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.
v
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
4.3.2 Shear strength in Month 2 132
4.3.3 Shear strength in Month 3 134
4.3.4 Shear strength in Month 4 138
4.3.5 Shear strength in Month 5 140
4.3.6 Shear strength in Month 6 144
4.3.7 Shear strength in Month 7 147
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
4.4.2 Root tensile strength in Month 2 170
4.4.3 Root tensile strength in Month 3 172
4.4.4 Root tensile strength in Month 4 173
4.4.5 Root tensile strength in Month 5 175
4.4.6 Root tensile strength in Month 6 177
4.4.7 Root tensile strength in Month 7 180
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4.4.8 Discussion and comparison 181
4.4.9 Summary of the results 189
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
4.5.5 Summary of the results 208
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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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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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
Mean root diameter, mean root breaking force and mean root
tensile strength over a range of diameter classes of Pennisetum
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4.11
4.12
4.13
4.14
4.15
4.16
setaceum „Rubrum‟ in Month 3
Mean root diameter, mean root breaking force and mean root
tensile strength over a range of diameter classes of Pennisetum
setaceum „Rubrum‟ in Month 4
Mean root diameter, mean root breaking force and mean root
tensile strength over a range of diameter classes of Pennisetum
setaceum „Rubrum‟ in Month 5
Mean root diameter, mean root breaking force and mean root
tensile strength over a range of diameter classes of Pennisetum
setaceum „Rubrum‟ in Month 6
Mean root diameter, mean root breaking force and mean root
tensile strength over a range of diameter classes of Pennisetum
setaceum „Rubrum‟ in Month 7
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
(Schiechtl & Stern, 1996)
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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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2.46
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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2.57
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
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3.31
3.32
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3.39
3.40
3.41
3.42
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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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3.52
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
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
200 mm depth
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
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
127
128
130
130
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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
Month 2 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
Month 2 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
200 mm depth
Month 2 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
300 mm depth
Month 2 average shear strength - displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟and one non-
rooted at three depths
Month 3 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
Month 3 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
200 mm depth
Month 3 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
300 mm depth
Month 3 average shear strength-displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟ and a non-
rooted at three depths
Month 4 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
Month 4 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
200 mm depth
Month 4 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
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135
135
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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
Month 4 average shear strength-displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟ and a non-
rooted at three depths
Month 5 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
Month 5 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
200 mm depth
Month 5 shear strength of a total set of two direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
300 mm depth
Month 5 average shear strength-displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟ and and one
non-rooted at three depths
Month 6 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
Month 6 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
200 mm depth
Month 6 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
300 mm depth
Month 6 average shear strength-displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟ and one non-
rooted at three depths
Month 7 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
Month 7 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
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142
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145
145
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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
Month 7 shear strength of a total set of three direct shear tests of P.
setaceum „Rubrum‟ and one non-rooted soil versus displacement at
300 mm depth
Month 7 average shear strength-displacement curves for direct
shear test on three replicates P. setaceum „Rubrum‟ and one non-
rooted at three depths
Soil moisture content versus planting month of rooted and control
samples for 100 mm depth. 'NR' indicates moisture content of
control sample
Soil moisture content versus planting month of rooted and control
samples for 200 mm depth. 'NR' indicates moisture content of
control sample
Soil moisture content versus planting month of rooted and control
samples for 300 mm depth. 'NR' indicates moisture content of
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
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 2
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 2
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149
150
151
152
153
154
155
161
162
169
170
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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
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 3
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 3
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 4
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 4
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 5
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 5
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 6
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 6
Relationship between applied breaking force of Pennisetum
setaceum „Rubrum‟ roots and root diameter in month 7
Relationship between root tensile strength of Pennisetum setaceum
„Rubrum‟ and root diameter for month 7
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
195
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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
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198
200
201
203
203
204
206
207
207
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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
The particle sizes corresponding to 30% passing value
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
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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
THIS’S THE TEMPLATE ASSOCIATED WITH TABLE OF CONTENS 2
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
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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
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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
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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
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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.
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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
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Hopefully, this study would complement similar studies revolved around
topic of soil bio- and eco-engineering, soil erosion control, slope stability and land
restoration.
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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
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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
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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
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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).
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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)
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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)
Figure 2.7: Schematic diagram of an
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)
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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
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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
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soil, exposed concrete is unsightly for some time. These precast blocks provides
immediate stabilising effect to the slope (Schiechtl & Stern, 1996).
Figure 2.14: Schematic diagram of an
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
(Schiechtl & Stern, 1996)
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
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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
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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)
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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).
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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
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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
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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).
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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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