1 KYAMBOGO UNIVERSITY FACULTY OF ENGINEERING DEPARTMENT OF CIVIL AND BUILDING ENGINEERING BACHELOR OF ENGINEERING IN CIVIL AND BUILDING ENGINEERING YEAR 3 SEMESTER TWO CE 323 HIGHWAY ENGINEERING I GROUP ASSIGNMENT MEMBERS NDOZIREHO RONNIE
07/U/187/ECD/GV
WEPUKHULU EFULONI JOSHUA
07/U/2980/ECD/PD
KANABU EVANS
08/X/1936/ECD/PD 08/X/1936/ECD/ PD
MURA DANIEL
06/U/169/ECD/GV
MURUNGI ELIZABETH
07/U/186/ECD/GV
LANGOLTUM PETER
07/U/09/ECD/PD/NPT 07/U/09/ECD/PD/ NPT
ACIDRI SAMUEL
08/U/1931/ECD/PD
LECTURER: MR OKELLO EUGENE FRANCIS
2 TABLE OF CONTENTS 1(a). Role of road infrastructure in economic development ……………………………………………....3 (b). Advantages and disadvantages of road transport over other modes……………………….....5 (c). Information expected in a feasibility study report…………………………………………………….....8 2(a). Necessity of economic evaluation of highway projects……………………………………………....11
(b). Major differences between economic and financial analysis………………………………………..11 (c). Criteria followed in selection of an appropriate economic evaluation method…………....12 (d). Economic analysis of given road………………………………………………………………………………….....13 3(a). Information expected from a desk study in a reconnaissance survey………………………...15 (b). Considerations influencing choice of centre line in preliminary location survey………....17 (c). Benefits analyzed in the feasibility study……………………………………………………………………...17 4(a). Considerations in drawing up geometric standards for a country...................... country................................19 ..........19 (b). Super elevation.........................................................................................................19 (c). Transition curve..........................................................................................................20 (d). Super elevation calculation.........................................................................................20 5(a). Vertical alignment......................................................................................................22 (b). Factors considered in the design of vertical curves.......................................................22 (c). Design vehicle, design speed, speed limit....................................................................23 (d). Parabolic vertical curve calculation..............................................................................23 6(a). Cross section elements..............................................................................................25 (b). Preliminary phase of design process of a junction........................................................28 (c). T-junction design........................................................................................................28 7(a). Characteristics that render roundabouts most effective.............................................31 (b). Design features of roundabouts.................................................................................31 (c). Roundabout design....................................................................................................32 8(a). Considerations in the choice between rigid and flexible pavements............. pavements....................... ..............35 ....35 (b). Meaning of pavement structure terms......................... terms............ ......................... .......................... ........................... .................... ....... ....35 (c). Flexible pavement design...................................................................................................37
3 Question 1 (a.) The word transport originates from the Latin word; transportare i) Trans meaning across, over or through, and ii) Portare meaning carry. Thus transportation can be defined as ... the act of carrying somebody or something from one place to another, usually in a vehicle (Microsoft Student with Encarta Premium [DVD], 2008). ... Transport infrastructure refers to the fixed installations necessary for transport. In relation to Uganda, the major elements of the road infrastructure consist of the following; roads which may be earth roads, gravel roads or paved roads,
terminals such as bus stations, warehouses, trucking terminals,
refuelling depots including fuel stations.
Other elements of Uganda’s road infrastructure include; earth retaining structures,
bridges, culverts and other drainage facilities,
safety systems such as roadside barriers and speed limiting devices such as speed humps,
road signs,
road lighting, and
noise barriers.
Economic development refers to to the ... promotion of more intensive and more advanced economic activity through such means as education, improved tools and techniques, more available financing, better transportation facilities, and creation of new businesses (Microsoft Student with Encarta Premium [DVD], 2008). ... Transportation is vital component of any society’s growth and development. According to
Mathew (2006), it exhibits a very close relation to the style of life, the range and location of activities and the goods and services which will be available for consumption. ... Uganda’s road infrastructure plays a number of roles in relation to Uganda’s economic
development. These roles include the following: I. Development of The Agriculture Sector: The agricultural sector requires a constant supply of agricultural inputs such as pesticides, fertilizers, tools and equipment, labour, seeds and so on. The agricultural sector would also be practically redundant without access to markets. Uganda’s road infrastructure promotes the development of the agricultural sector by fulfilling the above essential needs of the agricultural sector. II.
Facilitation of Social Activities, Market Growth And Land Usage: Uganda’s road infrastructure enables social activities to take place, such as the provision of
education and health services, sensitization sensitization workshops, political or religious rallies and so on.
III.
Facilitation of Administration:
4 The management of the country by the Ugandan government can only be successful when there are good transport linkages across the country. This eases monitoring of policy implementation, eases maintenance of law and order as well as easing service delivery. IV.
Facilitation of Specialization & Mass Production: Uganda’s road infrastructure promotes economic development by facilitating mass production of goods which subsequently promotes specialization. Goods can be produced from one location in large quantities and then transported to other locales where demand is high. This results in better quality goods produced at lower costs through economies of scale.
V.
Development of the Industrial Sector: The industrial sector requires a constant supply of industrial inputs such as raw materials, labour, plant, tools, equipment and so on. The industrial sector also needs to be able to deliver finished goods to retailers and consumers. Uganda’s road infrastructure promotes the development of the industrial sector by fulfilling the
above essential needs of the industrial sector. VI.
Transport Infrastructure Influences Both Place & Time Utility of Goods:
VII.
Transport improves place utility of goods. For instance milk in Mbarara is far cheaper than that in Kampala due to the additional transport costs. A better road network ensures increased efficiency and reduced transport costs incurred, hence, reducing the cost of the final product. A better transport network ensures increased time savings due to fewer bottlenecks, traffic jams and better travelling surfaces and therefore gives time utility to economic activities. Facilitation of Urbanisation: A good road infrastructure promotes economic growth by facilitating urbanisation. The personal mobility afforded by Uganda’s road infrastructure has enabled urban growth of towns and cities
as more people relocate from rural areas in search of work and better living conditions. The increase in urban populations is accompanied by a shift from agricultural production, mainly for the domestic market, to industrial production for both domestic and foreign markets. VIII.
Preservation of Quality of Goods: Uganda’s road infrastructure enables the quick transportation of perishable goods, such as plant
products and animal products, from producers to consumers before the goods are no longer fit for consumption. IX.
Cost of Goods: The cost of a good also includes additional factors apart from the raw material costs and labour costs. In order to achieve low-cost goods for both domestic and foreign markets it is necessary to cut additional costs such as transport costs. In Uganda’s case, this can only be achieved if a competitive advantage can be gained through the provision of an efficient road transport network.
X.
Defence & Strategic Needs: A well organized transport network is necessary for the rapid response of a country to threats to its strategic and defence needs. Uganda’s road infrastructure is necessary for Uganda to be able
5 to carry out logistics operations. For example, it is difficult for a paramilitary organisation to take over areas of Uganda due to the rapid response of Uganda’s military. XI.
Development of the Tourism Sector: Without adequate road infrastructure, Uganda’s potential earnings from the tourism sector will
not be realised as tourists will not be able to access the tourist sites and areas of scenic beauty. Uganda’s road infrastructure makes the tourist’s experience convenient and comfortable by
providing easy access to tourist sites such as the at the source of the Nile at Jinja, Queen Elizabeth National Park and Kabalega National Park. In this way Uganda’s road infrastructure
promotes the tourism sector. XII.
Bridging The Gap Between The Producers & Consumers: The production of goods or provision of services does not always coincide, in terms of location, with that of consumers. This problem is overcome by Uganda’s road infrastructu re, which enables the movement of products, consumers or both to a common meeting point for trade to occur.
XIII.
Facilitation Of The Exploitation Of Natural Resources: Since Uganda is a landlocked country, the most convenient means of transporting exports from the exploitation of natural resources is through over land transport. The road infrastructure of Uganda enables natural resources such as plant products, animal products, mineral deposits and soon fossil fuels, to be transported to foreign markets, thereby earning foreign exchange and promoting economic development.
XIV.
Creation of Employment: The construction and maintenance of Uganda’s road infrastructure is highly labour intensive.
This provides much needed employment to both skilled and unskilled labour as well as conserving foreign exchange which would have been spent on expensive mechanical equipment. XV.
Balanced Development: A well distributed road network allows more balanced distribution of services and hence a more balanced development of the country, keeping in view the special needs of inaccessible areas and underdeveloped societies in the country.
Question 1 (b.) The advantages of road transport over other modes of transport include the following; I. High Employment Potential Road transport has a high employment potential for both skilled and non-skilled labour compared to other modes of transport. Road transport is more labour based in its construction, operation and maintenance compared to other modes of transport. II.
Lower Initial Costs Compared to other modes of transport such as railway, it is initially less costly to construct a road. Also in road construction, it is possible to carry out stage construction, which may not be the case for other modes of transport.
6 III.
More Economical For Short Hauls Road transport is more economical for moving goods and people through short distances compared to other modes of transport. This is because for other modes of transport it is not cost-effective to construct terminals within a short distance of each other, for example railway stations for railway transport or airports for air transport.
IV.
Services Unique to the Individual Other modes of transport usually do not offer services tailored to suit individual needs or preferences. However, with road transport, road users are provided with services suited to the individual, such as individual insurance cover, care of cargo and so on.
V.
Less Complex Packaging & Restraints Road transport affords road users the ability to transport goods using less complex packaging and restraints compared to other modes of transport. Other modes of transport are subject to greater vibrations and oscillations during transport, this poses a greater risk of damage to cargo, thus the packaging and restraints for such modes must be more comprehensive.
VI.
Door to Door Services Road transport affords road users the ability to utilise almost any part of the road network as a starting or ending terminal. Goods and services can be transported from the initial point to the final point without having to make additional interconnections between. This enables road users to avoid inconvenient delays and confusion and additional costs incurred when using modes of transport that have fixed and limited terminals.
VII.
Quick &Assured Deliveries If other modes of transport are used, it becomes uneconomical to transport goods when the vehicle is not fully loaded. Thus there are always time constraints with other modes of transport as the vehicle must be fully loaded before departing. In order to enjoy economies of scale these loads are usually large and hence it takes the vehicle some time to fill. However, the vehicles used in road transport have much smaller capacities and hence take less time to fill before departing to the final destination.
VIII.
Flexibility Road transport affords road users the opportunity to select from a number of available routes to suit their particular needs; however other modes of transport are usually fixed in their choice of transport routes. Also road transport enables road users to move at their own convenience. They are free from the rigours of timetables. However, the movement goods and people in other modes of transport are controlled by schedules, which may not always match demand nor be convenient.
IX.
Personalized Travel Road transport affords road users the ability to travel by a vehicle of their choosing according to factors such as their economic constraints, environmental concerns, space requirements, comfort preferences and availability of transport vehicles. These transport vehicles may nonmotorised human powered means, non-motorised animal powered means or motorised means of transport.
7 X.
Wide Geographical Coverage Because of the low construction costs and lower technical restraints imposed on road traffic by terrain compared to other modes of transport, road transport has a wider geographical coverage compared to other modes of transport. It would be very uneconomical to construct such extensive geographical coverage using another mode of transport.
XI.
Ease of Response to Road Accidents
Accidents occurring on or near roads are easier for emergency services to reach compared to other modes of transport. The disadvantages of road transport include the following; I. High Maintenance Costs for Frequent Maintenance Roads are affected to a great degree by climatic factors such as rainfall and variations in temperature. Coupled with repetitive stresses imposed on the road pavement layers by traffic, roads tend to deteriorate faster than other modes of transport such as rail transport. Thus a higher frequency of maintenance work is needed for road transport, resulting in high maintenance costs for road transport. II.
Safety Due to the large volume of road users and the variety of vehicles operating on the roads, as well as the effect of climatic factors on road transport, there is a large volume of road accidents compared to other modes of transport. This has led to economic losses in the form of loss of skilled labour, loss of materials and other inputs, delays
III.
High Energy Consumption The energy consumption per passenger kilometre and per tonne kilometre has been found to be higher with road transport compared to other modes of transport. This is because in other modes of transport, the energy costs are split with a larger number of transport users per journey. For example a car moving from A to B may carry only 5 people while a train moving between the same terminals may carry 1000 people.
IV.
Environmental pollution In urban areas the main source of pollution and noise is road transport. This poses significant problems such as unwanted noises and vibrations, poor air quality, formation of acid rain and is also said to be a contributor to global warming. Other examples of environmental degradation include the destruction of areas of scenic beauty as roads are constructed.
V.
Long Hauls Other modes of transport are better suited to transport goods and services across large distances compared to road transport. For example, water transport can be used transport exceedingly larger amounts of cargo and personnel than road transport. This is especially useful for large, bulky non-perishable products. Also air transport can cover a large distance in a much shorter time and thus is more suited to meet urgent transport needs such as; mail delivery, transport of perishable goods, transport of important cargo, transport of humanitarian items and also transport of military units.
VI.
Parking Problem
8 Due to the large volume of road users in urban areas, providing adequate parking for all of the road users is not always possible. This causes a number of problems such as narrower streets, which in turn cause encourage congestion and subsequent delays.
Question 1 (c.) The feasibility study report marks the end of the appraisal process and should recommend whether the project should go ahead, and to what standards it should be built (ORN 5: A Guide to Road Project Appraisal, 1988). The kind of information expected to be found in a feasibility study report of any given road link would include the following; 1. Definition of objectives: A road project should comply with the objectives of a national or regional transport plan. The project's objectives must be defined in order to provide the basic framework for the analysis. The objectives of the road project being studied may be ... to support some other developmental activity, to provide fundamental links in the national or a district road network, or perhaps to meet a strategic need. Alternatively, a road improvement may be proposed to increase the structural or volumetric capacity of an existing road to cope with higher traffic flows. Often, there will be multiple objectives (ORN 5: A Guide to Road Project Appraisal, 1988). ... 2. Determination of alternative ways of meeting objectives: A number of proposed solutions are determined in context of the particular needs and problems encountered. Possible alternatives include; Upgrading a road in order to increase capacity.
New construction of a road.
Rehabilitation in case the road has deteriorated beyond the condition at which overlaying is a satisfactory engineering alternative.
Reconstruction of the road, possibly to provide a new alignment .
Stage construction, or
Maintenance in order to ensure that the road does not 'fail' before the end of its design life.
3. Preliminary considerations: A number of preliminary considerations must be made and these include; Analysis period and design life, this impacts the design standards of the road as well as
the residual value of the road at the end of the economic analysis period.
Uncertainty and risk, the feasibility study should be carried out with recognition of uncertainties in the future and make provisions to minimise risk. Choice of technology, a choice must be made between using technology dominated by mechanical equipment or dominated by labour. Institutional issues, the aspects that need to be considered are ... the organisation, staffing, training, procedures, planning, maintenance, funding and controls within the
9 agency responsible for the project, and also within other agencies who may need to make an input to the project (ORN 5: A Guide to Road Project Appraisal, 1988). ...
Socio-economic considerations, the aspects that need to be considered are social changes, the effect of construction on the indigenous community, the effect of the project on road accidents, severance of the community, special needs of minority groups, expertise and resources of the local design organisation and contracting industry, the reliability of data on the local social environment, the site conditions and climate.
Environmental conditions, the possible environmental effects of the project should be considered in qualitative terms. Environmental problems raised may include air pollution, noise and vibration, visual intrusion and destruction of ecosystems.
4. Assess traffic demand: For the purposes of geometric design and the evaluation of economic benefits, the volume and composition of current and future traffic needs to be known in terms of cars, light goods vehicles, trucks, buses, non-motorised vehicles, and so on (ORN 5: A Guide to Road Project Appraisal, 1988). 5. Design and cost different options: The costs involved in the project are based on a design carried out to an appropriate level of detail. The road design process consists of the four following phases. Route location, the corridor is normally chosen to balance maintaining user benefits by providing the shortest route consistent with engineering constraints of topography,
ground conditions, drainage, structures and the location of road building materials (ORN 5: A Guide to Road Project Appraisal, 1988).
Pavement design, this primarily deals with the choice of materials and their thickness for each pavement layer.
Geometric design, this involves the selection of suitable horizontal and vertical alignments and road widths. Design of structures, this deals with earth retaining systems, drainage systems and special provisions for erosion control as well as structures which allow the road to cross or be crossed by rivers, water courses, railways and other roads.
6. Determine benefits for each alternative: Estimates are made, not only of the costs connected with the project, but also of the benefits that are expected to occur. Benefits normally considered are ... direct savings on the costs of operating vehicles, economies in road maintenance, time savings by travellers and freight, reduction in road accidents, and wider effects on the economic development of the region (ORN 5: A Guide to Road Project Appraisal, 1988). ... 7. Economic analysis and comparison of alternatives:
10 A cost-benefit analysis is carried out using the NPV (Net Present Value) or the IRR (Internal Rate of Return) method, in order to ensure that an adequate return in terms of benefits results from making a capital investment. 8. Recommendations: The report will make recommendations on whether the project should go ahead, and to what standards it should be built may also recommend alternative designs or approaches to the project that would increase the rate of return in those areas where the original project is not viable. Depending on who the report is being written for, some aspects will have been covered in greater depth than others and the recommendation will be delivered in the context of those aspects.
11 Question two a) Purposes and significance of an economic evaluation
Economic evaluation of project cycles helps in determining the extent to which the project will contribute to the goals of raising the standard of living of the people and their general welfare. Economic evaluation also provides for a systematic and unbiased procedure for selecting different schemes for implementation. Economic evaluation also ensures that the most worthwhile projects are given the highest priority. To decide whether the scheme under consideration is worth the investment at all.
To rank schemes competing for scarce resources in order of priority.
To compare various alternative schemes, with the most economical of all being
selected.
To assist in phasing the programme (stage construction) depending on the availability of resources.
b) Major differences between economic analysis and financial analysis
Financial analysis of a project estimates the profits accruing to the project operating entity whereas economic analysis is used to measure the effect of the project on the national economy (overall benefits as compared to the costs) For a project to be economically viable, it must be financially sustainable as well as economically efficient. Therefore the economical benefits of a project cannot be realized if the project is not financially sustainable. Both types of analysis are conducted in monitory terms, the major difference being in the definition of costs and benefits. In financial analysis, all expenditures incurred under the project and revenues resulting from it are taken into account whereas economic analysis attempts to access the overall impact of a project in improving the economic welfare of all the citizens of the country concerned. Economic analysis also differs from financial analysis in terms of; the breath of the identification and evaluation of inputs and outputs, and the measure of costs and benefits. Economic analysis involves all members of the society and measures the positive and negative impacts of the project in terms of the willingness and ability to pay for the units of increased consumption and to accept compensation for the foregone units of consumption rather than the actual prices paid as is the case with financial evaluations. In financial evaluation, we consider taxes, insurance premium and inflation rate whereas they are not considered in economic evaluations.
12 c) Criteria to be followed in coming up with an appropriate evaluation method Coming up with an appropriate evaluation method is a relatively complex process however some of the criteria given below may help guide the processes. The requirements or conditions imposed by the financing agency of the project: Some financial agencies may dictate the method of evaluation so in the event that the project is accepted, the specific method shall have to be used The local practice and experience: Sometimes the local experience may favour a particular method of evaluation. Such that if a particular method is not dictated, the adopted method will by default be used Technology involved in the given method; some of the methods involve complex computations that may require a certain level of technology. The level of accuracy: Some methods like the internal rate of return method require a very high level of accuracy. Other methods like the benefit cost ratio where the discount rate is assumed are not as accurate. Depending on the lending agency or the level of accuracy required, a particular method may be chosen. The problem type to be decided on: Here, the decision of the type design standards and/or
alignment of the road to be desired among other parameters like the determination of the most appropriate timing of the investment and/or prioritisation of the most appropriate road projects is made.
The type of project: consider a case where the project to be implemented is say a new construction/upgrading, reconstruction/rehabilitation, stage construction, or maintenance projects, then we have to have to focus on the feasibility and urgency of constructing the road basing on the capacity and benefit to be experienced in ratio of time. For example, the FYRR is suited to an upgrading, rehabilitation since these projects are majorly time constrained that is to say; the date/time at which the project is needed is a major consideration since there is already an existing alternative. However, for reconstruction or new construction projects, NPV, benefit cost ratio and IRR are preferable methods since the existing option is unsuitable or there is no alternative respectively.
The objectives of the project: The objectives of a project must be reflected by the benefits to be included in the analysis. While the objectives of inter-urban trunk road projects normally is to reduce vehicle operating costs and road maintenance costs, the objectives of rural access road projects may include improving access to social services, stimulating agricultural productions, etc.
The time, economic resources, skills and data available: For most projects, an analysis period of 15 years from the date of opening is appropriate but, this should be tested by the evaluation. Choosing the same analysis period as the design life of the pavement of a paved road simplifies the calculation of the projects’ residual value.
13 d) Economic evaluation problem Given data Total length of existing road = 30km Cost of improving an existing road = US$ 2, 000, 0000 Present traffic consists of: (a) Commercial vehicles per day = 2000 (b) Cars per day
= 1500
(c) Two wheelers per day
= 1200
(d) Total vehicles per day
= 4700
Cost of operation (US$/km) on the existing road and improved road: Existing road Improved road (a) Commercial vehicle 5.00 4.00 (b) Cars (c) Two wheeler
3.00
2.50
0.60
The accident rate on existing road and improved road is as under: (a) Existing road = 1.5 per mil- veh- km (b) Improved road = 1.0 per mil- veh-km (c) The cost of one accident = US$ 100,000 Growth rate of traffic is 10% p.a The annual maintenance costs per km are: (a) Existing road = US$ 30, 0000 (b) Improved = US$ 40,0000 The analysis period is 10yrs The discount rate is 12% Calculation Present Flow per year (mil. Veh- km/yr): Commercial vehicles 2000×365×30 = 21.90 Car 1500×365×30 = 16.40 Two wheeler 1200×365×30 = 13.14 Total 51.44 th Predicted flow in 4 year (mil. Veh- km/yr): Commercial cars: 21.90(1.1) ^4 = 32.07 Car 16.40(1.1) ^4 =23.97 Two wheeler cars 13.14(1.1) ^4=19.23 Total 75.27 Operation cost benefit: Predicted operation cost in 4th year: Commercial cars: 1×32.07×10^6 = 32.07×10^6 Cars: 0.5×24.06×10^6 = 12.06×10^6 Two wheeler car: 0.1×19.23×10^6 = 1.92×10^6 Total = 45.98 ×10^6 Accident benefit: 4th year (1.5 – 1.0) ×75.27×100,000 = 3763500
0.50
14
BENEFITS year
1 2 3 4 5 6 7 8 9 10 11 12 13
Flow(F) Mil. Vehkm/yr
75.27 82.8 91.08 100.23 110.23 121.26 133.35 146.73 161.4 177.51
Accident cost savings US$/yr (×106)
3.764 4.14 4.55 5.01 5.52 6.06 6.67 7.34 8.07 8.87
Cost Operating cost saving US$/yr (×106)
45.98 50.59 55.62 43.31 45.92 52.38 81.43 89.61 98.57 108.42
Total user benefits US$/yr (×106)
49.744 54.73 60.17 48.32 51.44 58.44 88.1 96.95 106.64 117.29 TOTAL
Discounted Benefits (PVB) US$/yr (×106)
31.613 31.055 30.483 21.857 20.775 21.074 28.365 27.870 27.371 26.879 267.348
Construction & maintenance cost US$/yr
Discounted cost, (PVC) US$/yr
20,000,000 20,000,000 20,000,000 300,000 300,000 300,000 300,000 300,000 300,000 300,000 300,000 300,000 300,000 TOTAL
17857142.86 15943877.55 14235604.96 190655.4235 170228.0567 151989.3364 135704.7646 121164.9684 108183.0075 96591.97098 86242.83123 77002.52788 68752.25704 49243140.51
Computation of NPV & B/C Ratio: NPV
∑ ∑ ∑ ∑
B/C Ratio =
= = 267,348,000 – 49,243,140.51 = US$ 218,104,859.5
/
=
= 5.43
Concluding remarks: All the above indicators point to the economic strength of the project under examination. Its NPV at just over US$ 218 million is strongly positive and B/C ratio of 5.43 is well in excess of unity.
15
Question 3a
The Desk study is the first step in the reconnaissance survey. It is a preliminary study of the bands/corridors being evaluated within the region. Some information may already publicly available at low cost (as compared with the cost of carrying out, say, new subsurface investigations) at government institutions such as the ministry of works having been obtained from previous similar projects around the area. A good desk study will result in great savings in both time and resources, as it helps ensure the early recognition of difficult routes and allows for better planning and interpretation of the subsequent detailed site investigations. The following is a general check-list of the types of information that might be gathered for a desk study; 1. General land survey : (a) Location of site on published maps and charts. (b) Dated air photographs. (c) Site boundaries, outlines of structures and building lines. This is especially useful in planning roads around developed areas where existing buildings and other structures restrain the possible road path. (d) Ground contours and natural drainage lines. (e) Above-ground obstructions to view and flying, e.g. transmission lines. (f) Indications of obstructions below ground. These are mainly in the form of service conduits and information regarding these may be obtained from the relevant service providers i.e. telephone, power and water. (g) Records of differences and omissions in relation to published maps. (h) Positions of survey stations and benchmarks (the latter with reduced levels). The main purpose of these stations is to monitor the location of the centreline accurately as even the most modern survey equipment has limited accuracy over long distances. (i) Appropriate meteorological information. 2. Permitted use and restrictions: (a) Planning and statutory restrictions applying to the particular areas under District and Country Planning Acts administered by the appropriate local authorities. (b) Local authority regulations on planning restrictions, listed buildings and building by-laws; (c) (d) Rights of light, support and way including any easements. (e) Tunnels, mine workings (abandoned, active and proposed), and mineral rights. (f) Ancient monuments, burial grounds, etc. (g) Prior potentially contaminative uses of the site and of adjacent areas (landfills). (h) Any restrictions imposed by environmental and ecological considerations, e.g. sites of special scientific interest e.g. wetlands, bird sanctuaries, game reserves etc. 3. Approaches and access (including temporary access for construction purposes):
(a) Road (check ownership); (b) Railway (c) By water 4. Ground conditions:
(a) Geological maps, obtainable from Entebbe Geological Centre. (b) Geological memoirs. (c) Flooding, erosion, landslide and subsidence history. This is especially important in hilly terrain which is highly susceptible to failure of this sort. (d) Data held by central and local governmental authorities. (e) Construction and investigation records of adjacent sites.
16 (f) Seismicity. 5. Sources of material for construction:
(a) Natural materials. These must be analysed for quality to ensure durable construction. (b) Tips and waste materials. (c) Imported materials and transport cost considerations. 6. Drainage and sewerage:
(a) Names of the sewerage, land drainage and other authorities concerned, and their by-laws. (b) Locations and levels of existing systems (including fields, drains and ditches), showing sizes of pipe and whether they are foul, storm water or combined (c) Existing flow quantities and capacity for additional flow. (d) Liability to surcharging. (e) Charges for drainage facilities. (f) Neighbouring streams capable of taking sewage or trade effluent provided that they are purified to the required standard. (g) Disposal of solid waste. (h) Flood risk to, and/or caused by, the proposed works. 7. Water supply:
(a) NWSC offices and their regulations. (b) Locations, sizes and depths of mains. (c) Pressure characteristics of mains. (d) Water analyses. (e) Availability of water for additional requirements. (f) Storage requirements. (g) Water sources for fire fighting. (h) Charges for connections and water. (i) Possible additional sources of water. (j) Water rights and responsibilities. 8. Electricity supply:
(a) Umeme,UETCL, and regulations; (b) Locations, sizes and depths of mains. (c) The voltage, phases and frequency. (d) Capacity to supply additional requirements. (e) Transformer requirements. (f) Charges for installation and current. 9. Telecommunications:
(a) Local office addresses; (b) Locations of existing lines; (c) Charges for installation. (d) Proposed lines such as underground fibre optic cables. 10. Information relating to potential contamination: (a) History of the site including details of owners, occupiers and users, and of any incidents or accidents relating to the dispersal of contaminants; (b) Processes used, including their locations; (c) Nature and volumes of raw materials, products, and waste residues; (d) Waste disposal activities and methods of handling waste; (e) Layout of the site above and below ground at each stage of development, including roadways, storage areas, hard-cover areas, and the presence of any existing structures and services; (f) The presence of any waste disposal tips, abandoned pits and quarries
17 (g) The presence of nearby sources of contamination from which contaminants could migrate via air and/or groundwater onto the site.
3b
The preliminary location survey is a large-scale study of one or more feasible routes within a corridor, each typically 40 –240 m wide, which is made for the purpose of collecting all physical information that may affect the location of the proposed roadway. Many considerations influence the location of the centreline that is finally selected. They include: (1) In rural locales, locate the road along property edges rather than through them and maximize the use of existing right-of-ways (to minimize the loss of farmland and the need for subways for crossing animals and farm machinery). (2) Avoid alignments that result in the motorist driving into the rising (morning) or setting (evening) sun for long periods and, to relieve the monotony of driving on long straight sections, site the road so as to view a prominent scenic feature. (3) Minimize the destruction of man-made culture or wooded areas, and avoid cemeteries, places of worship, hospitals, old people’s homes, schools and playgrounds.
(4) Avoid highly-developed, expensive, land areas and seek alignments that cause the least amount of environmental (visual and audio) blight. (5) In hilly terrain, maximize low-cost opportunities to provide long overtaking sections (for single carriageways) and of using varying central reservation widths and separate horizontal and vertical alignments (for dual carriageways). (6) In hilly country also, avoid alignments that are shielded from the sun (so that rainwater on the carriageway can dissipate). (7) If a vertical curve is imposed on a horizontal curve, ensure that the horizontal curve is longer; (8) Avoid introducing a sharp horizontal curve at or close to the top of a pronounced crest vertical curve or the low point of a pronounced sag vertical curve, and make horizontal and vertical curves as flat as possible at junctions with other roads (for safety reasons). (9) Seek favourable sites for river crossings (preferably at right angles to stream centrelines), and avoid locating bridges or tunnels on or near curves. (10) Minimize the use of alignments that require (expensive) rock excavation. (11) Try to ensure that excavation quantities are in balance with embankment quantities (so that earthworks haulage is minimized). 3(c) i. Societal benefits The highway network of a country forms the backbone of its economy and represents a huge investment in millions of dollars that allows for the safe and efficient movement of people and goods. A route with wider effects on the economic development of a region is more likely to be chosen. These societal beneficial effects may be in the form of direct employment during construction and maintenance, improved trade and market growth around the areas served by the road and wider accessibility to services and goods. ii. Road user cost One of the principal benefits of transport project proposals is the anticipated reduction in travel time. The final route should achieve maximum time savings by avoiding bottlenecks and providing the shortest route possible.
18 A reduction in accidents reduces the costs of vehicle repair associated with minor crashes and the immeasurable cost of human life lost in fatal crashes thus offering a direct benefit to the final road user. A route with minimum likelihood to cause accidents is likely to be chosen. A safer alignment may be achieved by the employment of gradual changes in alignment and sufficient traffic signs. Another direct benefit to the road user is in the form of reduced vehicle operation costs. This may be achieved by; offering a better riding surface that reduces vehicle maintenance regularity,
a straighter more direct alignment which reduce fuel costs as compared to longer meandering routes,
iii.
Cost of maintenance of the facility
A major consideration in the analysis of any project is the maintenance and life cycle costs. These costs include annual expenditures for scheduled upkeep and preventive maintenance to keep a road in operable condition. A route with lower anticipated maintenance costs is more likely to be chosen as this is offers a reduction in costs and hence a benefit to the client. A reduction in the maintenance costs when upgrading an existing structure can be achieved by improving drainage slopes and sizes of ditches to reduce the effect of climatic factors as well as careful selection of materials. iv. Cost of construction of the facility Any route that minimises the overall construction cost of the proposed alignment is likely to be chosen. This can be achieved by selecting the shortest alignment with the least obstacle s to the road’s path such as valleys, river crossings e.t.c.
19 4(a) Considerations for drawing up geometric design standards of a country
The road project should meet the set environmental standards. The project should meet the economic standards set by the government or donor group. The different terrain in the country should be clearly defined to enable proper calculation of features such as sight distances. The road should be clearly classified to enable proper evaluation and planning. The design speed and speed limits to be used should be appropriately defined. It is also important to carefully identify a design vehicle to enable proper design to accommodate all the other vehicles.
4(b) Super elevation – is the inward tilt or transverse inclination given to the cross section of a carriage way through out the length of a horizontal curve to reduce the effects of centrifugal forces on a moving vehicle. The minimum value for super elevation recommended for drainage is 2.5%.
TRANSVERSE SECTION The diagrams above shows the attainment of super elevation In fig a) the camber is intact In fig b) the outer edge is raised by placing fill that are then compacted until the fill is as high as the centre line. In fig c) more fill is added and compacted until the required slope of super elevation is attained.
20
Figs d), e) and f) show the attainment of super elevation about the outer and inner edge. Here there are fill, cut and compaction processes to achieve this. Below is the longitudinal section of super elevation about the centre line
LONGITUDINAL VIEW 4(c) Transition curve – is a curve whose radius continuously changes along its length. It is used to join two curves with different radii or a straight to a curve. It is important to have a transition curve; To promote passenger comfort as it reduces tendencies of sway and shock. To provide convenient sections for pavement enlargement and super elevation. To improve the general appearance of the road. To reduce tendencies of vehicular skidding.
4(d)
=2.5% Length due to acceleration
Length due to super elevation
The radius is greater than 300m so the width of the carriageway needs widening
Super elevation
21
Therefore the length is 87.52m ii
iii
L = 132.39m
22 5(a) Vertical alignment – is the arrangement of tangents and curves which compose the longitudinal profile of the road. It is composed of a series of straight-line gradients connected by curves, normally parabolic in form. A vertical curve provides a smooth transition between two tangent grades. There are two types of vertical curves: crest vertical curves and sag vertical curves. The main aim of vertical alignment is to ensure that a continuously unfolding stretch of the road is presented to motorists so that their anticipation of directional change and future action is instantaneous and correct *Rogers, 2003; O’Flaherty, 2002+. DIAGRAM SHOWING VERTICAL CURVES
5(b) In design of vertical curves the following are considered;
The shape of the vertical curve i.e. it can be circular, elliptical or parabolic and in all cases the ratio of length to the radius should not be greater than 0.1. There should be provision of cross falls to enable run off on the road to drain away. This can be achieved by having a camber sloping from the road centre line or sloping from one side of the road to the other. The sight distances should be well established because the length of a curve depends on it. The terrain of the area and hence the gradients to be manipulated to come up with an economical and environmental friendly curve. The design speed and speed limits of a given section of the road should be well established.
23 5(c) Design vehicle – is a selected vehicle with maximum dimensions and operating characteristics which are used as a basis to establish highway design controls to accommodate other vehicles that will use the sae highway.
Design speed is the maximum safe speed that can be maintained at a given section of a road while speed limit is a statutory maximum allowable speed on a road section.
5(d) Length
Elevation of PVC =Elevation PVI + Elevation of PVC =59.34 +
Elevation of PVI = 59.34 + For any elevation This is the formula for finding reduced level of any point. Elevation of PVI = Elevation PVI +
24 The results of the above formulae are tabulated below; STATIONING 0+533.17 0+560.00 0+580.00 0+600.00 0+617.49 0+620.00 0+640.00 0+660.00 0+680.00 0+700.00 0+720.00 0+735.17
CHORD LENGTH 0 6.83 20 20 17.49 2.51 20 20 20 20 20 15.17
CURVE LENGTH 0 6.83 26.83 46.83 46.32 2.51 22.51 42.51 62.51 82.51 102.51 117.68
ELEVATIONS 60.948 60.794 60.531 60.550 60.797 60.887 61.307 61.849 62.476 63.186 63.981 64.639
In conclusion And its elevation is 64.639m which tally with the results above
25 6(a) Road reserve: This is the width of land secured and preserved in public interest for road development purposes. It should be adequate to accommodate all the elements that make up the cross section of the highway and may reasonably provide for future development. The width of the road reserve is measured at right angles to the centerline of the road and varies according to the classification of the road. Carriage way: This is the section of the pavement which covers the traffic lanes, any auxiliary lanes and shoulders. Its width governs the safety and convenience of traffic and has a profound influence on the road capacity. The factors that influence capacity of a carriageway are: a) The design volume, i.e. the greater the traffic volume the wider the carriageway and Normally, the greater the number of lanes; b) Vehicle dimensions, i.e. heavy commercial vehicles require wider carriageways to ensure Adequate clearances when passing each other; c) The design speeds, i.e. vehicles travelling at high speed, especially commercial vehicles, require wider carriageways to ensure safe clearances between passing vehicles; d) The road classification, i.e. the higher the road classification the greater the level of service (and width of carriageway) expected. Central reservation/ median strip: This the longitudinal space separating dual carriages to separate the opposing traffic stream, minimize headlight glare, provide stopping area in case of emergency and to include space for safe operation of crossing and turning vehicles at intersections at grade. The width should be adequate for giving working space around a stopped vehicle. The central reservations on high-speed heavily trafficked rural roads in the United States are typically 15m to 30m. In Europe they tend to be much narrower (say 4 – 10m) and to be used with safety barriers. In urban areas they can be as narrow as 1m, but 3m is preferred so that a crossing pedestrian pushing a pram or wheelchair has space to wait in safety *O’Flaherty, 2002+. On severely restricted arterial streets, where a narrow separator of 0.6 – 1.2m is feasible, it may be desirable to have few, if
any, openings in median except at intersections. Shoulders: Paved or unpaved width of the road between the edge of the carriageway and the shoulder break point. The shoulder provides side support for the pavement or gravel surface and allows vehicles to stop or pass in an emergency .The width of the shoulder should be adequate for giving working space around a stopped vehicle. American practice recommends a 3m width for high type facility and a width of 1.2m 2.4m for low type facilities. UK practice for rural roads recommends widths ranging from 1.2m to 3.65m depending upon the road type and nature of kerb treatment. . (a) Camber Camber, also known as cross fall, is designed to reduce or eliminate the accumulation of water ponding on the pavement surface; if the water thickness is too great for the tire to make contact with the pavement surface, hydroplaning can occur where the tire is effectively sliding along a water surface with almost no ability to generate frictional resistance. The pavement can have a crown or a high point in the middle with slopes downwards towards both edges. This is favoured on two-lane roads and wider undivided roads. On divided roads, the
26 individual carriageways may be centrally crowned separately or a unidirectional slope may be provided across the entire carriageway width. The amount of camber to be provided depends upon the smoothness of the surface and the intensity of rainfall. In the UK, a value of 2.5% is generally adopted for design. A cross fall for the shoulders should be generally steeper than for the pavement by about 0.3 – 0.5% to facilitate quick drainage. The UK practice is to provide 5% slope on the shoulder [Kadiyali, 2006].
CROSS SECTION TERMS
27
28
(b) The preliminary design phase. The preliminary design phase is essentially an iterative one. It involves preparing a number of possible intersection layouts and generally examining each in terms of its operating characteristics (especially safety and capacity), ease of construction and likely capital cost, and environmental and local impacts that might affect the design selection. The most promising of the rough layouts are then selected for further development and analysis (including road user and vehicle operating costs, if appropriate), refined and examined in greater detail until that considered most suitable for the intersection is selected for detailed design and preparation of final construction plans and specifications *O’ Flaherty, 2002]. (c) Summary of design data W =9m WCR =8m WB-A=4m WB-C =4m WC-B =6m VrB-A =70m VlB-A =85m VrB-C =70m
VrC-B =70m qA-C =750pcu/hr qC-A =800pcu/hr qA-B =350pcu/hr qB-A =100pcu/hr qB-C =300pcu/hr qC-B =400pcu/hr
(i) Capacities of turning movements qsB-A =D{627+14W CR –Y(0.364 qA-C+0.114 qA-B+0.229 qC-A+0.520 qC-B)} ……..(i) ……(ii) qsB-C =E{745-Y[0.364 qA-C +0.144 q A-B+} s …….(iii) q C-B =F{745-0.364Y [q A-C +qA-B ]}
29 Where; Y=[1-0.0345W] =[1-0.0345X9] =0.6895 D = [1+ 0.094(W B-A - 3.65)][1+ 0.0009(V rB-A - 120)][1+0.006 VlB-A -150)] = [1+ 0.094(4 - 3.65)][1+0.0009(70 - 120)][1+0.006 (85) -150)] = 0.1076 E = [1+ 0.094(W B-C -3.65)][1+ 0.0009(V rB-C -120)] = [1 + 0.094(4 - 3.65)][1+ 0.0009(70 -120)] =0.9864 F=[1+0.094(W C-B -3.65)][1+ 0.0009(V rC-B -120)] =[1+0.094(6 -3.65)][1+ 0.0009(70 -120)] =1.1660 Substituting these values in equations
(i), (ii), (iii)
30 qsB-A =0.1076{627+14(8)- 0.6895(0.364x750 + 0.114x350 + 0.229x800 + 0.520400)} =27pcu/hr B-C =0.9864{745 - 0.6895[0.364x750 + 0.144x350]} =515 pcu/hr s q C-B =1.1660{745 - 0.364x0.6895 [750 +350]} =547pcu/hr (ii) Assessment of junction arms Using design reference flow (DRF) to capacity ratio called RFC i.e. Reference to Flow Capacity ratio. Arm B-A =DRFB-A = 190 = 7.04 > 0.85 capacity is exceeded s RFC q B-A 27 Arm B-C =DRFB-C = 300 RFC qsB-C 5I5
= 0.58 < 0.85
arm has sufficient capacity
Arm C-B =DRFC-B = 400 RFC qsC-B 547
= 0.73< 0.85
arm has sufficient capacity
Based on the reference flow capacity ratios obtained, arm B-A has exceeded its capacity and needs to be redesigned. (iii) Potential junction improvement to accommodate design year flow. . Create auxiliary loads to accommodate waiting vehicles. Given sufficient space requirements, the junction can be upgraded to a round-about or signalized inter section. Increasing the average width of the road carriageway. Reducing the average width of each of the minor road lanes
31 Number 7 a) Characteristics that render roundabouts most effective at grade intersections Roundabouts are most effective as at grade intersections that have the following characteristics in rural or urban areas High portions and volumes of right turning traffic and with five or more intersecting legs. This is due to the fact that right turning traffic causes difficulty with signal controls when
their numbers are large, therefore making roundabouts more effective in this area Priority is not given to traffic from any particular road. This occurs when intersecting roads carry nearly equal amounts of traffic, or where the intersecting roads are both of high standard and are intersecting at approximately right angles
Presence of accidents involving crossing or turning movements. Junctions that have considerable accident threats call for the use of roundabouts, as it has been observed that about
⁄
of accidents occurring at signalized junctions occur at roundabouts. Roundabouts
also reduce the possible areas of conflicts at junctions, therefore minimizing the possibility
of accidents Traffic on minor roads is delayed by stop or give way signs
Where they cause less overall delay to vehicles. This is achieved by eliminating the necessity of stopping the crossing streams of vehicles
Where there is a marked change in road standard for example, when one is moving from a single to a dual carriage road b) Design features of roundabouts
o
For small roundabouts, the central island should be approximately diameter
o
⁄
of the inscribed circle
The circulatory width around the roundabout should be constant at about 1.0 to 1.2 times the highest entry width subject to a maximum of15 meters
o
o o
o
Entries should be flares. Single and two lane approaches should become 3 and four lanes respectively at give way lines Steep downhill gradients should be avoided at round about approaches Pedestrian guard rails should be used to control haphazard pedestrian crossing of traffic streams and to provide safety The range of suggested parameters for the design of roundabouts are given in the table below Table 7a Symbol e v l’ S D Φ r
Description Entry width Approach half width Average effective flare length Sharpness of flare Inscribed circle diameter Entry angle Entry radius
Recommended range for design 4.0 – 15.0m 2.0 – 7.3m 1.0 – 100.0 m 15 – 100m 10 - 60° 6.0 – 100.0m
32 c) Given data Table 7(b)
Geometric parameter Entry width Approach half width Average effective flare length Sharpness of flare Inscribed circle diameter Entry angle Entry radius
symbol e v l’
S D φ
r
unit m m m
Arm C 7.0 3.5 50.0
Arm D 12.0 5.5 50.0
m deg m
40.0 35.0 50.0
40.0 30.0 50.0
Table 7 (c)
From (origin)
To (Destination) A A 20 B 140 C 220 D 150
B 160 0 190 180
C 230 205 8 170
D 100 160 210 12
Facility to be opened to traffic in 2007 Design year 2026 Traffic growth rate r = 8% Geometric parameters of arms C and D are shown in the table 7 (a) Design life Y = (2026-2007) +1 =20 years I.
Design flows (DF) for the year 2026 were obtained by multiplying the values in table 7 (b) by the equation
DF =
1.125P (1+r) y Where P Y r
present flows in table 7(b) in pcu/hr design life (20 years) traffic growth rate
Multiplication produced these values in table 7(c) below Table 7(d) To (Destination) A B A 105 839 From B 735 0 (origin) C 1154 996 D 787 944
C 1206 1075 42 891
D 525 839 1101 63
33 II.
Entry capacity for arms C and D, Q e = K(F - f eQ c)
⁄
Where K = 1-0.00347(φ -30) – 0.978[( F = 303X2, where X2 = v +
and S=
⁄
f c = 0.21t D(1+0.2x 2), where t D = 1+ 0.5/(1+M) and M=exp[(D-60)/10] Arm C K= = =
⁄ ⁄ ⁄ ⁄
1 - 0.00347(φ -30) – 0.978[( 1 – 0.00347(35 – 30) – 0.978[( 1.01199
F = 303X2, where X2 = v + S
=
and S =
= 0.112
[
X2
= 3.5 +
F
= 6.3596 = 303 = 1926.93
f c = 0.21t D(1+0.2x 2), where t D = 1+ 0.5/(1+M) and M=exp[(D-60)/10] M t D
f c
= e([40-60)/10) = 0.1353 = 1 + 0.5/(1+0.1353) = 1.4404
= 0.21 = 0.687 Approach capacity, Q
= Q CA + Q CB + Q CC + Q CD = 1154 + 996 + 42 + 1101 = 3293
Circulating capacity Q c
= Q AA + Q BB + Q DD + Q BA + Q BD + Q AD = 105 + 0 + 63 + 735 + 525 + 839 = 2267
Entry capacity Q e
= K(F - f eQ c) = 1.01199 (1926.93 – (0.687 = 373.93
Arm D K
⁄ ⁄
= 1 - 0.00347(φ -30) – 0.978[( = 1 – 0.00347(30 – 30) – 0.978[( = 1.02934
) – 0.05]
34
F = 303X2, where X2 = v + S =
⁄
⁄
and S=
= 0.208
X2 = 5.5 +
= 10.0904
F = 303 = 3057.39
f c = 0.21t D(1+0.2x 2), where t D = 1+ 0.5/(1+M) and M=exp[(D-60)/10] M t D
f c
= e([40-60)/10) = 0.1353 = 1 + 0.5/(1+0.1353) = 1.4404 = 0.21 = 0.913
()
Approach capacity Q
= Q DA + Q DB + Q DC + Q DD =787 + 944 + 891 +63 =2685 Circulating capacity Q c = Q AA + Q BB + Q CC + Q CA + Q CB + Q BA = 105 + 0 + 42 + 1154 + 996 + 735 = 3032 Entry capacity Q e
III.
= K (F - f eQ c) = 1.02934 (3057.39 – (0.913 = 297.66
Checking which of the arms still has capacity
For sufficient capacity, RFC =
= 8.806 RFC =
Arm C, RFC = Arm D,
= 9.02 Since RFCc 8.806, and RFCD=9.02 are far greater than the upper limit of 0.85, queuing will occur during peak hours on these arms of the roundabout
35 (8a)
Disruptions due to maintenance. Traffic disruption due to maintenance last longer on rigid pavements than on flexible pavements.
Riding characteristics. Due to increased variability in ground conditions, rigid pavements tend to offer a better riding surface than a flexible pavement.
Ease and cost of repair. The cost and repair of flexible pavements is cheaper as compared to rigid pavements. Effect of climatic conditions. Flexible pavements are more susceptible to climatic effects than rigid pavements. Life time costs. The life time costs incurred in flexible pavements are high due to frequent maintenance. In rigid pavements, the initial capital investment is higher. Noise pollution. Bituminous surfacings are traditionally considered to be quieter and are preferred for use in locales where noise is deemed a problem. Construction time. Flexible pavements can be opened to traffic as soon as compaction is completed and the surfacings have cooled to the ambient temperature, whereas rigid ones formed from conventional mixes cannot be opened until they have gained sufficient strength. Variation in loading. Concrete pavements are generally better able to cope with unexpected loads and fuel spillages in industrial estates and service areas. For example, if a new housing or industrial estate is being built and it is a requirement that the roads be constructed first so that they can be used by both construction traffic and subsequent general traffic, concrete is considered by many to have a clear advantage.
8b The structure of a road pavement is composed of superimposed layers of selected and processed materials that are placed on the basement soil or subgrade. The main structural function of a pavement is to support the wheel loads applied to the carriageway and distribute them to the underlying subgrade.
SURFACING The uppermost layer of a flexible pavement is called the surfacing . The primary function of this layer is to provide a safe, smooth, stable riding surface, i.e. a carriageway, for traffic; its secondary functions are to contribute to the structural stability of the pavement and protect it from the natural
36 elements. The majority of modern-day surfacings in the western world are bitumen-bound; only minor road surfacings are now composed of soil –aggregate materials. When a surfacing is composed of bituminous materials it may comprise a single homogeneous layer or course; more usually, however, with heavily-trafficked roads, two distinct sublayers known as a wearing course and a basecourse are laid in separate operations. The wearing course forms the uniform carriageway surface upon which vehicles run. Ideally, it should; (i) offer good skid resistance, (ii) allow for the rapid drainage of surface water, (iii) minimize traffic noise, (iv) resist cracking and rutting, (v) withstand traffic turning and braking forces, (vi) protect the underlying road structure, (vii) require minimal maintenance, (viii) be capable of being recycled or overlaid, (ix) be durable and give value for money. No one material meets all of these requirements so, in practice, the selection of a wearing course material depends on the design needs at each site. The basecourse (also called a binder course) is a structural platform which regulates (i.e. makes even) the top of the underlying roadbase, thereby ensuring that the wearing course has a good riding quality when built; it also helps to distribute the applied traffic loads. If the wearing course is impervious, the basecourse can be composed of a more permeable material. The roadbase, which provides the platform for the surfacing, is the main structural layer in a flexible pavement. As the stresses induced in a flexible pavement by the applied wheel loads decrease with depth, the main function of the roadbase is to distribute the loads transmitted to it so that the strength capacities of the weaker subbase and subgrade are not exceeded. Roadbases in flexible pavements are normally designed to be very dense and highly stable, and to resist fatigue cracking and structural deformation. If a pavement is formed with layers of bitumen- and cementbound materials, the structure is often referred to as a ‘composite’ pavement. The rationale for
building a composite pavement is to combine the better qualities of both flexible and rigid pavements. In the UK, a flexible composite pavement is defined as having its surfacing and upper roadbase (if used) constructed from bituminous materials, supported on a roadbase or lower roadbase of cement-bound material. The material for the base course is typically unstabilized aggregates. A subbase is very often present in a flexible pavement as a separate layer beneath the roadbase. Whether it is present, or how it is used, depends upon its intended function(s). As a structural layer within the pavement the subbase further distributes the applied wheel loads to the weaker subgrade below. Whilst the subbase material is of a lesser quality (and, thus, is normally cheaper) than the roadbase material, it must be able to resist the stresses transmitted to it via the roadbase and it must always be stronger than the subgrade soil. Another major function of the subbase is to act as a working platform for, and protect the subgrade from, site and construction vehicles as a pavement is being built. This is especially important when the subgrade is of poor quality, e.g. clayey or silty, as the critical load-carrying period is when the heavy wheel loads used in the laying and compaction of the roadbase are applied to the subbase during construction. The subbase is mostly a local aggregate material.
37 (8c) Summary of design information Number of wet months, Number of dry months, Subgrade CBR, Traffic growth rate varies with vehicle class Design PERIOD 20 years Construction materials Surfacing material asphalt concrete Roadbase crushed stone Subbase Natural gravel Subgrade good quality gravel Determination of subgrade strength, S From table 5.1, the CBR range given as 13-15% falls in the range of 12-20% corresponding to subgrade class of S3 Determination of cumulative design traffic
Unidirectional flow, V Assuming a maximum directional split of 2:1 for two way traffic of the traffic volume or each class
VEHICLE CLASS
TOTAL TRAFFIC FLOW
UNIDIRECTIONAL TRAFFIC FLOW, V 0
GROWTH RATE(%)
PROJECTED FLOW, VP
MINI BUSES
2511
1682
7
2524
BUSES
322
216
5.5
298
PICKUPS 2 AXLE TRUCKS 3 AXLE TRUCKS
588
394
2.5
457
495
332
5.7
463
131
88
4.5
115
WEAR FACTOR, C
VEHICLE CLASS
GROSS WEIGHT
FRONT AXLE
REAR AXLE 1
REAR AXLE 2
WEAR FACTOR, C
MINI BUSES
29.43
9.81
19.62
0
0.0038
BUSES
147.15
29.43
58.86
58.86
0.6044
PICKUPS 2 AXLE TRUCKS 3 AXLE TRUCKS
29.43
9.81
19.62
0
0.0038
117.72
39.24
78.48
0
0.9840
196.2
39.24
78.48
78.48
1.9102
38 GROWTH FACTOR
] VEHICLE CLASS
ANNUAL GROWTH RATE, r (%)
GROWTH FACTOR, G
MINI BUSES
7.0
2.0498
BUSES
5.5
1.7434
PICKUPS
2.5
1.2772
2 AXLE TRUCKS
5.7
1.7811
3 AXLE TRUCKS
4.5
1.5686
FINAL COMPUTATIONS FOR D T VEHICLE CLASS
v
C
G
Ti
MINI BUSES
2524
0.0038
2.0498
0.1435
BUSES
298
0.6044
1.7434
2.2909
PICKUPS
457
0.0038
1.2772
0.0162
2 AXLE TRUCKS
463
0.984
1.7811
5.9237
3 AXLE TRUCKS
115
1.9102
1.5686
2.5067 DT=10.8810
From table 5.4, our cumulative design traffic of 10.8810 falls in the range of 8-20 msa which corresponds to a traffic class of T1. Required design structural number, DSN
* +⁄
Using chart number 2 with a subbase of natural gravel and a base of crushed stone, the following parameters are obtained; ,
⁄ * +
Layer thickness based on actual Design strength number, DSN
From design chart number 2, for a subgrade strength class of S3, and traffic class of T1, the corresponding asphalt thickness, h1is 80mm, and from table 5.6, the layer coefficients are;
The pavement should therefore be composed of; 1. Surfacing material : 80mm 2. Roadbase
:
175mm
3. Subbase
:
250mm
