Roots Anchorage Stability

REVIEW PAPER

Contribution of the Root to Slope Stability

Abdolhossein Khalilnejad Faisal Hj.Ali

Normaniza Osman

Received: 3 February 2011 / Accepted: 8 September 2011 / Published online: 28 September 2011

Springer Science+Business Media B.V. 2011

Abstract Land sliding is a geotechnical event that

includes a wide range of ground movements such as

rockfalls, deep failure of slopes and shallow debris

flows, and it can cause various problems in varied civil

fields such as roads and dams. Since most conven-

tional methods are neither inexpensive nor applicable

everywhere, attention has nowadays been drawn to

soil bioengineering using vegetation as the environ-

ment-friendly method for slope stabilization. Soil

bioengineering or using vegetation in civil engineer-

ing design is mostly applicable to shallow slope

stabilization projects characterized by unstable slopes

with surface movement. Vegetation has both a silent

effect on soil improvement to predict the landslide and

a mechanical role to increase shear and pulling-out

stress on the soil. During the last decade, many

researches have been carried out to clarify the effect of

vegetation on slope stability, but many questions still

remain to be answered.

Keywords Slope stability Land sliding Vegetation Matric suction Subtropical region

List of symbols

c Soil cohesion

/ Soil friction angleW Soil dilation anglem Poisson coefficientc Volumetric weight

1 Introduction

Soil bioengineering, use of vegetation in civil engi-

neering design, is now an established practice in many

parts of the world and is considered as a practical

alternative to more traditional methods of soil stabil-

ization such as soil nailing or geosynthetic reinforce-

ment. These methods are mostly utilized in shallow

slope stabilization projects characterized by unstable

slopes with surface movement. To analyse the contri-

bution of vegetation to the slope stability, one needs to

consider its hydrological, biological and mechanical

roles. However, throughout this study, focus will be

more specifically on the mechanical role.

Soil and roots show some similarities with respect

to structure and ductile reaction to strain. Both these

elements deform to a great extent before they break.

Their retaining capacity is not lost during deflection

and subsidence of the relevant slope.

A. Khalilnejad (&) Faisal Hj.AliDepartment of Civil Engineering, University of Malaya,

Lembah Pantai 50603, Kuala Lumpur, Malaysia

e-mail: [email protected];

[email protected]

Faisal Hj.Ali

Department of Civil Engineering, National Defense

University of Malaysia, Kuala Lumpur, Malaysia

N. Osman

Institute of Biological Sciences, University of Malaya,

Kuala Lumpur, Malaysia

123

Geotech Geol Eng (2012) 30:277288

DOI 10.1007/s10706-011-9446-5

Vegetation also has a key role both in soil moisture

extraction by evapotranspiration process and in rain

drop interception by foliage. Foliage and plant residues

absorb the rainfall energy and prevent soil detachment

by raindrop splash. Also, root systems physically bind

or restrain soil particles while the above-ground

portions filter the sediment out of the run-off; as a

result, the stems and foliage increase the surface

roughness and slow the velocity of the run-off. Plants

and their residues help to maintain soil porosity and

permeability, thereby delaying the onset to run-off.

Soil reinforcement by roots is studied by consider-

ing the contribution of the tensile force in a root

segment that intersects a potential slip surface in a

rootsoil system, where the roots mechanically rein-

force the soil by transferring shear stresses in the soil

to tensile resistance in the roots. Different types of root

systems of plants can provide different strengthening

effects on the stability of the slope via fibre reinforce-

ment near the slope surface and deeper-binding soil

structure effect through tap or lateral root networks.

The anchorage of the roots and the improvement in

slope stability depend on the properties of the root

systems such as root distribution and tensile strength

(Nicoll and Ray 1996; Stokes and Guitard 1997;

Stokes et al. 1998; Normaniza and Barakbah 2006; Li

et al. 2007) as well as soil conditions.

Even root architecture has long been considered as

a major component of root anchorage, but some

researchers (Wu and Sidle 1995, Waldron and Dakes-

sian 1981, Greenwood et al. 2004) suggest that the

reinforcing effect of vegetation can be considered in

conventional slope design by adding an additional root

cohesion term, cR, to the MohrCoulomb strength

envelope for soil. When the soil is permeated by fibres

(as in the case of roots), the displacement of soil, as a

consequence of shear tension, generates friction

between soil grains and fibre surfaces, causing the

fibres to deform and to mobilize their tensile strengths.

In this way, some of the shear tension can be

transferred from soil to fibres, producing a reinforce-

ment of the soil matrix itself.

On the other hand, vegetation can protect soil from

erosion via foliage; also, they can draw water from soil

via respiration and transpiration and consequently

cause an increase in the soil suction by reducing the

soil moisture, which will help increase the shear

strength in soil, as discussed by Faisal et al. (1999).

2 Slope Stability

Sloping ground can be unstable if the gravity forces

acting on a mass of soil exceed the shear strength

available at the base of the mass and within it (Barnes

2000). Skempton and Hutchinson (1969) classified

land sliding as shown in Fig. 1.

Generally, land sliding occurs when shear stress (s)in the slope overcomes the related shear strength (sf),and the safety factor F is

F sfs: 1

As mentioned earlier in the introduction, different

mechanical parameters can affect the shear strength of

the soil and consequently the slope safety factor, for

example pore water pressure, due to the fact that when

pore water pressure increases, safety factor decreases.

To analyse slope stability, there are different

methods depending on the method of movement.

2.1 Plane Translational Slide

As shown in Fig. 1, translational slides are commonly

controlled structurally by surface weakness such as

faults, joints, bedding planes, and contacts between

bedrock and upper soil layer. This method can be

applicable when the slip surface (bedding planes, etc.)

is parallel to the ground surface as shown in Fig. 2.

Barnes (2000) showed that if slip surface is under

the water table, safety factor will be:

F c0 tg/0 cos2 b cz cwz chhw

cz sinbcosb

2where b = slip surface angle, z = slip surface depth,c = balk and saturated soil unit weight, hw = watertable depth, cw = water unit weight c0 = effectivecohesion impact and /0 = effective angle of internalfriction.

As mentioned earlier, increase in c0 and /0 cancause an increase in the safety factor F.

2.2 Circular Arc Analysis

This method assumes that the slip surface is an arc that

cuts the ground surface in a certain point, as shown in

Fig. 3. Safety factor in this case will be given by:

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F shear resistance moment=overturning moment:

Over the turning moment, there is a moment caused by

the weight of the soil over the slip surface and shear

resistance moment caused by shear strength in the slip

surface.

Barnes (2000) shows that in such a case, safety

factor will be:

F cuR2h

Wd: 32.3 Effective stress analysis

Bishop and Morgenstern (1960) found out that there

existed a relationship between the safety factor and

pore pressure ratio ru:

Fig. 1 Type of mass movement (Skempton and Hutchinson 1969)

Fig. 2 Plane translational slide (Barnes 2000)

Geotech Geol Eng (2012) 30:277288 279

123

F m nru 4where m and n are termed as stability coefficients.

This method was used to be applied until Barnes

found out that there was a relationship between safety

factor and /0 (Barnes 2000):

F a btg/0 5where a and b are stability coefficients of the slope.

Coefficient a refers to (hw/H), and b to both

(hw/H) and (c0/cH), where H is the slope height.

3 The Influence of Vegetation on the Slope

Segment Stability

Greenway (1987) presented the hydromechanical

influence on slope stability as shown in Fig. 4.

Then, Coppin and Richard (1990) formulated the

main effects of vegetation on slope segment stability

(Fig. 5).

They gave the following formula for the calculation

of safety factor:

F c0 c0R cZ cwhw W cos2 bT sinh

tg/0 T cosh cZ W sinbD cosb6

where c = unit weight of soil (kN/m3), Z = verticalheight of soil above the slip plane (m), b = slopeangle (), cw = unit weight of water (9.81 kN/m3),hw = vertical height of groundwater table above the

slip plane (m), cR0 = enhanced effective soil cohesion

due to root matrix reinforcement by vegetation along

slip surface (kN/m2), c0 = enhanced effective soilcohesion due to soil suction due to evaporation by

vegetation along slip surface (kN/m2), W = surcharge

due to weight of vegetation (kN/m), D = wind

loading force parallel to slope (kN/m), T = tensile

root force acting at base of slice (kN/m).

4 Mechanism of Root Anchorage in the Soil Slope

Vegetation affects both the superficial and mass

stability of slopes significantly (Gray 1995). Soil and

roots show some similarities with respect to structure

and ductile reaction to strain. Both of these elements

deform to a great extent before they break. Their

retaining capacity is not lost during deflection and

subsidence of the relevant slope.

The shear strength function is defined in the stress

diagram by Mohr as the envelope of the circles of

rupture at different stressstrain states. This method

shows obviously that the common simplification of the

function by a straight line is only valid in case of small

extents of surcharge. The depth of the soil covered by

roots is usually not deeper than 1, 5 or 2 metres. On the

soil surface, there is no surcharge and the stresses are

not considerable as compared to deeper layers. The

respective values are close to the values in the stress

diagram mentioned above. The envelope is not a linear

function of the shear parameters / and c, which areparameters used to simplify calculation, but do not

effectively describe the quality of the material.

Tobias (1995) described data analysis with the

superposition of passive stress state, where it was

shown that shear strength in the root layer was 955%

higher than the underneath depending on the type of

the plant. Using a basic model for soilroot interac-

tion, Gray and Leiser (1980) discussed that shear

strength increases in the reinforced soil by roots. The

angle of the roots being 90 to the shear surface, theshear strength is contributed by root reinforcement,

and Sr (limit equilibrium) requires that (Fig. 6a):

Fig. 3 Circular arc analysis(Barnes 2000)

280 Geotech Geol Eng (2012) 30:277288

123

Sr T cos a sin atg/ =A 7Sr Ty Tz tg/

A 7a

where T = tensile force in root reinforcement, a =inclination of T, A = area of the section under

consideration and / = angle of internal friction of soil.

When written in terms of stress (rr), Eq. 7 becomes:

Sr rrAr cos a sin atg/ =A 7bwhere Ar = area of reinforcement.

Gray and Ohashi (1983) show that for 48\a\72,Eq. 7b is applicable and cosa ? sinatg/ & 1.2.

Fig. 4 Hydromechanicalinfluence on the slope

stability (Greenway 1987)

Geotech Geol Eng (2012) 30:277288 281

123

The simplest way is to assume that the root and soil

will be deformed together or it will have no effect on

the shear deformation, where a is determined by theshear stress in the soil (Fig. 6b). In this case, Eqs. 7, 7a

and 7b would still be valid, provided that the correct

value of T and rr is used.Abe and Ziemers (1991) experiment with the

reinforced wall showed that by increasing the bending

stiffness, the thickness of the shear zone increases and

reinforcement no longer deforms with the soil. To

consider the deformation and bending resistance on

the reinforcement, Oden and Ripperger (1981) used

the following equation for the tie (Fig. 6c):

EI d4u

dz4 Tz d2u

dz2

q 8where E and I = Youngs modulus and moment of

inertia of the root reinforcement, q = soil reaction and

u = displacement. This equation can be simplified to

flexible cable if gL = 2.5, where g = (Tz/EI) and

L = length of the tie (deformed portion of root

reinforcement) in this case:

Tz 0 T L 8aTy 0 qyL 8bu 0 qyL22Tz 0 : 8cThe amount of T is limited by ultimate tension. For the

roots perpendicular to the slope, small amounts of u,

a ? 90 or Tz ? 0 can be used, which representsinitial failure when the root yields. If the root is ductile

and does not fracture, u and T increase continuously

until the cable solution is applicable.

In addition, deep woody root is more effective in

preventing shallow mass stability failures. Roots

mechanically reinforce soil transferring shear stress in

Fig. 5 Major influence ofvegetation on slope segment

stability (Coppin and

Richards 1990)

Fig. 6 Simple models; a limit equilibrium; b flexible rein-forcement; c cable model (Tobias 1995)

282 Geotech Geol Eng (2012) 30:277288

123

the soil tensile resistance in the roots. Meanwhile,

anchored and embedded stems can act as buttress piles

or arch abutment to counteract down slope shear force.

What is more, the weight of vegetation can (in certain

instances) increase the stability via increased confin-

ing (normal) stress on the failure surface (Gray and

Sotir 1996). On the other hand, the roots provide better

connection between particles in the soil body (tensile

force on the surface), which results in some cemen-

tation forces of the mass of the soil.

However, a dense herbaceous cover is one of the

best protections against superficial rainfall and wind

erosion. Soil looses due to rainfall erosion can be

decreased a hundred-fold (Johansson 2000), maintain-

ing a dense herbaceous cover. This protection has a

significant role both in soil moisture extraction by

evapotranspiration process and in rain drop intercep-

tion by foliage. Foliage and plant residues absorb the

rainfall energy and prevent soil detachment by rain-

drop splash. Also, the root systems physically bind or

restrain the soil particles while the above-ground

portions filter the sediment out of run-off; therefore,

the stems and foliage increase the surface roughness

and slow velocity of the run-off. Plants and their

residues help to maintain soil porosity and permeabil-

ity, thereby delaying the onset of run-off.

Gray and Sotir (1996) described computed soil loss

(e.g. tons) per acre for a given storm. The time interval

(A) can be obtained by examining the Universal Soil

Loss Equation (USLE):

A R K LS C P 9where R = climatic factor, K = soil erodibility value,

LS = topographic factor, C = vegetation factor and

P = erosion control practice factor.

USLE equation provides a method of estimating the

soil losses and range of variability of each of the

parameters in order to change, manage or limit the soil

losses. Furthermore, Brenner (1973) showed that

evapotranspiration by vegetation can reduce pore

water pressures within the soil mantle on the natural

slopes, promoting the stability.

5 Effect of Vegetation on Slope Stability Via

the Effect on Soil Characteristics

As Lu described (2006), particle-scale equilibrium

analyses are employed to distinguish three types of

interparticle forces: (1) active forces transmitted

through the soil grain; (2) active forces at or near

interparticle contacts; and (3) passive, or counterbal-

ancing, forces at or near interparticle contacts. The

second type of force includes physicochemical forces,

cementation forces, surface tension forces and the

force arising from negative pore water pressure; all

these forces can be conceptually combined into a

macroscopic stress called suction stress.

Terzaghi (1943) in saturated soil showed that:

r0 r uw 10where r0 = effective stress, r = total stress anduw = pore water pressure.

On the other hand, Coulomb equation for shear

strength in saturated soil is:

s c0 r0tg/0 11where c0 = effective cohesion impact and /0 = effec-tive angle of internal friction.

With the replacement of r0 from Eq. 10 to 11, wewill have:

s c0 r uw tg/0: 12On the other hand, Skempton (1960) showed:

r0 r 1 cs=c uw 13where cs = compressibility of the grain and c = com-

pressibility of the granular skeleton.

As shown above, uw is present in both of the

equations, which caused capillary force in the

soil moisture. This force in macroscopic engi-

neering behaviour of the soil can be apparent by

the associated increase in shear and tensile

strength.

Bishop (1959) added one parameter to the Taraghis

equation:

r0 r ua x ua uw 14where (r - ua) is net normal stress, (ua - uw) ismatric suction and x is effective stress parameter

(considered to vary between zero and unity).

Jennings and Burland (1962) stated that mechanical

parameter in unsaturated soil is affected differently by

changes in the net normal stress than by matric

suction. In other words, increase in matric suction

results in increase in shear strength, which we describe

as /b.

Geotech Geol Eng (2012) 30:277288 283

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As Fredlund and Morgensterns (1978) indepen-

dent stress variable approach incorporates Eq. 12 and

MohrCoulomb circle, shear strength (s) would be:

s c0 r ua x ua uw tg/0: 15Fredlund and Morgenstern (1978) found out that the

effect of change in the total normal stress can be

separated from the effect of change in pore water

pressure as shown below:

s c0 r ua tg/0 ua uw tg/b 16where /b indicates the angle for the rate of increase inshear strength related to soil matric suction.

When matric suction (ua - uw) reaches zero (in

saturated soil), Eq. 16 will become Eq. 12.

They illustrated MohrCoulomb circles in a three-

dimensional manner in the case of unsaturated soil in

Fig. 7. In this model, they described the shear stress sas the ordinate and (r - ua) and (ua - uw) asabscissas. Since pore-air pressure replacing with

pore-water pressure in case of saturation, (r - ua)axis changes for (r - uw).

As shown in Table 1 and Fig. 7, the value of /b ismostly less than or equal to /0.

They show that shear stress has a direct relationship

with the matric suction as illustrated in Fig. 8.

As shown in the diagram, the equation for the line is:

c c0 ua uw f tg/b 17where c = total cohesion intercept and (ua - uw)f =

matric suction on the failure plane at failure.

When unsaturated soil is saturated parallel to the

saturation process, c is decreasing as shown in Fig. 9.

The cohesion inspects C1, C2 and C3, like total

cohesion, have a direct relationship with the matric

suction.

With the substitution of Eq. 12 for Eq. 11, the shear

strength (sff) will be:

sff c r ua f tg/0: 18Faisal et al. (2006a) announced that the soil water

characteristic curve is another important relation-

ship for unsaturated soil. SWCC is the relationship

between soil water content and matric suction. In

this research, they found out that increase in matric

suction in the unsaturated soil produces the same

increase in the shear strength as does an increase in

net normal stress; the increase in shear strength

with respect to matric suction becomes less than

the increase with respect to the net normal stress.

In fact, in this research, it was shown that the

stress state in an unsaturated soil can be repre-

sented by two independent stress tensors as

(Eqs. 19, 20):

ox ua sxy sxzsyx oy ua syzsxz szy oz ua

19

ua uw 0 00 ua uw 00 0 ua uw

: 20

Fig. 7 Extended MohrCoulomb failure envelope

for unsaturated soil

(Fredlund and Morgenstern

1978)

284 Geotech Geol Eng (2012) 30:277288

123

These researchers found that increasing the matric

suction causes an increase in the shear strength;

however, this increase is not the result of increase in

/0. On the other hand, they found almost the same/0 for different matric suctions.

Matyas and Radhakrishna (1968) presented the

volume change in a three-dimensional surface with

respect to the state parameters (ua - uw) and (r - ua).Anderson (1991) in his model for slope/hydrology

stability used the effect of increasing the water table in

Table 1 Experimental value of /b (Fredlund and Morgenstern 1978)

Soil type c0 (kPa) /0 () /b () Test procedure Reference

Compacted shale; w = 18.6% 15.8 24.8 18.1 Constant water content triaxial Bishop and Morgenstern (1960)

Boulder clay: w = 11.6% 9.6 27.3 21.7 Constant water content triaxial Bishop and Morgenstern (1960)

Dhanauri clay; w = 22.2%,qd = 1,580 kg/m

337.3 28.5 16.2 Consolidated drained triaxial Satija (1978)


320.3 29.0 12.6 Constant drained triaxial Satija (1978)


315.5 28.5 22.6 Consolidated water content

triaxial

Satija (1978)

Dhanauri clay: w = 22.2%,qd = 1,478 kg/m

311.3 29.0 16.5 Constant water content triaxial Satija (1978)

Madrid grey clay; w = 29%, 23.7 22.5a 16.1 Consolidated drained directshear

Escario (1980)

Undisturbed decomposed granite;

Hong Kong

28.9 33.4 15.3 Consolidated drained

multistage triaxial

Ho and Fredlund (1982a)

Undisturbed decomposed rhyolite;

Hong Kong


multistage triaxial

Ho and Fredlund (1982a)

Tappen-Notch hill silt;

w = 21.5%, qd = 1,590 kg/m3


multistage triaxial

Krahn el al. (1989)

Compacted glacial till;

w = 12.2%, qd = 1,810 kg/m3

10 25.3 725.5 Consolidated drained

multistage direct shear

Gan et al. (1988)

a Average value

Fig. 8 Line of interceptsalong the failure plan on the

s versus (ua - uw) plane(Fredlund and Morgenstern

1978)

Geotech Geol Eng (2012) 30:277288 285

123

tropical region due to infiltration, but he ignored the

increase in soil strength through suction effect

(Anderson and Lloyd 1991).

Faisal et al. (2006b) with the same scheme as above

simulated a change in the dynamic/hydrological

condition due to rainfall and discussed the responsi-

bility of pure water pressure change (negative and

positive) in the slope stability analysis. They showed

that in the tropical regions, the soils involved are often

residual soils and have deep water tables. The surface

soils have negative pore water pressures that play a

significant role in the stability of the slope.

Because of heavy rain during the rainy season in

this region, water table can be changed in a short

period of time, leading to slope instability (result of

wet and dry cycle). But mostly in the slope stability

analysis, suction stress was ignored. In this study, it

was shown that for a given rainfall intensity

qs = 1 9 10-6 m/s, the factor of safety of the slope

tended to decrease with the increase in permeability

(ks) of the soil. The factor of safety of the slope also

reduced with increase in the slope height. Also, it was

discussed that in the simple soil section, the factor of

safety has a linear relationship with rate of change in

shear strength with respect to suction stress, which is

shown below:

F f s tan /b 21where F = the factor of safety, f and s = stability

coefficients, and tan /b = the rate of change in shearstrength with respect to matric suction.

Faisal et al. (2006c) stated that vegetation in the soil

surface not only decreases the infiltration but also

changes the suction value.

They also found out that soil without surface cover

appears to have higher infiltration rate compared with

the soil covered with grass. It appears that the presence

of grass encourages more water pounding. Besides, the

root system also helps in increasing the rate of water

infiltration. Suction monitoring in this study shows

that the suction values at steady state for model with

grass as its surface cover are generally marginally

lower. This may be due to the effect of roots that

formed abnormal water passage for the water to

infiltrate.

6 The Impact of Pulling-Out on the Slope

As shown in Fig. 10, shear stress along the slope is

converted into the pulling-out force at the end of the

slope (plane area). The roots in this area show some

resistance against this kind of force, as shown

below.

This kind of resistance plays a key role in slope

stability as the root is protecting the soil at the end of

the slope against the pulling-out force. Roots show

some kind of resistance against the slope failure by

decreasing shear stress along the slope. The mecha-

nism of this effect fixes the plain part of the end of the

slope by increasing the pulling-out resistance in this

part.

Fig. 9 Horizontalprojection of contour lines

of the failure envelope onto

the s versus (r - ua)(Fredlund and Morgenstern

1978)

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7 Conclusion

The present study addresses the mechanism of roots

anchorage in soil slope. It is a priority because of the

effect of vegetation roots on the hazards of land

sliding, especially in subtropical and tropical areas

with dense herb coverage.

Since 1973, over 500 papers have been published

on the effect of root anchorage on slope stability. The

basic mechanisms to deal with the problem are

hydrological and mechanical mechanisms (Fig. 4).

The major influence of vegetation on slope segment

stability is shown in Fig. 5.

The following are the mechanical mechanisms of

root anchorage in slope stability:

Direct shear stress carried by the root (Fig. 6),

prevention of shallow mass stability failure, preven-

tion of pulling-out at the end of slope and effect of

vegetation on slope stability via the effect on soil

characteristics such as matric suction, effective stress

pore water pressure and cohesion intercept.

Acknowledgments The authors are really grateful toUniversity Malaya and Professor N. Shokrpour from Shiraz

University of Medical sciences, Iran, for editing the manuscript

for English.

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Contribution of the Root to Slope StabilityAbstractIntroductionSlope StabilityPlane Translational SlideCircular Arc AnalysisEffective stress analysis

The Influence of Vegetation on the Slope Segment StabilityMechanism of Root Anchorage in the Soil SlopeEffect of Vegetation on Slope Stability Via the Effect on Soil CharacteristicsThe Impact of Pulling-Out on the SlopeConclusionAcknowledgmentsReferences

Roots Anchorage Stability

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