muhamad firdaurs bin abdullaheprints.uthm.edu.my/id/eprint/9864/1/muhamad_firdaurs_abdullah.pdf ·...

SOIL-ROOTS PERFORMANCE OF PENNISETUM SETACEUM ‘RUBRUM’ ON

MECHANICAL SOIL STRENGTH

MUHAMAD FIRDAURS BIN ABDULLAH

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

i

SOIL-ROOTS PERFORMANCE OF PENNISETUM SETACEUM ‘RUBRUM’ ON

MECHANICAL SOIL STRENGTH

MUHAMAD FIRDAURS BIN ABDULLAH

A thesis submitted in

fulfillment of the requirement for the award of the

Degree of Master of Civil Engineering

Faculty of Civil and Environmental Engineering

Universiti Tun Hussein Onn Malaysia

SEPTEMBER 2017

iii

Dedicated to my beloved father and my late mother,

Mariah Othman, May Allah (SWT) forgive all her sins and

may He make Jannatul Firdaus to be her final abode

(Ameen)

And

All my family members, teachers right from

childhood up to now and friends

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ACKNOWLEDGEMENTS

All praise due to Allah, the Lord of the worlds, who in His infinite mercy gave me

the strength, ability and courage to complete my thesis successfully. The author

would like to express his deepest gratitude and appreciation to supervisor, Dr Nor

Azizi Yusoff for his close supervision, constructive suggestions, entrepreneurial

thinking and financial support during the course of this Master project. The author is

really proud and glad to be his protégé. Special thanks go to co-supervisors, Dr

Hanim Ahmad (MARDI Serdang) and Dr Hartini Kasmin for providing all necessary

information to carry out the research, publishing of conference papers and also

financial support. Many thanks to the sponsors; KPT (MyBrain15), MARA and

ORICC UTHM as the project was funded through GIPS grant (vot no. 1361), GPP

grant (B026) and MDR grant (U088). Thanks to Associate Prof. Dr Adnan

Zainorabidin, Dr Alvin John Lim Meng Siang, Associate Prof. Dr Saiful Azhar

Ahmad Tajudin and Associate Prof. Ir. Azizan Abd. Aziz for helpful comments and

suggestions on the thesis. Thanks to everyone at RECESS who has helped in

conducting research, attending conferences and also going bowling. Special thanks to

staff at the Lightweight Structures Engineering Laboratory, Faculty of Civil and

Environmental Engineering and Mr. Abu Hanifah A. Jalal, staff at Packaging

Laboratory, Faculty of Mechanical and Manufacturing Engineering. Without their

co-operation, it is hard to accomplish the testing.

Besides, the author would like to thank all friends and family for their support

during the course of the Master project. There have been many highs and lows but to

have reached the end has been an enjoyable experience and not possible without all

of you. Some of them are: Mohamad Azim, Siti Hajar, Azranasmaraazizi, Ameer

Nazrin, Nur Abidah, Mohd Jazlan, Siti Aimi Nadia, Mohamad Fazrin, Muhammad

Faridzal, Tuan Noor Hasanah, Mohamad Hanif, Mrs Salina and Mr Muhammad Rufi

Muhidin. Finally, the author wishes to thank all those who have contributed in one

way or another in making this thesis a possible one.

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ABSTRACT

The potential of Pennisetum setaceum ‘Rubrum’ root in soil reinforcement was

investigated. This African native perennial bunchgrass has been introduced in many

parts of the world as an ornamental plant and for soil stabilization. The traditional

civil engineering techniques such as concreting of welded wire walls for slope

stabilization may not be sustainable in the long term due to high initial capital cost. It

also looks harsh and unnatural to the road users. Alternatively, vegetation can be

used together with inert structure as a way of reducing the visual impact of civil

engineering works. Hence, the study is aimed towards the establishment of a flowery

plant that able to perform decent soil-root shear strength reinforcement. P. setaceum

‘Rubrum’ has been planted at the field plots at RECESS. A series of laboratory direct

shear tests was performed on rooted and non-rooted samples at 100, 200 and 300 mm

soil depth, every month throughout the seven months of study period. The roots

tensile strength was determined using an Instron Universal Testing Machine (Model

3369). Plant morphological data such as shoot biomass, root density and plant height

were also measured. The direct shear test results show that shear strength of rooted

sample of P. setaceum ‘Rubrum’ increases with time for all depths, with the highest

increment of 441 % over the control sample, that belong to one of rooted soil sample

of month 7 at 300 mm soil depth. The increment is due to high root tensile strength

(43.68 kPa ± 3 kPa) and root density (9.36 kg/m3). In term of average peak shear

stress, month 7 was highest at all depth. Its shear stress values were 307 ± 82 kPa

(100 mm), 181 ± 42 kPa (200 mm) and 179 ± 41 kPa (300 mm). Whereas, root

tensile strength decreased with increasing diameter of roots following the power

function with the highest average tensile strength of 50 ± 2 MPa (month 6). The

results of this paper improve the knowledge about biotechnical characteristics of root

systems of P. setaceum ‘Rubrum’ and indicate that this species could potentially

serve as soil reinforcement.

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ABSTRAK

Kajian pada akar spesies rumput Pennisetum setaceum „Rubrum‟ yang berpotensi

dalam pengukuhan tanah telah dijalankan. Tumbuhan rumput lebat yang berasal dari

Afrika ini telah diperkenalkan ke serata pelusuk dunia sebagai tumbuhan hiasan dan

juga sebagai penstabil tanah cerun. Kaedah tradisional kejuruteraan awam bagi

penstabilan cerun seperti tembok penahan konkrit mungkin tidak lestari bagi jangka

masa panjang kerana kos permulaan tinggi. Strukur itu juga tampak buruk dan tidak

mesra alam kepada pengguna jalan raya. Sebagai alternatif, tumbuhan boleh

digunakan bersama-sama dengan struktur tersebut bagi mengurangkan kesan

pemandangan konkrit yang terhasil oleh struktur kejuruteraan awam. Maka, kajian

ini bertujuan untuk mewujudkan suatu tumbuhan berbunga yang dapat menghasilkan

pengukuhan kekuatan akar-tanah. P. setaceum „Rubrum‟ telah ditanam di plot tanah

padang RECESS. Beberapa siri ujian “daya ricih terus” telah dijalankan pada sampel

tanah berakar dan tanpa akar pada kedalaman 100, 200 dan 300 mm setiap bulan

sepanjang tujuh bulan tempoh kajian. Kekuatan regangan akar ditentukan

menggunakan mesin Ujian Universal Instron (Model 3369). Data morfologi

tumbuhan seperti biojisim pucuk, ketumpatan akar dan tinggi tumbuhan juga diukur.

Keputusan “ujian ricih terus” menunjukkan kekuatan ricih tanah berakar bagi

P. setaceum „Rubrum‟ meningkat seiring dengan masa bagi semua kedalaman tanah,

dengan peningkatan tertinggi sebanyak 441 % berbanding sampel kawalan,

diperolehi oleh salah satu daripada sampel berakar bulan 7 pada lapisan 300 mm.

Peningkatan ini disebabkan oleh daya regangan akar yang tinggi (43.68 kPa ± 3 kPa)

dan ketumpatan akar yang tinggi (9.36 kg/m3). Bagi purata daya ricih tanah, bulan 7

adalah tertinggi bagi semua lapisan. Daya ricihnya ialah 307 ± 82 kPa (100 mm), 181

± 42 kPa (200 mm) dan 179 ± 41 kPa (300 mm). Sementara itu, kekuatan regangan

akar semakin menurun apabila diameter akar meningkat, mematuhi fungsi kuasa

dengan purata kekuatan regangan tertingginya ialah 50 ± 2 MPa (bulan 6). Dapatan

kajian ini meningkatkan lagi pengetahuan tentang ciri-ciri bioteknikal sistem akar

vii

bagi P. setaceum „Rubrum‟ dan menunjukkan spesies ini berpotensi dalam

mengukuhkan tanah.

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CONTENTS

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS AND ABBREVIATIONS

LIST OF APPENDICES

i

ii

iii

iv

v

viii

xiii

xv

xxvii

xxxi

CHAPTER 1 INTRODUCTION 1

1.1 Research background 1

1.2 Problem statement 3

1.3 Aim and objectives 4

1.4 Scope of research 4

1.5 Significant of research 7

CHAPTER 2 LITERATURE REVIEW 9

2.1 Introduction 9

2.2 Soil bioengineering definiton 9

2.3 Soil bioengineering stabilization 11

2.3.1 Hydroseeding 11

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2.3.2 Ground cover 11

2.3.3 Live staking 12

2.3.4 Live fascines 13

2.3.5 Brushlayering 14

2.3.6 Vetiver grass hedgerows 15

2.3.7 Vegetated crib wall 15

2.3.8 Vegetated geogrids 16

2.3.9 Precast concrete cellular blocks 16

2.3.10 Vegetated cellular grids 17

2.3.11 Coil rolls 18

2.4 Effects of vegetation on slope stability 18

2.4.1 Mechanical effects 20

2.5 The root system 24

2.6 Root types and root architecture 26

2.7 Interactions between soil and root 28

2.8 Soil compaction effects to the roots 30

2.9 Effect of extension rate during tensile test 32

2.10 Mechanism of root failure during the reinforcement of soils 33

2.11 Review of soil-roots shear strength of some species 34

2.12 Review of root tensile strength of some species 41

2.13 Species studied 52

2.14 Plant nomenclature system 54

2.15 Applications of Pennisetum setaceum „Rubrum‟ in Malaysia 57

2.16 Summary of literature 60

CHAPTER 3 RESEARCH METHODOLOGY 62

3.1 Introduction 62

3.2 Basic physical soil properties 64

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3.2.1 Wet sieving procedure 65

3.2.2 Hydrometer test procedure 68

3.2.3 Particle size analyser 71

3.2.4 Atterberg limits 73

3.2.5 Particle size distribution analysis, soil description and

classification 78

3.2.6 Field density test 81

3.3 Site preparation and planning 84

3.3.1 Study area 84

3.3.2 Field plots 85

3.4 Site monitoring and maintenance 89

3.5 Sampling procedure 90

3.5.1 Maintaning soil moisture content 90

3.5.2 Soil and vegetation sampling 92

3.5.3 Extrusion of soil-roots columns 93

3.5.4 Root sampling 96

3.6 Plants morphological data measurements 98

3.6.1 Root density, profiles and architecture 98

3.6.2 Shoot biomass procedures 100

3.6.3 Plant height 101

3.7 Direct shear test 101

3.7.1 List of direct shear testing 102

3.7.2 Description of the small direct shear box apparatus 104

3.7.3 Determination of vertical load and displacement rate106

3.7.4 Direct shear test procedure 109

3.8 Root tensile test 114

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CHAPTER 4 RESULTS AND DISCUSSION 118

4.1 Introduction 118

4.2 Basic physical soil properties 119

4.2.1 Particle size distribution 119

4.2.2 Atterberg limits 125

4.2.3 In-situ density test 127

4.3 Direct shear testing results 129

4.3.1 Shear strength in Month 1 129







4.3.8 Soil moisture content 150

4.3.9 Comparison of peak shear strength of rooted and non-

rooted soil 155

4.3.10 Discussion and comparison 156

4.3.11 Summary of the results 166

4.4 Root tensile strength results 167

4.4.1 Root tensile strength in Month 1 167







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4.4.8 Discussion and comparison 181


4.5 Relationship of plant morphological parameters towards soil

shear strength 190

4.5.1 Root density and root profiles 190

4.5.2 Shoot biomass 194

4.5.3 Plant height 200

4.5.4 Root architecture 205


CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 209

5.1 Introduction 209

5.2 Conclusion 209

5.2.1 Soil-roots shear strength performance 209

5.2.2 Root tensile strength determination 210

5.2.3 Relationship between plant morphological data and

shear stress 211

5.3 Recommendations for future research 212

REFERENCES 214

APPENDICES 230

VITA 247

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LIST OF TABLES

2.1

3.1

General relationship of soil bulk density to root growth based on

soil texture (USDA, 2014)

List of soil testing and standards

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64

3.2

3.3

3.4

3.5

3.6

Typical Atterberg limits for soils (Budhu, 2011)

Descriptive terms for soil classification (BS 5930: 1981)

Composite types of coarse soil

List of direct shear tests

Typical displacements for peak shear strength in 60 mm shearbox

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80

102

109

4.1

4.2

Analysis of particle size distribution of two soil types

Percentage of gravel, sand, silt and clay in laterite and clay soil

(according to BSCS)

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124

4.3 Plastic limit test results of clay sample 126

4.4 Results of in-situ field density test by means of sand replacement 128

4.5

4.6

4.7

4.8

4.9

4.10

Comparison of peak shear strength of rooted soil and non-rooted

soil (control)

Comparison of previous works with current study findings

Result of root tensile strength test of P. setaceum „Rubrum‟ in

month 1

Mean root diameter, mean breaking force and mean root tensile

strength over a range of diameter classes of P. setaceum

„Rubrum‟ in month 1

Mean root diameter, mean root breaking force and mean root

tensile strength over a range of diameter classes of Pennisetum

setaceum „Rubrum‟ in Month 2



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4.11

4.12

4.13

4.14

4.15

4.16














Growth form, mean root tensile strength and tensile strength

range of Pennisetum setaceum „Rubrum‟ and other compared

species found in literature

Morphological characteristic of sampled

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LIST OF FIGURES

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

2.12

2.13

2.14

2.15

2.16

2.17

Hydroseeding with a hydroseeder (Schiechtl & Stern, 1996)

Calopo‟s trifoliate leaves (US Forest Service, 2011)

Calopo is grown at slope along the Jalan Felda Aring, Kelantan

Schematic diagram of an established growing live stake installation

(Sotir & Gray, 1995)

Healthy, growing live stakes (DesCamp, 2004)

Live fascines bundles used to retain topsoil on slope (Salix, 2015)

Schematic diagram of an established live fascine installation (Gray

& Sotir, 1996)

Brushlayer installation (Sotir & Gray, 1995)

The same slope after 1 year (Sotir & Gray, 1995)

Vetiver hedgerows after 1 month of planting at East – West

Highway, Malaysia (Yoon, 1997)

The same vetiver hedgerows after 11 months of planting (Yoon,

1997)

Concrete crib wall during construction (Schiechtl & Stern, 1996)

Open-front concrete crib wall with plantings in openings (Sotir &

Gray, 1995)

Schematic diagram of an established geogrids wall (Gray & Sotir,

1996)

The willows are well established on geotextile reinforced slope

(Schiechtl & Stern, 1996)

Vegetated precast concrete cellular blocks at km 54, Jalan Gua

Musang – Cameron Highland

Small apertures of cellular blocks causing improper grassing effect


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2.18

2.19

2.20

2.21

2.22

2.23

2.24

2.25

2.26

2.27

2.28

2.29

2.30

2.31

2.32

2.33

2.34

Installation of vegetated expendable honeycomb cellular grid on a

slope at Jalan Kemaman – Dungun, Kijal, Terengganu

Empty cellular grids that expand into a large honeycomb-like array

(Terrafix Geosynthetics Inc, 2015)

Coil rolls are arranged horizontally parallel to the contour (JKR,

2011)

Coir rolls installed on a slope at km 21, Jalan Gua Musang –

Cameron Highland (JKR, 2011)

Mechanical effects of vegetation on slope stability (Coppin &

Richards, 1990)

Different patterns of root growth (Yen, 1972)

Modification of root distribution by site conditions

Fibrous roots of grass (Pennisetum setaceum „Rubrum‟) after 10

month of planting. Note that the densest root growth was in the top

50 mm

Main components of woody root system including lateral, tap, and

sinker roots (Gray & Sotir, 1996)

Principal morphological shapes of woody root systems (Wilde,

1958)

Two main root system types; (a) fibrous root and (b) tap root

(Loades, 2010)

Changes in root system when grown within different soils and

under different fertilizer regimes (Fehrenbacher et al., 1967)

A schematic diagram shows some of the factors affecting soil and

root strength (Loades, 2010)

Compacted plow layer inhibits root penetration and water

movement through the soil profile (USDA, 2014)

Compacted soil between rows as a result of wheeled equipment use

(USDA, 2014)

Shear stress–shear displacement curves for soil samples with and

without roots with shear plane depth of; (a) 0.2 m, (b) 0.4 m and (c)

0.6 m (Cazzuffi et al., 2014)

Stress-displacement curves of the test made during June 2007

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2.35

2.36

2.37

2.38

2.39

2.40

2.41

2.42

2.43

2.44

2.45

2.46

2.47

(Comino & Druetta, 2010)

Stress-displacement curves of the test made during April 2008

(Comino & Druetta, 2010)

Maximum shear stress versus normal stress for twelve-month-old L.

leucocephala (Ali & Osman, 2008)

Soil shear strength; (a) at 30 cm of soil depth, in four plots after 24

months and (b) at 10 cm and 30 cm of soil depth, in LL and LLSS

plots. Vertical bars represent standard deviation (Osman &

Barakbah, 2011)

Average shear stress against displacement curve for a direct shear

test on three replicates Guinea grass and one control soil of depth;

(a) 100 mm, (b) 200mm and (c) 300 mm (Zainordin et al., 2015)

Root tensile strength versus diameter for field experiment (A) and

glasshouse (B) (Loades et al., 2010)

Root tensile strength versus root diameter for; (a) L. corniculatus,

(b) M. sativa, (c) T. pratense, (d) F. pratensis and (e) L. perenne

(Comino et al., 2010)

Relationship between root tensile strength (MPa) and root diameter

(mm) for the six species studied (Burylo et al., 2011)

An arrow shows the Pennisetum setaceum „Rubrum‟ planted in

wooden plots (Fauzi, 2014)

Root tensile strength result; (a) maximum tensile force against

diameter and (b) tensile strength against diameter (Yusoff et al.,

2016)

Guinea grass; (a) planted in plots and (b) field grown (Zainordin et

al., 2015)

Root tensile strength result; (a) maximum tensile resistance against

diameter and (b) tensile strength against diameter (Zainordin et al.,

2015)

Tensile strength test results of herb/grass species grown both in

natural conditions (S-plant‟s name) and in pots (P-plant‟s name)

(Cazzuffi et al., 2014)

Pennisetum setaceum „Rubrum‟ hedgerows

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2.49

2.50

2.51

2.52

2.53

2.54

2.55

2.56

2.57

3.1

3.2

3.3

3.4

3.5

3.6

Pennisetum setaceum „Rubrum‟

Adventitious roots of P. setacum „Rubrum‟

The binomial system of nomenclature of studied plant species

Another genus of Pennisetum found during author‟s visit to MAHA

2016

An aerial view of TBS‟s interchange portraying the Pennisetum

setaceum „Rubrum‟ hedgerows planted in the curved shapes

(Google Maps, 2014b)

Combination of large palm trees, rock mattress together with

Pennisetum setaceum „Rubrum‟ hedgerows creating a stunning

view of the road interchange (Google Maps, 2014b)

The visual effect of the Pennisetum setaceum „Rubrum‟ hedgerows

is pleasing to the eye of motorist (Google Maps, 2014b)

Pennisetum setaceum „Rubrum‟ planted in a series of boxes along

the roadside at Cheras-Kajang Expressway, Selangor (Google

Maps, 2014a)

Pennisetum setaceum „Rubrum‟ planted along the roadside just

after passing by a flyover, at Cheras-Kajang Expressway, Selangor

(Google Maps, 2014a)

The beautiful purplish colour of Pennisetum setaceum „Rubrum‟

together with its flower spikes blooming along a roadside at

Cheras-Kajang Expressway, Selangor on September 30, 2008

(Yunus, 2008)

Flow chart of the study

Two soil samples retrieved from different soil strata; (a) Laterite

and (b) clay

Wet sieving of soil sample; (a) Soil was washed in the 2 mm sieve

using a jet of water and (b) material retained on the 63 µm sieve

(Gilson Company Inc., 2017)

The material passing 6.3 mm sieve was resieved through sieves of 5

mm down to 63 µm using a mechanical shaker

Vacuum filtration apparatus (Fung, 2013)

Measurements for calibration of hydrometer. Refer BS 1377-2:1990

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3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

3.15

3.16

3.17

3.18

3.19

3.20

3.21

3.22

3.23

3.24

3.25

for the details (British Standards Institution, 1998b)

A CILAS 1180 particle size analyser connected to a computer

(CILAS, 2017)

The way CILAS 1180 works, combination of lasers and CCD

camera (CILAS, 2017)

Cone penetrometer apparatus used for Liquid limit test

Tip of the cone just touched the surface of the soil

Portions of soil sample taken from the penetration area

Mixing of soil sample with distilled water on glass plate

Moulding, dividing and rolling of the soil sample

Soil particle size ranges of BSCS

Sand cone equipment and tools for excavating hole

Calibrations; (a) Determination of mass of sand before pouring, (b)

placing sand pouring cylinder on flat surface, (c) mass of sand in

the cone of pouring cylinder, (d) placing sand pouring cylinder

concentrically on top of calibrating container and (e) mass of sand

in the cone and calibrating container

Steps of sand replacement; (a) Leveling the surface, (b) placing

metal tray on the prepared surface, (c) soil excavated until 15 cm

depth, (d) excavated soil was collected, (e) the pouring cylinder

placed concentrically on the hole, (f) mass of sand left after pouring

cylinder was removed and (g) retrieving the sand

Maps and aerial photographs showing the exact location of the

research site (Google Maps, 2015)

Site layout grid for planting 40 plots of Pennisetum setaceum

„Rubrum‟ grass. SR stands for sand replacement point, M1 untill

M7 represent points for control samples of respective months

Designing and marking of field layout

Drilling holes using a power auger

After successfully drilling 40 holes

Woven geotextiles were laid on ground surface

P. setaceum 'Rubrum' was transplanted to hole at the field

After all P. setaceum 'Rubrum' being transplanted to the ground

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3.26

3.27

3.28

3.29

3.30

3.31

3.32

3.33

3.34

3.35

3.36

3.37

3.38

3.39

3.40

3.41

3.42

3.43

3.44

3.45

3.46

3.47

3.48

3.49

Site maintenance and monitoring

Maintaining soil moisture content; (a) sampling tube was pushed

gently into the ground and (b) pouring water into tube without

removing the foliage

Steel sampling tubes and its cap

Soil sampling procedures

Automatic Universal Horizontal Extruder NL-5045

Soil extruder set-up

Segmentation of soil-roots column of sample 1G(2)-M1-T5

Extrusion of soil sample and preparation of soil sample for direct

shear test

3D illustration of sampling tube cross-sectional diagram with

respect to shear planes

Washing the soil-roots clods over a sieve

Photographing of Pennisetum setaceum „Rubrum‟ roots mass

Oven-drying the foliage at 70 °C

Measuring the height of selected plants

Shear Trac II, a fully automated direct shear device (Geocomp,

2014)

The details of Shear Trac II shearbox device

Schematic diagram of the direct shear box in Shear Trac II device

(Wijeyesekera, Lim & Yahaya, 2013)

Derivation of time to failure from consolidation graph (soil sample

NR(2)-M1-T4-10)

The small shearbox apparatus (60 mm square)

Direct shear test procedure using Shear Trac II device

The cross-sectional diagram of shearbox assembly

Real-time consolidation graphs displayed on the monitor

Real-time shear graphs displayed on monitor

Sheared sample 3B-M7-T1 (20 cm); a) front view, (b) plan view,

(c) shear surface and (d) side view

Roots were cut into 100 mm in length and their diameter were

predetermined

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3.50

3.51

3.52

3.53

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

Instron Universal Testing Machine, Model 3369 (Illinois Tool

Works Inc., 2014)

A Pennisetum setaceum „Rubrum‟ root being pulled by 50 kN

wedge grips

Measuring diameter of the root at rupture point using digital

callipers

The root that broken near to the jaw faces was disregarded

Different soil strata (soil profile diagram) observed throughout the

research period

Variation in vegetated soil sample strata retrieved from sampling

tube

Variation in control sample strata retrieved from sampling tube

Variation in soil strata after the sampling tube being pulled-out

Particle size distribution curves for laterite and clay samples

Graph of cone penetration versus moisture content for

determination of liquid limit (wL)

Plastic limit test; (a) Laterite thread and (b) clay thread

Plasticity chart: British system (BS 5930: 2015)

Sand replacement method; (a) A hole being excavated up to 15 cm

depth, (b) medium pouring cylinder is centrally placed on top of the

hole, (c) sand has been spread out on the excavated hole and (d)

sand is being retrieved from the hole

Month 1 shear strength of a total set of three direct shear tests of P.

setaceum 'Rubrum' and one non-rooted soil versus displacement at

100 mm depth


setaceum 'Rubrum' and one non-rooted soil versus displacement at

200 mm depth


setaceum „Rubrum‟ and one non-rooted soil versus displacement at

300 mm depth

Month 1 average shear strength - displacement curves for direct

shear test on three replicates P. setaceum „Rubrum‟ and one non-

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116

117

117

120

122

122

122

123

125

126

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128

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4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

rooted at three depths



100 mm depth



200 mm depth



300 mm depth

Month 2 average shear strength - displacement curves for direct

shear test on three replicates P. setaceum „Rubrum‟and one non-




100 mm depth



200 mm depth



300 mm depth

Month 3 average shear strength-displacement curves for direct

shear test on three replicates P. setaceum „Rubrum‟ and a non-




100 mm depth



200 mm depth



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134

135

135

136

137

137

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4.25

4.26

4.27

4.28

4.29

4.30

4.31

4.32

4.33

4.34

4.35

300 mm depth


shear test on three replicates P. setaceum „Rubrum‟ and a non-




100 mm depth



200 mm depth

Month 5 shear strength of a total set of two direct shear tests of P.


300 mm depth


shear test on three replicates P. setaceum „Rubrum‟ and and one

non-rooted at three depths



100 mm depth



200 mm depth



300 mm depth






100 mm depth



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144

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4.36

4.37

4.38

4.39

4.40

4.41

4.42

4.43

4.44

4.45

4.46

4.47

4.48

200 mm depth



300 mm depth




Soil moisture content versus planting month of rooted and control

samples for 100 mm depth. 'NR' indicates moisture content of

control sample



control sample



control sample

Correlations between maximum shear strength and moisture content

of all control samples

Correlation between the peak shear strength and soil moisture

content of rooted samples

Average peak shear strength value recorded throughout month 1 to

7 at three depths, for rooted soil (n=3) and non-rooted soil (n=8).

Vertical bars indicate standard error

Linear relationship between shear stress values registered and

planting month after direct shear tests completed

Relationship between applied breaking force of Pennisetum

setaceum „Rubrum‟ roots and root diameter for month 1

Relationship between root tensile strength of Pennisetum setaceum

„Rubrum‟ and root diameter for month 1


setaceum „Rubrum‟ roots and root diameter in month 2



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152

153

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155

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4.49

4.50

4.51

4.52

4.53

4.54

4.55

4.56

4.57

4.58

4.59

4.60

4.61

4.62

4.63

4.64





















Mean root tensile strength and mean root breaking force of

Pennisetum setaceum „Rubrum‟ grown from month 1 until month 7.

Vertical bars indicate standard error

Breaking force curves plotted against root diameter of Pennisetum

setaceum „Rubrum‟ grown from month 1 until month 7

Root tensile strength curves plotted against root diameter of

Pennisetum setaceum „Rubrum‟ grown from month 1 until month 7

The relationship between root tensile strength and root diameter

for studied species (Pennisetum setaceum „Rubrum‟) and fifteen

other compared species found in literature

Vertical distribution of average root density (kg/m3) with respect to

soil depth and planting months. Error bars represent standard errors

Root profiles diagrams showing distribution of root density of

174

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176

176

178

178

179

180

182

182

184

185

186

189

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4.65

4.66

4.67

4.68

4.69

4.70

4.71

4.72

4.73

4.74

4.75

Pennisetum setaceum „Rubrum‟ for selected sampling tubes from

month 1 until month 7

Roots of Pennisetum setaceum „Rubrum‟ able to grow beyond 50

cm soil depth after three months of planting

Correlation between average peak shear strength and average root

density regardless of soil depths and planting months. Error bars

represent standard errors

Average shoot biomass of P. setaceum „Rubrum‟ from month 1

untill 7. Error bars represent standard errors

New tiller of P. setaceum emerged as the plant growing over the

time

Correlation between average peak shear strength (sum of all depths)

and average shoot biomass. Error bars represent standard errors

Average plant height of P. setaceum „Rubrum‟ from month 1 untill

7. Error bars represent standard errors

Correlation between average peak shear strength (sum of all depths)

and average plant height. Error bars represent standard errors

The series of photos captured the development of P. setaceum

„Rubrum‟ growth

Fibrous root system of Pennisetum setaceum „Rubrum‟

The whole parts of Pennisetem setaceum „Rubrum‟

The height of Pennisetum setaceum „Rubrum‟ with respect to

human

196

197

197

198

200

201

203

203

204

206

207

207

THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENSxxvii

LIST OF SYMBOLS AND ABBREVIATIONS

%

<

>

±

√

°C

Cc

cm

Cu

D

d

D10

D30

D60

E

g

g

ha

Ip

kg

kN

kPa

m

m

Mg

min

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Percent

Less than

Greater than

Plus-minus (indicates range value or tolerance)

Square root

Degree Celsius

Coefficients of curvature

Centimetre

Values of uniformity

Diameter

Cone penetration

The particle sizes corresponding to 10% passing value



East

Gram

acceleration due to gravity (10 ms-2

)

Hectare

Plasticity index

Kilogram

Kilonewton

Kilopascal

Mass

Metre

Megagram

Minute

THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENSxxviii

mm

MPa

N

N

N

n

n/a

p

R2/ r

2

s

Tr

tf

V

w

wL

wP

Δx

μm

ρ

σ

τ

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Milimetre

Megapascal

Nitrogen

Newton

North

Number of observations or replicates (statistics)

Not available

P-value (statistics)

Coefficient of determination

Second

Tensile stress

Time to failure

Volume

Moisture content

Liquid limit

Plastic limit

Horizontal displacement

Micrometre

Density (pronounce as „Rho‟)

Normal stress (pronounce as „Sigma‟)

Shear stress (pronounce as „Tau‟)

ASTM

BS

BSCS

BSI

C4

CCD

CPYRWMA

DSIR

e.g.

et al.

FAO

-

-

-

-

-

-

-

-

-

-

-

American Society for Testing and Materials

Bristish Standard

British Soil Classification System for engineering purposes

The British Standards Institution

4-carbon molecule

Charge coupled device

Choctawhatchee, Pea and Yellow Rivers Watershed Management

Authority

Department of Scientific and Industrial Research

for example (from latin 'exempli gratia')

et alia/ et aliii (used after group of names, avoid a long list names)

Food and Agriculture Organization of the United Nations

THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENSxxix

FBM

FOS

GIPS

Hons

Inc.

JKR

km

KPT

LCD

Ltd.

MARA

MARDI

MAHA

MDR

MSE

NPK

ORICC

PMR

RAR

RD

RECESS

RLD

Sdn. Bhd.

SERAS

sp.

SPAC

spp.

TBS

TMI

TVNI

UK

US/ USA

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Fibre Bundle Model

Factor of Safety

Geran Insentif Penyelidik Siswazah

Honours (used after the name of a university degree)

Incorporated (used after the name of a company in the US)

Jabatan Kerja Raya (Public Works Department)

Kilometre

Kementerian Pendidikan Tinggi

Liquid Crystal Display

Limited (used after the name of a British company or business)

Majlis Amanah Rakyat (Council of Trust for the Bumiputra)

Malaysian Agricultural Research and Development Institute

Malaysia Agriculture, Horticulture and Agrotourism Show

Multi Disciplinary Research

Mechanically Stabilized Earth

Nitrogen, Phosphorus and Potassium

Office for Research, Innovation, Commercialization and

Consultancy Management

Penilaian Menengah Rendah (Lower Secondary Assessment)

Root area ratio

Root density

Research Centre for Soft Soil

Root length density

Sendirian Berhad (used after the name of a company in Malaysia)

Scientific, Engineering, Response & Analytical Services

Species

Soil-plant-atmosphere continuum

Species (refer to all species in that given genus)

Terminal Bersepadu Selatan

Testing Machine, Inc.

The Vetiver Network International

United Kingdom

United States of America

THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENSxxx

USCS

USDA

UTHM

-

-

-

Unified Soil Classification System

United States Department of Agriculture Forest Service

Universiti Tun Hussein Onn Malaysia

THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENSxxxi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A 230

B 231

C 232

D

234

E

235

F

238

G

244

H

Wet sieving data

Hydrometer test data

Particle size distribution of RECESS's clay

British Soil Classification System for Engineering

Purposes (BSCS)

Results of direct shear tests performed from month 1 till

7 (as a whole)

Result of root tensile strength test of Pennisetum

setaceum in month 2 till 7

Results of root tensile strength tests performed from

month 1 till 7 (as a whole)

Vertical distribution of root density according to planting

months and within soil depths up to 60 cm 246

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CHAPTER 1

1 CHAPTER 1 INTRODUCTION

INTRODUCTION

1.1 Research background

The planet Earth has an erratic surface and landslides occur frequently. During the

early times, humans have tried to select relatively stable ground to make a settlement.

As population increases and human life becomes more urbanized, there is a need for

terraces and corridors to be created to make room for buildings and infrastructures

such as quays, canals, railways and roads. Hence, man-made slopes also known as

cut and fill slopes have to be formed to facilitate such developments (Cheng & Lau,

2014). For example, in the modernizing of Malaysia‟s routes, many expressways

were built to link many major cities and towns in western Peninsular Malaysia. Many

slopes have to be formed, therefore it requires protection from the erosion due to

rainfall and runoff. The solution is to have the vetiver hedgerows planted on the

slope of major highways in Malaysia such as the Kuala Lumpur-Karak, East-West,

North-South and Cameron Highland highways since 1993. This vetiver grass

(Chrysopogon zizanioides) can grow very fast, in some applications rooting depth can

reach 3 – 4 m in the first year if planted correctly (Truong, Van & Pinners, 2008). Some

of the cut slopes were up to 150 m in vertical height in areas where annual rainfall

exceeds 3000 mm. In the 1990s, following, the extensive research into vetiver root

strength by Diti Hengchaovanich, a geotechnical engineer of Thailand, he has

successfully used this vetiver hedgerows system in the stabilization of those major

highways in Malaysia (Truong, 2004).

According to Osuagwu (2012), the use of grasses, trees and other plants to

protect slopes from erosion, shallow landslides and improve the geotechnical

properties of soil is termed as „soil bioengineering‟. It is considered as a practical

alternative to more traditional methods of slope stabilization such as soil nailing and


geosythetic reinforcement. This bioengineering is now a well-known practice in

many parts of the world particularly in Europe, since it has been widely investigated

and discussed starting in the 1960s (Comino & Druetta, 2010).

Nowadays, it is highly demanded to incorporate the use of vegetation in

restoring the stability of hillslope especially to solve the problem associated with

shallow slope failure in both natural and man-made slope (Abdullah, Osman & Ali,

2011). Based on a manual for maintenance and service of unpaved roads outlined by

CPYRWMA (2000), the most efficient and cost-effective method of stabilizing

banks and slopes is grass seeding. The grass will reduce water movement and allow

more infiltration. It will effectively hold soil particles in place and more importantly

reducing sedimentation. Surface completely covered slope with grass will be more

stable because the roots grip the soil on the slopes and prevent it from sliding. Above

ground, the shoots can grow up to a few meters and when planted together near each

other, it will form a solid vegetative barrier that retards water flow, filters and traps

sediment in run-off water (Truong & Loch, 2004).

On the other hand, slope revegetation could be an economical and

environmentally friendly solution to enhance and remediate unstable soils. With an

increase in awareness of the environment in which human lives together with all

other living things, sustainable and ecologically friendly solutions like this are being

sought after, in order to solve problems in engineering (Loades, 2010). Eventhough

soil bioengineering technique has been regarded as one way to alleviate landslide and

erosion problems, this process of revegetation is severely time consuming. Hence, in

order to avoid further damage to environment, properties and more importantly, life,

the right propagation density and plant species, preferably the native one should be

considered (Osman, Ali & Barakbah, 2009).

Research carried out by Petrone & Preti (2010) gave emphasized on the use

of indigenous plants for riverbank protection and its effect on economic efficiency.

The research that took place in the humid tropics of Nicaragua proved that the use of

local species not only successful in environmental restoration, even in a hardship

area (by maximizing the contribution of the local labor force and minimizing the use

of mechanical equipment), but also economically sustainable. Nonetheless, not much

research was conducted to determine the appropriate plants, particularly grass species

that has a marked adaptability to stabilize slope embankment and offering an


aesthetically flowery appearance. Therefore, this study is initiated in order to provide

a technical understanding on these particular issues.

1.2 Problem statement

The use of conventional structures such as concrete gravity wall, tie-back wall and

rock buttress to stabilize the slope sometimes is objected due to its stark, harsh and

unnatural appearance. Moreover, the structures are costly (Gray & Sotir, 1992). The

alternative solution for the cut and fill stabilization is soil bioengineering techniques.

It provides attractive, cost-effective and environmentally compatible ways to protect

slopes against superficial erosion and shallow mass movement (Gray & Sotir, 1996).

Traditional civil engineering techniques known as „grey solutions‟, such as

concreting of welded wire walls for slope stabilization, that may not be sustainable in

the long term due to high initial capital expenditure and more importantly increasing

maintenance requirements overtime (Morgan & Rickson, 1995). Besides that, the

concrete itself is noted as material that impervious to water resulting in significant

increases in surface run off following rain events. With low residence times for water

on the surface, drainage channels and rivers can become over-burdened with water

resulting in flooding (Loades, 2010).

Therefore, in civil engineering, vegetation is can be used as a way of reducing

the visual impact of civil engineering works and improving the quality of the

landscape. This can be illustrated by having a beautiful scenery of flowering plants

growing along the highways, creating a vibrant roadway and preventing eyesore to

the drivers. Vegetation able to perform an important engineering function because of

its direct influence both at the surface and on the soil, protecting and restraining the

soil, and at the depth, increasing the strength and competence of the soil mass

(Coppin & Richards, 2007).

According to Morgan & Rickson (1995), carefully selected and implemented

bioengineering techniques are bound to be more sustainable over time as vegetation

is self-regenerating and able to respond dynamically and naturally to changing site

conditions, ideally without compromising or losing the engineering properties of

selected vegetation.

The economic differentials between conventional, grey solutions and the use

of vegetation may be significant in areas where the availability of products such as


concrete, sheet piling, rip-rap and gabions is severely restricted, as in inaccessible

areas of developing countries. The current studies found that bioengineering

techniques have been used in developing countries such as Nepal and Nicaragua

where experience has shown the conventional methods of slope stablization are

prohibitely expensive on implementation and in maintenance, as well as being

inappropriate to the local technology and expertise used to combat slope instability

of the area (Petrone & Preti, 2010).

According to Osman & Barakbah (2006), it is aware that the documentation

of plant contribution to slope stability is extensive in most part of the developed

country, but it is lacking in the developing world. Slope problems vary between

different geographical regions. Due to this variability, the solutions are also different

and have to be specifically tailored. Moreover, there is a severe lack of empirical data

regarding the attribution of plant cover on slope stability in Malaysia (Osman &

Barakbah, 2006). Hence, it is essential to establish various data on soil-roots

mechanical strength of potential flowery plant towards soil reinforcement.

1.3 Aim and objectives

Based on the problems elaborated, the research aims towards the establishment of a

flowery plant that able to perform a decent soil-root shear strength reinforcement for

7 months of planting period. The objectives of this study are stated as below:

i) To analyse the soil-roots shear strength performance of a flowery

plant throughout the 7 months of planting period.

ii) To determine the root tensile strength of single root specimen related

to its diameter over the 7 months of planting period.

iii) To examine the relationship between plant morphological data and

shear stress development at different planting period.

1.4 Scope of research

The study was carried out at a field of Research Centre for Soft Soil (RECESS),

Universiti Tun Hussein Onn Malaysia (UTHM) for period of 7 months. The mass


planting of studied species was carried out at the site that within the reach of

researcher, hence the selection of field study on laterite fills located inside the

university is reasonable and for the ease of the study.

The research was limited to:

i) The field of RECESS used to grow the studied species is made up of

laterite soil as platform fills on top of layer of clay. The topography of

the field area is relatively flat with the original ground about 1.35 m to

1.80 m above the mean sea level. It is situated on area which has water

table of 0.5 – 0.65 meter from the ground surface (RECESS, 2017).

ii) The flowery plant was chosen based on its vigorous, cheap and

flowery in Malaysia‟s climatic condition. For those criteria listed, the

plant used in this research is Pennisetum setaceum „Rubrum‟ with

common name known as „purple fountain grass‟.

iii) The mode of planting is monoculture where only one species is

allowed to be grown in the field, rather than mix-culture system.

iv) In contrast to usual practice in investigating soil-root reinforcement,

the plants used in this study were grown in a field rather than

laboratory designated plots.

v) The phenomenon being discussed will circulate around the problem of

superficial landslides which means it is less than 1 meter deep

landslide and also known as miniature debris flows (Burylo, Hudek &

Rey, 2011).

vi) It should be noted at the outset that this research confines itself

primarily to methods and techniques for protecting upland slopes

against superficial erosion and mass movement. Upland slopes stated

herein include natural slopes, embankment fills, highway and railroad

cuts, landfill slopes, gullies and ravines. Streambank or riverbank,

coastal dune and bluffs stabilization are not addressed (Gray & Sotir,

1996). Superficial erosion is often ascertainable in coarse grained

soils, compared to deep slides that often occur rather in fine grained

soil (Frei, 2009). Mass movement as decribed by Oostwoud Wijdenes

& Ergenzinger (1998) is miniature debris flows, consist of a mixture


of coarse marl fragments within a silty matrix, moving down slope as

slides, gravity and fluid driven flow

vii) Direct shear test was conducted based on BS1377-7:1990, using

small shear box apparatus (60 mm x 60 mm) in the laboratory

(laboratory test) rather than in-situ test (field test) that usually make

use of larger shear box. Small shear box is used for determining the

angle of shearing of cohesionless soils and the drained peak and

residual shear strength of cohesive soil. Meanwhile, large shear box is

used for determining the similar properties of gravelly soils or on

large block samples. It is also due the availability of the direct shear

apparatus at RECESS.

viii) Determination of root tensile strength based upon a single root, being

pulled up vertically using Universal Testing Machine (Instron, Model

3369).

ix) Assessment on soil-root reinforcement is carried out for planting

period of 7 months.

x) Several basic geotechnical and plant morphological testing are

conducted.

xi) The study was limited to empirical data (direct comparison of shear

stress gained by rooted and non-rooted soil) rather than theories/ soil

reinforcement model such Wu‟s model or FBM model. Hence the

soil-roots shear strength and root tensile strength will not be computed

as one in this study as can be found in those two models. However

notes about those models have been briefly discussed in Section

2.4.1.1. No slope stability analysis to determine factor of safety (FOS)

required in the study.

xii) The study only focus on the mechanical effects of the root rather than

hydrological effects. This is due to the time contraint and large

parameters will be required if hydrological data such as precipitation,

potential evoptranspiration, frequency of rain events, soil loss, run off

and canopy cover etc. are employed in the study.


1.5 Significant of research

Biotechnical and soil bioengineering stabilization provide attractive, cost-effective

and environmentally compatible ways to protect slopes against superficial erosion

and shallow mass movement. The research will bring value to practitioners in such

diverse fields as geotechnical engineering, geology, soil science, forestry,

environmental horticulture and landscape architecture (Gray & Sotir, 1996).

The use of soil bioengineering techniques are believed able to promote and

sustain the life of indigenous vegetation species, reduce costs and employ the local

labour force (Petrone & Preti, 2010). However, much information about the below

ground functions and properties of the various types of vegetation that is relevance to

the civil/ geotechnical/ environmental engineers need to be known. The challenge

was mainly due to the difficulties in extracting whole root systems, and the problems

of testing plant roots both in situ and in the laboratory for their strength and other

mechanical properties. The lack of precise information on plant root properties has

possibly discouraged the use of soil bioengineering in civil engineering works, with

civil engineers preferring exact numbers to enable quantification for design to take

place. Thus this study plays an important role in the efforts to enrich and fullfill the

knowledge of vegetation used in civil engineering structures in the country and

indirectly promoting sustainable approach to the construction works.

According to research undertaken by Loades (2010), with an increased

understanding of the fundamental concepts on root systems, a practitioner interested

in soil reinforcement by roots will be able to better identify technologies and predict

their impact on soil stability. Engineering applications for this research could

include:

i) River bank management

ii) Engineered embankments

iii) Flood defence

iv) River catchment management

v) Sport surface technology


Hopefully, this study would complement similar studies revolved around

topic of soil bio- and eco-engineering, soil erosion control, slope stability and land

restoration.


CHAPTER 2

2 CHAPTER 2 LITERATURE REVIEW

LITERATURE REVIEW

2.1 Introduction

Landslides are a widespread erosional process occurred in highland regions that

includes a wide range of ground movements such as rockfalls, deep failure of slopes

and shallow debris flow. These geotechnical problems occured due to steep slopes,

high weathering rates exacerbated by severe climatic conditions or lack of vegetation

(Burylo, Hudek & Rey, 2011). Thus, attention has nowadays been drawn to soil

bioengineering using vegetation as the environment-friendly method to mitigate the

lansdslide.

This chapter will discuss more details about the term of soil bioengineering,

and examples of its application on slopes as well as their effects towards the slope

stability. The effects can be divided into hydrological and mechanical factors, which

can be beneficial or adverse to the slope stability (Coppin & Richards, 1990).

Besides, root system and architecture can either promote or dissipate soil water

pressure, thus they may either enhance or decrease the potential of shallow landslides

(Ghestem, Sidle & Stokes 2011). More importantly, soil and root strength are

interrelated, for example root system changes are being affected by different soils

and treatments. Compaction of soil may also impede the root growth and alter root

architecture (Loades, 2010).

2.2 Soil bioengineering definiton

In the past decades, the searching for ecologically correct technologies for

environmental restoration has become very important. Many researchers has urged to


accommodate ecological approaches to what was formally done through rigid

engineering (Holanda & Rocha, 2011). Mitsch & Jørgensen (2003) brought the idea

of “ecological engineering” that involves creating and restoring sustainable

ecosystems that have value to both humans and nature. The authors stated that, it

combines basic and applied science for the restoration, design and construction of

aquatic and terrestrial ecosystems.

Meanwhile, “soil bioengineering”, or biotechnical slope protection, has been

defined variously as “the use of mechanical elements (or structures) in combination

with biological elements (or plants) to arrest and prevent slope failures and erosion”

(Gray & Leiser, 1982). Similarly, Campbell, Shaw, Sewell & Wong (2008) stated the

meaning as the use of living vegetation, either alone or in conjunction with non-

living plant material and civil engineering structures, to stabilize slopes and/or

reduce erosion. In the case of upland slope protection and erosion reduction, the term

means combination of mechanical, biological, and ecological concepts to arrest and

prevent shallow slope failures and erosion (Gray & Sotir, 1992).

Until recently, many practitioners have coined the terms soil bio and eco-

engineering, but confusion still exists as to the exact definition of each. It appears

that the term bioengineering was first used as the translation from the German word

„Ingenieurbiologie‟, created in 1951 by V. Kruedener when referring to projects

using both the physical laws of „hard‟ engineering and the biological attributes of

living vegetation, which described the work that encompassed both engineering and

biology (Stokes, Sotir, Chen & Ghestem, 2010). Over time in North America, it

became clear that the word „bioengineering,‟ which also referred to medical works,

was confusing. In 1981, after many discussions with Dr. Schiechtl and other

European practitioners, R. Sotir developed the new terminology „soil bioengineering‟

for North America. This terminology has also been accepted in other parts of the

world including Hong Kong and Malaysia (Stokes et al., 2010).

The differences between soil bioengineering and eco-engineering are largely

due to their effectiveness over time and space. In soil bioengineering, from the first

moment of installation, no erosion should occur, as this would be considered part of

the original criteria and may be alleviated by the angular arrangement and density of

the installed measures (Stokes et al., 2010). Still, Stokes et al. (2010) emphasized

that in eco-engineering, civil engineering techniques are not used, although local


organic material at the site, e.g. logs and stumps, may be positioned to prevent soil

runoff.

2.3 Soil bioengineering stabilization

In this section, two approaches to soil bioengineering techniques are presented:

vegetative system and vegetative systems combined with simple structures. Both

approaches are discussed cursorily, aided by suitable figures. The vegetative systems

are hydroseeding, ground covers, live staking, live fascines, brushlayering and

Vetiver grass hedgerows. The second approach is the conjunctive use of plant and

inert structures such as vegetated cribwall, vegetated geotextiles structure, precast

concrete cellular blocks, vegetated cellular grids and coil rolls. These techniques able

to improve the appearance and performance of structure (Sotir & Gray, 1995).

2.3.1 Hydroseeding

Hydroseeding or hydromulching is a technique in which seeds and nutrients are

sprayed over the ground as a slurry (Bache & MacAskill, 1984). It is the most

common method to stabilize natural hill, cut and fill slope (Florineth & Gerstgraser,

1996). Hydroseeding is used on steep slopes which have a smooth surface and mild

climate, mainly in forests. Seed of grass/ herb, organic fertilizer, mulch and an alga

product as glue are mixed in a special barrel with water and pumped out onto the

slope (Figure 2.1). It is advisable to fasten a jute mesh on the slope when it comes to

very steep slope, so that it can fix the hydroseed (Florineth & Gerstgraser, 1996).

2.3.2 Ground cover

A dense herbaceous or grass cover comprises one of the best defenses against soil

erosion. For many installations vegetation alone will provide adequate long-term

erosion protection (Gray & Sotir, 1996). In this case, the cover system is leguminous

plant named Calopo (Calopogonium mucunoides). It is also known as “wild ground

nut” and “kacang asu” in English and Bahasa Indonesia respectively. It can reach

several meters in length and form a dense, vigorous, creeping and tangled mass of


foliage, 30-50 cm deep (Figure 2.2). The root system is dense and shallow, at most

50 cm deep (FAO, 2011). This creeper plant is mainly used as cover crop, alone or in

mixture with other legumes (e.g. Centrosema pubescens, Pueraria phaseoloides),

especially in rubber, oil palm or in young forest plantations (Figure 2.3). Calopo is a

pioneer species, it provides soil protection against erosion, reduces soil temperature,

improves soil fertility and controls weeds (Cook et al., 2005). It was introduced in

Indonesia and Malaysia as a cover crop and became naturalized. It is considered a

weed in some regions (US Forest Service, 2011).

Figure 2.1: Hydroseeding with a

hydroseeder (Schiechtl & Stern, 1996)

2.3.3 Live staking

Live staking involves the insertion and tamping of live, rootable vegetative cuttings

perpendicularly into the ground (Figure 2.4). The live stake will root and leaf out if

correctly prepared and placed (Figure 2.5). Live stakes can be placed in rows across

a slope to help control shallow mass movement. They can also be tamped through

and used in conjuction with jute or coir netting. The cuttings are usually ½ to 1 ½

inches in diameter and 2 to 3 feet long. The materials must have side branches

cleanly removed and the bark intact (Gray & Sotir, 1996). This system of stakes

creates a living root mat that stabilizes the soil by reinforcing and binding soil

particles together and by extracting excess soil moisture (Sotir & Gray, 1995).


2.3.4 Live fascines

Live fascines are long bundles of branch cuttings bound together into sausage-like

structures, which are placed in shallow trenches parallel to the slope contour (Figure

2.6). The bundles are tied together with twine and anchored in the trench with

wooden stakes and/ or live stakes, as shown in Figure 2.7 (Gray & Sotir, 1996). Live

fascines serve to dissipate the energy of downward moving water by trapping debris

and providing a series of benches on which grasses, seedlings, and transplants

establish more easily. Portions of the live fascines also root and become part of the

stabilizing cover.

Figure 2.2: Calopo‟s trifoliate leaves (US

Forest Service, 2011)

Figure 2.3: Calopo is grown at slope

along the Jalan Felda Aring, Kelantan

Figure 2.4: Schematic diagram of an

established growing live stake

installation (Sotir & Gray, 1995)

Figure 2.5: Healthy, growing live stakes

(DesCamp, 2004)


2.3.5 Brushlayering

In the case of brushlayering, live branches or shoots of such woody species as shrub

willow, dogwood or privet are placed in successive layers with the stems generally

oriented perpendicular to the slope contour, as shown in Figure 2.8. Live branch

cuttings are placed in small benches excavated into the slope. The benches can range

from 2 to 3 feet wide. The portions of the brush that protrude from the slope face

assist in retarding runoff and reducing surface erosion. Brushlayering can improve

soil stability to depths of 4 to 5 feet (Sotir & Gray, 1995). It works better on fill as

opposed to cut slope because much longer stems can be used in the former method.

Usually, branches up to 12 feet in length can be used on fill slope brushlayering

installations (Gray & Sotir, 1996). After one year, vegetation cover has become

established (Figure 2.9).

Figure 2.6: Live fascines bundles used to

retain topsoil on slope (Salix, 2015)


established live fascine installation

(Gray & Sotir, 1996)

Figure 2.8: Brushlayer installation (Sotir

& Gray, 1995)

Figure 2.9: The same slope after 1 year

(Sotir & Gray, 1995)


2.3.6 Vetiver grass hedgerows

Vetiver (Chrysopogon zizanioides) is a non fertile, non-invasive Indian clump grass

cultivated for centuries for essential oil (TVNI, 2015). The grass works best when

planted in hedgerows on contour with the plants spaced approximately 15 cm apart

as shown in Figure 2.10 (Gray & Sotir, 1996). To produce quality hedgerows, quality

planting materials must be used which must always begin with mature and active

tillers cultivated from nursery. Vetiver grass cultivar aged 4 months is suitable for

transplanting. Vetiver hedgerows shall never be planted from cut-root slip. Only

container plants shall be used to ensure the success of the planting (Yoon, 1994).

This vetiver hedgerows have been proven to stabilize some of the major highway

slopes in Malaysia such as the Kuala Lumpur-Karak, East-West, North-South and

Cameron Highland highways since 1993 as shown in Figure 2.11 (Truong, 2004).

Figure 2.10: Vetiver hedgerows after 1

month of planting at East – West

Highway, Malaysia (Yoon, 1997)

Figure 2.11: The same vetiver

hedgerows after 11 months of planting

(Yoon, 1997)

2.3.7 Vegetated crib wall

A vegetated crib wall consists of a hollow, box like interlocking arrangement of

structural beams (Figure 2.12). In conventional cribwalls, the structural members are

fabricated from concrete, wood logs and dimensional timbers. This live crib walls is

an example of combination vegetative system and inert structure. The vegetation

provides an attractives screen or landscaping touch on the face of the crib wall

(Figure 2.13). In the live wooden crib wall, the structure is filled with a suitable

backfill material and layers of live branch cuttings. For the concrete crib walls, the


frontal spaces between the stretchers in walls provides opening through which

vegetative cuttings or rooted plant can be inserted (Gray & Sotir, 1996).

Figure 2.12: Concrete crib wall during

construction (Schiechtl & Stern, 1996)

Figure 2.13: Open-front concrete crib wall

with plantings in openings (Sotir & Gray,

1995)

2.3.8 Vegetated geogrids

A vegetated geogrid installation consists of live cut branches (brushlayers)

interspersed between layers of soil and wrapped in natural or synthetic geotextile

materials, as shown in Figure 2.14. The brush is placed in a crisscross or over

lapping pattern so that the tips of the branches protrude just beyond the face of the

fill. The foliage growing on the face of the fill will retard runoff velocity and filter

the sediment (Figure 2.15). Vegetated geogrid structures are constructed in much the

same way as a conventional mechanically stabilized earth (MSE) structural fill.

However, the stems that extend back into slope are living and root along their lengths

and act as horizontal slope drains (Gray & Sotir, 1996).

2.3.9 Precast concrete cellular blocks

Precast concrete cellular blocks are placed on the slope surface, similar to a simple

grating (Figure 2.16). They are fixed with iron pegs or achors. The voids of the

blocks are filled with topsoil which is seeded. However the grassing effect could be

very variable. The blocks with larger apertures would facilitate better grass

establishment compared to the small one (Figure 2.17). After filling the blocks with


soil, exposed concrete is unsightly for some time. These precast blocks provides

immediate stabilising effect to the slope (Schiechtl & Stern, 1996).


established geogrids wall (Gray & Sotir, 1996)

Figure 2.15: The willows are

well established on geotextile

reinforced slope (Schiechtl &

Stern, 1996)

Figure 2.16: Vegetated precast concrete

cellular blocks at km 54, Jalan Gua

Musang – Cameron Highland

Figure 2.17: Small apertures of cellular

blocks causing improper grassing effect


2.3.10 Vegetated cellular grids

A cellular grid is essentially a lattice like array of structural members that is fastened

or anchored to a slope as shown in Figure 2.18. The structural members may be

either concrete, timber or a three dimensional expandable polymeric web. The

polymeric web usually manufactured from polyethylene or polyester strips (Figure

2.19). The spaces within the lattice or honeycomb array are planted with suitable

vegetation. The purpose of installing the structure is to facilitate the establishment of


vegetation on steep, barren slope. It does not require the importance of select backfill

and cribfill (Gray & Sotir, 1996).

Figure 2.18: Installation of vegetated

expendable honeycomb cellular grid on a

slope at Jalan Kemaman – Dungun,

Kijal, Terengganu

Figure 2.19: Empty cellular grids that

expand into a large honeycomb-like

array (Terrafix Geosynthetics Inc, 2015)

2.3.11 Coil rolls

Coirlogs or coil-rolls are cylindrical shape erosion control product which is made of

100% compressed biodegradable coconut fibers, wrapped in a polymer exterior

netting to form a bioengineering solution known as the Coconut Coir Logs (Figure

2.20). This flexible structure provides protection for slope embankment and toe,

ensures stabilization on stream bank, enhances vegetation establishment while acting

as silt check and sediment control tool (Fibromat, 2016). Coil rolls are used to

prevent loss of nutrients from the soil due to water run-off and supply the shrub with

enough nutrients to grow. They are arranged horizontally on the slope surface,

parallel to the contour (Figure 2.21). Organic fertilizer in the bags that are placed on

top of the berms will seeps slowly during the rain to provide continuos nutrients

supply to the growing plant while apart of it will retain in the coil rolls (JKR, 2011).

2.4 Effects of vegetation on slope stability

The importance of vegetation in the role of improving soil stability has been

recognized for a long time (Morgan, 2005). There are two mechanisms of plant that

influence the stability of slope, namely hydrological and mechanical. Hydrological

mechanism is associated with hydrologic cycle that is interrelated with plant roles


While mechanical mechanism occured due to physical interactions between plant

shoots and its ambient surrounding or roots system and slope soil (Figure 2.22). It is

realized that, both hydrological and mechanical effects can be adverse or beneficial

to slope stability (Alfred, 2006; Ghestem et al., 2011). However, the most important

part of the vegetation is the root. It increases the resistance of the soil by modifying

its mechanical and hydrological properties (Gray & Sotir, 1996).

Figure 2.20: Coil rolls are arranged

horizontally parallel to the contour

(JKR, 2011)

Figure 2.21: Coir rolls installed on a slope at

km 21, Jalan Gua Musang – Cameron

Highland (JKR, 2011)

Figure 2.22: Mechanical effects of vegetation on slope stability (Coppin & Richards,

1990)


2.4.1 Mechanical effects

2.4.1.1 Root reinforcement

The most apparent way in which vegetation stabilizes soil is through root

reinforcement. It occurs when the tap and sinker roots penetrate down through the

soil mantle and mechanically anchor into the firmer underlying strata (Ronald, 1985).

Roots embedded in soil form a composite material consisting of fibres of relatively

high tensile strength and adhesion within a matrix of lower tensile strength. The

shear strength of the soil is therefore enhanced by the root matrix (Ali & Osman,

2008). This is analogous to the reinforced soil system, where a soil mass is stabilized

by the inclusion of metallic, synthetic or natural materials. The shear strength of the

rooted soil mass is enhanced due to the presence of a root matrix. Root reinforcement

of soil provides relief of local stress by transferring load to regions of lower stress,

through the interaction of semi-continuous root systems (Farshchi, 2009).

A lot of works on slopes demonstrated that when compared with non-root

permeated soils, even low root density can provide substantial increase in shear

strength and the magnitude additional apparent cohesion varies with the distribution

of the roots within the soil and with the tensile strength of the individual roots (Wu,

Mckinnell & Swanston, 1979; Abernethy & Rutherfurd, 2001; Ali & Osman, 2008).

Currently, there are two theoretical slope stability models incorporating the

soil-roots strength parameters, namely Wu‟s Model and Fibre Bundle Model (FBM).

The first model was developed by Tien H. Wu in 1976 and used extensively for the

last 30 years (Stokes et al., 2010). This model of additional cohesion taking into

account the contribution of roots and it assumes that all roots grow vertically and act

as loaded piles such that tension is transferred to them instantaneously as the soil is

sheared (De Baets et al., 2008). Various limitations with the model have led to the

development and use of a new model called the Fibre Bundle Model (FBM) (Pollen

& Simon, 2005). The second model argues that all roots crossing the shear plane will

break at the same time as claimed by Wu‟s model. It is because, the shear surface

may propagate progressively through the soil mass and some roots pull out rather

than break. These effects often result in an overestimation of root cohesion (Docker

& Hubble, 2008). Hence, the second model predicts soil-root reinforcement better

than the the first model (Loades et al., 2010).


2.4.1.2 Root tensile strength

Root tensile strength is an important factor to consider when choosing suitable

species for reinforcing soil on unstable slopes. Tensile strength has been found to

increase with decreasing root diameter. It is defined as “the maximum force per unit

area required to cause a material to break” (Genet et al., 2005). Not only is root

tensile strength important when considering soil reinforcement, but it can also affect

plant anchorage. In herbaceous species, plants must withstand grazing pressure,

whereby uprooting occurs in tension, therefore a higher root tensile strength will

enable the plant to remain anchored in the soil (Ennos & Fitter, 1992).

Wide variations in root tensile strength have been reported in the literature.

Kindly refer Section 2.12: Review of root tensile strength of some species, for the

details review of the root tensile strength of numerous species recorded by other

researchers. In addition, the comparison of tensile strength values between various

species has been mentioned in Section 4.4.8, in form of table and graphs.

The root tensile strengths appear to depend on species and site factors such as

local environment, season, root diameter and orientation (Gray & Sotir, 1996). Study

by Lindström & Rune (1999) showed that root resistance to failure in tension can be

influenced by the mode of planting e.g. naturally regenerated Scots pine (Pinus

sylvestris L.) had stronger roots than those of planted pines. The time of year has also

been found to affect tensile strength as roots being stronger in winter than in summer,

due to the decrease in water content (Turmanina, 1965). Tensile strength usually

decreases with increasing root size (Loades, 2010; Osman, Abdullah, & Abdullah,

2011; Zainordin et al., 2015) and this phenomenon has been attributed to differences

in root structure, with smaller roots possessing more cellulose per dry mass than

larger roots (Commandeur & Pyles, 1991).

2.4.1.3 Root area ratio (RAR)

Root area ratio (RAR) is defined as the area of roots in relation to the area of soil

(Loades, 2010). It is calculated in order to measure root distribution (Abernethy &

Rutherfurd, 2001), also very important to be used as one of the parameters in

determination of additional cohesion of rooted soil in Wu‟s root reinforcement


theoretical model (Wu et al., 1979). RAR has a high variability with species, site

condition and depth. It has been used as an index of root density by many authors

(De Baets et al., 2008; Comino & Marengo, 2010; Burylo et al., 2011). It was

reported that the upsurge in the RAR causing the increase of soil reinforcement

(Loades et al., 2010). Thus, many authors suggested to use RAR as a part of slope

stability characterization in their research (Avani, Lateh & Bibalani, 2013).

There is exponential reduction in root area quantity with distance away from

the tree stem at all depth and as well as decrease in their maximum lateral extends

with depth (Genet et al., 2005). Abdi et al., (2010) analyzed the RAR in ironwood

(Parrotia persica) and found that root density normally decreases with depth

according to an exponential function. Maximum RAR values were located within the

first 0.1 m layer. Furthermore, Naghdi et al., (2013) studied study the effect of alder

(Alnus subcordata) roots on hillslope stability. The results indicated that the root

density, number of roots and RAR decreased with increasing depth. The maximum

RAR values were located in the upper layers only. Sometimes, root density that is

calculated as roots dry weight over a volume of soil is used to estimate the root area

ratio (RAR). Similarly, the pattern of result shows root density also decreased

significantly with increasing depth (Genet et al., 2008).

2.4.1.4 Anchorage, arching and buttressing

Vegetation particularly from woody plants able to influence slope stability through

buttressing and soil arching of the trunks of trees growing in slopes. Arching occurs

when soil attempts to move through and around a row of trees firmly embedded in an

unyielding soil layer (Bache & MacAskill, 1984). The embedded stems also act as

buttress piles or abutments, restraining soil movement from trunks, thereby

counteracting the down-slope shear stress (Gray & Leiser, 1982).

The taproot and the sinker roots of many tree species penetrate into the deeper

soil layers and anchor them against down-slope movement. The trunks and the

principal roots acts in the same manner as toe stabilizing piles, further restraining the

down-hill movement of soil. The magnitude of the arching effects is influenced by

spacing, diameter, embedment of trees, thickness and inclination of the yielding

stratum of slope as well as shear strength properties of soil. Whereas trees that are


sufficiently close together, the soil between the unbuttressed parts of the slope may

gain strength by arching (Coppin & Richards, 1990).

2.4.1.5 Surcharging

Surcharge is the effect of the additional weight on a slope resulting from the presence

of vegetation and it is normally considered only for trees, since the weight of grasses

and most herbs and shrubs are comparatively small. Surcharge could have adverse

effects, although it can be beneficial depending on the slope geometry, the

distribution of vegetation cover and the properties of the soil. This surcharge induces

a downslope stress, which reduces stability and a normal stress to the slope, which

increases the slope resistance to movement (Gray & Leiser, 1982). However, some

researchers also discovered that increase in normal load had increased the shear

strength of soil, implying the additional load by vegetation contributed in improving

the slope stability (Abdullah et al., 2011; Docker & Hubble, 2008).

Surcharge at the top of slope can lead to reduction of overall stability,

whereas it can add to stability when applied at the bottom of the slope. This is proven

by a study carried out by (Ali, Farshchi, Mu‟azu & Rees (2012), which determined

the factor of safety (FOS) based on various tree positions on slope. They discovered

that the tree located at the toe of slope had the highest FOS value compared to when

is located at the crest or middle of the slope. Another study shows that in an infinite

slope, surcharge is beneficial when cohesion is low, groundwater level is high, soil

angle of internal friction is high and slope angle is small (Coppin & Richards, 1990).

2.4.1.6 Wind loading

Wind loading is usually only significant when the wind speed is stronger than 11m/s.

Both the up- or down-hill wind loadings can destablilize the slope especially in larger

trees with shallow roots. The forces induced in vegetation by wind can sufficient to

disturbed upper soil layer thus, initiate landslips. An up-hill wind if sufficiently

strong can cause a toppling of a tree and impart a destabilizing moment to the slope

and a greater possible destabilizing effect can result from increased water infiltration

through the scar created by an uprooted tree (Coppin & Richards, 1990).


This wind loading effect is best described by a study on soil-roots system of

Makino bamboo towards slope stability by Lin, Huang & Lin (2010). In 2004,

continuous attacks of two typhoons; Typhoons Mindulle on 2nd

July and Aere on 25th

August in central Taiwan causing a large area of slopeland covered with Makino

bamboo collapsed and eroded. The typhoons has strong wind velocity ranged from

30 – 48 m/s. It can be speculated that the tension cracks widespread over the slope

surface due to the wind loading acting on the bamboo stems and the sequential

rainwater infiltration is the dominating factor in the collapse failure of slopeland.

Moreover, the shallow root depth (0.8 - 1.0 m) and large growth height (over 10 m)

of Makino bamboo became extremely unfavorable to the slope stability.

2.5 The root system

While it has been proven that the vegetation is able to improve soil stability through

both its above-ground and below-ground biomass, few studies have focussed on the

significance of the root system. The root system is particularly important when the

aboveg-round vegetation is absent for some time e.g. after harvest, grazing, fire or

outside the growing period of the crop (Hudek, 2013).

The development of the rooting system is influenced by environmental and

genetic factors such as water availability (rainfall and/or irrigation), temperature,

seasons and altitude, soil moisture, structure, texture, depth and slope, tillage, organic

content and nutrients input, micro- and macro-organisms activity, lignin and

cellulose content, plant age, density and competition (Genet et al., 2005; Osman &

Barakbah, 2006; Fan & Su, 2008; Preti, Dani & Laio, 2010).

Coppin & Richards (1990) properly explained that the root systems vary from

very fine fibrous systems through branched systems to a vertical taproot. All plants

have a mat of surface roots as to collect nutrients and which grow in and around the

surface soil layers because this is where mineral nutrients are generally available.

Deeper roots are used for anchorage and for absorbing water. Large taproots are

often associated with the storage of food for over-wintering plants, especially where

the above-ground parts die back substantially. The taproots are thus perennial

structures whereas fine fibrous roots are subject to annual cycles of decay and

renewal.

214

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