SOILS AND FOUNDATIONS
Japanese Geotechnical Society
Vol. 48, No. 4, 587–596, Aug. 2008
SHEAR STRENGTH OF A SOIL CONTAINING VEGETATION ROOTS
FAISAL HAJI ALIi) and NORMANIZA OSMANii)
ABSTRACT
Vegetation can signiˆcantly contribute to stabilise sloping terrain by reinforcing the soil: this reinforcement depends
on the morphological characteristics of the root systems and the tensile strength of single roots. This paper describes an
investigation on the reinforcing eŠect of soil-root matrix in the laboratory using a modiˆed large shear box apparatus
(300 mm×300 mm). Four diŠerent species of plant namely Vertiveria zizanoides, Leucaena leucocephala, Bixa orellana and Bauhinia purpurea were planted in special boxes containing residual soil compacted to a known density. The
results show that roots signiˆcantly contribute to the increase in soil shear strength. The presence of the roots only
aŠects the apparent cohesion of the soil and no signiˆcant change in angle of friction is observed. L. leucocephala
shows the outstanding increase in its root strength in which the strength varies with depth and time e.g., under soil suction-free condition (matric suction＝0), the roots have increased the cohesion by 116.6z (0.1 m), 225.0z (0.3 m) and
413.4z (0.5 m) after six months of growth. In twelve months, it is observed that the increase in cohesion is more than
three-fold of the six months growth period at 0.1 m depth. The results also indicate that shear strength is in‰uenced by
root proˆle and to some extent, the physiological parameters of the plants.
Key words: reinforcement, root, shear strength, slope stability, soil (IGC: D6)
eŠect of vegetation on slope stability.
INTRODUCTION
Soil Reinforcement
The roots and rhizomes of the vegetation interact with
the soil to produce a composite material in which the
roots are ˆbres of relatively high tensile strength and adhesion embedded in a matrix of lower tensile strength.
The shear strength of the soil is therefore enhanced by the
root matrix. Field studies of forested slopes (O'Loughlin,
1984) indicate that it is the ˆne roots, 1–20 mm in diameter, that contribute most to soil reinforcement.
Grasses, legumes and small shrubs can have a signiˆcant
reinforcing eŠect down to depths of 0.75–1.5 m. Trees
have deeper-seated eŠects and can enhance soil strength
to depths of 3 m or more depending upon the root morphology of the species (Yen, 1972). Root systems lead to
an increase in soil strength brought about by their binding action in the ˆbre/soil composite and adhesion of the
soil particles to the roots.
A comparative study on ``bushy'' ecosystem and
``grass'' ecosystem by Normaniza and Barakbah (2006)
showed that the Point Shear Strength (PSS) was much in‰uenced by the root length density (Tables 1 and 2). The
PPS value was measured at 10 cm of soil depth by using a
ˆeld inspection vane tester (Normaniza and Barakbah,
2006). Root length density (Root length per unit volume
of soil) is the highest near the soil-atmosphere interface
and decreases, at ˆrst drastically in top 20 cm and then in
The use of vegetation for slope stabilization started in
ancient times. In more recent times, the roles of vegetation in some speciˆc geotechnical processes have been
recognised. Vegetation may aŠect slope stability in many
ways. Comprehensive reviews may be found in Greenway
(1987), Gray (1970), Gray and Leiser (1982), Coppin and
Richards (1990). The stability of slopes is governed by the
load, which is the driving force that causes failure, and
the resistance, which is the strength of the soil-root system. The weight of trees growing on a slope adds to the
load whilst the roots of trees serve as soil reinforcements
and increase the resistance. In addition, vegetation also
in‰uences slope stability indirectly through its eŠect on
the soil moisture regime. Vegetation intercepts rainfall
and draws water from the soil via evapotranspiration.
This reduces soil moisture and pore pressure, increases
the shear strength of the soil, thus increases the
resistance.
Several case studies have shown that slope failures may
be attributed to the loss of tree roots as soil reinforcement
(O'Loughlin, 1974; Riestenberg, 1987; Wu et al., 1979).
Field and laboratory studies have shown that vegetation
reduces water content and increases soil-moisture suction
in the soil (Greenway, 1987; Gray, 1970; Gray and Brenner, 1970; William and Pidgeon, 1983). Greenway (1987)
has given an extensive summary of observations on the
i)
ii)
Professor, Civil Engineering Department, University of Malaya, Kuala Lumpur, Malaysia (fahali＠um.edu.my).
Lecturer, Institute of Biological Sciences, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia.
The manuscript for this paper was received for review on May 21 2007; approved on May 19, 2008.
Written discussions on this paper should be submitted before March 1, 2009 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyoku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.
587
HAJI ALI AND OSMAN
588
Table 1. Variation of Root Length Density (RLD) (km m－ 3) with
depth (Normaniza and Barakbah, 2006)
Soil Depth (cm)
''Bushy'' ecosystem
''Grass'' ecosystem
00–10
42.2
7.9
11–20
6.6
1.9
21–30
3.7
0.3
31–40
1.6
0.6
41–50
0.6
0.3
51–60
0.7
0.2
61–70
0.4
0.1
Table 2. Point Shear Strengths (PSS) for diŠerent types of slope vegetation (Normaniza and Barakbah, 2006)
Type of Slope Vegetation
Point Shear Strength (kPa)
Thick Bush
(Diverse species; 1–3 m in height)
46.1± 3.4
Normal Bush
(Diverse species; º1 m in height)
182.8±12.5
Fern, Melastoma sp.
132.5± 9.6
Melastoma sp., grasses
110.7±11.5
25.7± 3.5
Grasses
a step-like manner, with increasing depth. The RLD of
the ``grass'' ecosystem was only about 20–30z that of
the ``bush'' ecosystem. The PSS at a depth of 9 cm into
the soil, when compared between various vegetation types, showed interesting result. PSS was highest in a ``normal'' bush, i.e., an open bush comprising many plant
species. Interestingly, PSS was very low in the thick bush
(this trend was repeatedly observed).
The magnitude of the diŠerence in PSS between thick
and normal bush was di‹cult to comprehend. This is especially so if one considers that the former was only
slightly higher than that of the thin grass. However, upon
analyzing the species components of the thick bush, the
result is easier to digest. The thick bush comprised mainly
various species of mosses and ferns which had successfully formed a ``closed'' system that did not allow other species, including tree species which is the main contributor
to shear strength, to colonize.
The pattern of the relationship between soil cohesion
and the roots is not known. Tengbeh (1989) found that
Loretta grass increased the strength of soil as a function
of root density (RD; Mg/m3) in a logarithmic relationship
that for a sandy clay loam soil:
c＝10.54＋8.63 logRD
(1)
and for a clay soil:
c＝11.14＋9.9 logRD
(2)
These studies show that root reinforcement can make signiˆcant contributions to soil strength, even at low root
densities and low shear strengths. The above equations
indicate that cohesion increases rapidly with increasing
root density at low root densities but Tengbeh (1989) also
found that increasing root density above 0.5 Mg/m3 on
the clay soil and 0.7 Mg/m3 on the sandy clay loam soil
has little additional eŠect. This implies that vegetation
can have its greatest eŠect close to the soil surface where
the root density is generally highest and the soil is otherwise weakest.
Since shear strength aŠects the resistance of the soil to
detachment by raindrop impact (Al-Durah and Bradford,
1982) and the susceptibility of the soil to rill erosion as
well as the likelihood of mass soil failure, root systems
can have a considerable in‰uence on all these processes.
The maximum eŠect on resistance to soil failure occurs
when the tensile strength of the roots is fully mobilized
and that, under strain, the behaviour of the roots and the
soil are compatible. This requires roots of high stiŠness
or tensile modulus to mobilize su‹cient strength and the
8–10z failure strains of most soils. The tensile eŠect is
limited with shallow-rooted vegetation where the roots
fail by pullout, i.e., slipping due to loss of bonding between the root and the soil, before peak tensile strength is
reached. The tensile eŠect is most marked with trees
where the roots penetrate several metres into the soil and
their tortuous paths around stones and other roots provide good anchorage.
Root failure may still occur, however, by rupture, i.e.,
breaking of the roots when their tensile strength is exceeded. The strengthening eŠect of the roots will also be
minimized in situations where the soil is held in compression instead of tension, e.g., at the bottom of hill slopes.
Root failure here occurs by buckling.
MATERIALS AND TEST METHODS
Soil
The soil was collected from a slope and subjected to a
number of standard tests to determine the basic physical
properties. Based on the grain size distribution curve the
soil is described as Silty Sand. Its physical properties and
grain size distribution are shown in Tables 3 and 4, respectively.
The reinforcing eŠects of the vegetation roots were
studied by carrying out laboratory shear box tests. The
laboratory tests were chosen instead of ˆeld tests so that
(i) identical soil samples can be prepared (ii) samples at
various depths can be obtained easily and (iii) saturation
of samples can be carried out. A large shear box apparatus (300 mm×300 mm) was fabricated and modiˆed such
that both saturated and partially saturated soil samples
containing roots can be tested with measurement of
deformation and load are done automatically. Four
diŠerent species of plant including vetiver grass were
planted in special boxes containing residual soil compacted to a known density. Each box was about 1 m high and
consisted of ˆve Perspex boxes with size 300×300×200
mm stacked up together. The soil was compacted in this
box by a specially designed mould to prevent the boxes
SHEAR STRENGTH OF A SOIL
Table 3.
Physical properties of the soil
Atterberg Limits
Liqiud Limit
Plastic Limit
Plasticity Index
26.9z
14.59z
12.31z
Linear Shrinkage
3.23
Speciˆc Gravity
2.61
Compaction
Optimum moisture content
Maximum dry density
Table 4.
13.5z
1.8515 Mg/m3
Grain size distribution
Type
Size distribution
Gravel (2 to 60 mm)
Sand (0.06 to 2 mm)
Silt (0.002 to 0.06 mm)
Clay(º0.002 mm)
10.0z
79.5z
7.5z
3.0z
Fig. 1.
Perspex boxes for preparation of samples
Fig. 2.
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from being damaged. The Perspex was chosen so that the
development of the roots could be observed.
The soils were compacted at a moisture content of 15z
and the compactive eŠort used was 29.43 Jkg－1. 10 kg of
soil was compacted by a rammer consisting of 2.5 kg
mass falling freely through 300 mm for 40 blows. The
processes were repeated until the 1m box was full of soil
(Fig. 1). The mean dry density of the soil samples was
1800 kgm－3. Some of the soil samples were used as control (i.e., samples with no vegetation roots). The plants
were allowed to grow for 6 and 12 months before the
samples were tested in the large direct shear box machine.
To remove the eŠect of soil suction on the shear
strength the samples were saturated in a specially
designed saturation box. The soil containing the roots
(inside the stacked boxes) was cut into ˆve parts. Samples
at depth 0.1, 0.3, 0.5 and 0.9 m were taken and each was
placed in the saturation box. A tip tensiometer was inserted into the soil to measure the soil matric suction. The
samples were soaked in de-aired water for around two
days and the box was connected to a vacuum pump until
the soil matric suction dropped to zero.
The shear strengths of the samples were determined using a specially fabricated 300×300×200 mm direct shear
box machine (Fig. 2). The shear box machine was
designed so that the sample can be soaked in water and
ˆxed with 8 units of tip tensiometer. Normal stresses were
applied using hydraulic pressure system through the top
plate. The normal stresses applied were 10 kPa, 20 kPa
and 30 kPa and the side friction was minimized by applying silicon grease to the sides of the box. An automatic
data logging system was designed to download all required data from the tensiometer, displacement transducer and load cell. All data was then transferred to the
personal computer and communicated with software
Large (300 mm ×300 mm) direct shear box machine
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HAJI ALI AND OSMAN
PC208W.
Root Proˆles and Physiological Parameters
Before the test, each plant sample was cut oŠ near the
lowest part of the stem prior to the measurement. The
root girth (circumference) was measured at the lowest
part of the stem using vernier calipers. After the test, the
root was washed from soil and debris. Root length (RL)
was measured using a leaf area image analyser. The root
was dyed using methyl violet in prior to measurement in
order to create a contrast under the image analyser. The
Root Length Density (RLD) was calculated as: total root
length/soil volume. Whilst shoot was partitioned into
stem and leaf and oven-dried at 809
C in order to obtain
shoot (stem and leaf) biomass.
Plant Materials
(a) Vetiveria zizanoides
This type of grass has good root system which is needed
to nail soil surface for providing some erosion control
(Grimshaw, 1994). Study has shown that vetiver can be
used to reduce erosion due to its extensive root system. It
has been proven that vetiver is also tolerant of a large
range of soil and ground water conditions. Its base tillers
can block the soil from eroding (Grimshaw, 1994). In
terms of the growth rate, the number of tillers can be increased to 7–8 tillers after four weeks of propagation.
(b) Bauhinia purpurea
This leguminous shrub is abundant in Malaysia and
has been used as landscape decorations in the housing
areas due to its attractive purplish orchid-like ‰owers. Its
big heart-shape leaf has a larger surface area (Photo 1)
which makes the plant have a higher photosynthetic rate
and resistance to drought condition (Ifa Herliani, 2006).
This easily grown shrub can reach a stem diameter of
about 18 cm and height of 5 m in less than 2 years (Ifa
Herliani, 2006).
(c) Leucaena leucocephala
It's a leguminous tree which can adequately improve
the quality of the soil via its nitrogen ˆxer capacity. It is a
multipurpose tree which profusely produces propagules
(beans; Photo 2) and has been used as an erosion control
plant (Parera, 1982). However, based on the fact there is
woeful lack of documentation on its contribution to slope
stabilization, this species had been chosen in this experiment.
(d) Bixa orellana
This shrub was chosen in this experiment due to the
fact that it has a higher growth rate and a good carbon
sink potential during the plant physiological screening
(Suraya, 1998), essential criteria as slope colonizer.
All plants, except vetiver, were germinated from seeds
and the seeds were inoculated with speciˆc Rhizobium
(Rubber Research Institute of Malaysia). The seeds were
placed at the centre of the Perspex box (e.g., Photos 1
and 2). Vetiver grass was propagated from three tillers
and also grown in the Perspex box (centre). All plants
were left to grow in the open air for su‹cient sunlight and
watered every morning (relative humidity: 70–90z, tem-
Photo 1. Bauhinia purpurea (upfront), Bixa orellana (thin arrow) and
Leucaena leucocephala (thick arrow) after three months of
growth–All plants were grown at the centre of Perspex box
Photo 2. Leucaena leucocephala (labelled LL06(L2))
propagules (arrow) after six months of growth
produced
perature: 32–389C and maximum Photosynthetically Active Radiation: 2100 mE m－2 s－1) until six and twelve
months of growing period.
RESULTS
Vetiver Grass (Vertiveria zizanoides)
The strengthening eŠects of the vetiver roots after 6
months and 12 months were observed (Figs. 3 and 4).
The presence of roots has improved the shear strength of
the soil. The results also show that the shear strengths at
SHEAR STRENGTH OF A SOIL
various depths have almost the same value (Fig. 3). At
this stage, the vetiver grass may have achieved maximum
strengthening eŠect of the roots.
Under soil suction-free condition (matrix suction＝0),
Fig. 3.
Fig. 4.
roots have increased the cohesion component of shear
strength (i.e., the value of intercept on the shear stress vs
normal stress plot) by 188z, 161z and 237z at the
depths of 0.1 m, 0.5 m and 0.9 m respectively after 6
months. The percentage is expressed as [(A－B)/B]×
100, where: A is the soil strength with roots and B is the
soil strength without roots.
After 12 months, the cohesion component has improved by 337z, 329z and 308z at depths of 0.1 m, 0.5
m and 0.9 m, respectively (Fig. 5). It shows that the roots
of vetiver grass play an important role in strengthening
the soil. The roots interact with the soil to produce a composite material in which the roots are ˆbres of relatively
high tensile strength and adhesion embedded in a matrix
of lower tensile strength.
Maximum shear stress vs. normal stress after six months
Maximum shear stress vs. normal stress after twelve months
Fig. 6.
591
Fig. 5.
Values of cohesion (vetiver) at various soil depths
Maximum shear stress vs. normal stress for L. leucocephala at various sample depths (after six months of growth)
HAJI ALI AND OSMAN
592
The shear strength of the soil is therefore enhanced by
the root matrix. Root systems lead to an increase in soil
strength through an increase in cohesion brought about
by their binding action in the ˆbre/soil composite and adhesion of the soil particles to the roots.
Small Plants (L. leucocephala, B. orellana and B. purpurea)
Six Month Growth Period
A typical plot of maximum shear stress against normal
stress was observed at various depths after six months of
growth (Fig. 6). The ˆgure indicates that the presence of
roots has signiˆcantly improved the shear strength of the
soil and it also shows that the eŠect is mainly on the cohesion. L. leucocephala is observed to be leading at all
depths; 0.1, 0.3 and 0.5 m (Fig. 7). Root of L. leucocephala has enhanced the cohesion component of shear
strength by 116.6z (0.1 m), 225.0z (0.3 m) and 413.4z
(0.5 m) as compared to the control (Fig. 6).
Enhancement by L. leucocephala is 6–7-fold (at 0.1 m
soil depth) of other species, 2–4-fold at 0.3 m soil depth
and 2–5-fold at 0.5m soil depth. B. purpurea exhibits better cohesion than B. orellana except at 0.3 m soil depth
(Fig. 7). The cohesion at any depth depends on the density of root (Tengbeh, 1989) where the cohesion increases
with increasing root density. Therefore, the diŠerent
trend in the results at 0.3 m may be due to the diŠerent
Fig. 8.
root proˆle between the two plant species. At this depth,
B. purpurea may have slightly lower root density as compared to B. orellana.
Twelve Months Growth Period
A typical plot of maximum shear stress against normal
stress is also observed at various depths after twelve
months of growth (Fig. 8). Cohesion component of shear
strength is enhanced by 674.5z (0.1 m), 272.2z (0.3 m)
and 493.4z (0.5 m) as compared to the control. Again L.
leucocephala is observed to be leading at all soil depths
Fig. 7. Values of cohesion at various sample depths (after six months
of growth)
Maximum shear stress vs. normal stress for L. leucocephala (after twelve months of growth)
SHEAR STRENGTH OF A SOIL
except at 0.3 m (Fig. 9). Results also reveal that the angle
of friction at every depth is almost similar except at the
0.3 m which is slightly higher. After considering the
results of all the tests (both after six and twelve months of
growth), generally it may be said that there is no signiˆcant eŠect of the presence of roots on the angle of friction.
Comparing the results of six months and twelve
months growth periods, it is observed that shear strength
(i.e., the cohesion) increases with time (for all species
studied). In twelve months, L. leucocephala is observed
to increase the cohesion by more than 3-fold of the sixmonth growth period at 0.1 m depth. However, there is
no distinct eŠect of growth period on cohesion at other
depths (0.3 m and 0.5 m). This may indicate that in six
months, L. leucocephala has achieved maximum growth
condition for the reinforcing eŠects at these depths.
Therefore, it implies that this species could be a good
Fig. 9. Values of cohesion at various sample depths (after twelve
months of growth)
Fig. 11.
593
pioneer on slope as it could reach maximum growth condition considerably faster, which is about six months
only, as compared to other species studied. Apart from
that, the considerable enhancement of strength at 0.1 m
implies that L. leucocephala is a good surface cover for
providing erosion control. The root systems of all species
studied do not seem to improve the friction of angle signiˆcantly with time.
Residual Strength
Residual strength is the shear resistance, which a soil
can maintain when subjected to large shear displacement
after the peak strength has been mobilised (Terzaghi and
Peck, 1968). A typical stress-displacement curve for
diŠerent species at a given depth is shown (Fig. 10) The
plots of `residual' cohesion (c!) at various depths was
also investigated (Fig. 11). The results show that in six
Fig. 10. Comparison of shear strength (kPa) among the species studied; L. leucocephala (LL), B. purpurea (BP), B. orellana (BX) and
control at soil depth of 30 cm after twelve months of growth
Values of cohesion factor of residual strength (kPa) at diŠerent depths for various types of plants ( 6 months ■ 12 months)
HAJI ALI AND OSMAN
594
months, root system of L. leucocephala has increased the
`residual' cohesion about 15 times that of the control at
0.1 m depth, almost 7 times and 10 times at 0.3 m and 0.5
m soil depth, respectively. In contrast, both B. purpurea
and B. orellana do not show much improvement in residual strength as compared to the control.
Whilst in twelve months, the `residual' cohesion of L.
leucocephala is only 1.5-fold (0.1 m) of those in sixmonths, 1.3-fold for both 0.3 and 0.5 m soil depths. It is
interesting to note that the increase in `residual' cohesion
between six months and twelve months of growth for
both B. purpurea and B. orellana is signiˆcantly high. As
before, there is no eŠect of root reinforcement on friction
Fig. 12.
Root proˆle of plant species studied
Fig. 13.
angle of residual strength in all species studied.
Shear Strength and Root Proˆle
In one-year observation, L. leucocephala shows the
highest RLD (i.e., length per unit volume) in all depths as
compared to other species studied. The average total
RLD of L. leucocephala is about ˆve times than that of
B. purpurea and ˆfteen times of B. orellana (Fig. 12). It
shows that L. leucocephala could display a prominent
root system, an imperative feature for stabilizing slope.
Most of the plants, except B. purpurea, exhibit high root
proˆles at the ˆrst 10 cm soil depth and decreases with
soil depth. For some reasons, the highest RLD of B. purpurea is found at 0.3 cm and 0.5 cm depth. Amongst the
species, B. orellana reveals the lowest root proˆle.
However, all the observations indicate that root system
of all species studied could play a role as surface erosion
control on slope.
There is a correlation between shear strength (at all
normal pressures) and RLD (Fig. 13), implying that high
root length density at the ˆrst 60 cm soil depth of all species studied could help reinforcing the soil by increasing
its cohesion. This would also be useful in reducing surface erosion. The increase in soil strength through an increase in cohesion particularly, is brought about by binding action in the ˆne roots or soil composite and adhesion
of the soil particles to the roots (Styczen and Morgan,
1995). Previous studies also indicated that shear strength
provided by ˆne roots, 1–20 mm in diameter, contributing most to soil reinforcement (O'Loughlin, 1984).
Correlations between shear strength and Root Length Density (RLD)
SHEAR STRENGTH OF A SOIL
Fig. 14.
595
Correlations between shear strength and physiological parameters
Shear Strength and Physiological Parameters
The results reveal some correlations between shear
strength and plant physiological parameters. A weak
relationship is observed between shear strength and root
girth (Fig. 14). The enhancement of shear strength may
arise from the eŠect of root branching, root geometry
(position and dimension) and root area but not much in‰uence from root diameter (Wu et al., 1988a). Meanwhile, shear strength increases with shoot biomass which
is less than 1.0 Kg. The value is almost constant beyond
that value (Fig. 14). However, no clear relationship between shear strength and plant height is observed in this
study (Fig. 14), implying plant height probably does not
grow proportional to the root proˆles.
DISCUSSION
The roots of all the species studied contribute signiˆcantly to enhancement of soil shear strength. The
results also indicate that the cohesion rapidly increases
with increasing root length density. The results imply that
the root matrix has a signiˆcant eŠect on the cohesion
and this eŠect varies with the depth depending on the root
length density. It has been reported that soil strength increases with depth due to the increase in interaction between the root and soil particles (Tobias, 1995; Wulfsohn
et al., 1996). Vegetation has its greatest eŠect close to the
soil surface where the root length density is generally the
highest. In the present study, it is also discovered that
there is no signiˆcant eŠect of the root matrix on the soil
angle of friction.
Amongst the species, only the root system of L. leu-
cocephala seems to achieve extensive soil-root reinforcement within six months of growth. This species also reveals the highest root reinforcement as indicated by the signiˆcant increase in the cohesion with respect to time. It
was found that the cohesion of L. leucocephala (after six
months) increased by almost double (at 0.1 m), triple (at
0.3 m) and ˆve fold (at 0.5 m) that of the control. Furthermore, the value of the cohesion (after twelve months)
was increased by almost eight fold (at 0.1 m depth), four
fold (at 0.3 m) and six fold (at 0.5 m) that of the control.
The increase between the six month and twelve month
periods is about 243z (at 0.1 m) and 10z at both the soil
depths of 0.3 m and 0.5 m. The extensive growth and development of taproots in L. leucocephala has probably
caused the tremendous enhancement of shear strength at
a soil depth of 0.1 m.
Amongst the species studied, L. leucocephala has the
highest percentage increase of root reinforcement at all
soil depths, except at 0.3 m, after twelve months of
growth. The results imply that the high capacity of the
root reinforcement of L. leucocephala rank it as an outstanding slope plant. The overall results suggest that the
species studied has a potential to play a major engineering
role in stabilising slopes and protecting the soil erosion.
The root system penetrates the soil mass, reinforces it,
bringing about an increase in cohesion and, hence, in soil
shear strength. Apart from that, the increase in cohesion
is probably partly due to an increase amount of ˆber
(ˆne) roots of the species studied as reported by Nilaweera (1994). In addition, a ˆne root mat close to the soil surface may act like a low-growing vegetation cover and protect the soil from erosion. Rickson and Morgan (1988)
596
HAJI ALI AND OSMAN
also reported that an increase in the percentage area occupied by ˆne roots produces an exponential decay in the
soil loss ratio. Thus, the combined eŠect of primary and
ˆne root systems resulted in the enhancement of soil-root
matrix shear strength.
CONCLUSIONS
This study shows that roots signiˆcantly contribute to
the increase in soil shear strength. The contribution
mainly arises from the cohesion but not the angle of friction. The eŠect varies with increasing depth and age of
plant depending on the root length density of the plants.
A higher residual strength observed in this study also indicates a high contribution of the root system to soil-root
reinforcement. Finally, the results also indicate that shear
strength is not only in‰uenced by the root proˆles but
also to some extent, the physiological parameters of the
plants.
ACKNOWLEDGEMENT
The authors would like to thank the University of
Malaya for the research funding throughout this project.
REFERENCES
1) Al-Durah, M. M. and Bradford, J. M. (1982): Parameters for
describing soil detachment due to single-waterdrop impact, Soil Sci.
Soc. Am. J., 46, 836–840.
2) Coppin, N. J. and Richards, I. G. (1990): Use of vegetation in civil
engineering, Butterworths, London.
3) Gray, D. H. (1970): EŠect of forest clear-cutting on the stability of
natural slopes, Bulletin Association of Engineering Geology, 7,
45–66.
4) Gray, D. H. and Brenner, R. P. (1970): The hydrology and stability
of cutover slopes, Proc. Interdis- ciplinary Aspects of Watershed
Management, ASCE, New York, 295–326.
5) Gray, D. H. and Leiser, A. T. (1982): Biotechnical Slope Protection
and Erosion Control, Van No-strand Reinhold Co., New York.
6) Greenway, D. T. (1987): Vegetation and slope stability, in Slope
Stability (eds. by M. G. Anderson and K. S. Richards), John Wiley,
New York, 187–230.
7) Grimshaw, R. G. (1994): Vetiver grass—its use for slope and structure stabilisation under tropical and semi tropical conditions,Vegetation and Slopes Stabilisation, Protection and Ecology (ed. by
Barker, D. H.), Thomas Telford House, London, 25–34.
8) Ifa Herliani, I. (2006): The eŠect of water stress on the growth of
Bauhinia purpurea, Insitute of Biological Sciences, Univ. of
Malaya, B. Sc. Thesis (unpublished).
9) Nilaweera, N. S. (1994): In‰uence of hardwood roorts on soil shear
strength and slope stability in Southern Thailand, Ph.D. Dissertation, Asian Institute of Technology, Bangkok.
10) Normaniza, O. and Barakbah, S. S. (2006): Parameters to predict
slope stability–soil water and root proˆles, Ecological Engineering,
28, 90–95.
11) O'Loughlin, C. L. (1974): The eŠects of timber removal on stability
of forest soils, J. Hydrology, New Zealand, 13, 121–134.
12) O'Loughlin, C. L. (1984): EŠectiveness of introduced forest vegetation for protecting against landslides and erosion in New Zealand's
steeplands, Symposium on EŠects of Forest Land Use on Erosion
and Slope Stability, Honolulu, Hawaii.
13) Parera, V. (1982): Leucaena for erosion control and green manure
in Sikka, Proc. Leucaena Research in the Asian-Paciˆc Region,
Singapore, 23–26 November 1982, 169–173.
14) Rauws, G. and Govers, G. (1988): Hydraulic and soil mechanical
aspects of rill generation on agricultural soils, J. Soil. Sci. 39,
111–124.
15) Rickson, R. J. amd Morgan, R. P. C. (1988): Approaches to
modelling the eŠects of vegetation on soil erosion by water, Commision of the European Communities DG (eds. by Morgan, R. P.
C. and Rickson, R. J.), VI EUR, 10860 EN, 237–254.
16) Riestenberg, M. M. (1987): Anchoring of thin colluvium on
hillslopes by roots of sugar maple and white ash, Ph.D Dissertation, University of Cincinnati, Cincinnati, Ohio.
17) Styczen, M. E. and Morgan, R. P. C. (1995): Engineering properties of vegetation, Slope Stabilization and Erosion Control: A
Bioengineering Approach (eds. by Morgan, R. P. C. and Rickson
R. J.), E and FN Spon, London, 5–58.
18) Suraya, D. (1998): Alleviation of the green house eŠect by increasing carbon sink, Insitute of Biological Sciences, Univ. of Malaya,
B. Sc. Thesis (unpublished).
19) Tengbeh, G. T. (1989): The eŠect of grass cover on bank erosion,
Ph.D Thesis, Silsoe College, Cranˆeld Institute of Technology.
20) Terzaghi, K. and Peck, R. B. (1968): Soil Mechanics in Engineering
Practice, 2nd Edition, New York, Wiley.
21) Tobias, Y. (1995): Shear strength of the soil root bond system,
Vegetation and Slope Stabilization, Protection and Ecology (ed. by
Barker, D. H.), Thomas Telford House, London, 280–285.
22) William, A. A. B. and Pidgeon, J. T. (1983): Evapotranspiration
and heaving clays in South Africa, Geotechnique, 33, 141–150.
23) Wu, T. H., McKinnell, W. P. and Swanston, D. N. (1979):
Strength of tree roots and landslides on Prince of Wales Island,
Alaska, Canadian Geotechnical Journal, 16, 19–33.
24) Wu, T. H., McKinnell, W. P. and Swanston, D. N. (1988a): Study
of soil-root interactions, Journal of Geotechnical Engineering
(ASCE), 114 (GT12), 1351–1375.
25) Wulfsohn, D., Adams, B. A. and Fredlund, D. G. (1996): Application of unsaturated soilmechanics for agricultural conditions,
Canadian Agricultural Engineering, 38, 173–181.
26) Yen, C. P. (1972): Study on the root system form and distribution
habit of the ligneous plants for soil conservation in Taiwan (preliminary report), J. Chinese Soil & Water Conservation, 3, 179–204.