stability of embankment dams under static loading …

STABILITY OF EMBANKMENT DAMS UNDER STATIC

LOADING CONDITIONS

Amjad Hussain Bhutto

PCE-010/5

A thesis submitted in fulfillment of the requirement

for the award of the degree of

Doctor of Philosophy

In

Civil Engineering

Department of Civil Engineering

Faculty of Engineering

Quaid-e-Awam University of Engineering, Science & Technology

Nawabshah

2020

i

AUTHOR’S DECLARATION

I declare that this thesis entitled “Stability of Embankment Dams Under Static

Loading Conditions” result from my own research except as cited in the

references. The thesis has not been accepted for any degree and is not

concurrently submitted in the candidature of any other degree.

Signature: ____________________________

Name of Student: Amjad Hussain Bhutto

Date: _______________________________

ii

DEDICATION

This dissertation is dedicated to my beloved parents. They always pray for my

success in every step of my life. The completion of this study is also part of their

prayers.

iii

ACKNOWLEDGEMENT

I am thankful to Almighty Allah for successful completion of my Ph.D work. I

am also greatful to my parents for their kindness and motivation for higher

studies.

The research work has been carried out under the supervision of Dr. Muhammad

Auchar Zardari, who provided every possible support which I needed during my

entire study period. I appreciate his patience, encouragement, and am fortunate to

have worked with him as a Ph.D student. Also, I am highly thankful to my co-

supervisor Prof. Dr. Bashir Ahmed Memon, for his guidance and valuable

suggestions from time to time. Especially his moral support and lessons of

patience encouraged me a lot.

Financial support from the Quaid-e-Awam University of Engineering Science

and Technology Nawabshah Pakistan, is greatly acknowledged. There are a

number of people who were involved directly or indirectly throughout my

research, I am thankful to all those for their support and I would like to

acknowledge them all. Thanks are also due to The Directorate of Postgraduate

Studies Quaid-e Awam University of Engineering Science and Technology for

their help and support to complete my work.

I would like to acknowledge Dr. Riaz Bhanbhro for his continuous guidance and

valuable suggestions throughout my research. I am also thankful to my younger

brother Rashid Hussain Bhutto and my colleague Shahnawaz Zardari for their

cooperation during my research.

Last but not least, I would like to thank my parents and my family for their moral

support throughout my study period.

iv

QUAID-E-AWAM UNIVERSITY OF ENGINEERING, SCIENCE

AND TECHNOLOGY NAWABSHAH

CERTIFICATE

This thesis written by Engr. Amjad Hussain Bhutto under the direction of his

supervisors and approved by all the members of the thesis committee, has been

presented to and accepted by the Dean, Faculty of Engineering, in fulfillment of

the requirements for award of the degree of Doctor of Philosophy in Civil

Engineering.

___________________________ ___________________________

Dr. Muhammad Auchar Zardari Prof. Dr. Bashir Ahmed Memon Supervisor Co-Supervisor

_______________________ _____________________________ Prof. Dr. Daddan Khan Bangwar Prof. Dr. Muhammad Munir Babar Internal Examiner External Examiner

___________________________ ______________________

Dr. Abdul Qayoom Jakhrani Prof. Dr. Noor Ahmed Memon

Director, Dean,

Postgraduate Studies& Research Faculty of Engineering

Date: ______________________

v

TABLE OF CONTENTS Description Page

DECLARATION i

DEDICATION ii

ACKNOWLEDGEMENT iii

CERTIFICATE iv

TABLE OF CONTENTS v

LIST OF TABLES ix

LIST OF FIGURES x

LIST OF ABBREVIATIONS xvi

ABSTRACT xviii

Chapter No. 1 Introduction

1.1 General 1

1.2 Problem statement 2

1.3 Research gap 5

1.4 Aim of research 5

1.5 Research objectives 5

1.6 Research methodology and work plan 6

1.7 Research outcomes and significance 7

1.8 Thesis layout 7

Chapter No. 2 Literature review

2.1 General 9

2.2 Historical development of embankment dams 9

2.3 Embankment dams 11

2.3.1 Zoned embankment dam 13

2.3.2 Components of zoned embankment dam 14

2.4 Soils used in construction of embankment dams 15

2.5 Soil varieties for embankments 16

2.6 Causes of slope failure 17

2.7 Causes of failure of embankment dams 19

vi

2.7.1 Rapid Drawdown mechanism in Embankment

Dams

21

2.7.2 Factors affecting slope-stability due to rapid

drawdown

24

2.8 Pore pressures in embankment dams 25

2.9 Stability conditions of embankment dams 26

2.9.1 End of construction and long-time stability 26

2.10 Slope stability analysis methods 29

2.10.1 Advantages of FEM in slope stability analysis 30

2.11 Main steps involved in FEM in Plaxis 2D 31

2.12 2D and 3D models 32

2.13 Factor of safety 34

2.13.1 Allowable safety factor 34

2.13.2 Importance of parametric analysis 35

2.14 Shear strength 35

2.14.1 Undrained shear strength 36

2.14.2 Drained shear strength 36

2.15 Staged construction of embankment dams 39

2.16 Settlement of embankment dams 39

2.17 Arching of embankment dams 41

2.18 Foundation treatment methods 46

2.18.1 Blanket grouting / consolidation grouting 47

2.18.2 Curtain grouting 47

2.19 Rainfall effect on slope stability 47

2.20 Types of embankment dams analysis 51

2.21 Soil models 52

2.21.1 Idealizations of plastic behavior 54

2.21.2 Linear elastic model 57

2.21.3 Mohr Coulomb Model (MCM) 58

2.21.4 Mohr-Coulomb Model Parameters 59

2.21.5 Application of Mohr Coulomb Model 63

vii

2.21.6 Hardening Soil Model 64

2.21.7 Applications of HSM 68

2.22 Summary 69

Chapter No. 3 Materials and Numerical Modelling

3.1 General 71

3.2 Project background and location 71

3.3 Materials and methods 72

3.4 Constitutive models 75

3.5 Stability of the dam 79

3.6 Settlement of the dam 80

3.7 Rapid drawdown of the dam 80

3.8 Effect of rainfall on stability of the dam 82

3.9 Summary 84

Chapter No. 4 Results and Discussion

4.1 General 85

4.2 Factors affecting dam stability 85

4.2.1 Effect of dam height on foundation soil 86

4.2.2 Effect of water filling on potential failure of

dam

88

4.3 Effect of friction angle of sandy gravel 89

4.4 Effect of friction angle of random fill 90

4.5 Effect of friction angle of clay core 91

4.6 Dam settlement comparison using MCM and

HSM models

92

4.6.1 Settlement at EoC and AFR 98

4.6.2 Dam settlement comparison w.r.t. depth 99

4.7 Variation effect of MOE of Sandy silt stone on

settlement w.r.t. depth

113

4.8 Comparison of long-term settlement for sandy

silt stone

115

viii

4.9 Rapid drawdown 116

4.9.1 Dam stability during drawdown under

Undrained Shear Strength of Clay = 20 kN/m2

117



119



121

4.10 Pore pressure under rapid drawdown 124

4.11 Effect of rainfall on slope stability of dam 132

Chapter No. 5 Conclusion and Suggestions

5.1 Conclusion 134

5.2 Recommendations for future studies 135

References

137

ix

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Summary of dams constructed in Pakistan 11

2.2 Dam classification based on capacity and height 12

2.3 Recommended values of safety factors 35

2.4 Parameters of Mohr Coulomb Model 58

2.5 Typical values soil permeability 63

2.6 HSM parameters 67

3.1 Material properties for Nai Gaj dam and its foundation 76

3.2 HSM parameters for four main zones of dam 77

3.3 Relationship b/w consistency index and undrained

shear-strength

81

4.1 Summary of dam's settlement comparison for EoC 97

4.2 Summary of dam’s settlement comparison for AFR 97

4.3 Summary of dam’s settlement comparison for EoC 105

4.4 Summary of dam’s settlement comparison for AFR 105

4.5 Variation effect of MoE of sandy siltstone on

settlement w.r.t. depth at EoC

112

4.6 Variation effect of MoE of sandy siltstone on

settlement w.r.t. depth for AFR

113

4.7 Reservoir drawdown rate vs. dam safety factor under

undrained shear strength of clay core = 20 kN/m2

123



123



123

4.10 Development and dissipation of excess pore pressure

under various drawdown rates

125

x

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Components of zoned type embankment dam

14

2.2 Slope instability when reservoir is lowered rapidly 23

2.3 Decrease of safety factor under rapid drawdown

scenario of Pilarcitos Dam

23

2.4 (a) Increase in pore pressure during construction time and

decrease during a long time (t)

28

2.4 (b) Change in safety factor during EoC and long-time 28

2.5 Potential failure zone in a dam with rigid foundation 28

2.6 Potential failure zone extending in embankment

and foundation

29

2.7 15 and 6- point nodal scheme and stress point scheme 31

2.8 Plain strain condition 33

2.9 Axisymmetric condition 33

2.10 Effect of increased clay content on bearing capacity of

footings on sand

38

2.11 Influence of increase in clay on slope's safety factor w.r.t

depths of failure surface

38

2.12 Effect of side slopes on arching ratio of clay core and shell 43

2.13 Effect of side slopes of core on arching ratio 43

2.14 Effect of increase of filter thickness on the arching ratio 44

2.15 Effect of shell core stiffness ratio with arching 44

2.16 Influence of inclined core on arching effect in a zoned

embankment dam

45

2.17 Effect of MoE of foundation on arching effect in a zoned

embankment dam

45

2.18 Rainwater infiltration through a dam 48

2.19 Rainfall infiltration through dam 49

2.20 Stress-strain curve of metals 54

xi

2.21 (a) Elastic and plastic behavior of material 55

2.21 (b) Elastic-linear-strain hardening plastic response of material 55

2.21 (c) Rigid plastic response of material 56

2.22 (a) Lightly over-consolidated clays and loose sands 56

2.22 (b) Over-consolidated clays and dense sands 57

2.23 Stress-strain behavior of an elastic, perfectly plastic model 58

2.24 Drained triaxial test results for 59

2.25 Dilatancy angle using the stress-strain curve obtained from

triaxial test

60

2.26 Determination of cohesion and friction angle 61

2.27 CD triaxial compression tests on three over consolidated

clays

62

2.28 Hyperbolic response in primary loading for triaxial test 66

2.29 Determination of from oedometer test 67

3.1 3D view of Nai Gaj Dam with earthen dykes 72

3.2 Cross section of Nai Gaj dam 75

3.3 FEM for Nai Gaj dam 79

3.4 FEM mesh of Nai Gaj dam 79

3.5 Highest reservoir level of Nai Gaj dam 79

3.6 10 m lowering of reservoir @ 1 m/day 82

3.7 Lowering of the reservoir up to 55 m depth @ 0.1 m/day 82

3.8 Average annual rainfall at Nai Gaj dam for four decades 83

3.9 Input of rainfall intensity at Nai Gaj Dam 83

4.1 Increase in pore pressure at 15 m dam raising 86




4.5 Potential failure zone of the dam at the EoC 89

4.6 Potential failure zone of the dam AFR 89

4.7 Effect of friction angle for sandy gravel on dam safety factor 90

xii

4.8 Effect of friction angle for random fill on dam safety factor 91

4.9 Effect of friction angle for clay core on dam safety factor 92

4.10 Dam settlement comparison for clay core at EoC 93

4.11 Dam settlement comparison for clay core at AFR 93

4.12 Dam settlement comparison for sandy gravel at EoC 94

4.13 Dam settlement comparison for sandy gravel at AFR 94

4.14 Dam settlement comparison for random fill at EoC 95

4.15 Dam settlement comparison for random fill at AFR 95

4.16 Dam settlement comparison for sandy silt stone at EoC 96

4.17 Dam settlement comparison for sandy silt stone at AFR 96

4.18 Settlement increase in % when MoE for sandy siltstone

varied from 70,000-125,000 kN/m2

98

4.19 Dam's central axis at which settlement was computed

w.r.t. depth at an interval of 20 m

100

4.20 Dam's settlement comparison w.r.t. depth at EoC applied to

clay core

100

4.21 Dam's settlement comparison w.r.t. depth at AFR applied to

clay core

101


sandy gravel

101


sandy gravel

102


random fill

102


random fill

103


sandy silt stone

104


sandy silt stone

104

4.28 Dam's settlement comparison w.r.t. depth for EoC applied to 106

xiii

sandy silt stone (MoE = 70,000 kN/m2)



107

4.30 Dam's settlement comparison w.r.t. depth for EoC applied to


107

4.31 Dam's settlement comparison w.r.t. depth for AFR applied to


108



108



109



109



110



110



111



111



112

4.40 Settlement at EoC by MCM and HSM models 114

4.41 Settlement for AFR by MCM and HSM models 114

4.42 Prediction of long-term settlement for sandy siltstone (MoE

= 125,000 kN/m2)

115

4.43 Prediction of long-term settlement for sandy siltstone (MoE

= 70,000 kN/m2)

116

4.44 Safety factor vs. time while lowering the reservoir under

undrained shear strength of clay = 20 kN/m2

118

xiv

4.45 Dam failure zone when reservoir is lowered @ 1 m/day to a

depth of 15 m (Su=20 kN/m2)

118

4.46 Dam failure zone when reservoir is lowered @ 0.1 m/day to

a depth of 20 m (Su=20 kN/m2)

119

4.47 Dam safety factor vs. time under undrained shear strength of

clay = 25 kN/m2

120

4.48 Dam failure zone when reservoir is lowered @ 1 m/day for a


120

4.49 Dam failure zone when reservoir is lowered @ 0.1 m/day for


121

4.50 Dam safety factor vs. time under undrained shear strength of

clay = 30 kN/m2

122

4.51 Dam failure zone when reservoir is lowered @ 1 m/day for a


124

4.52 Dam failure zone when reservoir is lowered @ 0.1 m/day for


124

4.53 Development of excess pore pressure under lowering of

reservoir @ 1 m/day for 10 m depth in 10 days (Su=20

kN/m2)

126


reservoir @ 0.1 m/day for 10 m depth in 100 days (Su=20

kN/m2)

126



kN/m2)

127



kN/m2)

127



kN/m2)

128

xv



kN/m2)

128



kN/m2)

129



kN/m2)

129



kN/m2)

130



kN/m2)

130



kN/m2)

131



kN/m2)

131

4.65 Effect of rainfall on dam stability 133

4.66 Dam failure zone due to 1-day precipitation 133

4.67 Dam failure zone due to 5-day precipitation 133

xvi

LIST OF ABBREVIATIONS

MOE = Modulus of elasticity

MCM = Mohr Coulomb Model

HSM = Hardening Soil Model

EOC = End of construction

AFR = After filling of reservoir

uo = Initial pore pressure

Δu = Change in pore pressure

tc = Construction time

t = Long time

LEM = Limit equilibrium method

FEM = Finite element method

2D = 2 Dimensional

3D = 3 Dimensional

FE = Finite element

kN/m2 = kilo Newton per square meter

kPa = kilo Pascal

m/s = Meter per second

= Arching ratio

= Total vertical stress in homogeneous dam

= Total vertical stress in zoned embankment dam

E = Young’s modulus of elasticity

v = Poisson’s ratio

G = Shear modulus

K = Bulk modulus

= Oedometer modulus

= Angle of internal friction

= Angle of dilatancy

c = Cohesion

xvii

= Elastic strain

= Plastic strain

E = Modulus of elasticity

Eo = Tangent modulus

E50 = Secant modulus/ primary loading stiffness modulus

Eur = Unloading-reloading modulus

C = Effective cohesion

= Effective friction angle

cm/sec = Centimeter per second

= Reference value of confining pressure

qf = Ultimate deviatoric stress

qa = Quantity of deviatoric stress

= Failure ratio

= Axial strain

= Stiffness modulus

= Tangent stiffness

Su = Undrained shear strength

= Major principal stress

= Minor principal stress

= Effective minor principal stress

m = Stress dependency power

= Coefficient of normal consolidation

= Uniaxial compressive strength

= Liquidity index

LL = Liquid limit

PL = Plastic limit

W = Natural moisture content

KPK = Khyber Pakhtunkhwa

xviii

ABSTRACT

The embankment dams require a massive quantity of fill materials. Embankment

dams have different zones, and the soils are generally obtained from different borrow

areas having different strength and stiffness properties, which may have effect on

stability and settlement. If the upper limit of the strength and stiffness properties of

various soils is used, there will be an overestimation of stability and underestimation

of settlement. The stability and settlement response of a 59 m high embankment dam

called Nai Gaj dam situated in District Dadu, Sindh, Pakistan is presented in this

study. The parametric analysis was conducted for three main zones of the dam:

upstream shell, core and downstream shell, which consisted of sandy gravel, clay and

random fill, respectively. The friction angle values of these materials were gradually

decreased to represent the soil conditions of different sources. It was observed that

the stability requirements for the end of construction and after filling of the reservoir

could be satisfied if friction angle values of the sandy gravel and random fill be

respectively utilized as 340 and 32

0 instead of 37

0 and 34

0. However, the value of the

friction angle of the core can be used as 300 without any reduction so that the dam

could be safe after filling the reservoir.

The settlement behaviour of the dam was analyzed by using the MCM and

Hardening Soil Model (HSM) models. The MCM model was applied to all material

zones and the HSM was used for four major material zones that occupied significant

volume. The settlement response of the dam was almost similar when computed with

the MCM and HSM for the three material zones (clay core, sandy gravel and random

fill), each having a modulus of elasticity (MOE) in the range of 25000 to 50000 kPa.

However, it was found that after the end of construction the MCM showed about

57% and 50% more settlement as compared to the HSM when MOE of sandy

siltstone was respectively varied from 70000 to 125000 kPa. The rate of increase of

settlement at the crest and at a depth of 120 m computed by MCM compared to HSM

was 53% and 82% respectively for after filling of the reservoir condition. The

settlement computed with MCM and HSM were 2.9% and 1.35% of the dam height.

It can be interpreted that the settlement predicted with MCM is unrealistically high

xix

due to the single constant value of modulus of elasticity (MOE). On the contrary, the

predictions of HSM are in agreement with the literature. besides, the long term

settlement computed using MCM is about 59% higher than that of HSM for after

filling of reservoir condition. The settlement of an embankment dam could be

predicted reliably by using HSM even when a limited number of stiffness data is

available.

The reservoir of the dam was lowered to a depth of 10 m up to 55 m in gradual

increments. It was found that the reservoir could be lowered up to a depth of 55 m at

a rate of 0.1 m/day when the undrained strength of clay core was 25 kN/m2. The dam

was also subjected to average daily rainfall intensity of 0.086 m/day continuously for

five days. The safety factor was reduced to 1.44 when the dam was subjected to the

rainfall duration for five days. This indicates that with the increase of duration of

rainfall, there is a reduction in the safety factor, because water enters through the

embankment and saturates the slopes. However, the stability of the dam is

satisfactory for the above-mentioned intensity and duration of the rainfall.

Keywords: embankment dam, slope stability, settlement, pore pressure,

consolidation, end of construction, filling of the reservoir, rapid drawdown, rainfall-

induced stability, parametric analysis, modulus of elasticity, Mohr Coulomb model,

Hardening Soil model.

CHAPTER 1

INTRODUCTION

1.1 GENERAL

Embankment dams have played a significant role in providing water for drinking and

agricultural purposes and generating electricity since ancient times. It is estimated that

about half of the embankment dams failures have occurred due to the build-up of

excess pore pressure, seepage, internal erosion, and excessive deformations [1]. These

instability problems may occur during routine and unusual conditions (e.g., heavy

rainfall, intense earthquake) during the service life of an embankment dam [2]. The

failure of a dam may cause loss of lives and financial resources. Hence, all dams must

function safely under routine everyday operations and under unusual conditions such

as floods and earthquakes. All embankment dams during their service life should be

systematically evaluated for safe performance under all operational conditions.

Most slope failures of embankment dams have occurred either during construction or

at the end of the construction stage. The increase of pore water pressures depends on

the availability of water content in the fill and construction rate [3]. The construction

of embankments at a rapid pace may increase total stress, as successive layers of fill

are placed. Due to soils' low permeability, the excess pore pressures might gradually

develop during the reservoir's raising and filling. The increase in excess pore

pressures may lead to a dam's slope instability or failure [2].

Water from rainfall may enter through the embankment into the dam body. High-

intensity rain may cause cracks in the crest and enter the embankment soils and join

the water table. As a result, the soils' suction and shear strength may reduce,

ultimately leading to instability conditions in embankment dams. The entry of

rainwater into clay core may have severe consequences on the embankment dams'

stability and settlement.

1

2

Besides the development of pore pressures, excessive deformations may affect dam

safety. The magnitude of deformations depends on several factors, such as soil

properties in various zones, the raising rate, and the dam's geometry. The dimensions

of various zones and their soils are designed in such a way as to minimize

deformations of an embankment dam concerning unique foundation conditions for

each site [4].

Pakistan is an agricultural country whose economy mainly depends on water

availability, which is also one of the primary sources of hydropower electricity. For

this purpose, various dams have been constructed, and some of the dams are planned

to be built in the future. It seems necessary to develop guidelines to ensure the

embankment dams' safety concerning local site conditions during their design life.

1.2 PROBLEM STATEMENT

The increase in pore water pressure during the construction of a dam and later during

filling of the reservoir stage may cause instability. This phenomenon leads to a

reduction in effective stress, which consequently reduces the shear strength. Thus, the

problem may lead to the dam's failure if the increase of pore pressures is not

appropriately controlled. Some of the failures of embankment dams have occurred

due to excessive generation of pore pressures during construction and filling of the

reservoir ( e.g., Teton dam USA 1976, Malpasset Dam France 1959, St. Francis Dam

USA 1928 [5]. The increase in pore pressure in an embankment dam may occur if

there is an intense rainfall for enough time, or the reservoir is lowered rapidly due to

some emergency.

Sometimes, an embankment dam's reservoir may be lowered rapidly due to

unavoidable conditions that may lead to the dam's instability. However, an

embankment dam's reservoir level needs to be lowered carefully at a slow rate to

avoid the upstream slope's instability. The rapid drawdown condition is dangerous for

the upstream slope because the water pressures do not counterbalance seepage forces

[6]. After drawdown, changes in pore pressure may occur due to changes in total

stress. Changes in pore pressure are dependent upon the stress-strain response of soils.

3

If the soils are stiffer, there may be a little change in pore pressure induced due to

applied stresses. The dissipation of pore pressure depends on initial conditions after

drawdown, permeability and stiffness of the soils.

The entry of water from rainfall through the crest of the dam to the phreatic level is

also a severe issue, as it may lead to a decrease in suction and shear strength due to an

increase in pore pressure. A gradual accumulation of rainwater in a slope may finally

result in slope failure [7].

The embankment dams are generally constructed as zoned structures involving

various materials with diverse stiffness and strength properties. Due to the difference

in stiffness of various zones, there may be the possibility of differential settlement. If

the dam is constructed on a weak foundation, there may be the risk of excessive

horizontal deformations and settlement.

Embankment dams' stability is affected by factors such as a change in the dam's

material properties and geometrical shape. The strength properties of embankment

dams depend upon cohesion and angle of internal friction [8].

It is evident from the literature that soil compaction with optimum water content on

the dry side may reduce shear strength significantly, particularly when fill material is

submerged. The resulting low shear strength may lead to slope instability. Also, the

optimum compaction of the dry side may result in the excessive settlement [9]. A

comparison of soil types also reveals that the clay core in an embankment dam

possesses low shear strength and high compressibility [10].

Since the embankment dams consist of a massive volume of fill materials, in such

conditions, generally different localities are chosen as the source of soil to be used as

the fill material in embankment dams. It is commonly observed that there is a

difference in stiffness and strength properties of soils collected from different borrow

areas. This difference in properties of collected soils may lead to problems related to

stability and settlement. Hence, it is highly essential to conduct a parametric study of

4

the soils concerning stiffness and strength to obtain a realistic estimate of an

embankment dam's stability and settlement.

The shear strength parameters, cohesion and friction angle, influence the stability of a

dam. The modulus of elasticity (MoE) of soils is a critical parameter that affects

settlement response. It is also common practice that the MoE of soils is not evaluated

based on laboratory or field tests. On the contrary, the value of MoE of soils is taken

from the literature based on correlations with other parameters. It is also evident that

the computations based on correlations are generally not reliable [11].

Presently, commercial finite element programs like PLAXIS 2D can be utilized to

computation stability and settlement behavior of embankment dams. This type of

software contains a library of soil models. The designer must choose a suitable kind of

constitutive model for a specific analysis performed for either stability or settlement.

Such constitutive models require input properties, mainly related to shear strength,

stiffness and permeability of soils. The stability and settlement of embankment dams

are directly dependent on the given input of material properties. Reliable estimates of

stability and settlement can be obtained if the suitable values of stiffness and strength

of various soils are given as input. As mentioned earlier, when the soils of different

zones of an embankment dam are obtained from different borrow areas, there may be

less reliability of the given input parameters.

First, it is necessary to identify appropriate constitutive models for computation of

stability and settlement response of embankment dams. Various types of constitutive

models are available in commercial finite element programs. If an advanced

constitutive model is selected for numerical analysis, there will be a need for more

advanced laboratory testing to properly evaluate soil properties. It is commonly

observed that there is a great lack of such advanced laboratory or field tests from

which reliable soil properties can be estimated. This is because the cost of advanced

laboratory and field tests may be higher than the allocated budget for the geotechnical

investigation. This implies a need for more studies on numerical modeling issues that

deal with obtaining reliable parameters of advanced constitutive models based on

5

limited laboratory or field data. Secondly, it is essential to select a suitable range of

stiffness and strength properties of soils used in various embankment dam zones.

For a zoned embankment dam, it is necessary to identify which types of soils have

more influence on settlement so that advanced constitutive models can be used for

their stress-strain behavior to obtain the reliable magnitude of settlement.

In addition to stability and settlement of an embankment dam, it is also essential to

numerically analyze (i) the safe rate of lowering of the reservoir if conditions warrant

for such a situation, and (ii) the effect of maximum possible rainfall, in the region

where the dam is located, on the stability of the embankment dam.

1.3 RESEARCH GAP

Based on the literature review, there is a significant need for more case studies on

stability and settlement issues in Pakistan's embankment dams.

1.4 AIM OF RESEARCH

Following is the main aim of the present study:

To address stability issues of embankment dams during construction, filling of the

reservoir and during service life.

1.5 RESEARCH OBJECTIVES

(i) To evaluate the stability of the embankment dam and its foundation during

gradual raising

(ii) To evaluate the stability of the embankment dam during the first filling of

the reservoir

(iii) To evaluate settlement of the dam and its foundation during construction,

filling of the reservoir and throughout the service life of the dam

(iv) To evaluate the stability of the dam for rapid drawdown condition and to

estimate the safe rate of lowering of the reservoir in case of emergency

6

(v) To evaluate the rainfall-induced stability of the embankment dam for

maximum possible rainfall when the reservoir is full.

1.6 RESEARCH METHODOLOGY AND WORK PLAN

A detailed literature review was conducted regarding stability and settlement issues

on embankment dams under static loading conditions. The focus of the literature

review was on the following main topics:

(i) Stability problems of embankment dams during construction, and filling of the

reservoir

(ii) Effect of excess pore pressure generated during gradual raising, lowering of

the reservoir, and rainfall on the stability of the embankment dams

(iii) Identification of material zones that have a significant influence on the

settlement of the dams

(iv) Use of suitable constitutive models for computation of stability and settlement

of embankment dams

(v) Identification of a proper range of strength and stiffness properties used in

various zones of embankment dams

A locally available embankment dam called Nai Gaj dam was identified as a case

study to perform numerical analysis to address stability issues, as mentioned in the

problem statement.

Numerical analysis of the embankment dam for the following conditions was

performed:

(i) Stability during gradual raising and filling of the reservoir

(ii) Stability during the rapid drawdown of the reservoir

(iii) Effect of maximum possible rainfall on the stability of the slopes

(iv) Settlement of the dam during the End of Construction (EoC) and After Filling

of the Reservoir (AFR)

7

1.7 RESEARCH OUTCOME AND SIGNIFICANCE

The proposed research outcome is hoped to benefit personnel working on

embankment dams, particularly concerning stability and settlement issues. The

following are regarded as the outcomes of the proposed research.

(i) Estimation of properties of filling material collected from different sources.

The same will be utilized in the constitutive modeling, especially when less

data regarding stiffness and strength of the soil is available

(ii) Identification of suitable models for different soil zones

(iii) Evaluation of stability and settlement of the dams

1.8 THESIS LAYOUT

The background of the research is presented in chapter No. 1, titled ―Introduction.‖ In

this chapter overall summary of the study, problem statement, the aim of the study,

objectives of the research, research methodology, and outcomes and consequence of

the study and thesis layout are described.

A review of the previously available state of the art by innumerable researchers is

presented in chapter No. 2. In this chapter, previous research work associated with the

research work presented in this thesis was reviewed. The literature review concerning

stability issues, settlement, different ratios of drawdown and rainfall effect of the

embankment dam using finite element program Plaxis 2D with constitutive models

explained in detail.

Complete details of dam location, dam geology, the primary purpose of dam socio-

economic condition materials used in the earthwork and the design of this dam, Finite

Element Model of the dam and associated material properties, stability, settlement, the

effect of lowering of the reservoir and maximum possible rainfall on the stability of

the dam are stated in chapter No. 3 titled ―Materials and Numerical Modelling.‖

8

Results and discussions are presented in chapter No. 4, ―Results and Discussion.‖ This

chapter explained all investigational work in tabular and graphical forms and

discussed them in detail. In the end, this chapter contains a summary of the results.

Chapter No. 5, ―Conclusions,‖ where detailed conclusions and suggestions for future

studies are described. In this chapter, all possible findings coined from the obtained

results are briefly presented along with potential suggestions.

9

CHAPTER 2

LITERATURE REVIEW

2.1 GENERAL

In the following sections, a review of available state-of-art is presented. The review

has been carried out concerning the objectives of the proposed research work. The

review's focus is on the instability conditions of embankment dams during

construction and operation due to increased pore pressure, drawdown and excessive

rainfall intensity, and identification of appropriate soil models that can be utilized to

analyze stability settlement response of embankment dams. Recent and relevant

literature has been included on the following main topics: historical development of

embankment dams zoned embankment dams and their components, materials of

construction of embankment dams, causes of failure of embankment dams,

development and dissipation of pore pressures under various loading conditions,

stability conditions of embankment dams, methods of slope stability analysis based on

limit equilibrium method, and finite element method.

2.2 HISTORICAL DEVELOPMENT OF EMBANKMENT DAMS

Egyptians were the pioneer in dam construction; they built the Sad-el-Kafara dam, 12

m high and 106 m long. After initial years of construction, the dam failed due to poor

construction and water overflow through the crest. Smith and Schnitter [12, 13]

observed that the dam's remains could be seen even today. In ancient times, the

Nimrud dam was constructed near Baghdad. The failure of this dam occurred in 1200

BC [14]. Sad-el-Arim dam, which initially was 4 m high, built along the River Marib

in Yemen. This dam was triangular with side slopes of 1:1. Sad-el-Arim dam was

constructed to provide water for irrigation purposes [15]. The failure of this dam

occurred in the 6th century due to poor maintenance for an extended period. Several

small earthen dams were constructed in eastern Arabia in ancient days.

9

10

In the olden days, several embankment dams were also constructed in India and

Ceylon. In 504 BC, the Padavil dam was built in Ceylon by making an embankment

that was 21.4 m high, having a foundation width of 61 m and a crest width of 8 m,

with a total length of 18 km [16]. Thirteen million m3 of earth fill was utilized for this

dam. The construction of dams had been given top priority during the reign of the

Roman Empire. One of the olden dams constructed by the Roman engineers was the

Cornalvo dam, 200 m long and 20 m in height [17]. The dam's inside was divided into

various sections by constructing stone walls in longitudinal and transverse directions.

The sections were latter filled with locally available materials, clay and stone. The

whole structure was covered with earth.

Masonry stone lining was used to construct dams in Germany, Austria, France, Great

Britain, and Italy. The necessity of dams construction was seriously felt during the

industrial revolution in the United Kingdom and other European countries.

Eventually, several dams were constructed in the United Kingdom and France at the

end of the 18th century and at the beginning of the nineteenth century [18].

In Great Britain, one of the largest dams was Todd Brook dam, which was 21 m high

[19]. In the 19th

century, various embankment dams were built in Europe and the USA

for water supply [20]. During the end of the 19th

century and beginning of the 20th

century, the industrial countries have had not enough experience of constructing

dams; therefore, failure of several dams occurred in those countries [21].

It is a common saying that necessity is the mother of invention. Having more need for

dams construction, attention was given to scientific knowledge for water flow through

soils. In 1856, Darcy presented a law for the flow of water through porous sandy soils.

Later, it was found that the distribution of water pressure through soils follows

Laplace’s differential equation. Karl Terzaghi, in the early 1920s, presented the basic

concepts of Soil Mechanics, which helped in understanding the behavior of dams built

of local construction materials. In Sweden, during that time, slopes' stability was

studied based on Soil Mechanics' principles.

11

Further advancement of Soil Mechanics theories regarding seepage, consolidation,

and settlement has led to better understating of embankment dams' design and

construction safely. The advancement of technology has also helped in the building of

large embankment dams. Modern dams are not only constructed for control of floods

and water supply, but also for the generation of hydroelectricity. Various dams have

been built in different parts of the world. For simplicity, some major dams constructed

in Pakistan are presented in Table 2.1.

Table 2.1: Summary of dams constructed in Pakistan.

Dam Height Location Type of dam

Mangla Dam 147 m Jhelum River Embankment Dam

Tarbela Dam 143.26 m Indus River Earth fill Dam

Gomal Zam dam 133 m Gomal River Curved gravity, roller

compacted concrete

Warsak Dam 76.2 m Kabul River,

Peshawar Gravity Dam

Allai Khwar Dam 51 m Allai Khwar River Reinforced cement concrete

Khan pur Dam 51 m Haro River, KPK Gravity dam

Hub dam 48 m Hub River Earth fill Dam

Mirani Dam 39 m Dasht River Concrete faced rockfill

Sabak Zai Dam 34.7 m Zhob River Earth and rockfill Dam

Duber Khwar Dam 32 m Duber Khwar River Gravity and roller

compacted concrete

2.3 EMBANKMENT DAMS

Dams are considered necessary for the socio-economic development and prosperity of

a nation. They are constructed to serve different functions such as water supply,

electricity, etc. Dams are built to act as a barrier to retain water. There are several

types of dams; embankment dams are the most commonly made [22]. Table 2.2

provides useful information regarding dams classification based on their storage

capacity and height [23].

12

Table 2.2: Dam classification based on capacity and height [23].

Size (m) Capacity (m3) Height (m)

Small Less than - 1,000,000 Less than 8

Medium 1,000,000 – 3,000,000 8 to 15

Large 3,000,000 – 20,000,000 15 to 30

Major Above - 20,000,000 More than 30

Soils and rocks are mainly used as materials for the construction of embankment

dams. Slopes of an embankment dam must remain stable during all operational stages

of its design life. There must be compatibility of soils and geometries with the

abutments of embankment dams [24].

The angle of inclination of embankments should be such that stability is assured. But

care should be taken in the design. Reducing the inclination angle up to 2-3° (to

horizontal) will cause a substantial increase of fill volume for a great dam [25].

There are several advantages to the construction of embankment dams. Some of the

main benefits of the embankment dams are described as follows [26]:

o If properly designed, these dams may be suitable for different climate,

topography and geological conditions.

o If suitable treatment is provided, embankment dams may remain stable even

on alluvial and pervious foundations.

o With the advancements of technology and existing knowledge of geotechnical

principles, a wide range of soils could be used to construct embankment dams.

o The construction of embankment dams does not involve the use of cement and

steel. This results in saving construction time and cost.

o Although an embankment dam may consume 4 to 6 times more fill volume

than a concrete dam, its total construction cost is 1/15 to 1/20 times lower than

the concrete dam.

o It has also been reported that the embankment dams offer more resistance to

seismic waves as compared to concrete dams.

The design of embankment dams should fulfill the following requirements:

13

(i) The main requirement for a dam to be safe is slope stability during all phases

of operation, i.e., EoC, filling of the reservoir, and long-term steady-state

seepage conditions.

(ii) Dams should be designed in such a way as to control excessive seepage,

internal erosion and uplift pressure.

(iii) It is necessary to provide enough freeboard to account for overtopping, post-

construction settlement of embankment and foundation.

(iv) If a dam is located in a seismic zone, it should have sufficient freeboard to

account for deformation due to the earthquake.

(v) The slopes should be protected with suitable and durable construction material

to resist the action of waves, rainfall, and wind.

2.3.1 Zoned Embankment Dam

Embankment dams may be homogenous, modified homogenous and zoned type. The

type of embankment dam is typically selected as zoned if there is the availability of

various types of soils. Embankment dams are heterogeneous, with various zones. The

fill material of a zoned embankment dam is usually obtained from local sites.

Embankment dams have a central impervious core. The upstream slope is usually

covered with a thin layer of rock called riprap, which protects it from water waves or

erosion. The downstream slope is normally vegetated to avoid erosion. Internal

drainage is provided to alleviate the harmful effects of pore water pressure. The

horizontal drainage is used to enhance the reduction of excess pore water pressures

[25]. This type of dam contains more than one material, e.g., clay, sand, and silt.

Various components of a zoned type embankment dam are shown in Figure. 2.1.

14

Figure. 2.1: Components of zoned type embankment dam [27].

2.3.2 Components of Zoned Embankment Dam

An embankment dam may consist of the following components [27]:

Core: It is an impervious wall of clay or fine material. The clay core is extended from

ground level to the highest flood level.

Shell: The particle size of the shell is coarser than the core. It is provided on both

sides of the core. It helps in the distribution of loads in the foundation and provides

support to the core.

Transition filter: It is placed in between the core and shell. With its help, the

movement of fine particles of the core is prevented from entering the shell's voids.

Upstream blanket: It is a layer of impervious material placed on the upstream side to

enhance the flow path and decrease seepage. As a result of this, excess pore pressures

and the seepage rate decrease on the downstream side.

Cut-off: It is an impervious barrier placed below the dam's core up to an impervious

stratum. It controls the flow of water. It is provided in those cases when the

foundation is incapable of resisting seepage.

Internal drain: It is provided to control seepage water, which may enter through the

core and cut-off. With the help of an internal drain, the downstream shell is kept

15

unsaturated. To prevent the carrying of fine particles, protective filters are also used in

addition to the internal drains.

Toe drain: It is generally provided at the downstream toe. The essential function of it

is the same as that of the internal drain.

Riprap: It is a protective layer that is generally provided to upstream sloped face and

sometimes to downstream slope to guard against the action of waves and rainfall,

respectively.

2.4 SOILS USED IN CONSTRUCTION OF EMBANKMENT

DAMS

For the construction of embankment dams, both fine-grained and coarse-grained soils

are used [28, 29]. The material mustn't contain more than 5% chloride salts, 2%

sulfate salts and 5% organic matter. The core of earth dams is constructed of

impervious earth materials having a permeability of 10-5

cm/s. The moisture content

of clay should be close to optimum moisture content or a little bit higher. An

embankment dam's construction work may become complicated and expensive if the

moisture content of clay is very wet or too dry. The shell of the dams could be

constructed with natural coarse-grained soils.

The rockfill materials which meet the following criteria be used for the construction

of embankment dams:

o Particle size smaller than 5 mm should not be more than 5%

o Particle size smaller than 20 mm should not be more than 30%

o The minimum permeability of the rockfill should be at least 10-2

m/s. The

shear strength of the rockfill should be high considering submergence in

water, breaking and fracture at high stresses.

Those, as mentioned above, are the suitable properties of rockfill. Several dams have

been constructed with rock fill, which does not satisfy suitable properties. It is usually

a difficult task for an engineer to opt for such a design in which rock fill having less

16

suitable properties is proposed to be used and to assure that the dam will function

safely over design life.

Transition zones, filter and subsoil drains can be constructed with sand, sandy gravel

material [30]. Sometimes, natural materials do not comply with all the criteria needed

for such zones. Therefore, several fractions of other materials are added, which may

result in 2-3 times more cost [31]. Dam slopes are usually protected from the action of

waves and weathering with stone fragments. Raising the dam's work is performed in

several layers, sprinkled with water around optimum moisture content and adequately

compacted with rollers.

2.5 SOIL VARIETIES FOR EMBANKMENTS

Fine-grained soils such as clays and silts with high water content may cause high pore

pressures to develop in an embankment dam. It is advisable to avoid clay as a

material for constructing an embankment dam if it has a liquid limit greater than 80.

Fine-grained soils are useful whose water contents are in the range of optimum

moisture content for compaction.

The reduction of water content of impermeable soils is generally difficult and

expensive. A borrow area may be rejected if the borrow area's impervious soils are 2–

5 % wet of optimum water content. Well-graded soils are generally given preference

to the soils of uniform particle sizes because the well-graded soils are stronger, less

compressible, and are less vulnerable to piping and erosion. The stones of sizes of

cobbles and boulders are removed from the embankment dam portion if their size is

larger than the layer's compacted thickness. Embankment soils may shrink upon

drying. Therefore, it is necessary to protect those shrinkable soils with non-shrinking

soils. The soils used in the core of embankment dams should have low permeability to

control the seepage rate through the embankments. The coefficient of permeability of

the core must be lower than 1×10 -5

cm/s. The shear strength of the shell material

must be sufficiently higher than that of the impervious core. Embankment

compressibility and foundation soils must be within tolerable limits to reduce the

dam's excessive settlement and deformation. The plasticity of embankment soils

17

should be high enough to curtail the possibility of cracks. The moisture content of

various zones of an embankment dam should be controlled close to the level of

optimum moisture content. The outer zone may be compacted at moisture content

below the optimum level, and the core is compacted at moisture higher than the

optimum one. It is a general recommendation that the content of particles smaller than

0.005 mm should be lower than 30% to maintain compaction quality control. It is

advisable to restrict the content of swelling and shrinking soils in embankment dams.

Generally, soils are considered heterogeneous material with the following properties:

o Soils exhibit stress-dependent stiffness and show nonlinear behavior.

o The stress-strain response of soils is dependent on confining pressure.

o Under loading conditions, the soils show elastoplastic behavior.

o Loose sands and normally consolidated clay show contractant behavior, dense

sands and overconsolidated clays exhibit dilatant response.

o It is generally reported that soils show different responses under primary

loading, unloading, and reloading conditions.

o Soils generally show anisotropic behavior.

2.6 CAUSES OF SLOPE FAILURE

Failure of a slope may occur due to changes in the properties of soils and loading

conditions. The fundamental condition for slope stability is that soils' shear strength

should be higher than the shear stresses that may cause failure. Therefore, it is

necessary to understand the factors that cause a decrease in shear strength and

increased shear stresses.

Factors causing a decrease in shear strength of a slope are [32]:

(i) Increase of pore pressures

Within slopes, a decrease of shear strength may be due to an increase in pore pressure

caused by the rise of groundwater level, heavy rainfall and seepage conditions.

Permeability of soils is the main factor by either increase and decrease of pore

18

pressures. In general, clayey soils have low permeability, but it has been observed that

such types of soils may show high permeability due to the presence of fissures,

cracks, and lenses of highly permeable materials. Consequently, pore pressures in clay

may sometimes change rapidly.

(ii) Cracking

There may be a possibility of the occurrence of cracks in the vicinity of the crest of a

slope. Since soil is weak in tension, these cracks may develop due to excessive tensile

stresses at the surface. With the development of cracks, the shear strength of the soil

is drastically decreased.

(iii) Swelling

Clayey soils may experience swelling if the water is added. The presence of water for

a long time and low confining pressure may cause swelling. It has been reported by

[33] that highway embankments of highly plastic clay have failed within a period of

10 to 20 years. The main reasons for the failure of such embankments were strength

loss, swelling and cracking.

(iv) Creep for constant loads

Continuous deformation may occur in highly plastic clays when acted by constant

loads. Failure of clays may occur under sustained loading, causing lower shear

stresses than the short-term strength. The creep behavior of clay may get intensified

under cyclic conditions of freezing-thawing and wetting-drying.

(v) Strain softening

Brittle soils show strain-softening behavior. As the strains are further increased

beyond the peak strength, a corresponding decrease occurs in the brittle soils' shear

resistance. Such type of response of brittle soils may facilitate the progressive failure

of slopes.

19

(vi) Cyclic loading

Cycling loading may occur due to an earthquake. As a result, loose saturated soils

become dense due to cyclic loading and pore pressure. Eventually, slope stability is

endangered with an increase of pore pressure.

Factors causing an increase in shear stress in a slope are [32]:

(i) Loads on a slope: If there are heavy loads on the crest of a slope, there will be

an increase in shear stress.

(ii) Presence of water in cracks of a slope: Sometimes, cracks may develop in a

slope. When water enters through cracks, it will make the cracks wide and

create increased pore pressure, which may increase shear stress and cause the

slope to destabilize.

(iii) The presence of water causes an increase in soil weight: When there is more

water content in a slope, there may be an increase in soil mass, which may

increase shear stress.

(iv) Excavation of a slope: If a slope is made steeper by excavation or erosion,

shear stress may increase.

(v) The decrease in water level: If the water level in any slope is rapidly

decreased, it may cause the slope's instability due to the removal of water

support. As a result, there will be an increase in shear stresses in the soil,

creating instability conditions.

2.7 CAUSES OF FAILURE OF EMBANKMENT DAMS

Embankment dams are constructed with compacted soil or small rock particles. These

dams are generally cheaper because locally available soils are utilized as the material

of construction. The stability of embankment dams depends upon the design and

material of construction [34]. To reduce the possibility of seepage, the central core of

embankment dams is constructed of fine-grained soil, i.e., clay and silt. Possible

failure modes of embankment dams could be due to hydraulic, seepage and structural

[35].

20

(i) Hydraulic failure: Hydraulic mode of failure of a dam may be due to wave

erosion, which may weaken soils and decrease the section of the dam, wind or

water erosion of the dam crest and downstream slope of the dam [36].

(ii) Seepage failure: Due to excessive seepage, there may be a possibility of slope

instability, uplift pressure, internal erosion, and foundation failure. Due to

extreme floods or storms, there may be chances of overtopping a dam [37].

Overtopping of dams may occur if the incoming flow is more than the

reservoir's storage capacity [38]. Overtopping of a dam may occur due to less

freeboard and excessive settlement of foundation/embankment soils. Due to

the low permeability of soils and placement water content during construction,

pore pressures may increase in the dam's embankment and foundation. If the

soils' permeability is less than 10-6

cm/s, the pore pressures do not dissipate as

rapidly as the construction work is carried out. If the pore pressures gradually

accumulate as the dam's successive layers are placed, it might create

conditions of low shear strength, which may lead to instability or failure of a

dam [39].

(iii) Structural failure: Some common structural failure signs are cracking,

excessive settlement, and slides [40]. This type of failure mode may occur due

to strong earthquakes [41] or erosive forces, which may weaken the

embankment and foundation soils. Besides, the roots of trees along the

embankment may weaken the soils and lead to the dam's instability. Saxena

and Sharma [14] have shown that 40 percent of the embankment dams have

failed due to overtopping.

For both the upstream and downstream slopes, the dam's safety factor must be

determined for the conditions considered to be most critical during the dam's life

span. It may be possible that the failure zone may occur within embankment soils or

may pass through both the dam and foundation. Two conditions are critical for an

upstream slope: EoC and AFR. For a downstream slope, EoC and steady-state

seepage conditions are essential stages. Slope stability is severely influenced by the

presence of pore water pressures in the dam body. In the normal routine, piezometers

21

are fixed in the dam body to record actual pore pressure generation to compare with

pore pressure [25]. Suppose the failure zone is likely to occur in the foundation, which

contains fissures and joints. In that case, there may be chances of progressive failure,

which may also occur due to difference in stress-strain characteristics of different

soils and non-uniformity of induced shear stresses.

Cracking of embankment dams is a severe problem, which may occur due to

differential settlement and compressibility characteristics of various soil zones.

Besides, there may be a likelihood of hydraulic fracturing of clay core under high

water pressures. Hydraulic fracturing may occur at those planes of the dam where

everyday stresses are lower than the water pressure. In the long-term scenario, clay

cores may settle concerning other zones in a dam. As a result, vertical stress in the

core may be decreased, leading to a hydraulic fracturing process [25].

In compacted embankment dams, strains develop due to differential settlement. As a

result, internal stress distribution changes and may increase stress at some portions

and reduce others. With the gradual increase of the reservoir's filling, the water

pressure exceeds the embankment stresses in the impervious zone and forms a

concentrated leak. With more deformation of the impermeable embankment material

due to the reservoir's filling, more cracks may occur. Eventually, the minor principal

stress becomes so low to create conditions to develop hydraulic fracturing [42].

In addition to the above-mentioned mode of failures, it has been reported that the

embankment dams may fail due to rapid drawdown.

2.7.1 Rapid Drawdown Mechanism in Embankment Dams

Sometimes due to an emergency, a reservoir may be lowered. In this situation, excess

pore pressure does not reduce as rapidly as the water is lowered, reducing the

stabilizing force. An embankment dam's upstream slope may become vulnerable to

failure due to the quick lowering of the water level [43]. Sometimes with the lowering

of the reservoir, instability of upstream embankment may not occur immediately.

22

Still, concerning time, seepage forces may develop, which may ultimately lead to

failure of the dam (Figure. 2.2).

When the water on the upstream side of the dam is removed, its effect as a stabilizer is

lost, and there will be an increase of shear stresses in the slope. If the drawdown rate

is fast, and the permeability of soils is low, then the undrained condition will increase

with excess pore pressure [44]. Even if water pressure is removed in unloading, there

may be a decrease in pore pressure, depending on whether soil shows contractant or

dilatant behavior [44]. It is difficult to correctly predict pore water pressures during

undrained loading or unloading conditions; therefore, the total stress method is

generally used [44].

Limit equilibrium analysis, along with undrained triaxial compression tests, has been

used to analyze dams' stability under drawdown conditions [45]. Apart from limit

equilibrium methods, the finite element method based on the total stress approach has

been used for computation of stability of Pilarcitos Dam under drawdown condition

[44]. In 1969, when the reservoir level of the Pilarcitos Dam was lowered by 10.7 m

in 43 days, the upstream slope suffered a rapid drawdown slide, which was about 5 m

deep and 28 m in length. Vandenberge [44] and Fredlund et al. [46] have also

conducted a useful stress analysis of the Pilarcitos Dam to simulate the stability of the

dam under drawdown condition (Figure. 2.3). The authors have also described the

failure mechanism of 60 feet high Walter Bouldin Dam under a rapid drawdown of 32

feet in 5.5 hours using effective stress analysis.

23

Figure. 2.2: Slope instability when reservoir is lowered rapidly [43].

Figure. 2.3: Decrease of safety factor under rapid drawdown

scenario of Pilarcitos Dam [46].

24

2.7.2 Factors Affecting Slope-Stability Due to Rapid Drawdown

When a reservoir is lowered, undrained conditions may develop with an increase in

excess pore water pressure, which may reduce concerning time. As a result, instability

of the upstream slope may occur, which may lead to failure. Several factors cause

instability of a slope during the drawdown process: drawdown rates, slope angles, soil

characteristics [47]. Factors such as geometry, soil characteristics, internal (pore

pressure) and external forces (surface water pressure) directly affect the slope

stability. The slope is affected by internal and external forces, and the water level

change due to partial or total submergence [48, 49]. Because of change in water level,

two types of pore pressures may develop inside a slope: (i) seepage pore pressure due

to transient flow, (ii) excess pore pressure due to loading. As the consolidation occurs,

excess pore pressures reduce with time [48]. Decrease of both the seepage pore

pressure and excess pore pressure depends on compressibility and permeability of

soils, and drawdown rate.

Stress-induced pore pressures mainly reduce during drawdown if the permeability of

soils is high. However, in low permeability soils, the dissipation rate of seepage pore

pressures and stress-induced pore pressures is not usually the same; therefore, the

soils behave as totally or partially undrained. If the external water level is dropped

quickly, it is called rapid drawdown without allowing proper time for drainage to

occur. As a result, soil resistance reduction will happen that may cause slope failure

[48, 50]. Such types of failures have occurred in natural and man-made slopes. Some

examples are Pilarcitos Dam, Walter Boudin Dam, and several river banks [45].

Therefore, it is necessary to analyze those slopes where there are chances of rapid

drawdown. Nowadays, stability analysis of slopes is commonly performed using

advanced numerical tools based on the finite element method, capable of considering

the nonlinear material response, complex boundary and loading conditions [49, 51,

52].

For slope stability under drawdown conditions, consideration is given to external

loads and seepage forces due to transient flow. The consolidation process can occur in

a slope during the drawdown, which depends on the rate and time of drawdown and

25

soils' permeability. During the drawdown, pore pressure changes occur, consisting of

static hydraulic pressure, seepage pressure, and excess pore pressure. These pore

pressures are not steady; they dissipate with time as the consolidation process occurs

[48]. The researchers [53-55] have developed charts based on limited state analysis to

compute safety factors under rapid drawdown conditions. The safety factor of a

partially submerged slope under rapid drawdown condition cannot be calculated using

charts developed by [53, 55]. On the contrary, such slopes' safety factor can be

computed by utilizing the finite element method [49].

2.8 PORE PRESSURE IN EMBANKMENT DAMS

The increase and reduction of excess pore pressures during construction and a long

time are governed by the consolidation process [56]. For large dams, high pore

pressures may occur within the core [57]. The safety of dams depends on how the

pore pressures develop during construction and decreases during a long time as the

consolidation process occurs [58].

In embankment dams, the pore pressures develop particularly in clay core as the

dam's raising is carried out. If the construction work of the dam is performed fast,

then the pore pressures will generate rapidly. The situation may get worse if the

reservoir is filled quickly. This type of scenario may create conditions that can lead to

hydraulic fracturing of the dam [59].The stability of embankment dams reduces with

the generation of pore pressure during construction. With the compaction process and

weight of the subsequent layers, the pore pressures get increased. In this way, the

shear strength and stability of the slope gets reduced. Concerning time, there may be

gradual dissipation of pore pressures, which may increase stability. For large

embankment dams, construction pore pressures are recorded during the raising of the

dam. The magnitude of pore water pressure depends on the dam's height, degree of

saturation, drainage system, and permeability of soils [60]. To determine the shear

strength of slope in terms of effective stress, pore pressures must be evaluated with

reliability. There are several sources of inaccuracy to estimate pore pressures in

embankment fill. When the reservoir level is increased, it causes more portions of the

26

soil to be saturated, increasing pore pressure and a decrease in shear strength and

slope stability [61].

Embankment slope instability and deformation can initiate with the increase in pore

pressures [62]. Deep cracks may develop due to long-term drought. During rainfall,

the developed cracks may enlarge and become channels for infiltration that may result

in internal erosion and reduced dam stability. Drought and flood cycles may cause

instability of embankment dams [63].

2.9 STABILITY CONDITIONS FOR EMBANKMENT DAMS

For the design and safe construction of embankment dams under different loading

conditions, slope stability is essential. The safety factor is evaluated with the help of

the limit equilibrium and finite element method. Slope stability depends upon the

shear strength of soils in embankment and foundation [64].

Under dry conditions, clay has high cohesion and friction angle as compared to the

wet state. Therefore, under dry conditions, embankment dams having central

impervious clay are satisfactory concerning slope stability. Several factors that affect

slope stability are characteristics of soils, side slopes of the embankment, seepage

forces, heavy surcharge, seismic load, excessive rainfall intensity. A natural stable

slope may fail due to the reduction of shear strength, an increase of external loads,

and a change of the slope's geometrical shape.

The stability of embankment slopes fluctuates with respect to time-dependent changes

in loads and shear strength of soils. Consequently, a slope's safety factor may decrease

or increase with respect to time-dependent material and loading conditions that occur

in a slope. Therefore, slope stability analyses are performed for different loading

conditions.

2.9.1 End of Construction and Long-time Stability

During the compaction process of fine-grained soils in the dam body, water is usually

sprinkled to facilitate compaction. As a result, the majority of the pores in the soil get

27

filled with water. Due to a load of incremental compacting layers and the rollers'

movement, there is an increase in pore pressure in those soils whose permeability is

low. The increase in pore pressure depends on several factors such as permeability

and compressibility of the soils, placement moisture content of layers, how fast the

dam is constructed, possible reduction of pore water due to drainage of soils.

The end of the dam construction may be considered a critical condition that may last

for a particular time because there is partial dissipation of pore pressures concerning

time. For embankment dams, a value of safety factor of 1.3 is considered satisfactory.

If there is uncertainty regarding various soil material properties, it is better to adopt a

higher safety factor value.

If the soils are of low permeability in an embankment dam, the dissipation of excess

pore pressures will take time. This implies that the soil would be in an undrained

condition at the end of the construction stage. For the calculation of stability, total

stress analysis will be utilized [25].

Figure. 2.4 (a) illustrates the increase in pore pressure during the construction stage of

a dam. Initially, the pore pressure is u0 at the start. The pore pressure then gradually

increases by an amount (Δu) during construction time (tc). After the end of the

construction stage, the pore pressures gradually dissipate as the soils are allowed for

consolidation to occur for a long time (t). Figure. 2.4 (b) shows that the safety factor

initially decreased at the EoC. The safety factor has increased due to the reduction of

excess pore pressures in a long time. This suggests that the critical stability condition

may occur at the end of a dam's construction stage.

28

Figure. 2.4 (a): Increase in pore pressure during construction

time and decrease during a long time (t) [25].

Figure. 2.4 (b): Change in safety factor during EoC

and long-time [25].

If an embankment foundation is rigid and robust, then a possible failure zone may

only initiate in embankment soils (Figure. 2.5). On the contrary, if the foundation is

weak, then the failure zone can develop both in the embankment and foundation

(Figure. 2.6). This implies that stability depends on both the shear strength of

embankment and foundation soils.

Figure. 2.5: Potential failure zone in a dam with rigid foundation [25].

29

Figure. 2.6: Potential failure zone extending in embankment

and foundation [25].

2.10 SLOPE STABILITY ANALYSIS METHODS

Two types of methods have been used for the calculation of slope stability of

embankments: the limit equilibrium method and finite element method:

(i) Limit Equilibrium Method (LEM)

The LEM is widely used to compute slope stability. Traditionally, slope stability was

performed using different methods based on conditions of equilibrium. Such

techniques consist of: the ordinary method of slices [65], Bishop's Modified Method

[66], force equilibrium methods [67], Janbu's generalized procedure of slices [68],

Morgenstern and Price's method [69] and Spencer's method [70]. One of the

disadvantages of these limit equilibrium methods is that the potential failure zone is

assumed in advance.

Several assumptions regarding normal and shear forces acting on various slices are

made in limit equilibrium methods of slope stability. This is due to several unknowns

as compared to the number of equilibrium equations. If the ordinary slices method is

used for slopes with high pore pressures, the calculated safety factor is 50% smaller

than the correct value. This is because only one condition of equilibrium is satisfied

[71]. Spencer method of slope stability is more accurate than the other limit

equilibrium methods [72].

30

The ordinary method of slices gives conservative values of safety factors in

comparison to other methods. The slopes designed on the standard method of slices

are always considered to be on the safer side. Some realistic assumptions of interslice

forces are made in methods like Ordinary Bishop's Method, Spencer's Method and

Morgenstern and Price as compared to the ordinary method of slices [73].

Morgenstern and Price method was extended by satisfying both moment and force

equilibrium conditions [68-70].

(ii) Finite Element Method (FEM)

With the advance of science and technology, the FEM has been extensively used in

geotechnical engineering to solve various problems [49]. In geotechnical engineering,

calculation of the stability of a slope plays a vital role. Very complex slope stability

problems have been solved with the help of FEM analysis. The stability of

embankment dams depends upon the geometry, properties of materials in

embankment and foundation. Besides, the stability of an embankment dam relies on

the level of the reservoir and its fluctuations. With the help of FEM settlement and

deformations, flow quantities can be determined for embankment dams. Generally,

two-dimensional FEM programs are used for calculation of slope stability of

embankments. At present, many geotechnical engineering software based on the FEM

is available commercially, which can be used to analyze dams' stability under various

operating conditions. The analysis results, such as settlement and deformation of an

embankment dam, mainly depend upon realistic input of material properties,

especially strength and stiffness parameters [74].

2.10.1 Advantages of FEM in Slope Stability Analysis

The following are the main advantages of FEM over the traditional limit equilibrium

methods of slope stability [49]:

(i) No assumptions are made for the position and shape of the failure zone. Failure

zone develops in those regions of the soil having lower shear strength than

applied shear stresses.

31

(ii) Equations of equilibrium and compatibility are satisfied.

(iii) No side forces are assumed because the failure zone is not divided into slices.

(iv) Realistic deformations of a soil mass could be computed if reliable parameters

of compressibility are utilized.

(v) FEM can capture progressive failure as well as overall shear failure.

2.11 MAIN STEPS INVOLVED IN FEM IN PLAXIS 2D

(i) Discretization of elements

The geometry of a geotechnical problem is divided into finite elements. These

elements are triangular in shape. There are two possibilities to select either 15 nodes

triangular element or 6-node triangular element, as shown in Figure. 2.7. There are 12

stress points and 3 stress points in 15 node elements and 6 node elements.

Figure. 2.7: 15 and 6- point nodal scheme and stress point scheme [75].

(ii) Approximation of primary (unknown) variables

Primary variables, such as displacement, is selected. The rules for variation of

primary variables over a finite element are mentioned.

32

(iii) Elemental equations

Element equations are derived and then combined into global equations.

(iv) Boundary conditions

Boundary conditions for displacement, loads and pore pressures are described. The

global equations are then modified with respect to boundary conditions.

(v) Solution of global equations

The global equations are obtained in large numbers as simultaneous equations. The

solution of the equation gives displacements at all nodes. Secondary quantities such as

stresses and strains are obtained from nodal displacements. The factors on which the

finite element analysis accuracy depends are the size of elements and the

displacement variation.

2.12 2D AND 3D MODELS

Generally, geotechnical problems are solved using three-dimensional numerical

techniques. 3D slope stability analysis is performed when there is a curved or

complicated geometry of an embankment dam [41]. Since the 3D problems involve a

huge geometry with a lot of a number of nodes. As a result, the solution of the 3D

problem using computer software takes substantial computational time. To reduce

computing time, the 3D problems are solved using plane strain and axisymmetric

conditions.

(i) Plane strain condition

For simplification, 3D geometry of geotechnical problems such as retaining walls,

continuous footings, and the stability of slopes, can be analyzed using plane strain

condition. It is assumed that the strain in the longitudinal direction is negligible

(Figure. 2.8). Therefore, a 3-dimensional geometrical problem can be reduced to a 2-

dimensional space. It is generally observed that in 2D situations, the factor of safety is

33

lower than 3D. So from a stability point of view, it is appropriate to use the 2D FE

method instead of 3D to save time [76].

Figure. 2.8: Plain strain condition [75].

(ii) Axisymmetric condition

Axisymmetric condition is adopted for circular structures having a radial cross-section

and loading on the central axis (Figure. 2.9). It is assumed that deformation and

stresses are similar in the radial direction. For geotechnical problems, which involve

axisymmetric condition, x-coordinate shows radius and y, and coordinate represents

the axial line of symmetry.

Figure. 2.9: Axisymmetric condition [75].

34

2.13 FACTOR OF SAFETY

Slope stability is usually determined with the help of the safety factor described as the

ratio of available shear strength of the soil to the shear strength required for

equilibrium. In the FE program PLAXIS 2D, the safety factor is defined using the

strength reduction technique. In this strength reduction technique, the tangent of the

friction angle and soils' cohesion is incrementally decreased until the soil mass's

failure. The safety factor is defined as follows [77]:

∑ =

Where is a safety factor, are friction angle, cohesion and undrained

shear strength of the soil.

2.13.1 Allowable Safety Factor

When the safety factor is greater than 1, it shows that resistive forces are more than

the driving forces. There are several uncertainties in determining material properties

with reliability; therefore, it is advisable to use safety factors greater than unity.

The slope stability of embankment dams is determined based on the factor of safety.

The following are the main points which provide basis why factory of safety is

necessary:

o Since the soil is heterogeneous material, there is less reliability of available

experimental techniques for reliable calculation of the strength properties of

soils

o There is some degree of uncertainty in the computation of pore pressures

within soils

o The magnitude of the safety factor gives a possible indication of the effect of a

dam's failure on losing lives and property.

o It depends on the height, storage capacity and foundation conditions of soils

o It also depends on how extensive the geotechnical investigation is performed

and how well the construction work is carried out.

35

The embankment dams' safety factor depends on the input values of shear strength

parameters of different soil types. It is indispensable that representative values of

shear strength parameters be obtained from field or laboratory tests. If the values of

strength parameters are overestimated, consequently, the safety factor can be

unrealistically higher. Allowable safety factors for various operating conditions of

embankment dams are mentioned in Table 2.3.

Table 2.3: Recommended values of safety factor [78].

Condition Safety factor Slope

End-of-construction 1.3 Upstream and Downstream

Long-term 1.5 Downstream

Rapid drawdown 1.1-1.3 Upstream

2.13.2 Importance of Parametric Analysis

A parametric analysis is carried out to investigate the effect of variation of specific

essential parameters on a study's results. For example, in geotechnical engineering, a

parametric analysis may be required for assessing the impact of strength parameters

(cohesion and friction angle of soils) on the overall slope stability of a dam. The

parametric analysis may also be required to assess the effect of stiffness properties on

the settlement response.

2.14 SHEAR STRENGTH

The shear strength of soils is internal resistance against the shear stresses along the

failure plane. The shear strength of soils depends on friction angle and applied normal

stresses. In addition to this, it also depends on the cementation of soils. The shear

strength analysis is conducted for two scenarios: drained and undrained conditions,

depending on soil loading and permeability. The drained condition develops under

loads applied very slowly, so excess pore pressures do not generate in the soil mass.

On the contrary, the undrained condition develops if the loading rate is fast, so there is

not enough time to reduce excess pore pressures [79].

36

2.14.1 Undrained Shear Strength

When soil is loaded to failure under undrained conditions, its corresponding strength

is called undrained shear strength. In the field, the undrained condition refers to loads,

which are applied faster than the soils' rate of drainage. On shearing, normally

consolidated soils exhibit increased pore pressure, and overconsolidated soils show

negative pore pressures under the undrained state. Undrained shear strength is

generally applicable to low permeability soils such as clay and silts [78].

Undrained shear strength is affected by soil type, moisture condition, and stress

history [80]. If the time is allowed for consolidation, the undrained shear strength of

clay during multi-stage loading increases [81].

It depends on the anisotropy of soils, stress history and strain rate [82]. The strength

of soils can be determined by laboratory tests and in-situ tests [82, 83]. It is to be

noted that there is some degree of disturbance observed in laboratory tests; therefore,

such type of tests do not represent actual in-situ conditions. In addition, laboratory

tests may be misleading because of the smaller size of the specimen that involves

fissures [84]. For undrained conditions, water remains in pores of soils. As a result,

with an increase in normal stress or shear stress, excess pore water pressure in fine-

grained soils increase. In practice, undrained shear strength is not determined as a

single value representative of soil conditions—the value of undrained shear strength

changes with stress direction [79].

2.14.2 Drained Shear Strength

Drained shear strength of soils shows the condition in which load is applied very

slowly so that there should not be an increase in excess pore pressures. In the field,

the drained condition refers to soil loading for a long term when pore pressures reach

equilibrium. Dense soils show dilatancy, and loose soils show contracting during

shearing under drained conditions. For conducting stability analysis of various

geotechnical structures, the shear strength is a major parameter. For reliable results of

stability analysis, correct shear strength values must be determined and given as input

37

to the finite element programs. In the case of non-cohesive soils (sands and gravels), it

is essential to assess correct values of effective friction angle from laboratory tests

[85].

Generally, the shear strength parameters of ideal soils, like, clay, and sand, are

presented in the literature [86]. It has been commonly observed in the field that a

variable amount of fines (silt and clay) is mixed with sandy soils. The load response

of sand-clay mixture soils may be lower than pure sands due to a change in shear

strength and stiffness properties. The sand-clay mixture soils may possess properties

which are intermediate between sand and clay [87].

The effect of fines content on the behavior of sandy soils was studied by many

researchers, e.g., [87-91]. In the above-mentioned studies, direct shear/triaxial tests on

different sand-clay mixtures were conducted. It was commonly observed that at a

particular confining pressure, there was a corresponding decrease in effective internal

angle of friction and an increase in effective cohesion by increasing clay content.

Furthermore, it was also observed that the plasticity index and compression index of

the sand-clay mixture increased with the addition of more percentage of clay.

Najjar et al. [85] report that the allowable bearing capacity of 1 to 4 m deep footings

on sand decreases due to the presence of clay content (Figure. 2.10). It can be

observed from the Figure. that there is a relatively decrease in the small percentage of

allowable bearing capacity when the clay content exceeds from 20% to 40%. Najjar,

Yaghi et al. [85] also mention that an infinite slope's safety factor at a given depth of

failure surface reduces with the clay content (Figure. 2.11).

38

Figure. 2.10: Effect of increased clay content on bearing

capacity of footings on sand [85].

Figure. 2.11: Influence of increase in clay on slope's

safety factor w.r.t depths of failure surface [85].

39

2.15 STAGED CONSTRUCTION OF EMBANKMENT DAMS

Staged construction is a technique in which step by step construction of various

geotechnical structures is analyzed using finite element programs. The construction of

dams is also modeled using staged construction techniques. In this approach, an

embankment dam can be raised gradually in layers using a specified period as it is

practiced on-site. One layer is placed during a specific period, and then the successive

layers are raised accordingly. If the permeability of soils involved in foundation and

embankment is low, there may be an increase in excess pore pressure, which can

decrease the dam's shear strength and overall stability. If the foundation of a dam is

not strong enough, then partial consolidation is allowed for some time so that the soils

can gain strength. In this way, with the finite element program, a designer can suggest

a proper time of raising and partial consolidation depending upon the strength and

permeability conditions of the foundation soils of a dam.

2.16 SETTLEMENT OF EMBANKMENT DAMS

Among all types of the number of earth and rockfill dams around the globe are more

than 85% of all the dam types. The excessive settlement is the main problem to which

these types of dams may be subjected. Therefore, it is essential to make a reliable

estimate of the settlement of such dams. Generally, maximum crest settlement of a

dam can be up to 2% of the height. The magnitude of a dam's settlement depends on

the self-weight of the embankment material and foundation geology [92].

The design engineer can predict a dam's behavior such as settlement, slope stability,

stress state and seepage based on geometry, soil properties involved in the dam and

foundation. Parameters such as settlement, stresses, and pore pressures are recorded

during construction and operation stages of a dam to determine whether the dam is

performing as per the predictions made during the design stage. The acceptable range

of behavior depends on the height of the dam and soil characteristics. A general rule is

that an embankment dam's post-construction settlement must not be greater than 1-2

% of the dam's height. After filling the reservoir, lateral deformation may occur at the

40

crest of the dam. If the loading condition is constant, then it is necessary that the rate

of settlement of a dam must decrease with respect to time [93].

If a rockfill dam is not compacted properly, then a settlement of 1 to 2 % of the dam's

height may occur. If the rockfill dams are well compacted, the probable settlement

could be lower from 0.5 to 0.7 % of the dam height [94]. When the settlement of an

embankment dam is analyzed, it must include the soft bedrock's depth. This is

because the total settlement of the embankment dam depends on the foundation

settlement [95].

The density and MoE of soils directly affect the deformation of the dam. This

indicates that the deformation of a dam will be more if the density and MoE of

different soil types are lower. A major portion of deformation occurs during the

construction process of a dam. Usually, in embankment dams, the core is considered

soft material as compared to the shell. When there is a significant settlement in a core,

the shells' load transfer occurs [96].

If the dams are constructed on foundations comprising thick layers of soft clay, there

can be issues related to stability and excessive deformations. Such types of problems

can lead to the development of cracks, fractures or collapse. Such types of issues can

be solved using staged construction in which partial consolidation occurs during the

raising process. The strength of soft clay soils depends on the embankment load, the

speed with which construction activities are performed, and waiting time between

different construction stages [3].

Ozcoban et al. [3] describe the staged construction of the Alibey dam (Turkey) on a

clay's thick deposit. Various instruments were installed on the site to record

settlement, deformation, and excess pore pressures in this dam. Based on the field

data, the dam's construction rate was adjusted, and the dam was safely constructed on

a soft clay foundation. The maximum recorded settlement at the main dam section

was 4.34 m that occurred during a time period of 24 years.

41

Javanmard et al. [97] conducted a settlement analysis of earth-fill dam Taham (Iran),

123 m high. The settlement computed using three different software packages

PLAXIS 2D, FLAC, and Geostudio was about 0.93 %, 0.97%, and 1.38% of the

dam's height, respectively.

Keyvanipour et al. [98] have analyzed the settlement response of a 35.5 m high bar

embankment dam (Iran) and compared the results with the field data. The authors

conclude that the settlement response predicted with the HSM was in close agreement

with the field data as compared to the Mohr-Coulomb Model (MCM). Rashidi and

Haeri [99] have conducted a numerical analysis of the Gavoshan rockfill dam

settlement, which is 123 m high. The calculated and measured settlement was about

2.2% of the height of the dam. The authors further describe that 88% of the dam's

total settlement occurred during the construction period.

According to available literature [93, 94, 100], an acceptable settlement of

embankment dams ranges from 1 to 2% of the dam height. Differential settlement of

embankment dams is also associated with the arching phenomenon, the details of

which are presented below:

2.17 ARCHING OF EMBANKMENT DAMS

The stability of embankment dams is related to stress conditions in the clay core as

compared to shell and other zones [101]. In zoned embankment dams, differential

settlement may occur due to differences in stiffness of clay core and shell soils [102].

As a result, vertical and horizontal stress in the core decreases and the load is

transferred to the shell. This situation may cause horizontal cracking of the core,

which is called arching of embankment dams. The transfer of load from the core to

adjacent soils is due to differences in stiffness of different soils [103, 104]. Compared

to the shell, the core is a soft material; therefore, the load is transferred from the core

to the shell. Due to this load transfer, pore water pressure in the core exceeds than the

total stresses [104, 105]. As a result, excessive settlement may occur in the core as

compared to the shell [101].

42

Arching of dams may occur during construction and may continue during operation.

When the reservoir is filled, water may enter the core and decrease effective stresses.

The combined action of arching and reducing effective stresses could pave the way

for hydraulic fracturing in the dam's clay core [102]. Concentrated leaks may occur

through the cracks in the core. Eventually, there may be gradual erosion of the core,

which may lead to the collapse of the dam [106]. For about 30-50% of embankment

dams' failure, progressive erosion is the main cause [1]. The failure of Teton Dam

occurred in the USA on 5 June 1976, within a few hours of filling the reservoir, which

is possibly considered to be due to leakage and hydraulic fracturing [107].

The arching ratio is defined as follows [108]:

Where arching ratio, total vertical stress in the depth of z in a

homogeneous dam and total vertical stress in depth z in a zoned embankment

dam.

Talebi et al. [106] studied various geometrical factors on a zoned embankment dam's

arching ratio. The authors conclude that a zoned embankment dam's side slopes have

a very negligible influence on the arching ratio (Figure. 2.12). The authors also

describe that if the side slopes of the core are flatter, and then there is a decrease in

the arching effect (Figure. 2.13). This implies that if the cores are thinner and steeper,

there may be chances for more differential settlement between core and shell. The

authors further mention that the arching effect decreases with the increase in filter

thickness (Figure. 2.14). The authors noted that the lower is the shell/core stiffness

ratio; the lower is the effect of arching (Figure. 2.15).

The authors describe that the inclined cores show a less arching effect than the

vertical cores of zoned embankment dams (Figure. 2.16). The authors conclude that

foundation compressibility has a negligible influence on the arching effect of a zoned

embankment dam (Figure. 2.17).

43

Figure. 2.12: Effect of side slopes on arching ratio

of clay core and shell [106].

Figure. 2.13: Effect of side slopes of core on arching ratio [106].

44

Figure. 2.14: Effect of increase of filter thickness on

the arching ratio [106].

Figure. 2.15: Effect of shell core stiffness ratio

with arching [106].

45

Figure. 2.16: Influence of inclined core on arching

Effect in a zoned embankment dam [106].

Figure. 2.17: Effect of MoE of foundation on arching

effect in a zoned embankment dam [106].

46

2.18 FOUNDATION TREATMENT METHODS

If the foundation of an embankment dam is on soft or weak soil/rock, there may be a

possibility of a large settlement and excessive deformations, leading to failure.

Grouting is a technique in which cement slurry is injected under pressure into the

underlying rock to fill joints, fractures, open bedding planes or other openings.

Grouting of dam foundations is carried out to decrease leaks, to reduce uplift pressure

and seepage. Generally, grouting is performed when a dam is newly constructed.

However, grouting could also be used for old dams as a remedial technique. Under a

dam foundation, grout is injected with high pressure to seal the joints to reduce

seepage.

Jafarzadeh and Garakani [109] have conducted the numerical analysis for the

effectiveness of consolidation grouting as foundation treatment of a 58 m high

embankment dam, Gerdebin (Iran). The authors conclude that with the application of

consolidation grouting below the clay core, there was a reduction of about 28% of the

dam's vertical deformation.

The main types of foundation grouting for embankment dams are curtain grouting and

blanket grouting. For new dams, curtain grouting is commonly used to reduce

seepage. In this method, boreholes are drilled into the bedrock parallel to the dam

axis. Blanket grouting is used at those sites where the rock is fractured at the

foundation contact. The purpose of blanket grouting is to make the foundation strong,

reduce seepage from the foundation bedrock and to decrease the possibility of internal

erosion of embankment and foundation materials. Commonly grout cap is provided at

about 3 feet wide and 3 – 8 feet deep.

Due to the presence of joints, cavities, interconnected fractures, and fissures in

bedrock, its permeability may be high. Therefore, grouting is provided to reduce

foundation permeability and decrease the risk of uplift pressure. If the permeability of

rocks is less than 1x10-5

cm/s, there may be less need for grouting.

47

2.18.1 Blanket Grouting/Consolidation Grouting

Blanket grouting is also termed as consolidation grouting. In this type of grouting, the

slurry is injected with low pressure to seal shallow bedrock below the embankment

dam's core. As a result, the bedrock gets harder. Blanket grouting is used in those

areas of the dam where the bedrock is highly fractured, having high permeability,

which may cause excessive seepage and internal erosion when the reservoir is filled.

Blanket grouting is provided only below the impervious core section of the dam. The

depth of blanket grouts is about 20 to 30 feet. The 20 feet depth of a blanket grout

may be satisfactory for a dam 100 feet high. If the dam's height is greater than 100

feet, then 30 feet blanket grout holes are drilled. Besides, the depth of blanket grout

depends on the geological conditions of the site. Twenty feet deep blanket grouts are

filled in one stage. On the contrary, 30 feet deep holes are grouted in two steps: 0 to

10 feet, and 10 to 30 feet.

2.18.2 Curtain Grouting

In curtain grouting, a zone of low permeability is constructed to a certain depth on the

dam's upstream side. The hazards such as piping and uplift pressure can be controlled

with the help of grout curtains in addition to the downstream drainage system.

If there is a possibility of much seepage under the dam's foundation, then curtain

grouts are used. These grouts are not thoroughly watertight. Such type of grouts just

reduces the quantity of seepage water. Usually, some arrangement of drainage is made

to collect the seeping water downstream of the curtain. The purpose of the grout

curtains is to reduce the quantity of seepage so that there is no loss of stored water and

no risk of the erosion of the foundation soils of the dam [110]. Reclamation

recommends grouting holes up to a depth of 0.5 to 1 times the reservoir water level.

2.19 RAINFALL EFFECT ON SLOPE STABILITY

When rainfall water enters through embankments, there will be a decrease in shear

strength due to excessive generation of pore pressures. As a result, there may be a

48

possibility of sliding off the slope [111]. With the entry of rainwater into the slopes,

unit weight, and moisture, the fill material's content can increase. There is a high

variation of rainfall rate with respect to time and space.

The following parameters have an effect on the slope stability due to rainfall:

(i) Permeability of soil along the depth of the embankment

(ii) Rate of infiltration of rainfall through the embankment

(iii) Depth of water that penetrates through the crest and slopes

The following are the conditions on which rainwater infiltration through the soil

depends: properties of fill soil such as permeability, moisture content, and porosity

[112, 113]. Initially, the surface layers of the embankment get saturated due to

rainfall. Pore water pressure develops in newly saturated layers. This process

continues until the rainwater reaches the phreatic level (Figure. 2.18) [42, 61].

Figure. 2.18: Rainwater infiltration through a dam [61].

If the intensity of rainfall is very low, the rainwater is absorbed by the soils. The rate

of infiltration depends upon the type of soils [114]. The depth of wetted soil depends

upon the intensity of rainfall and the rate of infiltration. For a partially saturated fill,

initially, the rate of infiltration is high. The rate of infiltration slowly decreases as the

saturation of fill soil occurs. Figure. 2.19 shows how the embankment soils are

saturated due to rainfall.

49

Figure. 2.19: Rainfall infiltration through dam [61].

In the unsaturated zones of embankment dams, there will be a decrease in suction,

increased permeability, and moisture content due to rainwater penetration. The

penetration of rainwater reduces the negative pore water pressure in slopes where the

groundwater level is initially low. This may cause a decrease in the safety factor.

Slope stability is affected by the intensity of rainfall and the position of the initial

groundwater table. If the initial water level in a slope is low, then there will be less

impact of rains on the slope's stability. High-intensity rainfalls may increase the water

level and decrease the negative pore pressure of slopes [115].

Several slope failures are triggered due to rainfall. In a conventional slope stability

analysis, the influence of rain is simulated by increasing the groundwater level or

changing water flow patterns. Several shallow failures of slopes suggest that there

was not any indication of an increase in water level in those slopes [116]. The main

reasons for failures are decreased shear strength and reduction of suction [117, 118].

Therefore, conventional techniques cannot perform a proper analysis of such failures

[119, 120].

Changes in pore water pressure and seepage forces due to rainfall are responsible for

the failure of slopes. Two different types of failure mechanisms are observed for

rainfall-induced failures.

(i) In the low area of the slope, there may be excessive development of positive

pore pressure. Movement of the soil along the sliding surface may decrease

50

effective stress to such a magnitude near zero, causing the soil mass to liquefy

[121].

(ii) The soil is in unsaturated state, the slope failure occurs due to a reduction in

shear strength, which also decreases soil suction [116, 117].

If the rainfall is more intense, a significant decrease of negative pore pressure is likely

to occur in a slope and an associated rise in the groundwater table [122, 123]. The

increase in water pressure and a decrease in suction may reduce shear strength, which

may cause the slope's failure. Depending upon the intensity of rainfall and the soil

type, the failures may be shallow and deep-seated slips. The safety factor of a slope

decreases if the infiltration of rainwater increases with time.

The pattern of the rise of water in a slope and steady-state pore pressure depends on

hydraulic features of the soil in the slope. If the initial volumetric moisture content of

a slope is high before rainfall, pore pressure will quickly increase if the rainwater

infiltrates in its soil.

Rainfall of high intensity and enough duration may cause the failure of slopes having

low permeability. On the contrary, a rainfall of high intensity for a shorter period may

cause a slope's inability to have high permeability. In coarse-grained soils, the

infiltration rate will be high, causing an increase in pore water pressure. As a result,

seepage forces may develop within the slope, which may ultimately lead to failure.

The infiltration rate is low in fine-grained soils; therefore, there is less development of

positive pore pressures. In this type of situation, loss of suction occurs, which may

cause a reduction in shear strength.

A safety factor of slopes decreases as the amount of infiltrated rainwater increases.

Rainfall duration, the permeability of the soil, and the slope's geometry have the main

influence on slopes' instability. If the soil slopes are highly permeable (10-4

m/s), a

small amount of rainfall is not likely to cause slope failure. If the soil slope's

permeability is very low (10-6

to 10-7

m/s), there may not be a significant pore

pressure change during the rainfall. After the wet period, there may be an increase in

pore pressures.

51

Rainwater can infiltrate into embankment dams. The stability of dams can be affected

depending upon the duration and intensity of the rainfall. Due to rainfall, the phreatic

level of embankment dams may change and create high pore pressures. Therefore, it

is essential to evaluate dams' stability when subjected to rainfall during full reservoir

conditions. Due to rainfall infiltration, pore pressures increase, which may add to the

seepage, resulting in the dam's instability. As compared to the intensity, the duration

of the rainfall is vital for the dams' stability. If the rain is of a long duration, there is

an increase in pore pressure and decreased safety.

2.20 TYPES OF EMBANKMENT DAMS ANALYSIS

The following types of analysis are performed for embankment dams: Consolidation

and settlement and stability analysis.

(i) Consolidation and settlement

Cohesive soils such as clay and silt have very poor permeability. These soils are

compacted at optimum moisture content. The pores of such soils are filled with water

due to impermeable nature. As the soil layers are successively placed and compacted

until the dam is raised to a final height, there is a gradual increase in pore pressures.

This is because the rate of loading in such soils is fast as compared to the rate of

dissipation of excess pore pressures. On the contrary, if enough time is allowed, there

may be a decrease in excess pore pressures due to consolidation. In addition to

deformation, in finite element programs, analysis of generation and dissipation of

pore pressures is also carried out [124].

Soils (in particular clay) generally settle slowly at a variable rate. The clays'

settlement rate depends upon how fast the excess pore pressure is generated and

dissipated due to the consolidation process. The development of excess pore pressures

in an embankment dam depends upon the placement water content during the

compaction process, weight of soil layers, rate of raising, the period over which no

construction work is carried out, permeability of core fill materials, and drainage

layers. The shear strength of various soils relies upon consolidation features of how

52

fast excess pore pressures are reduced. If the foundation is composed of low

permeability soils, pore pressure will increase, and the water will carry the loads. If

the foundation of a dam has low shear strength, then it is advisable to decrease the

loading rate, make the slopes flatter, or expedite the consolidation process by applying

drains. Therefore, consolidation analysis of embankment dams must be carried out to

account for the time-dependent variation of excess pore pressures in the dam body. As

the consolidation process occurs, the excess pore pressures reduce with time and soil

gets compressed under the applied loads.

The fine-grained soils' settlement comprises three phases: immediate, primary

consolidation, and secondary compression. Immediate settlement occurs with the

application of load. The consolidation settlement occurs due to the reduction of excess

pore pressures with time. Permeability of the soils controls the time-dependent

consolidation settlement process of the fine-grained soils. The soils' secondary

compression settlement is due to clays' creep behaviour with respect to time under

constant loading conditions. As the coarse-grained soils' drainage process is speedy,

the only immediate settlement is of concern in such soil types.

(ii) Stability analysis

The stability of embankment dams is normally checked for the following critical

conditions:

(a) Slope stability after the end of construction

(b) Slope stability of upstream side when the reservoir is filled

(c) Slope stability of the upstream side during the rapid drawdown of the reservoir

when it is at its full supply level

(d) Slope stability of both the upstream and downstream sides when an earthquake

shakes the embankment.

2.21 SOIL MODELS

Previously it was considered that soils exhibit elastic behavior. Over time, plasticity

theory was applied to soils, and it is now found that soils show elastoplastic behavior.

53

The stress-strain curve of a metal bar tested in tension is presented in Figure. 2.20

[125]. Up to OA, there is a linear relationship between stress and strain. If the bar is

unloaded at point A, then it will follow the same route during unloading as it was

during loading. If the bar's loading is continued after reaching point A, then stress-

strain relation is non-linear, but unloading is reversible. After the loading crosses

point B, the response of the bar changes from elastic to elastoplastic. Therefore, point

B is termed as the yield point of the bar. If the bar's loading is continued at point C,

and then the load is removed, the stress strain curve follows the path CD. It can be

noticed that even there was unloading, but at this stage, the permanent strain has

occurred in the bar, which is denoted by CD. The permanent strain in the metal

resulting from loading beyond the yield point is called plastic strain.

The behavior of the bar is considered as elastic up to point B. When the bar is loaded

up to point C, the total strain OE consists of two parts: (i) the elastic strain DE which

can be recovered if the bar is unloaded, (ii) the plastic strain OD, which is

unrecoverable. The slope of the unloading line CD is identical to elastic loading

behavior represented by OA.

If the bar is again reloaded at point D, it will follow the same unloading path up to

point C, termed the bar's new yield point. If the bar is further loaded beyond point C,

it will follow the original stress strain curve until failure occurs (point F). At point F,

the bar attains its ultimate tensile strength. If the yield point of a material increases,

then it is said that the material shows hardening behavior.

It is well known that soils show more complex behavior. The elastoplasticity

framework shall provide a reasonable basis for developing new mathematical models

that can realistically capture the soil behavior.

54

Figure. 2.20: Stress-strain curve of metals [125].

2.21.1 Idealizations of Plastic Behavior

Various attempts have been made to identify how metals' plasticity could be related to

soils' plastic behavior. It was assumed that essential properties of soils be captured,

and less critical features can be neglected. Elastic-perfectly plastic response is

presented in Figure. 2.21(a). The stress-strain graph is initially linear and elastic, and

with further loading, the material yields and deforms at constant yield stress by

showing no strain hardening. The elastic-linear-strain hardening plastic response of a

material is illustrated in (Figure. 2.21 b). After yielding the material, there is a

reduction in the linear stress-strain curve slope compared to the initial response.

(Figure. 2.21 c) presents the rigid plastic response of a material.

It is essential to determine how the idealizations, as mentioned above of plastic

behavior, could be applied to soils, showing complex stress-strain response.

55

Figure. 2.21 (a): Elastic and plastic behavior of material [125].

Figure. 2.21 (b): Elastic-linear-strain hardening plastic

response of material [125].

56

Figure. 2.21 (c): Rigid plastic response of material [125].

Depending upon the level of confining pressure, soils show compression, dilation, and

deformation at constant volume. Loose sands and lightly over-consolidated clay

exhibit compression (Figure. 2.22 a). On the contrary, dense to medium-dense sands

and heavily over-consolidated clays shown dilation (Figure. 2.22 b).

Figure. 2.22 (a): Lightly over-consolidated clays and loose sands [125].

57

Figure. 2.22 (b): Over-consolidated clays and dense sands [125].

2.21.2 Linear Elastic Model

This model follows Hooke’s law of isotropic elasticity. There are two fundamental

parameters of this model: (i) Young’s modulus of elasticity (E) and (ii) Poisson’s ratio

(v). This model is not reliable for soils because soils show an elastoplastic response.

This model represents the behavior of concrete and intact rocks. The essential

properties of soil, such as non-linearity and irreversibility, are not captured by the

Linear Elastic model. This model is represented by the following relationship [75]:

[

]

[

]

[

]

With the use of Hooke’s Law, following relationships between Young’s modulus (E),

shear modulus (G), the bulk modulus (K), and Oedometer modulus can be

obtained as follows:

58

2.21.3 Mohr-Coulomb Model (MCM)

This model shows the linear elastic, perfectly plastic behavior (Figure. 2.23). The

linear elastic part of the MC model is based on Hooke’s law of isotropic elasticity.

The perfectly plastic section of the model is based on the Mohr-Coulomb failure

criterion. This model is widely used for the calculation of the stability of slopes. There

are five material parameters which are given in Table 2.4. Most of the soil constitutive

models utilize the Mohr-Coulomb failure criterion. The MCM is not capable of

simulating the softening and effects of changes in density [126]. Geotechnical

practitioners and academics have widely used MCM for sand behavior [127].

Table 2.4: Parameters of Mohr-Coulomb Model [75].

Symbol Description Unit

E Modulus of elasticity [kN/m2]

v Poisson’s ratio [-]

c Cohesion [kN/m2]

Angle of internal friction [o]

Angle of dilatancy [o]

Figure. 2.23: Stress-strain behavior of an elastic,

perfectly plastic model [75].

59

2.21.4 Mohr-Coulomb Model Parameters

(i) Modulus of Elasticity (MoE)

Refer to the drained triaxial test (Figure. 2.24). The initial slope of the stress-strain

curve is represented by ) (tangent modulus) and illustrates the secant modulus

at half of the strength. Generally, (E50) is used as a MoE for loading conditions when

MCM is used. If unloading conditions are encountered, such as excavations and

tunneling, then the unload-reload modulus ( ) is used in place of .

If the confining pressure of soils is increased, there will be an increase in both first

loading modulus ( ) and unload-reload modulus ( ). This indicates that there will

be more stiffness for deep layers of soils than layers lying at the surface. As compared

to primary loading, there will be higher stiffness for unloading and re-loading

conditions. Soils show more stiffness during shearing in comparison to drained

compression. When a constant value of stiffness is used in a calculation, it should be

carefully selected to match stress intensity at a particular depth.

Figure. 2.24: Drained triaxial test results for [75].

60

(ii) Dilatancy angle

The heavily over-consolidated clays show dilatancy [128], whereas normally

consolidated soils' dilatancy is almost negligible. Density and friction angle are the

parameters on which the dilatancy of sands depends. The angle of dilatancy of soils is

lesser as compared to the angle of internal friction. The dilatancy angle of sands is

related to the friction angle as follows:

The dilatancy angle is almost zero if the value of the friction angle of sands is less

than 300. In drained condition, if the value of the dilatancy angle is positive, it

indicates that dilation will continue until the occurrence of shear deformation. This is

contrary to the behavior of soils in which shear deformation occurs at constant

volume. If the dilatancy angle's value is positive in undrained conditions, it will

generate tensile pore pressures with constant volume. Consequently, there will be an

overestimation of strength under undrained conditions. The procedure of finding of

dilatancy angle of soils is illustrated in Figure. 2.25.

Figure. 2.25: Dilatancy angle using the stress-strain curve

obtained from triaxial test [128].

61

(iii) Poisson’s ratio

It is the ratio of lateral strain to axial strain. For soils, it may be obtained from the

triaxial test under axial loading conditions. [129, 130] describe that the range of

Poisson’s ratio for granular soils is from 0.18 to 0.33. Newcomb and Birgisson [131]

mention that the Poisson’s ratio for loose and dense sands is in the range of 0.2- 0.4

and 0.3-0.45, respectively. Marchenko et al. [132] mention that Poisson’s ratio of clay

ranges from 0.2-0.45. The Poisson’s ratio of fine-grained soils is found to be 0.3 and

0.5.

(iv) Friction angle and cohesion of soils

The friction angle and cohesion of soils are termed as shear strength parameters.

Cohesion is a force that holds together particles of cohesive soils (clay and silt). The

strength parameters, friction angle and cohesion of various soils like clay, silt and

sand can be obtained from the shear box (Figure. 2.26) or triaxial test (Figure. 2.27).

The values of frictions and cohesion are generally affected by initial moisture

content, drainage, and saturation conditions. Compaction is the technique that

improves the shear strength of soils [133].

Figure. 2.26: Determination of cohesion and friction angle [134].

62

Figure. 2.27: CD triaxial compression tests on three

overconsolidated clays [135].

(v) Permeability of soils

Permeability refers to the ease with water that can flow through soils. Soils are porous

materials. Water can flow between voids of soils. Permeability is an important

physical property that controls the flow of water in soils. The rate of flow of water

through soils depends upon particle sizes and structural arrangement of particles.

Higher will be the flow of water if the particles are larger in size with more void

spaces. The flow of water will be lower if the particles are denser. Generally, a soil

becomes weaker if the water content is higher. The response of soils to loading

conditions depends upon the permeability of soils. The settlement of soils depends on

permeability and decrease of excess pore pressure.

Due to the random arrangement of solid particles in a soil mass, there may be

variation in permeability from point to point. For realistic determination of

permeability in the laboratory, least disturbed samples should be used. It is common

to realize that some disturbance to soil samples is expected to occur when the samples

are extracted, transported, and prepared in the laboratory. In addition, the value of

permeability depends on the sample size and soil structure. It has been observed that

63

most of the soils show anisotropic behavior in permeability in horizontal and vertical

directions. Generally, horizontal permeability is more as compared to the vertical

direction. For laboratory tests, the flow of water is in vertical direction. It can be

interpreted that the measured value of laboratory permeability is lower than the in-situ

permeability of the soil. In the laboratory, the permeability of a small volume of soil

is determined. Sometimes, disturbed soil samples are also used to find out

permeability. The permeability of a soil sample will be highly misleading if the

structure of the soil is altered. Therefore, for disturbed soil samples, there may be

chances that the value of soil permeability could not match with in situ soil

conditions. Typical values of permeability of soils are shown in Table 2.5 [136].

Table 2.5: Typical values soil permeability [136].

Permeability (cm/sec) Type of soils

Greater than 10-1

Coarse gravel

10-1

-10-3

Sand, fine sand

10-3

-10-5

Silty sand

10-5

-10-7

Silt

Less than 10-7

Clay

(vi) Unit weight

The unit weight of soils depends on material type and its density [135]. In zoned

embankment dams, generally, there are various soils having different unit weights.

The permeability of soils depends on unit weight. The higher the unit weight of a soil,

the lesser the voids will be the permeability [137].

2.21.5 Application of Mohr Coulomb Model

The MCM has been extensively used in the computation of slope stability of several

types of embankment dams.

Rabie [138] concluded that MCM can be used to computation slope stability under

heavy rainfall conditions. During rainfall, slope stability is affected by the reduction

64

of shear strength, the increase of bulk density, the increase of pore pressure and the

action of seepage force.

Wulandari and Tjandra [139] concluded that the MCM can be used to computation

safety factors for geotextile reinforced road embankment. Due to an increase in traffic

volume, road embankments on soft soils have been constructed. Such types of

embankments show large settlement and slope instability problems. To achieve

stability of a road embankment on soft soil, geotextile reinforcement is provided.

Singh et al. [140] conclude that the stability of rock slopes in road cut sections can be

evaluated with MCM's help in the landslide region.

Deliveris et al. [141] conducted a stability analysis of a mine waste dam using three

software, i.e., Plaxis 2D, FLAC and Phase2. The analysis was carried out under

drained and plain strain conditions. They determined that MCM can be utilized for the

safety evaluation of mine waste dams. Ouadif et al. [142] conclude that highway fill

materials' stability can be evaluated using the MCM. The authors analyzed the

stability of an embankment under wet and dry conditions, dynamic loads and

reinforcement. Ghanizadeh and Ghanizadeh [143] have used MCM for embankment

dams that involve soils which consisted of sand and gravel. Afiri and Gabi [144] have

used MCM for the stability of excavations, embankment dams and landfills. Vekli et

al. [145] conclude that the safety of slopes stabilized with stone columns can be

calculated by using MCM. Gaber et al. [146] conclude that the stability of reinforced

embankments on soft clay foundations can be modeled using MCM.

2.21.6 Hardening Soil Model (HSM)

The HSM for soils was developed based on the theory of plasticity [147]. This model

captures the response of clays, silts, gravel and sand [75]. In this model, vertical strain

and deviatoric stress are associated with a hyperbolic relationship (Figure. 2.28)

during primary loading. The constitutive equations of this model are based on a

standard drained triaxial test.

for q qf 2.8

65

Where and are the major and minor principal stresses, is Young’s modulus

at half of the shear strength, is ultimate deviatoric stress, is a quantity of

deviatoric stress, is the failure ratio (0.9).

For primary loading, the stress-strain relationship is nonlinear. Primary loading

stiffness is:

(

)

where is primary loading stiffness modulus, which depends on confining

pressure,

is stiffness modulus associated with confining pressure , m is the

stress dependency power. The value of power m is 1 for soft clays.

Janbu [148] describes that the value of m for Norwegian silts and sands is 0.5. Von

Soos [149] mention that the value of m can be in the range of 0.5 to 1.

For most of the soil conditions, the value of

Confining pressure-dependent stiffness modulus for unloading and reloading is:

(

)

is reference Young’s modulus for unloading and reloading.

66

Figure. 2.28: Hyperbolic response in primary loading

for triaxial test [147].

(

)

is tangent stiffness modulus than that can be taken from the Oedometer test

is tangent stiffness at vertical reference stress (100 kPa), the procedure of its

determination is illustrated in Figure. 2.29. The parameters of the HSM are

summarized in Table 2.6.

67

Table 2.6: HSM parameters [75].

Symbol Description Unit

Cohesion [kN/m2]

Friction angle [0]

Dilatancy angle [0]

Secant stiffness [kN/m2]

Tangent stiffness [kN/m2]

Unloading/reloading stiffness [kN/m2]

Power [-]

Poisson’s ratio [-]

Reference stiffness [-]

Normal consolidation [kN/m

2]

Failure ratio [-]

Figure. 2.29: Determination of

from oedometer test [75].

The HSM can capture the real response of soft and dense soils. For example, when

applied to soft soils and hard soils under undrained loading conditions, the HSM

decreases and increases mean effective stress [150]. With HSM's use, realistic

predictions about displacement and failure of various geotechnical structures can be

made [150]. The HSM limitation is that neither time-dependent behavior (creep) of

clay nor anisotropic strength and stiffness can be modeled. The HSM is also not

suitable for dynamic loading conditions [150].

68

2.21.7 Application of HSM

The HSM has been widely used for several geotechnical problems. A summary of the

main applications of the HSM is presented below.

Celik [151] has used HSM for the estimation of the settlement of tunnels. The author

concludes that deformations were estimated with reliability using the HSM compared

to the MCM. Teo and Wong [152] have analyzed the settlement of deep excavation

using HSM. The authors conclude that a realistic settlement of deep excavations can

be estimated using HSM compared to MCM. The soil behavior in deep excavations

simulated with the MCM can be unrealistic. The wall deflection and ground

movement calculated with the HSM is comparable with the measured data.

Pramthawee et al. [153] analyzed a rockfill dam. They conclude that the HSM can

give a realistic estimate of the deformation response of rockfill material. Ozkuzukiran

et al. [154] have analyzed settlement behavior of 133 m high concrete faced rockfill

dam using HSM. The authors conclude that the settlement of the rockfill dam was

consistent with field data.

In the HSM, the Mohr-Coulomb failure criterion is used. Therefore, the values of

safety factors computed with the HSM and MCM are almost the same [155]. Dickin

and Laman [156] have conducted a numerical analysis of strip anchors' uplift

response in cohesionless soil. They conclude that the HSM is capable of capturing

well the pre-peak uplift behavior of strip anchors in such soil types. Locat et al. [157]

have modeled landslide in Quebec, Canada, using HSM. The authors concluded that

the HSM captured the initial state of stress before failure due to land sliding. Singh

and Babu [158] concluded that the HSM could be applied to soil nail walls

constructed in soft soils if essential structures are adjacent. Phien-Wej et al. [159]

conclude that the deformation behavior of diaphragm walls constructed on soft soils

be modeled with HSM. Tschernutter and Kainrath [160] have used HSM for

numerical analysis of central reinforced concrete core embankment dam. The authors

conclude that the stress and deformation analysis of such type of dams can be

performed using HSM for embankment materials and linear elastic model for

concrete. Vahdati [161] have utilized HSM for computation of displacements in

69

earthfill and rockfill dams. They conclude that with HSM's use, calculated

deformations of embankment dams are consistent with field data. Honkanadavar and

Sharma [162] have compared results of triaxial tests with simulations performed with

HSM. They concluded that there was a good match between the triaxial behavior of

riverbed and blasted quarried rockfill materials and the response predicted with the

HSM. Beiranvand and Komasi [163] concluded that the generation of pore water

pressures in an earthen dam could be computed with HSM, which agrees with filed

data. Sukkarak et al. [164] concluded that HSM could be used for predicting the

behavior of rockfill materials in rockfill dams.

2.22 SUMMARY

The following main conclusions are drawn from the literature review of the stability

of embankment dams:

It is essential to assign different material properties to various soils used in zoned

embankment dams as reliable as possible. Because stress-strain response, stability,

and settlement of embankment dams are highly dependent on material properties'

input. It is necessary to include foundation soils in addition to embankment soils for

numerical modeling of stability and settlement.

The literature review regarding stability and settlement issues in embankment dams

suggests a need for more cases to address the effect of variation of material properties

obtained from different borrow areas. It is also necessary to investigate the impact of

lowering the reservoir on an embankment dam's stability. Numerical evaluations are

required to perform a safe rate of reducing of a reservoir so that the embankment

dams can be stable.

Regional rainfall in which an embankment dam is located may affect the stability of

the dam. Therefore, it is necessary to perform analysis to evaluate the effect of

regional rainfall on an embankment dam's stability.

In the literature, there are different models than can represent the stress-strain

behavior of soils. The MCM can be utilized for the computation of slope stability of

embankment dams. The HSM can be used for the estimation of settlement in

70

embankment dams. The HSM gives more reliable estimate of the settlement of

embankment dams as compared to the MCM.

The type of analysis performed for the embankment dam is stability and settlement

analysis and consolidation analysis. Due to the gradual raising of dam pore-pressure

could increase and reduce the shear strength and stability of embankment dams.

Groundwater level and placement moisture content of fill materials influence the

stability of dams.

Excessive settlement of embankment soils and foundation could be dangerous for the

stability of embankment dams. It is generally accepted that settlement of 1-2% of the

dam's height is considered within acceptable limits. If a dam's foundation is weak, it

may require treatment such as consolidation and curtain grouting.

71

CHAPTER 3

MATERIALS AND NUMERICAL MODELLING

3.1 GENERAL

Stability and settlement analysis of an earth dam called Nai Gaj Dam located in

district Dadu, Sindh Pakistan, is presented in this study. Initially, the project

background and location are described. The typical section of the main dam is

selected for numerical modeling. The Geotechnical Investigation report of the project

is reviewed to know about foundation conditions and different dam zones. Soil

properties are mainly obtained from the geotechnical investigation report, and some of

the properties are also obtained from the literature on similar types of soils. The finite

element model is developed using the finite element program Plaxis 2D. Material

properties and boundary conditions are assigned. Finally, the numerical calculations

were run, and the selected results are analyzed for stability and settlement of the dam.

The dam's stability was calculated for the EoC, after filling the reservoir, rapid

drawdown of the reservoir, and the effect of maximum possible rainfall on the dam's

slope stability.

3.2 PROJECT BACKGROUND AND LOCATION

Nai Gaj is a large stream in the Khirthar range of mountains in Sindh. The Nai Gaj

originates from Baluchistan, and after covering a distance of 160 km, it joins the

Khirthar range of mountains. It is located about 65 km north-west of Dadu city. The

bed material of the Gaj river consists mainly of gravel, pebbles, cobbles, clayey silty

and sand [165]. The rocks at the dam site are composed of conglomerate, sandstone,

siltstone and claystone.

71

72

3.3 MATERIALS AND METHODS

The proposed Nai Gaj Dam is an earth-fill type dam. It has been observed that the

reservoir rim is lower at many locations than the maximum flood level; for that

purpose, nine earthen dykes have been designed to protect the floodwater (Figure.

3.1).

Figure. 3.1: 3D view of Nai Gaj Dam with earthen dykes.

The cross-section of the Nai Gaj dam is shown in Figure. 3.2. The main embankment

of the dam is 59 m high. It is estimated that the total volume of the embankment fill is

about 7.8 million cubic meters. The type of dam is a zoned embankment. The dam has

a central impervious core (cf. Figure. 3.2) of clayey silt. The dam's material zones are

sandy gravel, random fill, central impervious clay core, sand filter, coarse filter, and

drainage blanket. The foundation of the dam mainly consists of sandy siltstone. The

dam is 1,137 m long, having a maximum design water level of 56.6 m. The

foundation of the dam consists mainly of sandy siltstone.

73

The clay core is constructed of clayey silt obtained from the borrow area on the

downstream side. The primary function of the clay core is to control seepage through

the dam. The proposed degree of compaction is 98% of standard proctor dry density.

The core is placed at wet of optimum moisture content to allow for post construction

settlement without cracking. The top width of the core is 4 m. To provide an effective

water barrier, the top of the core is extended to 0.8 m above the maximum water level.

The core's upstream slope is 1H to 3V, and the downstream slope of the core is 1H to

4V.

The upstream shoulder is composed of sandy gravel, which provides stability and

controls erosions. The downstream shoulder consists of random fill material, which

provides support to the core. The chimney drain is provided along the downstream of

the dam. A drainage blanket is used below the downstream shoulder. The inclination

of both upstream and downstream slopes is 1V to 2.5H. The freeboard is about 2.4 m.

The cut-off trench is extended up to sufficient depth in sound bedrock. A series of

curtain grouting and consolidation grouting is provided under the dam's clay core

section to prevent seepage. The diameter of each of the curtain and consolidation

grout is about 76 mm. The curtain grouting length is 16 m, while the length of

consolidation grouting is 45 m.

The crest width of the dam is about 10 m. The chimney drain is provided on the

downstream side of the core. It is 3 m thick and is provided to ease the construction at

the site and also hold the seepage from the cracks, which may usually develop in the

core.

Chimney drain is provided for the following purposes:

(i) To allow seepage water through the core so that there is no subtle particle

erosion.

(ii) To allow seepage water to pass through the drainage blanket without creating

a disturbance in the downstream shoulder.

74

A 0.5 m thick gravel drain is provided below the downstream shoulder between the

sand filter layers. The function blanket's collect seepage water from the chimney drain

and bedrock and dispose of the downstream toe. To reduce the wave action and flow

velocities, riprap is provided on the upstream side. The riprap is extended from crest

to the natural surface layers. Riprap consists of rock fragments that are dense, sound

and durable. A bedding material layer of 0.6 m thick is provided below the riprap

layer. The bedding layer aims to provide support to the riprap and protect the

embankment from the wave action. To protect the downstream slope, a 0.6 m thick

layer of cobbles and gravel is provided to protect the dam from erosion by wind or

rain. The size of cobbles and gravels is in the range of 75 mm to 200 mm. Relief wells

are provided at the downstream toe to decrease pore pressure, which may develop due

to seepage and may otherwise cause instability or piping. The spacing between the

two wells is 15 m center to center.

Drainage features, including a chimney drain and horizontal drainage blanket below

the downstream shell, have been incorporated to relieve the pore water pressures

within the dam body and dispose of the seepage water safely so that erosion and

boiling are prevented.

Numerical analyses were carried out on a typical section of the Nai Gaj dam for the

following conditions:

(i) Stability, settlement, and excess pore pressure analyses have been carried out

using a two-dimensional plane strain condition with finite element program

Plaxis 2D for two possible cases: gradual raising at the EoC and AFR.

(ii) Numerical analysis of the lowering of the reservoir is performed so that the

dam can be lowered safely without causing instability conditions.

(iii) Numerical analysis of the effect of rainfall on the slope stability of the dam is

performed.

75

Figure. 3.2: Cross-section of Nai Gaj dam [165].

3.4 CONSTITUTIVE MODELS

The linear elastic model was used for consolidation and curtain grouts, which are used

in the foundation of the dam. The Linear Elastic Model's material properties are the

MoE and Poisson’s ratio, whose values are 2x105 kN/m

2 and 0.33.

The MCM was used for all layers of the dam to computation pore pressures and safety

factors at the EoC. After filling the reservoir, the reservoir's rapid drawdown and

effect of maximum possible rainfall on the dam's stability. The material properties of

the MCM are described in Table 3.1. Mainly the material properties are obtained from

laboratory and field tests [165]. However, some of the material properties were taken

from the literature.

To ensure the safety of embankment dams, in addition to stability [166, 167],

settlement plays a critical role in being estimated with reliability [168, 169]. It has

been observed that the HSM is more reliable for calculation of settlement of the dam

as compared to the MCM. On the contrary, advanced laboratory tests are required to

evaluate the material properties of the HSM. However, the settlement of embankment

dams predicted with MCM may not be realistic for a different type of soils. The MCM

may show unrealistically more settlement. In such situations, there might be a need

76

for advanced constitutive models depending upon the types of soils involved in

embankment dam. The use of advanced soil models for computation of settlement of

an embankment dam, may require advanced testing which may result into more costs.

To obtain a reliable estimate of the settlement of an embankment dam by utilizing

material properties that are already determined during the geotechnical investigation

phase, it might be necessary to find out those material zones that may show variation

in settlement computed with the MCM in comparison to response predicted with an

advanced constitutive model like the HSM.

Therefore, it was decided to identify the dam zones, which occupy a considerable fill

material volume. The dam's cross-section shows that there are four zones, which

occupy most of the dam’s volume. These zones are clay core, sandy gravel, random

fill, and sandy siltstone. It is now important to investigate the effect of these zones on

the dam's overall settlement. For the computation of settlement in the dam during the

EoC and filling of the reservoir, HSM was used for four major zones: clay core, sandy

gravel, random fill, and sandy siltstone. For the remaining material zones, MCM was

used. The material properties of the HSM are presented in Table 3.2.

Table 3.1: Material properties for Nai Gaj dam and its foundation.

Material

type

Saturated

unit weight

(kN/m3)

Cohesion

(kN/m2)

Friction

angle

(deg)

MoE

(kN/m2)

Permeability

(m/day)

Clay 18.85[165]

9.57[165]

30[165]

50000[165]

0.000263[165]

Sandy

gravel 21.5

[170] 0 37

[171-173] 50000

[174] 86.4

[175]

Random

fill 18.85

[165] 0 34

[165] 50000

[165] 0.0263

[165]

Washed

gravel 21.5

[170] 0 37

[171-173] 45000

[176] 864

[135]

D/s slope

protection 19.5

[177] 0 34

[171-173]

40000[39, 135,

178]

8640[135]

Riprap 19.5[165]

0 34[165]

40000

[39, 175,

178]

8640[135]

Sand filter 18.85[165]

0 36[165]

40220[165]

26.33[165]

Drainage

blanket 21.5

[170] 0 37

[171-173] 45000

[135] 864

[135]

Sandy

siltstone 20.4

[165] 12

[165] 29

[165] 125000

[179] 0.00063

[165]

77

Table 3.2: HSM parameters for four main zones of dam [75].

Material Type

Clay core 50000 50000 150000

Random fill 50000 50000 150000

Sandy gravel 50000 50000 150000

Sandy siltstone 125000 125000 375000

Since the dam's settlement depends on the magnitude of the MoE of different soil

types, the effect of MoE variation of the four major zones of the dam is investigated.

For each of the three materials (clay core, sandy gravel and random fill), the MoE is

varied from 25000 to 50000 kN/m2 separately. While for the fourth material zone

(sandy siltstone), the MoE is increased from 70000 to 125000 kN/m2. Whenever the

effect of a particular material zone on the dam's settlement is investigated, the other

materials' properties are utilized as presented in Table 3.1 and Table 3.2. The MoE of

sandy siltstone was evaluated from its uniaxial compressive strength using the

following correlation [179].

Where E is the MoE (kN/m2) and is the uniaxial compressive strength (kN/m

2).

Most of the results show that the value of sandy siltstone's uniaxial compressive

strength at the dam site is less than 2 MPa. The minimum value is 0.35 MPa [165].

The following values of MoE of sandy siltstone are described for numerical analysis

of the present study:

E=200×0.35=70000 kN/m2 (minimum value) 3.2

E=350×0.35=125000 kN/m2 (maximum value) 3.3

In the above equations, the coefficient 200 is the minimum value of the prescribed

range of 200- 500. The coefficient 350 is the average of 200 and 500. The minimum

value of the MoE is 70,000 kN/m2,

and the maximum value is 125,000 kN/m2.

78

Since foundation material plays an essential role in the dam's settlement, the value of

the MoE was not actually determined from the laboratory or field tests. Preferably, the

value of MoE was obtained from correlation, as mentioned in equation 3.1. Therefore,

parametric analysis was performed to determine a suitable range of MoE of sandy

siltstone.

Coupled deformation and consolidation analysis were carried out to estimate the

development and dissipation of excess pore pressures, as a function of time, in the

dam.

The mechanical behavior of soils in the finite element program Plaxis 2D can be

described by fully drained and undrained conditions. Real soil behavior depends on

time. The equations describing the flow of water through soils are combined with the

equations governing the soil's deformation to capture such behavior. This type of

theoretical response is called coupled deformation and consolidation [180].

Besides, the safety analysis was conducted to compute the safety factors of the dam.

The raising of the embankment dam was decided based on construction work carried

on the site. The dam was mainly raised at a rate of three meters in 30 days.

The dam's finite element model was extended laterally by 100 m on each side to

minimize the effect of boundaries, as shown in (Figure. 3.3). The finite element mesh

consisted of 6619 triangular elements with 53463 nodes. The average element size

was 3.2 m. The mesh was sufficiently refined to such an extent that with further

refinement of the mesh, the results were almost the same (Figure. 3.4). The water

level in the finite element model is 15 m below the surface (Figure. 3.5). The highest

flood level of 56.6 m in the finite element model is shown in Figure. 3.6.

79

Figure. 3.3: FEM for Nai Gaj dam.

Figure. 3.4: FEM mesh of Nai Gaj dam.

Figure. 3.5: Highest reservoir level of Nai Gaj dam.

3.5 STABILITY OF THE DAM

As mentioned earlier, the stability of the dam was analyzed based on the factor of

safety. For this purpose, consolidation and safety analyses were performed using the

80

MCM for all the material zones, except consolidation grouting and grout curtain,

where the Linear Elastic model was used.

3.6 SETTLEMENT OF THE DAM

The settlement with respect to the dam's depth and long-term settlement was

calculated with the MCM and HSM. The purpose of the calculation of the dam's

settlement with both models was to check the reliability of the HSM compared to the

MCM.

It is relevant to describe here that the MoE is vital for the computation of settlement.

Real soils exhibit stress-dependent stiffness, whereas MCM uses a single MoE value,

resulting in the overestimation of settlement for embankment dams. On the other

hand, the HSM uses three different types of stiffness, which are stress-dependent.

The three types of stress-dependent stiffness used in the HSM are described in

Chapter 2.

3.7 RAPID DRAWDOWN OF THE DAM

The dam was also analyzed for rapid drawdown conditions due to the uneven

distribution of rainfall and continuous water discharge for drinking and irrigation

purposes. The stress-strain behavior of all the materials was modeled with the MCM.

The gradual raising process of the dam was carried out with consolidation analysis.

For the computation of safety at various stages during the dam's service life, safety

analysis was performed.

The rapid drawdown was carried out using coupled deformation and consolidation

analysis. Both the vertical boundaries and bottom boundary were closed for

groundwater flow in the finite element model. Before performing rapid drawdown

calculations, the effective strength and stiffness parameters (refer to table 1) of the

clay core were changed with undrained shear strength values of the clay core ranging

between 20 to 30 kN/m2. During the drawdown, the support provided by water to the

upstream embankment is lost. As a result, seepage forces develop on the upstream

81

embankment and extend to the clay core. It depends on the undrained strength of the

clay to resist seepage forces. If the undrained strength of clay is low, there will be

chances of the instability of the upstream embankment depending upon the generation

of pore pressures. The values of undrained shear strength were evaluated based on the

consistency index mentioned in reference [181]. The values were computed as

follows:

and

Based on the consistency index value of 0.726, the undrained shear strength (Su) of

soil lies in the range of 20 to 40 kN/m2 (Table 3.3) [181]. The drawdown calculations

were performed for undrained strength of clay as 20, 25 and 30 kN/m2. Besides, since

the clay became saturated, therefore, its reduced MoE was taken as 30000 kN/m2

instead of the initial value of 50000 kN/m2 [181].

The dam's drawdown was performed for different rates ranging from 1 m/day to 0.1

m/day. The lowest value of the drawdown was 10 m, and the highest value was 55 m,

as shown in Figure. 3.6 and Figure. 3.7, respectively.

Table 3.3: Relationship b/w consistency index and undrained shear-strength.

Strength

classification Consistency index (1-LI)

Undrained shear strength

(kN/m2)

Hard > 1.15 > 300

Very stiff 1.05 - 1.15 150 -300

Stiff 0.92 - 1.05 75 - 150

Firm 0.82 - 0.92 40 - 75

Soft 0.60 - 0.82 20 - 40

Very soft < 0.60 < 20

82

Figure. 3.6: 10 m lowering of reservoir @ 1 m/day.

Figure. 3.7: Lowering of the reservoir up to 55 m depth @ 0.1 m/day.

3.8 EFFECT OF RAINFALL ON STABILITY OF THE DAM

The stability of the dam was assessed for possible rainfall. During the last 40 years,

the maximum rainfall was 435 mm/year [184] Figure. 3.8; this rainfall was converted

to m/day and applied to the dam Figure's crest and downstream boundaries 3.9. The

average annual rainfall of 435 mm/year was used in five equal increments in five

days. The effect of rainfall was calculated on the stability of the dam.

83

Figure. 3.8: Average annual rainfall at Nai Gaj dam for four decades.

Figure. 3.9: Input of rainfall intensity at Nai Gaj Dam.

0

50

100

150

200

250

300

350

400

450

500

Ra

infa

ll i

nte

nsi

ty

Years

Average annual rainfall Nai Gaj Dam

84

3.9 SUMMARY

The main points described in this chapter are summarized as under:

o Background of the project and geological conditions of foundation

o Cross-section of the main dam

o Material properties of various soils used in dam and foundation required for

MCM and HSM

o Stability and the settlement of the dam during staged construction and AFR

o Stability of the dam during rapid drawdown

o Stability of the dam when subjected to rainfall.

85

CHAPTER 4

RESULTS AND DISCUSSION

4.1 GENERAL

Numerical analyses were performed for stability and settlement of an embankment

dam using finite element program PLAXIS 2D. The numerical modeling was utilized

to study the effect of retained water on the dam's stability by evaluating its stability

before and after filling up the reservoir. The other factors examined were the dam's

stability for the condition if the reservoir is lowered rapidly and how the stability is

affected if the maximum possible rainfall occurs in the reservoir area. Besides, the

dam's settlement was computed for the end of the construction stage, after filling the

reservoir and long-term. Parametric analyses are performed to investigate the effect of

change of strength properties (mainly friction angle) of three main material zones

(sandy gravel, random fill, and clay core) on the dam's overall stability during raising

and filling of the reservoir. For evaluation of the stability of the dam, the MCM was

used for all the material zones. For the computation of settlement, HSM was used for

four material zones (sandy gravel, random fill, clay core, and sandy siltstone). The

remaining material zones were analyzed with MCM. Parametric analyses were also

conducted by changing the stiffness parameter (i.e., MoE) of the four main zones to

compute the dam's settlement's reliable magnitude during raising and filling of the

reservoir conditions. Relevant results obtained from the numerical modeling process

are described and discussed in this chapter.

4.2 FACTORS AFFECTING DAM STABILITY

The dam's stability was determined based on the safety factor, which depends on the

development of excess pore pressure during raising and filling the reservoir

conditions.

85

86

4.2.1 Effect of Dam Height on Foundation Soil

The rate of loading by the embankment layers was 3 m in 30 days. The bottom of the

dam, where embankment layers are laid, is 300 m wide, and the dam's crest is 10 m in

width. The magnitude of excess pore pressures increased when the embankment was

raised to a level of 45 m (Figure. 4.1-4.3). When the dam was raised from 45 m to 59

m, the excess pore pressures mainly decreased in magnitude. This is because from the

bottom up to 45 m height (Figure. 4.3), the retained soil's weight is more than the

weight of the soil-filled from 45 to 59 m size of the dam (Figure. 4.4). As the dam is

being raised higher and higher, the width of layers gradually decreased. This indicates

that the weight of the top layers of the dam was lower as compared to the bottom

layers (Figure. 4.4). The increase of excess pore pressure at 15 m, 30 m and 45 m

raising of the dam was 46%, 5.2% and 1.2%, respectively. This shows that the

percentage increase in excess pore pressure decreased as the dam height was

increased. Finally, when the dam was raised from 45 m to 59 m, the excess pore

pressure instead of increasing dissipated at a 5% rate.

Figure. 4.1: Increase in pore pressure at 15 m dam raising.

87



88


4.2.2 Effect of Water Filling on Potential Failure of Dam

The dam's safety was computed to be 1.6 and 1.5 for the EoC and AFR conditions.

Due to the soils' saturation, the factor of safety after filling the reservoir is decreased

compared to the end of the construction condition. According to available guidelines

[78], the safety factors for the EoC and the reservoir at the normal operating levels are

1.3 and 1.5, respectively. This implies that the dam's stability at the EoC and AFR is

considered satisfactory.

In addition to the magnitude of safety factors, it is relevant to observe the

embankment dam's potential failure zones at the EoC and AFR. The dam's failure

zones at the EoC and AFR are shown in Figure’s 4.5 and 4.6, respectively. The

potential failure zone for the EoC developed at the upstream side of the dam.

Whereas, the failure zone after filling of the reservoir developed on the downstream

side. The modes of failure are consistent with the findings of the literature [32].

89

Figure. 4.5: Potential failure zone of the dam at the EoC.

Figure. 4.6: Potential failure zone of the dam AFR.

4.3 EFFECT OF FRICTION ANGLE OF SANDY GRAVEL

As the dam's upstream shell consists of sandy gravel material (cf Figure. 3.2), the

effect of the sandy gravel's friction angle on the dam's safety factor was examined

(Figure. 4.7). As expected, the safety factor increased with increasing values of the

sandy gravel's friction angle. For both the EoC and AFR conditions, the dam's

stability is satisfactory if the minimum value of sandy gravel's friction angle is taken

as 340. This implies that the sandy gravel's friction angle value could be 34

0 instead of

370

to be on the safe side. The safety factor's value at the EoC was higher than AFR

condition. The reason is that when the reservoir was filled with water, the soil became

saturated; as a result, the factor of safety was reduced.

90

Figure. 4.7: Effect of friction angle for sandy gravel on

dam safety factor.

4.4 EFFECT OF FRICTION ANGLE OF RANDOM FILL

As the downstream shell of the dam consists of random fill material (cf Figure. 3.2),

the effect of the friction angle of the random fill on the safety factor was analyzed

(Figure. 4.8). As expected, the safety factor increased with increasing the values of

the random fill's friction angle. After filling the reservoir condition, the friction angle

of random fill exceeding 320 showed no influence on the safety factor. This implies

that the random fill's friction angle could be 320 instead of 34

0 to remain on the safe

side. The safety factor values at the EoC were higher than AFR condition. The reason

is that when the reservoir was filled with water, the soil became saturated; as a result,

the factor of safety was reduced.

1.34

1.39

1.44

1.49

1.54

1.59

1.64

30 32 34 36

Fact

or

of

safe

ty

Friction angle of sandy gravel

End of construction After filling of reservoir

91

Figure. 4.8: Effect of friction angle for random fill on dam safety factor.

4.5 EFFECT OF FRICTION ANGLE OF CLAY CORE

As the center of the dam consists of clay core material (cf Figure. 3.2), the effect of

the clay core's friction angle on the dam's stability was investigated (Figure. 4.9). As

expected, the dam's safety factor increased with increasing values of the clay core's

friction angle for both the EoC and AFR conditions. In the partially dry state, the core

was stronger, and the factor of safety of the dam was more than sufficient, even the

value of friction angle utilized is 200 instead of 30

0. However, clay is more sensitive

to saturation. This implies that the friction angle of the core decreases with an

increase in moisture content. Therefore, it is necessary to fully compact the core and

utilize the maximum value of friction angle for the embankment dam's stability,

which is 300. The safety factor values at the EoC were higher than AFR condition.

The reason is that when the reservoir was filled with water, the soil became saturated;

as a result, the factor of safety was reduced.

1.42

1.44

1.46

1.48

1.5

1.52

1.54

1.56

1.58

1.6

1.62

30 31 32 33 34

Fa

cto

r o

f sa

fety

Friction angle of random fill (deg)


92

Figure. 4.9: Effect of friction angle for clay core on dam safety factor.

4.6 DAM SETTLEMENT COMPARISON USING MCM AND

HSM MODELS

All materials were modeled with the MCM (condition 1). Each of the four materials

(clay core, sandy gravel, random fill and sandy siltstone) were modeled separately

with HSM, and the rest of the materials were modeled with MCM (condition 2). The

dam's settlement calculations were performed for the EoC and AFR conditions. The

settlement response of the dam computed with MCM and HSM were compared for

the EoC and AFR in Figure’s 4.10-4.17.

The results (Figure’s 4.10-4.15) concluded that there is less variation in settlement

computed with the MCM and HSM for three major material zones (clay core, sandy

gravel and random fill) EoC and AFR conditions. The MCM overestimated the shear

strength for clay core, random fill and sandy gravel [75]. In undrained loading

conditions, the MCM shows a constant effective stress path to failure, while the real

soils show a curved effective stress path [75]. Therefore, the settlement predicted by

the MCM was slightly lower than the one computed with the HSM. On the other

hand, for sandy siltstone (foundation), the settlement predicted with the MCM is

higher than the one predicted with HSM (Figure’s. 4.16-4.17).

1.42

1.46

1.5

1.54

1.58

1.62

20 21 22 23 24 25 26 27 28 29 30

Fact

or

of

Safe

ty

Friction angle of clay core (deg)


93

A comparison of the dam's settlement computed with MCM and HSM for end of

construction, and after filling the reservoir, conditions are illustrated in Table 4.1 and

4.2, respectively.

Figure. 4.10: Dam settlement comparison for clay core at EoC.

Figure. 4.11: Dam settlement comparison for clay core at AFR.

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of clay core (kN/m2)

HSM MCM

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of clay core (kN/m2)

HSM MCM

94

Figure. 4.12: Dam settlement comparison for sandy gravel at EoC.

Figure. 4.13: Dam settlement comparison for sandy gravel at AFR.

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of sandy gravel (kN/m2)

HSM MCM

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of sandy gravel (kN/m2)

HSM MCM

95

Figure. 4.14: Dam settlement comparison for random fill at EoC.

Figure. 4.15: Dam settlement comparison for random fill at AFR.

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of random fill (kN/m2)

HSM MCM

1.5

1.6

1.7

1.8

1.9

2

2.1

25000 30000 35000 40000 45000 50000

Set

tlem

ent

(m)

MoE of random fill (kN/m2)

HSM MCM

96

Figure. 4.16: Dam settlement comparison for sandy silt stone at EoC.

Figure. 4.17: Dam settlement comparison for sandy silt stone at AFR.

0.6

0.9

1.2

1.5

1.8

2.1

2.4

70000 80000 90000 100000 110000 120000 130000

Set

tlem

ent

(m)

MoE of sandy siltstone (kN/m2)

HSM MCM

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

70000 80000 90000 100000 110000 120000

Set

tlem

ent

(m)


HSM MCM

97

Table 4.1: Summary of dam's settlement comparison for EoC.

Mat

eria

l MCM (EC) HSM (EC)

E=

25

000

(k

N/m

2)

E=

50

00

0 (

kN

/m2)

Set

tlem

ent

incr

ease

by

MC

M

(%)

as c

om

par

ed t

o H

SM

E=

25

00

0 (

kN

/m2)

Set

tlem

ent

incr

ease

by

MC

M

(%)

as c

om

par

ed t

o H

SM

E=

50

00

0

(kN

/m2)

E=

25

00

0 (

kN

/m2)

E=

50

000

(k

N/m

2)

Set

tlem

ent

incr

ease

by

HS

M

(%)

as c

om

par

ed t

o M

CM

E=

25

00

0 (

kN

/m2)

Set

tlem

ent

incr

ease

by

HS

M

(%)

as c

om

par

ed t

o M

CM

E=

50

000

(k

N/m

2)

Clay core 1.819 m 1.581 m

1.826 m 1.595 m 0.38 0.88

Sandy gravel 1.768 m 1.581 m

0.82 1.812 m 1.568 m 2.43

Random fill 1.724 m 1.581 m

0.89 1.929 m 1.567 m 10.6

Sandy

siltstone

E=70000 to

125000

(kN/m2)

2.33 m 1.581 m 56.65 49.40 1.01 m 0.8 m

Table 4.2: Summary of dam’s settlement comparison for AFR.

Mat

eria

l

MCM (AFR) HSM (AFR)

E=

25

000

(k

N/m

2)

E=

50

00

0 (

kN

/m2)

Set

tlem

ent

incr

ease

by

MC

M (

%)

as

com

par

ed t

o H

SM

E=

25

000

(k

N/m

2)

Set

tlem

ent

incr

ease

by

MC

M (

%)

as

com

par

ed t

o H

SM

E=

50

000

(k

N/m

2)

E=

25

00

0 (

kN

/m2)

E=

50

000

(k

N/m

2)

Set

tlem

ent

incr

ease

by

HS

M (

%)

as

com

par

ed t

o M

CM

E=

25

00

0 (

kN

/m2)

Set

tlem

ent

incr

ease

by

HS

M (

%)

as

com

par

ed t

o M

CM

E=

50

00

0 (

kN

/m2)

Clay core 1.891 m 1.702 m

1.978 m 1.751 m 4.40 2.8

Sandy

gravel 1.799 m 1.702 m

2.003 m 1.775 m 10.18 4.11

Random

fill 1.928 m 1.702 m

2.004 m 1.584 m 3.79 6.93

Sandy

siltstone

E= 70000

to 125000

(kN/m2)

2.547 m 1.702 m 58.85 50.30 1.055 m 0.844 m

98

4.6.1 Settlement at EoC and AFR

Figure. 4.18 compares dam settlement computed with the MCM and HSM for EoC

and AFR conditions. The MoE of sandy siltstone was varied from 70000 to 125000

kPa. It is observed that for EoC, the MCM showed 56.65% and 49.40% more

settlement than the HSM when the MoE of sandy siltstone increased from 70000

kN/m2 to 125000 kN/m

2 respectively. It is observed that for AFR, the MCM showed

58.85% and 50.30% more settlement than the HSM when the MoE of sandy siltstone

increased from 70000 kPa to 125000 kPa, respectively.

The MCM is suitable to determine the bearing capacity of foundations and slope

stability of embankment [151]. On the contrary, the MCM is unable to predict the

settlement of soils reliably [182]. The MCM uses constant soils' constant stiffness and

does not distinguish between initial loading, unloading, and reloading conditions in

soils. As a result, there is a significant difference between real soil settlement and the

predicted settlement with the MCM [151]. On the other hand, the HSM utilizes three

types of stiffness for loading, unloading and reloading situations. Therefore, the

settlement predicted with the HSM is more reliable for real problems [151].

Figure. 4.18: Settlement increase in % when MoE for sandy siltstone

varied from 70,000-125,000 kN/m2.

48

50

52

54

56

58

60

70000 80000 90000 100000 110000 120000 130000

Per

cen

tage

incr

ease

of

sett

lem

ent

of

the

dam

calc

ula

ted

wit

h M

CM

as

com

pare

d t

o H

SM


EOC AFOR

99

The settlement of the dam was calculated only at the crest of the dam. From the

results, it is concluded that the sandy siltstone (foundation) has more influence on

settlement than the other zones of the dam. According to a study on 134 such

embankments, it was found that most of the embankments settled as high as 1 to 2%

of the dam height [94]. As mentioned above, the sandy siltstone (foundation) is

sensitive to settlement when predictions were made with the MCM and HSM. The

MCM showed a settlement of 2.5 m at AFR when the MoE of sandy siltstone is

70000 kN/m2. This indicates that the predicted settlement with the MCM is about

4.2% of the dam height. The MCM showed a settlement of 1.7 m at AFR when the

MoE of sandy siltstone is 125000 kN/m2. This indicates that the predicted settlement

with the MCM is about 2.9% of the dam height.

The HSM showed a settlement of 1 m at AFR when the MoE of sandy siltstone is

70000 kN/m2, with a predicted settlement of about 1.69%, and a settlement of 0.8 m

at AFR when the MoE of sandy siltstone is 125000 kN/m2, showing the predicted

settlement of about 1.35%. This implies that the 0.8 m settlement of the dam is

considered to be a reasonable estimate.

4.6.2 Dam Settlement Comparison w.r.t. Depth

The settlement of the dam was computed by applying MCM to all zones of the dam.

For the sake of convenience, this is referred to as (condition 1). In the second case,

HSM is applied separately to four main zones (clay core, sandy gravel, random fill,

sandy siltstone), and the MCM was used for remaining materials (condition 2). The

settlements were determined along the dam's central axis and were computed up to a

depth of 120 m with an interval of 20 m, as shown in Figure. 4.19. The settlement

with respect to depth response of the dam when computed with MCM (condition 1)

and HSM applied to clay core, sandy gravel, random fill (condition 2) for EoC and

AFR are respectively given in Figure’s 4.20 to 4.25.

100

Figure. 4.19: Dam's central axis at which settlement was computed

w.r.t. depth at an interval of 20 m.

Figure. 4.20: Dam's settlement comparison w.r.t. depth

at EoC applied to clay core.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

101


at AFR applied to clay core.


at EoC applied to sandy gravel.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

102


at AFR applied to sandy gravel.


at EoC applied to random fill.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

103


at AFR applied to random fill.

From the results, it can be observed that there is less variation in the settlement

concerning depth when computed with MCM as compared to HSM for both EoC and

AFR for material zones (clay core, sandy grave and random fill).

Figure’s 4.26 and 4.27 respectively illustrate settlement concerning the depth of the

dam for end of construction and after filling of the reservoir when computed with

MCM (condition 1) and HSM (condition 2) applied to sandy siltstone (Foundation)

only.

Tables 3 and 4 respectively present the settlement concerning the dam's depth

computed with MCM and HSM for the end of construction and after filling the

reservoir.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

104


at EoC applied to sandy silt stone.


at AFR applied to sandy silt stone.

105

Table 4.3: Summary of dam’s settlement comparison for EoC.

Mat

eria

l MCM (EC) HSM (EC)

Set

tlem

ent

at

cres

t (m

)

Set

tlem

ent

at 1

20

m d

epth

Set

tlem

ent

incr

ease

by

MC

M (

%)

com

par

ed t

o

HS

M a

t cr

est

Set

tlem

ent

incr

ease

by

MC

M (

%)

com

par

ed t

o

HS

M a

t 1

20

m d

epth

Set

tlem

ent

(m)

at

Cre

st

Set

tlem

ent

(m)

at a

dep

th o

f

12

0 m

Set

tlem

ent

incr

ease

by

HS

M (

%)

as c

om

par

ed t

o

MC

M a

t th

e C

rest

Set

tlem

ent

incr

ease

by

HS

M (

%)

as c

om

par

ed t

o

MC

M a

t a

dep

th o

f 1

20

m

Clay core 1.574 0.55 4.18 1.612 0.527 2.36

Sandy

gravel 1.574 0.55 0.44 2.91 1.567 0.534

Random fill 1.574 0.55 0.51 1.566 0.562 2.41

Sandy

siltstone

E=70,000 to

125,000

(kN/m2)

1.574 0.55 49.17 79.64 0.8 0.112

Table 4.4: Summary of dam’s settlement comparison for AFR.

Mat

eria

l

MCM (EoC) HSM (AFR)

Set

tlem

ent

(m)

at t

he

Cre

st

Set

tlem

ent

(m)

dep

th o

f 1

20

m

Set

tlem

ent

incr

ease

by

MC

M (

%)

as c

om

par

ed

to H

SM

at

the

cres

t

Set

tlem

ent

incr

ease

by

MC

M (

%)

as

com

par

ed t

o H

SM

at

a d

epth

of

12

0

m

Set

tlem

ent

(m)

at t

he

Cre

st

Set

tlem

ent

(m)

at a

dep

th o

f 12

0 m

Set

tlem

ent

incr

ease

by

HS

M (

%)

as

com

par

ed t

o M

CM

at

the

Cre

st

Set

tlem

ent

incr

ease

by

HS

M (

%)

as

com

par

ed t

o M

CM

dep

th o

f 12

0 m

Clay core 1.7 0.568 1.733 0.571 1.90 0.53

Sandy gravel 1.7 0.568 0.41 1.693 0.586 3.07

Random fill 1.7 0.568 7.29 1.41 1.576 0.56

Sandy

siltstone

E=70000 to

125000

(kN/m2)

1.7 0.568 52.94 82.39 0.8 0.1

There is apparent variation in settlement both at the crest of the dam and at a depth of

120 m. Because in HSM, the modulus of soil elasticity increases with respect to the

depth, which is a real soil behavior. The available literature on embankment dams

106

suggests that most dams exhibit a settlement of about 1 to 2% of the dam's total height

[93, 94]. This dam's settlement computed with MCM and HSM after filling the

reservoir was found to be 2.9% and 1.35% of the dam height, respectively. It can be

interpreted that by utilizing the HSM parameters as mentioned in this study, the

settlement of the dam is considered to agree with the literature. From the above

results, it is observed that sandy siltstone has more influence on the settlement

concerning the depth of the dam as compared to the other material zones (clay core,

sandy gravel, random fill) when computed with MCM and HSM. Therefore, the

settlement response of sandy siltstone is investigated in more detail in Figure’s 4.28-

4.39. For this purpose, MoE of sandy siltstone was varied from 70000 to 125000

kN/m2 with an increment of 10000 kN/m

2 and settlement concerning depth was

calculated using both MCM and HSM at the EoC and AFR.

Tables 5 and 6 respectively present the effect of variation of MoE of sandy siltstone

on the settlement concerning depth using MCM and HSM for end of construction and

after filling of reservoir conditions.


For EoC applied to sandy silt stone (MoE = 70,000 kN/m2).

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

107


at AFR applied to sandy silt stone (MoE = 70,000 kN/m2).


for EoC applied to sandy silt stone (MoE = 80,000 kN/m2).

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5 3

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

108


for AFR applied to sandy silt stone (MoE = 80,000 kN/m2).


at EoC applied to sandy silt stone (MoE = 90,000 kN/m2).

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

109





0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

110





0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Dep

th o

f d

am

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

111





0

20

40

60

80

100

120

140

0 0.5 1 1.5 2D

epth

of

dam

(m

) Settlement (m)

HSM MCM

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2

Dep

th o

f d

am

(m

)

Settlement (m)

HSM MCM

112



Table 4.5: Variation effect of MoE of sandy siltstone on settlement

w.r.t. depth at EoC.

Mat

eria

l

MCM (EoC) HSM (EoC)

E =

(kN

/m2)

at t

he

cres

t (m

)

dep

th o

f 120 (

m)

Set

tlem

ent

incr

ease

by M

CM

(%

) as

com

par

ed t

o H

SM

at t

he

cres

t (m

)

Set

tlem

ent

incr

ease

by M

CM

(%

) as

com

par

ed t

o H

SM

dep

th o

f 120 (

m)

at t

he

cres

t (m

)

dep

th o

f 120 (

m)

Set

tlem

ent

incr

ease

by H

SM

(%

) as

com

par

ed t

o M

CM

at

the

Cre

st (

m)

Set

tlem

ent

incr

ease

by H

SM

(%

) as

com

par

ed t

o M

CM

dep

th o

f 120 (

m)

San

dy s

ilts

tone 70000 2.324 0.902 56.58 77.54 1.009 0.199

80000 2.12 0.79 55.24 77.85 0.949 0.175

90000 1.955 0.73 53.81 78.63 0.903 0.156

100000 1.82 0.673 52.42 79.20 0.866 0.14

110000 1.707 0.619 51.03 79.48 0.863 0.127

125000 1.574 0.55 49.17 79.64 0.8 0.112

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2D

epth

of

dam

(m

)

Settlement (m)

HSM MCM

113

Table 4.6: Variation effect of MoE of sandy siltstone on settlement

w.r.t. depth for AFR. M

ater

ial

MCM (AFR) HSM (AFR)

E =

(kN

/m2)

at C

rest

(m

)

dep

th o

f 120 (

m)

Set

tlem

ent

incr

ease

by M

CM

(%

) as

com

par

ed t

o H

SM

at t

he

cres

t (m

)

Set

tlem

ent

incr

ease

by M

CM

(%

) as

com

par

ed t

o H

SM

dep

th o

f 120 (

m)

at c

rest

(m

)

dep

th o

f 120 (

m)

Set

tlem

ent

incr

ease

by H

SM

(%

) as

com

par

ed t

o M

CM

at

the

Cre

st (

m)

Set

tlem

ent

incr

ease

by H

SM

(%

) as

com

par

ed t

o M

CM

dep

th o

f 120 (

m)

San

dy s

ilts

tone 70000 2.548 1.042 60.40 82.73 1.009 0.18

80000 2.298 0.886 58.70 82.28 0.949 0.157

90000 2.138 0.826 57.81 83.05 0.902 0.14

100000 1.976 0.732 56.22 82.92 0.865 0.125

110000 1.841 0.683 56.64 83.31 0.835 0.114

125000 1.7 0.568 52.94 82.39 0.8 0.1

4.7 VARIATION EFFECT OF MoE OF SANDY SILTSTONE ON

SETTLEMENT w.r.t. DEPTH

In this case, the comparison of the settlement of dam at the crest and up to a depth of

120 m was computed with MCM (condition 1), and HSM (condition 2) applied to

sandy siltstone (foundation ) only for the EoC and AFR as presented in Figure. 4.40

and 4.41 respectively. The MoE of sandy siltstone was varied from 70000 to 125000

kN/m2 with an increment of 10000 kN/m

2. The results indicate that the rate of

increase of settlement computed with MCM (compared with HSM) increased

concerning the dam's depth. At the dam's surface, the rate of increase of settlement

calculated with MCM (compared with HSM) slightly decreased with the rise in MoE

of sandy siltstone from 70000 to 125000 kN/m2. However, at a depth of 80 m and

downwards, the rate of increase of settlement computed with MCM (compared with

HSM) is almost the same irrespective of change in MoE of sandy siltstone.

114

Figure. 4.40: Settlement at EoC by MCM and HSM models.

Figure. 4.41: Settlement for AFR by MCM and HSM models.

0

20

40

60

80

100

120

140

35 40 45 50 55 60 65 70 75 80 85

Dep

th

of

da

m (

m)

Increase of settlement calculated by MCM compared to HSM (%) [EoC]

(MOE) Foundation 70000 kN/m2 (MOE) Foundation 80000 kN/m2



0

20

40

60

80

100

120

140

35 40 45 50 55 60 65 70 75 80 85

Dep

th

of

da

m (

m)

Increase of settlement calculated by MCM compared to HSM (%) [AFOR]




115

4.8 COMPARISON OF LONG-TERM SETTLEMENT FOR

SANDY SILT STONE

The long-term settlement of the dam was calculated since the dam's foundation

consists of sandy siltstone, which is reported to be weak, fragile with open bedding

planes. The settlement of the dam was calculated for 50 years for both the EoC and

AFR conditions using MCM (condition 1) and HSM (condition 2), as presented in

Figure’s 4.42- 4.43 respectively. The results suggest that the dam's long-term

settlement is almost the same as that occurred AFR. Even though the magnitude of the

settlement predicted with MCM is higher than that of HSM. However, both models

predicted that long-term settlement did not increase concerning the time after filling

the reservoir. The long-term settlement predicted with MCM for both EoC and AFR

conditions was about 51% and 59% higher than HSM. The magnitude of the

settlement is concentrated in clay core, sandy gravel and random fill. The clay core is

compacted during the construction process of the dam. However, clay is considered

compressible material; therefore, the clay's compression mostly occurred during the

construction and after filling the reservoir phases. Therefore, further settlement of the

clay did not happen during the long-term period of 50 years.

Figure. 4.42: Prediction of long-term settlement for sandy

siltstone (MoE = 125,000 kN/m2).

116

Figure. 4.43: Prediction of long-term settlement for sandy

siltstone (MoE = 70,000 kN/m2).

4.9 RAPID DRAWDOWN

The undrained shear strength of embankment soils, particularly the core, consists of

clay; have an essential role during the rapid drawdown process. This is due to less

time for excess pore pressures to dissipate. As a result, instability of the upstream

slope may occur due to the development of pore pressures. This implies that the

undrained condition is applied for both the strength and stiffness properties of clay.

Therefore, the effective strength parameters of clay were changed into undrained

shear strength. The undrained shear strength of clay was calculated based on

consistency index values [181], which are explained in detail in chapter 3. Besides,

the value of modulus of clay elasticity is taken as 30000 kN/m2 instead of 50000

kN/m2 to consider the effect of saturation of the clay when in contact with the

upstream water level.

Three cases have been discussed regarding the undrained shear strength of the clay

core. The values of undrained shear strength utilized are 20, 25 and 30 kN/m2. In

117

every case, the drawdown is shown at different increasing rates of 0.1, 0.125, 0.25,

0.5 and finally 1 m/day. The drawdown rates of 0.1 m/day and 1 m/day are

respectively called slow drawdown and the embankment dam's rapid drawdown.

4.9.1 Dam Stability During Drawdown under Undrained Shear Strength

of Clay = 20 kN/m2

Figure’s 4.44 shows the factor of safety of the embankment dam versus time at

different rates of drawdown varying from 0.1 m/day to 1 m/day, as mentioned above.

It can be observed that for a rapid drawdown of 1 m/day, the dam has shown enough

stability up to 15 days (15 m lowering of the reservoir depth) at which the factor of

safety was 1.2 [183]. When the dam's rapid drawdown at the rate of 1 m/day was

allowed continuously beyond 15 days, the dam showed instability in terms of the

safety factor, which continually reduced up to 0.92 at 35 days (35 m lowering of the

reservoir). If the reservoir was lowered beyond 35 m depth, then the computed results

showed complete collapse of the soils in the dam; this is because of the quick

lowering of the reservoir, resulting in an imbalance of the water's support the

upstream face.

When the drawdown rate was 0.5 m/day, the dam showed stability up to 32 days (16

m lowering of the reservoir), with a safety factor of 1.2. The dam became unstable

continuously when the reservoir was lowered continuously beyond 32 days.

Figure. 4.44 shows a limiting value of the factor of safety of 1.2 as a straight line. The

curves for the factor of safety plotted for a different rate of lowering the reservoir on

or above the limiting value of 1.2 are considered satisfactory. Further, if the dam is

lowered up to 20 m at a rate of 0.1 m/day up to 200 days, the dam would be safe with

a safety factor of 1.2.

118

Figure. 4.44: Safety factor vs. time while lowering the reservoir

under undrained shear strength of clay = 20 kN/m2.

The failure zones for rapid drawdown at a rate of 1 m/day (15 m depth of lowering of

the reservoir) and slow drawdown at a rate of 0.1 m/day (20 m depth of lowering of

the reservoir) are respectively shown in Figure’s 4.45-4.46. For the rapid drawdown

situation, the failure was initiated in the clay core and then extended to the

downstream side, which is less permeable than the upstream side. The failure zone

was created in clay core for slow drawdown and then extended to the dam's upstream

side.

Figure. 4.45: Dam failure zone when reservoir is lowered @ 1 m/day

to a depth of 15 m (Su=20 kN/m2).

0.9

1

1.1

1.2

1.3

1.4

10

35

60

85

110

135

160

185

210

235

260

285

310

335

360

385

410

435

460

485

Fac

tor

of

safe

ty

Time (days) 1 m/day 0.5 m/day

0.25 m/day 0.125 m/day

0.1 m/day Limiting values of factor of safety

119

Figure. 4.46: Dam failure zone when reservoir is lowered @ 0.1 m/day

to a depth of 20 m (Su=20 kN/m2).


of Clay = 25 kN/m2

The factor of safety versus time for the different lowering of the dam's reservoir is

shown in Figure. 4.47. It can be observed that when the dam is lowered at a quick

drawdown rate of 1 m/day, the dam showed stability up to 18 days (18 m lowering of

the reservoir), having a factor of safety of 1.2. Beyond 18 days, the reservoir lowered

at the same rate indicated instability of the dam.

When the drawdown rate was 0.5 m/day, the dam showed stability up to 42 days (22

m lowering of the reservoir), with a safety factor of 1.2. The dam became unstable

when the reservoir was lowered continuously beyond the 22 m lowering of the

reservoir.

For the slow rate of lowering the reservoir at a rate of 0.1 m/day, the dam showed

stability up to 550 days (55 m lowering of the reservoir). This indicated that the

reservoir could be lowered safely up to a depth of 55 m at a rate of 0.1 m/day.

120

Figure. 4.47: Dam safety factor vs. time under undrained

shear strength of clay = 25 kN/m2.



the reservoir) are respectively shown in Figure’s 4.48-4.49. The failure was initiated

in clay core for both situations and then extended to the dam's upstream side.


for a depth of 18 m (Su=25 kN/m2).

0.9

1

1.1

1.2

1.3

1.4

1.5

10

35

60

85

110

135

160

185

210

235

260

285

310

335

360

385

410

435

460

485

510

535

560

Fac

tor

of

safe

ty

Time (days) 1 m/day 0.5 m/day0.25 m/day 0.125 m/day0.1 m/day Limiting values of factor of safety

121




of Clay = 30 kN/m2

Figure. 4.50 illustrates that the dam is safe if its reservoir is lowered up to a depth of

55 m even at a rate of 0.25 m/day in 220 days. If the reservoir's lowering is performed

quickly at a rate of 1 m/day, the dam was stable up to a depth of 23 m only.

It is also interesting to note that for all rates of lowering of the reservoir, the factor of

safety initially decreased and then gradually increased even when the lowering was

continued beyond a depth of 35 m. This is because the clay core and upstream

material, that is, sandy gravel, are more comprehensive at the base as compared to the

top surface. Due to more width of clay core and upstream sandy gravel at more

profound layers of the dam, the soils became more hardened; therefore, safety was

increased.

In general, a decrease in the drawdown rate showed an increase in the dam's safety

factor. This indicates that the dam's stability became improved when the rate of

drawdown was relatively reduced.

122

Figure. 4.50: Dam safety factor vs. time under undrained

shear strength of clay = 30 kN/m2.

The depth of lowering of the reservoir for different drawdown rates for three cases of

undrained shear strength of clay core 20 kN/m2, 25 kN/m

2 and 30 kN/m

2 were

compared in tables 4.7, 4.8 and 4.9 respectively. It is observed that for a rapid

drawdown of 1 m/day, the permissible depth of the reservoir's lowering was 15 m, 18

m and 23 m, respectively, for the above-mentioned undrained shear strength.

For a slow drawdown of 0.1 m/day, the permissible depth of the reservoir's lowering

was 20 m (undrained shear strength 20 kN/m2) and 55 m for undrained shear strength

of 25 kN/m2 and 30 kN/m

2.

0.9

1

1.1

1.2

1.3

1.4

1.5

10

35

60

85

110

135

160

185

210

235

260

285

310

335

360

385

410

435

460

485

510

535

560

Fac

tor

of

safe

ty

Time (days) 1 m/day 0.5 m/day0.25 m/day 0.125 m/day0.1 m/day Limiting values of factor of safety

123

Table 4.7: Reservoir drawdown rate vs. dam safety factor under

undrained shear strength of clay core = 20 kN/m2.

Drawdown

(m/day) Lowering of the reservoir (m) Days Safety factor

1 15 15 1.2

0.5 16 32 1.2

0.25 18 72 1.2

0.125 19 152 1.2

0.1 20 200 1.2



Drawdown


1 18 18 1.2

0.5 22 42 1.2

0.25 24 96 1.2

0.125 55 440 1.22

0.1 55 550 1.24



Drawdown


1 23 23 1.2

0.5 26 52 1.2

0.25 55 220 1.2

0.125 55 440 1.26

0.1 55 550 1.3



the reservoir) are respectively shown in Figure’s 4.51 and 4.52. The failure was

initiated in clay core for both situations and then extended to the dam's upstream side.

124





4.10 PORE PRESSURE UNDER RAPID DRAWDOWN

In addition to stability during rapid drawdown and slow drawdown of the dam, it is

also necessary to observe how the excess pore pressures have developed and

dissipated concerning time.

Development and dissipation of excess pore pressures for slow drawdown (0.1 m/day)

and rapid drawdown (1 m/day) for various depths and undrained shear strength of 20,

25 and 30 kN/m2 are shown in (Figure’s 4.53-4.64).

It can be observed that for undrained shear strength of 20, 25 and 30 kN/m2, the

excess pore pressure reduced when the drawdown was carried out at a slow rate of

125

(0.1 m/day) up to a depth of 20 m as compared to rapid drawdown rate of (1 m/day).

On the other hand, if the depth of the reservoir's lowering was deeper (i.e., more than

20 m), then for undrained shear strength of 25 and 30 kN/m2, the excess pore

pressures were almost the same during both slow drawdown and rapid drawdown

rates. The increase of excess pore pressures during rapid drawdown (1 m/day) and

decrease of excess pore pressures during slow drawdown (0.1 m/day) is related to the

permeability of clay core [44]. For all the cases of undrained shear strength of 20, 25

and 30 kN/m2, the rate of dissipation of suction is lower during the slow drawdown;

this implies that the dam's slope is stabilized during the slow drawdown.

Table 4.10: Development and dissipation of excess pore pressure

under various drawdown rates.

Undrained

shear strength

Su (kN/m2)

Drawdown

rate

(m/day)

Depth of

drawdown

(m)

Days

Excess pore

pressure

(kN/m2)

Suction

(kN/m2)

20 1.0 10 10 360 280

20 0.1 10 100 225 225

20 1.0 20 20 370 160

20 0.1 20 200 280 140

25 1.0 20 20 325 175

25 0.1 20 200 275 150

25 1.0 55 55 540 20

25 0.1 55 550 540 20

30 1.0 20 20 300 175

30 0.1 20 200 275 150

30 1.0 55 55 540 20

30 0.1 55 550 540 20

126

Figure. 4.53: Development of excess pore pressure under lowering of

reservoir @ 1 m/day for 10 m depth in 10 days (Su=20 kN/m2).


reservoir @ 0.1 m/day for 10 m depth in 100 days (Su=20 kN/m2).

127





128





129





130





131





132

4.11 EFFECT OF RAINFALL ON SLOPE STABILITY OF

DAM

The stability of the dam was assessed for possible rainfall at the dam site. The

maximum rainfall during the last 40 years was 435 mm/year. The rainfall intensity of

435 mm/year was applied equally in 5 days to obtain the rainfall's worst effect on the

dam's stability. It is to be noted that the region in which the dam is located is

generally dry, and low rainfall intensity has occurred there in the last four decades.

This rainfall was converted to m/day and applied to the dam's crest and downstream

boundaries. The effect of rainfall was calculated for 1 to 5 days. The safety factor was

calculated for the impact of rainfall on the dam's stability, as shown in Figure. 4.65.

The dam's safety factor was reduced as the rainfall was continuously poured for 1 to 5

days. The dam's safety factor was 1.494 when it was subjected to rainfall intensity for

one day. The safety factor reduced from 1.494 to 1.439 when the rainfall continuously

poured for five days with the same intensity. The reason for the lowering of the safety

factor is that with the continuous pouring of rainfall on the crest and downstream

portion of the dam, the soils get saturated and increase pore pressures. As a result,

there was a reduction in strength and factor of safety. Overall, the dam's safety is

considered satisfactory even for the continuous rainfall of 5 days with the same

intensity.

The potential failure zones of the dam are shown in Figure’s 4.66-4.67, respectively.

It can be observed that the failure zones due to rainfall along with the highest

reservoir level have developed on the downstream side, and the effect of rainfall can

be seen on the clay core of the dam.

133

Figure. 4.65: Effect of rainfall on dam stability.

Figure. 4.66: Dam failure zone due to 1-day precipitation.

Figure. 4.67: Dam failure zone due to 5-day precipitation.

1.42

1.44

1.46

1.48

1.5

1 2 3 4 5

Fact

or

of

safe

ty

Time (Days)

134

CHAPTER 5

CONCLUSION AND SUGGESTIONS

5.1 CONCLUSION

A numerical analysis of an embankment dam's stability and settlement called Nai Gaj

Dam is presented in this study. Numerical simulations of stability and settlement for

staged construction and filling of the reservoir are illustrated. Parametric analysis of

stability was carried out to determine the strength parameters' effect on the dam's

stability. For the computation of stability, strength parameters such as the friction

angle of the main zones (sandy gravel, random fill, and clay core) were gradually

lowered. The values of the MoE of four zones that occupied more volume (sandy

gravel, random fill, clay core, and sandy siltstone) were incrementally lowered for

calculation of settlement. Besides, the lowering of the reservoir and the maximum

possible rainfall on the dam's stability are investigated. The main conclusions drawn

from this study are presented below:

The stability of the dam was evaluated in terms of the safety factor. The safety factor

values for the end of construction and after filling the reservoir are respectively 1.6

and 1.5. The results suggest that the dam's safety factor is satisfactory for both ends of

construction and after filling of the reservoir conditions [78].

Parametric analysis of the dam was conducted to adopt a reasonable magnitude of

MoE of those zones which have a significant influence on the settlement:

o The results suggest that out of the four major zones of the dam (clay core,

sandy gravel, random fill and sandy siltstone), sandy siltstone (Foundation

soil) has a significant influence on the settlement of the dam when computed

with MCM and HSM.

134

135

o As per the study's correlations, the maximum and minimum value of the sandy

siltstone's MoE was from 70,000 kPa to 125,000 kPa.

o Based on numerical analysis, the dam's settlement is predicted to be 1.35% of

the dam's height when the MoE of the sandy siltstone is 125000 kPa.

o The magnitude of the dam's predicted settlement agrees with existing available

literature when the value of MoE of sandy siltstone is taken as 125,000 kPa.

o It was observed that the safe rate of the lowering of the reservoir mainly

depends on the magnitude of the undrained shear strength of the clay core.

o The recommended value of the clay's undrained shear strength can be taken as

25 kN/m2. The computed results indicate that the dam's reservoir could be

lowered safely up to 20 m and 55 m depth if the drawdown rates are

respectively as 1 m/day (rapid drawdown) and 0.1 m/day (slow drawdown).

o For the drawdown mentioned above rates, the dam's obtained safety factor was

1.2, which is considered satisfactory as per guidelines.

o Based on the maximum possible rainfall intensity of 435 mm/year, the dam's

embankment was subjected to maximum rainfall intensity for 1 to 5 days. The

stability of the dam was affected as the duration of rainfall was extended. The

computed safety factors were 1.494 and 1.439, respectively, for 1 to 5 days of

rainfall. Overall, the stability of the dam was satisfactory during the rainfall-

induced response.

o This study shows that parametric analysis based on the finite element method

plays a vital role in estimating suitable values of MoE of the material zones

whenever the said values are not determined experimentally.

5.2 RECOMMENDATIONS FOR FUTURE STUDIES

Based on the results and discussions of the numerical analysis of stability and

settlement response of Nai Gaj dam, future studies may be conducted on the following

aspects of the numerical modeling:

o The effect of seepage on the dam's stability be assessed numerically, and the

results are compared with the field measurements of steady-state seepage

conditions.

136

o Numerical analysis of any possibility of internal erosion through the

embankment and its foundation be conducted.

o A numerical analysis of possible uplift pressure on the downstream side of the

dam is performed.

o The dam's stability is checked against shaking the maximum possible

earthquake in the dam area, and remedial measures are suggested to enhance

dam stability.

137

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