Drained and undrained slope stability analysis using GIS on a ...

UNIVERSITY GHENT 

UNIVERSITEIT 

GENT 

INTERUNIVERSITY PROGRAMME 

MASTER OF SCIENCE IN 

PHYSICAL LAND RESOURCES 

Universiteit Gent 

Vrije Universiteit Brussel 

Belgium 

<strong>Drained</strong> <strong>and</strong> <strong>undrained</strong> <strong>slope</strong> <strong>stability</strong> 

<strong>analysis</strong> <strong>using</strong> <strong>GIS</strong> on a regional scale 

September 2005 

Promotor: Master dissertation in partial fulfilment 

Prof. F. De Smedt of the requirements for the Degree of 

Master of Science in 

Physical L<strong>and</strong> Resources 

by: Prigiarto Hokkal Yonatan

Most true it is, that I have looked on truth 

Askance <strong>and</strong> strangely; but, by all above, 

These blenches gave my heart another youth, 

And worse essays proved thee my best of love. 

Shakespeare CX 

Het is zeker waar: ik zag oprechtheid, deugd 

met een scheel oog, maar hemel, alsjeblieft, 

dit dwalen bracht mijn hart een nieuwe jeugd, 

en jij bleek op mijn pad mijn zoetste lief. 

Shakespeare CX 

Il est vrai que j’ai regardé ce qui est vrai, 

etrangement de travers, mais après tout, 

ces faux regards ont donné une autre jeunesse 

à mon coeur; et les pires essays te montrent le meilleur. 

Shakespeare CX (Pierre Jean Jouve)

Acknowledgements i 

ACKNOWLEDGEMENTS 

This thesis on “Slope Stability Analysis Using <strong>GIS</strong> on a Regional Scale” is the final output of 

my advanced study in Physical L<strong>and</strong> Resources organized by Free University Brussels (VUB) 

<strong>and</strong> University of Gent (RUG). I would like to express my deepest appreciation <strong>and</strong> thanks to 

my promoter, Prof. Dr. Ir. F. De Smedt, for his encouragement, comments, suggestions <strong>and</strong> 

constant support throughout my study period <strong>and</strong> research work. It has been a privilege <strong>and</strong> a 

pleasure to be supervised by leading researcher in the department. 

I would like to express my best appreciation to Prof. Marc Van Molle for his valued support 

in giving direction for this thesis work. My sincere thanks also go to Mr. W. Solomon Tuccu, 

Mr. Corluy Jan <strong>and</strong> Mr. Hung Le Quock for their valuable support, criticism, guidance <strong>and</strong> 

help to make this manuscript finished. I have also been fortunate to have the support of Mr. Y. 

P. Ch<strong>and</strong>ra especially for sending me information <strong>and</strong> materials needed for finishing my 

thesis. 

My gratitude also goes to Anja Cosemans for her valuable support during my study. She has 

been a computer IT advisor, a good friend <strong>and</strong> also an advisor for many technical questions 

related to my study. 

This has been a wonderful year for me to have an experience studying in Belgium. This 

experience has been more colourful with many friends that support me during my study. My 

special thanks go to all my colleagues, especially Mr. Michael Ndemo Bogonko, for sharing 

computer room <strong>and</strong> accompanying me during my thesis work. I would like also to express my 

special gratitude to my best friend Mr. Pascal Nottet for encouragement, valuable support <strong>and</strong> 

especially sharing good <strong>and</strong> bad time together. Live in Belgium has never been wonderful 

without all of you. 

I would like to express my deepest gratitude also to my aunt, Mrs. Menny Indrawaty, for 

making everything possible <strong>and</strong> supporting me for studying in Belgium. My special gratitude 

also goes to my beloved brothers, Mr. Tjaja Hokmoro Jonatan <strong>and</strong> Mr. Sugiarto Hoklay 

Yonatan for their love <strong>and</strong> encouragement. 

<strong>Drained</strong> <strong>and</strong> Undrained Slope Stability Analysis Using <strong>GIS</strong> on a Regional Scale

Abstract ii 

ABSTRACT 

This study is the continuation of the previous study done by Ram Lakhan Ray, 2004, that 

applied <strong>stability</strong> model on an area of 341 km 2 of Dhading district, Nepal. In this study, a 

spatial distributed physically based <strong>slope</strong> <strong>stability</strong> model was presented <strong>and</strong> applied on 84 km 2 

of cohesive soil, covered about 25% of the original study area. Two methods of <strong>analysis</strong> were 

performed, i.e. total <strong>and</strong> effective stress analyses <strong>and</strong> Taylor <strong>and</strong> infinite <strong>slope</strong> methods were 

applied on the <strong>analysis</strong>. Critical height <strong>and</strong> safety factor maps were produced based on those 

analyses. Steady state <strong>and</strong> quasi dynamic conditions were considered for the present study 

with varying soil thickness. For quasi dynamic conditions, wetness index was applied based 

on direct rainfall infiltrations. Slope angle of 38° <strong>and</strong> 17° can be considered as the average 

mean <strong>slope</strong> angle to cause in<strong>stability</strong> <strong>and</strong> the lower most <strong>slope</strong> angle for stable conditions, 

respectively. This value was derived from the <strong>analysis</strong> based on half saturated conditions. It 

was also concluded that this case can serve as general conditions of safety factor map at the 

site where this case also has a similar result with models based on different return periods. 

Taylor method was not applicable for this study area since this method is only applicable for 

assessing safety factor with high <strong>slope</strong> angle. For short term safety factor map, completely dry 

conditions resulted from infinite <strong>slope</strong> method can be used as a short term applications. Half 

saturated case can be considered as general <strong>and</strong> long term safety factor map as this condition 

reveals similar result as given by various return periods. This study has proved that models 

developed with infinite <strong>slope</strong> models have given the best result even with some assumption. 

Keywords: <strong>stability</strong>, total stress <strong>analysis</strong>, effective stress <strong>analysis</strong>, Taylor method, infinite 

<strong>slope</strong> method, critical height, safety factor, steady state condition, quasi dynamic condition, 

short term safety factor map, long term safety factor map. 


Table of Contents iii 

Table of Contents 

ACKNOWLEDGEMENTS ................................................................................................. i 

ABSTRACT......................................................................................................................... ii 

Table of Contents................................................................................................................ iii 

List of Figures..................................................................................................................... vi 

List of Tables ...................................................................................................................... ix 

List of Abbreviations............................................................................................................x 

CHAPTER 1 : INTRODUCTION .......................................................................................1 

1.1 General..........................................................................................................................1 

1.2 Introduction to Study Area.............................................................................................2 

1.3 Scope of the Study.........................................................................................................4 

1.4 The Objective of the Study ............................................................................................4 

CHAPTER 2 : LITERATURE REVIEW ...........................................................................5 

2.1 General..........................................................................................................................5 

2.2 Slope Failure Mechanism...............................................................................................6 

2.2.1 Internal Factors Effecting Slope In<strong>stability</strong>.........................................................8 

2.2.1.1 Slope <strong>and</strong> Gravity Force ......................................................................9 

2.2.1.2 Influence of Groundwater ....................................................................9 

2.2.2 External Triggering Events.................................................................................9 

2.3 Fundamentals of Soil Parameters .................................................................................10 

2.3.1 Principle of Effective Stress .............................................................................10 

2.3.2 Failure Criterion...............................................................................................11 

2.3.3 <strong>Drained</strong> <strong>and</strong> Undrained Strength.......................................................................11 

2.3.3.1 Undrained Strength............................................................................12 

2.3.3.2 <strong>Drained</strong> Strength................................................................................14 

2.3.3.3 Residual Strength...............................................................................15 

2.3.4 Choice Between Total <strong>and</strong> Effective Stress ......................................................16 

2.4 Stability Analysis Methods ..........................................................................................17 

2.4.1 Infinite Slopes ..................................................................................................19 

2.4.1.1 Cohesive Material in Dry Condition...................................................19 

2.4.1.2 Cohesive Material with Groundwater Effect ......................................21 


Table of Contents iv 

2.4.1.3 Cohesionless Material in Dry Condition.............................................21 

2.4.1.4 Cohesionless Material with Groundwater Effect ................................22 

2.4.2 Total Stress Analysis........................................................................................22 

2.4.3 Wedge Analysis ...............................................................................................25 

2.4.4 Non-Linear Methods ........................................................................................25 

2.4.5 Model Based on Root Cohesion .......................................................................26 

2.5 L<strong>and</strong>slide Hazard Analysis with <strong>GIS</strong>............................................................................26 

2.5.1 Model Concept.................................................................................................27 

2.5.1.1 Using Infinite Slope with Total <strong>and</strong> Effective Stress ..........................28 

2.5.1.2 Using Taylor Method.........................................................................28 

2.5.1.3 Assessment of Stability Classes .........................................................29 

2.5.2 Hydrological Model .........................................................................................30 

CHAPTER 3 : MATERIALS AND METHOD.................................................................32 

3.1 General........................................................................................................................32 

3.2 Data Availability..........................................................................................................33 

3.2.1 Available DEM <strong>and</strong> Raster Maps .....................................................................33 

3.2.2 Available Hydrological Data ............................................................................37 

3.3 Applied Methodology ..................................................................................................37 

3.3.1 Soil Parameters Determination .........................................................................38 

3.3.2 Model Development.........................................................................................41 

CHAPTER 4 : RESULT AND DISCUSSION...................................................................46 

4.1 General........................................................................................................................46 

4.2 Ground Condition at the Study Area ............................................................................46 

4.3 Critical Height Maps....................................................................................................48 

4.3.1 Based on Total Stress Analysis (TSA)..............................................................48 


4.3.1.2 Using Infinite Slope Method ..............................................................50 

4.3.2 Based on Effective Stress Analysis (ESA) ........................................................52 

4.4 Safety Factor Maps......................................................................................................55 

4.4.1 Total Stress Analysis........................................................................................55 


4.4.1.2 Using Infinite Slope Method ..............................................................57 


Table of Contents v 

4.4.2 Effective Stress Analysis..................................................................................59 

4.4.2.1 Completely Dry Condition.................................................................60 

4.4.2.2 Half Saturated Condition....................................................................63 

4.4.2.3 Fully Saturated Condition ..................................................................65 

4.4.2.4 Based on Different Return Periods.....................................................68 

4.5 Discussion ...................................................................................................................71 

4.5.1 Total <strong>and</strong> Effective Stress Analyses..................................................................71 

4.5.2 Influence of Depth............................................................................................73 

4.5.3 Slope Angle .....................................................................................................73 

4.5.4 Selection of Maps.............................................................................................74 

4.5.4.1 Critical Height Map ...........................................................................74 

4.5.4.2 Safety Factor Map..............................................................................76 

4.5.5 Comparison with Root Cohesion Method .........................................................78 

CHAPTER 5 : CONCLUSIONS AND RECOMMENDATIONS ....................................82 

5.1 Conclusions .................................................................................................................82 

5.2 Recommendations........................................................................................................84 

REFERENCES................................................................................................................... ix 

APPENDICES .................................................................................................................. xiii 


List of Figures vi 

List of Figures 

Figure 1: Sensitive L<strong>and</strong>slides Area (Ray, 2004) ...............................................................3 

Figure 2 : Simplification Mass on Slope.............................................................................7 

Figure 3 : Results of Undrained Triaxial Tests on Saturated Clay .....................................12 

Figure 4 : Relationship between su/σ ' <strong>and</strong> plasticity Index (Bjerrum <strong>and</strong> Simons, 1960) ..13 

Figure 5 : Relationship between the Natural Shear Strength of Undisturbed Clays <strong>and</strong> 

Liquidity Index (Carter <strong>and</strong> Bentley, 1991) ......................................................14 

Figure 6 : Correlation between Effective Friction Angle <strong>and</strong> Plasticity Index for Fine- 

Grained Soils (NAVFAC DM-7)......................................................................15 

Figure 7 : The Concept of Residual Shear Strength...........................................................16 

Figure 8 : Forces on element of infinite <strong>slope</strong> (Cernica, 1995) ..........................................20 

Figure 9 : Total Stress Analysis........................................................................................23 

Figure 10 : Taylor's Stability Coefficients for φu = 0 (after Craig, 2004).............................24 

Figure 11 : Location of the Study Area (Ray, 2004) ...........................................................32 

Figure 12 : Digital Elevation Model (DEM) of the Study Area (Ray, 2004)........................34 

Figure 13 : Slope Map of the Study Area............................................................................34 

Figure 14 : Soil Map of the Study Area (Ray, 2004)...........................................................35 

Figure 15 : Clayey Soil in the Study Area...........................................................................35 

Figure 16 : L<strong>and</strong> Use Map of the Study Area (Ray, 2004) ..................................................36 

Figure 17 : Flow Chart for the Present Study......................................................................42 

Figure 18 : Previous <strong>and</strong> Present Study Assumption on Soil Thickness ..............................43 

Figure 19 : Map Calculation for Stability Coefficient (Ns) .................................................43 

Figure 20 : Map Calculation for Critical Height with Taylor Method..................................44 

Figure 21 : Map Calculation for Critical Height with Infinite Slope....................................44 

Figure 22 : Map Calculation for Safety Factor with Infinite Slope <strong>and</strong> TSA .......................45 

Figure 23 : Map Calculation for Safety Factor in Dry Condition.........................................45 

Figure 24 : Percentage Area of Each Soil Type for each L<strong>and</strong> Use Types...........................47 

Figure 25 : Slope Magnitude within the L<strong>and</strong> Use Type .....................................................48 

Figure 26 : Stability Coefficient Map for Taylor Method....................................................49 

Figure 27 : Critical Height based on Taylor Method...........................................................50 


List of Figures vii 

Figure 28 : Area of Critical Height for Each Soil Types Using Lower Undrained Shear 

Strength............................................................................................................51 

Figure 29 : Area of Critical Height for Each L<strong>and</strong> Use Types Using Lower Undrained Shear 

Strength............................................................................................................51 

Figure 30 : Critical Height Map with TSA..........................................................................52 

Figure 31 : Area of Critical Height based on ESA ..............................................................53 

Figure 32 : Area of Critical Height for Each Soil Types under Different Steady State 

Conditions........................................................................................................54 

Figure 33 : Area within Safety Factor Class with Taylor Methods......................................56 

Figure 34 : Safety Factor Map of Taylor Method with H = 5 m ..........................................56 

Figure 35 : Area of Stability Class under Different Soil Thickness for Infinite Slope Method 

with TSA..........................................................................................................57 

Figure 36 : Stability Area under Different Soil Types <strong>and</strong> Thickness with Infinite Slope <strong>and</strong> 

TSA .................................................................................................................58 

Figure 37 : Range of Slope Angle against Stability Class for Different Soil Thickness .......58 

Figure 38 : Safety Factor Map with Infinite Slope Method (TSA) for H = 2 m ...................59 

Figure 39 : Area of Stability Class for Dry Condition with ESA.........................................60 

Figure 40 : Relationship between Area Occupied by Stability Class <strong>and</strong> Soil Thickness.....61 

Figure 41 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Dry Condition ......61 

Figure 42 : Range of Slope Angle against Stability Class under Different Soil Thickness 

(Dry) ................................................................................................................62 

Figure 43 : Safety Factor Map of Completely Dry Condition for H = 4 m ..........................62 

Figure 44 : Area of Stability Class for Full Saturated Condition with ESA .........................63 

Figure 45 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Half Saturated 

Condition .........................................................................................................64 


(Half) ...............................................................................................................65 

Figure 47 : Safety Factor Map of Half Saturated Condition for H=5m................................65 

Figure 48 : Area of Stability Class for Full Saturated Condition with ESA .........................66 

Figure 49 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Full Saturated 

Condition .........................................................................................................67 


List of Figures viii 


(Full)................................................................................................................67 

Figure 51 : Safety Factor Map of Full Saturated Condition for H = 6 m..............................68 

Figure 52 : Wetness Index for Various Soil Thickness <strong>and</strong> Soil Types ...............................69 

Figure 53 : Rainfall Intensity with Various Return Periods.................................................69 

Figure 54 : Area of Safety Factor with Various Return Periods...........................................70 

Figure 55 : Stable Area with Various Soil Types <strong>and</strong> Return Periods with Soil Thickness of 

2m....................................................................................................................71 

Figure 56 : Comparison between Various Method Results..................................................72 


List of Tables ix 

List of Tables 

Table 1 : Classification of L<strong>and</strong>slides (Varnes, 1975)........................................................7 

Table 2: Consistency-Strength Relationship from Field Inspection (BS 8004: 1986) ......13 

Table 3 : Methods of Analysis.........................................................................................18 

Table 4 : Stability Clases.................................................................................................30 

Table 5 : Various Types of Soils <strong>and</strong> Corresponding Slope Angle...................................36 

Table 6 : Rainfall Prediction of Study Area with SMADA 6 Software (Ray, 2004) .........37 

Table 7 : Index Properties of Soil Based on Deoja et al. (1991).......................................39 

Table 8 : Undrained Shear Strength from Various References.........................................39 

Table 9 : Effective Stress Parameters for the Study Area.................................................40 

Table 10 : Soil Parameter Used for the Analysis ...............................................................41 

Table 11 : Tabulated Area of Soil Types for each L<strong>and</strong> Use Types ...................................46 

Table 12 : Summary of Critical Height Using Taylor Method ...........................................49 

Table 13 : Summary of Critical Height <strong>using</strong> Infinite Slope Method .................................51 

Table 14 : Range of Critical Height, Area <strong>and</strong> Slope Angle...............................................52 

Table 15 : Critical Height <strong>and</strong> Slope Angle under Different Steady State Condition..........54 

Table 16 : Range of Mean Slope Angle.............................................................................74 

Table 17 : Slope Angle for Unstable <strong>and</strong> Stable Conditions ..............................................74 

Table 18 : Summary of Critical Height..............................................................................75 

Table 19 : Percentage of Total Area of Safety Factor for TSA Result................................77 

Table 20 : Percentage of Total Area of Safety Factor for ESA Result................................78 

Table 21 : Previous Study Assumption on Soil Thickness for Cohesive Soil .....................78 

Table 22 : Lower Most Slope Angle Ca<strong>using</strong> In<strong>stability</strong> for Previous <strong>and</strong> Present Study...80 

Table 23 : Summary Comparison between Previous <strong>and</strong> Present Study.............................80 


List of Abbreviations x 

DEM Digital Elevation Model 

DoR Department of Roads 

ESA Effective Stress Analysis 

FS Safety Factor 

List of Abbreviations 

<strong>GIS</strong> Geographical Information System 

Inf. Infinite Slope Method 

Mod. Moderately 

Mst. Moderately Stable 

Qst. Quasi Stable 

RCM Root Cohesion Method 

St. Stable 

TSA Total Stress Analysis 

Ust. Unstable 


Chapter 1 : Introduction 1 

CHAPTER 1 : INTRODUCTION 

1.1 General 

Slope <strong>stability</strong> is a term used to explain the general immovability performance of a <strong>slope</strong> 

under natural conditions or man-made <strong>slope</strong>. A <strong>slope</strong> may be laterally unsupported earth 

mass, natural or man-made, whose surface forms an angle with the horizontal. Hills <strong>and</strong> 

mountains, riverbanks <strong>and</strong> coastal formations, earth dams, highway cuts, trenches <strong>and</strong> the like 

are examples of <strong>slope</strong>s. Every <strong>slope</strong> experiences gravitational forces <strong>and</strong> it may also possibly 

be subjected to earthquakes, glacial forces or water pressures. In turn, these phenomena may 

be direct influences on the <strong>stability</strong> of the <strong>slope</strong>. 

A distinction should be made between natural <strong>and</strong> man-made <strong>slope</strong>s where both of the <strong>slope</strong>s 

might have different effect on the <strong>stability</strong> performance. Man-made <strong>slope</strong>s are usually under- 

human controlled where dimensions, material characteristics <strong>and</strong> strength are controlled by 

several site tests <strong>and</strong> designs to adapt favourable <strong>slope</strong>. Natural <strong>slope</strong>s, on the other h<strong>and</strong>, are 

mainly natural occurrence of <strong>slope</strong>s where materials characteristics <strong>and</strong> strengths are 

generally un-controlled. Thus, in man-made <strong>slope</strong>s, the <strong>slope</strong> is designed in such a way to 

fulfil the characteristics <strong>and</strong> strengths of the materials, while for natural <strong>slope</strong>s, an attempt is 

used to maintain the <strong>slope</strong> from failure, which is caused by external triggering factor. 

Basically, the performance of immovability of a <strong>slope</strong>, safety factor, for both man-made <strong>and</strong> 

natural <strong>slope</strong>s is evaluated in relative terms of forces ratio that withst<strong>and</strong>s the <strong>slope</strong> from 

movements against that of causes failure. Among many internal <strong>and</strong> external forces, 

gravitational <strong>and</strong> seepage forces are the internal factors that mainly cause imbalance forces in 

soil or rock structures. Gravity is the force that acts everywhere on the earth’s surface, pulling 

everything in a direction toward the centre of the earth. While seepage or pore water pressure 

causes failure due to the rapid build up of pore water pressure. 

For an embankment, the evaluation is based on the controlled characteristics of the materials 

used for the embankment <strong>and</strong> an investigation of the underlying sub soils. However, the 

situation becomes complicated when the evaluation of <strong>stability</strong> incorporates huge areas or 

regional areas. The evaluation of safety factor or l<strong>and</strong>slide over a huge areas is generally 



called as L<strong>and</strong>slide Hazard Evaluation or Mapping. Complexity of the terrain <strong>and</strong> uncertainty 

in factors affecting failure of the <strong>slope</strong> are more substantial compared to local <strong>slope</strong>s. Thus, 

the need of evaluating l<strong>and</strong>slide hazard has led to the use of Geographical Information 

Systems (<strong>GIS</strong>), which are capable to analyze regional areas based on spatial distribution. 

However, the principle used for the evaluation of l<strong>and</strong>slide hazard remains the same as in 

conventional local <strong>slope</strong>, which evaluates imbalance in forces. The different is that in spatial 

analyzes the safety factor is evaluated in a pixel. Despite the difference, many deterministic 

methods can be applied for evaluating l<strong>and</strong>slide hazard <strong>and</strong> one of the most common methods 

is so-called limit equilibrium approach. In this method, a <strong>slope</strong> may be divided into a number 

of slices <strong>and</strong> the factor of safety is computed by solving the static equilibrium equations based 

on a set of assumptions (Ray, 2004). The parameters required for <strong>analysis</strong> includes <strong>slope</strong> 

geometry <strong>and</strong> conventional soil mechanics parameters. In most cases, the accuracy generally 

depends on a proper estimation of soil parameters, hydrogeology conditions <strong>and</strong> geometric 

conditions (Burton, 1998). However, consideration on the type of <strong>analysis</strong> either drained or 

<strong>undrained</strong> cases should be carefully taken into account, because these cases determined the 

chosen of parameters to be used in the analyses <strong>and</strong> the use of the outcome safety factor map. 

As the type of <strong>analysis</strong> shows different effect on the <strong>stability</strong> result, a decision must be made 

whether to use a total or an effective stress <strong>analysis</strong> especially, in clayey soils. The choice 

generally follows from the classification of a <strong>stability</strong> problem as short or long term. Slope 

failures generally result from a change of loading on the soil <strong>and</strong> if this occurs quickly, which 

is the case in hilly or mountainous areas, the <strong>stability</strong> during <strong>and</strong> immediately after the change 

may need to be assessed. This will be particularly important if the change of loading results in 

a change of pore-water pressure in the soil mass <strong>and</strong> the change is rapid compared to the 

consolidation time for the soil (Nash, 1987). Thus, in principle a total or an effective stress 

approach could be used to analyze any <strong>slope</strong>, although, in practice, the short term <strong>stability</strong> 

problems often simpler <strong>and</strong> regardless the fluctuation of groundwater table. 

1.2 Introduction to Study Area 

This study is a part of study that has been conducted by Ram Lakhan Ray as a part of his 

fulfilment of the requirements for the Degree of Master of Science in Physical L<strong>and</strong> 



Resources in Vrije Universiteit Brussel. Thus, materials <strong>and</strong> data used for this study are 

basically collected <strong>and</strong> re-used from the previous study done by Ram Lakhan Ray. 

The study area is located at Dhading district, Nepal. Nepal is located in the heart of the 

Himalayan arc <strong>and</strong> occupies nearly one third of the mountain range (Ray, 2004) with the 

longitude of 80°04’ to 88°12’ easting <strong>and</strong> latitude of 26°22’ to 30°27 northing. The previous 

study is a part of a project called “Slope Stability Analysis <strong>using</strong> <strong>GIS</strong> on a Regional Scale”, 

which lies in the Dhusa Village in Dhading district along the Prithvi Highway leading from 

the Western <strong>and</strong> Eastern parts of the country to Kathm<strong>and</strong>u, the national capital of Nepal. The 

study area itself is located in the mountainous district in Nepal where national road 

connecting major towns in some parts of Gorkha <strong>and</strong> Chitwan districts lies within this 

mountainous area with latitude of 27°45’ to 27°52’30” northing <strong>and</strong> longitude of 84°37’30” to 

84°52’30” easting. The latitude varies from about 242 to 1922m above sea level. Detail 

explanation related to the study area can be found in “Slope Stability Using <strong>GIS</strong> on a 

Regional Scale” by Ram Lakan Ray, 2004. Figure 1 presents the sensitive area where 

l<strong>and</strong>slides are frequently occurred. 

Figure 1: Sensitive L<strong>and</strong>slides Area (Ray, 2004) 

This area has been reported as the most critical area where many major l<strong>and</strong>slides occurred. 

One of the major l<strong>and</strong>slides in this area had been located at Krishna Bhir of Dhusa along with 

the Prithvi Highway. It was also reported that every year l<strong>and</strong>slide occurs during the rainy 



season <strong>and</strong>, because of that, the major national road that connects other major districts is 

closed for several weeks. Due to the frequently occurrence of l<strong>and</strong>slides within this area, the 

government has decided to develop mitigation plan for this area. 

1.3 Scope of the Study 

This study is mainly focused on to which extend the used of total stress <strong>analysis</strong> <strong>and</strong> effective 

stress <strong>analysis</strong> applicable for the proposed study area. Since, the study area is covered both by 

cohesive <strong>and</strong> cohesionless soil, while the total stress <strong>analysis</strong> is mainly applicable for 

cohesive soil. Thus the study is conducted only on cohesive soil presented in the study area. 

Two types of <strong>analysis</strong> was performed, i.e. total <strong>and</strong> effective stress <strong>analysis</strong>, <strong>using</strong> Taylor <strong>and</strong> 

infinite <strong>slope</strong> method. Critical height <strong>and</strong> safety factor maps were produced based on those 

analyses. Steady state <strong>and</strong> quasi dynamic conditions were considered for the present study 

with varying soil thickness. For quasi dynamic conditions, wetness index was applied based 

on direct rainfall infiltrations. 

1.4 The Objective of the Study 

Stability <strong>analysis</strong> on a regional scale have been investigated <strong>and</strong> studied by many researcher. 

However, the methods <strong>and</strong> assumption used are not well explained. Therefore, the present 

study aims to find a better approach for <strong>stability</strong> <strong>analysis</strong> over a regional area. The outcome of 

the study will be helpful in planning, designing <strong>and</strong> implementing the development paradigms 

of l<strong>and</strong>slide area. 

The l<strong>and</strong>slide hazard as an outcome of this study could then be used as a guidance to assists 

planners <strong>and</strong> administrators in making decisions related to the l<strong>and</strong>slide area. Furthermore, it 

can be used as an indication of <strong>stability</strong> conditions over the study area. Risk assessment <strong>and</strong> 

measurement can be interpreted based on the outcome. This will certainly provide useful 

information of <strong>stability</strong> on a project site in the early stage where necessary remedial action 

<strong>and</strong> design can be taken to avoid <strong>slope</strong> failure. In return, a good design <strong>and</strong> remedial action 

will reduce budget <strong>and</strong> also provide security on a project <strong>and</strong> society living nearby the project. 


Chapter 2 : Literature Review 5 

CHAPTER 2 : LITERATURE REVIEW 

2.1 General 

Slides may occur in almost every conceivable manner, slowly or suddenly <strong>and</strong> with or 

without any apparent provocation. The term l<strong>and</strong>slide is commonly used to denote the 

downward <strong>and</strong> outward movements of <strong>slope</strong>-forming materials along surfaces of separation 

by falling, sliding, <strong>and</strong> flowing at a faster rate. Although l<strong>and</strong>slides are primarily associated 

with mountainous regions they can also occur in areas of low relief, especially in surface 

excavations for highways, buildings <strong>and</strong> open-pit mines. The geological history <strong>and</strong> human 

activities often cause unstable conditions that lead to <strong>slope</strong> failure. 

A quantitative assessment of the <strong>stability</strong> of a <strong>slope</strong> is clearly important when a judgement is 

needed about whether the <strong>slope</strong> is stable or not, <strong>and</strong> decisions are to be made as a 

consequence. The quantitative assessment of the <strong>stability</strong> is referred to safety factor, which is 

calculated as a ratio between forces that withst<strong>and</strong> the structural soil mass from falling or 

resisting forces <strong>and</strong> forces that causes the structural soil to failure or driving forces. 

The safety factor evaluation is depended on a number of factors <strong>and</strong> the evaluation itself 

depends on the types of <strong>analysis</strong> used. The factors affecting <strong>slope</strong> in<strong>stability</strong> are generally 

influenced by gravity forces <strong>and</strong> seepage forces (Craig, 2004), while type of <strong>analysis</strong> to be 

used is depended on whether the safety factor is considered as short or long term applications. 

According to Nash (1987) both of <strong>analysis</strong> type can be applied for any <strong>slope</strong>s, eventhough, 

the consideration taken for short term application is much simpler <strong>and</strong> regardless the seepage 

forces. 

Deterministic, or physically based, models are based on physical laws of conservation of 

mass, energy or momentum. The parameters used in these models can be determined in the 

field or in the laboratory. Most deterministic models are site-specific <strong>and</strong> do not take into 

account the spatial distribution of the input parameters. Models which take into account the 

spatial distribution of input parameters are called ‘distributed models’ (Van Westen, 1994). 

Deterministic distributed models require maps which give the spatial distribution of the input 

data. The application of deterministic models for the zonation of l<strong>and</strong>slide hazard in larger 



areas, however, has never seen a more extensive development, due to the regional variability 

of geotechnical variables such as cohesion, angle of internal friction, thickness of layers, or 

depth to groundwater. Furthermore, the calculation of safety factors over larger areas involves 

an extremely large number of calculations, which could not be executed without the use of 

<strong>GIS</strong>. 

2.2 Slope Failure Mechanism 

The <strong>slope</strong> failure occurs because of in<strong>stability</strong> forces acting on a soil or rock mass. As all 

masses on earth’s surface are affected by gravity forces, the <strong>slope</strong>s, which are geometrically 

elevated above certain latitude <strong>and</strong> have a certain degree of <strong>slope</strong>, tends to slide to lower 

latitude. Once the balance of the forces is disturbed by internal changes or external triggering 

events, the mass structures are no longer able to withst<strong>and</strong> the forces that push the mass to a 

lower position. The movements of the mass from the original positions due to imbalance 

forces is called l<strong>and</strong>slide. 

The imbalance forces occurring on the soil or rock mass can be taken place due to internal 

forces or external forces. The internal forces include strengths between particles <strong>and</strong> pore 

water pressure, while external forces are the forces that act on the structural masses due to 

triggering events such as earthquakes. The strengths between soil or rock particles are the 

forces that generally withst<strong>and</strong> the soil mass from failure. Thus, in case of gravitation force 

only that acts on the structural mass, the tangential components of gravity force to the <strong>slope</strong> 

<strong>and</strong> the shear stress are the two forces that act inversely each other. Thus, if the shear stresses 

are larger than the tangential gravity force, the structural mass will not move or deform as 

illustrated in Figure 2. 



τ 

g p 

Not Moved 

g 

g t 

(a) Gentle Slope (b) Steep Slope 

Figure 2 : Simplification Mass on Slope 

Moved 

Based on the type of mass movements, Varnes (1958) classified gravity-induced movements, 

which was based on two variables, type of materials <strong>and</strong> type of movement. Movement types 

are divided into falls, topples, slides, flows <strong>and</strong> a combination of those movements, while the 

materials are divided into two classes, i.e. rocks <strong>and</strong> engineering soils, as listed in Table 1. 

Table 1 : Classification of L<strong>and</strong>slides (Varnes, 1975) 

<strong>Drained</strong> <strong>and</strong> Undrained Slope Stability Analysis Using <strong>GIS</strong> on a Regional Scale 

τ 

g p 

Type of Material 

Type of Movement Unconsolidated Sediment or Soil 

Bedrock 

Coarse Fine 

Falls Rock Fall Debris Fall Earth Fall 

Topples Rock Topples Debris Topples Earth Topples 

Slides 

Rotational Rock Slump Debris Slump Earth Slump 

Transitional Rock Block Slide Debris Slide Earth Slide 

Flows Rock Flow Debris Flow Earth Flow 

Complex Combination of two or more types 

g 

g t


In fall movements, the movements occur by free fall or a series of leaps <strong>and</strong> bounds down the 

steep <strong>slope</strong>. The movements are relatively free <strong>and</strong> lack of a slide plane. Depending upon the 

type of <strong>slope</strong> materials involved, it may be a rock-fall, soil fall, debris fall, earth fall, boulder 

fall, etc. 

Slide type of movements occurs when the materials move as a block mass along the failure 

plane. The failure plane is created as a result of imbalance forces that act in the plane in such 

away that the shear stresses of the particles are no longer capable to resist the soil or rock 

mass. There are two types of slides as depicted in Table 1, i.e. rotational <strong>and</strong> translational 

slides. The difference between those types is the type of the failure plane, translational slides 

occur when the failure plane is a planar parallel to the surface, while rotational slides occur 

when the failure plane is a circle. 

The other two movements, topple <strong>and</strong> flow, are considered less sliding because the 

movements are progressively. Topple type of movements occurs as a result of overturning of 

the blocks rather than sliding, while flows are the movements of materials progressively 

downward. 

A distinction should be made between the factor that affects the <strong>slope</strong> <strong>stability</strong> <strong>and</strong> the 

triggering factors that caused imbalances in forces. Both of the factors are explained in the 

following sections. 

2.2.1 Internal Factors Effecting Slope In<strong>stability</strong> 

It is very important to recognize the factors that effect in<strong>stability</strong> of a <strong>slope</strong> in order to know 

the mechanism of failure <strong>and</strong> possible assessment of l<strong>and</strong>slides. The factors that are those 

which lead to a slide without any change in surface conditions, which involve unaltered 

shearing stresses in the <strong>slope</strong> material (Ramiah <strong>and</strong> Chickanagappa, 1990) is called internal 

factors. The cause of such a condition is the decrease in shearing resistance brought about by 

excess pore water pressure, material softening, breakage of cementation bonds <strong>and</strong> ion 

exchange. Thus, l<strong>and</strong>slides caused by only internal factors are affected by two major forces, 

i.e. gravity force <strong>and</strong> pore water pressures. 



2.2.1.1 Slope <strong>and</strong> Gravity Force 

The angle at which material <strong>slope</strong>s is the major determining how much of the force of gravity 

is directed down<strong>slope</strong>. If a block of rock or soil is placed on a flat surface, gravity acts 

vertically <strong>and</strong> perpendicular to the flat surface <strong>and</strong> the full force of gravity is directed 

downward onto the surface. If the <strong>slope</strong> is rotated, some of the force of gravity is directed, or 

resolved, perpendicular to the <strong>slope</strong>d surface, called normal force, <strong>and</strong> part is resolved parallel 

to the surface, called shear force. As the angle of the <strong>slope</strong>d surface increases, the force of 

gravity remains the same however the amount of that force resolved as shear force increases 

<strong>and</strong> the amount resolved as normal force decreases as shown in Figure 2. At some point the 

ratio of shear or normal force, called the coefficient of sliding friction, reaches a critical level 

<strong>and</strong> the block begins to slide down the <strong>slope</strong>. Every material <strong>and</strong> <strong>slope</strong> type has an inherent 

angle at which the material becomes unstable, called the angle of repose. Most unconsolidated 

materials, such as soil or sediment, have angles of between 30 <strong>and</strong> 40 degrees. The angle of 

repose for solid rock materials depends on the smoothness of the <strong>slope</strong>d surface <strong>and</strong> the nature 

of the rock material, <strong>and</strong> can vary from 20 – 45 degrees. 

2.2.1.2 Influence of Groundwater 

Pore water is the water held within the void spaces, or pores, in the rock or sediment. Pore 

fluid has two distinct effects on mass wasting risk. Pore water has a tendency to liquefy <strong>and</strong> 

disaggregate unconsolidated materials, such as sediment or soil. Pore water tends to 

destabilize rock layers on <strong>slope</strong>d surfaces. When pore water is under pressure it reduces the 

normal force holding rock layer stable on the <strong>slope</strong>d surface without reducing the shear force 

that causes the downward motion of the rock. 

2.2.2 External Triggering Events 

External causes are those which produce an increase of the shearing stresses at unaltered 

shearing resistance of the material. They include steepening of the <strong>slope</strong>, deposition of 

material along the edge of <strong>slope</strong>s <strong>and</strong> earthquake forces. 

Earthquakes have been reported by many researchers as the most destructive environment 

phenomena. During an earthquake, the sudden ground shaking builds up rapid imbalance 

forces in the soil or rock structures in such away that reduction in normal stress <strong>and</strong> 



consequently also shear strength may occur. In rock materials, breaking of cementation in 

discontinuities or of intact rock may also occur. 

Steepening of the <strong>slope</strong> is considered as human interaction rather than environmental effect. It 

can be occurred when a mountainous area is cut for road, tunnel, aesthetic of residential, etc. 

Modification of a <strong>slope</strong> causes changing in <strong>slope</strong> angle so that it is no longer at the angle of 

repose. Then, the mass-wasting event happens in order to restore the <strong>slope</strong> to its angle of 

repose. 

2.3 Fundamentals of Soil Parameters 

A soil can be visualized as a skeleton of solid particles enclosing continuous voids which 

contain water <strong>and</strong> or air. For the range of stresses usually encountered in practice the 

individual solid particles <strong>and</strong> water can be considered incompressible; air, on the other h<strong>and</strong>, 

is highly compressible. The volume of the soil skeleton as a whole can change due to 

rearrangement of the soil particles into new positions, mainly by rolling <strong>and</strong> sliding, with a 

corresponding change in the forces acting between particles. The actual compressibility of the 

soil skeleton will depend on the structural arrangement of the solid particles. In a fully 

saturated soil, since water is considered to be incompressible, a reduction in volume is 

possible only if some of the water can escape from the voids. In a dry or a partially saturated 

soil a reduction in volume is always possible due to compression of the air in the voids, 

provided there is scope for particle rearrangement. 

The stress-strain relationship for any material is used for analyzing the <strong>stability</strong> of structures, 

<strong>slope</strong>, foundation, etc. Shear stress can be resisted only by the skeleton of solid particles, by 

means of forces developed at the interparticle contacts. Normal stress may be resisted by the 

soil skeleton through an increase in the interparticle forces. If the soil is fully saturated, the 

water filling the voids can also withst<strong>and</strong> normal stress by an increase in pressure. 

2.3.1 Principle of Effective Stress 

Effective stress in any direction is defined as the difference between the total stress in that 

direction <strong>and</strong> the pore-water pressure. The term effective stress is, therefore, a misnomer, its 

meaning being a stress difference (Simons <strong>and</strong> Menzies, 1977). Stresses are transmitted 

through a soil both by the soil skeleton <strong>and</strong> by the pore fluid. The soil skeleton can transmit 



normal stresses <strong>and</strong> shear stresses through the interparticle contacts, but the pore fluid can 

exert only all-round pressure. It is the stresses transmitted by the soil skeleton through the 

inter particle contacts that control the strength <strong>and</strong> deformation of the soil. Where stresses 

applied to the soil are wholly supported by the pore fluid pressure, they are not felt by the 

contacts between particles <strong>and</strong> hence the soil behaviour is not affected. The effective stress 

(σ’) acting on any plane is defined by the following equation : 

σ’ = σ - u (1 ) 

in which σ is the total stress acting on the plane <strong>and</strong> u is the pore pressure. 

2.3.2 Failure Criterion 

Numerous failure criteria have been proposed for the <strong>stability</strong> <strong>analysis</strong> of soil mass, but most 

of them are borrowed from basic mechanics. Since soil is a complicated material, some stress- 

strain-time behaviour is highly non-linear. However, for practical uses the linear elastic model 

<strong>and</strong> Mohr-Coulomb criterion <strong>and</strong> their shear equation are commonly used as expressed below: 

τ = c + σ tan φ (2 ) 

where τ is the shear strength, c is the cohesion, σ is the total stress <strong>and</strong> φ is the angle of 

internal friction. Depending on the type of <strong>analysis</strong>, total or effective stress <strong>analysis</strong>, the 

parameters of c, σ <strong>and</strong> φ should be substitutes with c’, σ’ <strong>and</strong> φ’. 

2.3.3 <strong>Drained</strong> <strong>and</strong> Undrained Strength 

A distinction should be made between drained <strong>and</strong> <strong>undrained</strong> strength of cohesive materials. 

As cohesive materials or clays generally posses less permeability compared to s<strong>and</strong>, thus, the 

movement of water is restricted whenever there is change in volume. So, for clay, it needs 

years to dissipate the excess pore water pressure before the effective equilibrium is reached. 

Shortly, drained condition refers to the condition where drainage is allowed, while <strong>undrained</strong> 

condition refers to the condition where drainage is restricted. Besides, the drained <strong>and</strong> 

<strong>undrained</strong> condition of cohesive soils, it should be noted that there is a decline in strength of 

cohesive soils from its peak strength to its residual strength due to restructuring. 



(a) Triaxial Undrained Test (b) Triaxial <strong>Drained</strong> Test 

2.3.3.1 Undrained Strength 

Figure 3 : Results of Undrained Triaxial Tests on Saturated Clay 

It has been found empirically that the strength of a saturated soil is constant if its volume 

remains unchanged. This description is given in Figure 3(a) which shows the result of testing 

several identical specimens of saturated clay in a triaxial apparatus with different confining 

pressures. If no drainage is allowed, the specimens have the same <strong>undrained</strong> shear strength 

<strong>and</strong> it appears that the clay is purely cohesive. The different by an amount equal to the 

difference in confining pressures, <strong>and</strong> hence the effective stresses are the same. This 

behaviour is in contrast to what happens if the drainage is not restricted; the specimens would 

have different drainage strengths as shown in Figure 3(b). 

Normally, the drained <strong>and</strong> <strong>undrained</strong> strength are derived by laboratory test by testing a 

specimen on a triaxial compression test. Then, the drainage condition is applied on the 

specimens whether drained or <strong>undrained</strong>, the strength result is comparable to drainage 

condition. 

However, to derive drained <strong>and</strong> <strong>undrained</strong> strength from the laboratory test takes a long time 

<strong>and</strong> costs a large amount of budget. To overcome this problem, some researchers proposed 

correlation for <strong>undrained</strong> shear strength, one of them are proposed by Skempton (1957). The 

following correlation between the ratio cu/σ’ <strong>and</strong> plasticity index, Ip, for normally 

consolidated clays was proposed by Skempton (1957): 



c u 

= 

σ' 

0. 

11 

+ 

0. 

0037 ⋅ I 

British St<strong>and</strong>ard gives a rough guide of <strong>undrained</strong> shear strength in relationships with the 

consistency as shown in Table 2. Bjerrum <strong>and</strong> Simons (1960) proposed the same correlation 

as proposed by Skempton in the form of chart as shown in Figure 4. Another correlation 

proposed by Carter <strong>and</strong> Bentley (1991) correlates natural <strong>undrained</strong> shear strength <strong>and</strong> 

Liquidity Index (LI) as shown in Figure 5. 

Table 2: Consistency-Strength Relationship from Field Inspection (BS 8004: 1986) 

Consistency Field Indications 


p 

Undrained Shear 

Strength (kPa) 

Very Stiff Brittle or very tough > 150 

Stiff Con not be moulded in the 

fingers 

Firm Can be moulded in the fingers 

by strong pressure 

75 - 150 

40 - 75 

Soft Easily moulded in the fingers 20 - 40 

Very Soft Exudes between the fingers 

when squeezed in the fist 

< 20 

Figure 4 : Relationship between su/σ ' <strong>and</strong> plasticity Index (Bjerrum <strong>and</strong> Simons, 1960) 

(3 )


The <strong>undrained</strong> shear strength usually uses when a total stress <strong>analysis</strong> is used. This correlation 

explains that the relationships between <strong>undrained</strong> shear strength increases to the depth. 

Figure 5 : Relationship between the Natural Shear Strength of Undisturbed Clays <strong>and</strong> Liquidity Index 

2.3.3.2 <strong>Drained</strong> Strength 

(Carter <strong>and</strong> Bentley, 1991) 

When the water movement is not restricted, a specimen placed on triaxial compression test 

will show different strengths for different confining pressures as shown in Figure 3. By 

referring to a triaxial test, the strength parameters of cohesive soils can be obtained by means 

of consolidated-drained tests or by means of consolidated-<strong>undrained</strong> tests with pore pressure 

measurement. Correlation given by Naval Facilities Engineering Comm<strong>and</strong> (NAVFAC), 

1986, gives a good estimation on the effective angle of shearing resistance as shown in Figure 

6. 



Figure 6 : Correlation between Effective Friction Angle <strong>and</strong> Plasticity Index for Fine-Grained Soils 

2.3.3.3 Residual Strength 

(NAVFAC DM-7) 

For <strong>analysis</strong> of shear characteristics of overconsolidated soils relating to <strong>stability</strong> problems, 

ordinary shear tests are not suitable because they give too high a shear value. Skempton 

(1964) showed that the strength remaining in laboratory samples after large shearing 

displacement corresponded closely with the computed strength from actual l<strong>and</strong>slides; 

therefore, he proposed a residual strength concept as shown in Figure 7. Because of the peak 

or residual shear parameters are relatively time consuming <strong>and</strong> expensive, for practical uses 

some simple experimental equations <strong>and</strong> correlations for estimating these strength parameters 

have been proposed by numerous in investigators such as proposed by Jamiolkowski <strong>and</strong> 

Pasqualini as cited by CRRI (1979) as below: 

φ’r = 453.1 (LL -0.85 ) (4 ) 



Figure 7 : The Concept of Residual Shear Strength 

2.3.4 Choice Between Total <strong>and</strong> Effective Stress 

A decision must be made when analysing <strong>slope</strong> <strong>stability</strong> whether to use a total or an effective 

stress <strong>analysis</strong>. The choice generally follows from the classification of a <strong>stability</strong> problem as 

short or long term. Slope failures generally result from a change of loading on the soil <strong>and</strong> if 

this occurs quickly, the <strong>stability</strong> during <strong>and</strong> immediately after the change may need to be 

assessed. This will be particularly important if the change of loading results in a change of 

pore-water pressure in the soil mass <strong>and</strong> the change is rapid compared to consolidation time of 

the soil or if the loading is a natural fluctuation of groundwater levels as occurs in natural 

<strong>slope</strong>s the problem is considered to be long term. 

Theoretically, both total <strong>and</strong> effective stress analyses could be applied to analyze any <strong>slope</strong>, 

although since soils are predominantly frictional materials an effective stress <strong>analysis</strong> seems 

inherently more logical especially for the <strong>analysis</strong> of long-term problems. In practice for 

short-term <strong>stability</strong> problems a total stress <strong>analysis</strong> is often simpler <strong>and</strong> more convenient as 

there is usually difficulty in predicting pore-pressure changes. 

In specifying the shear strength parameters for a total stress <strong>analysis</strong> it is assumed that for 

saturated soils φu = 0 <strong>and</strong> cu is the <strong>undrained</strong> shear strength, i.e. the soil behaves as if it were 

purely cohesive. In an effective stress <strong>analysis</strong> the effective strength parameters, c’<strong>and</strong> φ’, are 

used <strong>and</strong> the pore pressure must be specified as an independent variable. 



Another explanation related to total <strong>and</strong> effective stress is given by the permeability of the soil 

structure. If the permeability of the soil is low, a considerable time will elapse before any 

significant dissipation of excess pore water pressure will have taken place. At the end of 

construction the soil will be virtually in the <strong>undrained</strong> condition <strong>and</strong> a total stress <strong>analysis</strong> will 

be relevant. In principle an effective stress <strong>analysis</strong> is also possible for the end-of-construction 

condition <strong>using</strong> the appropriate value of pore water pressure for this condition. However, 

because of its greater simplicity, a total stress <strong>analysis</strong> is generally used. It should be realized 

that the same factor of safety will not generally be obtained from a total stress <strong>and</strong> an effective 

stress <strong>analysis</strong> of the end-of-construction condition. In a total stress <strong>analysis</strong> it is implied that 

the pore water pressures are those for a failure condition, while in an effective stress <strong>analysis</strong> 

the pore water pressures used are those predicted for a non-failure condition. 

2.4 Stability Analysis Methods 

The <strong>stability</strong> <strong>analysis</strong> methods are categorized into two basic approaches, i.e. (1) Limit 

Equilibrium Analysis <strong>and</strong> (2) Deformation <strong>analysis</strong>, <strong>and</strong> It is also depended on the type of 

<strong>analysis</strong> used, i.e. (1) Total Stress Analysis <strong>and</strong> (2) Effective Stress Analysis. So far, limit 

equilibrium methods are the most common used for assessing <strong>slope</strong> <strong>stability</strong>, while the type of 

<strong>analysis</strong> can be used both total <strong>and</strong> effective stress <strong>analysis</strong>. 

Limit equilibrium approach postulates that the <strong>slope</strong> might fail by a mass of soil sliding on a 

failure surface. When the failure occurs, the shear strength is fully mobilized all the way along 

the failure plane, <strong>and</strong> the overall <strong>slope</strong> <strong>and</strong> each part of it are in static equilibrium. In the 

<strong>analysis</strong> of stable <strong>slope</strong>s the shear strength mobilized under equilibrium conditions is less than 

the available shear strength, <strong>and</strong> it is conventional to introduce a factor of safety F defined by: 

Available Shear Strength 

FS = (5 ) 

Shear Strength required for <strong>stability</strong> 

Equation (5) is the basic formula in assessing safety factor in limit equilibrium methods. 

Depending on the method used, the slip surfaces are usually defined <strong>and</strong> the safety factors are 

calculated based on the selected slip surface. The smallest safety factor from the defined 

failure planed is considered as the safety factor of the <strong>slope</strong>. The failure plane itself can be 



curve or plane section, thus, it is necessary to consider the likely shape of the failure surface. 

Table 3 presents the various method of limit equilibrium <strong>and</strong> their formed of failure planed. 

The chosen of <strong>analysis</strong> type determines the shear strengths should be used for the <strong>analysis</strong>. 

The shear strength of the soil is normally given by the Mohr-Coulomb failure criterion as 

follow : 

s = cu = su (for <strong>undrained</strong> total stress analyses) (6 ) 

s = c’ + σ’ tan φ’ (for drained effective stress analyses) (7 ) 

where, cu or su are the <strong>undrained</strong> shear strengths <strong>and</strong> c’ <strong>and</strong> φ’ are the effective cohesion <strong>and</strong> 

the effective friction angle, respectively. 

Table 3 : Methods of Analysis 

Method Circular Non-Circular 

Infinite Slope 

Wedge Analysis 

Total Stress Analysis 

Ordinary or Swedish 

Method 

Bishop's Method of Slices 

Janbu Simplified 

Spencer's Method 

Janbu Rigorous 

* 

* 

Assumption about 

Interslice force 

* Parallel to Slope 

* Defined Inclination 

Resultant parallel to 

base of each slice 

* (*) Horizontal 

* * Horizontal 

* (*) Constant Inclination 

* * Define thrust line 

As listed in Table 3, there are many limit equilibrium methods available; however, only linear 

<strong>and</strong> total stress <strong>analysis</strong> methods are discussed in detail. The methods of <strong>analysis</strong> which are 

most amenable to h<strong>and</strong> calculation are the infinite <strong>slope</strong> <strong>analysis</strong>, total stress <strong>analysis</strong> <strong>and</strong> the 

wedge or sliding block <strong>analysis</strong>. These methods are simple to use since in each there is a 

linear equation for the factor of safety <strong>and</strong> thus it is considered as linear methods. 



2.4.1 Infinite Slopes 

Infinite <strong>slope</strong> is one of the simplest approaches for <strong>slope</strong> <strong>stability</strong> <strong>analysis</strong>. According to 

Skempton <strong>and</strong> Delory, 1957, a l<strong>and</strong>slides of a planar mass of soil occurs in slip surface which 

is approximately parallel to the ground surface can be analyzed effectively <strong>using</strong> the infinite 

<strong>slope</strong> <strong>analysis</strong>. The name infinite-<strong>slope</strong>s is given to earth masses of constant inclinations of 

unlimited extent <strong>and</strong> uniform conditions at any given depth below the surface. Thus, in this 

<strong>analysis</strong> the soil is assumed to slide on a plane slip surface which is parallel to the ground 

surface <strong>and</strong> the <strong>slope</strong> is assumed to be infinite in extent at a certain inclination to the 

horizontal (Nash, 1987). Even though, such assumptions adopted by infinite <strong>slope</strong>s are 

realistically never taken place, infinite <strong>slope</strong> method provides a good general idea about the 

<strong>stability</strong> of a <strong>slope</strong>. Based on the type of materials <strong>and</strong> groundwater occurrence, infinite <strong>slope</strong> 

can be determined in several cases as elaborated below. 

2.4.1.1 Cohesive Material in Dry Condition 

As shown in Figure 8, a case of <strong>slope</strong> with slip failure parallel to the ground surface is applied 

with the <strong>slope</strong> is infinite extent <strong>and</strong> no seepage is assumed. The gravity force (W) of a column 

soil mass with thickness b is given by γHb. As a consequence of angle i, the weight of the 

column mass can be divided into two components namely S, the force along the inclination of 

the block <strong>and</strong> N, the force normal to the inclination of the block. Both of the force can be 

expressed as follow, while forces acting parallel to the slip surface, F1 <strong>and</strong> F2 are assumed 

equal <strong>and</strong> opposite, <strong>and</strong> are therefore ignored in the <strong>analysis</strong>. 

Normal Force (N) = W cos i = γHb cos i (8 ) 

Shear Force (S) = W sin i = γHb sin i (9 ) 



Figure 8 : Forces on element of infinite <strong>slope</strong> (Cernica, 1995) 

Resolving the two forces in Equation (8) <strong>and</strong> (9), the normal <strong>and</strong> shear stress can be derived 

by dividing the two forces by the width of the soil mass on a plane failure, which is b/cos i. 

Thus, the normal stress is given by : 

<strong>and</strong> the shear stress is given by : 

where, γ is the unit weight of soil. 

N 2 

σ = = γ H cos i 

(10 ) 

b cos i 

S 

τ = = γ H sin i cos i 

(11 ) 

b cos i 

In case of dry condition, where pore water pressure does not present, the shear resistance 

shown in Equation (7) becomes as follow : 

s = c + σ tan φ (12 ) 



where, c <strong>and</strong> φ are the cohesion <strong>and</strong> internal friction angle, respectively. Thus, substituting 

Equation (11) <strong>and</strong> (12) into Equation (5), the safety factor for this condition becomes as 

follow: 

c + σ tan φ c tan φ 

FS = = 

+ 

(13 ) 

γ H sin i cos i γ H sin i cos i tan i 

For clayey soil, it is interesting to defined a critical height (Hc) of the clay stratum, which can 

be expressed by the formula : 

c sec i 

H c = 

(14 ) 

γ tan i − tan φ 

2.4.1.2 Cohesive Material with Groundwater Effect 

For a condition with groundwater effect, the pore pressure at a depth H equals γw Hw cos 2 i. 

The effective pressure is (γ H - γw Hw) cos 2 i, where γw is the unit weight of water <strong>and</strong> Hw is the 

height of water above the failure plane. Assuming that the thickness of water above the failure 

plane equals to mH, then the shear resistance is given by : 

s = c + (γ H - γw Hw) cos 2 i tan φ 


2 

s = c + (γ H - γw mH) cos 2 i tan φ = c + (γ - γw m) H cos 2 i tan φ (15 ) 

The factor m, in the above equation termed as the wetness index gives the condition of 

saturation of the soil. If m equals to one, the soil is in a completely saturated condition while 

the value zero indicates dry conditions of the soil. Similar to the procedure described above, 

the safety factor in this condition is calculated by the following relationship. 

( γ − γ m) 

2.4.1.3 Cohesionless Material in Dry Condition 

2 

c + w H cos i tan φ 

FS = (16 ) 

γ H sin i cosi 

Cohesionless soils are completely different with cohesive soil in terms of cohesion. 

Cohesionless soils do not exhibit cohesion characteristics as in cohesive soil. Thus, in the case


of cohesionless soil in dry condition, the c <strong>and</strong> m in Equation (16) become zero <strong>and</strong> the safety 

factor for this condition is given by : 

tan φ 

FS = (17 ) 

tan i 

Equation (17) expresses that for cohesionless soil the critical angle of the <strong>slope</strong> is equal to the 

internal friction angle under dry condition. 

2.4.1.4 Cohesionless Material with Groundwater Effect 

Looking at Equation (16), for this condition, the wetness index, m, is no longer zero because 

there is an effect of groundwater table. Thus, solving Equation (16) for this condition, the 

safety factor becomes, 

2.4.2 Total Stress Analysis 

( γ − γ m) 

w tan φ 

FS = (18 ) 

γ tan i 

The permeability of clays is very much less than that of s<strong>and</strong>s <strong>and</strong> this inhibits the movement 

of water if there is tendency to change volume. As a result it may take years after a change of 

surface loading on a deposit of clay for excess pore pressures to dissipate <strong>and</strong> for the effective 

stresses to reach equilibrium. In this case, the condition of the soil is <strong>undrained</strong> where the 

excess pore water pressures are unable to dissipate. However, the shear strength of a soil is 

dependent on the effective stresses whatever the condition of drainage. Thus, when movement 

of the pore water is restricted, the pore pressure increases in a soil which is trying to contract 

<strong>and</strong> decreases in one trying to dilate. The change of pore pressure directly affects the effective 

stresses <strong>and</strong> hence the shear strength. 

When considering the field problems in which the loading or unloading occurs sufficiently 

rapidly that drainage does not occur, the <strong>undrained</strong> shear strength may be applied in the 

<strong>stability</strong> <strong>analysis</strong> when a total stress <strong>analysis</strong> is used (Nash, 1987) for clayey soil. The 

<strong>undrained</strong> shear strength of clay may be determined in the laboratory, or in-situ in the field. 



The <strong>stability</strong> <strong>analysis</strong> calculated by infinite <strong>slope</strong> for cohesive soil can be applied on total 

stress <strong>analysis</strong> by assuming the internal friction angle (φ) equals to zero. The explanation 

about this <strong>analysis</strong> is given in Section 2.5.1.1. Another method for total stress <strong>analysis</strong> is 

developed by Taylor (after Craig, 2004), which is assumed fully saturated clay under 

<strong>undrained</strong> conditions as shown in Figure 9. 

Figure 9 : Total Stress Analysis 

As shown in Figure 9, only moment equilibrium is considered in the <strong>analysis</strong> <strong>and</strong> <strong>undrained</strong> 

shear strength are used. In section, the potential failure surface is assumed to be a circular arc. 

A trial failure surface (centre O, radius r <strong>and</strong> length La) is shown in Figure 9. Thus, the safety 

factor can be expressed as follow, 

c u L a r 

FS = (19 ) 

W d 

where, cu is the <strong>undrained</strong> shear strength, La is the total length of the failure plane, r is the 

radius of the failure plane, W is the weight of the block <strong>and</strong> d is horizontal distance of the 

weight force to the centre of the circle. 



Based on the principle of geometric similarity, Taylor (after Craig, 2004) published <strong>stability</strong> 

coefficients for the <strong>analysis</strong> of homogeneous <strong>slope</strong>s in terms of total stress. For a <strong>slope</strong> of 

height H the <strong>stability</strong> coefficient (Ns) for the failure surface along which the factor of safety is 

a minimum is as follow, 

<strong>and</strong> the safety factor can be expressed as follow: 

N 

s 

c u 

= (20 ) 

FS γ H 

c u 

FS = (21 ) 

N γ H 

The coefficient Ns depends on the <strong>slope</strong> angle β <strong>and</strong> the depth factor D, where DH is the 

depth to a firm stratum. Figure 10 shows the Taylor’s <strong>stability</strong> charts. 

Figure 10 : Taylor's Stability Coefficients for φ u = 0 (after Craig, 2004) 

The use of <strong>undrained</strong> shear strength in this <strong>analysis</strong> implies that pore pressures <strong>and</strong> effective 

stresses in the soil have not had time to reach equilibrium under an applied loading. Thus it 

can be applied appropriately for natural <strong>slope</strong>s, where generally, <strong>slope</strong> in<strong>stability</strong> are caused 

by heavy rain that the rapid increases of groundwater table are not able to dissipate the excess 


s


pore water pressure. However, this method should be used with caution due to generalization 

in pore water pressures. It might be possible to use this method with assumption that the 

clayey soils are heavily impermeable <strong>and</strong> thus, the groundwater pressures are not easily 

dissipated. 

2.4.3 Wedge Analysis 

There are situation in which the slip surface can be approximated by two or three straight 

lines. This may occur when the <strong>slope</strong> is underlain by a strong stratum such as rock or there is 

a weak stratum included within or beneath the <strong>slope</strong>. In these circumstances an accurate 

assessment of the <strong>stability</strong> may be made by splitting the <strong>slope</strong> into several blocks of soil <strong>and</strong> 

examining the equilibrium of each block. 

In this method, the trial sliding mass is divided into two or three large sections or wedges. The 

upper wedge is called the driving or active wedge, while the lower wedge is called the 

resisting or passive wedge. In a three-wedge system, the middle segment is sometimes 

referred to as the sliding block. The potential failure surface is simplified to a series of planes. 

2.4.4 Non-Linear Methods 

There are numerous non-linear methods, however, all of those non-linear methods has the 

same assumption of failure plane that this method considers non-linear failure planes. One of 

these methods is called as Method of Slices. There are also many methods of slices developed 

by researcher such as General Formulation developed by Fredlund <strong>and</strong> Krahn, Bishop’s 

Routine Method, Janbu’s Simplified Method, etc. 

Despite the fact that there are many methods of slices, however, they share the same principle 

that the <strong>slope</strong> being analyzed is divided into a number of slices. First of all, an assumed non- 

linear failure plane is determined either circular or a combination between block <strong>and</strong> circular. 

Then, the slices are determined within the ground surface <strong>and</strong> the defined failure plane. The 

forces are resolved for every slice with the same principle as in infinite <strong>slope</strong>. 

Depending on the method of slices, some of them are only considers vertical forces, while 

horizontal forces occurred on both side of a slice are assumed to be equal <strong>and</strong> thus, it was 

neglected such as Bishop’s routine method. Method of slices also considers moment balance 



based on an assumed central point of sliding. The safety factor is then determined as the 

balance between forces that ca<strong>using</strong> sliding against the central point <strong>and</strong> that of withst<strong>and</strong>ing 

the block against failure. 

2.4.5 Model Based on Root Cohesion 

This method is adapted by Montgomery <strong>and</strong> Dietrich (1994), Van Westen <strong>and</strong> Terlien (1996) 

<strong>and</strong> de Vleeschauwer <strong>and</strong> De Smedt (2002), which combined the <strong>stability</strong> <strong>analysis</strong> with the 

cover type of the l<strong>and</strong>. Since, <strong>stability</strong> of a <strong>slope</strong> is not only depended on the internal factor 

but also external factors, the method adopt the effect of external factor such as surcharge 

pressure <strong>and</strong> root cohesion. By applying root cohesion, it means that the method also take into 

account the possibility of translational failure because of l<strong>and</strong> cover type. This method can be 

expressed by the following formula: 

Cs 

+ C r γ w tan φ 

FS = + 1 − m 

(22 ) 

γ Dsin 

i γ tan i 

e 

where, FS is the safety factor, Cs <strong>and</strong> Cr are the effective soil <strong>and</strong> root cohesion governed by 

the vegetation type, respectively; D is the depth of the soil above failure plane; φ is the angle 

of internal friction; i is <strong>slope</strong> angle; γw is the unit weight of water <strong>and</strong> γe is the effective unit 

weight of soil as defined by Westen <strong>and</strong> Terlien (1996). 

Actually, this method was developed based on infinite <strong>slope</strong>, however there are differences in 

assumption <strong>and</strong> the philosophy behind the formula. First, the assumption of soil depth is taken 

as the thickness of soil above the failure plane <strong>and</strong> it is perpendicular to the failure plane, 

while in ordinary infinite <strong>slope</strong> the soil depth is the vertical depth against failure plane. 

Secondly, there is a new parameter introduced in the formula that is root cohesion. By 

introducing this parameter, the formula are no longer satisfy ordinary infinite <strong>slope</strong> equation, 

but it serves as a method that takes into account erosions as a factor ca<strong>using</strong> in<strong>stability</strong> of 

<strong>slope</strong>. 

2.5 L<strong>and</strong>slide Hazard Analysis with <strong>GIS</strong> 

L<strong>and</strong>slide hazards assessment tools are becoming a popular tool not only for the disaster 

prevention or mitigation purposes but also for l<strong>and</strong> use planning, resources development <strong>and</strong> 


e


infrastructure development (Joshi, 2002). The l<strong>and</strong>slide potential mapping are becoming 

useful for watershed management <strong>and</strong> they are proving themselves a good assistant to help 

decision makers for careful development of hill <strong>slope</strong> which eventually can reduce the 

economic <strong>and</strong> social losses, reducing the damage potential. Protection plans require the 

description of scenarios that can be defined by means of simulation with mathematical 

models, which incorporates the occurrence conditions of the failure including the triggering 

mechanism 

Regional l<strong>and</strong>slide evaluation <strong>and</strong> mapping have been actively pursued by research 

institutions <strong>and</strong> government agencies for a long time. Among different techniques of l<strong>and</strong>slide 

hazard model such as statistical approach, one widely used technique now a day is 

deterministic approach. This approach seems to be superior because it has direct linkage to 

physics. Evolution of fast processing computers <strong>and</strong> Geographic Information System (<strong>GIS</strong>) 

has enhanced its capacity of mapping. <strong>GIS</strong> technologies could provide a powerful tool to 

model the l<strong>and</strong>slide hazards for their spatial <strong>analysis</strong> <strong>and</strong> prediction. This is because the 

collection, manipulation <strong>and</strong> <strong>analysis</strong> of the environmental data on l<strong>and</strong>slide hazard can be 

accomplished much more efficiently <strong>and</strong> cost effectively (Carrara <strong>and</strong> Guzzetti, 1999 <strong>and</strong> 

Guzzetti et al., 1999). Many <strong>GIS</strong>-based <strong>analysis</strong> models <strong>and</strong> quantitative prediction models of 

l<strong>and</strong>slide hazard have been proposed since the beginning of <strong>GIS</strong> application in geohazards 

research in the late 1980s (Carrara, 1983; Van Westen, 1994; Carrara et al., 1991; Carrara et 

al., 1995; Carrara <strong>and</strong> Guzzetti, 1999; Jade <strong>and</strong> Sarkar, 1993; Chung et al., 1995; Chung <strong>and</strong> 

Fabbri, 1998 <strong>and</strong> Chung <strong>and</strong> Fabbri, 1999). 

2.5.1 Model Concept 

The <strong>analysis</strong> of <strong>slope</strong> <strong>stability</strong> <strong>using</strong> <strong>GIS</strong> requires the overlying of various thematic maps such 

as <strong>slope</strong> map derived from the Digital Elevation Model (DEM), l<strong>and</strong> use map <strong>and</strong> soil map. 

While for rainfall-triggered l<strong>and</strong>slides, there are two main approaches for rainfall-triggered 

l<strong>and</strong>slide prediction: (1) use statistical correlations <strong>and</strong> forecasting techniques to establish the 

empirical relationships between rainfall <strong>and</strong> l<strong>and</strong>slide; (2) use a deterministic model coupling 

mechanistic <strong>slope</strong> <strong>stability</strong> model with a hydrological model to model groundwater recharge 

<strong>and</strong> pore water pressure changes caused by rainfall. Many researchers have been engaged in 

the <strong>slope</strong> failure or l<strong>and</strong>slide hazard <strong>analysis</strong> with models similar to second approach (Dietrich 



et al., 1995; Montgomery <strong>and</strong> Dietrich, 1994; Wu <strong>and</strong> Sidle, 1995 <strong>and</strong> Pack et al., 1998). 

However, most models are valuable for certain applications <strong>and</strong> certain region. 

The following sections discuss how the methods explained in Section 2.4 are applied for the 

<strong>analysis</strong> of <strong>stability</strong>. The study mainly focuses on the <strong>stability</strong> for cohesive soil with emphasis 

on Infinite Slope Method <strong>and</strong> Taylor Method by applying two stress cases, i.e. total <strong>and</strong> 

effective stress. 

2.5.1.1 Using Infinite Slope with Total <strong>and</strong> Effective Stress 

The difference between total <strong>and</strong> effective stress <strong>analysis</strong> is the use of strength parameters <strong>and</strong> 

the used of pore water pressures. For cohesive soil under effective stress <strong>analysis</strong>, the 

cohesion should be replaced by effective cohesion (c’) <strong>and</strong> if the cohesive soil is subjected to 

internal friction angle, then it should be replaced by effective internal friction angle (φ’). On 

the other h<strong>and</strong>, for cohesive soil under total stress <strong>analysis</strong>, <strong>undrained</strong> shear strength (cu) 

might be used <strong>and</strong> angle of internal friction (φ) equals to zero (Nash, 1987) with pore pressure 

being zero. Thus, the formulas for cohesive soil in dry condition (Total Stress Analysis) 

becomes : 

c u 

FS = (23 ) 

γ H sin i cos i 

<strong>and</strong>, the cohesive soil with groundwater influence (Effective Stress Analysis), the formula 

becomes: 

( γ − γ m) 

c'+ 

w H cos i tan φ' 

FS = (24 ) 

γ H sin i cos i 

For effective stress <strong>analysis</strong>, m is the soil wetness index, which is defined the relative height 

of water above the slip plane. So, if m equals to one, then the water table is at the ground 

surface, while if m equals to zero, then the water table is at the slip plane. 

2.5.1.2 Using Taylor Method 

For Taylor Method, the formula shown in Equation (21) has shown the used of total stress 

<strong>analysis</strong> because there is no effect of pore water pressure. Thus, by <strong>using</strong> Taylor Method, the 


2


consideration is only for total stress <strong>analysis</strong>. Equation (21) can be used to estimate the safety 

factor by applying <strong>stability</strong> coefficient as shown in Figure 10, which is depended on angle of 

the <strong>slope</strong> <strong>and</strong> thickness of the stratum. 

2.5.1.3 Assessment of Stability Classes 

There is no general rule on how the safety factor should be classified. For instance, Van 

Westen <strong>and</strong> Terlien, 1996, categorized safety factor into 3 classes, below one, which means 

unstable, between 1 <strong>and</strong> 1.5, which means moderately stable, <strong>and</strong> above 1.5, which means 

stable. SINMAP, Stability Index Mapping, an extension computed added modelling for <strong>slope</strong> 

<strong>stability</strong> in ArcView, uses 6 classes for safety factor including division of safety factor below 

1. 

In the design of <strong>slope</strong>s, the factor of safety on shear strength traditionally has several 

functions : 

1. To take into account uncertainty of shear strength parameters due to soil variability, <strong>and</strong> 

the relationship between the strength measured in the laboratory <strong>and</strong> the operational field 

strength. 

2. To take into account uncertainties in the loading on the <strong>slope</strong> such as surface loading, unit 

weight, pore pressures, etc. 

3. To take into account the uncertainties in the way the model represents the actual 

conditions in the <strong>slope</strong>, which includes (a) the possibility that the critical failure 

mechanism is slightly different from the one which has been identified, <strong>and</strong> (b) that the 

model is not conservative. 

4. To ensure deformation within the <strong>slope</strong> are acceptable. 

Thus, a safety factor of 1 does not indicate that failure of a <strong>slope</strong> is necessarily imminent. The 

real safety factor is strongly influenced by minor geological details, stress-strain 

characteristics of the soil, actual pore-pressure distribution, initial stresses, progressive failure 

<strong>and</strong> numerous other factors. However, in the practice, it is convenient to assume that a safety 

factor of 1 is defined as the critical condition where the forces in the condition of balance. 

However, all of the classification proposed by researcher has a certain threshold safety factor, 

which is FS=1 <strong>and</strong> FS=1.5, the first explains the critical conditions <strong>and</strong> the former explains 



the stable conditions. Safety factor classes used by Westen <strong>and</strong> Terlien (1996) is strictly 

categorized a <strong>slope</strong> being unstable, moderately stable or stable, however, for <strong>analysis</strong>, it is 

necessary to quantify the area falls in safety factor between 1 to 1.5. Thus, it is convenient to 

classify the safety factor in four classes as shown in Table 4. 

Table 4 : Stability Clases 

Safety Factor Slope Stability Class Remarks 

FS >1.5 Stable 

1.25 < FS < 1.5 Moderately Stable 

1 < FS < 1.25 Quasi Stable 

Only major destabilising factors lead to 

in<strong>stability</strong> 

Moderate destabilising factors lead to 


Minor destabilising factors can lead to 


FS < 1 Unstable Stabilising factors are needed for <strong>stability</strong> 

2.5.2 Hydrological Model 

One of the possible triggering mechanisms of <strong>slope</strong> failure is caused by the rapid increase of 

ground water table, which finally affect the increasing pore water pressure. Beven <strong>and</strong> 

Kirkby, 1979, developed soil saturation in function of hill <strong>slope</strong> topography as the wetness 

index as follow, 

a 

m = ln 

(25 ) 

tan θ 

where a is the contributing area per unit contour length <strong>and</strong> θ is the <strong>slope</strong> of the pixel. 

However, this equation does not consider the hydrological characteristic of the soil <strong>and</strong> 

rainfall events, which are in the case of <strong>slope</strong> <strong>stability</strong>, very important. Thus, the following 

formula is more appropriate to be used because it expresses the rainfall intensity. 



D 

+ R ⋅S 

m = 

2 

(26 ) 

D 

where, D is depth of soil [m], R is recharge or maximum daily rainfall [m], <strong>and</strong> S is Specific 

Yield of soil [-]. 


Chapter 3 : Materials <strong>and</strong> Methods 32 

CHAPTER 3 : MATERIALS AND METHOD 

3.1 General 

As this study is the continuation of the previous study done by Ram Lakan Ray, 2004, thus, 

the necessary data for the <strong>analysis</strong> is collected by the previous <strong>analysis</strong>. In general, the study 

area shown in Figure 11 has shown active l<strong>and</strong>slides as reported by Ram Lakan Ray at 

Krishna Bhir. It is covered not only by soil but also rocks (cliff), however, the existence of 

rock is very small compared to soil. Besides, due to this study mainly focuses on clayey soils, 

thus the existence of rock does not affect the result. 

Figure 11 : Location of the Study Area (Ray, 2004) 

The study mainly focuses on the applicability of total <strong>and</strong> effective stress <strong>analysis</strong> method by 

applying infinite <strong>slope</strong> <strong>and</strong> Taylor methods on the study area. To be able to compare 

objectively between the two analyses cases, the study is only conducted on a clayey soil. Even 

though, effective stress <strong>analysis</strong> is also applicable for non-cohesive soil. 

This chapter discusses the materials used <strong>and</strong> the method applied on the study area. It is 

covered how the available data derived by previous study <strong>and</strong> also how both of the analyses 

cases are applied on the study area. 



3.2 Data Availability 

Analyzing <strong>slope</strong> <strong>stability</strong> on a regional area requires two types of data, i.e. geotechnical 

including topographical <strong>and</strong> hydrological data. Both of the data are equally important since 

the geotechnical data represent the characteristics of the materials, while the hydrological data 

represent the amount of rainfall in the area. However, sometimes it is difficult to collect such 

information especially in rural area of a developing country, where information on earth 

resources is always connected to the budget provided <strong>and</strong> development priority given by the 

government. It is also the case that research <strong>and</strong> collection of data in a developing country are 

not well organized. 

Unfortunately, the situation is the same in Nepal for the study area. There is no soil map, l<strong>and</strong> 

use map, records of soil parameter <strong>and</strong> meteorological station inside the study area. The soil 

map was then interpreted based on the Project Report prepared by Department of Roads 

(DoR), Ministry of Works <strong>and</strong> Transport, Nepal. For l<strong>and</strong> use map, it was produced by aerial 

photographs prepared by Department of Survey. While for hydrological data, it was derived 

from four meteorological stations around the study area, which is located at Dhading, Aru 

Ghat, Gorkha <strong>and</strong> Rampur. 

Since there is no actual measurement on soil parameters for this study area, thus, the soil 

parameters were interpreted <strong>and</strong> adapted from various relevant books <strong>and</strong> papers. Even 

though, the interpretation for soil parameters from various publications is quite useful to be 

used for the <strong>analysis</strong>; however, the approach might not be accurate <strong>and</strong> involve a big 

assumption due to the variation of soil parameters on the site. 

For this study, the available data used from the previous study consist of four maps <strong>and</strong> a set 

of hydrological data. The maps are DEM, <strong>slope</strong> map, l<strong>and</strong> use map <strong>and</strong> soil map, while the 

hydrological data has been calculated <strong>using</strong> statistical software as explained in Section 3.2.2. 

3.2.1 Available DEM <strong>and</strong> Raster Maps 

The available DEM map has a grid size of 20 m covering an area of 341 km 2 with an 

elevation ranging from 245 m to 1895 m as shown in Figure 12. From the <strong>slope</strong> map, the 

study area has a <strong>slope</strong> ranging from 0.011° to 61° as shown in Figure 13. The maps have 701 



rows <strong>and</strong> 1237 columns, covering the area between 561524m to 586264 m Easting <strong>and</strong> 

3070318 m to 3084338m Northing. The unit of the map is in meters. 

Figure 12 : Digital Elevation Model (DEM) of the Study Area (Ray, 2004) 

Figure 13 : Slope Map of the Study Area 



From the soil map, it was identified that the study area is covered by 11 soil types as shown in 

Figure 14. There are three types of cohesive soil in the study area, i.e. Inorganic Silt, Organic 

Silt <strong>and</strong> S<strong>and</strong>y Clay as shown in Figure 15, covering a total area about 84.057 km 2 . 

Figure 14 : Soil Map of the Study Area (Ray, 2004) 

Figure 15 : Clayey Soil in the Study Area 



Table 5 : Various Types of Soils <strong>and</strong> Corresponding Slope Angle 

Soil-Code Soil Type Count Area (km2) 

Angle (degree) 

Min Max 

1 Clayey S<strong>and</strong> 241513 96.6052 0.0591 49.9658 

2 Poorly G. S<strong>and</strong> 82881 33.1524 0.3526 51.0144 

3 Silty Gravel 107882 43.1528 0.1908 59.7071 

4 Gravelly S<strong>and</strong> 20053 8.0212 0.0106 50.925 

5 S<strong>and</strong>y Clay 117980 47.192 0.2843 60.945 

6 Rock 1408 0.5632 1.7308 46.4667 

7 Inorganic Silt 85819 34.3276 0.0193 57.7121 

8 Poorly G. Gravel 35238 14.0952 0.244 57.958 

9 Organic Silt 6344 2.5376 1.2014 43.4273 

10 Silty S<strong>and</strong> 84749 33.8996 0.1215 52.2431 

11 Clayey Gravel 68815 27.526 0.3208 60.4487 

Total 852682 341.0728 

From the l<strong>and</strong> use map, it was identified that the study area is covered by 9 types of l<strong>and</strong> use 

as shown in Figure 16. The study area is covered majority by three types of l<strong>and</strong> cover, 

agricultural l<strong>and</strong>, bush l<strong>and</strong> <strong>and</strong> forest with percentage of 48 %, 29 % <strong>and</strong> 20 %, respectively. 

Figure 16 : L<strong>and</strong> Use Map of the Study Area (Ray, 2004) 



3.2.2 Available Hydrological Data 

There are four meteorological stations surrounding the study area. Of the four meteorological 

stations surrounding the study area, only three of them were analyzed for developing 

hydrograph. The data derived from meteorological stations at Rampur was not considered 

because it is located in a plain area where climate <strong>and</strong> rainfall patterns are completely 

different than the study area. However, only the closest rainfall stations to the study area were 

considered, i.e. Dhading. The rainfall data collected from the Department of Hydrology, 

HMG, Nepal, consists of yearly rainfall data from 1956 to 1996. The rainfall frequency 

<strong>analysis</strong> is developed <strong>using</strong> SMADA 6.0 software with a Log Pearson Type III distribution. 

Table 6 presents the results for the study area. 

Table 6 : Rainfall Prediction of Study Area with SMADA 6 Software (Ray, 2004) 

Exceedence Return Period Daily Rainfall St<strong>and</strong>ard 

Probability (years) (mm) Deviation (mm) 

0.995 200 370 102 

0.990 100 322 74 

0.980 50 277 52 

0.960 25 235 35 

0.900 10 185 20 

0.800 5 150 13 

0.667 3 124 9 

0.500 2 103 8 

3.3 Applied Methodology 

As explained in the previous chapters, the safety factor for a regional area can be derived with 

the use of <strong>GIS</strong> where the information related to the spatial data is stored in various map such 

as topography, soil <strong>and</strong> l<strong>and</strong> use map. The spatial information of a map in <strong>GIS</strong> is stored in 

attribute tables of the respective map. Then the calculation of the safety factor for every grid 

cell is done by applying the method in every grid. 

As the study is mainly focused on cohesive soil, two methods are used for determining the 

safety factor of cohesive soil in the study area. The methods are the Taylor method <strong>and</strong> the 

infinite <strong>slope</strong> method. The Taylor method is only applicable for total stress <strong>analysis</strong>, while the 

infinite <strong>slope</strong> method can be applied to both total <strong>and</strong> effective stress <strong>analysis</strong>. As the method 

is applied on the same cohesive soil, a comparison between the methods is easy. 



As explained in Chapter 2, the Taylor method is a total stress <strong>analysis</strong> where the safety factor 

is calculated based on a <strong>stability</strong> coefficient (Ns) expressed in Equation (21). The <strong>stability</strong> 

coefficient developed by Taylor (1948) is expressed in terms of <strong>slope</strong> angle with different 

thickness of soil. For this study, the thickness of the soil is assumed to be infinite <strong>and</strong> thus 

only one line of the <strong>stability</strong> coefficient developed by Taylor is used, i.e. the line with D = ∞. 

To be able to calculate the <strong>stability</strong> coefficient in spatial <strong>analysis</strong>, the <strong>stability</strong> coefficient for 

D = ∞ was first digitized. The data were then correlated <strong>using</strong> polynomial regression to be 

able to derive the mathematical equations. As shown in Figure 10, the <strong>stability</strong> coefficient for 

D = ∞ can be divided into two parts, i.e. constant <strong>and</strong> a polynomial function, for <strong>slope</strong> angle 

of 0° to 52.8° <strong>and</strong> above 52.8°, respectively. The mathematical equations derived from the 

polynomial regression for <strong>stability</strong> coefficient of D = ∞ are as follow: 

Ns = 0.183 for 0 < β ≤ 52.8° (27 ) 

Ns = 6.10 -7 β 3 – 10 -4 β 2 + 0.0079 β - 0.0263 for β > 52.8° (28 ) 

For infinite <strong>slope</strong> methods, both total <strong>and</strong> effective stress <strong>analysis</strong> are applied with different 

soil parameters. Total stress <strong>analysis</strong> applied on infinite <strong>slope</strong> uses <strong>undrained</strong> cohesion <strong>and</strong> 

φ = 0, while effective stress <strong>analysis</strong> applied on infinite <strong>slope</strong> uses effective shear strengths. 

3.3.1 Soil Parameters Determination 

As explained previously, soil parameters for the study area were not available, thus, to 

estimate the soil parameters, published references were used. There are many references 

related to soil parameters of cohesive soil. Three strength parameters should be determined 

<strong>using</strong> the available references, i.e. <strong>undrained</strong> shear strength (su), effective cohesion (c’) <strong>and</strong> 

effective angle of internal friction (φ’), for three cohesive soil types identified in the study 

area. The three cohesive soils identified in the study area are S<strong>and</strong>y Clay (CL), Inorganic Silts 

(ML or MH) <strong>and</strong> Organic Silts (ML). 

For <strong>undrained</strong> shear strength (su), the available correlations explained in Chapter 2 are based 

on either Liquidity Index (LI) or Plasticity Index (PI), given by Deoja et al. (1991). For the 

three cohesive soils, the liquid limit (LL) ranges from 30% to 68% with plastic limit (PL) 



ranging from 17 % to 38 % <strong>and</strong> thus, the plasticity index (PI) ranges from 4 % to 30 % as 

shown in Table 7. 

Soil 

Code 

5 

7 

9 

Soil 

Code 

5 

7 

9 

Table 7 : Index Properties of Soil Based on Deoja et al. (1991) 

Soil Type Classification Water 

Content 

S<strong>and</strong>y 

Clay 

Inorganic 

Silts 

Organic 

Silts 

Soil Type Classification 


Clay 

Inorganic 

Silts 

Organic 

Silts 

Unit Weight Atterberg Limit (%) 

Total Dry 

Liquid 

Limit 

(LL) 

Plastic 

Limit 

(PL) 

Plasticity 

Index 

(PI) 


Liquidity 

Index 

(LI) 

(%) (kN/m 3 ) (kN/m 3 ) (%) (%) (%) (-) 

CL 19 18.50 15.55 33 17 16 0.1 - 0.4 

ML 27 18.50 14.60 30 26 4 0.2 - 0.4 

MH 48 17.00 11.49 68 38 30 0.3 - 0.5 

OL 24 13.50 10.89 42 29 13 0.4 - 0.7 

Table 8 : Undrained Shear Strength from Various References 

Plasticity 

Index 

(PI) 

Liquidity 

Index 

(LI) 

Soil 

Thickness Skempton 

Undrained Shear Strength (su) 

Bjerrum 

& 

Simons 

(1953) 

Carter & 

Bentley 

(1959) 

(%) (%) (m) (kN/m 2 ) (kN/m 2 ) (kN/m 2 ) 

CL 16 0.1-0.4 

ML 4 0.2-0.4 

MH 30 0.3-0.5 

OL 13 0.4-0.7 

1 3.13 3.24 

2 6.26 6.48 

3 9.39 9.71 

4 12.52 12.95 

1 2.31 1.85 

2 4.62 3.70 

3 6.93 5.55 

4 9.24 7.40 

1 1.59 1.66 

2 3.18 3.31 

3 4.77 4.97 

4 6.36 6.62 

1 0.58 0.59 

2 1.17 1.18 

3 1.75 1.78 

4 2.34 2.37 

20 - 60 

20 - 40 

15 - 30 

10 - 20


Based on the atterberg limit derived from Deoja et al. (1991), the <strong>undrained</strong> shear strengths 

were determined <strong>using</strong> the available correlations. Undrained shear strengths given by Bjerrum 

<strong>and</strong> Simons (1960) <strong>and</strong> Skempton (1957) share the same correlation based on the effective 

overburden pressures. However, those correlations show very low value as shown in Table 8 

compared to the one given by Carter <strong>and</strong> Bentley (1959). This is caused by the fact that both 

of correlations are mainly applicable only for normally consolidated clay or marine clay, 

which is not applicable for this mountainous area. Thus, the correlations developed by Carter 

<strong>and</strong> Bentley (1991) are more appropriate to be used. 

The effective internal friction angle (φ’) was determined from the correlation chart explained 

in Chapter 2 <strong>and</strong> compared to the one given by Deoja et al. (1991). Again, the correlations 

given by NAVFAC DM7 are higher compared to the one given by Deoja et al. (1991). 

However, the effective internal friction angle given by Deoja et al. (1991) seems to be at the 

lower bound of the correlations given by NAVFAC DM7. Thus, the average values of the 

correlations between both are used for further <strong>analysis</strong>. Table 10 presents the parameters of 

the soil used for the <strong>analysis</strong> of safety factor. 

Soil 

Code 

5 

7 

9 

Soil 

Type 


Clay 

Table 9 : Effective Stress Parameters for the Study Area 

Classification 

Plasticity 

Index 

(PI) 

Effective Strength 

from Deoja, et. al 

(1991) 

Friction 

Cohesion 

Angle 

Effective 


Angle * 


Used 

Effective 


Angle 

(%) (kN/m 2 ) (°) (°) (°) 

CL 16 20 28 32 30 

Inorganic ML 4 7 32 35 

Silts MH 30 10 25 28 

Organic 

Silts 

Note: * Determined from NAVFAC DM7 

OL 13 10 25 33 28 

30


Soil 

Code 

5 

7 

9 

Soil 

Type 


Clay 

Classification 

Inorganic ML 

Silts MH 

Organic 

Silts 

3.3.2 Model Development 

Table 10 : Soil Parameter Used for the Analysis 

Total 

Unit 

Weight 

Undrained 

Shear 

Strength 

(su) 

Effective Strength 

Cohesion Friction 

Angle 

Specific 

Yield 


Conductivity 

(kN/m 3 ) (kN/m 2 ) (kN/m 2 ) (°) (m/day) 

CL 18.5 20 - 60 20 30 0.12 1.E-08 

18.5 20 - 40 10 30 0.18 1.E-05 

OL 13.5 10 - 20 10 28 0 1.E-06 

The current study follows the flow chart illustrated in Figure 17. Basically, there are 2 groups 

of map produced, i.e. the critical height (Hc) maps <strong>and</strong> safety factor (FS) maps. The critical 

height maps are determined based on total <strong>and</strong> effective stress <strong>analysis</strong> by applying either 

taylor method on total stress <strong>analysis</strong> (TSA) or infinite <strong>slope</strong> method on total <strong>and</strong> effective 

stress <strong>analysis</strong> (ESA). However, the critical height maps for ESA are only calculated for 

conditions of dry, half saturated <strong>and</strong> fully saturated. For safety factor maps, the same analyses 

are also applied with TSA for Taylor <strong>and</strong> infinite <strong>slope</strong> methods <strong>and</strong> ESA for infinite <strong>slope</strong>. 

The assumption of soil depth for the present study is that the base of the soil or the top surface 

of the rigid layer is infinite. With this assumption, the slip plane may occur within the ground 

surface <strong>and</strong> the top surface of the rigid layer. Since the calculation of <strong>stability</strong> requires a 

thickness of soil where the slip plane takes place, thus the calculation of safety factors for the 

present study was done by assuming various depth of slip planes. The various depths of slip 

planes were expressed by taking various soil thicknesses. The calculation of the safety factor 

was then stopped when all of the study area became unstable. Figure 18 explains the 

difference between the previous <strong>and</strong> the present study related to the assumption taken for the 

soil thickness.


For effective stress <strong>analysis</strong>, besides safety factor maps for dry, half saturated <strong>and</strong> completely 

saturated conditions, the safety factor maps for different return periods were also calculated. 

The wetness index (m) for the infinite <strong>slope</strong> method was developed on the basis of Equation 

(26). 

The calculation of safety factor <strong>and</strong> critical height maps was done with the help of ArcView 

3.2. The development of both maps in the environment of ArcView is a kind of repetition 

process where different scenarios were conducted <strong>using</strong> map calculator in ArcView. Some of 

the calculation <strong>using</strong> map calculator are shown in Figure 19 to Figure 23. 

# 

# 

! 

# # 

$ 

# 

! 

# 

! 

! " 

! 

! 

& 


# 

! 

$ 

# # 

Figure 17 : Flow Chart for the Present Study 

# 

# 

# 

! ! %


` 

ι 

1 

ι 

2 

(a) Previous Study Assumption (b) Present Study Assumption 

Figure 18 : Previous <strong>and</strong> Present Study Assumption on Soil Thickness 

Figure 19 : Map Calculation for Stability Coefficient (Ns) 


ι 

1 

ι 2


Figure 20 : Map Calculation for Critical Height with Taylor Method 

Figure 21 : Map Calculation for Critical Height with Infinite Slope 



Figure 22 : Map Calculation for Safety Factor with Infinite Slope <strong>and</strong> TSA 

Figure 23 : Map Calculation for Safety Factor in Dry Condition 


Chapter 4 : Result <strong>and</strong> Discussion 46 

CHAPTER 4 : RESULT AND DISCUSSION 

4.1 General 

There were two types of maps produced in the current study that focuses on cohesive soil, i.e. 

Critical Height (Hc) maps <strong>and</strong> Safety Factor (FS) maps. Both of the maps were developed by 

total <strong>and</strong> effective stress maps by applying Taylor <strong>and</strong> infinite <strong>slope</strong> methods on TSA <strong>and</strong> 

applying infinite <strong>slope</strong> methods on ESA. For the development of safety factor maps in quasi 

dynamic condition, the hydrological model based on rainfall direct infiltration was used for 

calculating wetness index of different return periods. While steady state conditions on ESA, 

three conditions were considered with completely dry conditions (m = 0), half saturated soils 

(m = 0.5) <strong>and</strong> completely saturated conditions (m = 1). Since the depth of the rigid base in this 

study was assumed to be infinite (see Figure 18), thus the calculation for the safety factor 

maps was based on different depth of slip plane, i.e. different soil thicknesses. The calculation 

was stopped until the area being studied was completely unstable or until the maximum 

critical height identified by TSA was reached. 

4.2 Ground Condition at the Study Area 

Three types of cohesive soils were identified at the study area as presented in Table 10. In 

total, the three soil types covered about 25 % of the total area of 341 km 2 . Of the three soil 

types, s<strong>and</strong>y clay <strong>and</strong> inorganic silts have the biggest area of 47 km 2 <strong>and</strong> 34 km 2 , respectively, 

while only about 3 km 2 of the study area is covered by organic silts. The rest of the study area 

of 257 km 2 is covered by granular soils. 

Table 11 : Tabulated Area of Soil Types for each L<strong>and</strong> Use Types 

L<strong>and</strong> use Type S<strong>and</strong>y 

Clay 

Inorganic 

Silts 

Area (km 2 ) 

Organic 

Silts 

Total 

Built up Area 0 0 0.1 0.1 

Agricultural 

L<strong>and</strong> 

28.1 24.4 1.0 53.5 

Forest 0.6 5.7 1.4 7.8 

Grass 0 0.1 0 0.1 

Bush 18.4 4.1 0 22.5 

Barren L<strong>and</strong> 0 0 0 0 

Total 47.2 34.3 2.5 84.1 



Based on the l<strong>and</strong> use type, there were 9 types of l<strong>and</strong> use, however only 6 types of l<strong>and</strong> use 

were present on the cohesive soil. These were built up area, agricultural l<strong>and</strong>, forest, grass 

l<strong>and</strong>, bush <strong>and</strong> barren l<strong>and</strong> as listed in Table 11. Among the 6 types of l<strong>and</strong> use, agricultural 

l<strong>and</strong> has the biggest area of 53.5 km 2 , while bush l<strong>and</strong> use type is only about 22.5 km 2 . Forest 

l<strong>and</strong> use type found in the study area was only about 8 km 2 <strong>and</strong> the rest of the l<strong>and</strong> use was 

less than 1 km 2 . 

Agricultural l<strong>and</strong> has the biggest area on the study area <strong>and</strong> most of this l<strong>and</strong> use type falls 

within cohesive soil as shown in Figure 24. Forest l<strong>and</strong> use type is mainly covered by organic 

silts, while bush l<strong>and</strong> use type is covered by s<strong>and</strong>y clay. As much as 24 % of the total area of 

cohesive soil falls within the <strong>slope</strong> angle of 20° to 30° as shown in Figure 25. This <strong>slope</strong> 

magnitude occurs within agricultural l<strong>and</strong> cover. However, higher <strong>slope</strong> magnitudes were also 

identified with less percentage within forest, bush <strong>and</strong> agricultural l<strong>and</strong> cover. 

Percentage of Area against Total 

Area of Each Soil Type (%) 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Built up Area 

S<strong>and</strong>y Clay Inorganic Silts Organic Silts 

Agricultural L<strong>and</strong> 

Forest 

Grass 

L<strong>and</strong> Use Type 


Bush 

Barren L<strong>and</strong> 

Figure 24 : Percentage Area of Each Soil Type for each L<strong>and</strong> Use Types


Percentage Area against Total 

Area of Cohesive Soil (%) 

50 

40 

30 

20 

10 

4.3 Critical Height Maps 

0 

Built up Area Agricultural L<strong>and</strong> 

Forest Grass 

Bush Barren L<strong>and</strong> 

0 - 10 10 - 20 20 - 30 30 - 40 40 - 50 50 - 60 > 60 

Slope Range (degree) 

Figure 25 : Slope Magnitude within the L<strong>and</strong> Use Type 

The critical height (Hc) maps can be used as an indication on how the <strong>slope</strong> behaves without 

support <strong>and</strong> it also explains the ability of a <strong>slope</strong> to withst<strong>and</strong> imbalances. The critical height 

can be assumed as the height when safety factor equals to 1. For this study, the critical height 

maps were determined <strong>using</strong> TSA <strong>and</strong> ESA with the Taylor <strong>and</strong> the infinite <strong>slope</strong> methods. 

4.3.1 Based on Total Stress Analysis (TSA) 

The critical height maps produced by means of TSA were based on Taylor <strong>and</strong> Infinite Slope 

Methods. The map produced by Infinite Slope Method was analyzed by taking the angle of 

internal friction as zero. However, these maps were only produced for steady state conditions, 

<strong>and</strong> the results are explained below. 


The critical height under TSA shows that the critical height for the cohesive soil ranges from 

4 m to 6 m <strong>and</strong> from 8 m to 18 m <strong>using</strong> lower <strong>and</strong> upper <strong>undrained</strong> shear strength, 

respectively. However, most of the area falls within critical height of 5.5 m to 6 m for lower 

<strong>undrained</strong> shear strength <strong>and</strong> 10 m to 11 m <strong>and</strong> 17 m to 18 m for upper <strong>undrained</strong> shear 

strength as shown in Table 12. Due to the majority occurrence of the <strong>slope</strong> magnitude in 

cohesive soils falls below 52.8°, the <strong>stability</strong> coefficient (Ns) becomes constant throughout the 

study area as shown in Figure 26. As a consequent the critical height did also show a constant 



value throughout the area with some small variation due to different lower <strong>undrained</strong> shear 

strength used as shown in Figure 27. 

Table 12 : Summary of Critical Height Using Taylor Method 

Critical Height 

Class (m) 

Undrained Shear 

Strength Used 

Area (km 2 ) 

4.0 - 4.5 Lower 2.5 

4.5 - 5.0 Lower 0 

5.0 - 5.5 Lower 0.1 

5.5 - 6.0 Lower 81.4 

8.0 - 9.0 Upper 2.5 

9.0 - 10.0 Upper 0 

10.0 - 11.0 Upper 34.3 

14.0 - 15.0 Upper 0 

15.0 - 16.0 Upper 0.1 

16.0 - 17.0 Upper 0 

17.0 - 18.0 Upper 47.1 

Figure 26 : Stability Coefficient Map for Taylor Method 

Total stress <strong>analysis</strong> is very good in giving an indication to which extends the <strong>analysis</strong> should 

be conducted in terms of soil thickness. Usually, the effective stress <strong>analysis</strong> gives lower 



safety factor, thus the <strong>analysis</strong> conducted with infinite soil thickness can be done within the 

critical height derived from total stress <strong>analysis</strong>. 

4.3.1.2 Using Infinite Slope Method 

Figure 27 : Critical Height based on Taylor Method 

The critical height derived with the infinite <strong>slope</strong> method shows that for cohesive soil the 

critical height ranges from 1 m to greater than 10 m for both lower <strong>and</strong> upper <strong>undrained</strong> shear 

strength. However, most of the cohesive soil has a critical height between 2 m to 4 m <strong>using</strong> 

lower <strong>undrained</strong> shear strength. Total areas covered by this critical height are about 40 km 2 

within s<strong>and</strong>y clay soil, about 45% total area of cohesive soil, <strong>and</strong> about 25 km 2 present within 

inorganic silts soil, about 30% of total area of cohesive soil. Lower <strong>and</strong> higher critical height 

than this range also occurred with total area less than 20 km 2 . While <strong>using</strong> upper <strong>undrained</strong> 

shear strength, the critical height ranges mostly between 6 m to 8 m with significant areas 

falling within critical height of 4 m to 6m <strong>and</strong> 8 m to greater than 10 m. Figure 28 presents the 

area of critical height for each soil types <strong>using</strong> lower <strong>undrained</strong> shear strength. 

Agricultural <strong>and</strong> bush l<strong>and</strong> have a critical height of 2 m to 4 m with area of about 40 km 2 <strong>and</strong> 

20 km 2 , respectively, as shown in Figure 29. Figure 30 presents the map of critical height 

<strong>using</strong> lower <strong>undrained</strong> shear strength. 



Table 13 : Summary of Critical Height <strong>using</strong> Infinite Slope Method 


Class (m) 

Area (km 2 ) 

Area (km 2 ) <strong>using</strong> Undrained Shear 

Strength 

Lower Upper 

1 - 2 1.5 - 

2 - 4 66. 6 6.4 

4 - 6 9.5 18.1 

6 - 8 2.7 31.5 

8 - 10 1. 12.6 

> 10 2.6 15.4 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

H = 1 - 2m H = 2 - 4m 

H = 4 - 6m H = 6 - 8m 

H = 8 - 10m H > 10m 

S<strong>and</strong>y Clay Inorganic Silt Organic Silts 

Soil Type 

Figure 28 : Area of Critical Height for Each Soil Types Using Lower Undrained Shear Strength 

Area (km2) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

Built up Area 

Agricultural L<strong>and</strong> 

Forest 

Grass 

L<strong>and</strong> Use Type 

H = 1 - 2m H = 2 - 4m 

H = 4 - 6m H = 6 - 8m 

H = 8 - 10m H > 10m 


Bush 

Barren L<strong>and</strong> 

Figure 29 : Area of Critical Height for Each L<strong>and</strong> Use Types Using Lower Undrained Shear Strength


Figure 30 : Critical Height Map with TSA 

Table 14 presents the summary of critical height class <strong>and</strong> their respective area <strong>and</strong> <strong>slope</strong> 

angle <strong>using</strong> lower <strong>undrained</strong> shear strength. As shown in the table, about 66.5 km 2 of the 

study area is occupied by critical height of 2 m to 4 m with <strong>slope</strong> angle ranging from 11° to 

61°. About 13 km 2 of the study area is occupied by critical height of 4 m to 10 m <strong>and</strong> only 

about 2.5 km 2 of the area is occupied by critical height of greater than 10 m with <strong>slope</strong> angle 

ranging from 0° to 6°. 

Range of 


(m) 

Table 14 : Range of Critical Height, Area <strong>and</strong> Slope Angle 

Area (km2) 

Angle (degree) 

Min Max 

1 - 2 1.539 23.9125 43.4273 

2 - 4 66.558 10.8803 60.945 

4 - 6 9.530 7.195 16.3685 

6 - 8 2.690 5.3424 10.5664 

8 - 10 1.190 4.3061 7.8435 

> 10 2.550 0.0193 6.2438 

4.3.2 Based on Effective Stress Analysis (ESA) 

The critical height for effective stress <strong>analysis</strong> was developed only for steady state conditions 

with completely dry condition (m = 0), half saturated condition (m = 0.5) <strong>and</strong> fully saturated 



condition (m = 1). The shear strength parameters, i.e. cohesion <strong>and</strong> angle of internal friction, 

use effective stress parameters, i.e. effective cohesion <strong>and</strong> effective angle of internal friction. 

The critical height based on ESA ranges from 1 m to greater than 10 m for all steady state 

cases. The calculation of critical height with ESA results in negative value of the critical 

height because the term (tan i – tan φ) in Equation (14) becomes negative when the <strong>slope</strong> 

angle is less than the angle of internal friction. In this case, the negative value should be 

considered as infinite critical depth, because if the <strong>slope</strong> angle is less than the angle of internal 

friction, the failure is unlikely to occur. As shown in Figure 31, most of the study area for 

completely dry <strong>and</strong> half saturated conditions, almost 60% <strong>and</strong> 40%, respectively, has an 

infinite critical height. For fully saturated condition, most of the study area has a critical 

height between 2 m to 4 m. The figure also shows that the area with infinite critical depth 

decreases when the soil becomes more saturated. For instance, completely full saturated 

condition has a larger area for critical depth between 2 m to 4 m than that of infinite critical 

depth. 

Area (km 2 ) 

60 

50 

40 

30 

20 

10 

0 

H = Infinite 

H = 1 - 2 m 

H = 2 - 4 m 

H = 4 - 6 m 

H = 6 - 8 m 

Critical Height Class (m) 

Dry Half Fully 

H = 8 - 10 m 

Figure 31 : Area of Critical Height based on ESA 

Under different soil types, most of the soil types have an infinite critical height as shown in 

Figure 32. For s<strong>and</strong>y clay under different steady state conditions, the critical height ranges 

from 2 m to greater than 10 m. As shown in Table 15, the range of <strong>slope</strong> angle in which the 

critical depth is infinite decreases from dry to fully saturated conditions. Thus, the most 

unstable condition is fully saturated conditions. 

H > 10 m 



Area (km 2 ) 

30 

25 

20 

15 

10 

5 

0 

H = ~ H = 1 - 2m 

H = 2 - 4m H = 4 - 6m 

H = 6 - 8m H = 8 - 10m 

H > 10m 


Soil Type 


Area (km 2 ) 

30 

25 

20 

15 

10 

5 

0 

H = ~ H = 1 - 2m 

H = 2 - 4m H = 4 - 6m 

H = 6 - 8m H = 8 - 10m 

H > 10m 


Soil Type 

(a) Completely Dry Condition (b) Half Saturated Condition 

Area (km 2 ) 

30 

25 

20 

15 

10 

5 

0 

H = ~ H = 1 - 2m 

H = 2 - 4m H = 4 - 6m 

H = 6 - 8m H = 8 - 10m 

H > 10m 


Soil Type 

(c ) Completely Saturated Condition 

Figure 32 : Area of Critical Height for Each Soil Types under Different Steady State Conditions 

Table 15 : Critical Height <strong>and</strong> Slope Angle under Different Steady State Condition 

Critical 

Height 

Class (m) 

Slope Angle (degree) 

Dry Condition Half Saturated Full Saturated 

Min Max Min Max Min Max 

Infinite 0 30.0 0.0 23.0 0.0 15.2 

1 - 2 52.8 57.7 42.8 57.7 33.4 57.7 

2 - 4 38.6 60.9 30.5 60.9 19.5 60.9 

4 - 6 35.5 52.7 26.2 42.8 15.6 33.4 

6 - 8 33.6 42.1 24.2 34.6 13.7 26.4 

8 - 10 32.4 38.6 23.1 31.4 12.6 23.4 

> 10 28.0 36.7 18.7 29.6 8.3 21.6


4.4 Safety Factor Maps 

The safety factor maps are used as an indication for <strong>slope</strong> <strong>stability</strong>, which can be used by 

planner <strong>and</strong> government official as a preliminary judgment when construction is needed in a 

certain area. However, different safety factor maps may indicate different usages of the maps 

depending on the type of <strong>analysis</strong>, method <strong>and</strong> assumption used for developing the maps. 

In this study, two types of analyses were used with two different methods. The analyses being 

used were total <strong>and</strong> effective stress <strong>analysis</strong> with the Taylor <strong>and</strong> the Infinite Slope Methods. 

For effective stress <strong>analysis</strong> where groundwater effect presented, two conditions were 

considered, i.e. steady state conditions <strong>and</strong> quasi steady state conditions with different return 

periods as discussed in the following sections. 

4.4.1 Total Stress Analysis 

For total stress <strong>analysis</strong>, Taylor <strong>and</strong> Infinite Slope Methods were used for developing safety 

factor maps. The Taylor Method follows Equation (21) with <strong>stability</strong> coefficient developed by 

Taylor, shown in Figure 10. On the other h<strong>and</strong>, Infinite Slope Method follows equations (23) 

with cu (= su) as <strong>undrained</strong> shear strength. 


The safety factor under Taylor Method is completely governed by thickness of the soil due to 

the <strong>stability</strong> coefficients (Ns) are almost constant throughout the study area because areas 

having <strong>slope</strong> angle larger than 52.8° are limited. While the other parameters, su <strong>and</strong> unit 

weight, are constant for a certain soil type. Thus, the results show that for the soil thickness up 

to 3 m, the study area is mostly in stable condition <strong>and</strong> only a small area being in moderately 

stable condition because of the small <strong>undrained</strong> shear strength used. On the other h<strong>and</strong>, for a 

soil thickness of 6 m, the entire study area becomes completely unstable. 

The total stress <strong>analysis</strong> is very good in defining the influence depth of <strong>stability</strong> due to its 

simplicity. Thus, this method can be used to determine the extent to which the <strong>analysis</strong> should 

be conducted. For this study area, it is shown in Figure 33 that the <strong>stability</strong> should be 

analyzed at least up to 5 m depth, where at this soil thickness the study area is quasi stable. 

This critical height was produced <strong>using</strong> lower <strong>undrained</strong> shear strength. Figure 34 presents 



one of the safety factor maps by the Taylor Method with soil thickness of 5 m <strong>using</strong> lower 

<strong>undrained</strong> shear strength. 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m 

H = 2m 

H = 3m 

H = 4m 

H = 5m 

H = 6m 

Unstable Quasi Stable Mod. Stable Stable 

Safety Factor Class 

Figure 33 : Area within Safety Factor Class with Taylor Methods 

Figure 34 : Safety Factor Map of Taylor Method with H = 5 m 

However, total stress <strong>analysis</strong> based on Taylor method does not express the safety factor in 

terms of <strong>slope</strong> angle for <strong>slope</strong> magnitude less than 52.8°. Thus, the effect of <strong>slope</strong> angle in 



medium magnitude of 20° to 52.8° is not taken into account in the calculation. So, this 

calculation should be used with caution whenever the <strong>slope</strong> magnitude in the area is medium, 

the calculation might lead to over estimation since this range of <strong>slope</strong> angle might also cause 

failure. 

4.4.1.2 Using Infinite Slope Method 

Infinite Slope Method with total stress <strong>analysis</strong> might result in more reliable safety factor 

map, since the method takes into account the <strong>slope</strong> magnitude. As shown in Equation (23), the 

<strong>slope</strong> magnitude inversely affects the safety factor. Thus, the smaller the <strong>slope</strong> the higher the 

safety factor will be. 

Based on the Infinite Slope Method with TSA <strong>using</strong> lower bound of <strong>undrained</strong> shear strength, 

the <strong>slope</strong> tends to be unstable whenever the soil thickness is greater than 2 m as shown in 

Figure 35. The study area is in completely stable conditions for soil thickness of 1 m, however 

if the soil thickness becomes larger, the area exponentially decreases in stable, quasi stable 

<strong>and</strong> moderately stable conditions. When <strong>using</strong> upper bound of <strong>undrained</strong> shear strength, the 

<strong>slope</strong> starts to be unstable from soil thickness of 4 m <strong>and</strong> the <strong>slope</strong> is completely stable for 

soil thickness of 1 m <strong>and</strong> 2 m as shown in Figure 35(b). 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m H = 2m 

H = 3m H = 4m 

H = 5m H = 6m 


Stability Class 


Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m H = 2m 

H = 3m H = 4m 

H = 5m H = 6m 



(a) Using Lower bound Undrained Shear Strength (b) Using Upper Bound Undrained Shear Strength 

Figure 35 : Area of Stability Class under Different Soil Thickness for Infinite Slope Method with TSA 

Figure 36 presents area of each <strong>stability</strong> class for each soil type <strong>using</strong> lower bound <strong>undrained</strong> 

shear strength. The safety factor for s<strong>and</strong>y clay tends to be greater than 1 for soil thickness up 

to 2 m, while for inorganic silts <strong>and</strong> organic silts, the safety factor tends to be less than 1


when the soil thickness is 3 m as shown in Figure 36. The shifting from quasi stable condition 

to unstable conditions seems to exponentially increase for all types of soil. 

Area (km 2 ) 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

Unstable Quasi Stable 

Mod. Stable Stable 


Soil Types 


Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(a) H = 2m (b) H = 3 m 




Soil Types 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(c) H = 4m (d) H = 5 m 

Figure 36 : Stability Area under Different Soil Types <strong>and</strong> Thickness with Infinite Slope <strong>and</strong> TSA 

Figure 37 : Range of Slope Angle against Stability Class for Different Soil Thickness


Figure 37 presents typical range of <strong>slope</strong> angle for various <strong>stability</strong> class developed <strong>using</strong> 

lower bound <strong>undrained</strong> shear strength. The mean angle ca<strong>using</strong> unstable conditions are about 

30° for different soil thickness as shown in Figure 37. Under quasi stable, moderately stable 

<strong>and</strong> stable conditions, the mean angle ca<strong>using</strong> those conditions exponentially decreases. The 

<strong>slope</strong> angle below 6° can be considered as a limit line for all <strong>stability</strong> class under different soil 

thickness. Figure 38 shows an example of safety factor map developed with the Infinite Slope 

Method <strong>and</strong> TSA for soil thickness of 2 m <strong>using</strong> lower bound of <strong>undrained</strong> shear strength. 

Figure 38 : Safety Factor Map with Infinite Slope Method (TSA) for H = 2 m 

4.4.2 Effective Stress Analysis 

Effective stress <strong>analysis</strong> is used when considering long-term applications where <strong>slope</strong> failure 

is usually caused by the movement of water. It is also the case in natural <strong>slope</strong> where the 

changing of loading results in a change of pore water pressure in the soil mass. The changing 

itself is rapid compared to the consolidation time for the soil, particularly in cohesive soil 

where permeability is very small. Thus, the excess pore water pressure is not able to be 

dissipated <strong>and</strong> causes decreasing shear strength. In the long term, the pore pressures will 

increase to their equilibrium values, thus resulting in a further reduction in the effective 

stresses in the clay, <strong>and</strong> hence a reduction in its strength <strong>and</strong> thus in the <strong>stability</strong> of the <strong>slope</strong> 

(Nash, 1987). 



In this study, the effective stress <strong>analysis</strong> was conducted for steady state <strong>and</strong> quasi dynamic 

conditions. For steady state condition, three cases were considered with completely dry, half 

saturated <strong>and</strong> fully saturated conditions. Quasi dynamic conditions were considered by 

applying wetness index with different return periods of rainfall. 

4.4.2.1 Completely Dry Condition 

Theoretically, the completely dry condition is not realistic in a hilly area with tropical climate 

such as Nepal. However, this condition can be considered as the most stable condition as there 

is no effect of excess pore water pressures that decreases the soil strength. Under this 

condition, the safety factor is governed only by cohesion, angle of internal friction, <strong>slope</strong> 

magnitude <strong>and</strong> soil thickness. Among those three parameters within one soil type, only <strong>slope</strong> 

angle <strong>and</strong> soil thickness can be different from one to another location. Thus, the <strong>slope</strong> 

magnitude <strong>and</strong> soil thickness might govern the safety factor for the study area. A very steep 

<strong>slope</strong>, under very low effective soil strength parameters, can result in a very low safety factor 

<strong>and</strong> large thickness of soil will also result in a very low safety factor. 

As there is no saturation influencing the <strong>slope</strong>s, the parcels with stable condition occupy the 

largest area. However, the area occupied by stable condition reduces, due to the effect of soil 

thickness as shown in Figure 39. This is caused by the fact that soil thickness governs the 

safety factor for dry condition. The larger the soil thickness, the smaller the safety factor will 

be. 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m H = 2m H = 3m 

H = 4m H = 5m H = 6m 

H = 7m H = 8m H = 10m 

H = 12m H = 15m H = 20m 



Figure 39 : Area of Stability Class for Dry Condition with ESA 



The relationship between area occupied by <strong>stability</strong> class <strong>and</strong> the respective soil thickness is 

best described by Figure 40. As shown in the figure, for stable condition, the relationship 

decreases exponentially towards infinite. 

Area (km 2 ) 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 



0 

0 20 40 60 80 100 120 

Soil Thickness (m) 

Figure 40 : Relationship between Area Occupied by Stability Class <strong>and</strong> Soil Thickness 




Soil Types 


Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(a) H = 2m (b) H = 3m 




Soil Types 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(c) H = 4m (d) H = 5m 

Figure 41 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Dry Condition


Under dry condition, s<strong>and</strong>y clay becomes unstable when the soil thickness is greater than 4 m, 

while inorganic silts tend to be unstable when the soil thickness is greater than 2 m. On the 

other h<strong>and</strong>, organic silts were found to be the most unstable in the study area in which for soil 

thickness of 2 m the safety factor start to be less than 1 as shown in Figure 41. 

Figure 42 : Range of Slope Angle against Stability Class under Different Soil Thickness (Dry) 

Figure 43 : Safety Factor Map of Completely Dry Condition for H = 4 m 



The mean angle ca<strong>using</strong> unstable conditions is about 42° for different soil thickness as shown 

in Figure 42. Under quasi stable, moderately stable <strong>and</strong> stable conditions, the mean angle 

ca<strong>using</strong> those conditions decreases for different soil thickness. The <strong>slope</strong> angle below 24° can 

be considered as a safe limit line for all <strong>stability</strong> class under soil thickness up to 6 m. Figure 

43 shows an example of safety factor map under dry condition with soil thickness of 4 m. 

4.4.2.2 Half Saturated Condition 

Half saturated condition may describe the real condition at the site, where the rise of ground 

water from other parcels or direct infiltration of rain from the surface occurs. This case is also 

more reliable for tropical areas such as Nepal. However, the assumption of wetness index 

being half for the entire study area seems to be illogical. Thus, the <strong>analysis</strong> result will only 

serve as an indication of <strong>slope</strong> failure under the influence of groundwater being half saturated. 

With half saturated condition, the lower <strong>and</strong> upper most safety factor ranges from 0.564 to 

1.554 <strong>and</strong> 1985 to 3326, respectively, for soil thickness ranging from 1 m to 6 m. About 27.5 

km 2 of the cohesive soils are associated with stable condition under 6 m soil thickness as 

shown in Figure 44. This value accounts for about 55.6%, 43.2% <strong>and</strong> 1.3% within s<strong>and</strong>y clay, 

inorganic silts <strong>and</strong> organic silts, respectively. 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m H = 2m H = 3m 

H = 4m H = 5m H = 6m 



Figure 44 : Area of Stability Class for Full Saturated Condition with ESA 



Area (km 2 ) 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 


Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(a) H = 2m (b) H = 3m 




Soil Types 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(c) H = 4m (d) H = 5m 

Figure 45 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Half Saturated Condition 

S<strong>and</strong>y clays are the strongest cohesive soils in the study area in which the safety factor 

becomes less than 1 when the soil thickness is greater than 3 m for half saturated condition. 

This value accounts for less than the one show by dry condition case. Inorganic silts <strong>and</strong> 

organic silts, on the other h<strong>and</strong>, are not able to support imbalances for soil thickness greater 

than 2 m as some parcel tend to be unstable for soil thickness of 2 m as shown in Figure 45. 

An average mean <strong>slope</strong> angle of 38° will cause unstable conditions for different soil thickness 

as shown in Figure 46. Under quasi stable, moderately stable <strong>and</strong> stable conditions, the mean 

angle ca<strong>using</strong> these conditions decreases for different soil thickness. The <strong>slope</strong> angle below 

17° can be considered as a safe limit for all <strong>stability</strong> class under soil thickness up to 6 m. An 

example of safety factor map under half saturated condition with soil thickness of 5 m is 

given in Figure 47.


Figure 46 : Range of Slope Angle against Stability Class under Different Soil Thickness (Half) 

Figure 47 : Safety Factor Map of Half Saturated Condition for H=5m 

4.4.2.3 Fully Saturated Condition 

Once more, fully saturated condition is not a real condition, especially in mountainous areas 

where failure usually occurs before saturation is reached. Thus, this condition serves as the 

worst condition ever happening in mountainous areas. This condition will then only serve as 

the lower limit of safety factor for the study area. Thus, the safety factor in reality should be 

larger than the safety factor shown by this condition. 



Under fully saturated condition, the cohesive soil shows moderately stable <strong>and</strong> stable 

condition for soil thickness of 1 m. The cohesive soil becomes unstable when the soil 

thickness is 2 m or higher as shown in Figure 48. However, up to 6 m height of soil thickness, 

the cohesive soil still shows stable condition with an approximate area of about 14 km 2 . This 

area belongs to s<strong>and</strong>y clay, inorganic silts <strong>and</strong> organic silts with approximate percentage of 10 

%, 7 % <strong>and</strong> less than 1 % of the total area of cohesive soil, respectively. 

Area (km 2 ) 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

H = 1m H = 2m H = 3m 

H = 4m H = 5m H = 6m 



Figure 48 : Area of Stability Class for Full Saturated Condition with ESA 

Although s<strong>and</strong>y clays are the strongest cohesive soils in the study area, the capability of 

supporting its weight under fully saturated condition is no longer superior. The safety factor 

becomes less than 1 showing this phenomenon when the soil thickness is 3 m or higher as 

shown in Figure 49. This value accounts for the lowest value for all steady state conditions. 

Inorganic silts <strong>and</strong> organic silts, on the other h<strong>and</strong>, are not able to support imbalances for soil 

thickness of 2 m or higher as some parcel tend to be unstable for soil thickness of 2 m. 

As this condition serves as the worst case, the average mean <strong>slope</strong> angle ca<strong>using</strong> unstable 

condition also shows the lowest value of 34° than the other two steady state cases as shown in 

Figure 50. Under quasi stable, moderately stable <strong>and</strong> stable conditions, the mean angle 

ca<strong>using</strong> these conditions decreases for different soil thickness. The <strong>slope</strong> angle below 10° can 

be considered as a safe limit line for all <strong>stability</strong> class under soil thickness up to 6 m. An 



example of safety factor map under half saturated condition with soil thickness of 6 m is 

given in Figure 51. 

Area (km 2 ) 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 


Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(a) H = 2m (b) H = 3m 




Soil Types 

Area (km 2 ) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 




Soil Types 

(c) H = 4m (d) H = 5m 

Figure 49 : Stability Area under Different Soil Types <strong>and</strong> Thickness in Full Saturated Condition 

Figure 50 : Range of Slope Angle against Stability Class under Different Soil Thickness (Full)


Figure 51 : Safety Factor Map of Full Saturated Condition for H = 6 m 

4.4.2.4 Based on Different Return Periods 

This section deals with the <strong>analysis</strong> of safety factor <strong>using</strong> wetness index based on different 

rainfall return periods explained in the preceding chapters. The <strong>analysis</strong> incorporates wetness 

index with return periods of 2, 10, 25 <strong>and</strong> 50 years. The wetness index is developed <strong>using</strong> 

formulas based on direct infiltration of rainfall. 

The difference between steady state condition <strong>and</strong> quasi dynamic condition is the wetness 

index (m) that is calculated by means of direct rainfall infiltration. For this <strong>analysis</strong>, the 

wetness index controls the safety factor calculation. Unfortunately, the wetness index for the 

study area is not very much different for various soil thicknesses as shown in Figure 52. The 

highest <strong>and</strong> lowest wetness index occurs in inorganic silts <strong>and</strong> organic silts soils with m value 

ranging from 0.52 to 0.56 <strong>and</strong> 0.5 to 0.505, respectively, under different return periods. These 

insignificant differences are caused by the fact that the calculated rainfall values based on 

statistics are also not significantly different for the return periods as shown in Figure 53. This 

wetness index was not different from the wetness index for steady state condition with half 

saturated condition, for which the m value is 0.5. 



Wetness Index 

0.57 

0.56 

0.55 

0.54 

0.53 

0.52 

0.51 

0.5 

H = 1 m H = 2 m 

H = 3 m H = 4 m 

H = 5 m H = 6 m 

0 10 20 30 40 50 60 70 80 90 100 110 

Return Periods (years) 

0.5 

0 10 20 30 40 50 60 70 80 90 100 110 


Wetness Index 

0.57 

0.56 

0.55 

0.54 

0.53 

0.52 

0.51 

H = 1 m H = 2 m 

H = 3 m H = 4 m 

H = 5 m H = 6 m 


(a) S<strong>and</strong>y Clays (b) Inorganic Silts 

Wetness Index 

0.57 

0.56 

0.55 

0.54 

0.53 

0.52 

0.51 

H = 1 m H = 2 m 

H = 3 m H = 4 m 

H = 5 m H = 6 m 

0.5 

0 10 20 30 40 50 60 70 80 90 100 110 


(c) Organic Silts 

Figure 52 : Wetness Index for Various Soil Thickness <strong>and</strong> Soil Types 

Rainfall (mm) 

400 

350 

300 

250 

200 

150 

100 

50 

0 

0 50 100 150 200 250 


Figure 53 : Rainfall Intensity with Various Return Periods


Although there are differences between wetness indices of different return periods, the 

difference is not significant to affect drastically the safety factor. As shown in Figure 54, the 

difference in area occupied by various <strong>stability</strong> classes with various return periods <strong>and</strong> with 

various soil thicknesses is relatively small. However, the effect of soil thickness still 

consistently shows that the higher the soil thickness the higher the safety factor will be, shown 

by the amount of area occupied by that safety factor. Significant decrease of area occupied by 

stable condition is also noticed for soil thickness of 1 m to 3 m as shown in Figure 54(d). 

Area (km 2 ) 

Area (km 2 ) 

120 

100 

80 

60 

40 

20 

0 

120 

100 

80 

60 

40 

20 

0 

H = 1m H = 2m H = 3m H = 4m 

H = 5m H = 6m 

RP 2 yr RP 10 yr RP 25 yr RP 50 yr 

Return Periods (year) 


Area (km 2 ) 

120 

100 

80 

60 

40 

20 

0 

H = 1m H = 2m H = 3m H = 4m 

H = 5m H = 6m 



(a) Unstable Condition (b) Quasi Stable Condition 

H = 1m H = 2m H = 3m H = 4m 

H = 5m H = 6m 



Area (km 2 ) 

120 

100 

80 

60 

40 

20 

0 

H = 1m H = 2m H = 3m H = 4m 

H = 5m H = 6m 



(c) Moderately Stable Condition (d) Stable Condition 

Figure 54 : Area of Safety Factor with Various Return Periods 

The same tendency also shows if the area of each soil type is plotted against <strong>stability</strong> class for 

various return periods that insignificant difference occur. Figure 55 shows relatively small 

differences of area occupied by stable condition with various return periods for soil thickness 

of 2 m. This is caused by the fact that there are relatively small differences between safety 

factors <strong>using</strong> various return periods.


Area (km 2 ) 

50 

40 

30 

20 

10 

0 

RP 2 yr RP 10 yr 

RP 25 yr RP 50 yr 


Soil Types 

Figure 55 : Stable Area with Various Soil Types <strong>and</strong> Return Periods with Soil Thickness of 2m 

Compared to the result given by steady state with half saturated condition in which the 

wetness index was 0.5, the calculations of safety factor based on various rainfall return 

periods give similar result. Thus, the steady state with half saturated condition can serve as a 

general safety factor map for this study area with various return periods. The wetness indices 

for various return periods of rainfall are not significantly different from the one obtained with 

half saturated condition. The differences are only about 0.06. 

4.5 Discussion 

Owing to the ever-increasing capabilities of hardware <strong>and</strong> software, electronic geographical 

data processing is becoming a common tool in a wide range of research or production 

activities. This technology has brought <strong>GIS</strong> for evaluating natural hazards such as l<strong>and</strong> slides. 

However, the extent to which this technology is applicable still remains a big issue for users 

<strong>and</strong> researchers. Actually, the crucial issue in hazard assessment is the input data which 

remain fundamentally inadequate in quantity <strong>and</strong> quality for the task to be accomplished. 

Thus, a good underst<strong>and</strong>ing of geology, hydrology, <strong>and</strong> soil properties is central to applying 

<strong>slope</strong> <strong>stability</strong> principles properly. Analyses must be based upon a model that accurately 

represents site subsurface conditions, ground behavior, <strong>and</strong> applied loads. Good judgments 

regarding acceptable risk or safety factors must be made to assess the results of analyses. 

4.5.1 Total <strong>and</strong> Effective Stress Analyses 

It is clear from the theory of the difference between total <strong>and</strong> stress <strong>analysis</strong> that total stress 

<strong>analysis</strong> does not take into account pore water pressure effect in the calculations. This also 



means that loss of strength in time does not take that into considerations, as there is possibility 

of strength loss in time due to fluctuation of groundwater or dissipation of excess pore water 

pressure. Therefore, both safety factor <strong>and</strong> critical height based on this principle should result 

in a higher value. 

However, the calculation for total stress <strong>analysis</strong> shown above with lower bound <strong>undrained</strong> 

shear strength (TSA-Inf.-Lower) gives lower result compared to effective stress <strong>analysis</strong> as 

indicated in Figure 56(a). The reason behind this phenomena is that the <strong>undrained</strong> shear 

strengths are used as a constant value when applying into Equation (23), while the 

denominator (γ × H × sin i × cos i) can increase with depth <strong>and</strong> thus give lower value of safety 

factor or critical height. 

As shown in Figure 56(a), the result given by total stress <strong>analysis</strong> with infinite <strong>slope</strong> <strong>and</strong> <strong>using</strong> 

upper bound <strong>undrained</strong> shear strength is higher than effective stress with completely dry 

condition. However, after a certain soil thickness, the area for stable condition becomes less 

than for completely dry condition. The same tendency is also seen for the Taylor Method if 

higher values of <strong>undrained</strong> shear strength were used. However, the result was not shown in 

Figure 56. 

Percentage of Stable Area (%) 

140 

120 

100 

80 

60 

40 

20 

0 

Taylor TSA-Inf.-Lower 

TSA-Inf.-Upper ESA-Inf-Dry 

ESA-Inf-Half ESA-Inf-Full 

H = 1m H = 2m H = 3m H = 4m H = 5m H = 6m 

Soil Thickness 


Percentage of Unstable Area (%) 

140 

120 

100 

80 

60 

40 

20 

0 

Taylor TSA-Inf.-Lower 

TSA-Inf-Upper ESA-Inf-Dry 

ESA-Inf-Half ESA-Inf-Full 

H = 1m H = 2m H = 3m H = 4m H = 5m H = 6m 

Soil Thickness 

(a) Stable Condition (b) Unstable Condition 

Figure 56 : Comparison between Various Method Results 

Hence, the Taylor Method might be not applicable since it is giving very high values of safety 

for small soil thickness, while in reality, failure reported in this study area occurs for small 

soil thickness. Therefore, it is only applicable for identifying the depth of influence for further 

<strong>analysis</strong>. On the other h<strong>and</strong>, the results given by the Infinite Slope Method with TSA


produces lower <strong>and</strong> higher safety factors if lower <strong>and</strong> upper <strong>undrained</strong> shear strengths are 

used. In this case, this model can only be applied as the uppermost <strong>and</strong> lower most safety 

factor for the area. The two models should also be confirmed with l<strong>and</strong>slide occurrence at the 

site. However, it was not possible to calibrate the two models because there is no information 

about failure on cohesive soil that was recorded. Failure occurring at this study area happened 

on the granular soils. 

The three models resulting from effective stress <strong>analysis</strong> showed a good result with a 

tendency of decreasing <strong>stability</strong> with soil thickness. Completely dry condition gives the 

highest value compared to the other two conditions. It is also confirmed by the results that 

fully dry conditions <strong>and</strong> infinite <strong>slope</strong> with upper <strong>undrained</strong> shear strength of TSA are close 

together, except for soil thickness higher than 4 m. 

4.5.2 Influence of Depth 

The <strong>stability</strong> <strong>analysis</strong> has been found to be directly influenced by the depth of the soil. 

Generally, all the models consistently show that the safety factor decreases against soil 

thickness as also shown in Figure 56. Especially the result given by Taylor method is clearly 

shown in this relationship; the <strong>slope</strong> is safe up to soil thickness of 3 m, however, when the soil 

thickness is larger than 3 m, the safety factor becomes completely unstable. 

The model given by the Infinite Slope Method with TSA (TSA-Inf.-Upper) also clearly shows 

the influence of depth as shown in Figure 56(a). The results indicate that up to 4 m soil 

thickness, the relationship between safety factor <strong>and</strong> soil thickness gradually decreases. 

However, for soil thickness greater than 4 m, the decrease of safety factor becomes very large 

as the line of this model is crossing the other models. The reason is that the soil thickness 

plays a major role in total stress <strong>analysis</strong> with infinite <strong>slope</strong>. In Equation (23) the numerator is 

a constant value <strong>and</strong> the denominator increases with depth. 

4.5.3 Slope Angle 

Range of mean <strong>slope</strong> angle for various <strong>stability</strong> class <strong>and</strong> analyses methods are presented in 

Table 16. In general, the Infinite Slope Method with completely dry conditions gives the 

highest values of mean <strong>slope</strong> angle, while total stress <strong>analysis</strong> with infinite <strong>slope</strong> give the 

lowest mean <strong>slope</strong> angles. The mean <strong>slope</strong> angle is also observed decreasing from dry to fully 



saturated conditions. The same tendency is also observed for average mean angle to cause 

in<strong>stability</strong> <strong>and</strong> lower most <strong>slope</strong> angle for stable condition that it decreased from fully dry to 

completely saturated conditions as summarized in Table 17. 

Stability 

Class 

TSA-Inf.- 

Lower 

Table 16 : Range of Mean Slope Angle 

TSA-Inf.- 

Upper 

Mean Angle (degree) 

ESA-Inf-Dry ESA-Inf-Half ESA-Inf-Full 

Unstable 28 - 32 42 - 37 41 - 55 37 - 45 32 - 38 

Quasi 

Stable 

12 - 36 33 - 29 35 - 42 29 - 37 22 - 33 

Mod. Stable 9 - 42 42 - 34 30 - 36 24 - 49 18 - 40 

Stable 5 - 26 26 - 24 18 - 26 14 - 26 10 - 25 

Description 

Average mean <strong>slope</strong> 

angle ca<strong>using</strong> In<strong>stability</strong> 

Lower most <strong>slope</strong> angle 

ca<strong>using</strong> in<strong>stability</strong> 

Lower most <strong>slope</strong> angle 

for stable Condition 

4.5.4 Selection of Maps 

Table 17 : Slope Angle for Unstable <strong>and</strong> Stable Conditions 

TSA-Inf.- 

Lower 

TSA-Inf.- 

Upper 

ESA-Inf- 

Dry 

ESA-Inf- 

Half 


ESA-Inf- 

Full 

30 37 43 38 34 

9 24 36 26 16 

6 14 24 17 10 

This section deals with the choice of critical height maps <strong>and</strong> safety factor maps that are 

applicable for the study area. As there are many methods that can be applied for l<strong>and</strong>slide 

assessment, the issues still remains which of those represent general conditions for the study 

area <strong>and</strong> to which extend these maps can be used. The selection itself depends on several 

factors such as the application of the map <strong>and</strong> method used for developing the map. 

4.5.4.1 Critical Height Map 

A critical height map can serve as a general guidance to facilitate planners <strong>and</strong> administrators 

to construct correct decisions at the planning stage of a development project. This map 

explains the behavior of the ground under no supporting structure to which extend the soil is


able to withst<strong>and</strong> imbalance forces. It can serve also as a general guidance to decide to which 

height a <strong>slope</strong> can be cut without failure. Furthermore, the <strong>analysis</strong> can also identify under 

which magnitude the <strong>slope</strong> will not fail. 

In this study two methods have been used for assessing critical height map: the Taylor 

Method <strong>and</strong> the Infinite Slope Method with total <strong>and</strong> effective stress <strong>analysis</strong>. Under total 

stress <strong>analysis</strong>, two method were used, i.e. the Taylor <strong>and</strong> the Infinite Slope Methods, while 

under effective stress <strong>analysis</strong>, the Infinite Slope Method was applied as summarized in Table 

18. 

Type of Stress 

Analysis 

Total Stress 

(Lower su) 

Effective Stress Infinite Slope 

Table 18 : Summary of Critical Height 

Method Case 

Critical Height, H c (m) 

Ranges 


Most 

Occurrence 

Taylor - 4 - 6 4 - 6 

Infinite Slope - 1 - >10 2 - 4 

Dry 2 - ∞ Infinite 

Half Saturated 1 - ∞ Infinite 

Fully Saturated 1 - ∞ 2 - 4 

The result for all type of analyses shows that the critical height ranges from 1 m to infinite. 

However, the infinite critical depth resulting from the effective stress <strong>analysis</strong> when the <strong>slope</strong> 

magnitudes are less than the angle of internal friction results in an infinite critical height value 

as for a flat area also. Thus, the infinite critical height should not be considered as a general 

rule for assessing critical height in this area. 

Generally, the critical height for cohesive soil in this study area ranges from 2 m to 4 m <strong>and</strong> 4 

m to 6 m based on result given by the Infinite Slope <strong>and</strong> Taylor Methods, respectively, as 

shown in Table 18. Although there are differences in applying stress <strong>analysis</strong>, a critical height 

of 2 m to 4 m can be used as a rule of thumb for critical height of cohesive soil in the study 

area. Meanwhile, the result given by the Taylor Method can be used as a general guidance to 

which extent the <strong>analysis</strong> of safety factor should be conducted.


Even though general guidance given by the Taylor Method is very useful, the methods were 

not able to explain any spatial distribution of critical height. An infinite <strong>slope</strong> maps give a 

better description of the distribution of critical height over the study area. Comparison result 

between the Infinite Slope with total <strong>and</strong> effective stress analyses shows that the result given 

by total stress <strong>analysis</strong> is the most conservative. Finally, it is concluded that this map can 

serve as base map of critical height for the study area as shown in Figure 30. 

4.5.4.2 Safety Factor Map 

Stability conditions of a <strong>slope</strong> on a regional scale can be accessed through safety factor map. 

Planners <strong>and</strong> administrators both from government or private offices might use this map for 

early planning of a project. This will certainly provide useful information of the <strong>stability</strong> on a 

project site in the early stage where necessary remedial action <strong>and</strong> design can be taken to 

avoid <strong>slope</strong> failure. In return, a good design <strong>and</strong> remedial action will reduce the budget <strong>and</strong> 

also provide security for the project <strong>and</strong> society living nearby the project. 

The same simulations as for critical height map have been applied for the safety factor map. 

The result from total stress <strong>analysis</strong> can be considered as a short term safety factor map. Short 

term safety factor map refers to <strong>stability</strong> factors within a short time frame, as for instance 

short term construction periods. While the result from effective stress <strong>analysis</strong> can be 

considered as long term safety factor map due to its nature that allows decreasing of soil 

strengths in long term periods. 

Short term safety factor map resulted from the Taylor Method is considered inapplicable 

because it does not take into account <strong>slope</strong> angle less than 52.8°. On the other h<strong>and</strong>, the 

Infinite Slope Method is more reliable as <strong>slope</strong> magnitudes are taken into account. However, 

both of the methods express the same depth of influence of about 6 m in which the entire 

study area becomes unstable as shown in Table 19. 



Soil 

Thickness Unstable 

Table 19 : Percentage of Total Area of Safety Factor for TSA Result 

Infinite Slope Method (Lower Bound Su) Taylor Method 

Quasi 

Stable 

Mod. 

Stable 

Stable Unstable Quasi 

Stable 

Mod. 

Stable 


Stable 

H = 1m 0 0 0 100 0 0 0 100 

H = 2m 2 37 23 38 0 0 0 100 

H = 3m 62 16 8 14 0 0 3 97 

H = 4m 81 8 4 8 0 3 97 0 

H = 5m 89 4 2 5 3 97 0 0 

H = 6m 92 3 1 4 100 0 0 0 

Long term safety factor maps should be developed with effective stress parameters <strong>and</strong> 

performed by effective stress <strong>analysis</strong>. Three steady state conditions were performed with 

varying soil thickness, while under quasi dynamic conditions, the result is considered similar 

as the one showed by steady state condition with half saturated case. The summary of the 

results from steady state condition are shown in Table 20. 

In general, the three conditions can serve as a base map for practical used as the safety factor 

will not exceed this range, i.e. within dry <strong>and</strong> fully saturated condition. However, it should be 

noted that this is under hypothesis that the assumption taken related to soil strength 

parameters <strong>and</strong> soil types are reliable. In this case, dry <strong>and</strong> fully saturated conditions serve as 

the upper <strong>and</strong> lower limit of safety factor in the study area, respectively. On the other h<strong>and</strong>, 

half saturated safety factor map can be used as a general safety factor map in the study area.


H (m) 

Table 20 : Percentage of Total Area of Safety Factor for ESA Result 

Completely Dry Half Saturated Fully Saturated 

Ust. Qst. Mst. St. Ust. Qst. Mst. St. Ust. Qst. Mst. St. 

1 0 0 0 100 0 0 0 100 0 0 5 95 

2 0 2 7 91 1 8 10 82 8 11 18 63 

3 1 7 16 76 7 16 21 56 21 27 18 34 

4 3 14 19 64 14 24 19 43 40 23 12 25 

5 6 19 19 56 23 24 16 37 52 18 10 20 

6 9 21 19 51 30 23 14 33 60 15 8 17 

Note : H = Soil Thickness Mst. = Moderately Stable 

Ust. = Unstable St. = Stable 

Qst. = Quasi Stable 

4.5.5 Comparison with Root Cohesion Method 

The previous study conducted by Ray (2004) was based on the model with root cohesion 

developed by Montgomery <strong>and</strong> Dietrich (1994), Van Westen <strong>and</strong> Terlien (1996) <strong>and</strong> de 

Vleeschauwer <strong>and</strong> De Smedt (2002). This model combines cohesion between soil <strong>and</strong> root 

cohesions as explained in Equation (22). The study also assumed constant soil depth 

according to a depth factor of 1, 0.75 <strong>and</strong> 0.5 for <strong>slope</strong>s up to 30°, 30° to 45° <strong>and</strong> 45° to 61°, 

respectively. Thus, as shown in Table 21, soil thickness assumption for the previous study 

ranges from 1 m to 2 m. This means almost 73% of cohesive soil falls within soil thickness of 

2 m. Moreover, it can be concluded that the previous study was conducted with effective 

stress <strong>analysis</strong> with different steady state conditions <strong>and</strong> quasi dynamic conditions. 

Table 21 : Previous Study Assumption on Soil Thickness for Cohesive Soil 

Soil Type 

Total 

Area 

(km 2 ) 

Minimum 

(m) 

Coverage 

Area 

(km 2 ) 

Soil Depth 

Maximum 

(m) 

Coverage 

Area 

(km 2 ) 

S<strong>and</strong>y Clay 47. 2 1 18.4 2 28.8 

Inorganic Silts 34.3 1 4.2 2 30.1 

Organic Silts 2.5 2 2.5 2 2.5 



The difference between the previous <strong>and</strong> the present study besides the model being used is the 

assumption of the soil depth as depicted in Figure 18. The previous study assumed constant 

soil depth, while the present study assumed infinite soil depth. As a consequence, the 

evaluation of the safety factor for both of the studies was also different. As the depth of the 

soil was assumed to be constant for the previous study, then the safety factor was evaluated at 

the base of the soil layer. Contrary, as the depth of the soil was assumed infinite for the 

present study, so the evaluation of safety factor was based on different slip plane or soil 

thickness. Therefore, in order to compare the previous <strong>and</strong> the present studies, the 

comparisons conducted in this study were only done for soil thickness of 2 m as this soil 

thickness covered almost 73% of cohesive soil in the previous study. The comparison was 

done by evaluating the area occupied by a certain <strong>stability</strong> class within cohesive soil only 

under various steady state conditions. 

In completely dry condition, about 42 km 2 , 32 km 2 <strong>and</strong> 2 km 2 for s<strong>and</strong>y clay, inorganic silts 

<strong>and</strong> organic silts respectively, were reported previously to be in stable conditions. It was also 

concluded that clayey s<strong>and</strong> <strong>and</strong> s<strong>and</strong>y clay types of soils are more stable even on steep <strong>slope</strong> 

with unmanaged cultivation practice (Ray, 2004). The present study also indicates the same 

tendency, where in completely dry condition the area occupied by s<strong>and</strong>y clay is about 47 km 2 

with soil thickness of 2 m. However, slightly differences in area occupied by s<strong>and</strong>y clay were 

observed of about 5 km 2 . It was also confirmed by the present study that the s<strong>and</strong>y clay soil 

were more stable due to the fact that up to 5 m soil thickness about 60% of s<strong>and</strong>y clay soil 

was still in stable condition (Figure 41). 

However, the previous study concluded that the <strong>slope</strong> angle ca<strong>using</strong> in<strong>stability</strong> is 37° for all 

soil types, while the present study concluded that the average mean <strong>slope</strong> angle is 43° for 

cohesive soil only. However, the lower most <strong>slope</strong> angle ca<strong>using</strong> in<strong>stability</strong> identified in this 

study is very similar with the previous study as shown in Table 22. 

The difference becomes significant when half <strong>and</strong> fully saturated conditions are compared. As 

shown in Table 23, about 20% <strong>and</strong> 26% of differences are observed for half <strong>and</strong> fully 

saturated conditions, respectively, on s<strong>and</strong>y clay soils. However, the difference observed in 

inorganic silts was less than 10% for both of cases. A significant difference of stable area in 



s<strong>and</strong>y clay soils might be caused by the low value strength parameters used by the previous 

study. Thus when pore water pressure was considered, the shear stresses become less than the 

normal stress. Hence, this would result in low safety factor for the previous study. 

Table 22 : Lower Most Slope Angle Ca<strong>using</strong> In<strong>stability</strong> for Previous <strong>and</strong> Present Study 

Steady State 

Conditions 

Lower Most Slope Angle 

Ca<strong>using</strong> In<strong>stability</strong> 

Previous 

Study 

Present 

Study 

Dry 37 36 

Half Saturated 27 26 

Fully Saturated 21 16 

In terms of <strong>slope</strong> angle ca<strong>using</strong> in<strong>stability</strong>, according to previous study, <strong>slope</strong> angle of 27° <strong>and</strong> 

21° was observed for half <strong>and</strong> fully saturated conditions. The present study indicates that 

average mean <strong>slope</strong> angle of 37° <strong>and</strong> 34° causes in<strong>stability</strong> of <strong>slope</strong> for half <strong>and</strong> fully 

saturated conditions. However, the lower most <strong>slope</strong> angle ca<strong>using</strong> in<strong>stability</strong> give similar 

result for half saturated but not in fully saturated case as shown in Table 22. 

Soil 

Types 


Clay 

Inorganic 

Silts 

Total 

Area 

Total 

Area 

(km2) 

Table 23 : Summary Comparison between Previous <strong>and</strong> Present Study 

Ray 

(2004) 

Stable Area Occupied for Each Steady State Conditions 

Fully Dry Condition Half Saturated Condition Full Saturated Condition 

Present 

Study 

% 

Difference 

Ray 

(2004) 

Present 

Study 

% 

Difference 

Ray 

(2004) 


Present 

Study 

% 

Difference 

47.192 42 47 6 30 46 20 17 38 26 

34.3276 32 27 6 27 21 8 15 14 1 

81.5196 

Both results from the present <strong>and</strong> the previous study share the same tendency that models 

developed based on direct infiltration produced similar result with model based on half 

saturated condition. As the rainfall intensity for various return periods developed with 

statistical software does not significantly different, so the wetness index was not significantly


discrete also. As a consequence, the safety factor resulted from this wetness index is not 

significantly different. Thus, it is concluded that for present study area, the wetness index 

does not significantly affect the safety factor, since the amount of rainfall is not significantly 

discrete. 

Another explanation of similar result between models based on various return periods <strong>and</strong> half 

saturated conditions is basically due to the concept <strong>and</strong> philosophy of cohesive soil. 

Theoretically, cohesive soil is different than cohesionless soil in terms of shapes <strong>and</strong> reaction 

against water. These two important differences distinguish the behaviour of the soils in shear. 

Cohesive particles are normally plate-formed, while cohesionless particles are normally 

rounded-formed. As the shape between these two particle types is different, it constitutes 

different phenomena whenever there is a movement of water. The movement of water inside 

of soil particles is determined by its permeability. In terms of permeability, cohesive soil 

reserves a lower value than cohesionless soil. As the permeability of cohesive soil is very 

small, the movement of water in clay particles is very slow. In hydrology, the movement of 

water affected only by gravitation force is explained by its specific yield, which also shares 

the same tendency as permeability. 

Shortly, specific yield phenomenon in cohesive soil has a very small affects on the movement 

of water from rainfall to reach the groundwater. This caused the amount of rainfall falls 

within clayey soil is reduced by the specific yield to reach the groundwater. As a 

consequence, the wetness index is not very much different. 


Chapter 5 : Conclusion <strong>and</strong> Recommendation 82 

CHAPTER 5 : CONCLUSIONS AND RECOMMENDATIONS 

5.1 Conclusions 

Natural <strong>slope</strong> in<strong>stability</strong> is a major concern in a mountainous area where failures might cause 

catastrophic destruction on the surrounding area. The failures might be triggered by internal 

or external factors that cause imbalance natural forces. Internal triggering factor is the factor 

that causes failure due to internal changes, such as increasing pore water pressure <strong>and</strong> or 

imbalance forces developed due to expansion of soil mass. External triggering factor, on the 

other h<strong>and</strong>, might be either caused by human activities or natural events, such as earthquakes. 

This study is the continuation of the previous study done by Ram Lakhan Ray, 2004, that 

applied <strong>stability</strong> model on an area of 341 km 2 of Dhading district, Nepal. In this study, a 

spatial distributed physically based <strong>slope</strong> <strong>stability</strong> model was presented <strong>and</strong> applied on 84 km 2 

area located in the same study area. It covered only about 25% of the original study area as 

the present study was mainly conducted only on cohesive soil present in the study area. Two 

methods of <strong>analysis</strong> were performed, i.e. the total <strong>and</strong> effective stress analyses <strong>and</strong> the Taylor 

<strong>and</strong> the Infinite Slope Methods were applied on the <strong>analysis</strong>. Critical height <strong>and</strong> safety factor 

maps were produced based on those analyses. Steady state <strong>and</strong> quasi dynamic conditions were 

considered for the present study with varying soil thickness. For quasi dynamic conditions, 

wetness index was applied based on direct rainfall infiltrations. 

It is concluded that total stress <strong>analysis</strong> give a good indication of the depth to which extend 

the <strong>analysis</strong> should be conducted. Theoretically, a total stress <strong>analysis</strong> should give the most 

critical case, however, due to lack of soil strength parameters, the <strong>analysis</strong> resulted in strongly 

varying results if the lower or upper bound of <strong>undrained</strong> shear strength was used <strong>and</strong> applied 

on the Infinite Slope Method. In this case, the model can be used to find the upper most <strong>and</strong> 

lower most of safety factor. On the other h<strong>and</strong>, the Taylor Method which also applied on this 

study area, produce a very large safety factor for very small soil thickness. This means that 

the method is not useful to examine <strong>stability</strong> when failure normally occurs with shallow 

depth. Effective stress <strong>analysis</strong> with steady state conditions gave more realistic results with a 

tendency of decreasing safety with increasing soil thickness. Complete dry condition gives the 



highest safety factor, while fully saturated condition gives the lowest safety factor, as 

expected. 

In general, all models consistently show decreasing safety factors with increase of soil 

thickness. However, the influence of soil thickness is more strongly shown in total stress 

<strong>analysis</strong> than in effective stress <strong>analysis</strong>. Again, due to lack of soil parameters, the 

assumptions taken for the strength parameters might not represent natural conditions at the 

site, which in return will affect the safety factor considerably. 

Slope angles of 38° <strong>and</strong> 17° can be considered as the average mean <strong>slope</strong> angle to cause 

in<strong>stability</strong> <strong>and</strong> the lower most <strong>slope</strong> angle for stable conditions respectively. These values 

were derived from the <strong>analysis</strong> based on half saturated conditions. It is also concluded that 

these cases can serve as general conditions for a safety factor map because similar results are 

obtained with models based on different return periods. 

The root cohesion method conducted by the previous study gave lower results compared to 

the present study. The comparison was conducted only for cohesive soil with a soil thickness 

of 2 m. The difference between the previous <strong>and</strong> the present study might be caused by the 

different concept <strong>and</strong> principle. The root cohesion method uses small values of soil cohesion, 

but in addition it adds root cohesion. Even though, there are differences in the concept, the 

result shown for completely dry conditions gave similar result. 

This l<strong>and</strong>slide hazard map is made based on the Infinite Slope Method, which is predominantly 

applied only for translational slides on the contact of the upper soil <strong>and</strong> the underlying bedrock. 

Hence, this map is only applied for determining translational slides hazard within the study area. 

In reality, any forms of sliding might happen due to natural activities, such as block sliding, 

circular sliding or topple. Consequently, the resulting map should be used with caution. Any 

sliding occurring within the study area should be carefully examined whether it is correlated to 

this map <strong>and</strong> further study is required in order to determine appropriate l<strong>and</strong> slide hazard maps. 

Furthermore, soil strength parameters in this study were taken to be constant for certain soil 

types. Even though, this assumption was useful in predicting <strong>slope</strong> in<strong>stability</strong>, this does not 

represent the spatial variability of strength parameters throughout the study area or even 



within one soil type. Although, this assumption might give conservative values for the safety 

factor due to conservative soil parameters being used, the results will be over estimated. 

Conservative value used in the <strong>analysis</strong> might also lead to in-correct conclusions that the 

effect of other factors might not be seen due to the fact that their effect is masked. 

5.2 Recommendations 

Detailed soil explorations <strong>and</strong> hydrological investigations are strongly recommended this 

active l<strong>and</strong>slide area. Detailed soil explorations should include developing soil maps <strong>and</strong> soil 

parameter data-bases, while hydrological studies should include spatial variability of rainfall 

data. 

However, for a detailed exploration, large amount of budgets are needed. Therefore, soil 

exploration could be organized with only shallow depths of 2 m to 4 m, which was indicated 

by the present study as the major critical depth within cohesive soil. Laboratory test on 

undisturbed soil samples may be conducted to determine soil strength or can be replaced by 

in-situ measurements as the St<strong>and</strong>ard Penetration Test or Cone Penetration Test. In-situ 

measurement of soil consistency does not measure strength parameters such as performed in 

the laboratory by means of triaxial compression tests. However, there are many correlations 

that have been scientifically proved, such as correspondence between St<strong>and</strong>ard Penetration 

Test <strong>and</strong> <strong>undrained</strong> shear strength. 

L<strong>and</strong>slide inventory throughout the area is also very important to identify the behaviour <strong>and</strong> 

type of sliding occurring within the area. Occurring l<strong>and</strong>slides should be compared with the 

available safety factor models for obtaining a more accurate safety factor map for the study 

area. 

Natural hazard, such as earthquakes, has been widely reported as a cause of <strong>slope</strong> failure. 

Even though, the occurrence of an earthquake is exceptional, the damage caused by 

earthquakes is tremendous <strong>and</strong> sometimes hazardous to human life, especially in a 

mountainous area such as Nepal. Hence, for a good hazard map the effects of earthquakes 

should be included in the hazard map (Van Westen <strong>and</strong> Terlien, 1996). 


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Appendices xiii 

APPENDICES 


Appendices xiv 

Critical Height (Hc) Map based on Infinite Slope Method with Total Stress Analysis 


Appendices xv 

Safety Factor Map based on Infinite Slope Method with Total Stress Analysis for H = 2 m 


Appendices xvi 

Safety Factor Map of Completely Dry Condition for H = 4 m 


Appendices xvii 

Safety Factor Map of Half Saturated Condition for H = 5 m 


Appendices xviii 

Safety Factor Map of Full Saturated Condition for H = 6 m

Drained and undrained slope stability analysis using GIS on a ...

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