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i) Professor, Civil Engineering Department, University of Malaya, Kuala Lumpur, Malaysia (fahali＠um.edu.my).ii) 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, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.

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SOILS AND FOUNDATIONS Vol. 48, No. 4, 587–596, Aug. 2008Japanese Geotechnical Society

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 dependson the morphological characteristics of the root systems and the tensile strength of single roots. This paper describes aninvestigation 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 orel-lana and Bauhinia purpurea were planted in special boxes containing residual soil compacted to a known density. Theresults show that roots signiˆcantly contribute to the increase in soil shear strength. The presence of the roots onlyaŠects the apparent cohesion of the soil and no signiˆcant change in angle of friction is observed. L. leucocephalashows the outstanding increase in its root strength in which the strength varies with depth and time e.g., under soil suc-tion-free condition (matric suction＝0), the roots have increased the cohesion by 116.6z (0.1 m), 225.0z (0.3 m) and413.4z (0.5 m) after six months of growth. In twelve months, it is observed that the increase in cohesion is more thanthree-fold of the six months growth period at 0.1 m depth. The results also indicate that shear strength is in‰uenced byroot proˆle and to some extent, the physiological parameters of the plants.

Key words: reinforcement, root, shear strength, slope stability, soil (IGC: D6)

INTRODUCTION

The use of vegetation for slope stabilization started inancient times. In more recent times, the roles of vegeta-tion in some speciˆc geotechnical processes have beenrecognised. Vegetation may aŠect slope stability in manyways. Comprehensive reviews may be found in Greenway(1987), Gray (1970), Gray and Leiser (1982), Coppin andRichards (1990). The stability of slopes is governed by theload, which is the driving force that causes failure, andthe resistance, which is the strength of the soil-root sys-tem. The weight of trees growing on a slope adds to theload whilst the roots of trees serve as soil reinforcementsand increase the resistance. In addition, vegetation alsoin‰uences slope stability indirectly through its eŠect onthe soil moisture regime. Vegetation intercepts rainfalland draws water from the soil via evapotranspiration.This reduces soil moisture and pore pressure, increasesthe shear strength of the soil, thus increases theresistance.

Several case studies have shown that slope failures maybe 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 vegetationreduces water content and increases soil-moisture suctionin the soil (Greenway, 1987; Gray, 1970; Gray and Bren-ner, 1970; William and Pidgeon, 1983). Greenway (1987)has given an extensive summary of observations on the

eŠect of vegetation on slope stability.

Soil ReinforcementThe roots and rhizomes of the vegetation interact with

the soil to produce a composite material in which theroots are ˆbres of relatively high tensile strength and ad-hesion embedded in a matrix of lower tensile strength.The shear strength of the soil is therefore enhanced by theroot matrix. Field studies of forested slopes (O'Loughlin,1984) indicate that it is the ˆne roots, 1–20 mm in di-ameter, that contribute most to soil reinforcement.Grasses, legumes and small shrubs can have a signiˆcantreinforcing eŠect down to depths of 0.75–1.5 m. Treeshave deeper-seated eŠects and can enhance soil strengthto depths of 3 m or more depending upon the root mor-phology of the species (Yen, 1972). Root systems lead toan increase in soil strength brought about by their bind-ing action in the ˆbre/soil composite and adhesion of thesoil 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). ThePPS value was measured at 10 cm of soil depth by using aˆeld inspection vane tester (Normaniza and Barakbah,2006). Root length density (Root length per unit volumeof soil) is the highest near the soil-atmosphere interfaceand decreases, at ˆrst drastically in top 20 cm and then in

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Table 1. Variation of Root Length Density (RLD) (km m－3) withdepth (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 vege-tation (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

Grasses 25.7± 3.5

588 HAJI ALI AND OSMAN

a step-like manner, with increasing depth. The RLD ofthe ``grass'' ecosystem was only about 20–30z that ofthe ``bush'' ecosystem. The PSS at a depth of 9 cm intothe soil, when compared between various vegetation type-s, showed interesting result. PSS was highest in a ``nor-mal'' bush, i.e., an open bush comprising many plantspecies. Interestingly, PSS was very low in the thick bush(this trend was repeatedly observed).

The magnitude of the diŠerence in PSS between thickand normal bush was di‹cult to comprehend. This is es-pecially so if one considers that the former was onlyslightly higher than that of the thin grass. However, uponanalyzing the species components of the thick bush, theresult is easier to digest. The thick bush comprised mainlyvarious species of mosses and ferns which had successful-ly formed a ``closed'' system that did not allow other spe-cies, including tree species which is the main contributorto shear strength, to colonize.

The pattern of the relationship between soil cohesionand the roots is not known. Tengbeh (1989) found thatLoretta grass increased the strength of soil as a functionof root density (RD; Mg/m3) in a logarithmic relationshipthat 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 sig-niˆcant contributions to soil strength, even at low root

densities and low shear strengths. The above equationsindicate that cohesion increases rapidly with increasingroot density at low root densities but Tengbeh (1989) alsofound that increasing root density above 0.5 Mg/m3 onthe clay soil and 0.7 Mg/m3 on the sandy clay loam soilhas little additional eŠect. This implies that vegetationcan have its greatest eŠect close to the soil surface wherethe root density is generally highest and the soil is other-wise weakest.

Since shear strength aŠects the resistance of the soil todetachment by raindrop impact (Al-Durah and Bradford,1982) and the susceptibility of the soil to rill erosion aswell as the likelihood of mass soil failure, root systemscan have a considerable in‰uence on all these processes.The maximum eŠect on resistance to soil failure occurswhen the tensile strength of the roots is fully mobilizedand that, under strain, the behaviour of the roots and thesoil are compatible. This requires roots of high stiŠnessor tensile modulus to mobilize su‹cient strength and the8–10z failure strains of most soils. The tensile eŠect islimited with shallow-rooted vegetation where the rootsfail by pullout, i.e., slipping due to loss of bonding be-tween the root and the soil, before peak tensile strength isreached. The tensile eŠect is most marked with treeswhere the roots penetrate several metres into the soil andtheir tortuous paths around stones and other roots pro-vide good anchorage.

Root failure may still occur, however, by rupture, i.e.,breaking of the roots when their tensile strength is exceed-ed. The strengthening eŠect of the roots will also beminimized in situations where the soil is held in compres-sion instead of tension, e.g., at the bottom of hill slopes.Root failure here occurs by buckling.

MATERIALS AND TEST METHODS

SoilThe soil was collected from a slope and subjected to a

number of standard tests to determine the basic physicalproperties. Based on the grain size distribution curve thesoil is described as Silty Sand. Its physical properties andgrain size distribution are shown in Tables 3 and 4, re-spectively.

The reinforcing eŠects of the vegetation roots werestudied by carrying out laboratory shear box tests. Thelaboratory tests were chosen instead of ˆeld tests so that(i) identical soil samples can be prepared (ii) samples atvarious depths can be obtained easily and (iii) saturationof samples can be carried out. A large shear box appara-tus (300 mm×300 mm) was fabricated and modiˆed suchthat both saturated and partially saturated soil samplescontaining roots can be tested with measurement ofdeformation and load are done automatically. FourdiŠerent species of plant including vetiver grass wereplanted in special boxes containing residual soil compact-ed to a known density. Each box was about 1 m high andconsisted of ˆve Perspex boxes with size 300×300×200mm stacked up together. The soil was compacted in thisbox by a specially designed mould to prevent the boxes

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Table 3. Physical properties of the soil

Atterberg LimitsLiqiud Limit 26.9zPlastic Limit 14.59zPlasticity Index 12.31z

Linear Shrinkage 3.23

Speciˆc Gravity 2.61

CompactionOptimum moisture content 13.5zMaximum dry density 1.8515 Mg/m3

Table 4. Grain size distribution

Type Size distribution

Gravel (2 to 60 mm) 10.0zSand (0.06 to 2 mm) 79.5zSilt (0.002 to 0.06 mm) 7.5zClay(º0.002 mm) 3.0z

Fig. 1. Perspex boxes for preparation of samples

Fig. 2. Large (300 mm×300 mm) direct shear box machine

589SHEAR STRENGTH OF A SOIL

from being damaged. The Perspex was chosen so that thedevelopment of the roots could be observed.

The soils were compacted at a moisture content of 15zand the compactive eŠort used was 29.43 Jkg－1. 10 kg ofsoil was compacted by a rammer consisting of 2.5 kgmass falling freely through 300 mm for 40 blows. Theprocesses were repeated until the 1m box was full of soil(Fig. 1). The mean dry density of the soil samples was1800 kgm－3. Some of the soil samples were used as con-trol (i.e., samples with no vegetation roots). The plantswere allowed to grow for 6 and 12 months before thesamples were tested in the large direct shear box machine.

To remove the eŠect of soil suction on the shearstrength the samples were saturated in a speciallydesigned saturation box. The soil containing the roots(inside the stacked boxes) was cut into ˆve parts. Samplesat depth 0.1, 0.3, 0.5 and 0.9 m were taken and each wasplaced in the saturation box. A tip tensiometer was insert-ed into the soil to measure the soil matric suction. Thesamples were soaked in de-aired water for around twodays and the box was connected to a vacuum pump untilthe soil matric suction dropped to zero.

The shear strengths of the samples were determined us-ing a specially fabricated 300×300×200 mm direct shearbox machine (Fig. 2). The shear box machine wasdesigned so that the sample can be soaked in water andˆxed with 8 units of tip tensiometer. Normal stresses wereapplied using hydraulic pressure system through the topplate. The normal stresses applied were 10 kPa, 20 kPaand 30 kPa and the side friction was minimized by ap-plying silicon grease to the sides of the box. An automaticdata logging system was designed to download all re-quired data from the tensiometer, displacement trans-ducer and load cell. All data was then transferred to thepersonal computer and communicated with software

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Photo 1. Bauhinia purpurea (upfront), Bixa orellana (thin arrow) andLeucaena leucocephala (thick arrow) after three months ofgrowth–All plants were grown at the centre of Perspex box

Photo 2. Leucaena leucocephala (labelled LL06(L2)) producedpropagules (arrow) after six months of growth


PC208W.

Root Proˆles and Physiological ParametersBefore the test, each plant sample was cut oŠ near the

lowest part of the stem prior to the measurement. Theroot girth (circumference) was measured at the lowestpart of the stem using vernier calipers. After the test, theroot was washed from soil and debris. Root length (RL)was measured using a leaf area image analyser. The rootwas dyed using methyl violet in prior to measurement inorder to create a contrast under the image analyser. TheRoot Length Density (RLD) was calculated as: total rootlength/soil volume. Whilst shoot was partitioned intostem and leaf and oven-dried at 809C in order to obtainshoot (stem and leaf) biomass.

Plant Materials(a) Vetiveria zizanoidesThis 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 beused to reduce erosion due to its extensive root system. Ithas been proven that vetiver is also tolerant of a largerange of soil and ground water conditions. Its base tillerscan block the soil from eroding (Grimshaw, 1994). Interms of the growth rate, the number of tillers can be in-creased to 7–8 tillers after four weeks of propagation.(b) Bauhinia purpurea

This leguminous shrub is abundant in Malaysia andhas been used as landscape decorations in the housingareas due to its attractive purplish orchid-like ‰owers. Itsbig heart-shape leaf has a larger surface area (Photo 1)which makes the plant have a higher photosynthetic rateand resistance to drought condition (Ifa Herliani, 2006).This easily grown shrub can reach a stem diameter ofabout 18 cm and height of 5 m in less than 2 years (IfaHerliani, 2006).(c) Leucaena leucocephala

It's a leguminous tree which can adequately improvethe quality of the soil via its nitrogen ˆxer capacity. It is amultipurpose tree which profusely produces propagules(beans; Photo 2) and has been used as an erosion controlplant (Parera, 1982). However, based on the fact there iswoeful lack of documentation on its contribution to slopestabilization, this species had been chosen in this experi-ment.(d) Bixa orellana

This shrub was chosen in this experiment due to thefact that it has a higher growth rate and a good carbonsink potential during the plant physiological screening(Suraya, 1998), essential criteria as slope colonizer.

All plants, except vetiver, were germinated from seedsand the seeds were inoculated with speciˆc Rhizobium(Rubber Research Institute of Malaysia). The seeds wereplaced at the centre of the Perspex box (e.g., Photos 1and 2). Vetiver grass was propagated from three tillersand also grown in the Perspex box (centre). All plantswere left to grow in the open air for su‹cient sunlight andwatered every morning (relative humidity: 70–90z, tem-

perature: 32–389C and maximum Photosynthetically Ac-tive Radiation: 2100 mE m－2 s－1) until six and twelvemonths 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 ofthe soil. The results also show that the shear strengths at

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Fig. 3. Maximum shear stress vs. normal stress after six months

Fig. 4. Maximum shear stress vs. normal stress after twelve months Fig. 5. Values of cohesion (vetiver) at various soil depths

Fig. 6. Maximum shear stress vs. normal stress for L. leucocephala at various sample depths (after six months of growth)


various depths have almost the same value (Fig. 3). Atthis stage, the vetiver grass may have achieved maximumstrengthening eŠect of the roots.

Under soil suction-free condition (matrix suction＝0),

roots have increased the cohesion component of shearstrength (i.e., the value of intercept on the shear stress vsnormal stress plot) by 188z, 161z and 237z at thedepths of 0.1 m, 0.5 m and 0.9 m respectively after 6months. The percentage is expressed as [(A－B)/B]×100, where: A is the soil strength with roots and B is thesoil strength without roots.

After 12 months, the cohesion component has im-proved by 337z, 329z and 308z at depths of 0.1 m, 0.5m and 0.9 m, respectively (Fig. 5). It shows that the rootsof vetiver grass play an important role in strengtheningthe soil. The roots interact with the soil to produce a com-posite material in which the roots are ˆbres of relativelyhigh tensile strength and adhesion embedded in a matrixof lower tensile strength.

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Fig. 7. Values of cohesion at various sample depths (after six monthsof growth)

Fig. 8. Maximum shear stress vs. normal stress for L. leucocephala (after twelve months of growth)


The shear strength of the soil is therefore enhanced bythe root matrix. Root systems lead to an increase in soilstrength through an increase in cohesion brought aboutby their binding action in the ˆbre/soil composite and ad-hesion of the soil particles to the roots.

Small Plants (L. leucocephala, B. orellana and B. pur-purea)Six Month Growth Period

A typical plot of maximum shear stress against normalstress was observed at various depths after six months ofgrowth (Fig. 6). The ˆgure indicates that the presence ofroots has signiˆcantly improved the shear strength of thesoil and it also shows that the eŠect is mainly on the cohe-sion. L. leucocephala is observed to be leading at alldepths; 0.1, 0.3 and 0.5 m (Fig. 7). Root of L. leu-cocephala has enhanced the cohesion component of shearstrength 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 msoil depth) of other species, 2–4-fold at 0.3 m soil depthand 2–5-fold at 0.5m soil depth. B. purpurea exhibits bet-ter cohesion than B. orellana except at 0.3 m soil depth(Fig. 7). The cohesion at any depth depends on the den-sity of root (Tengbeh, 1989) where the cohesion increaseswith increasing root density. Therefore, the diŠerenttrend in the results at 0.3 m may be due to the diŠerent

root proˆle between the two plant species. At this depth,B. purpurea may have slightly lower root density as com-pared to B. orellana.

Twelve Months Growth PeriodA typical plot of maximum shear stress against normal

stress is also observed at various depths after twelvemonths of growth (Fig. 8). Cohesion component of shearstrength 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

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Fig. 9. Values of cohesion at various sample depths (after twelvemonths of growth)

Fig. 10. Comparison of shear strength (kPa) among the species stud-ied; L. leucocephala (LL), B. purpurea (BP), B. orellana (BX) andcontrol at soil depth of 30 cm after twelve months of growth

Fig. 11. Values of cohesion factor of residual strength (kPa) at diŠerent depths for various types of plants ( 6 months ■ 12 months)


except at 0.3 m (Fig. 9). Results also reveal that the angleof friction at every depth is almost similar except at the0.3 m which is slightly higher. After considering theresults of all the tests (both after six and twelve months ofgrowth), generally it may be said that there is no sig-niˆcant eŠect of the presence of roots on the angle of fric-tion.

Comparing the results of six months and twelvemonths growth periods, it is observed that shear strength(i.e., the cohesion) increases with time (for all speciesstudied). In twelve months, L. leucocephala is observedto increase the cohesion by more than 3-fold of the six-month growth period at 0.1 m depth. However, there isno distinct eŠect of growth period on cohesion at otherdepths (0.3 m and 0.5 m). This may indicate that in sixmonths, L. leucocephala has achieved maximum growthcondition for the reinforcing eŠects at these depths.Therefore, it implies that this species could be a good

pioneer on slope as it could reach maximum growth con-dition considerably faster, which is about six monthsonly, as compared to other species studied. Apart fromthat, the considerable enhancement of strength at 0.1 mimplies that L. leucocephala is a good surface cover forproviding erosion control. The root systems of all speciesstudied do not seem to improve the friction of angle sig-niˆcantly with time.

Residual StrengthResidual strength is the shear resistance, which a soil

can maintain when subjected to large shear displacementafter the peak strength has been mobilised (Terzaghi andPeck, 1968). A typical stress-displacement curve fordiŠerent species at a given depth is shown (Fig. 10) Theplots of `residual' cohesion (c!) at various depths wasalso investigated (Fig. 11). The results show that in six

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Fig. 12. Root proˆle of plant species studied

Fig. 13. Correlations between shear strength and Root Length Density (RLD)


months, root system of L. leucocephala has increased the`residual' cohesion about 15 times that of the control at0.1 m depth, almost 7 times and 10 times at 0.3 m and 0.5m soil depth, respectively. In contrast, both B. purpureaand B. orellana do not show much improvement in resid-ual 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 six-months, 1.3-fold for both 0.3 and 0.5 m soil depths. It isinteresting to note that the increase in `residual' cohesionbetween six months and twelve months of growth forboth B. purpurea and B. orellana is signiˆcantly high. Asbefore, there is no eŠect of root reinforcement on friction

angle of residual strength in all species studied.

Shear Strength and Root ProˆleIn one-year observation, L. leucocephala shows the

highest RLD (i.e., length per unit volume) in all depths ascompared to other species studied. The average totalRLD of L. leucocephala is about ˆve times than that ofB. purpurea and ˆfteen times of B. orellana (Fig. 12). Itshows that L. leucocephala could display a prominentroot system, an imperative feature for stabilizing slope.Most of the plants, except B. purpurea, exhibit high rootproˆles at the ˆrst 10 cm soil depth and decreases withsoil depth. For some reasons, the highest RLD of B. pur-purea is found at 0.3 cm and 0.5 cm depth. Amongst thespecies, B. orellana reveals the lowest root proˆle.However, all the observations indicate that root systemof all species studied could play a role as surface erosioncontrol on slope.

There is a correlation between shear strength (at allnormal pressures) and RLD (Fig. 13), implying that highroot length density at the ˆrst 60 cm soil depth of all spe-cies studied could help reinforcing the soil by increasingits cohesion. This would also be useful in reducing sur-face erosion. The increase in soil strength through an in-crease in cohesion particularly, is brought about by bind-ing action in the ˆne roots or soil composite and adhesionof the soil particles to the roots (Styczen and Morgan,1995). Previous studies also indicated that shear strengthprovided by ˆne roots, 1–20 mm in diameter, contribut-ing most to soil reinforcement (O'Loughlin, 1984).

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Fig. 14. Correlations between shear strength and physiological parameters


Shear Strength and Physiological ParametersThe results reveal some correlations between shear

strength and plant physiological parameters. A weakrelationship is observed between shear strength and rootgirth (Fig. 14). The enhancement of shear strength mayarise 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). Mean-while, shear strength increases with shoot biomass whichis less than 1.0 Kg. The value is almost constant beyondthat value (Fig. 14). However, no clear relationship be-tween shear strength and plant height is observed in thisstudy (Fig. 14), implying plant height probably does notgrow proportional to the root proˆles.

DISCUSSION

The roots of all the species studied contribute sig-niˆcantly to enhancement of soil shear strength. Theresults also indicate that the cohesion rapidly increaseswith increasing root length density. The results imply thatthe root matrix has a signiˆcant eŠect on the cohesionand this eŠect varies with the depth depending on the rootlength density. It has been reported that soil strength in-creases with depth due to the increase in interaction be-tween the root and soil particles (Tobias, 1995; Wulfsohnet al., 1996). Vegetation has its greatest eŠect close to thesoil surface where the root length density is generally thehighest. In the present study, it is also discovered thatthere is no signiˆcant eŠect of the root matrix on the soilangle of friction.

Amongst the species, only the root system of L. leu-

cocephala seems to achieve extensive soil-root reinforce-ment within six months of growth. This species also rev-eals the highest root reinforcement as indicated by the sig-niˆcant increase in the cohesion with respect to time. Itwas found that the cohesion of L. leucocephala (after sixmonths) increased by almost double (at 0.1 m), triple (at0.3 m) and ˆve fold (at 0.5 m) that of the control. Fur-thermore, the value of the cohesion (after twelve months)was increased by almost eight fold (at 0.1 m depth), fourfold (at 0.3 m) and six fold (at 0.5 m) that of the control.The increase between the six month and twelve monthperiods is about 243z (at 0.1 m) and 10z at both the soildepths of 0.3 m and 0.5 m. The extensive growth and de-velopment of taproots in L. leucocephala has probablycaused the tremendous enhancement of shear strength ata soil depth of 0.1 m.

Amongst the species studied, L. leucocephala has thehighest percentage increase of root reinforcement at allsoil depths, except at 0.3 m, after twelve months ofgrowth. The results imply that the high capacity of theroot reinforcement of L. leucocephala rank it as an out-standing slope plant. The overall results suggest that thespecies studied has a potential to play a major engineeringrole 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 soilshear strength. Apart from that, the increase in cohesionis probably partly due to an increase amount of ˆber(ˆne) roots of the species studied as reported by Nilaweer-a (1994). In addition, a ˆne root mat close to the soil sur-face may act like a low-growing vegetation cover and pro-tect the soil from erosion. Rickson and Morgan (1988)


also reported that an increase in the percentage area occu-pied by ˆne roots produces an exponential decay in thesoil loss ratio. Thus, the combined eŠect of primary andˆne root systems resulted in the enhancement of soil-rootmatrix shear strength.

CONCLUSIONS

This study shows that roots signiˆcantly contribute tothe increase in soil shear strength. The contributionmainly arises from the cohesion but not the angle of fric-tion. The eŠect varies with increasing depth and age ofplant depending on the root length density of the plants.A higher residual strength observed in this study also in-dicates a high contribution of the root system to soil-rootreinforcement. Finally, the results also indicate that shearstrength is not only in‰uenced by the root proˆles butalso to some extent, the physiological parameters of theplants.

ACKNOWLEDGEMENT

The authors would like to thank the University ofMalaya for the research funding throughout this project.

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