SOIL MOISTURE VARIATION DUE TO GRASS WATER UPTAKE

IN SHALLOW SLOPE

SITI SAIDATUL AZWEEN BINTI ISMAIL

A thesis submitted in fulfillment of the

requirements for the award of the degree of Master of Engineering (Civil -Geotechnics)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

JUNE 2015

iii

I would like to dedicate my thesis to my beloved

husband Mohd Muzri Bin Mohamad and to my

lovely daughter Siti Amni Nasuha Binti Mohd

Muzri.

iv

ACKNOWLEDGEMENT

I am using this opportunity to express my gratitude to everyone who

supported me throughout the course of this research master project. I am thankful for

their aspiring guidance, invaluably constructive criticism and friendly advice during

the project work. I am sincerely grateful to them for sharing their truthful and

illuminating views on a number of issues related to the project.

First of all, I would like to take this opportunity to express my highest

gratitude to all who gave me the support physically and mentally to complete this

thesis successfully especially to my husband Mohd Muzri bin Mohamad, my

daughter Siti Amni Nasuha bt Mohd Muzri and my family. I want to thank my

supervisor, Prof Madya Dr. Nazri Ali for his helpful advices and knowledge

throughout the year in completion of my project. His valuable encouragement, time

spent and patience dedicated to my study are fully appreciated. Without him, this

report will not more than a book than empty paper.

Lastly, I warmly thank to my friends, for their love, patience and continuous

support during my study period

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ABSTRACT

Shallow landslide is the slope failure phenomenon that frequently occurs in

tropical rainforest region such as Malaysia. While fine root distribution for grass with

depth is consistent and easily defined, the fine root network plays an important role

as geosynthetic mesh by providing apparent enhanced cohesion that increases the

strength of the soil. This study is aimed to evaluate the effects of grass on shallow

slope stability in terms of soil moisture variation and to develop 1D soil moisture

suction due to grass water uptake. The numerical simulation modelling was applied

based on the literature review results to obtain the most appropriate condition to

replicate the grass water uptake within the soil slope. The transpiration was solely

based on six types of grass which have significantly altered the matric suction or

moisture variation distribution on an unsaturated soil slope. The types of grass were

Axonopus Compressus, Pennisetum Purpureum, Andropogon Gayanus, Brachiaria

Humidicola, Melinis Minutiflora and Digitaria Eriantha. The assessment of slope

stability due to the influence of six type of grasses induced suction was provided in

this research. The length of grass roots are different for all grasses, ranging from

0.3048 m up to 4.000 m. Results of safety factor demonstrated that shallow slope

stability is sensitive to pressure head. The factors of safety for all grasses are ranging

from 2.8800 until 3.1860. A value which is greater than 1 represents the stability of

the slope. This research delivers a strongly belief that Axonopus Compressus grass

absorb water way faster from others with factor of safety of 3.0670 and is beneficial

in maintaining stability of unsaturated soil.

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ABSTRAK

Tanah runtuh cetek adalah fenomena kegagalan cerun yang sering berlaku

masalah geoteknikal di kawasan hutan hujan tropika seperti Malaysia. Bagi rumput

di mana pengagihan akar bergantung pada kedalaman adalah konsisten dan mudah

ditakrifkan, rangkaian akar memainkan peranan yang penting sebagai jaringan

geosintetik dengan menyediakan panduan jelas yang dipertingkatkan untuk

meningkatkan kekuatan tanah. Kajian ini adalah untuk menilai kesan rumput pada

kestabilan cerun cetek dari segi perubahan kelembapan tanah dan membangunkan

1D untuk sedutan kelembapan tanah mengenai pengambilan air dari rumput yang

terlibat. Pemodelan simulasi berangka telah digunakan berdasarkan keputusan kajian

literatur untuk mendapatkan keadaan yang paling sesuai untuk mendapatkan kadar

pengambilan air rumput di cerun tanah. Transpirasi adalah berdasarkan kepada enam

jenis rumput di mana ia telah mengubah dengan ketara matrik pengedaran sedutan

atau kelembapan perubahan pada cerun untuk tanah tak tepu. Kajian ini dapat

disimpulkan bahawa enam rumput seperti Axonopus compressus, Pennisetum

purpureum, Andropogon Gayanus, Brachiaria Humidicola, Melinis Minutiflora dan

Digitaria Eriantha telah digunakan. Penilaian kestabilan cerun disebabkan oleh

pengaruh dari enam jenis rumput yang disebabkan sedutan terdapat dalam kajian ini.

Panjang akar rumput adalah berbeza untuk semua rumput, dari 0.3048 m sehingga

4m digunakan. Keputusan faktor keselamatan menunjukkan bahawa kestabilan cerun

cetek adalah sensitif kepada turus tekanan. Faktor-faktor keselamatan untuk semua

rumput adalah dalam julat 2.8800 hingga 3.1860. Nilai lebih besar dari 1 mewakili

kestabilan cerun. Kajian ini menyatakan bahawa rumput Axonopus Compressus

menyerap air lebih cepat daripada enam jenis rumput yang lain dengan faktor

keselamatan 3.0670 dan bermanfaat dalam mengekalkan kestabilan tanah tidak tepu.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE OF PROJECT i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xviii

LIST OF APPENDICES xx

1 INTRODUCTION

1.1 Background of Study 1

1.2 Problem Statement 2

1.3 Objective 3

1.4 Scope of Study 4

1.5 Significant of Study 4

1.6 Thesis Layout 5

2 LITERATURE REVIEW

2.1 Introduction 7

2.2 Vegetation in Civil Engineering 7

2.2.1 Plant Form and Structure 8

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2.2.2 Root Systems 10

2.2.3 Grass 12

2.2.3.1 Axonopus Compressus Grass 13

2.2.3.2 Pennisetum Purpureum Grass 15

2.2.3.3 Andropogon Gayanus Grass 16

2.2.3.4 Brachiaria Humidicola Grass 18

2.2.3.5 Melinis Minutiflora Grass 20

2.2.3.6 Digitaria Eriantha Grass 21

2.3 Shallow Slope Stability 23

2.4 Soil Water Characteristic Curve (SWCC) 24

2.5 Transpiration 27

2.6 Field Monitoring 28

2.7 The Root-Water Uptake Process 30

2.8 Numerical Simulation Of Grass Water Uptake

Model 31

2.9 Concluding Remarks 33

3 METHODOLOGY

3.1 Introduction 35

3.2 Chart Methodology 37

3.3 Data Collection 38

3.3.1 Numerical Simulation 40

3.3.1.1 Hydrological Mechanism 44

3.3.2 Site Measurement 51

3.3.3 Validate between Numerical Simulation

and Site Measurement 52

3.4 Analysis of Safety Factor on Slope Stability 53

3.5 Concluding Remarks 58

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4 VALIDATION BETWEEN NUMERICAL SIMULATION AND FIELD MEASUREMENT RESULT

4.1 Introduction 59

4.2 Computer Simulation 59

4.3 Result validation between computer simulation

and field measurement result 61

4.4 Concluding Remarks 63

5 SOIL MOISTURE VARIATION DUE TO GRASS

WATER UPTAKE

5.1 Introduction 64

5.2 Simulation 5-1 – Result for Axonopus

Compressus Grass 65

5.3 Simulation 5-2 – Result for Pennisetum

Purpureum Grass 68

5.4 Simulation 5-3- Result for Andropogon

Gayanus Grass 71

5.5 Simulation 5-4 – Result for Brachiaria

Humidicola Grass 74

5.6 Simulation 5-5- Result for Melinis

Minutiflora Grass 77

5.7 Simulation 5-6- Result for Digitaria

Eriantha Grass 79

5.8 Combination for Six Types of Grass 82

5.9 Concluding Remarks 85

6 ANALYSIS OF SAFETY FACTOR ON SLOPE

STABILITY

6.1 Introduction 87

6.2 Pressure Head 87

6.2.1 Differences in Pressure Head for Six

Type of Grasses 91

6.3 Analysis of Safety Factor on Slope Stability 97

x

6.4 Concluding Remarks 101

7 CONCLUSIONS AND RECOMMENDATION

7.1 Overall Conclusion and Summary 104

7.2 Recommendations for Future Research 106

REFERENCES 108

APPENDICES A-G 117

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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Length of grasses root 12 2.2 Common Names of Axonopus Compressus

(Barnard ,1969; Bogdan, 1977, Evans et al, 1988 and Wong, 1990) 13

2.3 Common Names of Pennisetum Purpureum (Bogdan, 1977) 15 2.4 Comman Names of Andropogus Gayanus (Department

of Agriculture, Fisherie, 2013) 17

2.5 Common Names of Brachiaria Humedicola (Chippendall et al, 1955; Chippendall et al, 1976; CIAT, 1992; Lenne et al,1994; Miles et al, 1996) 19

2.6 Common Names of Melinis Minutiflora (Bogdan, 1977) 20 2.7 Common Names of Digitaria Eriantha (Bogdan, 1977) 22 3.1 Basic soil properties (N. Ali & Rees, 2008) 38 3.2 Time step 49 3.3 Experimental Values of b , modified after Fredlund and

Rahardjo (1993) 53

3.4 Material Properties (Bishop et al, 1960) 58 4.1 Field Measurement (Razi, 2014) 61 5.1 Percentage of water absorb with time 84 5.2 Rankings of Moisture Content 85 6.1 Results of differences between Pennisetum Purpureum

grass and Axonopus Compressus grass 94

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6.2 Results of differences between Andropogon Gayanus

grass and Axonopus Compressus grass 95

7.2 Results of differences between Brachiaria Humidicola grass and Axonopus Compressus grass 95

7.3 Results of differences between Melinis Minutiflora grass and Axonopus Compressus grass 96

7.4 Results of differences between Digitaria Eriantha grass and Axonopus Compressus grass 96

6.6 Calculation of FOS without Pressure Head 98 6.7 Calculation of FOS Axonopus Compressus 98 6.8 Calculation of FOS Pennisetum Purpureum 99 6.9 Calculation of FOS Andropogon Gayanus 99 6.10 Calculation of FOS Brachiaria Humidicola 100 6.11 Calculation of FOS Melinis Minutiflora 100 6.12 Calculation of FOS Digitaria Eriantha 101 6.13 Increase in FOS 102

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LIST OF FIGURES

FIGURES NO. TITLE PAGE

2.1 Forms of trees and shrubs with respect to different stem (Coppin and Richards, 1990) 9

2.2 Forms of grasses and herbs with respect to different stem (Coppin and Richards, 1990) 9

5.1 Different type of root growth patterns (Yen, 1971) 10 2.4 Modification of root distribution by subsurface

conditions (Coppin and Richards, 1990) 11

5.2 Grass root 11 2.6 Axonopus Compressus (Barnard, 1969) 14 2.7 Pennisetum Purpureum (Bogdan, 1977) 16 2.8 Andropogus Gayanus (Department of Agriculture,

Fisherie, 2013) 18 2.9 Brachiaria Humedicola (Miles, 1996) 19 2.10 Melinis Minutiflora (Bogdan, 1977) 21 2.11 Digitaria Eriantha (Bogdan, 1977) 23 2.12 Typical herbaceous plant root orientation with respect

to shallow slope stability (Coppin and Richards, 1990) 24 2.13 Typical soil water characteristic curves (after Fredlund and Xing, 1994) 25

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2.14 Soil water characteristic curves for sandy soil, a silty soil,

and a clayey soil (Fredlund and Xing, 1994) 27 3.1 Flow chart Methodology 37 3.2 Water Retention Curve (Ali, 2007) 39 3.3 Linear variation of extraction rate (Ali, 2007) 40 3.4 General shape of the alpha as a function of the absolute

value of the capillary potential, modified after Feddes et al (1978) 43

3.5 1D Finite Element Method (25 element and

128 node) 45 3.6 1D Finite Element Method (From 1 to 5 element

and node) 46 3.7 1D Finite Element Method (From 6 to 10 element

and node) 46

3.8 1D Finite Element Method (From 11 to 15 element and node) 47

3.9 1D Finite Element Method (From 16 to 20 element

and node) 47

3.10 1D Finite Element Method (From 20 to 25 element and node) 48

3.11 Textpad (dat file) 50 3.12 Fotran 2.0 50 3.13 Textpad (out file) 51 3.14 Tensiometer installed at study area (Razi, 2014) 52 3.15 Forces acting on a slice through a sliding mass with

a circular slip surface, modified after Fredlund and Rahadjo (1993) 54

xv

4.1 Andropogon Gayanus 60 4.2 Percentage difference of moisture content for day 15 62 5.1(a) Numerical simulation moisture content variation

with time for Axonopus Compressus day 15 66 5.1(b) Numerical simulation moisture content variation with

time for Axonopus Compressus day 30 66 5.1(c) Numerical simulation moisture content variation with

time for Axonopus Compressus day 60 67 5.1(d) Numerical simulation moisture content variation with

time for Axonopus Compressus day 90 67 5.2(a) Numerical simulation moisture content variation with

time for Pennisetum Purpureum day 15 69 5.2(b) Numerical simulation moisture content variation with

time for Pennisetum Purpureum day 30 69 5.2 (c) Numerical simulation moisture content variation with

time for Pennisetum Purpureum day 60 70 5.2(d) Numerical simulation moisture content variation with

time for Pennisetum Purpureum day 90 70 5.3(a) Numerical simulation moisture content variation with

time for Andropogon Gayanus day 15 72 5.3(b) Numerical simulation moisture content variation with

time for Andropogon Gayanus day 30 72 5.3(c) Numerical simulation moisture content variation with

time for Andropogon Gayanus day 60 73 5.3(d) Numerical simulation moisture content variation with

time for Andropogon Gayanus day 90 73 5.4(a) Numerical simulation moisture content variation with

time for Brachiaria Humidicola day 15 75 5.4(b) Numerical simulation moisture content variation with

xvi

time for Brachiaria Humidicola day 30 75 5.4(c) Numerical simulation moisture content variation with

time for Brachiaria Humidicola day 60 76 5.4(d) Numerical simulation moisture content variation with

time for Brachiaria Humidicola day 90 76

5.5(a) Numerical simulation moisture content variation with time for Melinis Minutiflora day 15 77

5.5(b) Numerical simulation moisture content variation with

time for Melinis Minutiflora day 30 78 5.5(c) Numerical simulation moisture content variation with

time for Melinis Minutiflora day 60 78 5.5(d) Numerical simulation moisture content variation with

time for Melinis Minutiflora day 90 79 5.6(a) Numerical simulation moisture content variation with

time for Digitaria Eriantha day 15 80 5.6(b) Numerical simulation moisture content variation with

time for Digitaria Eriantha day 30 80 5.6(c) Numerical simulation moisture content variation with

time for Digitaria Eriantha day 60 81 5.6(d) Numerical simulation moisture content variation with

time for Digitaria Eriantha day 90 81 5.7 Combination of six types of grass at a depth 1m moisture

content with time 82 5.8 Combination of six types of grass at a depth 5m moisture

content with time 83

6.1 Pressure head for Axonopus Compressus 81 6.2 Pressure head for Pennisetum Purpureum 89 6.3 Pressure head for Andropogon Gayanus 90

xvii

6.4 Presure head for Brachiaria Humidicola 91 6.5 Presure head for Melinis Minutiflora 92 6.6 Presure head for Digitaria Eriantha 93 6.7 Slope Geometry 97

xviii

LIST OF SYMBOLS

C Specific moisture capacity (cm-1)

Ks Saturated hydraulic conductivity (cm/s)

σ3 Pressure cell

ua-uw Different suctions

qu Undrained compressive strength

N Total normal force on the base of the slice (kN)

Smax, rzSzSS ,,,,, Sink term (cm3/cm3/s)

T, Tj Potential Transpiration rate (cm/s)

t Time (s)

V, Vw Volume of water (cm3)

W Total weight of a slice (kN)

nml ,,, Soil specific parameter

() Moisture content

(ua) Air pressure

(uw) Water phase pressure

Gs Specific gravity of soil solids

Void ratio

Volumetric moisture content (%)

1 , 2 Contact angle of an interface

r Residual water content (%)

s Saturated water content (%)

Capillary potential (cm)

Total potential for moisture flow

Water flux at the boundary (cm/s)

P Pressure Head

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b Angle indicating the rate of increase in shear strength

relative to matric suction (degrees)

xx

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Axonopus Compressus Grass 117 B Pennisetum purpureum Grass 119 C Andropogus Gayanus Grass 121 D Brachiaria Humidicola Grass 123 E Melinis Minutiflora Grass 125 F Digitaria Eriantha Grass 127 G CODING 129

CHAPTER 1

INTRODUCTION

1.1 Background of Study

Shallow landslide or shallow slope failure is the slope failure phenomenon

that occurred frequently in tropical rainforest region such as Malaysia. It is a

common geotechnical problem faced by slope engineers. Shallow slope failure

usually occurred within 2 m of the ground surface and normally taken to be

translational, rotational failures or a combination of both (Coppin and Richards,

1990). There are many factors that induce the landslides in residual soil slopes and

rainfall has been considered to be the cause of majority landslides that occur in

regions experiencing high seasonal rainfalls (Brand, 1984).

Slope can be generally classified as natural slope and man-made slope.

Natural slope consist of residual soil slope and rock slope. Meanwhile, man-made

slope normally are cut-back slope and filled slope with well compacted filled

material. Surface protection including of vegetation and surface concreting should be

conducted on man-made slope with proper surface drainage such as surface channels,

catch pits and sand traps. This is to protect slope surface from rainfall-induced

erosion. Subsurface stabilization which includes soil nailing and ground anchors with

2

subsurface drainage such as weep hole are required depending on the design to keep

the slope stable in position.

Normally, vegetation have two main functions in stabilizing slope stability,

(i) via mechanical reinforcement, and (ii) via hydrological mechanism which causing

the increased evapotranspiration and hence increase suctions or soil moisture

extraction (MacNeil et al., 2001, Coppin and Richards, 1990, Greenwood et al.,

2004). The stabilizing effects depend on the type of vegetation with different root

distribution including dense cover of sod, grasses, herbs, shrubs and trees (Gray and

Sotir, 1996, Coppin and Richards, 1990). This study focuses on the effect of soil

moisture variation due to water uptake.

1.2 Problem Statement

Earthworks are a necessary part of many civil engineering projects such as

road construction, riverbanks, mining operations, landfills and grass involving

engineering slope. Stability of engineered landscape slope should be well analysed

and evaluated continuously and professionally during construction and post

construction for safety purposes. There are many shallow slope failures or landslides

induced by heavy rainfall which cause soil erosion and rapid increase of water table.

Shallow slope failures mostly occurred during intense increase of soil moisture and

pore water pressure due to antecedent and prolonged rainfall. The increasing soil

moisture or pore water pressure can be seen as the main contributing factor for

decreasing soil shear strength thus leading to the weakening of the slope stability,

resulting in factor of safety (FOS) reduction. Although the concrete surfacing

provide adequate erosion control and hence improve stability, but it has its own

disadvantages in terms of cost and aesthetic value. Vegetation cover is another option

for slope surface which include trees and grass turfs.

3

The hydrological effect is closely related to soil moisture variation and can be

directed through transpiration. This effect is found to be important as hydrological

effect provides significant increase in soil strength that will definitely improve slope

stability in certain conditions.

The current work is aimed at providing a framework for the simulation of

moisture migration due to vegetation that will be of potential value for a range of geo

engineering applications and totally about 6 types of grasses in Malaysia (tropical

country). This study explores the numerical simulation capable of representing the

extraction of water from the soil by the roots of grass – the so called water-uptake

process.

1.3 Objective

Detailed objectives of this investigation are:

a) To develop 1D numerical simulation model soil moisture suction due to grass

water uptake.

b) To analysis six types of grasses for grass water uptake due to soil moisture

variation using numerical simulation.

c) To validate soil moisture variation between numerical simulation and field,

laboratory test from previous study.

d) To analyse the factor of safety of slope due to grass water uptake.

4

1.4 Scope of Study

In this study, the related parameters of study on enhanced parameters of grass

effect are determined using the methods that will be described in the methodology

section. A data geometry is drawn up and will be filled into the spreadsheet and will

be transferred from established Fortran Code program for stability calculation.

Patterns of soil moisture transfer and migration due to the influence of tree root

water-uptake on the unsaturated soil condition are presented. It focuses on the

hydrological aspect on soil moisture pattern within the vicinity of the tree. The

mechanical aspect of the tree roots such as tensile strength bonding between root and

soils which can lead to increasing soil strength are negligible in this study. The

current work would consider the effect of tropical grasses such as Axonopus

Compressus, Pennisetum Purpureum, Andropogon Gayanus, Brachiaria Humidicola,

Melinis Minutiflora and Digitaria Eriantha. The root growth will be excluded in this

work scope and only consider on transpiration by grass. The approach to encounter

soil moisture variation was based on numerical simulation computer. A series of

validation were analysed with field and laboratory test with previous study for one

type of grass. Apart from that, the field monitoring result brought significant result to

compare with numerical simulation to analyse the soil moisture content. The

comparison of these works revealed that tree moisture transfer is beneficial in

maintaining stability of unsaturated soil

1.5 Significance of Study

This study is aimed to produce the results of additional hydrological

parameters of grass in increasing soil strength, and hence providing the efficiency of

grass in preventing shallow slope failure in terms of moisture content. Geotechnical

5

engineers can then evaluate the suitability of grass as vegetation option on slope

cover. The benefit that would be gained from this study may include the following;

a) Create database for suction due to grass water uptake

b) Numerical method for grass water uptake

c) Factor of safety for slope due to grass water uptake

1.6 Thesis Layout

An overview of related research work to the analysis of grass root water

uptake in unsaturated soil is presented in Chapter 2. The review provides a

commentary on the general significance of the grass and also water-uptake process.

It then provides a summary of the key mechanisms involved and aims to provide

some background information that can be utilised in subsequent work.

The methodology for describing the flow in a moisture variation is presented

in Chapter 3. Some of the fundamental concepts used to describe moisture flow due

to water uptake plant by roots are also introduced. This chapter describes the

derivation of the soil moisture variation on hydrological mechanism calculation and

hence increases suctions or soil moisture extraction.

Chapter 4 presents a validation of grass Andropogon Gayanus on

hydrological mechanism soil moisture variation using two methods between result

from laboratory test and numerical computer simulation. Data for field monitoring

6

and laboratory test was also obtained from the previous researcher of the site which

the information was determined by site survey method.

Chapter 5 presents about numerical simulation of the soil moisture variation

due to grass water uptake and all parameter are included in the spreadsheet to

conclude the moisture content and this particular chapter analyzes and discusses the

6 types of grasses.

Chapter 6 discusses about pressure head and parameter to be included to

determine factor of safety of slope due to grass water uptake

Meanwhile, discussions in Chapter 7 focuses on objectives regarding the

research and recommendations for future study.

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