Application of soil nail method for slope stability purpose

Faculty of Engineering and Information Technology School of Civil and Environmental Engineering

Application of Soil ailing for Slope Stability Purpose by Victor Yeung

Student Number:

10240810

Project Number

S08 – 097

Major:

Civil Engineering

Supervisor: Dr. Behzad Fatahi

A 6 Credit Point Project submitted in partial fulfillment of the Requirement for the Degree of Bachelor of Engineering

21 November 2008

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Statement of Originality The work contained in this thesis report is the sole work of the author. Fragments of texts that’s that were used from other sources have been properly acknowledged and the theories, results and designs that have been used in this report have been appropriately referenced and all sources of assistance have been acknowledged.

Victor st

Yeung

21 ovember , 2008



























Contents 1.0

Page

Introduction 1.1

Statement of Problem

1

1.2

Objective

1

1.3

Structure of Dissertation

1

2.0

Literature Review 2.1

Principle Theory of Slope Failure

3

2.2

Factors affect the slope stability

4

2.3

Slope failure hazard

7

2.4

Landslide hazard identification

9

2.5

Consequence-to-life Consequence-to-life Category

10

2.6

Australia Landslide risk zone category

11

2.7

Major landslide in past history (Hong Kong & Australia)

12

2.7.1

Hong Kong

12

2.7.2

Australia

13

2.8

Past method of slope failure fail ure prevention

14

2.9

Current method of slope prevention in present

15

3.0

2.9.1

Soil nail

15

2.9.2

Bio-Engineering

15

2.9.3

Soil Re-compaction & No-fine N o-fine Replacement

16

2.9.4

Other method

17

Application of Soil nailing for slope stabilization 3.1

Principle theory of soil nailing

3.2

Soil nail history and Development

19

3.3

Function of soil nail

20

3.4

Different between soil nail and soil anchor

21

3.4.1

4.0

18

Maintenance

21

3.5

Soil nail application in different construction purpose

22

3.6

Advantage of Soil nail for slope stability

23

3.7

Limitation of soil nail

24

Construction Construction method of soil s oil nails



























4.4.1

Setting out of soil nail position

28

4.4.2

Drilling

28

4.4.3

Soil nail steel bar installation

30

4.4.4

Grouting

30

4.4.5

Procedure of pull-out test nail sample

30

4.4.6

Soil Nail Head

31

4.5

Quality Specification

33

4.6

Testing on soil nail

34

4.6.1

Soil nail steel bar

34

4.6.2

Cement Grout

35

4.6.3

Soil nails

37

4.7

5.0

Other type of soil nail installation techniques

43

Design of soil nails 5.1

Concept of Factor of safety

5.2

Introduce the Circular slip and Method of slices

45 46

5.2.1

Circular slip analysis method

46

5.2.2

Method of slices (Ordinary method or Swedish method)

47

5.3

Soil nail calculation method

48

5.4

Analysis slope stability with soil nail element

50

5.5

Slope stability analysis computer program - Slope/W

50

5.6

Design parameter

51

5.7

Design procedure

52

6.0

Case Study 6.1

Case study ( Hong Kong)

53

6.1.1

Geotechnical assessment

55

6.1.2

Slope/W

59

6.1.3

Hand calculation using Swedish Method of Slices

60

6.1.4

Estimated Slip surface

63

6.1.5

Soil nail design

64

6.1.6

Summary

69

6.2

Stability Analysis

Case Study ( Australia)

70

6.2.1


73

6.2.2

Slope/W Stability Analysis

76

6.2.3


77



























7.0

Conclusion 7.1

Summary and concluding remarks

7.2 Recommendations

8.0 9.0

Bibliography List of Appendices Appendix A – Previous Boreholes Log Records (Case study 1) Appendix B – Previous Laboratory Test Recods (Case Study 2) Appendix C – Slope/W Analysis Analysis Data ( Case study 1) Appendix D – Classification Guide ( Case Study 2) Appendix E – Slope/W Analysis Analysis Data ( Case Study 2)

86 87 88

90



























List of Figures Figure 1 :

Typical Typical circular / rotational shaped slip surface

3

Figure 2

Typical channelisation flow (CEDD ,1990)

7

Figure 3

Typical Slide type landslide (CEDD, 1995)

7

Figure 4

Landslide in main access road of Hong Kong International Airport (Appledaily news ,2008)

8

Figure 5

Landslide in Hong Kong (Appledaily news ,2008)

8

Figure 6

Sau Mau Ping Landslide, (CEDD, 1976)

12

Figure 7

Sau Mau Ping Landslide, (CEDD,1976)

12

Figure 8

Kotewall Road. Landslide, (CEDD,1976)

12

Figure 9

Wong Chuk Hang Landslide, (CEDD,1995) (CEDD, 1995)

12

Figure 10

Thredbo 1997 landslide (EMA disaster DB,1997)

13

Figure 11

Thredbo 1997 landslide (EMA disaster DB,1997)

13

Figure 12

Sea Cliff Bridge (http://seacliffbridge.co (http://seacliffbridge.com/) m/)

13

Figure 13

Landslide on Lawrence Hargrave Drive (EMA disaster DB,1988)

13

Figure 14

Shotcrete surface

14

Figure 15

Masnory surface

14

Figure 16

Chuman surface

14

Figure 17

Typical Typical Soil nailing method (Maunsell.Geotechnical (Maunsell.Geotechnical ltd ,2003)

15

Figure 18

Typical Typical Soil nailing method (IECA, 1995)

15

Figure 19

Root orientation with respect to shallow slope failure (Coppin ,1990)

15

Figure 20

Vetiver Grass System,

15

Figure 21

No-Fine concrete replacement (Maunsell geotechnical geotechnical Ltd. ,2005)

Figure 22

Completed no-fine replacement slope (After landscaping)

( Toyo Greenland Greenlan d Co., Ltd , 2008)

( Maunsell geotechnical geotechnical Ltd. , 2005) Figure 23

Active and Passive zone (Abramson, 2002)

Figure 24

Typical tie-back for deep excavation excavatio n (deepexcavation.org (deepexcavat ion.org , 2008)

Figure 25

Typical permanent Tie-back wall

16 16 18 22

(Office of Geotechnical, California, 2008)

22

Figure 26

Soil nail reinforcement bar

25

Figure 27

Typical Centralisers Central isers

25

Figure 28

Steel plate and Steel nuts

25

Figure 29

Typical Typical soil nail head reinforcement

25

Figure 30

Drilling Rig

26

Figure 31

Air compressor

26





























Figure 36

Steel bar installation

30

Figure 37

Grouting process

30

Figure 38

Excavated soil nail head

31

Figure 39

Typical buried soil nail head

31

Figure 40

Shotcreting soil nail head

31

Figure 41

Typical Typical detail of soil nail and soil nail head (Hong Kong CEDD standard drawing, 2008 )

32

Figure 42

Steel bar test sample pieces

34

Figure 43

Bleeding test

35

Figure 44

Flow cone test

35

Figure 45

Typical section of flow cone test equipment (ASTM C939, 2002)

35

Figure 46

Typical sample record sheet for Bleeding Test and Flow Cone Test ( Maunsell Geotechnical Geotechnical services Ltd ,

2008 )

36

Figure 47

Square cement grout cube

37

Figure 48

Compressive strength test

37

Figure 49

Pull out test

37

Figure 50

Dial Gauge

38

Figure 51

Typical sample data sheet for Pull out test (Maunsell Geotechnical services Ltd , 2008)

Figure 52

Typical Typical sample plotting sheet for pull out test (Maunsell Geotechnical services Ltd ,2008)

Figure 53

40

Typical sample data sheet for proving test (Maunsell Geotechnical services Ltd, 2008)

Figure 54

39

41

Typical Typical sample plotting sheet for Proving test (Maunsell Geotechnical services Ltd ,2008)

42

Figure 55

Self drilling (Dipl.-Wirt.Ing, 2008)

43

Figure 56

Jet Grouting (Dipl.-Wirt.Ing, 2008)

44

Figure 57

Soil nail launch machine (soil nail launcher Ltd. , 2008 )

44

Figure 58

Circular slip model (Liu.(2008)

46

Figure 59

Swedish Method Model

47





























Figure 69

Soil nail design section detail

66

Figure 70

General view of slope

70

Figure 71

Elevation View

70

Figure 72

Side View

70

Figure 73

Silty clay at slope toe

71

Figure 74

Silty clay at slope crest

71

Figure 75

Pocket Penetrometer

72

Figure 76

Pocket Penetrometer

72

Figure 77

Slope location plan

74

Figure 78

Sample collection position

74

Figure 79

Critical Cross Section A-A

75

Figure 80

Critical slip surface

76

Figure 81

Swedish Method of Slices model

77

Figure 82

Estimated slip surface

79

Figure 83

Soil nail slope FOS analysis

81

Figure 84

FOS comparison

81

Figure 85

Soil nail design section detail

82





























List of Tables Table 1

Typical Examples of Facilities Affected by Landslides in Each Consequence-to-Life Consequence-to-Life Category ( CEDD, 2007)

Table 2

10

Summary of landslide risk categories and development controls (Wilson ,2004)

11

Table 3

Other drilling method for soil nail

(Elias & Juran , 1991)

29

Table 4

Comparison of Consequence-to-life Consequence-to-life Category

53

Table 5

Design parameter

56

Table 6

Section A-A FOS result

59

Table 7

Two methods FOS result comparison table

61

Table 8

Swedish Method of Slices Calculation Spreadsheet Spreadsheet

61

Table 9

FOS results table

63

Table 10

Soil nail parameter

64

Table 11

FOS result (after soil nail installed)

65

Table 12

Design Assumptions

66

Table 13

Tension Failure of the Steel Bar calculation spreadsheet spreadsheet

67

Table 14

Bond Failure between Grout and Steel Bar calculation spreadsheet

68

Table 15

Bond Failure between Grout and soil calculation spreadsheet spreadsheet 1

68

Table 16

Bond Failure between Grout and soil calculation spreadsheet spreadsheet 2

69

Table 17

Final Soil Nail design schedule table

69

Table 18

Final result table

69

Table 19

Hand penetrometer test results

72

Table 20

Design parameter

73

Table 21

Section A-A FOS result

76

Table 22

Two methods FOS result comparison table

78

Table 23

Swedish Method of Slices Calculation Spreadsheet Spreadsheet

78

Table 24

FOS results table

79

Table 25

Soil nail parameter

80

Table 26

FOS results (after soil nail installed)

81





























Abstract Landslides are a common natural disaster which take place around the world. They have claimed many human lives and much damage has occurred from different types of landslides. Through the last couple of decades, different kinds of landslide preventive measures have been developed for reducing these hazards. Each preventive measure involves a unique technique and application application benefit. benefit.

One of the most common slope stabilisation stabilisation methods methods is

soil nailing. The soil nail application has been developed in the last 30 years. This method is growing rapidly and becoming more popular due to its advantages. Use of the soil nail method for reinforcing unstable slopes is one of the most favourable solutions in geotechnical engineering practice.

Thus, there would be a lot of benefit for future use which would be

associated with the development of the soil nail application for slope stabilisation. This project will present the application of soil nail for slope stabilisation. The benefits and limitations of soil nail and its construction procedures are described. In addition, design requirements and quality control control specifications specifications are explained. explained.

Slope stability analysis using

“SLOPE/W” code is demonstrated and a design method of soil nail using Slope/W is described in detail. Two selected case studies, located in Hong Kong and Australia, are presented to demonstrate the effectiveness of the soil nail system for slope stabilisation. These case studies present a typical design method used for soil nail walls. A simplified hand calculation method is compared with the limit equilibrium approach used in Slope/W code.





























Acknowledgements I wish to express my sincere gratitude to my supervisor Dr. Behzad Fatahi for his inspiring discussions, without which I would not successfully been able to complete this thesis. Furthermore, I would like to thank Dr. Behzad Fatahi for his invaluable personal time spent with me through numerous conversations. He has not only taught me how to approach my Capstone project; but more importantly, he has provided invaluable insight which will help me in my journey to be a professional Geotechnical engineer. He has assisted in my development of a large knowledgebase of geotechnical engineering concepts and some interesting ideas such as bio-engineering. I am also thankful to my wife, Maggie Leung. Sharing her Geotechnical experience provided much support and assistance, both of which have contributed in some way to the journey of writing this thesis. Special thanks to my previous employer, Maunsell Geotechnical Services Ltd for the invaluable assistance and some sample data information. Special thanks John Marsh for his advice in proof reading and correcting some grammatical mistake in my thesis.































Application of Soil Nailing for slope stability purpose

1.0

Introduction

1.1 Statement of Problem This Capstone project topic is Application of soil nailing for slope stability purpose. The project involves the literature li terature review for design, analysis, research related to slope stabilisation methods. This project demonstrates different aspects associated with soil nail application. In Australia and Hong Kong there is high risk of slope failures. Some of them are associated with high risk of hazard for public in densely populated areas. Therefore, slope investigation and classification are important for the community. Thus, both cities developed their own landslip risk reduction programs following a similar independent path, resulting in a large amount of experience gained in dealing with rainfall triggered landslides in densely populated areas.

1.2 Objective This project reviews different methods of slope stabilisation. This project presents the current knowledge and known benefits of soil nail as a slope stabilisation method. In addition, various factors that may trigger slope failure is discussed. Through the use of case studies, the design and construction methods of soil nailing is described in this






























application for different construction purposes such as deep excavating. The last part of Chapter 3

notes the advantages advantages for selecting selecting soil nailing nailing as an initiative to

improve slope instability and this was compared to the other methods are described. Chapter 4 will cover the construction methods and the procedure involved in soil nailing, including the equipment used, and procedures. Quality control is also an essential procedure for soil nail construction. This part will present the quality control criteria in the whole soil nail installation process. Chapter 5 is about design of the soil nail. Here, the design criteria and principle theory are presented. In this part, the use of the computer design program ( SLOPE/W with Morgenstern-price Morgenstern-price method) is i s also discussed. Chapter 6 is about case studies for soil nail application. This section concentrates on two separate separate case studies.

The first case case study investigates a slope in Hong Kong.

The other case study related to application of soil nailing in Australia. Two different design standard have been used for these case studies. The first case study in Hong Kong will use Hong Kong GEOguide for design standard and the second case study Australia Standard AS4678-2002 is used for its design standard. For both case studies, Slope/W computer software is used for stability analysis. The factor of safety is an important outcome for the classification of slopes. Hand calculations using Swedish Method of Slices will also be provided in both case studies.





























Application of Soil Nailing f r slope stability purpose

2.0 Literatur Review 2.1

Principle

heory of Slope Failure

Every year there are ap proximately a thousand slope failure cases around the globe. On average, a death t oll of many thousands of people, as wel economic losses related to landslide events are common.

as astronomical

Therefor , it is evident that

there is a clear need to i vestigate the cause of devastating slope fail res. Slope failure is related o various causes, these include: the rise of round watertable, soil properties and geol gical characteristics of slopes. These cause s of slope failures are often interrelated

nd can influence each other, collectively deteriorating the

stability of the slope.

he combination of these failure modes fo rms the principle

elements related to slop failure. Principle Theory

Slope failure is driven by slope slip surface which is caused by gravitational and seepage forces that pus

the slip surface and causes slope instabili ty (Ortigao,2004)

According to Abramso

(2002), there are various types of slope failure which are

driven by slip surfaces, namely: circular/rotational slip, non-circular slip, translational slip and compound slip.






























2.2

Factors Affecting the Slope Stability

There are many factors which affects the slope stability. According to Ortigao, (2004) described that one of the main factors is the geometrical changes. This is described as a change in the gravitational force. The main force responsible for movement is gravity. Gravity is the internal force that acts on body, pulling mass object in a direction toward the center of the earth. If the object is on a flat surface then the gravitational force will act downward. In another words, if the objects is located on the flat surface it will not move under the gravity force. However, in the case of a sloping ground, according to Ortigao (2004) described that the force of gravity can be divided into two vector components, one component is acting normal to the slope and the other component is acting tangent to the slope. The slope gains its stability from the strength properties of the soil. These include the shear strength, frictional resistance and cohesion among the soil particles that make up the soil mass (Ortigao, 2004). As the applied shear stress which occurs under gravitational force becomes greater than the combination of forces holding the soil mass on the slope, the object will move down the slope. In geotechnical engineering, this movement is called slope failure or landslide. Thus, this slope movement is favored by steeper slope angles which increase the shear stresses on the soil. The slope stability stabilit y is threatened by anything that reduces the shear






























which the sand slope can remain stable (Liu ,2008). In this case, the stability of the sand slope is purely dictated by the frictional contact between the soil grains. In general, the friction angle increases with increasing grain size. However, different soil types contain different soil friction angles. This mechanical soil parameter can be usually obtained from experiments, for example, Triaxial test and direct shear test . In the partially saturated soil, water particle and the sand particle are interlocked by an internal suction force between them. This suction force assists in building up apparent cohesion in cohesionless material. It should be noted that, excessive water will break the suction force between the soil particles. The other factor that affects the slope stability is the additional loads (surcharge) applied on the top of the slope. This external loading can increase the disturbing force and cause slope instability. Another reason that affecting slope stability is water pressure. Water pressure is common on a general slope where a watertable might usually exist. When water pressure increases, the effective stresses , shear strength decrease and can l ead to slope failure. An increase in the water pressure may be due to many uncertain reasons. Usually, the most common reasons that cause slope failure relate to water pressure increases due to elevated rainfall intensity and increases in the water content in slope, such as water pipe leakage. These are the main factors that can affect the slope stability. These are also the main
































According to CEDD (2008) & Ortigao, (2004), the causes of slope instability can be summarised as follows: External force that causes slope instability:



Geometrical changes (Undercutting, erosion, changes in slope height, length and steepness)



Surcharge (Addition of material, Increase in slope height and increase development at slope crest)



Shocks and vibrations (earth quake)



Drawdown (lowering of water in lake or reservoir)



Change in water regime ( rainfall , increase in weight , pore pressure )

Internal forces that causes slope instability: 

Progressive failure (following lateral expansion of fissuring and erosion)



Weathering (reduction of cohesion, desiccation)






























2.3

Slope Failure Hazard

According to J.A.R. Ortigao, 2004 , Landslips can be classified into 3 main types of landslides; these are described below according to their kinematics of the slide. Fall type landslide

Usually occurring in rock slopes, rock displacement and rock falls with a very fast movement (EMA ,2008). ,2008). Usually a topple t opple fall is classified cl assified into this category. Slide type landslide

Slides are usually caused by mass movements that present a well-defined failure wedge and surface. According to their failure wedge and surface geometry, it can be classified into shallow slides or deep slides (Ortigao, 2004).






























Regardless of the type of landslide failure mode, in some areas of high population density, a landslip can cause a large disaster. If the landslide is a minor one, it might cause damage and displacement of a building’s foundation or break the frame structure of the building. This displacement or settlement can disrupt the building’s structural stability and cause the building to collapse. In the case of a major landslide flow, a whole building can be overwhelmed. Usually this type of major flow will have a high casualty rate if it occurs in a high population density area. For example, Hong Kong has a unique geological environment which mainly consists of volcanic rock with a mountainous region and few flat land areas. This scenarios left many developers with few options, one of which was t o build skyscrapers on hillsides. The cost of land is very high as the developers often need to bulldoze mountains to carry out site formation and form more flat lands for the construction of the buildings, which are often over 30 stories. Thus, many of the man-made slopes are very close to buildings, as this helps to save on the land cost, therefore simultaneously stretching the profit margin of a lot of land.






























2.4

Landslide hazard identification

According to Hong Kong housing authority (1999) reported that the identification of landslide hazard involved following procedure: 

Desk study- An aerial photograph is an important aspect of landslide hazard

identification. The study of aerial photographs assists in cataloguing of historical landslides, describing and evaluating the geomorphology and determining the site history particularly with respect to human activities on natural slopes. 

Engineering geological reconnaissance Mapping- The mapping provided

additional landslide information data which was not visible on the aerial photos and enables ground truthing of some of the geomorphological interpretations made from aerial photographs. 

Ground Investigation – In order to understand the ground model better, ground

investigation was carried out to explore the soil properties and the condition of the groundwater regime. 

Site investigation – site visits and field measurements were taken of the slope






























2.5

Consequence-to-life Consequence-to-life Category

This guidance is used to identify the level of risk of a human loss in relation to the type of facilities that are affected by landslides. According to the following table, an engineer would be able to improve on the stability of the slope by examining the types of facilities used on the top of the slope. Hence, this would prevent catastrophic damage and the loss of human lives. Table 1 : Typical Examples of Facilities Affected by Landslides in Each

Consequence-to-Life Consequence-to-Life Category ( CEDD, 2007) Consequence Group

Facilities

to-life Category

(a) Heavily Used Buildings – residential building, commercial office, store and shop, hotel, factory, school, power station, ambulance depot, market, hospital, pol yclinic,clinic, welfare centre
































2.6

Australia Landslide Risk Zone Category

The Australia Geomechanics subcommittee has developed a classification of consequences of landsliding. AGS(2000) which classified the hazard slope as Exempt (EX), Low(L) , Medium(M0) , Medium(M1), Medium (M2) and High (H) according their different trigger factor, is shown as follows. Table 2

: Summary of landslide l andslide risk categories and development controls






























2.7 2.7.1

Major landslide in past history (Hong Kong & Australia) Hong Kong

According to CEDD reported that between 1925 and 2007, more than ten thousand landslides have occurred and every year have about 300 slope failures occur in cut slope areas. In the past half century, at least 300 people died in 24 landslides in Hong Kong. (CEDD,2008) On 18th June 1972, a major landslide took place in Hong Kong at mid-level Kotewall






























2.7.2

Australia

According to EMA Disasters Database, 2008, there have been 48 recorded landslide events which have collectively resulted in the death of 39 people , and 19 casualties out of the 7,586 victims in Australian landslide history since 1897. One of Australia’s worst landslides was held in 30th July 1997. A large section of the steep mountainside below the Alpine Way road collapsed and overwhelmed a section of the Thredbo Ski Village in NSW. About 1,000 tonnes of landslide material slipped rapidly down the steep slope and shearing the Carinya lodge off its foundations and slamming it into the Bimbadeen Lodge. It was recorded that 18 people had fallen






























2.8

Past Method of Slope Failure Prevention

Before 1990, chuman surface and non-reinforcing shotcrete surfaces were a common use of material for slope stability improvement. For some steep slopes, a stone pitching surface was most widely used, or masonry facing for rigid surface cover. Some of them were installed “weepholes” to reduce the pore water pressure inside the slope. However, the main purpose of this was to achieve an impervious interface for prevention of the surface erosion and the rainfall entry into the slope in order to reduce the pore water pressure inside the slope. This method is easy in terms of construction






























2.9

Current Method of Slope Failure Prevention

In the past 20 years, slope improvement technology has advanced significantly. The slope improvements are now focusing on the slope stability design and environmental protection. Many different types of slope retaining methods are used in slope improvement construction and design. The most commonly used methods are Soil-nailing and Bio-Engineering.

2.9.1

Soil ailing is a new technique in which soil slopes, excavations or

retaining walls are reinforced by the t he insertion steel reinforcing bars. According to Ortigao (2004) noted that the first use of the soil nailing application was in 1972 and
































2.9.3

Soil Re-Compaction and o-fine Replacement






























2.9.4

Other Method of Slope Failure Prevention






























3.0

Application of Soil nailing for Slope Stabilisation






























3.2

Soil nail History and Development






























covered by hydroseeding surface on top of soil nail head to provide a natural and environmentally friendly slope surface.












































































































































































































































































































































































































































































































































































































































































5.6

Design parameters

In soil nail calculation and SLOPE/W, soil properties and conditions are the essential components for both of these calculations. Therefore, site investigation and visual inspection are needed for design data collection purposes. Site investigation:

Boreholes - Rotary drilling methods are commonly used in slope site investigation. Through the soil data logging, a description of the soil and properties such as soil type, colour, consistency and soil structure are determined. This also can provide an un-disturbed sample for a triaxial test. Ground water levels can also be measured in the boreholes recorded. In-situ test – A standard penetration test or cone penetration test is used to define the relative density of the soil and relative strength. Other in-situ test such as vane shear test and pressuremeter test may be used which vary depending on the soil type t ype and data collection. Laboratory test – Tri-axial tests are more commonly used in ground investigation which determine the soil friction angle value and cohesive value. Moisture content tests are commonly used in Australia which can provide an easy and economical method to estimate the soil type and property through common practice. Visual inspection: inspection:

Surface Stripping – This is a commonly used method in visual inspection which removes a narrow strip on the slope surface. This is an easy method for determining the skip layer of soil property. Slope geometry geometry -

Slope height height and slope angle angle should be measured measured and this data can be used to model the slope profile in SLOPE/W and soil nail design.

Other data

Such as surcharge utilities, slope surface seepage and tension




























5.7

Design procedure

The following procedure will be used for soil nail slope improvement design. 1. Design parameter collection 2. Geotechnical assessment, modeling of the slope profile according to the design parameters 3. Use SLOPE/W program to analyse the critical section Factory of Safety 4. If the analysis is not satisfactory in terms of the required FOS, use trial and error method to determine the failure-resisting force until the slope analysis is satisfactory the required FOS. 5. Use soil nail calculation methods are used to determine the size of steel bar, inclination angle and horizontal and vertical spacing required, the bond length and size of the soil nail head. Also, check the maximum allowable tensile force, max. allowable bond stress and total force mobilised which needs to met the FOS requirements. Use a trial and error method to determine the most efficient design. 6. Input the designed parameter of soil nail into SLOPE/W software to re-analyse the most critical of factory of safety.




























6.0

Case Study

Introduction This part of the case study will represent a sample of soil nail application for slope stability improvement. Slope/W software is used as well as the soil nail calculation method to briefly design the soil nail. The first part of the analysis will choose one of Hong Kong Cut slope which is of a high Consequence-to-life Category (Cat 1) . The other part of case study will choose one of Australia Cut slope which also is of a high risk category of Consequence-to-life. Because the type of soil properties in the two geological different areas vary, Case study (Hong Kong) will use Geoguide standard and Case study (Australia) will use Australia Standard AS 4678-2002.

6.1 Case Study Analysis

(Hong Kong)

There are 3 sample cut slopes which have been selected for this case study. Comparing these 3 cut slopes, the most high risk for slope failure is located next to a sports centre. If slope failure were to occur, soil mass may flow into sport centre and may damage the building structure. The worst case scenario would involve the whole building collapsing. Therefore, I choose this slope for slope stability analysis. Table 4

Compare Consequence-to-life Category Crest facility – Un-development green belt Toe facility – playground , Pavilion

Category group 3 Consequence-to-life Category

2

Middle risk Crest facility - Un-development green belt Toe facility - road with heavy vehicular or pedestrian

traffic density

Category group 2b Consequence-to-life Category

2

Middle risk Crest facility – Main Access road Toe facility - indoor games or sport hall

Category group 2a




























Slope Background The selected slope is a soil cut slope which is located at next to sports centre. According to background information from the previous study (CEDD, 1993), the caption slope was formed in 1975 by cutting in association with the opening of access road along crest . Figure 60

General View of slope

Site description This cut slope is located at east of Shek Kip Mei Sports Centre. The slope is about 80m long with a maximum height of 12m. This slope has divided into two portions, an upper portion and lower portion which are separated by a berm. The slope angle in upper portion is approximate 60 o and lower portion is approximate 55 o. The slope is covered with a vegetation surface which provided a minor surface improvement. The crest facility is low a traffic road which is the main access of the sports centre. The toe facility is an indoor sport hall name Shek Kip Mei Park Sports Centre which is located approximately 5m away from the slope toe.

Visual Inspection The site inspection on the caption slope was carried out in July 2008. During site observation, no seepage or leakage was observed on the slope or surrounding area. The slope has been divided into two batters by a one meter wide berm. The slope is covered with a vegetation surface and no surface erosion has occurred. The slope appears to be in good condition and no adverse signs of distress were observed. Surface channels were found at the berm and toe of slope. The drainage condition appears to be in good condition.

Ground Investigation During the desk study stage, there were three previous ground investigations which were carried out. One in 1984 , one in 1993 and the other in 2005. This previous records are open to the public, as it is able to be accessed at the CEDD Geotechnical Information Unit Library. Combining these investigation records, we got a total of 6




























Laboratory test During the desk study stage, there are previous laboratory tests which have been carried out in 2005 by Gold Ram Engineering and Development Limited . These previous laboratory test reports are open to the public which can collect in CEDD Geotechnical Information Unit Library, in order to obtain soil parameters for further stability assessment and identify the material from ground investigation work. These previous laboratory tests contained information regarding particle size distribution and single stage tri-axial compression tests under undrained conditions. From the result of particle size distribution, the result showed that the completely decomposed granite in the vicinity of the slope was solely composed of sandy materials. From the single-stage triaxial compression tests under undrained conditions. The p’-q plot for completely decomposed granite was generated according to the previous test result which carried out in 2005. The triaxial test results from previous laboratory tests are shown in Appendix B

6.1.1


Critical section

According to the site inspection, the minimum distance between slope toe and toe facility had a uniform spacing of around 5 meters and the slope angle has a uniform value of approximately 55-60 degrees. Therefore, the critical section is controlled by the maximum height of 12m. The critical section plan is shown in figure 1. Ground conditions

According to the previous borehole log record, From DH1 and DH2, completely decomposed granite (CDG) was found at around 0.04m below the ground. Thus, CDG was found immediately on the slope. The thickness of CDG layer is 14.79m at DH1 and 6.77m at DH2. In the borehole log record, completely decomposed granite is described as extremely weak, brownish yellow, occasionally reddish yellow and brown spotted grey and brown, completely decomposed medium grained granite (silty fine to coarse sand with some angular to subangular fine to medium gravel of granite and quartz) base on Geoguide standard. Groundwater condition






























Parameter for analysis

The soil strength parameters adopted for the stability analysis and soil nail design are based on the laboratory test results of the consolidated undrained single stage triaxial tests on the soil samples. From the results of the p’-q plot in table 6.1, the c’ =0kpa and φ’ =38o. However, according to CEDD Geoguide 1 standard, the recommended minimum value of cohesion for Completely Decomposed Granite (CDG) is 5kPa. Therefore, c’ =5 kpa is recommended in caption slope. The shear strength parameters adopted in the stability analysis for the caption slope are shown as follows: Design parameter

Table 5

Soil Type

Unit weight γ’ KN/M3

Cohesion c’ (kPa)

Friction Angle φ’

CDG

20

5

38

500 450 400 350 ) a P k ( q

300 250 200

DH1

150

DH3

100 50 0 0

100

200

300

400

500

600

700

800

P' (kPa)

Figure 61

p–q

plot graph

( Gold Ram Engineering and Development Limited ., 2005)



























































A A n o i t c e S s s o r C l a c i t i r C 3 6 e r u






























6.1.2

Slope/W

Stability Analysis

The minimum FOS of slip surface is generated under Morgenstern and Price analysis using SLOPE/W software. The soil layer distribution is made up by previous borehole log records DH1, DH2, and BH6. Other assumptions are according to design assumptions. All design assumptions and soil properties are according to CEDD Geoguide 7 (2008) standards.

Figure 64

Critical Slip surface

The critical slip surface is generated under automatic Grid and radius generation,






























6.1.3


According to the Morgenstern and Price analysis using SLOPE/W software, critical slip surface plates are generated and the circular arc, centre and radius are computed. In order to compare the FOS with Morgenstern and Price analysis, hand calculation for the method of slices is also carried out to present the basic theory of slope stability analysis for the same slip. This method has assumed that the slip wedge is divided by vertical planes into a series of slices of a certain width. The base of each slice is assumed to be a straight line. For any slice the inclination incli nation of base to horizontal is α. In order to make the result more accurate and consistent with mechanics, the slope will be divided into 30 slices and the arc length and inclination angle of each slice is measured. On the other hand, because the ground water table is below the slip surface, no pore water pressure will affect the slope and the boundary water force can be ignored in the method of slices equation.

w
































Swedish Method of Slices result

The following table lists the results of the hand calculation method and compares with Slope/W calculation results using Morgenstern and Price analysis. Table 7

Two methods FOS result comparison table Morgenstern and Price

Factor of safety ( FOS)

Swedish Method of Slices

method

method

0.986

0.802

From the table 6.3, the FOS is not consistent. There is about an 18% difference between each method. It is because in the Method of Slices all interslice forces are ignored. Also, this method is only for c’=0. Therefore, some errors may occur in this analysis. According to Krahn, (2004) described that, usually the error arrange is within the range 5-20% compare with Morgenstern and Price method. The calculation spreadsheet is shown in following table: Table 8

Swedish Method of Slices Calculation Spreadsheet

Soil Unit weight of soil=20

Soil friction angle Φ =38

Water table table below distance below ground, ground,

Tan Φ =0.78128

no pore water water pressure pressure






























16

0.48

16.49

40.00

0.77

0.64

12.63

10.60

9.87

17

0.46

15.98

39.00

0.78

0.63

12.41

10.05

9.70

18

0.46

18.06

38.00

0.79

0.62

14.23

11.12

11.12

19

0.11

4.70

37.00

0.80

0.60

3.75

2.83

2.93

20

0.51

24.07

36.00

0.81

0.59

19.47

14.15

15.21

21

0.51

26.55

35.00

0.82

0.57

21.75

15.23

16.99

22

0.51

26.16

33.00

0.84

0.54

21.94

14.25

17.14

23

0.51

22.78

32.00

0.85

0.53

19.32

12.07

15.09

24

0.51

19.39

31.00

0.86

0.52

16.62

9.99

12.99

25

0.46

14.40

30.00

0.87

0.50

12.47

7.20

9.75

26

0.46

11.78

28.00

0.88

0.47

10.40

5.53

8.13

27

0.46

9.17

27.00

0.89

0.45

8.17

4.16

6.38

28

0.46

6.55

26.00

0.90

0.44

5.88

2.87

4.60

29

0.46

3.93

25.00

0.91

0.42

3.56

1.66

2.78

30

0.46

1.31

24.00

0.91

0.41

1.20

0.53

0.93

Σ

381.90

306.22

F= Σ(N)*tan(φ)/ΣT (=w*sin(α))= 0.80182






























6.1.4

Estimated Slip Surface

At the beginning of the soil nail design procedure, we need to estimate the different shapes of the slip surface. This is because if we use the minimum FOS (0.986) to design the soil nail, the reinforced slope may have another shape of slip surface for which the FOS is smaller than 1.4. For example, if the slip no. 1 FOS is smaller than 1 and the slope is not safe, after installing the soil nail with the bond length is just passing through the slip surface of slip no. 1, after that analysis the reinforced slope of slip 1 and the FOS will rise to meet the requirement. The slip 1 seems safe. However, the soil nail may not be contributing a resisting force to slip no. 2,3,4,5. Therefore the overall slope will still not be safe. Therefore, the slope will typically be distributed over 5 different shapes of slip surface and analysed for all of the FOS values (Slope/W FOS analysis data shown in Appendix C )






























6.1.5

Soil nail design

Soil nail length and bond length prediction

From the previous analysis, the minimum FOS is for slip no. 2. The FOS for the other slip nos. 1 and 3 are smaller than the required required FOS =1.4.

For the soil nail concept concept

prediction, if the soil nail bond length passes through the slip 3 failure plate, the soil nail resisting force is then functional to the soil mass. Therefore, for the same reason, the soil nail is also functional for slip no.1, and 2. On the other hand, avoid the reinforced slope minimum FOS fallback to slip no. 3. Thus, the bond length of soil nail will start from Slip no. 3 failure plate. In the trial length estimation, the soil nail resisting force and soil nail length are estimated and a trial and error approach is used to determine the required FOS value. The position of the soil nail is estimated according the slope profile and slope parameters. A 2m horizontal and vertical spacing with a staggered format is recommended. The first row of soil nails is 2m from ground and third row of soil nail is 1m above the berm. Therefore, 5 rows of soil nails are formed uniformly over the 12m slope height.

Estimated design






























Soil nail reinforced slope analysis The soil property and soil profile remain as previously modeled. Three rows of 8m soil nails and two rows of 12m soil nail at inclination 15 o are added in SLOPE/W slope profile. Bond length and forces are predicted and applied. The minimum FOS of 5 nos slip surface is generated under the Morgenstern and Price analysis using SLOPE/W software.
































Figure 68

FOS comparis n Soil ail detail Calculati on

In order to calculate the oil nail detail parameters, we need to use a trial and error method to find out the nai l bar size, nail length, bond length, inclinati n, spacing and required resisting force. It is recommended to use an EXCEL spreads eet to compare the parameters and check he outcome

to achieve the most suitable d sign.






























Soil nail design calculation for Section A-A

(Final Result)

This part investigates the tensile strength of the bar, bond failure between grout & steel bar and bond bond failure between between grout grout & soil. soil.

This step is an essential part of the

design to avoid the failure of grout and soil bonding and failure of steel bar and grout bonding. Design Parameters

Soil Type = CDG , C’=5kPa ,

γ=20kN/m3 , φ ' = 38ο

Drill Hole diameter = 0.1m Soil Nail inclination angle = 15 o Unit weight of water = 9.81kN/m 9. 81kN/m3

Part A

- Tension Failure of the Steel Bar

Yield stress f y = 460Mpa Maximum tensile stress = 0.5 f y = 230Mpa






























Part B

-

Bond Failure between Grout and Steel Bar

Cube strength of cement grout at 28 days f cu = 32Mpa For type 2 deformed bar β = 0.5 Factor of safety adopted SF = 3 Allowable bond stress = Ultimate bond stress / SF

= [ β (f ccuu)1/2 ] / SF

2

= 942.81k/m

Maximum allowable force between steel and grout =

[ β (f ccuu)

Where

1/2

] × π × (d - 4) × L e / SF

Eq ( 5.9)

Le : Effective bond length

Table 14

Bond Failure between Grout and Steel Bar calculation spreadsheet Max. Bar

Bar Size Horizontal

Free

Level Row No.

Length (mPD)

(d)

Spacing

length

La

Bond

Force per

length Le m Width

Force Allowable Required Force

Tmax > Tr






























Table 16

Bond Failure etween Grout and soil calculation spreadshe et 2 Effective Vertical Stress

Row No.

σv (kPa)

Force Mobilised Tf (kN)= ( 1.571 +

Total Force

Force

Mobilised

Required

F.O.S.

F.O.S. > 2

0.140 σ’v ) x Le Le

CDG

CDG

Tf (kN)

Tr (kN)

Tf / Tr

Row E

68.00

36.65

36.65

16.00

2.29

O.K.

Row D

106.00

62.45

62.45

30.00

2.08

O.K.

Row C

144.00

93.58

93.58

40.00

2.34

O.K.

Row B

180.27

220.16

220.16

100.00

2.20

O.K.

Row A

158.57

230.92

230.92

110.00

2.10

O.K.

Table 17

Final Soil Nail design schedule table






























6.2

Case Study Analysis

(Australia)

Introduction In Australia, some cut slopes may be discovered near some railway tracks or on a highway road side area. These cut slopes usually formed when highways and railways are constructed. Some of them are sandstone based original cut slope. Due to the soil property of sandstone, there is a lower landslide hazard in sandstone cut slope. However, some of them consist of weak sandstone, silt clay slope, silt sand slope etc. When these soil properties are in a slope which is formed to a steep angle, there may be a high risk of slope failure. In this case study in Australia, a sample of a silt clay slope is presented for demonstration using soil nail for slope stability purpose. This slope is located at Sydney suburban area - Hurstville, which is on the lllawarra line railway side cut slope.

The toe facility is a railway and the crest facility is

moderate use traffic road. However, if the slope failure, the railway services may be required to stop, or in the worst case this could cause a train derailment. This may
































by cutting in for railway work construction to Waterfall by Sydney Railway Company (SRC)

Site description This cut slope is located at north of Railway lllawarra line. The slope is about 50m long with a maximum height of 6m which according to GPS height record. This slope toe contains a 1.5m high solid pile wall. The slope has two different slope angles. The slope angle in upper portion is approximate 45 o and lower portion is






























Penetrometer test In order to provide more precise soil property data, a penetrometer in-situ test was taken on site. The equipment used in this field test was Pocket Penetrometer. The pocket penetrometer is a device used by geotechnical engineers to estimate unconfined compressive strengths of in situ soils. The pocket pocket penetrometer penetrometer is a spring-loaded spring-loaded penetrometer penetrometer..

The spring spring is calibrated

against unconfined compressive strength (typically measured in k g/cm 2). The mark at which the indicator is located is taken as the unconfined compressive strength of the soil.






























6.2.1


Critical section

According to the site inspection, the minimum distance between slope toe and toe facility is of a uniform spacing of approximately 0 meters and the slope angle on the surface is uniformly approximate at 45-50 degree. Therefore, the critical section is controlled by the maximum height of 6m with solider pile wall. The critical section plan is shown in figure 6.11.






























Design assumptions 

The ground water table is assumed as a 1 in 10 year rainfall intensity and the groundwater table is assumed to be at 1/3 of t he slope height.



The surcharge of the crest traffic road is assuming 20kPa uniform loading.



The slope is assumed to consist of one layer of silty clay soil.





























































6.2.2

Slope/W

program

Stability Analysis






























6.2.3


According to the Morgenstern and Price analysis using SLOPE/W software, critical






























Swedish Method of Slices result






























6.2.4

Estimated Slip surface






























6.2.5

Soil nail design





















































































































































































































































Application of soil nail method for slope stability purpose

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