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

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 Yeung

21st ovember , 2008

Contents Page

1.0 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 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 prevention 14

2.9 Current method of slope prevention in present 15

2.9.1 Soil nail 15

2.9.2 Bio-Engineering 15

2.9.3 Soil Re-compaction & No-fine Replacement 16

2.9.4 Other method 17

3.0 Application of Soil nailing for slope stabilization

3.1 Principle theory of soil nailing 18

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 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

4.0 Construction method of soil nails

4.1 The compound of the soil nail 25

4.2 Introduce the soil nail construction equipment 26

4.3 Soil nail construction procedure summary 27

4.4 The major procedure of soil nails construction 28

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 Other type of soil nail installation techniques 43

5.0 Design of soil nails

5.1 Concept of Factor of safety 45

5.2 Introduce the Circular slip and Method of slices 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 Stability Analysis 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 Case Study ( Australia) 70

6.2.1 Geotechnical assessment 73

6.2.2 Slope/W Stability Analysis 76

6.2.3 Hand calculation using Swedish Method of Slices 77

6.2.4 Estimated Slip surface 79

6.2.5 Soil nail design 80

6.2.6 Summary 85

7.0 Conclusion

7.1 Summary and concluding remarks 86

7.2 Recommendations 87

8.0 Bibliography 88

9.0 List of Appendices 90

Appendix A – Previous Boreholes Log Records (Case study 1)

Appendix B – Previous Laboratory Test Recods (Case Study 2)

Appendix C – Slope/W Analysis Data ( Case study 1)

Appendix D – Classification Guide ( Case Study 2)

Appendix E – Slope/W Analysis Data ( Case Study 2)

List of Figures

Figure 1 : 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) 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.com/) 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 Soil nailing method (Maunsell.Geotechnical ltd ,2003) 15

Figure 18 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, ( Toyo Greenland Co., Ltd , 2008) 15

Figure 21 No-Fine concrete replacement (Maunsell geotechnical Ltd. ,2005) 16

Figure 22 Completed no-fine replacement slope (After landscaping)

( Maunsell geotechnical Ltd. , 2005) 16

Figure 23 Active and Passive zone (Abramson, 2002) 18

Figure 24 Typical tie-back for deep excavation (deepexcavation.org , 2008) 22

Figure 25 Typical permanent Tie-back wall

(Office of Geotechnical, California, 2008) 22

Figure 26 Soil nail reinforcement bar 25

Figure 27 Typical Centralisers 25

Figure 28 Steel plate and Steel nuts 25

Figure 29 Typical soil nail head reinforcement 25

Figure 30 Drilling Rig 26

Figure 31 Air compressor 26

Figure 32 Grouting machine 26

Figure 33 Shotcrete machine 26

Figure 34 Mobile drilling rig 28

Figure 35 Typical drilling rig 28

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 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 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) 39

Figure 52 Typical sample plotting sheet for pull out test

(Maunsell Geotechnical services Ltd ,2008) 40

Figure 53 Typical sample data sheet for proving test

(Maunsell Geotechnical services Ltd, 2008) 41

Figure 54 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 60 General View of slope 54

Figure 61 p – q plot graph

( Gold Ram Engineering and Development Limited., 2005) 56

Figure 62 Slope location plan & Bore hole location 57

Figure 63 Critical Cross Section A-A 58

Figure 64 Critical Slip surface 59

Figure 65 Swedish Method of Slices analysis 60

Figure 66 Estimated Slip Surface 63

Figure 67 Soil nail slope FOS analysis 65

Figure 68 FOS comparison 66

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 Category ( CEDD, 2007) 10

Table 2 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 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 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 67

Table 14 Bond Failure between Grout and Steel Bar calculation spreadsheet 68

Table 15 Bond Failure between Grout and soil calculation spreadsheet 1 68


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 78

Table 24 FOS results table 79

Table 25 Soil nail parameter 80

Table 26 FOS results (after soil nail installed) 81

Table 27 Design Assumptions 82

Table 28 Tension Failure of the Steel Bar calculation spreadsheet 83

Table 29 Bond Failure between Grout and Steel Bar calculation spreadsheet 84



Table 32 Final Soil nail design schedule Table 85

Table 33 Final Result table 85

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 benefit. One of the most common slope stabilisation 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 specifications are 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.

It should be noted that soil nailing is one of the methods used for stabilising medium size

slopes. Enhancing public education for the landslide hazard is the most desirable way to

prevent human loss and property damage in high landslide risk areas. In this study, some

recommendations regarding increasing public awareness about landslide hazards are

described as well.

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.

Lastly, I would like to thank all my friends for their continuing and unconditional support

and assistance.

Application of Soil Nailing for slope stability purpose

1

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 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

project.

1.3 Structure of Dissertation

In this project, Chapter 2 will discuss how the slope instability can affect society, as

well as provide a technical review of the factors that affect the slope instability. This

review discusses the different methods used to reduce these hazards will be discussed.

Furthermore, a discussion follows, noting the wide range of traditional stabilisation

methods available to engineering are presented. This ranges from a simple methods

such as to flatten and drain a slope , to more complex methods, such as anchors and

soil nail, bio-engineering vegetation and the most common practice of methods

involving shotcrete surfaces, masonry facing and so on.

A cost-effective solution for stabilisation is the application of soil nailing, which is

discussed in chapter 3.0. The first part of this chapter mainly focuses on the literature

review of soil nailing and also reviews the principle theory of soil nailing, including

it’s history and development of nailing. Furthermore, it will also discuss the soil nail


2

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

of Chapter 3 notes the advantages for selecting soil 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 method) is also discussed.

Chapter 6 is about case studies for soil nail application. This section concentrates on

two separate case studies. The first 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.

In Chapter 7, conclusion at the study is presented. Furthermore, recommendations for

an innovative design method for slope improvement will be briefly described.


2.0 Literature Review

2.1 Principle Theory of Slope Failure

Every year there are approximately

On average, a death toll of

economic losses related to landslide

there is a clear need to investigate the cause of

Slope failure is related to various causes, these include

soil properties and geological characteristic

are often interrelated and can influence

stability of the slope. The combination of

elements related to slope failure.

Principle Theory

Slope failure is driven by slope slip surface

seepage forces that push

According to Abramson

driven by slip surfaces, namely

slip and compound slip.

The most common type

described as a circular shaped slip surface

isotropic soil condition, whereas a non

non-homogenous condition

(2004) described that slope failure driven by translational and compound slip surface

is developed due to the presence of a rigid layer (for example a bedrock layer), or the

presence of discontinuiti

Figure 1 -


Literature Review

Principle Theory of Slope Failure

approximately a thousand slope failure cases around the globe

n average, a death toll of many thousands of people, as well as astronomical

economic losses related to landslide events are common. Therefore, it is evident that

investigate the cause of devastating slope failure

Slope failure is related to various causes, these include: the rise of ground

geological characteristics of slopes. These causes of slope failures

and can influence each other, collectively

. The combination of these failure modes forms the principle

slope failure.

Slope failure is driven by slope slip surface which is caused by gravitational and

seepage forces that push the slip surface and causes slope instability

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

, namely: circular/rotational slip, non-circular slip, translational

type of slope failure mode is circular/rotational slip

a circular shaped slip surface which is mobilised across a homogenous

isotropic soil condition, whereas a non-circular slip surface is mobilize

homogenous condition (Ortigao, 2004). On the other hand, according to

slope failure driven by translational and compound slip surface


presence of discontinuities such as fissures and pre-existing slips.

- Typical circular / rotational shaped slip surface

3

slope failure cases around the globe.

of people, as well as astronomical

Therefore, it is evident that

slope failures.

the rise of ground watertable,

causes of slope failures

deteriorating the

forms the principle

caused by gravitational and

slope instability (Ortigao,2004)

here are various types of slope failure which are

circular slip, translational

rotational slip. This is

ed across a homogenous &

circular slip surface is mobilized in a

according to Ortigao,

slope failure driven by translational and compound slip surface


Typical circular / rotational shaped slip surface


4

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 is threatened by anything that reduces the shear

strength, such as lowering the cohesion among the particles or lowering the frictional

resistance. The tenancy of slope failure is expressed in terms of the ratio of shear

strength to shear force, which is known as Safety Factor (Cornforth,2005)

Safety Factor = Shear Strength/Shear force

If the safety factor becomes less than 1.0, slope failure is expected.

The other factor that causes slope failure is an increase in water pressure. This is

caused by the increase in groundwater level. Consequently, an increase of water

pressure adds an increased internal water force inside the slope. Although water is not

always directly involved as the transporting medium in mass-wasting processes

(Ortigao, 2004), it does play an important role. For exemplary reasons, a sand castle

on the beach may be used. If the sand is dry, it is impossible to build a steep face like a

castle wall. If the sand is wet, vertical wall can be build. If the sand is too wet, then it

flows like a fluid and cannot stay as a wall.

For the case of dry sand, the sand can form a slope with a slope angle relative to the

flat ground that is equal to its Friction angle. The friction angle is the steepest angle at


5

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 lead 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

items which one has to focus on when dealing with reducing the presence of slope

instability.

There is another factor that can induce instability to a slope, which is an earthquake.

However this factor is relatively uncommon when compared to the other factors

mentioned above. Slope instability caused by an earthquake only happens during

earthquakes in active earthquake zones, such as in China and Japan. This factor causes

slope displacement and changes the gravity condition of slope material. During the

displacement and change of gravity of slope, the body of slope mass no longer is in a

balance condition, and slope will no longer be in a stable condition.

In many seismic regions of the world, slope displacements caused by earthquakes have

led to disaster situations. Examples of magnitude 7.8 earthquake-induced landslides

are the landslide events in the area of Sichuan in China, which were caused by a major

earth movement event near the belt of Sichuan region in May 2008.


6

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)

� Seepage erosion (solution , piping)

Moreover, there are some other non-natural factor cause slope instability:

� Removal of vegetation;

� Interference with, or changes to, natural drainage;

� Modification of slopes by construction of roads, railways, buildings, etc;

� Overloading slopes;

� Mining and quarrying activities;

� Vibrations from heavy traffic, blasting, etc; and

� Excavation or displacement of rocks.


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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). Usually a topple fall is classified 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).

Flow type landslide

A flow landslide is a continuous viscous slide involving soil or rock (Emergency

Management Australia - 2008 ). According to Ortigao (2004) explained that if material

is clay or fine soil material, this flow is termed a mud flow. Flow slides usually

include saturated soil or mud mix with water (also called liquefaction) and are usually

initiated from the summit of a hill or mountain due to high rainfall or water leakage

and flow downward by channelisation. Sometimes, this slide is also triggered by rapid

ground motion and commonly occurs during earthquakes. Unfortunately, the flow will

result in major economic loss and major landslide casualties if it happens in a densely

populated area.

Figure 2 Typical channelisation flow

(CEDD ,1990)

Figure 3 Typical Slide type landslide

(CEDD, 1995)


8

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 to 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.

At times where land availability is limited, a surplus in population often leads to a city

being overdeveloped. This would elevate the risk of landslide failure, as developers

are left with no choice but to cut back on the slope to form flat land. By doing so, the

new slope would decrease the safety factor, leaving a very steep angle and a lack of

surface protection. As this is becoming a widespread global situation, landslides are

not unusual in urban areas. This is evident with the even that occurred on 7th June

2008, when a series of landslips occurred in Lantau Island due to heavy rainfall. These

serious landslips are mainly located near the main access road of Hong Kong

International Airport. This disaster severely affected the operation of the airport.

Figure4 Landslide in main access road

of Hong Kong International Airport

(Appledaily news ,2008)

Figure 5 Landslide in Hong Kong

(Appledaily news ,2008)


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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

geometry (eg. Slope height, angle, seepage). Therefore, the collected data can be

used to provide the most precise information and representative the real slope

geometry for further design.

� Engineering Geological synthesis – An engineering geological synthesis of the

finding from the desk study, engineering geological mapping, ground

investigation fieldwork, site investigation fieldwork and laboratory tests was

conducted to produce a geological model and representative geological sections

Development of Landslide Risk Assessment in Australia

In the recognition of the challenge between development pressures and landslide

hazards, in the year 2000, Australian Geomechanics Society Published a series of

guidelines called ‘Landslide Risk Management Concepts and Guidelines (AGS2000)’.

This is a benchmark technical paper for development of landslide assessment. In 2004,

Landslide Likelihood Research had been undertaken to investigate the likelihood of a

landslide in residential areas. The aim of this research is to develop the probability

estimates for landslide hazards in Australia.

Development of Landslide Preventive Measure Program in Hong Kong

Prior to 1976, due to the high risk for landslide in Hong Kong, The Geotechnical

Engineering Office has been responsible for studies and upgrading works in respect of

old (i.e. pre-GEO) substandard slopes under a long term program- Landslip Preventive

Measures (LPM) Program. According to CEDD (2008) reported that this long term

program will be targeting over 5,000 high-priority substandard Government man-made

slopes, and will carry out safety-screening studies for another over 10,000

high-priority private man-made slopes by the year 2010.


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2.5 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 Category ( CEDD, 2007)

Group Facilities

Consequence

to-life

Category

1

(a) Heavily Used Buildings

– residential building, commercial office, store and shop, hotel, factory, school,

power station, ambulance depot, market, hospital, polyclinic,clinic, welfare centre

1 (High)

(b) Others

– cottage, licensed and squatter areas

– bus shelter, railway platform and other sheltered public waiting area

– dangerous goods storage site (e.g. petrol stations)

– road with very heavy vehicular or pedestrian traffic density

2

(a) Lightly Used Buildings

– indoor car park, building within barracks, abattoir, incinerator, indoor games’

sport hall, sewage treatment plant, refuse transfer station, church, temple,

monastery, civic centre, manned substation

(b) Others

– major infrastructure facility (e.g. railway, tramway, flyover, subway, tunnel

portal, service reservoir)

– construction site (if future use not certain)

– road with heavy vehicular or pedestrian traffic density 2 (Middle)

3

– heavily used open space and public waiting area (e.g. heavily used playground,

open car park, heavily used sitting out area, horticulture garden)

– road with moderate vehicular or pedestrian traffic density

4

– lightly used open-air recreation area (e.g. district open space, lightly used

playground, cemetery, columbarium

– non-dangerous goods storage site

– road with low vehicular or pedestrian traffic density 3 (Low)

5 – remote area (e.g. country park, undeveloped green belt, abandoned quarry)

– road with very low vehicular or pedestrian traffic density


11

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 risk categories and development controls

(Wilson ,2004)

Landslide risk zone category

Exempt (Ex) Low(L) Medium(M0) Medium(M1) Medium(M2) High(H)

Ap

pli

cab

le g

eolo

gy

& s

lop

e

Alluvium &

Colluvium<5%

Alluvium &

Colluvium

5-20% ;

Tertiary<15%;

Bedrock<20%

Granite 20-40% Other Bedrock

20-40%

Colluvium

20-50%;

Tertiary>15%;

Rhyodacite

20-40%;

Granite & other

bedrock >40%

Known

Landslides and

similar terrain.

Colluvium>50%

Rhyodacite>50%

Def

init

ion

of

risk

cate

go

ry

Likelihood of

landslide of

natural slopes is

extremely low

Landslide

without

development is

very unlikely

Landslide is unlikely without development. The

likelihood of instability without development is greater

in M2 than for the M0 & M1 zones

There is a

likelihood of a

landslide without

development

A s

imp

le

exp

lan

ati

on

You possibly

have a flooding

problem

Worry about

something else

Watch out for

springs.

Comply with

Guidelines

Comply with

the guidelines

Comply the

guidelines.

They are there

for a purpose.

You probably

have a problem

Req

uir

ed s

ite

spec

ific

geo

tech

nic

al

info

rma

tio

n

Confirmation of risk category by the shire using geotechnical information

submitted to classify the site for soil reactivity.

Slope stability assessment by an

experienced geotechnical practitioner

to confirm or change the regional

classification plus additional

geotechnical investigation where

considered necessary.

Dev

elo

pm

ent

con

tro

ls Good

engineering

practice

Good Hillside

practice

site and project specific controls

where applicable including specific

attention to drainage & erosion

control

site and project

specific controls

where

applicable

If confirmed as

High H , it is

unlikely permit

will be issued.


12

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

2.7.1 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

Road. Two high-rise residential buildings collapsed due to a large landslide which was

responsible for the death of 67 people (CEDD, 2008). In the same year, another

major landslide event caused many fatalities which occurred in Sau Mau Ping village.

This devastating landslide event caused major debris flow which overwhelmed a large

section of Sau Mau Ping Village (CEDD , 2008).

Figure 6 Sau Mau Ping Landslide,

(CEDD, 1976)

Figure 7 Sau Mau Ping Landslide,

(CEDD,1976)

In 13th Aug 1995, the large Wong Chuk Hang landslide occurred and the landslide

material slipped rapidly down the steep slope and destroyed the seaside shipyards.

Two people died in this landslide (CEDD,2008)

Figure 8 Kotewall Road. Landslide,

(CEDD,1976)

Figure 9 Wong Chuk Hang Landslide,

(CEDD,1995)


13

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

victims in this disaster which also caused multimillion dollars in damage (EMA ,2008)

Figure 10 Thredbo 1997 landslide

(EMA disaster DB,1997)

Figure 11 Thredbo 1997 landslide

(EMA disaster DB,1997)

According to Australia National Landslide Database,(2007) reported that on 30 April

1988 in Coledale, a small coal mining town near Wollongong, a landslide resulted

from a combination of human interference and two weeks of heavy rainfall. A 20

meter high railway embankment collapsed after earth and rock ballast used to fill an

old mine dam became saturated, resulting in severe undermining and subsidence. A

sudden rush of mud and rock smashed into a house below, turning it through a 60

degree angle before it was demolished. The occupants a young mother and her baby

son, were killed.

Some landslide hazards have led to the re-development of infrastructure for some

geological reason. For example, the purpose of Sea Cliff Bridge is to replace a section

of Lawrence Hargrave Drive that was permanently closed in July 2003 due to a great

landslide hazard reason. Therefore, landslides in Australia not only cause human loss,

but also cause economic loss which due to leak of landslide hazard assessment .

Figure 12 Sea Cliff Bridge

(photo: http://seacliffbridge.com/)

Figure 13 Landslide on Lawrence

Hargrave Drive 1988


14

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

and maintenance and was also cost efficient.

However, if the slope had inherent instability due to internal soil, shear failure and

sliding would still occur. This method would not provide an enough structural external

force against the movement of the slope failure wedge. On the other hand, this method

usually uses a concrete or stone base construction material, which is usually grey or

white in colour. This triggers an environmental problem, as the finish is very

inconsistent with the surrounding natural landscape.

The following lists are the conventional slope stabilisation methods.

� Shotcrete surface method:

Shotcrete is a process where concrete is sprayed onto

slope surface using a shotcrete feeder gun to form

rigid surface. Usually, shotcrete surface slopes have

approximate 50-150mm thick and provide wire mash

reinforcement to prevent surface crack and shrinkage. Figure 14 Shotcrete surface

� Masnory surface method

Use stone pitching as a rigid surface cover for prevent

erosion and surface runoff. This method is easy for

maintenance and construction.

Figure 15 Masnory surface

� Chuman surface method

Use of cement sand mix material for surface

protection. No reinforcement and wire mash required.

Poor crack and shrinkage resistance.

Figure 16 Chuman surface


15

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 insertion steel reinforcing bars. According to

Ortigao (2004) noted that the first use of the soil nailing application was in 1972 and

now this method is a well-established technique around the world. Sometimes, soil

nailing can combine different type of retaining methods such as soil nailing on

retaining walls and with greening surfaces. Soil nailing can provide a cost efficient,

quick and standard technique for slope improvement solution. Thus, according to

CEDD (2008) reported that soil nailing methods dominate about 70% of all soil slope

improvement constructions in Hong Kong.

Figure 17 Typical Soil nailing method

(Maunsell.Geotechnical ltd ,2003)

Figure 18 Typical Soil nailing method

(IECA, 1995)

2.9.2 Bio-Engineering is one of the most innovative technologies for slope

improvements in the world. According to Coppin (1990) described that

Bio-Engineering includes the use of tree roots or plant roots to retain shallow slope

failure. This method has an advantage as it is natural and environmental friendly

(Coppin,1990). However, many factors can influence the effectiveness of

Bio-engineering for slope stabilisation. This method is in an early stage of

development, and needs a period of time for technology proving and development.

Figure 19 Root orientation with

respect to shallow slope failure

(Coppin ,1990)

Figure 20 Vetiver Grass System,

( Toyo Greenland Co., Ltd , 2008)


16

2.9.3 Soil Re-Compaction and o-fine Replacement

For some loose material slopes such as fill slope, soil nailing is not a suitable

stabilisation method. Some technologies such as soil re-compaction and soil

re-placement are more suitable and are usually applied. Soil re-compaction involves

the excavation of the loose soil, backfilling and re-compacting to improve the friction

angle. However, the soil re-compaction method has some restrictions such as every

backfill and re-compaction has to be carried out in a 300mm thick layer (Geoguide 7,

2008), layer by layer, and every single layer needs an individual soil test for

compaction ratio checking. Moreover, this method is highly influenced by weather

conditions. The soil has to be placed in thinner lifts and requires moisture control for

compaction. As a result, this method will increase the construction cost and period.

The other method is the soil replacement method. This design approach includes using

other materials such as no-fine concrete or gravel to replace the loose soil. Removal of

the original loose soil on the slope is carried out, then forming a slope with a design

slope angle by backfilling with no-fine concrete or gravel. After that, a thin layer of

soil with hydroseeding is applied to the surface as a cover and for landscaping. This

method can reduce the construction period, hence alleviating labour costs and

operation costs which then compare with the soil re-compaction method.

However, these replacement and re-compaction methods are constrained in that the

construction sequence has to be scheduled for the dry season when the groundwater

levels are lower than they were at the time of active landsliding. Alternatively,

temporary groundwater lowering through the use of a raking drain may be needed

prior to, and during construction work.

Figure 21

No-Fine concrete replacement

(Maunsell geotechnical Ltd. ,2005)

Figure 22

Completed no-fine replacement slope

(After landscaping)

( Maunsell geotechnical Ltd. , 2005)


17

2.9.4 Other Method of Slope Failure Prevention

Subsurface Drainage

Of all stabilisation methods considered for the prevention of landslides, a reduction of

pore water pressure behind the slope is the most important. According to

Cornforth(2005) described that the subsurface drainage method can reduce the

destabilising hydrostatic and seepage water pressures on the slope as well as the risk of

sliding or flow. For large, unstable slopes, a drainage tunnel can be applied to draw

down the water table and minimise the risk of slope failure. In Hong Kong, the Lung

Fu Shan drainage tunnel and vertical drainage system is under construction. This

drainage tunnel can prevent the failure of a 200m high natural slope which could be

triggered by water pressure. Other subsurface drainage methods include: Drain blanket,

Trenches, Cut-off drains, Horizontal Drains, Relief Drains and Raking Drains.

Stone Columns

Based on Cornforth, (2005) described that this ground improvement method can

increase the average shear resistance of soil along a potential slip surface by replacing

or displacing the in situ soil with a series of closely spaced and large diameter columns

of compacted stone. However, this method requires the use of a boring machine and

material delivery, which would result in an access problem if the slope is inaccessible.

This method is not common use in Hong Kong. Usually, vertical soil nailing can

provide the same results as stone columns.

Shear Piles

According to Cornforth, (2005) described that shear piles are reinforced concrete

cylindrical piles that pass through the slide plant and anchored at lower end stable soils

or bedrock. This shear pile anchorage can provide lateral bearing resistance near the

base of ground movement (Cornforth, 2005). This method is effective for a large

instability zone and can provide the flexibility of selecting an installation location.

However, this method has limitations such as being costly and cannot be installed in

moving landslide.


18

3.0 Application of Soil nailing for Slope Stabilisation

3.1 Principle theory of soil nailing

A slope can be described in terms of geological theories, according to Abramson (2002)

described that the soil mass behind the slope surface can be divided into an active and

passive zone which are separated by a shear face call slip surface. The slope stability

analysis for a soil nailed slope considers the stabilising effect of nails acting on the slip

surface, however, this differs with respect to the shape of the slip surface, the forces

act on a nail and the method used for calculation of stability.

Based on Ortigao (2004) described that Soil nailing consists of reinforcing the instable

soil mass by the series of elements called nails to resist tension, bending and shear

forces. These nail elements are usually made of galvanized steel bar and protected by

cement grout. Nails are installed sub-horizontally and closely spaced in a parallel

fashion (usually 1.5m to 2.0m in spacing) into soil mass in a pre-drilled hole to

improve stability of slope.

According to CEDD (2008) described that soil nailing provides pullout resistance

force and tension over their entire length. The angle, length and diameter of soil nails

are dependent on soil condition and design criteria. Usually, soil nails are installed for

permanent slope improvement. Therefore, the corrosion-resistant treatment is similar

to soil anchors and requires galvanizing.

The soil nail system for mechanical stabilisation against the instability force can be

categorised as a limit equilibrium analysis (Abramson,2002). This is a conventional

slope stability calculation method with potential slip surfaces modeled, such as

circular arc slip surface. Abramson,(2002) stated that this potential slip surface model

approximately represents the critical surface of maximum tensile load. Limit

equilibrium analysis can examine the slip surface and others to determine the lowest

factor of safety after the slope is reinforced.

Figure 23 Active and Passive zone (Abramson, 2002)


19

3.2 Soil nail History and Development

Soil nailing methods are widely use in geotechnical construction work. Nowadays,

these technologies can be used in Tie-back retailing wall, Temporary support, ground

anchor and Tunneling support. Therefore, soil nailing has a great contribution in

geotechnical construction.

Based on Ortigao (2004) noted that , in the late 60’s, soil nailing developed used in

tunneling shotcrete supporting method. This method used a flexible lining that enabled

soil deformation around the excavation, which had been reinforced by a number of

bolts or nailing. An active zone is formed around the excavation and the lining is

subjected to reduced loading (Ortigao, 2004). This technique is the traditional

tunneling technique method for preventing soil deformation and reducing the

subjected ground pressures of tunnel.

As reported by Ortigao (2004), the first time nailing was used in tunneling

construction work was in 1970 in Brazil. After that, this nailing method is widely used

in France, Canada, Germany, UK and in the USA, among other countries.

The soil nailing for slope stability method is similar to the tunneling support method

(tieback). The difference is they are installed non-tensioned at a slight downward

inclination on slope. Such construction work used soil nailing for slope improvement

work in Versailles (France) for first time in 1972 (Cornforth, 2005). According to

Ortigao (2004) reported that one of the first national guideline publications for soil

nailing was produced in Japan in 1987; the USA has produced national guideline

publications through the Federal Highway Administration on this subject in 1996.

The Geotechnical Engineering Office (GEO) of Hong Kong extensively uses soil

nailing to stabilize thousands of man-made slopes in residual and saprolitic soils and

in 1996 presents its prescriptive design method. CEDD, (2008) mentioned, since 1995,

over ten thousand of such soil nailing structures have been constructed in Hong Kong

through LPM program to stabilize slopes in residual soil.

Regarding the development of the soil nail head, in the early 90’s , an exposed soil nail

head was commonly used in the soil nailing system. With a large size and exposed

head, it was possible to transfer the component of load from the slope face to soil nail.

However, buried soil nail heads are now common, since the late 90’s. Hidden into the

slopes surface and with a small size, the soil nail head (approximate 0.6m-0.8m) is the

main element of the design in soil nailing system. This type of soil nail head can be


20

covered by hydroseeding surface on top of soil nail head to provide a natural and

environmentally friendly slope surface.

3.3 Function of the Soil ail

Soil nail - in general, these are a form of in situ non-tensioned reinforcement, acting

similarly to strip reinforcement (Abramson, 2002). Typically, soil nails usually have a

diameter of 25-32mm. The length and inclination are both dependent on the design

calculation and factor of safety. They are installed in drillholes and bonded into place

with low pressure grout. Stress is transferred from the ground to the nail over its full

length and there is a shear stress reversals as in reinforced earth.

According to Ortigao (2004) described that ,when considering a very steep slope in a

granular or cohesive soil, many factors may influence the soil, causing it to not have

sufficient internal strength to stand at such an angle. Therefore, for the face to remain

stable the force exerted by soil mass sliding must be resisting by a reinforcement

structure. In previous chapters, it has been mentioned that the stability method can be

achieved through the implementation of structural elements (such as skip wall) , or

through the inclusion of reinforcement in the soil (such as soil nail). The aim of the

inclusions is to interact with soil mass in a stabilising manner. An active inclusion is

like a stressed soil anchor , it exerts a force on the soil mass through the tension in

anchor. In the chapter of Principle theory of soil nailing, Abramson (2002) mentioned

that the two zones can be identified, an active zone and a passive zone. The stabilising

manner relies on the soil frictional force between the soil nail surface and soil which is

generated by the surrounding soil mass in passive zone. If the soil mass had to stand at

a very steep angle and had insufficient shear strength, the soil mass would deform.

Therefore, this deformation may exert a force which would act on any structural

element placed in the soil.

Based on Cornforth (2005) described that the main aim of the soil nailing method is

the structural element which is used to resist this deformation force. Hausmann, (1992)

mentioned that Soil nail contains two forces when the soil mass undergoes

deformation. The first is friction between the deformation soil mass and the inclusion.

This interaction length can be termed the “bond length”. The second is derived from

the normal stress which exerted by soil on the inclusion. There are four possible

actions of this force: tension, compression , shear and bending. For general slope,

bending and shear are commonly used in slope soil nail design.


21

3.4 Differences between Soil nail and Soil anchor

The soil nail and soil anchor are similar in structure. Both of them are able to take

tension and resist the soil mass sliding as the earth retailing structure. However, they

are two different types of structural. Significant conceptual differences exist, as

described in the following section.

Soil Anchor –Anchor structure for slope stability which is only able to resist tension

forces. The nail or tendon are usually are Prestressed in a high loads. According to Das,

(1990) described that Soil anchor nails contain two parts: Free length and Bonded

length. Free length usually are ungrouted length or un-bonded, and bonded length

usually are grouted or bonded into the soil (Das, 1990). In this type of structure,

tendons are taking the tensile force , which is transmitted from the anchor head to the

anchorage zone. As the tendon is located in the free length, it does not have any grout

protection. The corrosion protection control of tendon is very important for this

reason.

Soil nail- Soil nails involve the rigid reinforcing of a soil mass. These nails can resist

tension, shear forces and bending moment which imposed by slope movement. The

nail inside the soil is fully grouted and usually Non-prestressed and relatively closely

spaced. No force will act on soil nail system until the soil mass failure. Usually soil

nails involve a more simplistic installation technique than soil anchors and are easier

to construct.

3.4.1 Maintenance

Typically, soil anchors need to keep the tensile force in the tendon at a constant level.

Many factors can trigger the prestress loss. Therefore, maintenance of re-prestress

process may be necessary and thus, result in an increase in the overall maintenance

cost. On the other hand, soil nail reinforcement bars inside of the soil are fully

protected by cement grout and are usually non-prestressed. If the reinforcement bar

corrosion protection control keeps the nail in good condition, the soil nail needn’t be

actively maintenaned, hence reducing the maintenance cost.


22

3.5 Soil nail application in different construction purposes

In global construction, soil nails are widely used in construction sites as an anchor

system. Not only are soil anchors used as a slope stability retaining structure, but also

for other purposes as follows

� Tie- back wall – In this case, the soil nails are used to provide a tension force to

the back of the wall to increase the passive pressure of retaining wall system. It’s

conceptually very similar to geo-synthetic soil nail (Ortigao, 2004). In order to

minimize wall movement and ground settlement, tieback walls are designed to

achieve an efficient earth retaining structure within economical considerations.

� Ground Anchor – Using soil nails to provide the tensile force in the ground.

Typically, they are used to prevent the overturning or floatation of structures such

as footing or structures in water.

� Deep excavation support - Usually this involves using soil nails as a temporary

measure for deep excavation stabilisation. Similar, to the tie-back wall method, in

deep excavation, vibration sheet pile will be installed for supporting the vertical

cut slope. The deeper the excavation , the higher the active pressure that will be

generated and act on the pile wall. Therefore, structural supporting on upper

portions of the pile are necessary. Soil nails can provide these external tensile

forces to help resist the deformation of pile wall.

Figure 24

Typical tie-back for deep excavation

(deepexcavation.org , 2008)

Figure 25

Typical permanent Tie-back wall

(Office of Geotechnical, California,

2008)


23

3.6 Advantages of Soil nailing for Slope Stability

Soil nailing presents the following advantages that have be contributed to the

widespread use of this technique in many countries in more recent times.

� Economy : Steel bar reinforcement is inexpensive. The concrete or shotcrete for

the soil nail head is relatively small and inexpensive. Construction techniques are

simple and quick. Skilled labor can be minimized. According to Cornforth (2005)

mentioned that, soil nailing can result in a cost saving of 10 to 30 percent when

compared to tieback walls.

� Rate of construction: Fast rates of construction can be achieved if adequate

equipment is employed.

� Light construction equipment: Soil nailing can be done using a conventional

drilling rig and grouting equipment. Thus, equipment can be delivered to site

easily even in areas with difficult access or limited working space constraints.

� Adaptability to different soil type : Soil nails can still be used in heterogeneous

ground where boulders or hard rocks may be encountered in the soil slope. Soil

nailing generally is more feasible than other techniques. This is because it

involves only small-diameter drilling for the installation of the inclusions.

� Flexibility : Soil nailing retaining structures are more flexible than classical

cast-in-place reinforced concrete retaining structures. Soil nails can be

incorporated with other earth retaining system such as Tie-back wall, Skill wall

etc. Also soil nails can limit the deformation or settlement in the vicinity of

existing structures such as a foundation (Cornforth, 2005). This characteristic of

soil nailing can help to provide economical retaining structures on unstable

slopes.

� Reinforcement redundancy: Based on Ortigao (2004) stated that, if any one soil

nail becomes overstressed for any reason, it will not cause failure of the slope. It

will redistribute the overstress to the adjoining nails system.


24

3.7 Limitation of Soil ail

Although soil nails are widely use for slope stability, there are some limitations

regarding the application of soil nail.

� Unsuitable soil: Cohesionless soil slopes are not suitable for soil nails for

increasing slope stability. This is because during the drilling of the hole, the

un-grouted hole may collapse. Usually, casing drilling may be applied during the

drilling process.

� Groundwater: Soil nailing has to occur above groundwater level. When soil nail

holes are drilled, the drilled hole may collapse because hole surfacing soil is

saturated or is filled with water. Therefore, a drilled hole cannot support itself and

in result the hole will collapse. Furthermore, when the soil nails are being grouted,

groundwater inside the drilled hole may affect the water/cement ratio of the

cement grout. This may affect the grout quality and reduce the cement grout

strain capabilities.

� Utilities: soil nails are drilled inside the slope. Behind of slope may contain

utilities such as buried water pipes, underground cables and drainage systems.

There are some limitations that state that soil nails must have a safe distance

between soil nails and these utilities. Therefore, a soil nail must change its

inclination or length or spacing to achieve this distance.

� Vibration sensitive structure: During the drilling procedure, vibration may

occur and cannot be avoided. Some building structures are vibration sensitive

such as Historical Buildings. Therefore, soil nailing is not the suitable method for

slope improvement in these cases.

� Rock base slope: Some cut slope contain only few meters of top soil. During site

investigation the deep layer soil type or a large boulder may be undetected (which

would be possible with ground investigation, indicating it’s importance). When

drilling the soil nail holes and the rock layer is reached, dust and stone powder

may affect the environment and public health.


25

4.0 Construction method of soil nails

4.1 The Compound of the Soil ails

These following component parts are the main elements of the soil nail system:

� Soil nail reinforcement – Steel galvanized bar which usually would have a high

grade 500N or equivalent strength capability, usually a galvanized steel, deformed

bar.

� Centralizers – PVC material which are fixed to the soil nail and ensure that soil nail

is centered in the drill hole.

� Grout tube – Use to transfer the cement grout from grouting machine to the bottom

of soil nail.

� Steel Plate – A square shape steel plate which use to transfer the bearing from soil

nail to the soil nail head.

� Steel �uts – Used to fix the steel plate on the soil nail steel bar. Usually each soil

nail contains 2 steel nuts to fix the position of steel plate.

� Soil nail head – A square shape concrete structure which includes the steel plate,

steel nuts, and soil nail head reinforcement. This part of structure provides the soil

nail bearing strength, and transfers bearing loads from the soil mass to soil nail.

Figure 26 Soil nail reinforcement bar

Figure 27 Typical Centralisers

Figure 28

Steel plate and Steel nuts

Figure 29

Typical soil nail head reinforcement


26

4.2 Introduce the Soil ail Construction Equipment

Some special equipment is using in soil nail construction. The following details

describe the essential equipment involved in soil nail construction work.

Figure 30 Drilling Rig

Drilling rig is a machine which creates holes in

the slope. Most of them are powered by

compressed air. For soil nail purposes, the

length of drilling rig is about 2m max

length .Therefore, it has a small size, easy

delivery and the benefit of high level of

mobility.

(More drilling methods in Table 3)

Figure 31 Air compressor

Air compressor is a machine that provides

compressed air to drilling rig for the power

source. It also provides the air wash through the

drilling bit which spreads air pressure inside the

hole to remove the soil debris.

Figure 32 Grouting machine

Grouting machine is a machine that provides

the grout material and pump into soil nail drill

hole. It contains two tanks, one is a mixing tank

which used to mix the cement and water to

form the liquid grout. The other one is a holding

tank which is used to store the grout from

mixing tank and high pressure pump to hole

through grout tube.

Figure 33 Shotcrete machine

Shotcrete machine is used to construct the

soil nail head. For some steep slope and

high slope, traditional case in situ concrete

method is not suitable for concreting the

soil nail head. Therefore, Shotcrete is the

most suitable in concreting soil nail work

because of its flexibility and mobility.


27

4.3 Soil ail construction procedure summary

This section will discuss some of the more practical aspects of soil nailing

construction and some of the various techniques available for soil nail installation.

Soil nail installation involves different parts of a procedure as follows:

Trial test process:

� Constructed a trial soil nail (Test nail) for a pull-out test to ensure the soil

condition is the same as in the design assumption.

Preparation work:

� Erect and setup the working platform.

� Setup all necessary equipment

� Fix the centralisers and grout tube on the steel reinforcement.

Drilling process:

� Use air wash drilling machine or coring machine to drill the hole for the soil nail

(Hole diameter approximate 100mm to 150mm)

Installation the soil nail:

� Use pressure air wash to ensure the drilled hole is clear.

� Install the soil nail steel reinforcement into drilled hole.

Grouting process:

� Use grouting machine to mix the cement and water to a designed water/cement

ratio. Bleeding test must be carried out in this stage.

� Grouting the soil nail from bottom to top with a suitable grout pressure.

� Flow cone test must be carried out during the grouting process.

Testing process:

� Random proving tests must be carried out after 3 to 7 days of the grouting

process to prove the soil nail can withstand the acceptance load.

Soil nail head construction:

� After the cement grout hardens, install the nuts and steel plate into the soil nail

steel bar.

� Install the soil nail reinforcement and concreting the soil nail head structure.


28

4.4 The Major procedure of Soil nails construction

4.4.1 Setting out of soil nail position

The soil nail position should be located identical to that of the working plan, although

soil nail locations may be offset within the tolerances of specification agreed by design

engineering. Before the drilling stage, cable detection and visual inspection shall be

provided to avoid any buried utilities.

4.4.2 Drilling

The drilling rig is the equipment for drilling the soil nail drill hole. Before starting the

drilling process, the inclination and angle of drill mast should be checked to ensure

that they are the same as the design specified angle. Moreover, the drilling bit also

needs to meet the diameter requirement according the specification. Usually, most soil

nail drilling uses a drill rig machine due to its easy delivery and simple maintenance

schedule. Some of them use a small hydraulic and mobile platform. But this

equipment is very expensive for operation, especially if the site is not accessible or the

space of working area is not available for drilling rig. Coring machines are also

suitable for coring the soil nail hole. Traditional drilling rigs are powered by

compressed air which is provided by an air compressor machine. Compressed air has

two functions in the drilling rig. One is as a power source to drive the rotation of the

drill bit and push the drilling mast inward into the slope. The other function is when

the drilling rig is operating; the compressed air can force the soil debris out of the hole.

The drilled hole diameter usually should be in the range of 100mm to 150mm,

depending on the size of the soil nail steel bar. In some case, an iron casing is provided

when the soil property is loose dense and the drilled hole collapses easily.

Un-grouted drill holes in soil should be kept open only for short periods of time.

According to (CEDD GS vol2 section 7, 1992) standard, un-grouted drill holes should

not kept open for four days and for only 24 hours according to the Australian Standard.

The longer the hole is left open, the greater the risk of collapse of drilled hole. If the

hole is collapsed and unable for soil nail installation, a re-drill of the holes is

necessary.

Figure 34 Mobile drilling rig Figure 35 Typical drilling rig


29

Other type of drilling method

Table 3 Other drilling method for soil nail (Elias & Juran , 1991)

Dri

ll r

ig T

yp

e

Drilling method Open

Hole Cased

Drill

hole

diameter

(mm)

Drill Bit

Type

Cutting

removal Comments

Au

ger

Lead Flight Kelley

Bar Driven Yes No

100-300 Rock,

soil ,drag

Mechanical

Hydraulic rotary

auger methods for

drilling competent

soils or weathered

rock.

Sectional solid-stem Yes No

Sectional

Hollow-stem Yes Yes

Mechanical (Air

support)

Continuous Flight

Solid-stem Yes No Mechanical

Continous Flight

Hollow-Stem Yes Yes

Mechanical (Air

support)

Ro

tary

Single-stem Air Yes No

100-200

Button,

Roller,

Drag

Compressed air

Hydraulic rotary

methods for

drilling competent

soils, rock, or

mixed ground

conditions

(Pneumatic

hammers

available)

Duplex Air Rotary Yes Yes

Sectional Solid-Stem

Augers Yes No

Sectional

Hollow-Stem Augers Yes Yes 100-300

Rock,

soil, drag

Mechanical Hydraulic rotary

auger methods for

drilling competent

soils or weathered

rock.

Mechanical (Air

support)

Air

tra

ck

Single stem air

rotary Yes No 100-300

Button,

Roller,

Drag

Compressed air

Pneumatic rotary

methods for

drilling

non-caving

competent soils or

rock


4.4.3 Soil nail steel bar installation

length and diameter must be carried out before install

(Geoguide 7, 2008). After all, the steel bar must be installed

angle with the drilled hole and handled carefully to avoid damag

and to avoid collapsing the hole

4.4.4 Grouting

In the grouting process, it must be

gap in the grouted column. Hence, the grout should be injected through

which was previously fixed to the steel bar

low pressure pump to the grout tube from

drilled hole fully contains

grout evenly and completely fills the hole from bottom to surfac

voids. To make sure the quality of grout met the specification requirement

for cement grout should be carried

described in the section on

Figure 36 Steel bar installation

Figure 37 Grouting process


Soil nail steel bar installation

According to CEDD Geoguide 7 (2008)

requirement, before the steel bar

the hole, the centraliser and grout tube should

be provided and fixed tightly to

prevent any movement which could be

by the grouting pressure. Spacing and size of

the centralisers needs to meet the requirement

of design specifications and ensure that the

steel bar is located in the center in the drill

hole. Checking of the galvaniz

must be carried out before installing the soil nail steel bar

. After all, the steel bar must be installed in the same direction and

angle with the drilled hole and handled carefully to avoid damaging the drilled hole

collapsing the hole.

Grouting is the concreting procedure

which provides the bond over the length of

nail. The grouting material should be mix of cement

and water with an acceptable water/cement ratio.

Too high or too low of a water cement ratio may

cause a higher risk of failure in the

and bleeding test. Therefore, according to CEDD

Geoguide 7 (2008) requirement, a 0.35

of water/cement ratio is common for

grouting. In some cases, Water Reducing

are added into the grout to provide a higher strength

of cement grout in order to reduce the water content.

it must be ensured that the soil nail does not have any void or

gap in the grouted column. Hence, the grout should be injected through

ed to the steel bar. The grout should be delivered with a

low pressure pump to the grout tube from the bottom of the drilled hole until the

cement grout (Geoguide 7, 2008). This will

grout evenly and completely fills the hole from bottom to surface without any air

make sure the quality of grout met the specification requirement

for cement grout should be carried out in the grouting process. The test

the section on Testing on soil nail.

Steel bar installation

Grouting process

30

According to CEDD Geoguide 7 (2008)

steel bar is installed in

and grout tube should

the steel bar to

could be caused

by the grouting pressure. Spacing and size of

to meet the requirement

and ensure that the

center in the drilled

zing, steel bar

the soil nail steel bar

same direction and

the drilled hole

procedure of the soil nail

length of the soil

nail. The grouting material should be mix of cement

acceptable water/cement ratio.

water cement ratio may

the flow cone test

according to CEDD

0.35-0.45 range

use in soil nail

educing Admixture

higher strength

of cement grout in order to reduce the water content.

not have any void or

gap in the grouted column. Hence, the grout should be injected through the grout tube

delivered with a

bottom of the drilled hole until the

ensure that the

e without any air

make sure the quality of grout met the specification requirement, some tests

grouting process. The tests will be


4.4.5 Procedure of pull-

The installation procedure of

construction procedure. However,

length. Test nails require partial grouting

Therefore, in the grouting stage

For the purpose of the test

to the free-bonded length.

4.4.6 Soil ail Head

purpose of the bearing plate is to distribute the force at

head and the steel reinforcement

Usually the steel reinforcement for

For the concreting of soil nail head, There are two common

and the other one is traditional ready mix c

depends on the height of slope, angle of slope,

material delivery. Both method

requirements of specification

bearing plates are placed at the half stage of shotcrete process to avoid

the soil nail head. Therefore, bearing plate

reaches the half depth mark

Figure 39 Typical buried soil nail

Figure 38

Excavated soil nail head


-out test for nail sample

procedure of the pull-out test nail is similar to a general soil nail

construction procedure. However, a pull-out test nail needs to provide a free bonded

. Test nails require partial grouting of the nail to develop a bonded length.

grouting stage, the grout should applied to the bonded length only.

the test, packer should be provided to limit the grout material flow

bonded length.

This part of the structure provides the soil bearing

strength and transfers bearing loads from soil mass

to soil nail. Usually the size of soil nail head about

400mm x 400mm and min depth is 250mm. In

soil nail system, there are two type of soil nail head:

Exposed soil nail head and buried soil nail head.

Exposed soil nail heads are located on

surface and buried soil nail head are buried inside

the slope surface. Therefore, excavation

heads is necessary with buried option

head mainly comprises of three components, the

bearing plate, nuts and steel reinforcement. The

bearing plate is to distribute the force at the nail end to the whole nail

steel reinforcement prevents the shrinkage and crack of soil nail.

Usually the steel reinforcement for the soil nail head is 16mm diameter mild steel.

For the concreting of soil nail head, There are two common methods. One is shotcrete

and the other one is traditional ready mix concreting. Which method will be used

on the height of slope, angle of slope, accessibility for ready mix track and

material delivery. Both methods of soil nail head production shall

of specifications through a compressive strength test. Moreover, steel

are placed at the half stage of shotcrete process to avoid

the soil nail head. Therefore, bearing plates should be installed when the shotcrete

mark of the soil nail head.

Typical buried soil nail head Figure 40 Shotcreting soil nail head

31

general soil nail

needs to provide a free bonded

bonded length.

bonded length only.

, packer should be provided to limit the grout material flow

the soil bearing

from soil mass

to soil nail. Usually the size of soil nail head about

400mm x 400mm and min depth is 250mm. In the

soil nail system, there are two type of soil nail head:

buried soil nail head.

are located on the slope

surface and buried soil nail head are buried inside

the slope surface. Therefore, excavation for soil nail

option. The soil nail

three components, the

, nuts and steel reinforcement. The

nail end to the whole nail

and crack of soil nail.

16mm diameter mild steel.

. One is shotcrete

oncreting. Which method will be used

for ready mix track and

shall meet the

pressive strength test. Moreover, steel

are placed at the half stage of shotcrete process to avoid a gap behind

should be installed when the shotcrete

Shotcreting soil nail head


32

Fig

ure

4

1 T

yp

ical

det

ail

of

soil

nai

l an

d s

oil

nai

l h

ead

(Ho

ng

Ko

ng

CE

DD

sta

nd

ard

dra

win

g,

20

08

)


33

4.5 Quality Specification

Specifications are an explicit set of requirements and standards to be satisfied , set by

design criteria on soil nail installation. It must be ensured that the soil nail

construction is up to the technical standards and met the quality requirements.

According to CEDD General Specification vol 2 (1992) and Australia standard R64

(2007), The main specifications in soil nail installation work are briefly summarised

as following:

� The tolerances on drilled hole diameters are not in excess of 10mm with

minimum thickness of grout cover being 30mm at all locations. (R64, 2007)

� The depth of the drilled hole shall not be in excess of 100mm of the designed

depth(CEDD GS Vol2, 1992)

� Maximum offset to the marked location not excess 100mm vertically and

300mm horizontally. (CEDD GS Vol2, 1992)

� The tolerances on outside diameter of centralizers on steel bar shall be within

5mm. (CEDD GS Vol2, 1992)

� The spacing of the centralisers shall not be excess 1.5m c/c in Hong Kong

(CEDD GS Vol2, 1992) standard and 2m c/c in the Australian standard.

� Soil nail installation and grouting shall be carried out within 24 hours after

the holes are drilled (Australian standard), or four days after the holes are

drilled (Hong Kong standard) (CEDD GS Vol2, 1992)

� Water used in grouting shall be clean and free from oil, acids, alkali, organic

or vegetable matter and from any ingredients harmful to steel or cement grout.

� Water temperature used in grout shall be measured at mixer and shall not be

less than 5OC and not more than 27

OC (CEDD GS Vol2, 1992)

� Cement grout shall be passed through a 2.36mm sieve aperture.

� The Grout shall be used as soon as possible after mixing and within 30

minutes of adding cement. (CEDD GS Vol2, 1992)


34

4.6 Testing on Soil ails

According to CEDD CS2 (1992) noted that different tests are needed to carry out

during the soil nail installation stage. These different tests have a main purpose of

ensuring that the quality of work is satisfactory in terms of the standard requirements.

In the whole soil nail construction process, the following tests must be carried out for

quality assurance.

� Soil nail steel bar – tensile test, bending test, re-bend test, galvanizing test.

� Cement Grout – Bleeding test, flow cone test, compressive strength test.

� Soil nail – Pull-out test (suitability test ), Proving test (acceptance test)

4.6.1 Soil nail steel bar

The soil nail steel bar is the most

essential element of soil nail system

and withstands tension, bending and

shear force in soil mass. Therefore, all

ranges of testing are needed to be

carried out before soil nail installation.

Usually, take five samples of a 1 meter

length from stock of soil nail steel bar

for test pieces.

� Tensile test

Tensile strength, yield stress and elongation are given out in these tests. According to

Hong Kong Construction Standard 2 (1995), the tensile stress shall be at least 10%

greater than the actual yield stress measured in tensile test. The acceptable

elongation shall not be over 12% of 5 time diameter of test piece in high stress steel

bar. (CEDD CS2, 1995)

� Bending Test

This is the test for bending of a steel bar to meet the bending requirements. According

to CEDD CS2 (1995) noted that the test piece shall withstand being bent through

180o around a former of a specific diameter. The test specimens shall satisfy the

requirement which states that no sign of cracks on visual examination are evident.

� Re-bend test

This test is for bending in opposite direction after same process of bending test and

acceptable require the test specimens shall not break into two pieces.

� Galvanizing test

This is the measure of the content of galvanized material which has been painted or

spread on the steel bar (CEDD CS2, 1995). The galvanized material can prevent the

corrosion of the steel bar which may be caused by ground water or saturated soil.

Figure 42 Steel bar test sample pieces


35

4.6.2 Cement Grout

Cement grout is the surrounding material of the soil nail which can protect the steel

bar against ground water and transfer the frictional force from soil to steel bar.

Therefore, cement grout quality concrete is essential.

Bleeding Test - This is the measure of

water bleeding from cement grout.

According to CEDD CS2 (1992) & AS

R64 mentioned that bleeding shall not

exceed 1% of volume at 60 minutes after

mixing when measured at 20OC temperate

in a covered 100mm diameter cylinder.

The bleeding rate is dependent on the

humidity and temperature. Therefore, a 20oC constant temperate and covered cylinder

are necessary for this test. Moreover, vibration should be avoided during the period of

test.

Flow Cone Test – According to ASTM C939 –

(2002) noted that this test is used to determine the

fluidity, or viscosity, of the grout. The fluidity is an

indication of how well the grout mix will flow when

it is pumped into the grout tube. According to CEDD

GS vol2 (1995), the grout mix should pass through

this flow cone in at least 15 seconds but should not

exceed 30 second. Grout mix that is too thick or too

viscous may not be able to be pushed through the

length of the tendon, and if the grout mix that is too

thin means the grout may contain too much water

and affect the water/cement ratio. This test is

typically required to be run twice every two hours or

randomly during grouting operations

Figure 45 Typical section of flow cone test equipment (ASTM C939, 2002)

Figure 43 Bleeding test

Figure 44 Flow cone test


36

Fig

ure

4

6 T

yp

ical

sam

ple

rec

ord

sh

eet

for

Ble

edin

g T

est

and F

low

Co

ne

Tes

t

( M

aunse

ll G

eote

chnic

al s

ervic

es L

td , 2008 )


Compressive strength test

Through the compressive strength test

of cement grout can

strength

mechanical

nail grout requirement. The specimen sample can be

100mm sided

cylinder

(1992

samples are used

sample

failure load divided by cross

and reported in force per

4.6.3 Soil nails

Soil nail tests are carried out

construction of the soil nail head stage

that the soil nail test involved two type

suitability test ) and the other one is proving test

these tests involve applying

slope and measure the degree of resistance

Therefore, the rate of load application shall be in the range of 3

Figure 48

Compressive

strength test

Figure 49 Pull out test


Compressive strength test

Figure 47 Square cement grout cube

Through the compressive strength test, the specimen sample

of cement grout can provide an indication of compressive

strength of the material which provides an indication of

mechanical and durability properties, in order to meet the soil

nail grout requirement. The specimen sample can be

100mm sided square cube or 100mm dia x

cylinder. According to CEDD General Specification vol.2

1992) and Australia standard R64(2007) required that

samples are used for square cube samples and three

samples. The compressive strength is calculated from the

failure load divided by cross-sectional area resisting the load

and reported in force per unit area.

carried out after the completed grouting stage and before the

the soil nail head stage. According to CEDD CS2 (1992) described

oil nail test involved two types of tests. One is pull-out test

suitability test ) and the other one is proving test (also call acceptance test). Both of

ing a force which is trying to pull the soil nail out of the

slope and measure the degree of resistance with the soil mass.

Pull-out test – Test on soil nail which

failure to allow the measurement of the ultimate

bond strength at the soil mass interface. This test is

a destroyable test method because the soil nail steel

bar and grout are at failure at the ultimate load.

According to CEDD GS vol2 (1995) noted that t

maximum test load should not exceed

steel bar ultimate tensile capacity, in order to avoid

any accident by sudden failure of steel bar.

Therefore, the rate of load application shall be in the range of 3-5kN/minute

37

quare cement grout cube

, the specimen sample

of compressive

an indication of the

to meet the soil

nail grout requirement. The specimen sample can be a

mm dia x 250mm long

General Specification vol.2

(2007) required that six

and three cylinder

. The compressive strength is calculated from the

sectional area resisting the load

grouting stage and before the

According to CEDD CS2 (1992) described

out test (also call

cceptance test). Both of

the soil nail out of the

est on soil nail which is taken to

measurement of the ultimate

soil mass interface. This test is

because the soil nail steel

failure at the ultimate load.

According to CEDD GS vol2 (1995) noted that the

exceed 90% of the

in order to avoid

accident by sudden failure of steel bar.

minute until this


final load is reached. Usually, the period of pull

cycle’s designed load depend

example, for the maximum test load, if the factor of safety for pull

test load must be at 150% of the allowable pull out capacity.

hold about 60mins for observed

in stages that should not

should record the overall

60mins. When the test reaches the maximum load, it should be unloaded in three

intervals.

Based on CEDD GS2 (1995) & AS R64(2007),t

� A load test should not be initiated before the grout reaches

compression strength of 25MPa in 3 days

compressive strength

GS vol2, 1995)

� A sudden failure of steel bar can cause serious accidents and should be

avoided. Therefore, a safe distance for operation from test setup and barriers

should be provided.

� If the soil nail steel bar are connected together using coupler. It should be

ensured that the connect

connection.

Acceptance criteria

1. Measured displacement

2. The test result graph tension load VS

acceptance range. (CEDD GS vol2, 1995)

Pull out test on soil nails are taken up to failure in soil. Therefore, This test also can

find out the soil geometry

The failure friction (qs) is calculated by

where D = soil nail hole diameter, L

Figure 50 Dial Gauge


Usually, the period of pull-out contains a 3 cycle

esigned load depends on the percentage of maximum working load.

the maximum test load, if the factor of safety for pull-out is 1.5, then the

150% of the allowable pull out capacity. Each cycle should be

observed the displacement. Also, the test should be carried out

in stages that should not exceed 20% of the maximum load and at each stage one

should record the overall displacement at 1-5 min intervals for at least 30mins to


Based on CEDD GS2 (1995) & AS R64(2007),there are some criteria in

A load test should not be initiated before the grout reaches

compression strength of 25MPa in 3 days, which is established

compressive strength test series with a minimum of three samples.

A sudden failure of steel bar can cause serious accidents and should be

Therefore, a safe distance for operation from test setup and barriers

should be provided.

If the soil nail steel bar are connected together using coupler. It should be

that the connection of steel bar is secure to avoid the failure

Displacement Measurement

Ortigao, (2004) mentioned that two or three gauges are

necessary to measure the displacement of soil nail

under loading. They are positioned along the axis of

the measurements and at the bottom of pull

equipment which reduced the affect of soil

compression displacement during pull-out operation.

Measured displacement stabilises under the maximum test load

The test result graph tension load VS displacement are within the range of

(CEDD GS vol2, 1995)


of failure friction.

is calculated by ��

��

diameter, Lb= bonded length , Tf= Failure tension load

38

3 cycle period. Each

of maximum working load. For

out is 1.5, then the

ach cycle should be

the displacement. Also, the test should be carried out

maximum load and at each stage one

east 30mins to


in pull-out test:

A load test should not be initiated before the grout reaches minimum

is established in a

a minimum of three samples. (CEDD

A sudden failure of steel bar can cause serious accidents and should be

Therefore, a safe distance for operation from test setup and barriers

If the soil nail steel bar are connected together using coupler. It should be

avoid the failure of the

wo or three gauges are

measure the displacement of soil nail

under loading. They are positioned along the axis of

the measurements and at the bottom of pull-out

which reduced the affect of soil

out operation.

are within the range of


Eq. (4.1)

= Failure tension load


39

Figure 51 Typical sample data sheet for Pull out test (Maunsell Geotechnical services Ltd , 2008)


40

Fig

ure

5

2 T

yp

ical

sam

ple

plo

ttin

g s

hee

t fo

r pu

ll o

ut

test

(M

aunse

ll G

eote

chnic

al s

ervic

es L

td ,2008)


41

Proofing Test

The method and equipment is similar to the pull-out test but the proofing test is not a

destroyable test method. The proofing test is used to ascertain the function of soil nail

and prove the soil nail conditions have not changed after construction. This test

indicates that the completed soil nail can safely withstand the design loads without

any excessive movement or long term creep over its service life.

This test is a single cycle test in which the load is applied in increments to a test load.

According to CEDD General Specification vol.2, 1992 and Australia standard

R64(2007), the design test load should be 150% of the design load capacity and rate

of load application shall be in range of 3-5kN/min (same as pull out test). At the

maximum test load, the period of observation shall be 60 min for displacement

measurement and elongation measurement.

Figure 53 Typical sample data sheet for proving test

(Maunsell Geotechnical services Ltd, 2008)


42

Fig

ure

54 T

yp

ical

sam

ple

plo

ttin

g s

hee

t fo

r P

rovin

g t

est

(M

aunse

ll G

eote

chnic

al s

ervic

es L

td ,

2008)


43

4.7 Other Type of Soil nail installation techniques

There are different methods of soil nail installation which are used internationally.

The common method is the drilled and grouted soil nail method as previously

described. The following list will briefly introduce the other methods of soil nail

installation.

� Drilled and grouted soil nails method

These are approximately 100mm and 150mm diameter nail holes drilled into

the slope. The space of the holes typically about 1.5m to 2m, and they are

arranged in a staggered pattern on the slope. Steel bars are placed in the center

of the holes and use the grouting method to grout the hole (Ortigao, 2004). This

method is most commonly used in Hong Kong soil nail construction projects

and also for Australian soil nail construction. This method can be used as a

temporary and permanent application. Also this method is the most mature

technology of soil nail method.

� Driven soil nail method

According to United States Federal Highway Administration (2006) described

that these install method are relatively small in diameter and are mechanically

driven into the slope. They are usually spaced approximately 1 to 1.2m apart.

This method allows for a faster installation when compared with the drill and

grout method. However, This method is not able to provide good corrosion

protection. Furthermore, this method cannot be used in narrow construction

sites. Therefore, driven nails are only used in the United States for temporary

applications. Permanent soil nail cannot be used in this method.

� Self-drilling soil nail method

Based of Oliver Freudenreich, (2008) described

that these soil nails consist of hollow bars that can

be drilled and grouted in one operation. In this

method, the grout is injected through the hollow

bar and drilling takes place at the same time.

Therefore, the grout will fill the void from top to

bottom of the drill hole. Rotary percussive drilling

techniques which are mentioned in table 3 are used

with this method. This method allows for a faster

installation which compared with drill and grout

method. Unlike with driven method, some level of

corrosion protection is provided. However, this

method is similar to the driven method, in that it

Figure 55

Self drilling

(Dipl.-Wirt.Ing, 2008)


44

cannot be used with permanent soil nails. This method is commonly used for

temporary nails.

� Jet grouted soil nail method

Ortigao, (2004) introduced that jet grouting

is used to erode the ground and allow the

hole for the nail and reach the final location.

After that, a vibro-percussion drilling

method is used to installed the nail bar.

However, if the soil is high in plasticity, or

clay or bounder inside the slope, this method

cannot be used.

� Launched soil nail method

As introduced by Soil nail launcher Ltd.,(2008), this method involves the

launching of the soil nail bar into the slope in a very high speed manner which

uses a firing mechanism machine powered by compressed air. Usually, the

launch bar diameters are around 19mm to 25mm and up to 8m in length. This

method allows for a fast installation with

little impact to project site. However, with

this method, it may be difficult to control the

length of launched nail inside the slope. Also,

this method cannot be used in highly plastic

clay material. Therefore, this method is only

used for temporary nails and widely used for

road repair and railroad-related landslides in

the United Kingdom and Western Europe.

(http://soilnaillauncher.com/dnn/ )

Figure 56

Jet grouting

(Dipl.-Wirt.Ing, 2008)

Figure 57

Soil nail launch machine

(soil nail launcher Ltd. , 2008 )


45

5.0 Design of Soil ails

5.1 Concept of Factor of Safety

Based on Cornforth, (2005) discussed that in the limit equilibrium design approach,

shear strength, pore water pressure, slope geometry and other soil and slope

properties are established in soil mechanics calculations. In slope stability

calculations, the result needs to obviously ensure that the resisting forces are greater

than the force tending to cause slope failure. According to Liu.(2008) defined that the

ratio between these relationships is called the factory of safety (FOS) (Liu.2008). In

circular slip plate method, FOS is defined as the ratio of total resisting forces to total

disturbing forces or total resisting moment. In general, the lower the quality of the

site investigation, the higher the value of the FOS, since the degree of risk is

influenced by previous experiences. Therefore, the actual magnitude of FOS used in

design will vary with requirement of material type and performance.

There are three typical of FOS definitions due to different type of analysis method

required

fFOS

ττ

= (Total Stress) Eq (5.1)

' ' tan '

required

cFOS

σ φτ

+=

(Effective stress) Eq (5.2)

Summation of resisting force=

Summation of mobilized forceFOS

Resisting moment=

Overturning momentFOS

R=

Wx

fdsFOS

τ∫ Eq (5.3)


46

5.2 Introduce the Circular slip and Method of slices

5.2.1 Circular slip analysis method

According to Liu.(2008) described, this method is the simplest circular analysis based

on assumption that a cylindrical block will fail by rotation about its circular center

and the shear strength along the failure plate is defined in an undrained condition.

Therefore, under this undrained assumption, the friction angle is assumed to be zero

( Φu=0). The FOS for this method may be analysed by taking the ratio of resisting

moment and overturning moment about the center of the circular failure plate.

The FOS for this method may be described according this equation :

Since Φu=0 , Therefore, Cu=τf

w1 w 2

L RFOS =

W PS P d P b

f × ×

χ + − −

τ

Eq (5.4)

If water pressure is below the toe of slip

plate, the equation can simplified as

follows

Figure 58

Circular slip model (Liu.(2008)

L RFOS =

W

f × ×

χ

τ

Eq (5.5)

where τf = undrained shear strength , R= radius of circular slip surface , W= Weight

of sliding mass

χ =Horizontal distance between circle center , and O = center of the sliding mass

However , Liu.(2008) mentioned that in some case , when Φu>0, this method is not

suitable for analysis in this situation because it is more complicated. Therefore, the

method of slices shall be used when Φ is not equal to zero.


47

5.2.2 Method of slices (“Ordinary” method or “Swedish” method)

From the above paragraph, the circular slip method is only applicable for the

undrained condition and when the friction angle is to be zero. However, if the

strength for cohesive soil and friction angle are to be calculated during site

investigation, lab test, etc., the distribution of the effective normal stresses along the

failure surface must be known. According to Liu. (2008) stated that this analysis is

usually carried out by discretizing the mass of failure slope into smaller slices and

treating each individual slice as a unique sliding block. This method is used by most

computer programs and many different types of program methods have been

developed, such as Janbu’s method and Morgenstern-Price method, etc. However, in

its general form, it is a complex method and therefore many procedures have been

proposed to simplify. Hence, for hand calculation purposes, the Swedish method of

slices will be used in the case study and compared with computer program results in

this report and based on Krahn,(2004) defined that the interslice forces are assumed

to be zero.

In the Swedish method of slices, it is assumed that the FOS value is the ratio of

resisting moment to disturbing moment. Any moments are taken around the centre of

slip circle plate. The equations of the FOS for this method are as follows:

total stress analysis : F � ∑ ( ��.�� ∝� .�� ∅�)�

� !

∑ �� .��∝�� !

Eq (5.6)

Effective stress analysis : F � ∑ ( �".�� ∝�#$ .�� ∅" )�

� !

∑ �� .��∝�� !

Eq( 5.7)

Figure 59 Swedish Method Model

This method does have several advantages such as different soil layers, water

pressure and surcharges can be readily taken into account in the calculations. The

distribution of forces around the failure surface is defined and the solution is in

equilibrium for the assumed interslice behaviour. However, according to Krahn,

(2004) described that this method is only the simplistic method for hand calculations,

as the interslice forces are ignored. The slice weight is only resolved into forces

which are parallel and perpendicular to the slice base. Therefore, slope analysis may

be not accurate and not the most efficient in soil nail design calculation. In

1 1

3

4

5

6 7

O


48

engineering industry, because of the unrealistic factors of safety and consequently

should not be used in geotechnical firm.

5.3 Soil nail calculation method

Soil nail calculation is an independent part of slope stability improvement design.

The result of the calculation is the only suitable soil nail parameter and the FOS of

internal stability. Over all stability analysis of soil nail applied slope need to be

calculated for final stability checking.

Australian Standard AS4678-2002 Appendix C provide the accepted standards for

soil nail design procedure. From the design, we can determine the length of soil nail

required, bar size, inclination angle and soil nail head size. All soil property data is

collected from ground investigations. For the industry based design, a trial and error

method will be used for checking the tensile stress of steel bar, the bond failure

between grout and steel bar and shear failure of soil nail. The calculation result is

based on the FOS in the internal stability analysis. The objective of the internal

stability analysis is to ensure that for any failure mechanism the outward thrust of the

soil within the failure zone is balanced by the tensile restraint of the soil nail.

FOS= Available Force / Required Force

In the procedure of soil nail design, slope parameter will be applied according to

critical section of the slope. If the slope has different critical criteria, then the design

will contain different critical sections of design. Thus, the slope will be separated into

different zones in terms of soil nail parameters.

In soil nail design, based on Hausmann (1992) and MGSL Ltd (2006) the following

equations are noted.

Maximum allowable tensile force of steel bar:

Ta = (Φfy) (d - 4)2 × π / 4 Eq ( 5.8)

where Φ = stress reduction factor according to AS 4100:1998 ,

fy= Yield stress of steel bar.

d= diameter of steel bar


49

Maximum allowable force between steel & grout:

[ β (fcu)1/2

] × p × (d - 4) × Le / SF Eq (5.9)

where β = 0.5 for type 2 deformed bars ,

fcu = cube strength of the cement grout at 7 days = 32MPa

SF = factor of safety adopted ,

Le = effective bond length (grout length)

Maximum allowable force between soil & grout:

[(πD C' + 2D Kα σν' tanΦ) Le] / SF Eq ( 5.10)

where D = diameter of the drill hole,

C’= effective cohesion of the soil

Kα=coefficient of lateral pressure(α=Inclination angle) =

1-(α/90)(1-Ko)=1-(α/90)(sinΦ)

σν'=theoritical vertical stress in soil calculated at mean depth of

reinforcement

Φ = friction angle


50

5.4 Analysis of slope stability with soil nail element

When the soil nail is applied into the slope, theoretically the slope is stabilised.

According to Abramson (2002), the instability force will transmitted across the

interslice boundaries through the soil nail element. Therefore, all forces acting on the

interslice (including the forces in the soil and water and the reinforcement forces

transmitted through the interslice boundary) are represented by a total interslice force

(Abramson, 2002). Then the element of the Factor of Safety that is the soil and

water force acting on an interslice and a designed soil nail reinforcement force can be

determined. Bromhead, (1992) mentioned that the soil and water force acting on an

interslice is calculated by subtracting the designed soil nail reinforcement force from

the calculated total interslice force. Another way to interpret the relationship of

forces in the interslice during the calculation of the factor of safety is to consider the

soil and water force acted on the interslice and the reinforcement force acted on the

interslice separately (Bromhead, 1992). However, for slope analysis purposes, this

analysis is complex and difficult to carry out by hand. Therefore, computer software

will be applied for slope reinforced analysis.

5.5 Slope stability analysis computer program - Slope/W

Slope/W is the computer software for geotechnical analysis which was developed by

GEO-SLOPE International Ltd. According to Krahn, (2004) introduced that there are

many methods of analysis which are based on general limit equilibrium methods such

as the ordinary method, bishop method, Janbu’s method, spencer method and

Morgenstern-price method etc. For industry base, Morgenstern-price method is the

most common method for slope analysis. However, based on Krahn (2004) described

that the ordinary method of slices is only used for teaching purposes and could not be

used in practice due to potential unrealistic FOS values. Therefore,

Morgenstern-price’s method will be used for case study design.

Morgenstern-Price’s method

this method was developed and improved by Morgenstern and Price (1965, 1967).

According to Ortigao (2004) introduced that the essence of the method is to divide

the sliding mass into a relatively small number of linear sections or wedges which are

vertical-sided in the conventional way. Within each of these sections, Krahn (2004)

explained that interslice forces are considered and the conditions of force equilibrium

can be satisfied taking directions normal and parallel to slip surface. Compared with

other method, Morgenstern-Price’s method is the closest to the equilibrium approach.

Therefore, this method will be used in soil nail design in order to form an economic

and efficiently design .


51

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 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:

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 - Slope height and slope angle should be 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

cracks etc. These uncertain data can influence the accuracy of the

design and affect the design assumption.


52

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.


53

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


Middle risk

Crest facility – Main Access road

Toe facility - indoor games or sport hall

Category group 2a


High risk


54

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 60o 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

bore holes relative the slope. According to these records, the borehole log had

indicated that the slope was composed of completely decomposed granite and highly

decomposed granite base on CEDD Geoguide 5 standard. The location of boreholes

and borehole log records are shown in appendix A


55

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 Geotechnical assessment

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

According to the borehole record, DH1, DH2, DH3 noted that no groundwater was

observed. Therefore, the design groundwater table adopted for this slope stability

analysis is to be estimated to be at one-third of the slope height to represent the

assumed 1 in 10 year design groundwater table.


56

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:

Table 5 Design parameter

Soil Type Unit weight γ’ KN/M3 Cohesion c’ (kPa) Friction Angle φ’

CDG 20 5 38

Figure 61 p – q plot graph

( Gold Ram Engineering and Development Limited., 2005)

Design assumptions

� Based on CEDD Geoguide 1 , the ground water table is assumed as a 1 in

10 year rainfall intensity case and the groundwater table is assumed to be at

1/3 of the slope height.

� The surcharge of the crest access road is assumed to be a 20kPa uniform

load.

� Because of a minor highly decomposed granite (HDG) soil layer at the

bottom of CDG, c’ and φ’ are assumed to be 5 and 42 respectively

0

50

100

150

200

250

300

350

400

450

500

0 100 200 300 400 500 600 700 800

q (

kP

a)

P' (kPa)

DH1

DH3


57

Figure 62 : Slope location plan & Bore hole location


58

Fig

ure

6

3 C

riti

cal

Cro

ss S

ecti

on A

-A


59

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.

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

because sliding may occur along any number of possible surfaces. Therefore,

computer generated numbers of slip are used to find out the minimum FOS which is

recommended. The minimum Factor of safety (FOS) at section A-A obtained are as

following table:

In the result, the minimum FOS for soil slope at section A-A does not meet the

minimum requirement of 1.4 (according to the Geoguide 7 (2008) standard) for the caption

slope having a Consequence-to-Life Category 1. Therefore, further slope stability

improvement work is necessary.

Section A-A

Minimum Factor of safety (FOS) 0.986

Figure 64 Critical Slip surface

Table 6 Section A-A FOS result


60

6.1.3 Hand calculation using Swedish Method of Slices

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 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

T N

Figure 65 Swedish Method of Slices analysis


61

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

method

Swedish Method of Slices

method

Factor of safety (FOS) 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 Tan Φ =0.78128

Water table below distance below ground, no pore water pressure

Slice

No

Arc

Length

Weight

(W)

Angle

α , degree Cos α Sin α N=W*cos(α) T=W*sin(α) N*tan(Φ)

1 1.04 4.04 66.00 0.41 0.91 1.64 3.69 1.28

2 1.03 12.11 63.00 0.45 0.89 5.50 10.79 4.29

3 1.03 20.18 60.00 0.50 0.87 10.09 17.48 7.88

4 0.79 27.71 58.00 0.53 0.85 14.69 23.50 11.47

5 0.69 29.63 56.00 0.56 0.83 16.57 24.57 12.95

6 0.58 26.51 54.00 0.59 0.81 15.58 21.45 12.17

7 0.58 26.30 53.00 0.60 0.80 15.83 21.01 12.37

8 0.52 28.44 51.00 0.63 0.78 17.90 22.10 13.98

9 0.52 27.05 50.00 0.64 0.77 17.38 20.72 13.58

10 0.52 25.65 49.00 0.66 0.75 16.83 19.36 13.15

11 0.52 24.26 47.00 0.68 0.73 16.55 17.74 12.93

12 0.52 22.87 46.00 0.69 0.72 15.89 16.45 12.41

13 0.52 21.48 44.00 0.72 0.69 15.45 14.92 12.07

14 0.52 20.09 43.00 0.73 0.68 14.69 13.70 11.48

15 0.48 18.23 42.00 0.74 0.67 13.55 12.20 10.59


62

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


63

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 )

Slip No FOS

1 1.112

2 0.986 (minimum)

3 1.157

4 1.367

5 1.572

Figure 66 Estimated Slip Surface

Table 9 FOS results table


64

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 FOS =1.4. For the soil nail 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

In the preliminary design, soil nail length is estimated as 8m in length for Row A to

Row E. However, when checking for shear failure of the adjacent ground (Bond

stress between soil and grout), Row A and Row B do not satisfy the safety

requirements. Therefore, the bar length is finally changed to 12m to satisfy the shear

failure adjacent ground checking.

Table 10 Soil nail parameter

Row Nail Length

(m)

Bond

Length (m)

Inclination

Angle

(degree)

Nail

Spacing

(m)

Design

resisting

force KN

A 12 3.3 15 2 55

B 12 3.8 15 2 50

C 8 4.3 15 2 20

D 8 4.2 15 2 15

E 8 5.7 15 2 8


65

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 15o

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.

Slip no.1 introduces some errors during Morgenstern and price analysis. The FOS of

slip no.1 cannot be generated in this method. The reason for this problem is the row B

soil nail design resisting force is too large and causes a force which pushes the soil

mass upward towards the berm. However, from the data sheet in Appendix C the FOS

for slip no. 1 calculated by ordinary method the FOS is 3.084. Therefore, FOS for slip

no. 1 generated by Morgenstern and price analysis is ignored in this case.

Table 11 FOS result (after soil nail installed)

Slip No FOS

1 error

2 1.534

3 1.529

(Minimum)

4 1.671

5 1.835

Figure 67 Soil nail slope FOS analysis


Soil ail detail Calculation

In order to calculate the soil nail detail parameter

method to find out the nail bar size

required resisting force. It is recommended to use

the parameters and check t

Soil ails design assumption

� Because of the caption slope is location at Hong Kong area

on BS8110 for the reinforcement

� At the preliminary design

horizontal is assumed to be

� For internal mode of failure, the following modes of failure were checked and the safety

factors were adopted as follows:

Table 12 Design Assumptions

Modes of Failure

(a) Tensile Failure of the steel bar

(b) Bond Failure between grout and steel bar

(c) Shear Failure of adjacent ground

Figure 69


Calculation

soil nail detail parameters, we need to use a

method to find out the nail bar size, nail length, bond length, inclination

force. It is recommended to use an EXCEL spreadsheet to compare

the outcome to achieve the most suitable design.

assumptions

Because of the caption slope is location at Hong Kong area, the design is ba

reinforcement bar.

design stage, the angle of inclination of soil nail from

d to be 15° downward.

For internal mode of failure, the following modes of failure were checked and the safety


Design Assumptions

Modes of Failure Min. Factor of Safety

(a) Tensile Failure of the steel bar fmax = 0.5

Failure between grout and steel bar 3

(c) Shear Failure of adjacent ground 2

Figure 68

FOS comparison

Soil nail design section detail

66

a trial and error

, inclination, spacing and

EXCEL spreadsheet to compare

most suitable design.

he design is based

he angle of inclination of soil nail from

For internal mode of failure, the following modes of failure were checked and the safety

Min. Factor of Safety

= 0.5fy

comparison


67

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 failure between grout & 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 = 15o

Unit weight of water = 9.81kN/m3

Part A - Tension Failure of the Steel Bar

Yield stress fy = 460Mpa

Maximum tensile stress = 0.5 fy = 230Mpa

Maximum allowable tensile force of steel bar

Ta = (0.5 fy) (d - 4)2 × ππππ / 4 Eq (5.8)

Force per m Width data is Trial and error from Slope/W

Bar length is use trial and error from Slope/W analysis

Table 13 Tension Failure of the Steel Bar calculation spreadsheet

Row No.

Level

(mPD)

Bar Length

(m)

L

Bar Size (d)

(mm)

D

Horizontal

Spacing

(m)

S

Force per m

Width F

(kN)

F

Force

Required

=F x S

Max.

Allowable

Tensile

Force

Ta > Tr

Tr (kN) Ta (kN) Check

Row E 58.80 8.0 25 2.0 8.00 16.00 79.66 O.K.

Row D 56.80 8.0 25 2.0 15.00 30.00 79.66 O.K.

Row C 54.80 8.0 25 2.0 20.00 40.00 79.66 O.K.

Row B 52.80 12.0 32 2.0 50.00 100.00 141.62 O.K.

Row A 50.80 12.0 32 2.0 55.00 110.00 141.62 O.K.


68

Part B - Bond Failure between Grout and Steel Bar

Cube strength of cement grout at 28 days fcu = 32Mpa

For type 2 deformed bar β = 0.5

Factor of safety adopted SF = 3

Allowable bond stress = Ultimate bond stress / SF

= [ β= [ β= [ β= [ β (fcu)1/2 ] / ] / ] / ] / SF = 942.81k /m

2

Maximum allowable force between steel and grout

= [ β β β β (fcu)1/2

] × π π π π × (d - 4) × Le / SF Eq ( 5.9)

Where Le : Effective bond length

Table 14 Bond Failure between Grout and Steel Bar calculation spreadsheet

Part C - Shear Failure of Adjacent Ground (Bond Failure between Grout and soil)

Resisting Zone - for soil nail design

Mobilisation Force, Tf = (ππππ D c' + 2 D Kαααα σσσσv' tanφφφφ) × Le

Inclination Factor, Kαααα = 1 - (αααα / 90) (1 - Kοοοο) = 1 - (αααα / 90) (sinφφφφ)

Completely Decomposed Granite (CDG)

Kαααα = 0.897

Tf =(ππππ D c' + 2 D Kαααα σσσσv' tanφφφφ) × Le = ( 1.571 + 0.140 σ σ σ σ’v ) × Le Eq( 5.10)

Table 15 Bond Failure between Grout and soil calculation spreadsheet 1

Row No.

Resisting Zone

Effective bond length in CDG layer (m)

Le

Depth to mid-point of the effective bond length (m)

CDG Zone

CDG WATER

Row E 3.30 3.40 0.00

Row D 3.80 5.30 0.00

Row C 4.30 7.20 0.00

Row B 8.20 9.70 1.40

Row A 9.70 9.40 3.00

Row No. Level

(mPD)

Bar

Length

(m)

Bar Size

(d)

(mm)

Horizontal

Spacing

(m)

Free

length La

(m)

Bond

length Le

(m)

Force per

m Width

F (kN)

Force

Required

Max.

Allowable

Force

Tmax > Tr

Tr (kN) Tmax (kN)

Row E 58.8 8.0 25 2.0 4.70 3.30 8.00 16.00 205.26 O.K.

Row D 56.8 8.0 25 2.0 4.20 3.80 15.00 30.00 236.36 O.K.

Row C 54.8 8.0 25 2.0 3.70 4.30 20.00 40.00 267.46 O.K.

Row B 52.8 12.0 32 2.0 3.80 8.20 50.00 100.00 680.06 O.K.

Row A 50.8 12.0 32 2.0 2.30 9.70 55.00 110.00 804.46 O.K.


Table 16 Bond Failure between Grout and soil

Row No.

Effective Vertical

Stress

σv (kPa)

CDG

Row E 68.00

Row D 106.00

Row C 144.00

Row B 180.27

Row A 158.57

Table 17 Final Soil Nail

Row No.

Level

(mPD)

Bar Length

(m)

Row E 58.80 8.0

Row D 56.80 8.0

Row C 54.80 8.0

Row B 52.80 12.0

Row A 50.80 12.0

6.1.6 Summary

In this case study assessment, the caption slope

condition before adding soil nail

Morgenstern and Price analysis is 0.986

(According to CEDD Geoguide

FOS of the slope is upgrade

meet the slope stability requirement

Table 18 Final result table

Section A-A

Minimum FOS

The slope is up to require FOS


Bond Failure between Grout and soil calculation spreadsheet 2

Force Mobilised Total Force

Mobilised

Force

Required Tf (kN)= ( 1.571 +

0.140 σ’v ) x Le

CDG Tf (kN) Tr (kN)

36.65 36.65 16.00

62.45 62.45 30.00

93.58 93.58 40.00

220.16 220.16 100.00

230.92 230.92 110.00

ail design schedule table

Bar Size (d)

(mm)

Horizontal

Spacing

(m)

La

(m)

Le

(m)

Force per m

Width F

(kN)

Force

Required

Tr

25 2 4.70 3.30 8.00 16.00

25 2 4.20 3.80 15.00 30.00

25 2 3.70 4.30 20.00 40.00

32 2 3.80 8.20 50.00 100.00

32 2 2.30 9.70 55.00 110.00

ase study assessment, the caption slope was in an unstable state in its initial

ore adding soil nail. The minimum FOS using SLOPE/W software under

rice analysis is 0.986. This is a smaller than the require FOS 1.4

Geoguide 7 standard ,2008). After applying the

FOS of the slope is upgraded to a minimum of 1.529 which has been increased to

slope stability requirements.

Final result table

Before Upgrading Soil nail applied

0.986 1.529

up to require FOS after soil nail installation.

69

calculation spreadsheet 2

F.O.S. F.O.S. >

2

Tf / Tr

2.29 O.K.

2.08 O.K.

2.34 O.K.

2.20 O.K.

2.10 O.K.

Force

Required

r (kN)

16.00

30.00

40.00

100.00

110.00

was in an unstable state in its initial

SLOPE/W software under

smaller than the require FOS 1.4

ing the soil nails, the

has been increased to

Soil nail applied

1.529


70

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

cause a loss of human life, as well as substantial economic losses.

Figure 70 General view of slope

Crest facility –road with moderate

vehicular traffic

Toe facility –railway

Category group 1b


High risk

Slope Background

Figure 71 Elevation View

Figure 72 Side View

The selected slope is a soil cut slope which is located at the side of a railway track

(S33o57’54.91”, E151

o05’40.62”). According to background information from a

previous information search (SRC, 2008), the caption slope was formed since 1902


71

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 45o and lower portion is

approximate 50 o

. The slope is covered with vegetation and the

surface is in good condition. The crest facility consists of a moderate vehicular

trafficked road about 3m away from the slope crest. The toe facility is a railway line

which located adjacent to the slope toe.

Visual Inspection

The site inspection on the caption slope was carried out in October 2008. During the

site observation, no seepage or leakage was observed on the slope or surrounding area.

The slope is covered with a vegetation surface and no surface erosion occurred. The

slope appears to be in good condition and no adverse signs of distress were observed.

No surface channels were found around the slope. A 1.2m high solid pile wall was

observed at toe of the slope.

Site Investigation

Because there was no previous study relative the caption slope, no previous ground

investigation record was able to be collected for this case study. All soil layers and

soil types are according the assumption under Geo standard AS4678-2002. The soil

type identification is under the field excavation test. According to the inspection of

excavated disturbed soil sample at slope toe and at top of the slope, the slope surface

is loose sand material and about 0.01 m depth is the in-situ original soil. The soil

sample is classified as silty clay. (Classification guide shown in Appendix D) The

sample collection location is shown in figure 78.

Figure 73 Silty clay at slope toe Figure 74 Silty clay at slope crest


72

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 penetrometer is a spring-loaded penetrometer. The spring is calibrated

against unconfined compressive strength (typically measured in kg/cm2). The mark at

which the indicator is located is taken as the unconfined compressive strength of the

soil.

This Penetrometer test was taken with the data from 10 relative soil layers. An

average value of 10 samples was determined and provided an estimated soil

properties. The collected data is shown in following table.

Table 19 Hand penetrometer test results

Reading kg/cm2 1 kg/cm

2 = 100kPa

Test no1 - 2.9kg/cm2 290 kPa










From this data, the average value is 2.75 kg/cm2. From relative analysis, the cohesion

value of soil will be high.

Due to a lack of more accurate borehole log information and soil laboratory data, And

due to conservative reasons, this case study will use the assumption under standard

AS4678-2002 table D4 to predict the typical soil property under the soil type

identification

Figure 75 Pocket Penetrometer Figure 76 Pocket Penetrometer


73

6.2.1 Geotechnical assessment

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.

Ground condition

According the site investigation, the soil composition of the slope toe and crest under

few centimeters is a silty clay material. Therefore, under these parameters, it is

assumed that the soil type for the slope from top to bottom is also a silty clay layer.

According to the Australian geological property, it is assumed that the bedrock is a

sandstone base.

Groundwater condition

According to site inspection noted that no groundwater and seepage was observed.

Therefore, design groundwater table adopted for slope stability analysis is to be

estimated at one-third of the slope height to represent the assumed 1 in 50 year design

groundwater table in Australia and water pipe leakage at slope crest.

Parameter for analysis

The soil strength parameters adopted for the stability analysis and soil nail design are

based on the typical soil assumptions for the collected soil samples. According to the

soil type identification the soil sample is define as a silty clay material. Hence, In the

Australian standard AS4678-2002 table D4 the soil group for silty clay is classified as

poor grade. According to the typical soil assumption in Table D4 the soil parameter

is assume the c’=0-5 kpa and φ’ =17 - 25o. Based on these assumptions, for this

caption slope use c’ =5 kpa and φ’ =25o for the soil design parameter.

The unit weight of silty clay are base on the assumption according to AS4678-2002

Table D1. The moist bulk weight of silty clay is 18kN/m3

.

Therefore, the shear strength parameters adopted in stability analysis for the caption

slope are as following list:

Table 20 Design parameter (based on assumption according AS4678-2002 )

Soil Type Unit weight γ’ kN/m3 Cohesion c’ (kPa) Friction Angle φ’

Silty Clay 18 5 25


74

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 the 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.

Figure 77 : Slope location plan

Figure 78 : Sample collection position

Caption slope

Section A-A


75

Fig

ure

7

9 C

riti

cal

Cro

ss S

ecti

on A

-A


76

6.2.2 Slope/W program Stability Analysis

The minimum FOS of slip surface is generated under Morgenstern and Price analysis

using SLOPE/W software. The soil layer distribution are made up of soil type

estimation from practical experience . All design assumptions and soil properties are

based on Australia Standard AS 4678-2002 Table D1 and Table D4.

Figure 80 Critical slip surface

The critical slip surfaces are generated under automatic grid and radius generation.

Because sliding may occur along any number of possible surfaces, computer

generation numbers of slip to find out minimum FOS are recommended. The

minimum Factor of safety (FOS) at section A-A obtained are as following table:

Table 21 Section A-A FOS result

In the result, the minimum FOS for soil slope at section A-A does not meet the

minimum requirement of 1.5 for the caption slope. Therefore, further slope stability

improvement work is necessary.

Section A-A

Minimum Factor of safety (FOS) 1.031


77

6.2.3 Hand calculation using Swedish Method of Slices

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, a hand calculation

using the method of slices to present the basic theory of slope stability analysis in the

same slip is also carried out.

This method assumes that the slip wedge is divided by vertical planes into a series of

slices of uniform width. The base of each slice is assumed to be a straight line. For

any slice, the inclination of base to horizontal is α . Slope will be divided into 6 slices

and the arc length and inclination angle of each slice is measured. Because the ground

water table is below the slip surface., no pore water pressure effects the slope and the

boundary water force can be ignored in the method of slice equations.

Figure 81 Swedish Method of Slices model


78

Swedish Method of Slices result

The following table lists the results of the hand calculations and compares with the

Slope/W calculation using Morgenstern and Price analysis.

Table 22 Two methods FOS result comparison table

From the table, the FOS is not consistent. There is about a 38% difference between

the two methods. This is because in the Method of slices all of the interslice forces

are ignored. Also, this method is only for c’=0. Therefore, some errors may occur in

this analysis. Usually the error is within the range 5-20% compare with Morgenstern

and Price method.

The calculation spreadsheet is shown in following table:

Table 23 Swedish Method of Slices Calculation Spreadsheet

Unit weight of soil= 18 friction angle Φ = 25 Tan Φ = 0.4663

Water table below distance below ground

Slice No Arc Length Weight Angle

α Cos α Sin α N=W*cos(α) T=W*sin(α) N*tan(Φ)

1 1.89 15.60 59.00 0.52 0.86 8.03 13.37 3.75

2 1.54 39.52 50.00 0.64 0.77 25.40 30.27 11.85

3 1.82 67.84 41.00 0.75 0.66 51.20 44.51 23.87

4 1.59 63.90 33.00 0.84 0.54 53.59 34.80 24.99

5 1.47 53.48 25.00 0.91 0.42 48.47 22.60 22.60

6 1.03 24.45 18.00 0.95 0.31 23.25 7.55 10.84

Σ 153.11 97.90

F= Σ(N)*tan(φ)/ΣT=w*sin(α))= 0.6394060.6394060.6394060.639406

Morgenstern and price

method

Swedish Method of Slices

method

Factor of safety (FOS) 1.031 0.639


79

6.2.4 Estimated Slip surface

At the beginning of soil nail design procedure, we need to estimate the different

shape of the slip surface to analyse the FOS. This is because if we use the minimum

FOS (1.031) to design the soil nail, the reinforced slope may have another shape of

slip surface which the FOS is smaller than 1.5. For example, if the slip no. 1 FOS is

smaller than 1 and the slope is not safe, after installing the soil nails with the bond

length being just enough to pass 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. The slope will still not be safe. Therefore, the slope will be

distributed into 4 typical different shapes of slip surface and all will be analysed in

terms of their FOS. From the analysis, the FOS for slips 1,2,3 and 4 do not satisfy the

FOS requirement of 1.5. Therefore, the soil nail must pass through slip 4 for ensue

that slip 1,2,3,4 all of them are in resisting by soil nail. ( Slope/W FOS data sheet

shown in Appendix E )

Slip No FOS

1 1.241

2 1.107 (Minimum)

3 1.118

4 1.119

Figure 82 Estimated slip surface

Table 24 FOS results table


80

6.2.5 Soil nail design

Soil nail length and bond length prediction

From the previous analysis, the minimum FOS is with slip no. 2. Other slip no. 1,3,4

have a FOS which is smaller than the required FOS of 1.5 .For the soil nail concept

prediction, if the soil nail bond length passes through the slips 3 and 4 failure plates,

the soil nail resisting force is then functional to the soil mass. Therefore, for the same

reason, the soil nail will also be functional to the slip no.1,3 and 4. On the other hand,

avoid the reinforced slope minimum FOS fallback to slip no. 3,4. Thus, the bond

length of the soil nail will start from the Slip no. 2 failure plate. In the trial length

estimation, the soil nail resisting force and soil nail length are estimated trial and

error is used to determine the up to standard FOS.

The position of the soil nail is estimated according the slope profile and slope

parameters. A 1.5m horizontal and vertical spacing with a staggered format is

recommended. The first row of soil nails is 1m from the ground and third row of soil

nails is 1m high from slope crest. Therefore, 3 rows of soil nails are formed

uniformly in 5m slope height.

Estimated design

In the preliminary design, soil nail lengths are estimated to be 8m length for Row A

to Row C. However, in checking of Shear Failure of Adjacent Ground, all of the rows

from A to B are not satisfy the requirement. Therefore, the bar length is adjusted to

12m to satisfy the shear failure adjacent ground checking.

Table 25 Soil nail parameter

Row Nail Length

(m)

Bond

Length (m)

Inclination

Angle

(degree)

Nail

Spacing

(m)

Design

resisting

force KN

A 12 10.03 15 1.5 35

B 12 9.8 15 1.5 25

C 12 10.16 15 1.5 18


1.241 1.107 1.118 1.119

2.2481.712 1.509

0

0.5

1

1.5

2

2.5

1 2 3

FOS Change


The soil property and soil

soil nail at inclination 15

forces are predicted. The

Morgenstern and Price analysis using

Slip no.1 introduces some error

slip no.1 cannot be generated

C soil nail design resisting force

mass upward to slope crest. However, from the data sheet in (Appendix E) the FOS

for slip no. 1 calculated by ordinary method the FOS is 9.831. Therefor

no. 1 generated by Morgenstern and

Figure 83 Soil nail slope FOS analysis

Figure 84 FOS


1.119

1.509

4

FOS Change

Reinforced slope

Pre-reinforced

slope


The soil property and soil profiles remain as previously modeled. Three

at inclination 15o

are added in SLOPE/W slope profile. Bond

The minimum FOS of 4 nos slip surface is generated under

analysis using SLOPE/W software.

some errors during Morgenstern and Price analysis. The FOS of

slip no.1 cannot be generated with this method. The reason of this problem

C soil nail design resisting force is too large and causes a force which


for slip no. 1 calculated by ordinary method the FOS is 9.831. Therefor

no. 1 generated by Morgenstern and Price analysis is ignored in this case.

Slip No

1

2

3

4

Soil nail slope FOS analysis

FOS comparison

Table 26

(after soil nail installed)

81

Three rows of 12m

Bond length and

is generated under

rice analysis. The FOS of

this method. The reason of this problem is the row

which pushes the soil


for slip no. 1 calculated by ordinary method the FOS is 9.831. Therefore, FOS for slip

rice analysis is ignored in this case.

FOS

error

2.248

1.712

1.509

(Minimum)

FOS results

(after soil nail installed)


Soil ail detail Calculation

In order to calculation of soil nail

method to find out the nail bar size

required resisting force. It is recommended to use

the other parameters and to ensure that the most suitable design is achieved

Soil ails design assumption

� Because of the caption slope is location at Australia area

AS4100 for reinforcement

� At the preliminary design

horizontal is assumed to be

� For internal mode of failure, the following modes of failure were checked and the safety


Table 27 Design Assumptions

Modes of Failure

(a) Tensile Failure of the steel bar

(b) Bond Failure between grout and steel bar

(c) Shear Failure of adjacent ground

Figure


Calculation

In order to calculation of soil nail parameter details, we need to use a

method to find out the nail bar size, nail length, bond length, inclination

It is recommended to use an EXCEL spreadsheet to compare

to ensure that the most suitable design is achieved

assumptions

Because of the caption slope is location at Australia area, the design is ba

for reinforcement bar.

design stage, the angle of inclination of soil nail from

d to be 15° downward.

internal mode of failure, the following modes of failure were checked and the safety


Design Assumptions

Modes of Failure Min. Factor of Safety

(a) Tensile Failure of the steel bar fmax = 0.5

Failure between grout and steel bar 3

(c) Shear Failure of adjacent ground 2

Figure 85 Soil nail design section detail

82

a trial and error

, inclination, spacing and

EXCEL spreadsheet to compare

to ensure that the most suitable design is achieved.

he design is based on

he angle of inclination of soil nail from

internal mode of failure, the following modes of failure were checked and the safety

Min. Factor of Safety

= 0.5fy


83

Soil nail design calculation for Section A-A (Final Result)

This part aims to check the bar and grout bond and tensile strength of steel bar, which

is an essential part of checking of soil nail design to avoid the failure of steel bar and

failure of the bar and grout bonding.

Design Parameters

Soil Type = Silty Clay , C’=5kPa , γ=18kN/m3

, φ ' = 25ο

Drill Hole diameter = 0.1m

Soil Nail inclination angle = 15o

Unit weight of water = 9.81 kN/m3

Part A - Tension Failure of the Steel Bar

Yield stress fy =500MPa (Assume use OneSteel for steel reinforcement)

Maximum tensile stress = 0.5 fy = 250Mpa

Maximum allowable tensile force of steel bar

Ta = (0.5 fy) (d - 4)2 × ππππ / 4 Eq (5.8)

Force per m Width data is Trial and error from Slope/W

Bar length is use trial and error from Slope/W analysis

Table 28 Tension Failure of the Steel Bar calculation spreadsheet

Row No.

Level

(mPD)

Bar Length

(m)

L

Bar Size (d)

(mm)

D

Horizontal

Spacing

(m)

S

Force per m

Width F

(kN)

F

Force

Required

=F x S

Max.

Allowable

Tensile

Force

Ta > Tr

Tr (kN) Ta (kN) Check

Row C 74.00 12.0 25 1.5 18.00 27.00 86.59 O.K.

Row B 72.50 12.0 25 1.5 25.00 37.50 86.59 O.K.

Row A 71.00 12.0 25 1.5 35.00 52.50 86.59 O.K.


84

Part B - Bond Failure between Grout and Steel Bar

Cube strength of cement grout at 28 days fcu = 32Mpa

For type 2 deformed bar β = 0.5

Factor of safety adopted SF = 3

Allowable bond stress = Ultimate bond stress / SF

= [ β= [ β= [ β= [ β (fcu)1/2 ] / ] / ] / ] / SF = 942.81k /m

2

Maximum allowable force between steel and grout

= [ β β β β (fcu)1/2

] × π π π π × (d - 4) × Le / SF Eq ( 5.9)

Where Le : Effective bond length

Table 29 Bond Failure between Grout and Steel Bar calculation spreadsheet

Part C - Shear Failure of Adjacent Ground (Bond Failure between Grout and soil)

Resisting Zone - for soil nail design

Mobilisation Force, Tf = (ππππ D c' + 2 D Kαααα σσσσv' tanφφφφ) × Le

Inclination Factor, Kαααα = 1 - (αααα / 90) (1 - Kοοοο) = 1 - (αααα / 90) (sinφφφφ)

Silty Clay

Kαααα = 0.93

Tf =(ππππ D c' + 2 D Kαααα σσσσv' tanφφφφ) × Le = ( 1.571 + 0.087 σ σ σ σ’v ) × Le Eq(5.10)

Table 30 Bond Failure between Grout and soil calculation spreadsheet 1

Row No.

Resisting Zone

Effective bond length in Silty

Clay layer (m)

Le

Depth to mid-point of the effective bond length (m)

Silty Clay Zone

Silty Clay WATER

Row C 10.03 2.62 0.00

Row B 9.80 4.34 0.58

Row A 6.78 5.35 1.23

Row No. Level

(mPD)

Bar

Length

(m)

Bar Size

(d)

(mm)

Horizontal

Spacing

(m)

Free

length La

(m)

Bond

length Le

(m)

Force per

m Width

F (kN)

Force

Required

Max.

Allowable

Force

Tmax > Tr

Tr (kN) Tmax (kN)

Row C 74.0 12.0 25 1.5 1.97 10.03 18.00 27.00 623.87 O.K.

Row B 72.5 12.0 25 1.5 2.20 9.80 25.00 37.5 609.56 O.K.

Row A 71.0 12.0 25 1.5 1.84 10.16 35.00 52.5 631.96 O.K.


Table 31 Bond Failure between Grout and soil

Row No.

Effective

Vertical Stress

Force

Mobilised

σv (kPa)

Tf (kN)

( 1.571 +

0.087

x L

Silty Clay Silty Clay

Row C 47.16 56.76

Row B 72.43 76.93

Row A 84.23 60.16

Table 32 Final Soil nail

Row No.

Level

(mPD)

Bar Length

(m)

Row C 74.00 12.0

Row B 72.50 12.0

Row A 71 12.0

6.1.6 Summary

In this second case study

adding the soil nail is un-stab

the Morgenstern and Price analysis is

1.5 for slope stability. After

upgraded to minimum 1.50

Table 33 Final Result table

Section A-A

Minimum FOS

The slope is up to require FOS


Bond Failure between Grout and soil calculation spreadsheet 2

Force

Mobilised Embedded

Rock Length

L (m)

Total

Force

Mobilised

Tf (kN)

Force Required(kN)=

( 1.571 +

0.087 σ’v )

x Le

Bond in

rock

T ( kN)

Silty Clay Tr (kN)

56.76 0.00 0 56.76 27

76.93 0.00 0 76.93 37.5

60.16 3.38 530.93 591.03 52.5

nail design schedule Table

Bar Size (d)

(mm)

Horizontal

Spacing

(m)

La

(m)

Le

(m)

Force per m

Width F

(kN)

Force

Required

Tr

25 1.5 1.97 10.03 18.00 27.00

25 1.5 2.20 9.80 25.00 37.5

25 1.5 1.84 10.16 35.00 52.5

study assessment, the caption slope in its initial condition

stable. The minimum FOS using SLOPE/W software under

rice analysis is 1.031. This is smaller than the require

fter the application of the soil nails, the FOS of the

09 which meets the requirement.

Final Result table

Before Upgrading Soil nail applied

1.031 1.50

up to require FOS after soil nail installation.

85

calculation spreadsheet 2

Force Required F.O.S. F.O.S. > 2

Tr (kN) Tf / Tr

27 2.10 O.K.

37.5 2.05 O.K.

52.5 11.26 O.K.

Force

Required

r (kN)

27.00

37.5

52.5

in its initial condition before

SLOPE/W software under

smaller than the required FOS of

the FOS of the slope was

Soil nail applied

1.509


86

7.0 Conclusions

7.1 Summary and concluding remarks

Throughout this project, it has been shown that landslide hazards do not only occur in

hill sides. Many cut slopes in urban areas also may face the slope instability problem.

There are many solutions for slope improvement and stability work. However,

through the literature review we can find that soil nailing is the most efficient,

environmentally friendly, and simplest method of slope stability improvement.

Soil nail construction has been shown to be a simple technology and does not need

complex machines. Therefore, this method can provide a lower construction period

and can distribute more resources to stabilise other instable slopes.

In conclusion of the two case studies, two slopes are located at two different

geological areas. However, it was shown that the soil nail application can be used in

these two different cases. Compared which other new technologies such as

Bio-engineering, these two case studies have demonstrated that soil nailing is a

diverse method that can be applied to any type of soil, in a variety of climates and

with any slope angle. Furthermore, using software to analyse the slope is much more

accurate than hand calculations and this is the most commonly used method for slope

analysis. From the result generated by Slope/W , we can find the different of FOS

values before adding soil nails and after adding soil nails. This shows that soil nails

can upgrade the FOS for stability purposes and that all parameters of soil nails are

designable. That means that the design of soil nails can result in a higher level of

efficiency that just using a standardized design.

The city is growing and more and more new technologies for slope stability methods

are developing. However, the soil nail method can provide some unique aspects over

some other methods such as reliability and designability. Therefore, the soil nail

method may not be discontinued in the future. The more innovate design of soil nails

may combine with other new technologies such as use soil nails for slip prevention

measures and be covered with Geotexile or mulching systems for surface erosion and

shallow slide preventive measures. From these innovative technologies, There will be

more and more slope stability methods combined with soil nail technology and

Bio-Engineering technology in the future.


87

7.2 Recommendations

It should be noted that soil nailing is one of the main methods used for stabilising

medium sized slopes. Enhancing public education for the landslide hazard is the most

desirable way for preventing human loss and property damage in high landslide risk

areas. Some recommendations on increasing public awareness about landslide hazard

are described as well.

The following points are some recommended action items :

� Create a database or slope record system similar to the Hong Kong Slope

information System and classification the degree of slope hazard.

� Consider that the slope hazard information is open to public - let

householders know the risks which exist in their surrounding area.

� Improve the education about the landslide hazards and increase the public

alertness.

� Educate the public in terms of simple inspection of slopes, increasing their

slope failure alertness.

� Educate the private slope householder in terms of maintaining their slope,

such as drainage clearance and protecting the slope surface to avoid surface

erosion etc.


88

8.0 Bibliography

ASTM C939-(2002) “Standard test method for flow of grout”

Civil Engineering and Development Department (CEDD), GEOTECHNICAL

ENGINEERING OFFICE. (2000). “ Technical Guidelines on Landscape

treatment and Bio-engineering for man-made slope and retaining walls” ,

Hong Kong.


ENGINEERING OFFICE .(1992). “ General Specification Vol 2”, Hong Kong.


ENGINEERING OFFICE .(1995). “ Construction Specification 2” ,Hong

Kong.


ENGINEERING OFFICE ( 2008). “Geoguide 7 Guide to soil nail design and

Construction”, Hong Kong.


ENGINEERING OFFICE ( 2008). “Geoguide 5 Guide to Slope maintenance

and inspection”, Hong Kong.

CHENG LIU. JACK B. EVETT. (2008). “ Soils and Foundations”. 8th

edn.

Civil Engineering and Development Department (CEDD) “Manuals, Guides and R &

D Reports”, viewed 21th September 2008 – 5th

October 2008,

< http://www.cedd.gov.hk/eng/publications/manuals/index.htm >.

DERECK.CORNFORTH (2005) “ Landslides in practice: investigation, analysis,

remedial and preventive options in soils.”

E.N.BROMHEAD. (1992). “The stability of slopes”.

Emergency Management Australia Database, “ Australia Landslide historic events”

viewed 1st September 2008,

<http://www.ema.gov.au/ema/emadisasters.nsf/webEventsByCategory?OpenVie

w&Start=1&Count=30&Expand=14#14 >.


89

GEOSCIENCE AUSTRALIA , “landslide hazard”, (viewed 30th

August 2008 )

< http://www.ga.gov.au/hazards/landslide/ > .

Guide to the field description identification and classification soil (2007)

“Geotechnical site investigation procedure Manual 5/2007”, Australia .

Ground investigation report (2005), “Drill hole record”, Gold Ram Engineering

and Development Limited, Hong Kong.

Ground investigation report (1993), “Drill hole record”, Vibro Limited. , Hong

Kong.

Hong Kong Slope safety website, “Slope information system”, viewed 11th

September

2008 , < http://hkss.cedd.gov.hk/hkss/eng/whatsnew/index.htm >.

J.A.R.ORTIGAO. A.S.F.J.SAYAO (2004). “ Handbook of slope stabilisation”.

JOHN KRAHN. (2004). “Stability modelling with SLOPE/W” First Edn Revision 1 ,

GEO-SLOPE/W International, Ltd. , Canada. , pt. 7-57.

J.MICHAEL DUNCAN, STEPHEN G. WRIGHT. (2005). “Soil strength and Slope

Stability”

LEE ABRAMSON,THOMAS S.LEE , SUNIL GLENN M (2002). “ Slope stability

and stabilisation methods.”

M.R.HAUSMANN. (1992). “ Soil and rock anchorage, rock bolting, soil nailing &

dowelling “.

N.J. COPPIN , I.G.RICHARDS (1990), “ Use of Vegetation in Civil Engineering”

STANDARD AUSTRALIA (2002) “ Earth retaining structures AS 4678-2002”

Standard Australia.

United states department of agriculture, Soil bioengineering for upland slope

protection and erosion reduction (1995), Engineering Field Handbook, chapter

18 pt 4-21


90

List of Appendices

Appendix A - Previous boreholes Log record ( Case study 1)

Appendix B - Previous laboratory test record ( Case study 1)

Appendix C – Slope/w analysis data ( Case study 1)

Appendix D - Classification guide ( Case study 2)

Appendix E - Slope/w analysis data ( Case study 2)

application of soil nail method for slope stability purpose

Documents