carry out an embankment test to determine the number of roller passes required to achieve a given compaction in the field. Table A.11: DCP test at Station 0+165 On Left Hand Side Initial Reading at 3.0 m 930 No. of Blows Reading Cummulative Penetration Station mm Blows mm 0 930 0 0 0+165 10 830 10 100 10 785 20 145 10 735 30 195 10 695 40 235 10 645 50 285 10 605 60 325 10 560 70 370 10 515 80 415 10 475 90 455 10 420 100 510 10 365 110 565 5 330 115 600 5 290 120 640 5 255 125 675 5 230 130 700 5 200 135 730 5 165 140 765 5 125 145 805

On Right Hand Side at 3.0 m No. of Blows Reading Cummulative mm Blows 0 930 0 10 870 10 10 825 20 10 785 30 10 730 40 10 680 50 10 620 60 10 570 70 10 515 80 10 430 90 10 370 100 10 295 110 10 200 120 10 100 130

Penetration mm 0 60 105 145 200 250 310 360 415 500 560 635 730 830

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Table A.12: DCP test at Station 0+520 On Left Hand Side Initial Reading at 3.0 m 930 No. of Blows Reading Cummulative Penetration Station mm Blows mm 0 930 0 0 0+520 5 830 5 100 5 740 10 190 5 690 15 240 6 615 21 315 6 585 27 345 5 565 32 365 10 530 42 400 10 495 52 435 10 430 62 500 10 345 72 585 10 295 82 635 10 225 92 705 10 150 102 780

On Right Hand Side at 3.0 m No. of Blows Reading Cummulative Penetration mm Blows mm 0 930 0 0 5 855 5 75 5 820 10 110 5 790 15 140 5 755 20 175 5 725 25 205 5 705 30 225 10 685 40 245 10 665 50 265 10 640 60 290 10 620 70 310 10 585 80 345 10 565 90 365 10 535 100 395 10 500 110 430 5 470 115 460 5 430 120 500 5 405 125 525 10 385 135 545 10 330 145 600 5 295 150 635 5 275 155 655 5 260 160 670 10 225 170 705 10 170 180 760

Table A.13: DCP test at Station 0+825 On Left Hand Side Initial Reading On Right Hand Side at 3.0 m at 3.0 m 930 No. of Blows Reading Cummulative Penetration Station No. of Blows Reading Cummulative Penetration mm Blows mm mm Blows mm 0 930 0 0 0 930 0 0 0+825 10 890 10 40 10 890 10 40 10 870 20 60 10 855 20 75 10 840 30 90 10 820 30 110 10 815 40 115 10 775 40 155 10 785 50 145 10 730 50 200 10 750 60 180 10 680 60 250 10 705 70 225 10 620 70 310 10 660 80 270 10 555 80 375 10 600 90 330 10 475 90 455 10 530 100 400 10 395 100 535 10 470 110 460 10 315 110 615 10 400 120 530 10 230 120 700 10 335 130 595 10 145 130 785 47 10 245 140 685 10 150 150 780


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Table A.14: DCP test at Station 1+130 On Left Hand Side Initial Reading On Right Hand Side at 3.0 m at 3.0 m 930 No. of Blows Reading Cummulative Penetration Station No. of Blows Reading Cummulative Penetration mm Blows mm mm Blows mm 0 930 0 0 0 930 0 0 1+130 5 900 5 30 10 885 10 45 5 870 10 60 10 855 20 75 5 850 15 80 10 825 30 105 10 845 25 85 10 800 40 130 10 825 35 105 10 765 50 165 10 800 45 130 10 725 60 205 10 785 55 145 10 670 70 260 10 735 65 195 10 600 80 330 10 710 75 220 10 525 90 405 10 665 85 265 10 480 100 450 10 605 95 325 10 460 110 470 10 535 105 395 20 420 130 510 10 460 115 470 20 395 150 535 10 360 125 570 20 360 170 570 5 300 130 630 5 245 135 685 5 170 140 760 5 145 145 785

4.8 Bibliography 1. Arora, K. R., 2000, Soil Mechanics and Foundation Engineering, 5th Edition. 2. ELE International, 2006, Construction Materials Testing Equipment, 10th Edition, Hertforshire HP2 7HB, England. 3. Ministry of Works, and Transport, 2005. Road Design ManualVol.III, Pavement Design Manual, Republic of Uganda, Kampala. 4. Okello, F.E., 2006, Upgrading of Kansanga Lukuli (Soweto) Road to a Paved Bituminous Surface, Final Year Project Report, Department of Civil Engineering, Kyambogo University. 5. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil Engineering, Kyambogo University. 6. Transport Research Laboratory, 1993, A Guide to Design of Bitumen Surfaced Roads in Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England.

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Chapter Four:

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Chapter Four:

Selection of Pavement Materials 5.1 General Pavement materials include materials like gravel, aggregates, bitumen, tars, asphalts, stabilizers (like lime and cement) and natural soils. When materials are stabilized, they are referred to as bound materials, otherwise they are unbound materials. Local materials should be used as much as possible before giving thought to imported material. Construction costs are greatly minimised when a complete inventory of all available materials such as stone, gravel, sand is carried out prior to road construction [TRL, 1993].

5.2 Unbound Pavement Materials Selection of unbound materials for use as roadbase, sub-base, capping and selected subgrade layer normally depends on the properties of unbound materials. The main categories with a brief summary of their characteristics are shown in table 4.1 below. Table 4.1: Properties of Unbound Materials Code Description GB1,A Fresh, crushed rocks

Summary of Specification Dense graded un-weathered crushed stones. Non-plastic parent fines.

GB1,B

Crushed rocks, gravel or boulders

Dense grading, PI<6. Soil or parent fines.

GB2,A

Dry-bound macadam

Aggregate properties as for GB1,B; PI<6

GB2,B

Water-bound macadam

Aggregate properties as for GB1,B; PI<6

GB3

Natural coarsely graded granular materials including processed and modified gravels GS Natural gravel GC Gravel or gravel-soil Source: TRL (1993)

Dense grading, PI<6, CBR after soaking >80 CBR after soaking >30 Dense graded, CBR after soaking >15

Note: (i) These specifications are sometimes modified according to site conditions, material type and principal use. (ii) GB = Granular roadbase, GS = granular sub-base, GC = granular capping layer.

a)

Roadbase Materials (GB) A wide range of materials can be used as unbound roadbases including crushed quarried rock, crushed and screened, mechanically stabilised, modified or naturally occurring `as dug' gravels. Their suitability for use depends primarily on the design traffic level of the pavement and climate but all Roadbase materials must have a particle size distribution and particle shape which provide high mechanical stability and should contain sufficient fines (amount of material passing the 0.425mm sieve) to produce a dense material when compacted. In circumstances where several types of roadbase are suitable, the final choice should take into account the expected level of future maintenance and the total


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costs over the expected life of the pavement. The use of locally available materials is encouraged, particularly at low traffic volumes (i.e. categories T1 and T2). Their use should be based on the results of performance studies and should incorporate any special design features which ensure their satisfactory performance. As a cautionary note, when considering the use of natural gravels a statistical approach should be applied in interpreting test results to ensure that their inherent variability is taken into account in the selection process. For lightly trafficked roads the requirements set out below may be too stringent and in such cases reference should be made to specific case studies, preferably for roads under similar conditions. Aggregates used in macadam roadbases are most usually non-flaky crushed rock or gravel. All primarily rely for their strength and resistance on aggregate interlock. These materials should be protected from weather elements before compaction due to the danger of segregation after changing their moisture content. If smooth wheeled or rubber rollers are used for compaction, compacted layers should not be more than 150mm deep. If vibratory rollers are used then single layers of up to 225mm compacted thickness can be laid satisfactorily.

i)

Crushed Stone Graded Crushed Stone (GB1, A and GB1, B) Two types of material are defined in this category. One is produced by crushing fresh, quarried rock (GB1,A) and may be an all-in product, usually termed a ‘crusher-run’, or alternatively the material may be separated by screening and recombined to produce a desired particle size distribution. The other is derived from crushing and screening natural granular material, rocks or boulders (GB1, B) and may contain a proportion of natural fine aggregate. Typical grading limits for these materials are shown in table 4.2. After crushing, the material should be angular in shape with a Flakiness Index (British Standard 812, Part 105 (1990)) of less than 35 per cent. If the amount of fine aggregate produced during the crushing operation is insufficient, non-plastic angular sand may be used to make up the deficiency. In constructing a crushed stone roadbase, the aim should be to achieve maximum impermeability compatible with good compaction and high stability under traffic. To ensure that the materials are sufficiently durable, they should satisfy the criteria given in table 4.3. These are a minimum Ten Per Cent Fines Value (TFV) (British Standard 812, Part 111 (1990)) and limits on the maximum loss in strength following a period of 24 hours of soaking in water. The likely moisture conditions in the pavement are taken into account in broad terms based on climate. Other simpler tests e.g. the Aggregate Impact Test (British Standard 812, Part 112, 1990) may be used in quality control testing provided a relationship between the results of the chosen test and the TFV has been determined. Unique relationships do not exist between the results of the various tests but good correlations can be established for individual material types and these need to be determined locally.

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Table 4.2: Grading Limits for crushed stone base materials (GB1,A; GB1,B)

Source: TRL (1993) Table 4.3: Mechanical strength requirements for the aggregate fraction of crushed stone roadbases (GB1,A; GB1,B) as defined by the Ten Percent Fines Test

Source: TRL (1993)

The fine fraction of a ‘GB1, A’ material should be non-plastic. For ‘GB1, B’ materials the maximum allowable PI is 6. When producing these materials, the percentage passing the 0.075mm sieve should be chosen according to the grading and plasticity of the fines. For materials with non-plastic fines, the proportion passing the 0.075 mm sieve may approach 12 per cent. If the PI approaches the upper limit of 6 it is desirable that the fines content be restricted to the lower end of the range. To ensure this, a maximum Plasticity Product (PP) value of 45 is recommended where:

0.075

… . 3.1

In order to meet these requirements it may be necessary to add a low proportion of hydrated lime or cement to alter the properties of the fines. Such materials are commonly referred to as modified materials. These materials may be dumped and spread by grader but it is preferable to use a paver to ensure that the completed surface is smooth with a tight finish. The material is usually kept wet during transport and laying to reduce the likelihood of particle segregation. The in-situ dry density of the placed material should be a minimum of 98 per cent of the maximum dry density obtained in the British Standard (Heavy) Compaction Test, 4.5 kg rammer, or the British Standard Vibrating Hammer Test (British Standard 1377, Part 4 (1990)). The compacted thickness of each layer should not exceed 200 mm. When properly constructed, crushed stone roadbases will have CBR values well in excess of 100 per cent. In these circumstances there is no need to carry out CBR tests.


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Dry-Bound Macadam (GB2, A) Dry-bound macadam is a traditional form of construction, formerly used extensively in the United Kingdom, and is comparable in performance with a graded crushed stone. It has been used successfully in the tropics and is particularly applicable in areas where water is scarce or expensive to obtain. It is also suitable where labour intensive construction is an economic option. The materials consist of nominal single-sized crushed stone and non-plastic fine aggregate (passing the 5.0 mm sieve). The fine material should preferably be well graded and consist of crushed rock fines or natural, angular pit sand. The fine screenings are graded from 5mm to less than 10% passing the 0.075mm sieve. This limits segregation during stockpiling and transportation and provides for more uniform construction at a moderate cost. The dry-bound macadam process involves laying single-sized crushed stone of either 37.5 mm or 50 mm nominal size in a series of layers to achieve the design thickness. The compacted thickness of each layer should not exceed twice the nominal stone size. At the construction site, dry coarse material is spread to a uniform thickness of 75 to 100mm with preliminary rolling of two (2) passes and shaping is carried out with an 8 to 10 tonne smooth wheeled roller. Each layer of coarse aggregate should be shaped and compacted and then the fine aggregate spread onto the surface and vibrated into the interstices to produce a dense layer. Any loose material remaining is brushed off and final compaction carried out, usually with a heavy smooth-wheeled roller. This sequence is then repeated until the design thickness is achieved. To aid the entry of the fines, the grading of the 37.5 mm nominal size stone should be towards the coarse end of the recommended range. Economy in the production process can be obtained if layers consisting of 50 mm nominal size stone and layers of 37.5 mm nominal size stone are both used to allow the required total thickness to be obtained more precisely and to make better overall use of the output from the crushing plant. Water-Bound Macadam (GB2, B) Water-bound macadam is similar to dry-bound macadam. It consists of two components namely a relatively single-sized stone with a nominal maximum particle size of 50 mm or 37.5 mm and well graded fine aggregate which passes the 5.0 mm sieve. The coarse material is usually produced from quarrying fresh rock. The crushed stone is laid, shaped and compacted and then fines are added, rolled and washed into the surface to produce a dense material. Care is necessary in this operation to ensure that water sensitive plastic materials in the sub-base or subgrade do not become saturated. The compacted thickness of each layer should not exceed twice the maximum size of the stone. The fine material should preferably be non-plastic and consist of crushed rock fines or natural, angular pit sand. Typical grading limits for the coarse fraction of ‘GB2, A’ or ‘GB2, B’ materials are given in table 4.4. The grading of M2 and M4 correspond with nominal 50 mm and 37.5 mm single-sized roadstones (British Standard 63 (1987)) and are appropriate for use with mechanically crushed aggregate. M1 and M3 are broader specifications. M1 has been used for hand-broken stone but if suitable screens are available, M2, M3 and M4 are preferred. Aggregate hardness, durability, particle shape and in situ density should each conform to those given above for graded crushed stone.

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Table 4.4: Typical Coarse aggregate gradings for Dry-bound (GB2,A) and Water-bound Macadam (GB2,B)

Source: TRL (1993)

ii) Naturally Occurring Granular Materials Normal Requirements for Natural Gravels and Weathered Rocks (GB3) A wide range of materials including latentic, calcareous and quartzitic gravels, river gravels and other transported gravels, or granular materials resulting from the weathering of rocks can be used successfully as roadbases. Table 4.5 contains three recommended particle size distributions for suitable materials corresponding to maximum nominal sizes of 37.5 mm, 20 mm and 10 mm. Table 4.5: Recommended Particle size distribution for mechanically stable natural gravels and weathered rocks for use as roadbases (GB3)

Source: TRL (1993)

Only the two larger sizes should be considered for traffic in excess of 1.5 million equivalent standard axles. To ensure that the material has maximum mechanical stability, the particle size distribution should be approximately parallel with the grading envelope. To meet the requirements consistently, screening and crushing of the larger sizes may be required. The fraction coarser than 10 mm should consist of more than 40 per cent of particles with angular, irregular or crushed faces. The mixing of materials from different sources may be warranted in order to achieve the required grading and surface finish. This may involve adding fine or coarse materials or combinations of the two. All grading analyses should be done on materials that have been compacted. This is especially important if the aggregate fraction is susceptible to breakdown under compaction and in service. For materials whose stability decreases with breakdown, aggregate hardness criteria based on a minimum soaked Ten Per Cent Fines Value of 50kN or a maximum Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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soaked Modified Aggregate Impact Value of 40 may be specified (British Standard 812, Part 112 (1990)). The fines of these materials should preferably be non-plastic but should normally never exceed a PI of 6. As an alternative to specifying PI, a Linear Shrinkage not exceeding 3 may be specified. If the PI approaches the upper limit of 6 it is desirable that the fines content be restricted to the lower end of the range. To ensure this, a maximum PP of 60 is recommended or alternatively a maximum Plasticity Modulus (PM) of 90 can be used where:

0.425

… . 3.2

If difficulties are encountered in meeting the plasticity criteria, consideration should be given to modifying the material by the addition of a low percentage of hydrated lime or cement. When used as a roadbase, the material should be compacted to a density equal to or greater than 98 per cent of the maximum dry density achieved in the British Standard (Heavy) Compaction Test, 4.5 kg rammer. When compacted to this density in the laboratory, the material should have a minimum CBR of 80 per cent after four days immersion in water (British Standard 1377, Part 4 (1990)). Arid and Semi-arid Areas In low rainfall areas in the tropics, typically with a mean annual rainfall of less than 500 mm, and where evaporation is high, moisture conditions beneath a well sealed surface are unlikely to rise above the optimum moisture content determined in the British Standard (Heavy) Compaction Test. In such conditions, high strengths (CBR>80 per cent) are likely to develop even when natural gravels containing a substantial amount of plastic fines are used. In these situations, for the lowest traffic categories (TI, T2) the maximum allowable PI can be increased to 12 and the minimum soaked CBR criterion reduced to 60 per cent at the expected field density. Materials of Basic Igneous Origin Materials in this group are sometimes weathered and may release additional plastic fines during construction or in service. Problems are likely to worsen if water gains entry into the pavement and this can lead to rapid and premature failure. The state of decomposition also affects their long term durability when stabilised with lime or cement. The group includes common rocks such as basalts and dolerites but also covers a wider variety of rocks and granular materials derived from their weathering, transportation or other alteration. Normal aggregate tests are often unable to identify unsuitable materials in this group. Even large, apparently sound particles may contain minerals that are decomposed and potentially expansive. The release of these minerals may lead to a consequent loss in bearing capacity. There are several methods of identifying unsound aggregates. These include petrographic analysis to detect secondary (clay) minerals, the use of various chemical soundness tests e.g. sodium or magnesium sulphate (British Standard 812 Part 121 (1990)), the use of dye adsorption tests or the use of a modified Texas Ball Mill Test. Indicative limits based on these tests include: • A maximum secondary mineral content of 20 per cent; • A maximum loss of 12 or 20 per cent after 5 cycles in the sodium or magnesium sulphate tests respectively ; • A Clay Index of less than 3, and; • A Durability Mill Index of less than 90. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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In most cases it is advisable to seek expert advice when considering their use, especially when new deposits are being evaluated. It is also important to subject the material to a range of tests since no specific method can consistently identify problem materials. Materials Marginal Quality In many parts of the world, as-dug gravels which do not normally meet the normal specifications for roadbases have been used successfully. They include latentic, calcareous and volcanic gravels. In general their use should be confined to the lower traffic categories (i.e. T1 and T2) unless local studies have shown that they have performed successfully at higher levels. Successful use often depends on specific design and construction features. The calcareous gravels, which include calcretes and marly limestones, deserve special mention. Typically, the plasticity requirements for these materials, all other things being equal, can be increased by up to 50 per cent above the normal requirements in the same climatic area without any detrimental effect on the performance of otherwise mechanically stable bases. Strict control of grading is also less important and deviation from a continuous grading is tolerable.

b)

Sub-base Materials (GS) The sub-base is an important load spreading layer in the completed pavement. It enables traffic stresses to be reduced to acceptable levels in the subgrade, it acts as a working platform for the construction of the upper pavement layers and it acts as a separation layer between subgrade and roadbase. Under special circumstances it may also act as a filter or as a drainage layer. In wet climatic conditions, the most stringent requirements are dictated by the need to support construction traffic and paving equipment. In these circumstances the sub-base material needs to be more tightly specified. In dry climatic conditions, in areas of good drainage, and where the road surface remains well sealed, unsaturated moisture conditions prevail and sub-base specifications may be relaxed. The selection of sub-base materials will therefore depend on the design function of the layer and the anticipated moisture regime, both in service and at construction.

i)

Bearing Capacity of the Sub-base A minimum CBR of 30 per cent is required at the highest anticipated moisture content when compacted to the specified field density, usually a minimum of 95 per cent of the maximum dry density achieved in the British Standard (Heavy) Compaction Test, 4.5 kg rammer. Under conditions of good drainage and when the water table is not near the ground surface the field moisture content under a sealed pavement will be equal to or less than the optimum moisture content in the British Standard (Light) Compaction Test, 2.5 kg rammer. In such conditions, the sub-base material should be tested in the laboratory in an unsaturated state. Except in Category (3) areas, if the roadbase allows water to drain into the lower layers, as may occur with unsealed shoulders and under conditions of poor surface maintenance where the roadbase is pervious, saturation of the sub-base is likely. In these circumstances the bearing capacity should be determined on samples soaked in water for a period of four days. The test should be conducted on samples prepared at the density and moisture content likely to be achieved in the field. In order to achieve the required bearing capacity, and for uniform support to be provided to the upper pavement, limits on soil plasticity and particle size distribution may be required. Materials which meet the recommendations of tables 4.6 and 3.7 will usually be found to have adequate bearing capacity.


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Table 4.6: Recommended Plasticity characteristics for granular Sub-bases (GS)

Source: TRL (1993)

Table 4.7: Typical Particle Size distribution for sub-bases (GS) which meet strength requirements

Source: TRL (1993)

ii)

Use of the Sub-base as a Construction Platform In many circumstances the requirements of a sub-base are governed by its ability to support construction traffic without excessive deformation or ravelling. A high quality sub-base is therefore required where loading or climatic conditions during construction are severe. Suitable material should possess properties similar to those of a good surfacing material for unpaved roads. The material should be well graded and have a plasticity index at the lower end of the appropriate range for an ideal unpaved road wearing course under the prevailing climatic conditions. These considerations form the basis of the criteria given in tables 4.6 and 4.7. Material meeting the requirements for severe conditions will usually be of higher quality than the standard sub-base (GS). If materials to these requirements are unavailable, trafficking trials should be conducted to determine the performance of alternative materials under typical site conditions. In the construction of low-volume roads, where cost savings at construction are particularly important, local experience is often invaluable and a wider range of materials may often be found to be acceptable.

iii)

Use of the Sub-base as a Filter or Separating Layer This may be required to protect a drainage layer from blockage by a finer material or to prevent migration of fines and the mixing of two layers. The two functions are similar except that for use as a filter the material needs to be capable of allowing drainage to take place and therefore the amount of material passing the 0.075 mm sieve must be restricted.


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The following criteria should be used to evaluate a sub-base as a separating or filter layer. D15 (Coarse Layer) • The ratio should be less than 5 D85 ( Fine Layer) Where D15 is the sieve size through which 15 per cent by weight of the material passes and D85 is the sieve size through which 85 per cent passes.

D50 (Coarse Layer ) should be less than 25 D50 ( Fine Layer ) For a filter to possess the required drainage characteristics a further requirement is: • The ratio

D15 (Coarse Layer) should lie between 5 and 40 D15 ( Fine Layer) These criteria may be applied to the materials at both the roadbase/sub-base and the subbase/subgrade interfaces. • The ratio

c)

Selected Subgrade Material and Capping Layer (GC) These materials are often required to provide sufficient cover on weak subgrades. They are used in the lower pavement layers as a substitute for a thick sub-base to reduce costs. The requirements are less strict than for sub-bases. A minimum CBR of 15 per cent is specified at the highest anticipated moisture content measured on samples compacted in the laboratory at the specified field density. This density is usually specified as a minimum of 95 per cent of the maximum dry density in the British Standard (Heavy) Compaction Test, 4.5 kg rammer. In estimating the likely soil moisture conditions, the designer should take into account the functions of the overlying sub-base layer and its expected moisture condition and the moisture conditions in the subgrade. If either of these layers is likely to be saturated during the life of the road, then the selected layer should also be assessed in this state. Recommended gradings or plasticity criteria are not given for these materials. However, it is desirable to select reasonably homogeneous materials since overall pavement behaviour is often enhanced by this. The selection of materials which show the least change in bearing capacity from dry to wet is also beneficial.

5.3 Bitumen Bound Pavement Materials a) Introduction This chapter describes types of bituminous materials, commonly referred to as premixes, which are manufactured in asphalt mixing plants and laid hot. In situ mixing using either labour intensive techniques or mechanised plant can also be used for making roadbases for lower standard roads but these methods are not generally recommended and are not discussed in detail here. The most important pavement materials include bitumen and tar, cement and lime, rock and gravel aggregates. The surfacing course consists of coarse aggregates, fine aggregate, bitumen / tar and/or cement or lime. The coarse aggregate should be produced from crushed, sound, un-weathered rock or natural gravel. Crushed aggregates should be from crushed rock or natural sand clean and free from organic impurities. The filler (material passing 0.075mm sieve) may be crushed rock fines, Portland cement or hydrated lime. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Cement or lime is added to natural filler in quantities of 1 -2 % by mass of total mix to assist adhesion of bitumen to the aggregate. Good bituminous mixes should have the following qualities; high resistance to deformation, high resistance to fatigue and ability to withstand high strains (i.e. they need to be flexible), sufficient stiffness to reduce the stresses transmitted to the underlying pavement layers, good durability, low permeability to prevent water and air penetration, and good workability to allow adequate compaction.

b) Bituminous Road Binder Materials Two basic types of bituminous binder exist: • Bitumen – obtained from the oil refining process; • Tar – obtained from the production of coal gas or manufacture of coke. With the decreased availability of tar, bitumen is the most commonly used binding/water resisting material from the oil refining process.

c)

Bitumen Bitumen is a viscous liquid or semi solid material consisting hydrocarbons and their derivatives which are soluble in trichloroethylene. Bitumen is available as penetration grade bitumen, cutback bitumen, and bitumen emulsions. Most bitumen used on roads are penetration grade products of fractional distillation of petroleum products at refineries. Penetration grade refinery bitumen is designated by the number 0.1mm units that a special needle penetrates the bitumen under standard loading conditions, with lower penetration depths being associated with harder bitumen. Penetration grade bitumen range from 15 pen (Hardest) to 450 pen (softest). The medium grades (35-70 pen) are used in hot rolled asphalts and the softer grades (100-450 pen) in macadams. They are black or brown in colour, possess waterproofing qualities and adhesive properties and soften gradually when heated i.e. its binding effect eliminates the loss of material from the surface of the pavement and prevents water penetrating the structure.

i)

Modified Binders In order to apply the binder effectively, its stiffness must be modified during the construction phase of the pavement. Two such binder modifications used during surface dressing are cutback bitumen and bitumen emulsion.

ii) Cutback Bitumen Penetration grade bitumen is normally heated to very high temperatures (typically 140180oC) for use in road pavements. There are instances where it is neither necessary nor desirable to use a penetration grade binder. Instead, cutback bitumen capable of being applied at ambient temperatures with little or no heating is applied. Cutback bitumen can be classified as slow-curing, medium curing and rapid curing depending on the nature of volatile solvent used to prepare them. Medium curing cutback bitumen is applied in surface patching dressing, maintenance patching purposes and open textured bitumen macadams that allow the solvent to evaporate quickly because they are porous. The medium curing cutback is produced by blending kerosene or creosote with a 100, 200 or 300 pen bitumen. After application, the solvent dissipates into the atmosphere leaving the cementitious bitumen behind. Such solutions are termed as cutbacks and Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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the process of evaporation of the volatile solvents is called curing. In practice an adhesive agent is usually added in the formulation of the cutback bitumen. iii) Bitumen Emulsions Bitumen can be made easier to handle by forming it into an emulsion where particles of it become suspended in water. In most cases their manufacture involves heating the bitumen and then shredding it in a colloidal mill with a solution of hot water and an emulsifier. The particles are imparted with an ionic charge that makes them repel each other. When sprayed onto the road surface, the charged ions are attracted to opposite charges on the surface, causing the emulsion to begin “breaking” with the bitumen particles starting to coalesce together. The break down process is complete when the film of bitumen is continuous. These emulsions are applied in surface dressing and premix work. Their advantage is that they can be applied on damp surfaces. iv) Some Common Bitumen Tests In addition to the specifications for design and construction of bituminous pavements, it is imperative that the properties of the binder used should be adequately controlled. A number of tests exist to ensure that the binder has the correct properties for use in the upper layers of a pavement. Some of the tests carried out include: 1) Viscosity; is a property of a fluid that retards its flow. The viscosity of a fluid slows down its ability to flow and is of particularly significance at high temperatures when the ability of the bitumen to be sprayed onto or mixed with aggregate material is of great significance. If the viscosity is too low at mixing, the aggregate will be easily coated and during transportation, the binder may drain off. If the viscosity is too high, the mix may be unworkable on reaching the site. If too low a viscosity is used in surface dressing, the result may be bleeding or a loss of chippings from the surface. The determination of viscosity is determined using a sophisticated apparatus called a viscometer. Viscosity is reported in Pascal Seconds (Pa.S). Other authors report the viscosity in terms of centistokes. 2) Penetration test; measures the depth to which a standard needle will penetrate bitumen under standard conditions of temperature (25oC), load (100g) and time (5s). The penetration test is in no way indicative of the quality of the bitumen but does allow the material to be classified. The result obtained is expressed in penetration units where one unit equals 0.1mm. Thus, if the needle penetrates 10mm within the five second period the result is 100 and the sample is designated as 100 pen. The lower the penetration the more viscous and therefore the harder the sample. 3) The softening point test; determines the temperature at which bitumen changes from semi-solid to fluid. The softening point is the temperature at which all refinery bitumens have the same viscosity (about 1200Pa.s). The test involves taking a sample of bitumen which has been cast inside a 15mm diameter metal ring and placing it inside a water bath with an initial temperature of 5oC. A 25mm clear space exists below the sample. A 10mm steel ball is placed on the sample and the temperature of the bath and the sample wihin it is increased by 5oC per minute. As the temperature is raised, the sample softens and therefore sags under the weight of the steel ball. The temperature at which the weakening binder reaches the bottom of the 25mm vertical gap below its initial position is known as its softening point. The mixing temperature of bitumen should be 110oC above its softening point. A bituminous binder should never reach its softening point under traffic. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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The penetration and softening point tests are the two most prominent bituminous tests. They both indirectly measure the viscosity of the bitumen sample. The results from the two tests enable the designer to predict the temperatures necessary to obtain the fluidity required in the mixture for effective use within the pavement [Rogers, 2003]. 4) The flash point test; is carried out by heating a bitumen sample at a uniform rate while periodically passing a small flame across the material. The temperature at which the vapours first burn with a brief flash is the flash point of the binder. The flash point indicates the maximum temperature to which a binder can be safely heated. The flash point of most penetration grade bitumens lies in the range 245 – 335oC. This test helps to ensure the safety of the mixing group on site and care is necessary when dealing with rapid and medium curing cutback bitumens whose flash points are quite low. Other tests include: loss on heating test, solubility test, permittivity test, rolling thin film test etc.

d) Road Tar Road tar is a black viscous liquid with adhesive properties that is obtained by the destructive distillation of coal, wood, and shale at temperatures well beyond 600oC. Destructive distillation is the application of heat in the absence of air. The major difference in their manufacturing processes is that bitumen is obtained from the oil refining process while tar is obtained from the production of coal gas or the manufacture of coke [Rogers, 2003]. The other differences between bitumen and tar are under listed below: • Tar coats aggregates and retains it better in the presence of water than bitumen; • Tar is less susceptible than bitumen to the dissolving action of petroleum solvents and would last longer in places like parking yards that are susceptible to fuel spills; • Tar is more temperature susceptible than bitumen and has a narrower working temperature range; • Tar is more readily oxidized than bitumen when used in surfacing materials unless very well compacted. e)

Aggregates For road construction, aggregates play a role in bearing the main stresses occurring in the road pavement as a result of application of static, traffic or dynamic loads, the necessity of the geological production and testing of aggregate properties and characteristic must be carefully assessed if the aggregates are to meet the required purpose. Aggregates are obtained from natural rocks that occur as rock outcrops, gravel or sand. The physical properties governing the suitability of aggregates for use differs not only widely in each group but also often show considerable variation in samples taken at different times from the same parent rock. Below are some aggregate properties and their significance: • Road aggregates should be strong enough to withstand stresses caused by traffic loads; • Offers resistance to abrasive action of traffic, normally in the wearing of coarse; • They take up subjected wheel impact loading; • Aggregates should be capable of standing test of time by resisting weathering agents e.g. rain during the design life of the road; • Cubical-angular aggregates are normally preferred because of their high affinity for bitumen and water.


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The coarse aggregates used for making premix should be produced by crushing sound, unweathered rock or natural gravel. The specifications for the aggregates are similar to those for granular roadbases. The aggregate must be clean and free of clay and organic material. To obtain good mechanical interlock and good compaction the particles should be angular and not flaky. Rough-textured material is preferable. Gravel should be crushed to produce at least two fractured faces on each particle. The aggregate must be strong enough to resist crushing during mixing and laying as well as in service. Aggregates which are exposed to traffic must also be resistant to abrasion and polishing. Highly absorptive aggregates are wasteful of bitumen and give rise to problems in mix design. They should be avoided where possible but if there is no choice, the absorption of bitumen must be taken into account in the mix design procedure. Hydrophillic aggregates which have a poor affinity for bitumen in the presence of water should also be avoided. They may be acceptable only where protection from water can be guaranteed. The fine aggregate can be crushed rock or natural sand and should also be clean and free from organic impurities. The filler (material passing the 0.075 mm sieve) can be crushed rock fines, Portland cement or hydrated lime. Portland cement or hydrated lime is often added to natural filler (1-2 per cent by mass of total mix) to assist the adhesion of the bitumen to the aggregate. Fresh hydrated lime can help reduce the rate of hardening of bitumen in surface dressings and may have a similar effect in premixes. Suitable specifications for the coarse and fine mineral components are given in tables 4.8 and 4.9. Table 4.8: Coarse Aggregate for Bituminous mixes

61 Source: TRL (1993)



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Table 4.9: Fine Aggregate for Bituminous Mixes

Source: TRL (1993)

Laboratory tests for Aggregates a) Shape Tests Flaky material is described as material which is usually angular and it’s thickness is small relative to the width and/or length. The flakiness index (BS 812: Section 105) is determined by separating the flaky particles and expressing their mass as a proportion of the total sample. The test must be carried out on all particle sizes in the sample. Aggregate particles are deemed flaky when they have a thickness (smallest dimension) less than 0.6 of the mean sieve size. This shape is taken as the means of limiting sieve sizes used for determining the size fraction in which the particle occurs. The test is not applicable to material passing a 6.3mm test sieve. The flakiness index is reported as the sum of masses of aggregate passing through the various thickness gauges expressed as a percentage of the total mass of the sample that is being gauged. The lower the index the more cubicle the aggregates will be. Chippings which are flaky and elongated have a tendency to crush under a roller or during trafficking. b) Strength Tests (i) Aggregate Impact Value The aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock e.g. under a vibratory roller. It’s carried out on an aggregate passing the 14mm sieve and retaining the 10mm sieve to 15 blows of a 14kg hammer falling through a height of 380mm. Following completion of the series of blows the material passing the 2.36mm sieve is expressed as a percentage of the sample and recorded as the aggregate impact value. An aggregate impact value of less than 25 is usually regarded as appropriate. Greater values than this mean the material is unsuitable for use in pavements. (ii) Ten Percent Fines Value (TFV) Test The ten percent fines value test is the load (in kN) required to produce a ten percent of fine material when subjected to a gradually applied compressive load. The test is carried out on material passing the 14mm sieve and retained on the 10mm sieve. The aggregate is placed in a standard mould and using a specified procedure and then loaded for uniformly for10 minutes. This action causes a degree of crushing resulting in fines. The mass of fines passing the 2.36mm sieve is weighed and expressed as a percentage of the total sample. The Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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force of causing this degree of crushing is also noted. The procedure is repeated a number of times and the formula applied to the results that will cause 10% fines. This is the ten percent value (TFV). The laboratory tests on aggregates can be summarised as below: 1. Particle size distribution; this is a fundamental property that governs how an aggregate will perform. In simple terms any asphalt is a particular grading of aggregate with a specified amount of particular bitumen. 2. Relative density; this parameter is important in asphalt mix design. Relative density is also used in calculation of aggregate abrasion value to adjust for loss of volume. 3. Cleanliness; Dust (actually defined as material passing the 0.075mm sieve) clings to larger aggregate particles and inhibits binder sticking to the surface of the aggregate. 4. Shape; a cubicle shape is required to ensure texture depth in hot rolled asphalt (HRA) and surface dressings. Also see notes on crushed stone base material. 5. Strength; the strength tests include; • Aggregate impact value (AIV); • Aggregate crushing value (ACV); • Ten Percent Fines value (TFV) • Los Angels Abrasion value (LAAV) 6. Abrasion; here the aggregate abrasion value (AAV) measures the resistance of an aggregate to surface wear by dry abrasion. Aggregate which is exposed at the road surface must be resistant to wear caused by trafficking. Texture depth is a measure of the roughness of a surface course and is required to facilitate the removal of water. The higher the texture depth the lesser the distance required for a vehicle to brake in wet conditions. 7. Soundness; some aggregates thought to be suitable in terms of their strength have been found to fail in use. Soundness is a measure of the durability of an aggregate in service. The Sodium and magnesium sulphate soundness value test is used to measure this parameter.

5.4 Bituminous Surfacings The most critical layer of the pavement is the bituminous surfacing, and the highest quality material is necessary for this layer. Where thick bituminous surfacings are required, they are normally constructed with a wearing course laid on a basecourse (sometimes called a binder course) which can be made to slightly less stringent specifications. To perform satisfactorily as road surfacings, bitumen aggregate mixes need to possess the following characteristics:• High resistance to deformation; • High resistance to fatigue and the ability to withstand high strains i.e. they need to be flexible; • Sufficient stiffness to reduce the stresses transmitted to the underlying pavement layers; • High resistance to environmental degradation i.e. good durability; • Low permeability to prevent the ingress of water and air; • Good workability to allow adequate compaction to be obtained during construction. In the tropics, higher temperatures and high axle loads produce an environment which is more severe thereby making the mix requirements more critical and an overall balance of properties more difficult to obtain. High temperatures initially reduce the stiffness of mixes, making them more prone to deformation, and also cause the bitumen to oxidise and harden more rapidly, thereby reducing its durability. Unfortunately the requirements for improved Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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durability i.e. increased bitumen content and lower voids, usually conflict with the requirements for higher stiffness and improved deformation resistance. As a result, the tolerances on mix specifications need to be very narrow and a high level of quality control at all stages of manufacture is essential. The requirements are so critical for wearing course mixes that different mix designs are often necessary for different conditions on the same road. For example, mixes suitable for areas carrying heavy, slow-moving traffic, such as on climbing lanes, or areas where traffic is highly channelled, will be unsuitable for flat, open terrain where traffic moves more rapidly. A mix suitable for the latter is likely to deform on a climbing lane and a mix suitable for a climbing lane is likely to possess poor durability in flat terrain. In severe locations the use of bitumen modifiers is often advantageous. The age hardening of the bitumen in the wearing course is much greater at the exposed surface where the effect of the environment is much more severe and it is this hardened, brittle skin that usually cracks early in the life of the surfacing. In areas where the diurnal temperature range is large, for example in most desert areas, thermal stresses can significantly increase the rate at which cracking occurs. The risk of premature cracking can be greatly reduced by applying a surface dressing to the wearing course soon after it has been laid, preferably after a few weeks of trafficking by construction traffic. This provides a bitumen-rich layer with a high strain tolerance at the point of potential weakness whilst also providing a good surface texture with improved skid resistant properties. If such a surface dressing is used, some cost savings can often be made by using a basecourse material in place of the wearing course.

a) Premixed Surfacings For severely loaded sites, such mixes can be designed to have a high resistance to deformation and under these conditions a surface dressing is essential if early cracking is to be prevented. It has also been shown that 40/50, 60/70 and 80/100 penetration grade bitumens in the surface of wearing courses all tend to harden to a similar viscosity within a short time. It is therefore recommended that 60/70 pen bitumen is used to provide a suitable compromise between workability, deformation resistance and potential hardening in service. If possible, bitumen should be selected which has a low temperature sensitivity and good resistance to hardening as indicated by the standard and extended forms of the Rolling Thin Film Oven Test [TRL, 1993]. i)

Types of Premix in Common Use The most important bituminous materials used within highway pavements are categorized into: • Asphalt mixes [i.e. Asphaltic concrete (AC), Hot rolled Asphalt (HRA)]; • Coated Macadams [i.e. Dense bitumen macadam (DBM), High Density Macadam (HDM), Pervious Macadam (PA)]. The main types of premixes used in the tropics are; asphaltic concrete, bitumen macadam and hot rolled asphalt. Each type can be used in surfacings or roadbases. Their general properties and specifications suitable for tropical environments are described below. A design procedure based on ‘refusal density’ is suggested to enhance the standard Marshall procedure (Appendix D of TRL, 1993).

ii) Mechanisms by which Asphalt and Macadams Distribute Traffic Stresses Asphalt Mixes consist of single sized coarse aggregate blended with fine aggregate and filler to produce a gap graded material or a graded course aggregate blended with fine aggregate and filler to give a semi gap graded mix. Since they are not well graded, asphalts lack good aggregate interlock and mainly derive their strength and stability from the finesKyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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filler-binder mortar. The filler and binder contents in the mix are high. The binder should be of high viscosity and high filler content is required to stiffen the binder so that when the fines-filler-binder combination hardens, it produces a mortar with high stiffness modulus able to resist wheel track deformation. The role of coarse aggregate in an asphalt mix is to bulk the material and provide additional stability to the hardened mortar. Because the binder used in asphalt mixes must be hard, they must be hot laid and hot compacted. Compacted asphalt layers have low air voids contents, are nearly impervious to the entrance of water and are durable. The high mortar content of the asphalt mix does not provide good skidding resistance so it is important to apply a surface dressing of pre-coated chippings to improve skid resistance. Coated Macadam materials tend to have higher coarse aggregate contents and to be more continuously graded than asphalt materials. The strength and stability of a coated macadam material is primarily derived from particle to particle contact, inter-particle friction and aggregate interlock. It is therefore important that the aggregate particles are sufficiently tough to ensure they do not break down under rollers during compaction as well as under traffic action during service. The role of the binder in these materials is mainly to lubricate aggregate particles during compaction while acting as a bonding and waterproofing agent when the pavement is in service. In dense coated macadam materials the filler causes an increase in binder viscosity and this reduces the risk of the binder flowing from the aggregate during transport. The voids content of a coated macadam material should be within the specified range after compaction. This is important in a wearing course material where a high void content leads to fretting and a very low void content leads to deformation. A high voids content increases ageing of the binder through oxidation ultimately leading to brittle fracture of the wearing course at low pavement temperatures. The predominant type of macadam used is dense bitumen macadam (DBM). iii) Asphalt Concrete (AC) Asphaltic concrete (AC) is a dense, continuously graded mix which relies for its strength on both the interlock between aggregate particles and, to a lesser extent, on the properties of the bitumen and filler. The mix is designed to have low air voids and low permeability to provide good durability and good fatigue behaviour but this makes the material particularly sensitive to errors in proportioning, and mix tolerances are therefore very narrow. The particle size distributions for wearing course material given in table 4.10 have produced workable mixes that have not generally suffered from deformation failures but they are not ideal for conditions of severe loading e.g. slow moving heavy traffic and high temperatures. This is because the continuous matrix of fine aggregate, filler and bitumen is more than sufficient to fill the voids in the coarse aggregate and this reduces the particle to particle contact within the coarse aggregate and lowers the resistance to deformation. A particle size distribution that conforms to the requirements for asphaltic concrete or a close graded bitumen macadam basecourse (BC1 in table 4.10 or BC2 in table 4.13) is recommended for use as the wearing course in severe conditions but such mixes must be sealed. It is common practice to design the mix using the Marshall Test and to select the design binder content by calculating the mean value of the binder contents for (i) maximum stability, (ii) maximum density, (iii) the mean value for the specified range of void contents and (d) the mean value for the specified range of flow values. Compliance of properties at this design binder content with recommended Marshall Criteria is then obtained (table 4.11). A maximum air voids content of 5 per cent is recommended to reduce the potential age hardening of the bitumen but on severe sites the overriding criteria is that a minimum air voids of 3 per cent at refusal density should be achieved. This is equivalent to the condition which will arise after heavy trafficking and is designed to ensure that serious deformation does not occur. For such a mix it is unlikely that it will also be possible to reduce the air voids content at 98 Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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per cent of Marshall Density to 5 per cent and therefore it is recommended that a surface dressing is applied to the wearing course to provide the necessary protection against age hardening. It is frequently found that mixes are designed to have the highest possible stabilities. This usually means that the binder content is reduced resulting in mixes which are more difficult to compact and are less durable. It is important to note that there is a relatively poor correlation between Marshall stability and deformation in service, and durability should not be jeopardised in the belief that a more deformation resistant mix will be produced. A better method of selecting the Marshall Design binder content is to examine the range of binder contents over which each property is satisfactory, define the common range over which all properties are acceptable, and then choose a design value near the centre of the common range. If this common range is too narrow, the aggregate grading should be adjusted until the range is wider and tolerances less critical. Table 4.10: Asphaltic Concrete Surfacings

Source: TRL (1993) Table 4.11: Suggested Marshall Test Criteria

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Source: TRL (1993) Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514


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To ensure that the compacted mineral aggregate in continuously graded mixes has a void content large enough to contain sufficient bitumen, a minimum value of the voids in the mineral aggregate (VMA) is specified, as shown in table 4.12 below. Table 4.12: Voids in Mineral Aggregate (VMA)

Source: TRL (1993)

The Marshall Design procedure is based on the assumption that the densities achieved in the Marshall Test samples represent those that will occur in the wheel paths after a few years of trafficking. If in situ air voids are too high, rapid age hardening of the bitumen will ensue. Conversely, on severely loaded sites the air voids may be reduced by traffic leading to failure through plastic flow. In this latter situation the method of designing for a minimum air voids in the mix (VIM) at refusal density should be used (to be discussed later).

iv) Dense Bitumen Macadam (DBM) Dense bitumen macadams (DBMs) (also known as close graded bitumen macadams) are continuously graded mixes similar to asphaltic concretes but usually with a less dense aggregate structure. Conventionally DBMs are well graded with typically 3-8% air voids when compacted. They are made to recipe specifications without reference to a formal design procedure. Aggregates which behave satisfactorily in asphaltic concrete will also be satisfactory in DBMs. Suitable specifications for both wearing course and basecourse mixes are given in table 4.13. Sealing the wearing course with surface dressing soon after laying is recommended for a long maintenance-free life. Close graded bitumen macadam mixes offer a good basis for the design of deformation resistant materials for severe sites and in these cases they should be designed on the basis of their refusal density. Recipe mixes are not recommended in these circumstances and the Marshall Design criteria in table 4.14 should be used. At the time of construction the air voids content is virtually certain to be in excess of five per cent and therefore a surface dressing should be placed soon after construction.

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Table 4.13: Bitumen Macadam Surfacings

Source: TRL (1993)

Table 4.14: Suggested Marshall Criteria for Close Graded Bitumen Macadams or DBMs

Source: TRL (1993)

v) Hot Rolled Asphalt (HRA) Hot rolled asphalt (HRA) surfacing is a dense, low air void content (3-6%) nearly impervious gap graded material composed of filler (<0.075mm), fine aggregate (0.075 2.36mm) in which coarse aggregate (> 2.36mm) is dispersed. It contains little medium sized (2.36mm to 10mm) aggregate and has relatively high binder content. The primary function of the coarse aggregates in the mortar is to bulk the mortar and reduce/lower the cost of asphalt. The mechanical stability of the hot rolled asphalt is controlled by the quality of the fines – filler-bitumen mortar. The influence of the coarse aggregate on stability and density increases as the proportion of coarse aggregate in the mix increases above approximately 55%. Crushed rock and gravel are permitted as coarse aggregate (>2.36mm) in HRA wearing course and base courses. The maximum size of aggregate in either course is Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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69

controlled by its thickness. The usual practice is to limit the maximum size to one third or one half of the thickness of the compacted layer to achieve good compaction. HRA wearing courses can have aggregate contents of 0%, 15%, 30%, 35% or 55%. The basecourse mixes have high coarse aggregate contents of 50% or 60%. They are typically laid 50mm or 55mm thick beneath and HRA wearing course. The fines (0.075-2.36mm) used in HRA are natural sands or crushed rock fines. Crushed rock fines give the mix less workability but are more stable than the natural sands. The grading of sand and rock aggregates must be such that no more than 5% and 10% by mass respectively is retained on the 2.36mm sieve and no more than 8% and 17% respectively can pass through the 0.075mm sieve. At least 85% of filler material (<0.075mm) must pass the 0.075mm sieve. Most of the filler is in the form of added limestone dust, hydrated lime or Portland cement. The filler stiffens the bitumen. The compositions of suitable mixes are summarised in table 4.15 below. Table 4.15: Hot Rolled Asphalt (HRA) Surfacings

Source: TRL (1993)

vi) Flexible Bituminous Surfacing (RB) It is essential that the thin bituminous surfacings (50mm) recommended for structures described in Charts 3, 4 and 7 of the structural catalogue are flexible. This is particularly important for surfacings laid on granular roadbases. Mixes which are designed to have good durability rather than high stability are flexible and are likely to have “sand” and bitumen contents at the higher end of the permitted ranges. In areas where the production of sandsized material is expensive and where there is no choice but to use higher stability mixes, additional stiffening through the ageing and embrittlement of the bitumen must be prevented by applying a surface dressing.

vii) Design to Refusal Density Under severe loading conditions asphalt mixes must be expected to experience significant secondary compaction in the wheel paths. Severe conditions cannot be precisely defined but will consist of a combination of two or more of the following; • High maximum temperatures Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Bituminous Roadbases • • •

70

Very heavy axle loads Very channelled traffic Stopping or slow moving heavy vehicles

Failure by plastic deformation in continuously graded mixes occurs very rapidly once the VIM are below 3 per cent therefore the aim of refusal density design is to ensure that at refusal there is still at least 3 per cent voids in the mix. For sites which do not fall into the severe category, the method can be used to ensure that the maximum binder content for good durability is obtained. This may be higher than the Marshall optimum but the requirements for resistance to deformation will be maintained. Where lower axle loads and higher vehicle speeds are involved, the minimum VIM at refusal can be reduced to 2 per cent. Refusal density can be determined by two methods; • Extended Marshall compaction; • Compaction by vibrating hammer. Details of the tests and their limitations are given in Appendix D of ORN31.

5.5 Bituminous Roadbases Satisfactory bituminous roadbases for use in tropical environments can be made using a variety of specifications. They need to possess properties similar to bituminous mix surfacings but whenever they are used in conjunction with such a surfacing the loading conditions are less severe, hence the mix requirements are less critical. Nevertheless, the temperatures of roadbases in the tropics are higher than in temperate climates and the mixes are therefore more prone to deformation in early life, and ageing and embattlement later. i)

Principal Mix Types Particle size distributions and general specifications for continuously graded mixes are given in table 4.16. No formal design method is generally available for determining the optimum composition for these materials because the maximum particle size and proportions of aggregate greater than 25 mm precludes the use of the Marshall Test. Suitable specifications for gap-graded rolled asphalt roadbases are given in Table 4.17. All these specifications are recipes which have been developed from experience and rely on performance data.

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71

Table 4.16: Bituminous Macadam Roadbase

Source: TRL (1993)

Table 4.17: Rolled Asphalt Roadbase

Source: TRL (1993)

71 ii)

Principles of Handling Bituminous Mixes The following principles should be adopted for all bituminous layers but are particularly important for recipe type specifications:


Bituminous Roadbases • •

• • iii)

72

Trials for mix production, laying and compaction should be carried out to determine suitable mix proportions and procedures. Durable mixes require a high degree of compaction and this is best achieved by specifying density in terms of maximum theoretical density of the mix or, preferably, by using a modification of the Percentage Refusal Test with extended compaction time (British Standard 598, Part 104 (1989). Mixing times and temperatures should be set at the minimum required to achieve good coating of the aggregates and satisfactory compaction. The highest bitumen content commensurate with adequate stability should be used.

Manufacture and Construction General guidance on the design, manufacture and testing of bitumen macadams and rolled asphalts can be found in the British Standards, BS 4987 (1988) for macadams and BS 594 (1985) and BS 598 (1985) for rolled asphalts. Similar guidance for asphalt concrete is given in the publications of the Asphalt Institute, SS-1 (1980), MS-2 (1988) and MS-22 (1983), and the US Army Corps of Engineers (1991). It is normal practice to carry out preliminary design testing to determine the suitability of available aggregates and their most economical combination to produce a job-mix formula. The job mix particle size distribution should be reasonably parallel to the specified grading envelope and is the target grading for the mix to be produced by the asphalt plant. Loss of fines may occur during the drying and heating phase and, therefore, tests on aggregates which have passed through the asphalt plant in the normal way should be used to establish a job-mix formula which meets the specified Marshall Test criteria. The importance of detailed compaction trials at the beginning of asphalt construction work cannot be over emphasised. During these trials, compaction procedures and compliance of the production-run asphalt with the job-mix formula should be established. Adjustments to the job-mix formula and, if necessary, redesign of the mix are carried out at this stage to ensure that the final job-mix satisfies the mix design requirements and can be consistently produced by the plant. Tolerances are specified for bitumen content and for the aggregate grading to allow for normal variation in plant production and sampling. Typical tolerances for single tests are given in Table 4.18. Good quality control is essential to obtain durable asphalt and the mean values for a series of tests should be very close to the job-mix formula which, in turn, should have a grading entirely within the specified envelope. Mixing must be accomplished at the lowest temperatures and in the shortest time that will produce a mix with complete coating of the aggregate and at a suitable temperature to ensure proper compaction. The ranges of acceptable mixing and rolling temperatures are shown in Table 4.19. Very little additional compaction is achieved at the minimum rolling temperatures shown in the table and only pneumatic tyred rollers should be used at these temperatures. Rolled asphalts are relatively easy to compact but bitumen macadams and asphaltic concretes are relatively harsh and more compactrve effort is required. Heavy pneumatic tyred rollers are usually employed, the kneading action of the tyres being important in orientating the particles. Vibratory compaction has been used successfully but care is needed in selecting the appropriate frequency and amplitude of vibration, and control of mix temperature is more critical than with pneumatic tyred rollers. Steelwheeled deadweight rollers are relatively inefficient and give rise to a smooth surface with poor texture but are required to obtain satisfactory joints. Rolling usually begins near the shoulder and progresses towards the centre. It is important that directional changes of the roller are made only on cool compacted mix and that each pass of the roller should be of slightly different length to avoid the formation of ridges. The number


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of joints to cold, completed edges should be minimised by using two pavers in echelon or a full-width paver to avoid cold joints between adjacent layers. If this is not possible, repositioning of the paver from lane to lane at frequent intervals is another option. If a layer is allowed to cool before the adjacent layer is placed, then the Asphalt Institute method of joint formation is recommended. The edge of the first layer must be “rolled over” and thoroughly compacted. Before laying the second lane the cold joint should be broomed if necessary and tack coated. The paver screed should be set to overlap the first mat by a sufficient amount to allow the edge of the rolled over layer to be brought up to the correct level. Coarse aggregates in the material overlapping the cold joint should be carefully removed. The remaining fine material will allow a satisfactory joint to be constructed. Table 4.18: Job-mix Tolerances

Table 4.19: Manufacturing and rolling temperature (in degrees centigrade)

Source: TRL (1993)

5.6 Surface Dressing a) Introduction Surface dressing is a simple, highly effective and inexpensive road surface treatment if adequate care is taken in the planning and execution of the work. The process is used throughout the world for surfacing both medium and lightly trafficked roads, and also as a maintenance treatment for roads of all kinds. Surface dressing comprises a thin film of binder, generally bitumen or tar, which is sprayed onto the road surface and then covered with a layer of stone chippings. The thin film of binder acts as a waterproofing seal preventing the entry of surface water into the road structure. The stone chippings protect this film of binder from damage by vehicle tyres, and form a durable, skid-resistant and dust-free wearing surface. In some circumstances the process may be repeated to provide double or triple layers of chippings. Surface dressing is a very effective maintenance technique which is capable of greatly extending the life of a structurally sound road pavement if the process is undertaken at the optimum time. Under certain circumstances surface dressing may also retard the rate of failure of a structurally inadequate road pavement by preventing the ingress of water and thus preserving the inherent strength of the pavement layers and the subgrade. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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In addition to its maintenance role, surface dressing can provide an effective and economical running surface for newly constructed road pavements. Existing roads with bituminous surfacings, carrying in excess of 1000 vehicles/lane/day, have been successfully surfaced with multiple surface dressings. For sealing new roadbases traffic flows of up to 500vehicles/lane/day are more appropriate, although this can be higher if the roadbase is very stable or if a triple seal is used. A correctly designed and constructed surface dressing should last at least 5 years before resealing with another surface dressing becomes necessary. If traffic growth over a period of several years necessitates a more substantial surfacing or increased pavement thickness, a bituminous overlay can be laid over the original surface dressing when the need arises. The success of a surface dressing depends primarily on the adhesion of the chippings to the road surface, hence both the chippings and the road surface must be clean and free from dust during the surface dressing process. Inappropriate specifications, poor materials, and bad workmanship, can also drastically reduce the service life of a surface dressing. b) Surface Treatments i) Prime and Tack Coats It is essential that good bonding is achieved between the surface dressing and the existing road surface. This means that non-bituminous materials must be primed before surface dressing is carried out. A prime coat is a thin layer of bitumen sprayed onto the surface of an existing layer, usually of unbound or cement/lime bound material. Its purpose can be summarised as follows: • It assists in promoting and maintaining adhesion between the roadbase and the bituminous surfacing by pre-coating the surface of the roadbase and by penetrating the voids near the surface. • It helps to seal the surface pores in the roadbase, thus reducing the absorption of the first spray of bitumen of a surface dressing. • It helps to bind the finer particles of aggregate together in the surface of the roadbase. • If the application of the surfacing is delayed for some reason, it provides the roadbase with temporary protection against the detrimental effects of rainfall and light traffic. Low viscosity, medium curing cutback bitumen such as MC-30, MC-70, or in rare circumstances MC-250, can be used for prime coats (alternatively low viscosity road tar can be used if this is available). The depth of penetration should be about 3-10 mm and the quantity sprayed should be such that the surface is dry within two days. The correct viscosity and application rate are dependent primarily on the texture and density of the surface being primed. The application rate is likely to lie within the range 0.3-1.1 kg/m2. Low viscosity cutbacks are necessary for very dense cement or lime-stabilised surfaces, and high viscosity cutbacks for untreated coarse-textured surfaces. It is .usually helpful to spray the surface lightly with water before applying the prime coat as this helps to suppress dust and allows the primer to spread more easily over the surface and to penetrate. Bitumen emulsions are not suitable for priming because they tend to form a skin on the surface. The primary function of a tack coat is to act as a glue to assist bonding of a new surface layer to a previously primed surface, bituminous roadbase, or basecourse that has been left exposed for some time. Tack coats should be extremely thin and it is appropriate to use a dilute bitumen emulsion spread to give less than 0.2 kg /m2 of residual bitumen with continuous cover. When temperature conditions are satisfactory, it is possible to obtain a thin layer by lightly spraying the undiluted emulsion with a handlance Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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and then spreading it with a pneumatic tyred roller to obtain complete coverage [TRL, 2000]. ii) Types of Surfacing Dressing Surface dressings can be constructed in a number of ways to suit site conditions. The common types of dressing are illustrated in Figure 4.1 below.

Single Surface Dressing When applied as a maintenance operation to an existing bituminous road surface a single surface dressing can fulfil the functions required of a maintenance re-seal, namely waterproofing the road surface, arresting deterioration, and restoring skid resistance. A single surface dressing would not normally be used on a new roadbase because of the risk that the film of bitumen will not give complete coverage. It is also particularly important to minimise the need for future maintenance and a double dressing should be considerably more durable than a single dressing. However, a 'racked-in' dressing may be suitable for use on a new roadbase which has a tightly knit surface because of the heavier applications of binder which is used with this type of single dressing.

Double Surface Dressing Double surface dressings are robust and should be used when: • A new roadbase is surface dressed. • Extra 'cover' is required on an existing bituminous road surface because of its condition (e.g. when the surface is slightly cracked or patched). • There is a requirement to maximise durability and minimise the frequency of maintenance and resealing operations. The quality of a double surface dressing will be greatly enhanced if traffic is allowed to run on the first dressing for a minimum period of 2-3 weeks (and preferably longer) before the second dressing is applied. This allows the chippings of the first dressing to adopt a stable interlocking mosaic which provides a firm foundation for the second dressing. However, traffic and animals may cause contamination of the surface with mud or soil during this period and this must be thoroughly swept off before the second dressing is applied. Such cleaning is sometimes difficult to achieve and the early application of the second seal to prevent such contamination may give a better result. Sand may sometimes be used as an alternative to chippings for the second dressing. Although it cannot contribute to the overall thickness of the surfacing, the combination of binder and sand provides a useful grouting medium for the chippings of the first seal and helps to hold them in place more firmly when they are poorly shaped. A slurry seal may also be used for the same purpose.

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Figure 4.1: Type of Surface Dressing Source: TRL (2000)

76 Tripple Surface Dressing A triple surface dressing (not illustrated in Figure 4.1) may be used to advantage where a new road is expected to carry high traffic volumes from the outset. The application of a


Surface Dressing

77

small chipping in the third seal will reduce noise generated by traffic and the additional binder will ensure a longer maintenance-free service life.

Racked-in Surface Dressing This system is recommended for use where traffic is particularly heavy or fast [TRL, 1996]. A heavy single application of binder is made and a layer of large chippings is spread to give approximately 90 per cent coverage. This is followed immediately by the application of smaller chippings which should ‘lock-in' the larger aggregate and form a stable mosaic. The amount of bitumen used is more than would be used with a single seal but less than for a double seal. The main advantages of the racked-in surface dressing are: • Less risk of dislodged large chippings. • Early stability through good mechanical interlock. Good surface texture.

i)

Other Types of Surface Dressing Sandwich Surface Dressings These are principally used on existing binder rich surfaces and sometimes on gradients to reduce the tendency for the binder to flow down the slope.

ii) Pad Coat Surface Dressings These are used where the hardness of the existing road surface allows very little embedment of the first layer of chippings, such as on a newly constructed cement stabilized roadbase or a dense crushed rock base. A first layer of nominal 6mm chippings will adhere well to the hard surface and will provide a 'key' for larger l0mm or l4mm chippings in the second layer of the dressing. c)

Surface Dressing Design In order to design the surface dressing, consideration must be taken of the existing road surface, traffic, available chippings and climate. A surface material can be applied as a single or double surface dressing. A single surface dressing is suitable and adequate when applied to a bituminous layer while a double surface dressing is recommended for non bituminous layers. The quality of double surface dressing is enhanced if traffic is allowed to run on the first dressing for a period of 2-3 weeks. This allows the chippings of the first dressing to adopt a stable mosaic that provides a firm foundation for the second dressing [TRL, 1993].

i) Road Surface Hardness Embedment of chippings under traffic is dependent upon the hardness of the layer to be sealed and the size of the chippings. The assessment of layer hardness can be based on descriptive definitions or measured using a simple penetration test probe. Details of surface category, penetration values and descriptive definitions are shown in the table 4.20 below [TRL, 1993].

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Table 4.20: Category of Road Surface Hardness Surface Penetration Definition Category

o

at 30 C (mm)

Very Hard

0-2

Hard

2-5

Surfaces such as concrete or chemically stabilised road base into which negligible penetration of chippings will occur un heavy traffic Granular roadbase into which chippings will penetrate only slightly under heavy traffic

Normal

5-8

Bituminous roadbase or basecourse into which chippings w penetrate moderately under medium and heavy traffic

Soft

8-12

Bitumen rich asphalts into which chippings will penetrate considerably under medium and heavy traffic

Source: TRL (1993)

ii) Determination of Traffic Categories Traffic categories for surface dressing are considered in terms of approximate number of vehicles whose unladen weight is greater than 1.5 tonnes (per day) in the lane under consideration. The traffic categories are defined in table 4.21 below. It should be noted that, this differs from the traffic class used in the selection of pavement structure [TRL, 1993]. Table 4.21: Traffic Categories for Surface Dressing

Category

1 2 3 4 5

Approximate number of vehicles with unladen weight greater than 1.5 tonnes (per day) Over 2000 1000-2000 200-1000 20-200 Less than 20

Source: TRL (1993)

iii) Selection of Chippings The selection of chippings is based on the fact that the sizes of chippings chosen should match the level of traffic and hardness of the underlying surface as shown in table 4.22 below. Table 4.22: Recommended maximum chipping size (mm)

Surface

Traffic Category

Category 1 2 Very hard 10 10 Hard 14 14 Normal 20 14 Soft * 20 * Not suitable for surface dressing

3 6 10 14 14

4 6 6 10 14

5 6 6 6 10

Source: TRL (1993) Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Samples of the chippings should be tested for grading, flakiness index, aggregate crushing value and, when appropriate, the polished stone value and aggregate abrasion value. Sampling and testing should be in accordance with the methods described in British Standard BS 812 (1985, 1989a, 1989b 1990a, 1990b) [TRL, 2000]. In order to decide on the nominal size of chippings for double surface dressings, the size of chippings for the first layer is chosen on the basis of the hardness of the existing surface and the traffic category as indicated in table 4.22. The nominal size of chipping selected for the second layer should then be about half the nominal size of the first layer to promote good interlock between the layers.

iv) Selection of Bitumen for Surface Dressing The correct choice of bitumen for surface dressing work is critical. The bitumen must fulfil a number of important requirements. They must: • be capable of being sprayed; • 'wet' the surface of the road in a continuous film; • not run off a cambered road or form pools of binder in local depressions; • 'wet' and adhere to the chippings at road temperature; • be strong enough to resist traffic forces and hold the chippings at the highest prevailing ambient temperatures; • remain flexible at the lowest ambient temperature, neither cracking nor becoming brittle enough to allow traffic to 'pick-off' the chippings; and • resist premature weathering and hardening. Some of these requirements conflict, hence the optimum choice of binder involves a careful compromise. For example, the binder must be sufficiently fluid at road temperature to 'wet' the chippings whilst being sufficiently viscous to retain the chippings against the dislodging effect of vehicle tyres when traffic is first allowed to run on the new dressing. Figure 4.2 below shows the permissible range of binder viscosity for successful surface dressing at various road surface temperatures. In the tropics, daytime road temperatures typically lie between about 25oC and 50oC, normally being in the upper half of this range unless heavy rain is falling. For these temperatures the viscosity of the binder should lie between approximately l04 and 7 x l05 centistokes. At the lower road temperatures cutback grades of bitumen are most appropriate whilst at higher road temperatures penetration grade bitumens can be used. The temperature/viscosity relationships shown in figure 4.2 do not apply to bitumen emulsions. These have a relatively low viscosity and 'wet' the chippings readily, after which the emulsion 'breaks', the water evaporates and particles of high viscosity bitumen adhere to the chippings and the road surface. Depending upon availability and local conditions at the time of construction, the following types of bitumen can be used in the tropics: • Penetration grade; • Cutback; • Emulsion; • Modified bitumen.


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Penetration grade bitumens Penetration grade bitumens vary between 80/100 to approximately 700 penetration. The softer penetration grade binders are usually produced at the refinery but can be made in the field by blending appropriate amounts of kerosene, diesel, or a blend of kerosene and diesel. With higher solvent contents the binder has too low a viscosity to be classed as being of penetration grade and is then referred to as a cutback bitumen which, for surface dressing work, is usually an MC or RC 3000 grade. In very rare circumstances a less viscous grade such as MC or RC 800 may be used if the pavement temperature is below 15oC for long periods of the year. (Read about the other types of bitumen used in surface dressing). v)

Rate of Spread of Chippings An estimate of the rate of application of the chippings assuming the chippings have a loose density of 1.35Mg/m3 can be obtained from the equation below or from figure 4.4. Where; ALD = R =

/

,

1.364

… . 4.1

average least dimension of the chippings (in mm) Basic rate of spread of chippings (kg/m2)

Determination of Average Least Dimension (ALD) The ALD of chippings is a function of both the average size of the chippings, as determined by normal square mesh sieves, and the degree of flakiness. The ALD may be determined in two ways. Method A. A grading analysis is performed on a representative sample of the chippings in accordance with British Standard 812:1985. The sieve size through which 50 per cent of the chippings pass then is determined (i.e. the ‘median size'). The flakiness index is then also determined in accordance with British Standard 812:1985. The ALD of the chippings is then derived from the nomograph shown in figure 4.3. Method B. A representative sample of the chippings is carefully subdivided (in accordance with British Standard 812:1985) to give approximately 200 chippings. The least dimension of each chipping is measured manually and the mean value, or ALD, is calculated. Note: The chipping application rate should be regarded as a rough guide only. It is useful in estimating the quantity of chippings that is required for a surface dressing project before crushing and stockpiling of the chippings is carried out [TRL, 1993].

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Figure 4.2: Surface temperature/choice of binder for surface dressings Source: TRL (2000)

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Figure 4.3: Determination of average Least Dimension Source: TRL (2000)

vi) Rate of Application of Binder To determine the rate of application of binder, an appropriate factor should be selected from table 4.23 for each of the four sets of conditions listed. The four factors are then added together to give the total weighting factor, F. The Average Least Dimension of the chippings and the total weighting factor obtained from the condition constants in table 4.4 are then used with the formula below or figure 4.4 to obtain the rate of application of binder [TRL, 1993].

.

,

0.625

0.023

0.0375

0.0011

… . 4.2 82

Where; F ALD R

= = =

Overall weighting factor the average least dimension of the chippings (mm) Basic rate of spread of bitumen (kg/m2)


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Table 4.23: Condition Constants for determining the rate of application of Binder

Traffic Very Light Light Medium Medium Heavy Heavy Very Heavy

Existing Surface Untreated/primed roadbase Very lean bituminous lean bituminous average lean bituminous very rich bituminous

Vehicles /day Constant Type of chippings 0-50 50-250 250-500 500-1500 1500-3000 3000+

+3 +1 0 -1 -3 -5

+6 +4 0 -1 -3

Constant

Round/dusty

+2

Cubical

0

Flaky

-2

Pre-coated

-2

Climatic Conditions Wet and cold Tropical (wet and hot) Temperate Semi-arid (dry and hot) Arid (very dry and very hot)

+2 +1 0 -1 -2

Source: TRL (1993)

NB: The traffic considered here is that for all vehicle classes as emphasized in ORN 3 [TRL, 2000].

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Figure 4.4: Surface Dressing Design Chart Source: TRL (1993)


84

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vii) Adjusting the Rates of Spray for Maximum Durability The spray rate which will be arrived at after applying the adjustment factors in table 4.24 will provide very good surface texture and use an 'economic' quantity of binder. However, because of the difficulties experienced in many countries in carrying out effective maintenance, there is considerable merit in sacrificing some surface texture for increased durability of the seal. For roads on flat terrain and carrying moderate to high speed traffic it is possible to increase the spray rates obtained by applying the factors given in table 4.24 by approximately 8 per cent. The heavier spray rate may result in the surface having a 'bitumen-rich' appearance in the wheel paths of roads carrying appreciable volumes of traffic. However, the additional binder should not result in bleeding and it can still be expected that more surface texture will be retained than is usual in an asphalt concrete wearing course [TRL, 2000]. Table 4.24: Typical Bitumen Spray Rate Adjustment Factors Binder Grade

Basic Spray Rate from figure

Flat Terrain, moderate

High Speed Traffic,

Low speed Traffic,

A4 or appropriate equation

traffic speeds

down-hill grades >3%

up-hill grades >3%

MC-3000

R

R

R*1.1

R*0.9

300 pen 80/100 pen Emulsion1

R R R

R*0.95 R*0.90 R*(90/%binder)

R*1.05 R*0.99 R*(99/%binder)

R*0.86 R*0.81 R*(81/%binder)

1

%binder is the percentage of bitumen in the emulsion

Source: TRL (2000)

viii) Plant and Equipment Methods of distributing binder The success of a surface dressing is very dependent on the binder being applied uniformly at the correct rate of spread. The method adopted for distributing binder must therefore; • be capable of spreading the binder uniformly and at the predetermined rate of spread; and • be able to spray a large enough area in a working day to match the required surface dressing programme. The use of hand-held containers such as watering cans, perforated buckets etc, has a place for minor works. Any type of binder from penetration grades to emulsion can be applied in this way but uniform spreading of predetermined amounts cannot be achieved by this method and hence it is not recommended for anything other than small-scale work. A rather more controllable method of hand application is to use hand-lances. If skilfully used, they can produce an acceptably uniform rate of spread but it is very difficult to achieve a specified rate of spread with them. They cannot therefore be recommended for other than in small-scale work and limited maintenance operations. The use of either of these hand methods of binder application for larger scale work invariably results in waste of valuable binder and a poor quality surface dressing which will have a short 'life'. The spreading of binder on a larger scale requires the use of a bulk binder distributor, which may be either a self propelled or a towed unit (British Standards BS 1707:1989, and BS 3136: Part 2:1972). There are two basic types of bulk binder distributors; the pressurised tank, constant rate of spread, constant volume, and constant pressure machines. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Example 4.1: Surface Dressing

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Constant volume distributors These distributors are fitted with positive displacement pumps, the output of which can be pre-set. All the binder delivered by the pump is fed to the spray-bar when spraying is in progress and there is no by-pass arrangement for re-circulating binder to the tank. For a spray bar of given length and output, the rate of spread of binder on the road is inversely proportional to the forward road speed of the distributor. On most constant volume machines it is possible to preheat the spray bar by circulating hot binder to it before spraying commences but this facility is not available on all machines. Constant volume distributors can spray a wide range of types of binder and they are quite common in tropical developing countries. Disadvantages of constant volume distributors are; • Calibration involves three inter-related variables, i.e. the pump output, the road speed and the spray bar width; hence the calibration procedures need to be extensive if, for example, it is required to vary spray bar width to allow for different lane widths. However, some constant volume machines have a limited but useful degree of automatic control of bitumen pump speed to compensate for variation in road speed. • The relative mechanical complexity of the machines means that they are not suitable for operation by partly skilled operators. Most distributors manufactured in the USA are constant volume machines. Constant pressure distributors In these machines a pump of adequate capacity delivers binder to the spray bar at a preset pressure. A relief valve regulates the pressure and permits binder to bypass the spray bar and return to the tank. The pressure in the spray bar is not affected by the number of jets in use, and hence re-calibration is not required when spray bar extensions are fitted or the number of jets are reduced. As with constant volume machines, the rate of spread of binder varies inversely with the road speed of the distributor. Most distributors made in the UK are of the constant pressure type.

5.7

Example 4.1: Surface Dressing The Kansanga – Lukuli Road is situated in an area with a tropical climate. It is due for upgrading from a gravel road to a pavement consisting of a lime stabilised sub-base, a natural gravel, GB3 roadbase and surface dressing layer for the surfacing course. The road is carrying an ADT (for all vehicles) of 3154veh/day and an ADT (for commercial vehicles) of 2207veh/day. The surface dressing is to be laid on a bituminous primed roadbase into which chippings will penetrate moderately under medium and heavy traffic. The chipping sizes available from the proposed quarry in Muyenga are shown in table 4.1E below. Table 4.1E: Proposed Aggregate Sizes Chipping Size Median Sieve Grading (mm) Flakiness Index, FI 20 12.0 4.0 14 10.0 12.0 10 7.5 20.0

The chippings from this quarry have been found to be cubical in shape and the bitumen to be used in the surface dressing operation is designated 80/100 pen. As a Consultant Engineer, you have been asked to recommend the maximum chipping sizes to be used in the surfacing layers, the rate of spread of chippings and the rate of spread of binder. Adjust Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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87

the binder spray rates for flat terrain with moderate traffic speeds, down hill grades >3% for high speed traffic and uphill grades >3% for low speed traffic.

Solution 1.0

Design Information a) ADT (All Vehicle classes) b) ADT (Commercial vehicles) c) Type of binder pen d) Type of chippings e) Existing Surface: First layer Second Layer f) Climatic Conditions

2.0

= = :

3154veh/day 2207veh/day Penetration grade bitumen 80/100

: : : :

Cubical Primed Roadbase Lean Bituminous Tropical

Recommended Maximum Size of Chippings Surface Hardness Category : Normal a Traffic Category : 2b Notes: -The aggregate is to be laid on a bituminous primed roadbase into which chippings will penetrate moderately under medium and heavy traffic (See table 3.1). b –The Number of commercial vehicles/lane = ADT/2 = 2207/2 = 1104 veh/day. This traffic volume lies in the range 1000-2000veh/day (See table 3.3). a

From table 3.2, knowing that the surface hardness category of the project road is Normal and that the traffic category is 2, then the recommended chipping size for the first layer is 14mm Since this road is to be upgraded from a gravel to a bituminous surface road, it should receive a double surface dressing. This means that the second layer will have a chipping size of about half the normal size of the first layer i.e.; Chipping size of second layer ≈ 0.5 (14) mm = 7mm However, 7mm chippings are not common on the market so we take 10mm chippings for second layer. 10mm chippings also have added advantage of increasing the skid resistance of the road surface. Below are a summary of the recommended maximum chipping sizes and ALD are shown below. Table 4.7: Recommended maximum chipping sizes Surfacing Layer First Layer Second Layer

Chipping Sizes 14mm 10mm

ALD 8.0mm 5.5mm

3.0 Chippings Spread Rate An estimate of the rate of application of the chippings assuming that the chippings have a loose density of 1.35Mg/m3 can be obtained from the following equation: Chipping application rate (kg/m2) = 1 .364*ALD First Layer = 1.364x8.0 = 10.912 kg/m2 ≅ 10.9 kg/m2 Second Layer =

1.364x5.5

=


7.502 kg/m2

87


≅

88

7.5 kg/m2

Alternatively, the surface dressing chart in figure 4.4 can be used to determine the chippings spread rate. A horizontal line is drawn from the ALD Value obtained until it strikes the AB line. An upward vertical line is then drawn to strike the appropriate chippings application rate at the top of the chart. Determination of Average Least Dimension (Method A)

2nd layer ALD = 5.5mm

1st layer ALD = 8.0mm

4.0 Binder Spray Rate Using the ALD and 'F' values (from table 3.4) in the equation below will give the required basic rate of spread of binder. R = 0.625+ (F*0.023) + [0.0375+ (F*0.0011)] ALD Where; F = ALD = R =

Overall weighting factor the average least dimension of the chippings (mm) Basic rate of spread of bitumen (kg/m2)


88

Questions

89

Alternatively, the two values can be used in the design chart given in figure 4.4. The intercept between the appropriate factor line and the ALD line is located and the rate of spread of the binder is then read off directly at the bottom of the chart. Table 4.8: Determination of Overall weighting Factor ‘F’ for first layer of surface dressing Condition Constant Description Factor for first layer Traffic Volume (veh /day) -3 ≥ 3000 Type of Chippings: Cubical 0 Type of Existing Surface Primed Roadbase 6 Climatic Conditions Tropical 1 Total Weighting Factor, F 4 Table 4.9: Determination of Overall weighting Factor ‘F’ for Second layer of surface dressing Condition Constant Description Factor for first layer Traffic Volume (veh /day) 3000 -3 ≥ Type of Chippings: Cubical 0 Type of Existing Surface Lean Bituminous 0 Climatic Conditions Tropical 1 Total Weighting Factor, F -2

Therefore the binder spread rate for: First Layer R = 0.625+ (4x0.023) + [0.0375+ (4x0.0011)] x8 ≅ 1.05 kg/m2 Second Layer R =

≅

5.0

=

1.0522

0.625+ (-2x0.023) + [0.0375+ (-2x0.0011)] x5.5 = 0.77315 0.77 kg/m2

Adjustment of Binder Spray Rate According to ORN 3 [TRL, 2000], the best bitumen spray results are obtained if the basic rate of spread of binder is adjusted to take account of traffic speed and road gradient. Below are the appropriate adjustment factors for the various terrain types as obtained from table 3.4 for an 80/100 pen bitumen binder. Table 4.10: Adjustment of Spray rates Basic Spray rate, Flat Terrain Layer R (kg/m2) R*0.90 (kg/m2) 0.9450 First Layer 1.05 0.6930 Second Layer 0.77

Uphill Grades >3% Down Hill Grades >3% R*0.99 (kg/m2) R*0.81(kg/m2) 1.0395 0.8505 0.7623 0.6237

5.8 Questions Question one A wide range of unbound materials can be used as roadbase, sub-base, capping layer and selected subgrade layers within a flexible pavement. It has now to come to Mr. Fernández’s attention that for the pavement to perform satisfactorily during its design life, a number of parameters need to be tested to ensure material compliance. a) As a Consultant Engineer, you have been assigned the duty of selecting suitable natural gravel for the sub-base. The sub-base will perform as a construction platform. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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90

What performance criteria would you look out for? State the laboratory tests that would apply to each criterion. b) As a Resident Engineer supervising the contractor on site, what two major tests would you carry out and what values would you expect from these tests to ensure a well constructed sub-base? c) What is the limit on the plasticity expected in good natural gravel for the roadbase? If the material being tested exceeded this limit, what treatment would you give it prior to application on site? d) Mr. Fernández who has been involved in carrying out the feasibility study and detailed design of the Kansanga-Lukuli road would now like to decide between a “cement or lime stabilised gravel material” and a “graded crushed stone material” for the roadbase. Two materials coded BP1 and BP2 from different proposed borrow pits have been tested. Their results are shown in Table 4.1Q below. Table 4.1Q: Borrow Pit Test Results Material % Passing 0.075 mm sieve Plasticity index (%)

BP1 50 36

BP2 18 4

Which of these materials would you recommend for lime stabilisation and which would you recommend for cement stabilization. In each case, explain why? e) A recent report from the Ministry of Works and Transport (MoW&T) has indicated that the Kampala – Jinja road is in a state of failure due to the premature hardening of the bitumen binder used in the premix material applied to the surfacing course. Briefly explain the root cause of this kind of pavement failure and describe the failure path. Propose a feasible precaution that can be used by Mr. Fernández’s, at the construction stage of the road, to prevent this kind of failure. f) For road construction, aggregates play a role in bearing the main stresses in the road pavement as a result of application of static, traffic and/or dynamic loads. Briefly explain why a materials engineer is normally concerned about the following types of aggregates: • Highly absorptive aggregates; • Hydrophilic aggregates. g) Assuming a premix option for the surfacing course was to be considered, explain how the following premixes attain their structural stability: • Asphalts; • Coated Macadams.

Question Two a) As a consultant engineer assigned to the above project road, where would you apply the following types of surface dressing and why? • Sandwich surface dressing, and Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Questions •

91

Pad coats

b) A prime coat is a thin layer of bitumen sprayed onto the surface of an existing layer, usually of unbound or cement/lime bound material. Describe at least three functions of a prime coat pertaining to highway construction. c) The Kansanga – Lukuli road is due for upgrading from a gravel road to a pavement consisting of a lime-stabilised sub-base, a natural gravel GB3 material and a surface dressing layer for the surfacing course. It is situated in an area with a tropical climate. Assume the baseline traffic survey carried out at station ‘A’ (refer to question one) is representative of the entire 1.32 km stretch, and that the surface dressing is to be laid on a pavement surface whose penetration value is 6.5 mm at 30oC. Mr. Fernández, the Spannish highway consultant, has proposed Muyenga quarry as the source of chippings to be used on the project road. The aggregate test results from this quarry are summarized below. Table 4.1: Quarry Test Results Chipping Sise 20 mm 14 mm 10 mm

Median Sieve Size (mm) Flakiness Index, FI (%) 17.5 8.0 15.0 7.0 12.5 21.0

The chippings from this quarry have been found to be cubical in shape and the bitumen to be used in the surface dressing operation is designated 80/100 pen. As a consultant engineer, you have been asked to recommend the chipping sizes to be used in the surfacing layers, the rate of spread of chippings and the rate of spread of binder. Adjust the binder spray rates for flat terrain with moderate traffic speeds, downhill grades > 3% for high speed and uphill grades > 3% for low speed traffic.

Question Three a) What factors would you consider in order to justify the use of a given road base material (i.e. GB1, GB2 or GB3) on a given project road. b) What are the benefits associated with proper grading of fines in a ‘GB2, A’ material? What would be the expected grading limits for these fines? c) What is the difference between bitumen and tar binders? Briefly discuss the various types of bitumen available on the Ugandan market. d) Describe the different physical properties of aggregates that govern their suitability for use on flexible pavements. Outline at least eight aggregate tests you would carry out to ensure material compliance. e) What are the differences between asphaltic concrete (AC), hot rolled asphalt (HRA) and dense bitumen macadams (DBMs)? f) Differentiate between recipe and design mix specifications as applied to premixes. Describe the five major parameters you would look at if you asked to come up with a recipe specification for hot rolled asphalt (HRA). Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Bibliography

92

g) What are the four major factors you would consider in the design of surface dressing? Differentiate between ‘constant volume’ and ‘constant pressure’ distributors. How would you check whether the computed rate of bitumen spray has been achieved on site?

5.9 Bibliography 1. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and Maintenance of Pavements. 4th Edition, Oxford, Butterworth Heinemann. 2. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil Engineering, Kyambogo University. 3. Transport Research Laboratory, 1993, A Guide to Design of Bitumen Surfaced Roads in Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England. 4. Transport Research Laboratory, 2000, A guide to surface dressing in tropical and subtropical countries, Overseas Road Note 3, Crowthorne, England.

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Chapter Five:

93

Chapter Five:

The Structure Catalogue 5.1 Basis for the Structure Catalogue The structure catalogue in the fourth edition of Overseas Road Note (ORN) 31 shows the various pavement structures encountered in design. It is predominantly based on: • The results of full-scale experiments where all factors affecting performance have been accurately measured and their variability quantified; • Studies of the performance of as-built existing road networks. Where direct empirical evidence was lacking, designs were interpolated or extrapolated from empirical studies using: • Road performance models; and • Standard analytical, mechanistic methods. In view of the statistical nature of pavement design caused by the large uncertainties in traffic forecasting and the variability in material properties, climate and road behaviour, the design charts have been presented as a catalogue of structures, each structure being applicable over a small range of traffic and subgrade strength. Such a procedure makes the charts extremely easy to use but it is important that the reader is thoroughly conversant with the notes applicable to each chart [TRL, 1993].

5.2 How to use the Structure Catalogue The information necessary to use the Structure Catalogue is contained in the previous chapters of these notes. The cells of the catalogue are defined by ranges of traffic (Chapter 2) and subgrade strength (Chapter 3) and some of the materials are described in Chapter 4. A summary of requirements and reference chapters relevant to each design chart is given in Table 5.1. Although the thicknesses of layers should follow the designs whenever possible, some limited substitution of materials between sub-base and selected fill is allowable based on the structural number principles outlined in the AASHTO guide for design of pavement structures [AASHTO, 1986]. Where substitution is allowed, a note is included with the design chart. The charts are designed so that, wherever possible, the thickness of each lift of material is obvious. Thus, all layers less than 200 mm will normally be constructed in one lift and all layers thicker than 300 mm will be constructed in two lifts. Occasionally layers are of intermediate thickness and the decision on lift thickness will depend on the construction plant available and the ease with which the density in the lower levels of the lift can be achieved. The thickness of each lift need not necessarily be identical and it is often better to adjust the thickness according to the total thickness required and the maximum particle size by using a combination of gradings from Table 4.2. In Charts 3, 4 and 7 where a semi-structural surface is defined, it is important that the surfacing material should be flexible and the granular roadbase should be of the highest quality, preferably GB1,A. In traffic classes T6, T7 and T8 only granular roadbases of Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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How to use the Structure Catalogue

94

type GB1 or GB2 should be used; GB3 is acceptable in the lower traffic classes. For lime or cement stabilised materials, the charts already define the layers for which the three categories of material may be used. The choice of chart will depend on a variety of factors but should be based on minimising total transport costs as discussed in Section 7.2. Factors that will need to be taken into account in a full evaluation include: • The likely level and timing of maintenance; • The probable behaviour of the structure; • The experience and skill of the contractors and the availability of suitable plant; • The cost of the different materials that might be used; • Other risk factors. It is not possible to give detailed guidance on these issues. The charts have been developed on the basis of reasonable assumptions concerning the first three of these, as described in the text, and therefore the initial choice should be based on the local costs of the feasible options If any information is available concerning the likely behaviour of the structures under the local conditions, then a simple risk analysis can also be carried out to select the most appropriate structure. With more detailed information, it should be possible to calibrate one of the road investment models such as HDM-111 or RTIM-2 and then to use the model to calculate the whole life costs associated with each of the possible structures thereby allowing the optimum choice to be made. For many roads, especially those that are more lightly trafficked, local experience will dictate the most appropriate structures and sophisticated analysis will not be warranted [TRL, 1993]. Table 5.1: Summary of Material Requirements for the Design Charts

94 Source: TRL, 1993


Key to Structural Catalogue

95

5.3 Key to Structural Catalogue

Source: TRL, 1993


95

Pavement Design Charts

96

5.4 Pavement Design Charts Chart 1: Granular Roadbase / Surface Dressing

Source: TRL, 1993

96



97

Chart 2: Composite Roadbase (Unbound & cemented) / Surface Dressing

Source: TRL, 1993

97



98

Chart 3: Granular Roadbase / Semi-Structural Surface

Source: TRL, 1993

98



99

Chart 4: Composite Roadbase / Semi-Structural Surface

Source: TRL, 1993

99



100

Chart 5: Granular Roadbase / Structural Surface

Source: TRL, 1993

100



101

Chart 6: Composite Roadbase / Structural Surface

Source: TRL, 1993

101



102

Chart 7: Bituminous Roadbase / Semi-Structural Surface

Source: TRL, 1993

102



103

Chart 8: Cemented Roadbase / Surface Dressing

Source: TRL, 1993

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Questions

104

5.5 Questions Question One a) The Kansanga-Lukuli road has undergone a traffic survey that has established a traffic loading of 3.5 million equivalent standard axles. The subgrade tests taken at 0.5 km intervals along a 2.5 km section have established the following results. Table 1.1Q: Subgrade Test Results Chainage 80 + 500 81 + 000 81 + 500 8.0 9.0 11.0 CBR (%)

82 + 000 7.0

82 + 500 5.0

83 + 000 6.0

Assuming that the material to be used for the base is graded crushed stone of category GB1, design the different pavement layers in terms of thicknesses and clearly name each layer and indicate its design thickness. For the surfacing, only indicate the type of surface.

5.6 Bibliography 1. Transport Research Laboratory, 1993, A Guide to Design of Bitumen Surfaced Roads in Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England.

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Chapter Six:

105

Chapter Six:

Highway Drainage 6.1 General Drainage is very important to the life of a road pavement and the safety of road users. Its main objective is to protect the road and the adjacent land against potential damage from storm and subsurface water. Drainage elements should generally be free of obstructions and should maintain their design cross sections and grades [Kadiyali, 2006]. Improper drainage of the road increases the risk of skidding and long braking distances in wet conditions, reduced wind screen visibility as a result of splashing from the tyres of other vehicles especially commercial vehicles. It further causes deterioration by weakening the soil around drainage structures and softening it thereby causing loss of strength in terms of reduced bearing capacity [Singh, 2001]. Therefore in considering drainage design, engineers must take account of both surface and sub surface drainage depending on the existing conditions. The former takes care of the process of safely dispersing surface water from the road prism while the latter caters for water beneath the pavement that may weaken it if not provided with drainage paths to lead it away from underlying pavement layer. Cross drainage is used to control storm water within a watershed area [Ruhweza, 2005].

6.2 Main functions of Drainage A good and well maintained drainage system plays the following main functions: • To convey rain water from the surface of the carriageway to outfalls; • To control the level of water table in the subgrade beneath the carriageway; • To intercept ground and surface water flowing towards the road; • To convey water across the line of the road in a controlled fashion. The first three functions are performed by longitudinal drainage components; in particular side drains while the fourth function requires cross drainage structures such as culverts, fords, drifts and bridges [Singh, 2001].

6.3 Highway Drainage Terminologies The Labour Based Maintenance Contractor Training Module (MoWH&C, 2003) defines the following drainage terms: a) Side drains; Side drains run along the road and collect the water from the carriage way and adjoining land and transport it to a convenient point of disposal. b) Mitre drains; Mitre drains (or turnout drains) let the water out of the side drains and safely dispose it on adjoining land. Mitre drains should be provided as often as possible so that the accumulated water volume in each drain is not too high and does not cause erosion to the adjoining land. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Surface Drainage

106

c) Catch water drains; Where the road is situated on a hill side, a significant amount of rain water may slow down the hill towards the road. This may cause damage to the cut face (back slope) of the road and even cause land slides. Catch water drains intersect or ‘catch surface water flowing towards from the adjacent land and lead it away. d) Scour Checks; Scour checks prevent erosion on side drains from stiff gradient by slowing down the water. Scour checks are usually built using local available material, such as stones or wooden sticks. e) Culvert; A culvert is a transverse drain built under the road and its function is to lead water from the upper, up hill side to the lower, valley side. In tropical countries with high rainfall, 3-4 culverts are required per kilometre. Culvert rings are usually made of concrete or prefabricated corrugated steel rings [MoWH&C, 2003]. Road drainage works may be classified into the following three categories which are to be discussed hereafter: • Surface drainage; • Sub-surface drainage; and • Cross drainage.

Figure 6.1: Road Drainage features Source: TRL (1993)

6.4 Surface Drainage The object of surface drainage is to remove storm water from the roadway so that traffic can move safely and efficiently. Under this category, surface water is intercepted and diverted into a natural channel or depression. If it’s not done, the surface water will flow along the road or across it causing erosion. The major types of surface drainage are;


106

Surface Drainage •

Open or ‘Over the edge drainage’ which is constructed over embankment slopes and into open ditches or preformed channel blocks. It is mainly used in rural roads and should never be used where a footpath is adjacent to the carriageway.

•

Kerbs and Gutters; here, vertical kerbs and sloping gutters are used to form triangular channels that carry the runoff water to inlets in gulley pits. They are normally used in urban areas [Ruhweza, 2005].

107

a) Forms of Open (or Over the Edge) Road Drainage According to Thagesen (1996), longitudinal drainage can be classified into various forms of open road side drainage channels basing on the various functions they perform. These include: i) Ditches: - these are channels provided to remove the run off from the road pavement, shoulders, and ‘cut and fill’ slopes. Its depth should be sufficient to remove the water without risk of saturating the pavement subgrade. It may be lined to control erosion. Unlined ditches should preferably have slopes not steeper than 1:4. ii) Gutters: - they are the channels at the edges of the pavement or the shoulder formed by curb or by a shallow depression. They can be paved with concrete, bricks, stone blocks or other structural materials. iii) Turnouts: - they are sometimes referred to as ‘mitre drains’. They are short open and skewed ditches or gutters. They are used to reduce the sizes of the side ditches and minimise velocity of water and thereby the risk of erosion. They are provided at intervals depending on the runoff, permissible velocity of the water and slope of the terrain. iv) Chutes: - they are also open, lined channels or closed pipes used to convey water from gutters and side ditches down fill slopes and from intercepting ditches down cut slopes. Their interval of placing depends on the capacity of gutter or ditches. v) Intercepting ditches: - they are sometimes referred to as ‘cut off drains’. They are located on natural ground near the top edge of a cut slope or along the edge of the right – of – way. They serve to intercept the runoff from hillsides before it reaches the road. Intercepting the surface flow reduces erosion of cut slopes and roadside ditches, lessens silt deposition and infiltration in the roadbed area, and decreases the likelihood of flooding the road in severe storms [Thagesen, 1996]. Generally, the design of road drainage is basically concerned with selecting a design storm, estimating the likely run off from the storm resulting from the catchment area, and deciding how to collect and remove the water to a suitable discharge point so that it can be disposed off safely and economically. b) Factors Affecting Design Floods The major factors affecting design floods are; i)

Rainfall: - rainfall varies throughout Uganda. A large amount of rainfall comes from high intensity short duration thunderstorms. The Uganda road design manual (1994) makes the following assumptions: • The flood peak will occur when a stationary storm is of sufficient duration to ensure that run off from each portion of the catchment will contribute to the flood


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Surface Drainage

108

peak simultaneously. This means the critical storm duration (Ts) should at least equal to the time of concentration (Tc). •

Run off reaching the catchment exit after the time of concentration (Tc) will be too late to contribute to flood peak.

Intensity-Duration-Frequency (IDF) curves are based on a stationary storm. In many instances the prevailing storm direction over the catchment is known. In cases where the storm is known to travel downstream, the Uganda Road Design Manual (1994) recommends that the time of concentration be reduced by 20%. However, this is an arbitrary figure due to lack of local data to prove otherwise. ii) Mean Annual Precipitation: - the higher the annual precipitation the higher the design floods anticipated. Mean annual rainfall is location specific and in this regard, data on the same should be obtained from the responsible meteorological department. iii) Flood Return Period: - If an event has a return period of T years, it means that the probability of that event occurring in any specific year is 1/T. In flood estimation, this event is commonly defined as that equalling or exceeding flow rate. The return period of floods should be determined based on passed experience and an economic evaluation. A long return period will result in building of more expensive drainage infrastructure whereas a short return period will result into a greater frequency of upstream flooding at the culvert inlet [Ocen, 2005]. The following design storm frequencies or return periods as adopted from the Kafu – Masindi road project can be used for the hydrological studies. Table 6.1: Run off coefficient for the rational method

Structure Major Bridges Minor Bridges Slab and Box Culverts Pipe Culverts Ditches Source: SABA, 2003

Return Period (in Years) 25 20 15 10 5

iv) Other factors Include: • Topography; According to Duggal (1991), the topography of the watershed includes factors such as; extent of area drained, slope of area, nature of soil, number of available ditches in the area and shape of the area (i.e. fan shaped areas drain away discharge more quickly than oblong shaped areas). •

c)

Humidity, wind and Temperature; greater humidity, high winds and warm temperatures tend to reduce the storm water flow [Duggal, 1991].

Estimation of Surface Runoff There are many formulae proposed to measure the amount of run-off from a storm over a given area. Some of the methods work well for small catchments while others work on larger catchments [Wilson, 1994].


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109

According to SABA consultants (2003) the three common methods used are; • the rational method (usually recommended for small catchments of area < 25km2); • the triangular unit hydrograph or SCS (Soil Conservation Service) method (usually recommended for larger catchments); • the Giandotti method (for areas > 30km2). i) The Rational Method of Estimating Surface Runoff It is the most widely used method for predicting peak discharges on un-gauged catchments. It is recommended that the direct application of the rational formula should be limited to catchments less than 25km2 in area [Gichaga, 1998]. This is because of the fact that it gives reliable results for such catchments. The main assumption in this method is that the maximum rate of flow results from a uniform rainfall intensity over the entire drainage area where the rainfall has taken place. The method relates the peak rate of runoff from a given catchment to rainfall of a given average intensity by means of the equation below: . . 3.6

/ … . 5.1

= = = =

Flood peak at catchments exit in m3/s Rational run off coefficient Average rainfall intensity over the whole catchment in mm/hr Catchment Area in km2

Where; Q C I A

ii) Time of Concentration The intensity of rainfall in any area is dependent on the duration of the storm. In the widely accepted rational approach, it is assumed that at the peak flood, the duration is equal to the time of concentration. The time of concentration is a function of the length of water course and height difference from the source to outflow. The following relationship (as proposed by the U.S Bureau of Reclamation – also known as Kirpichs’ formula) can be adopted in estimating the time of concentration, TC. .

0.87

… . 5.2

Where, Tc L H

= = =

Time of concentration; Length (in km) of the longest water course from exit; Height difference (in m) from source to exit;

When Tc is determined, the corresponding rainfall intensity can then be obtained from the Intensity-Duration-Frequency curve [Ruhweza, 2005]. The underlying principle behind the use of time of concentration can be explained as follows; when rain falls on drainage catchments, part of the water may be prevented from reaching the catchment exit while some may be delayed during flow to the exit. Losses result from infiltration, evaporation, storage in surface depressions and interception by vegetal cover. The principal factor used to link rainfall and run off is the Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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time taken for a catchment to respond to the rainfall input also known as the time of concentration (Tc). It is this time that is adopted as the measure of the catchment response time. It is taken as the time taken for the surface runoff from the hydraulically most remote part of the catchment area to reach the point being considered [Duggal, 1991]. Table 6.2: Run off coefficient for the rational method Rural Areas, C =Cs + Ck + Cv Factor

MAP(mm) > 600

Component

Cs

Average slope of hillsides in Catchment

Soft to Moderate (3.5‐11%)

0.08

Ck

Permeability of the soil

Impermeable

0.16

Cv

Vegetation

Cultivated land

0.11

Source: Uganda Road Design Manual (1994)

Note: MAP is translated as ‘Mean Average Rainfall’ in mm.

d) Hydraulic Design of Open Channels In a drainage channel the discharge (Q), the depth (d), and the velocity (v) depend on the channel shape, roughness factor (n) and slope (s). The relationship between these is expressed by Manning’s equation as shown below [Edwards, 2006]: .

1

.

.

… . 5.3

Where; V n R S A Q

= = = = = =

Flow velocity (m/s); Manning’s roughness coefficient; Hydraulic radius (m); Stream bed slope (m); Flow cross section area (m2); Flow Discharge (m3).

The required drainage structure waterway area is governed by the allowable headwater at the inlet or outlet. After determining the design discharge (Q), then the capacity of an open channel is calculated according to mannings’ equation, which gives a reliable estimate of uniform flow condition [Gupta, 1995]. Table 6.3: Maximum Permissible velocity in open channels Soil /Lining Permissible Velocity (in m/s) Rock 4.5 – 6.0 Earth: Gravel 2.0 Sand and Silt 0.3 – 1.0 Clay 0.6 – 1.5 Turfed slope 1.6 – 2.0 Rip – rap 4.5 Brick lining 3.0

Concrete Lining

6.0

Source: Kadiyali (2000)


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It should be noted that where the longitudinal gradient of the roadway has to be near zero, the depth of the side drains may have to be varied to obtain sufficient gradient of the ditch. The longitudinal gradient should therefore preferably not be less than 0.3% for curbed pavements and not less than 0.2% in very flat terrain [Thagesen, 1996]. Normally capacities of channels should allow for a maximum velocity of 0.6 m/s for earth channels. The channel free board is the difference in height between the flow surface level and the top of the channel or gutter. The following free boards are recommended for drainage channels along road ways: • 100mm on side drains (as recommended by MoWH&C); • Zero on mountable kerbs; • 50mm where barrier kerbs are used [Ruhweza, 2005]. Table 6.4: Manning’s n Values Type of Conduit Wall & Joint Description Concrete Pipe Good joints, Smooth walls Good joints, Rough walls Poor joints, rough walls

Manning’s n 0.013 0.016 0.017 Concrete Box Good joints, Smooth finished walls 0.012 Poor joints, rough finished walls 0.018 Ordinary Earth 0.02 Earth having Vegetation 0.05‐0.1 Rough rubble pitching 0.04 Source: DeKalb County storm water management Manual (2006) & Singh (2001)

6.5 Sub-Surface Drainage Under this category, the seepage or subsurface water is intercepted and removed to a safe place by installation of intercepting drains and provision of drains to keep the water table about 1.5 metres below the formation. Therefore the main aim sub surface drainage is to prevent changes in moisture content of the subgrade as the increase in the moisture content reduces the bearing strength of subgrade. A road with a poorly drained sub-surface will undergo pavement distress in the form of surface cracking, rutting and potholes in the outer parts of the pavement especially in the wheel paths of heavy commercial vehicles. When the moisture content of the subgrade increases, its strength decreases. The variations in moisture content are caused by: seepage of water from higher adjoining ground, penetration of moisture through the pavement, and percolation of water from shoulders, pavement edges, and soil formation slopes. These are ways in which free water enters the pavement. As concerns ground water entry, moisture variation is caused by rise or fall of underground water table, capillary rise of moisture in retentive types of soils like clay and transfer of moisture vapour through soils. In controlling seepage flow, if seepage level reaches a depth of 0.60m – 0.90m from the road subgrade, it should be intercepted to keep the seepage line at a safe depth below the road subgrade. This is done where the surface of the ground and the impervious layer embedded below it is sloping towards the road. In controlling capillary rise, the water table should be lowered by placing a granular layer of suitable thickness can be inserted between the subgrade and the highest level of water table during construction. The thickness of the granular layer should be such that the capillary rise of the water remains within this layer. However, it should be noted that if the Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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water table is more than 1.5m below the subgrade of the road, it will not require any sub soil drainage [Singh, 2001].

6.6 Cross Drainage Under this category of drainage, water of natural drainage under the road is intercepted and disposed off using road drainage structures like culverts and bridges for high discharge and greater linear way [Gupta, 1995]. According to Thagesen (1996), cross drainage structures can be very costly and it is therefore important to analyse all major cross drainage along an alignment before final selection of the new road alignment. Where there is a choice in the selection of the position of a stream crossing, it is desirable that, as far as possible, the stream is located • • • • •

On a straight reach of the stream, away from bends; As far as possible from the influence of large tributaries; On a reach with well defined banks; At a site which makes a straight approach road feasible; At a site which makes a right angle crossing possible [Thagesen, 1996].

In order to determine the requirements for cross drainage, information must be collected and predictions made about level of traffic and the likely flow of water passing under the road. The following types of structures should be considered: i) Ford; this utilizes a suitable river bed and is appropriate for shallow slow moving water courses with little probability of flash floods, traffic volumes up to 100veh/ day. ii) Drift; it consists of a concrete slab constructed in the river bed which would otherwise be unable to carry vehicles. It is suitable as a crossing for rivers that are prone to flash floods, traffic volumes up to 100 vehicles/ day. iii) Culverts; it consists of a concrete or steel pipe or a reinforced concrete box, placed under the road within an embankment to provide a suitable means of conveying streams, or the contents of side drains under the road with no restriction on traffic. iv) Bridge; this may have a super structure on timber ,concrete, and /or steel on masonry, concrete or timber, abutments and will be required for crossing streams or rivers where cross culverts would provide insufficient capacity, or where the road crosses an obstruction such as a railway or canal protected. However, for this study, culverts will be considered as the cross drainage structure. They are a means of conveying water from streams below the road and carry water from one side-ditch to the other. Culverts can be made of concrete or steel pipes. The common forms of concrete culverts in Uganda are the Portland Cement Concrete type with sizes ranging from 600mm- to- 1200mm and the reinforced concrete box culvert. Common steel pipes include corrugated galvanised steel pipes also known as ‘Armco’ culverts [Thagesen, 1996]. 112


Culverts

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6.7 Culverts a) Introduction A culvert is a covered channel of relatively short length designed to pass water through an embankment (e.g. a highway, a railroad, and a dam). The design requires a hydrological study of the upstream catchment to estimate the maximum (design) discharge and the risks of exceptional (emergency) floods. The dimensions of a culvert are based on hydraulic, structural and geotechnical considerations. Their impact on the environment must also be considered e.g. flooding of the upstream plain. Culverts are hydraulically designed to operate at peak flows with a submerged inlet to improve hydraulic efficiency. The culvert constricts the flow of the stream and may cause ponding at the upstream or inlet end. These effects of ponding and flow appurtenant structures, embankments, and adjacent properties are important considerations in culvert design. Structurally, culverts are buried in soil and are designed to support the dead load of the soil over the culvert as well as live loads of traffic. Live loads on culverts are generally not as significant as the dead loads unless the cover is shallow. In most culvert designs, the soil or embankment material surrounding the culvert plays an important structural role. Lateral soil pressures enhance the culverts ability to support vertical loads. The stability of the surrounding soil is important to the structural performance of most culverts. In terms of maintenance, culverts are usually designed to constrict flow, there is an increased potential for waterway blockage by debris and sediment, especially for culverts subject to seasonal flow. Multi barrel culverts are particularly susceptible to debris accumulation. Scour caused by high outlet velocity or turbulence at the inlet end is of concern. As a result of the above factors, routine maintenance for culverts primarily involves the removal of obstructions and the repair of erosion and scour. Other defects that require routine maintenance include those due to weathering, loading and aging [Ruhweza, 2005]. b) Basic Types and Characteristics of Culverts The most common types of culverts include pipe culverts, slab culverts, box culverts, and arch culverts. Most of them begin from upstream with the inlet structure and terminate downstream with the outlet structure which comprises of: i) Inlet structure: - allows in the storm water through the culvert. It may be constructed as a single unit called headwall or as a combination of various units such as wing walls, or drop-in chambers. A combination of materials can be used to construct an inlet structure, which includes plain concrete, masonry walls in brick, blocks or stone blocks. ii) Barrel: - this is the entire arrangement of culverts to a certain gradient to form a tunnellike structure for storm water passage. It joins both the inlet and outlet structures together. iii) Outlet Structure: - it serves the purpose of discharging the storm water from the culvert to the outlet channel. Its construction is similar to that of the inlet structure. iv) Apron: - it is a concrete slab constructed at both the inlet and outlet structures. It provides the transition from the channel to the culvert (inlet) and from culvert to channel (outlet). Sometimes, it is constructed with or without cut-off wall (toe wall). v) Culvert Bed: - this is a layer upon which the culvert units are laid to gradient. It can be constructed out of natural gravels, sand, or other granular material placed properly over the subgrade [Thagesen, 1996]. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Hydraulic Design of Culverts The hydraulic performances of a culvert are based on the design discharge, Qd, the upstream total head and the maximum (acceptable) head loss ∆H. Head loss must be minimised to reduce upstream flooding. The primary design constraints are; minimum cost, minimum rise of water level above normal free surface level upstream of the culvert, the embankment height (either given or part of the design) and a scour protection which may be considered particularly if a hydraulic jump is expected to take place near the culvert outlet [Chanson, 2000]. i) Engineering design Criteria Generally, the following engineering criteria, as adopted from JICA consultants (1997) can be considered for most culvert designs. Frequency of the flood; as already mentioned, this is based on experience and economic evaluation. In Uganda, a 10 year return period can be adopted as recommended by the Kampala Drainage Master Plan. Velocity Limitations; both minimum and maximum velocities should be considered when designing a culvert the maximum velocity being consistent with channel stability requirements at the culvert outlet. The maximum allowable velocity for pipe flowing full is 4.6 m/s. However, it is recommended that culverts be laid to grades which produce non – silting non – erosive velocity between 1 – 3.5 m/s [MoWH&C, 1994]. Length and Slope; the culvert length and slope should be chosen to approximate the existing topography and to the degree practicable. In addition, the culvert invert should be aligned with the channel bottom and the skew angle of the stream, carefully matching it to the geometry of the roadway embankment. The maximum slope using concrete pipe is 10% and for corrugated metallic pipe is 14%. Gradients less than 1% should be avoided if possible and those less than 0.5% should not be used as they cause maintenance problems resulting from silting of the culvert. Minimum Size of Culvert; it is recommended that the minimum diameter of culvert on any given project should be taken as 600mm (for access culverts and minor cross drainage) and 900mm (for major cross drainage channels). Headwater Limitations; the allowable headwater elevation is determined from an evaluation of land use upstream of the culvert and proposed or existing roadway elevation. Headwater is the depth of water above the culvert invert at the entrance end of the culvert [Ruhweza, 2005]. ii) Headwater – depth relationships According the MoWH&C (2005), all culverts should be designed to carry the design frequency flood with a headwater depth that does not materially increase the size of the flood anticipated in upstream area. Allowable headwater depth is thus determined by the maximum permissible elevation of the headwater pool at the culvert for the design discharge. It is limited by one of the following factors: • non-damaging to upstream property; • below the traffic lines of interest or no higher than the shoulder or 0.5 m below the edge of the shoulder; • equal to an Hw/D no greater than 1.5; • no greater than the low point in the road grade; and


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Culverts •

115

equal to the elevation where flow diverts around the culvert.

A headwater-culvert depth ratio (Hw/D) equal to 1.2 is recommended for cases where insufficient data is available to predict the flooding effect from headwater depth [MoWH&C, 2005]. iii) Steps in Culvert Design Procedure i) The first step involves Listing all the design data ii) It is followed by determining the trial culvert size by assuming a trial velocity 1-3.5 m/s and computing the culvert area, A = Q/V. The culvert diameter D can then be determined. iii) Next is to find the actual headwater (HW) depth for the trial size culvert for both inlet and outlet control. • For inlet control, the inlet control nomograph is entered with D and Q and HW/D for the proper entrance type is then determined. • HW is computed and if too large or too small another culvert size is tried before computing HW for outlet control. • For outlet control, the outlet control nomograph is entered with the culvert length, entrance loss coefficient, and trial culvert diameter. • To compute HW, the length scale for the type of entrance condition and culvert diameter scale are connected with a straight line, a pivot is made on the turning line, and a straight line drawn from the design discharge through the turning point to the head loss scale H. The headwater elevation HW is then computed from the equation: .

… . 5.4

Where; ho = ½(critical depth + D), or tailwater depth whichever is greater. D = culvert diameter or depth of box culvert. iv) The computed head waters are then compared and the higher HW nomograph (see figure 5.2 and 5.3) is used to determine if the culvert is under inlet or outlet control. If outlet control governs the culvert design and HW is unacceptable, a lager trial size is selected and another HW found with the outlet control nomographs. Since the smaller size culvert had been selected for HW by the inlet control nomographs, the control for the larger pipe need not be checked. v) The exit velocity is finally calculated and the expected stream bed scour determined in order to ascertain if an energy dissipater (in the form of weirs or rip rap line along the channel) is needed along the channel. It should be noted that the use nomographs requires a trial and error solution. It should also be remembered that velocity, hydrograph routing, roadway overtopping, and outlet scour require additional computations beyond what can be obtained from the nomographs [Thagesen, 1996]. 115


Culverts

116

Figure 6.2: Nomograph for the Calculation of Headwater Depth with Inlet Control Source: Uganda Road Drainage Manual (2005)

116


Culverts

117

Figure 6.3: Headwater Losses for Concrete Pipe Culverts Flowing Full Source: Uganda Road Drainage Manual (MoW&T, 2005)

117


Culverts

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Figure 6.4: Headwater Losses for Concrete Pipe Culverts Flowing Full Source: Uganda Road Drainage Manual (MoW&T, 2005) Table 6.5: Entrance loss Coefficient (Outlet Control, Full or Partially full)

_________________________________________________

118

Type of Structure and Design of Entrance Coefficient ke ________________________________________________________________________

Pipe, concrete Mitered to conform to fill slope

0.7


Questions End-section conforming to fill slope* Projecting from fill, square cut end Headwall or headwall and wingwalls Square-edge Rounded (radius = 1/12D) Socket end of pipe (groove-end) Projecting from fill, socket end (groove-end) Beveled edges, 33.7˚ or 45˚ bevels Side- or slope-tapered inlet

119

0.5 0.5 0.5 0.2 0.2 0.2 0.2 0.2

Pipe, or pipe-arch, corrugated metal Projecting from fill (no headwall) Mitered to conform to fill slope, paved or unpaved slope Headwall or headwall and wingwalls square-edge End-section conforming to fill slope* Beveled edges, 33.7˚ or 45˚ bevels Side- or slope-tapered inlet

0.9 0.7 0.5 0.5 0.2 0.2

Box, Reinforced Concrete Wingwalls parallel (extension of sides) square-edged at crown 0.7 Wingwalls, 10˚ to 25˚ or 30˚ to 75˚ to barrel, square-edged at crown 0.5 Headwall parallel to embankment (no wingwalls) Square-edged on 3 edges 0.5 Rounded on 3 edges to radius of 1/12 barrel dimension 0.2 Beveled edges on 3 sides 0.2 Wingwalls at 30˚ to 75˚ to barrel, crown edge rounded to radius of 1/12 barrel dimension, or beveled top edge 0.2 Side- or slope-tapered inlet 0.2 _________________________________________________________________________

.

2

… . 5.5

6.8 Questions a) Discuss the main dangers associated with poor drainage of flexible pavement. Describe the three major categories of drainage works that can be used to mitigate the risks associated with poor drainage. b) Describe the major factors considered when estimating design floods in preparation for drainage design. c) Discuss in detail at least three methods used to estimate surface runoff from a storm over a given catchment area. d) What do you understand by the following drainage structures: fords, drifts, culverts and bridges? e) Discuss at least three engineering criteria you would consider in the design of culvert structures clearly stating the reasons for your choices. f) M&E consultants have just completed a detailed drainage study on the kansanga Lukuli road. Below is a summary of drainage data obtained. • Catchment area = 1.74km2 • Average slope of the hill sides in catchment = 8% • Surrounding soil type semi-permeable • Surrounding vegetation Cultivated land Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Bibliograhy • • • • • • • •

Length of the longest water course from exit Contour difference from exit Allowable headwater depth ratio AHD/D Tail water depth TW Culvert slope Free board Permissible velocity Entrance type for: Pipe Culvert Box Culvert 75o

= = = = = = = = =

120

0.710km 58m 1.5 1.2m 1.2% 0.45m 1 - 3.5m/s Groove end with headwall wing wall flare of 30-

Table 6.1Q: Rainfall Intensity in the project Area Return Period (in Years) Duration (in minutes) 10 20 40 60 80 100 120

2

5

10

25

Intesity in mm/hr 115.5 80.8 54.0 42.1 35.1 30.4 27.0

149.3 104.1 69.6 54.2 45.2 39.2 34.8

174.6 121.7 81.4 63.4 49.0 43.8 40.9

208.1 145.1 97.0 75.6 63.0 54.6 48.5

Table 6.2Q: Area reduction factors A (Km2) f

0 1

1 0.93

2 0.88

3 0.85

4 0.83

5 0.8

You are required to design the culvert structure and test its hydraulic performance. Ascertain whether the culvert system is operating under inlet or outlet control. Illustrate your final answer with a neat sketch of this structure with clear dimensions and annotations.

6.9 Bibliograhy 1. Bindra, S.P, 1999, A Course in Highway Engineering, 4th Edition, Dhanpat Rai Publishers, New Delhi. 2. Chanson, H, 2000, Introducing originality and innovation in engineering, www.tandf.co.uk/journals, accessed 29th march 2006. 3. Duggal K.N, 1991, Elements of Public Health Engineering. 4th Edition, Rajendra Ravinda printers (Pvt) Ltd, New Delhi. 4. Gupta, B.L, 1995, Roads, railways Bridges and Tunnels engineering, 4th edition, Standard publishers Distributors, Nai sarak, Delhi. 5. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including Expressways and Airport Engineering), 4th Edition. Khanna Publishers, New Delhi. 6. Ken Edwards, 2006, Trapezoidal open Channel design calculations, www.lmnoeng.com/channels/trapezoid.htm, accessed 10th may 2006. 7. Ministry of Local government, 2002, Kampala Urban Transportation Improvement plan KUTIP, Kampala, Uganda. 8. Ministry of Works, and Transport, 2003. Labour based Maintenance Contractor Training Module, Republic of Uganda, Kampala. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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121 9. Ministry of Works, and Transport, 2005. Road Design ManualVol.III, Pavement Design Manual, Republic of Uganda, Kampala. 10. Ministry of works, housing and communications, 1994, Uganda Road Design Manual, Republic of Uganda, Kampala. 11. O’Flaherty C.A., 2002. Highways: The Location, Design, Construction and Maintenance of Pavements. 4th Edition, Oxford, Butterworth Heinemann. 12. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil Engineering, Kyambogo University. 13. SABA Consultants, 2002, Design Study Review and Construction Supervision Services for Upgrading Kafu-Masindi Road, Project Report. 14. Singh, G, 2001, Highway Engineering, 3rd edition, Standard publishers and Distributors, Delhi. 15. Thagesen, B., 1996, Highway Engineering in Developing Countries, 1st edition; Alden press, Great Britain. 16. Wilson, E.M, 1994, Engineering Hydrology, 4th Edition. Macmillan press Ltd, Kent.

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Chapter Seven:

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Chapter Seven:

Conclusion 7.1 Road Deterioration The purpose of structural design is to limit the stresses induced in the subgrade by traffic to a safe level at which subgrade deformation is insignificant whilst at the same time ensuring that the road pavement layers themselves do not deteriorate to any serious extent within a specified period of time. By the nature of the materials used for construction, it is impossible to design a road pavement which does not deteriorate in some way with time and traffic, hence the aim of structural design is to limit the level of pavement distress, measured primarily in terms of riding quality, rut depth and cracking, to predetermined values. Generally these values are set so that a suitable remedial treatment at the end of the design period is a strengthening overlay of some kind but this is not necessarily so and roads can, in principle, be designed to reach a terminal condition at which major rehabilitation or even complete reconstruction is necessary. However, assessing appropriate remedial treatments for roads which have deteriorated beyond a certain level is a difficult task. In most design methods it is assumed that adequate routine and periodic maintenance is carried out during the design period of the road and that at the end of the design period a relatively low level of deterioration has occurred. Acceptable levels of surface condition have usually been based on the expectations of road users. These expectations have been found to depend upon the class of road and the volume of traffic such that the higher the geometric standard, and therefore the higher the vehicle speeds, the lower the level of pavement distress which is acceptable. In defining these levels, economic considerations were not considered by the Transport Road Research Laboratory (TRRL) because there was insufficient knowledge of the cost tradeoffs for an economic analysis to be carried out with sufficient accuracy.

7.2 Economic Considerations In recent years a number of important empirical studies have shown how the costs of operating vehicles depend on the surface condition of the road. The studies have also improved the knowledge of how the deterioration of roads depends on the: • Nature of the traffic; • Properties of the road-making materials; • Environment; and • Maintenance strategy adopted. In some circumstances it is now possible to design a road in such a way that provided maintenance and strengthening can be carried out at the proper time, the total cost of the transport facility i.e. the sum of construction costs, maintenance costs and road user costs, can be minimised. These techniques are expected to become more widespread in the future. Also, with the introduction in many countries of pavement management systems in which road condition is monitored on a regular basis, additional information will be collected by TRRL to allow road performance models to be refined. Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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Effects of Climate

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Pavement structural design could then become an integral part of the management system in which design could be modified according to the expected maintenance inputs in such a way that the most economic strategies could be adopted. Whilst these refinements lie in the future, the research has provided important guidance on structural designs suitable for tropical and sub-tropical environments and has been used, in part, in preparing the fourth edition of Road Note 31. For the structures recommended in this Note, the level of deterioration that is reached by the end of the design period has been restricted to levels that experience has shown give rise to acceptable economic designs under a wide range of conditions. It has been assumed that routine and periodic maintenance activities are carried out to a reasonable, though not excessive, level. In particular, it has been assumed that periodic maintenance is done whenever the area of road surface experiencing defects i.e. cracking, ravelling, etc, exceeds 15 per cent. For example, for a 10 year design period, one surface maintenance treatment is likely to be required for the higher traffic levels whereas for a 15 year design period, one treatment is likely to be required for the lower traffic levels and two for the higher. These are broad guidelines only and the exact requirements will depend on local conditions.

7.3 Effects of Climate Research has shown how different types of road deteriorate and has demonstrated that some of the most common modes of failure in the tropics are often different from those encountered in temperate regions. In particular, climate related deterioration sometimes dominates performance and the TRRL research emphasises the overriding importance of the design of bituminous surfacing materials to minimise this type of deterioration. Climate also affects the nature of the soils and rocks encountered in the tropics Soilforming processes are still very active and the surface rocks are often deeply weathered. The soils themselves often display extreme or unusual properties which can pose considerable problems for road designers.

5.7 Variability in Material Properties and Road Performance Variability in material properties and construction control is generally much greater than desired by the design engineer and must be taken into account explicitly in the design process. Only a very small percentage of the area of the surface of a road needs to show distress for the road to be considered unacceptable by road users. It is therefore the weakest parts of the road or the extreme tail of the statistical distribution of 'strength' which is important in design. In well controlled full-scale experiments this variability is such that the ten per cent of the road which performs best will carry about six times more traffic before reaching a defined terminal condition than the ten per cent which performs least well. Under normal construction conditions this spread of performance becomes even greater. Some of this variability can be explained through the measured variability of those factors known to affect performance. Therefore, if the likely variability is known beforehand, it is possible, in principle, for it to be taken into account in design. It is false economy to minimise the extent of preliminary investigations to determine this variability. In practice it is usually only the variability of subgrade strength that is considered and all other factors are controlled by means of specifications i.e. by setting minimum acceptable values for the key properties. But specifications need to be based on Kyambogo University | P. O. Box 1, Kampala-Uganda CE413 – Highway Engineering II, © FEO- 2010. E-mail: [email protected] . Mobile No.: (256) 712 806514, (256) 701 806514

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easily measurable attributes of the materials and these may not correlate well with the fundamental mechanical properties on which behaviour depends. As a result, even when the variability of subgrade strength and pavement material properties are taken into account, there often remains a considerable variation in performance between nominally identical pavements which cannot be fully explained. Optimum design therefore remains partly dependent on knowledge of the performance of in-service roads and quantification of the variability of the observed performance itself. Thus there is always likely to be scope for improving designs based on local experience. Nevertheless, it is the task of the designer to estimate likely variations in layer thicknesses and material strengths so that realistic target values and tolerances can be set in the specifications to ensure that satisfactory road performance can be guaranteed as far as is possible. The thickness and strength values described in this ORN 31 are essentially minimum values but practical considerations require that they are interpreted as lower ten percentile values with 90 per cent of all test results exceeding the values quoted. The random nature of variations in thickness and strength which occur when each layer is constructed should ensure that minor deficiencies in thickness or strength do not occur one on top of the other, or very rarely so. The importance of good practice in quarrying, material handling and stock-piling to ensure this randomness and also to minimise variations themselves cannot be over emphasised.

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