5.1 Introduction
Incorporating the vegetation effect in slope stability has been used for many years in geotechnical engineering. The vegetation effect on slope stability usually ignored in conventional slope analysis and it is considered as a minor effects. Although the vegetation effect on slopes qualitatively appreciated after the pioneer quantitative research. The vegetation cover is recognized in urban environment and it is generally utilized along transportation corridors such as highways and railway, river channels, canals, mine waste slopes and artificially made sloping ground.

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There are some remedial techniques for soil stabilizations in civil engineering practice such as geosynthetic reinforcement or soil nailing are often used at slopes at great expense, but now many parts of the world considered sustainable alternative methods such as using the vegetation cover or soil bioengineering in civil engineering applications. This method reduces the cost and local labour force and it is environmental friendly method.
The vegetation cover, the roots draw out moisture from soil slopes through evapo-transpitation leads to shrinking and swelling in soil. After prolonged wet and dry period, it is possible to foam cracks at dry period due to reduction of moisture content from vegetation covers.
5.2 Influence of vegetation
The vegetation effect influence on soil slopes, generally classified into two types, they are mechanical and hydrological effects. The hydrological effect is responsible for soil moisture content, increasing the evapo-transpiration and resulting increasing the soil matric suction. Water is removed from the soil region in several ways, either evaporation from the ground surface or by evapo transpiration from vegetation cover. The process produces upward flux of the water out of the soil. The mechanical effects from the vegetation root responsible for physical interaction with soil structure
5.2.1 Hydrological effects
The influence of vegetation cover in soil moisture content in different ways. The rain water evaporates back to atmosphere ultimately reduce the amount of water infiltrate into the soil slope. The vegetation roots extract moisture from the soil and this effects leads to reducing the soil moisture content. The reduction in moisture content in soil, it will help to increase the matrix in unsaturated soil or decrease the pore water pressure condition in saturated soil. Both of this action ultimately improves the soil stability. The vegetation’s moisture reduction ability is well recognized. The root reinforcement is most important factor, it is generally considered in vegetation effects on slope analysis, thought the recent studies shows the importance of hydrological effects on slopes by Simon& Collision (2002). They studied the pore water pressure and matric suction in soil over for one cycle of wet and dry cycle under different vegetation covers. This result shows the significant effects of vegetation hydrological effects are soil structure.
5.2.2 Mechanical effects
The vegetation’s root matrix system with high tensile strength can increase the soil confining stress. The soil’s root reinforcement is described with root’s tensile test and adhesional properties. The additional shear strength of soil is given by the plant root bound together with the soil mass by providing additional apparent cohesion of the soil.
The slope contain large trees need to consider the weight of the tree. The additional surcharge to the slope may give from larger trees. This surcharge increases the confining stress and down slope force. The surcharge from larger trees could be beneficial or adverse condition depending of the location on soil slope. If the trees located slope toe, the slope stability will be improved due to additional vertical load. On the other hand, if the trees located at upper surface of the slope, hence overall stability reduced due to vertical down slope force
Furthermore, the wind loading to larger trees increasing the driving force acting on the slope. In the wind load is sufficiently large it may create the destabilizing moment on the soil slope from larger trees. Larger trees roots penetrate deeper strata and act as stabilizing piles. The effects of surcharge, wind loading and anchoring usually considered only larger trees.
5.3 Vegetation effects on soil slope numerical study
In this parametric study, the effect of vegetation on the stability of slope has been investigated using the SLOPE/W software tool. In this study only consider the parameter root cohesion known as apparent root cohesion (CR). This coefficient incorporated with Mohr-Coulomb equation.
5.3.1 Model geometry
20 m
10 m
20 m
10 m
20 m
Figure 5. 1 Slope geometry
20 kN/m3
c = 15 kPa
20°In this parametric study 10 m height 2:1 homogenous slope (26.57°) is used to investigate the vegetation effect on stability analysis, as shown in Figure 5.1. The soil properties are as follows:
5.3.2 Vegetation covers arrangement for the numerical model
Case
Slope geometry
Description
01
No vegetation cover
02
1 m height vegetation cover-entire ground surface
cohesion 1 kPa to 5 kPa
03
2 m height vegetation cover-entire ground surface
cohesion 1 kPa to 5 kPa
04
3 m height vegetation cover-entire ground surface
cohesion 1 kPa to 5 kPa
05
vegetation cover only at the slope surface
06
vegetation cover only at the slope surface and upper surface
Figure 5. 2 Vegetation covers arrangement for the numerical model
5.3.3 The root cohesion values from previous researchers
Source
Vegetation, soil type and location
Root cohesion c’ v (kN/m2)
Grass and Shrubs
Wu‡ (1984)
Sphagnum moss (Sphagnum cymbifolium), Alaska, USA
3.5 – 7.0
Barker in Hewlett
Boulder clay fill (dam embankment) under grass in concrete block reinforced
3.0 – 5.0
et al. † (1987)
cellular spillways, Jackhouse Reservoir, UK
Buchanan & Savigny * (1990)
Understorey vegetation (Alnus, Tsuga, Carex, Polystichum), glacial till soils, Washington, USA
1.6 – 2.1
Gray § (1995)
Reed fiber (Phragmites communis) in uniform sands, laboratory
40.7
Tobias † (1995)
Alopecurus geniculatus, forage meadow, Zurich, Switzerland
9.0
Tobias† (1995)
Agrostis stolonifera, forage meadow, Zurich, Switzerland
4.8 – 5.2
Tobias† (1995)
Mixed pioneer grasses (Festuca pratensis, Festuca rubra, Poa pratensis), alpine, Reschenpass, Switzerland
13.4
Tobias† (1995)
Poa pratensis (monoculture), Switzerland
7.5
Tobias† (1995)
Mixed grasses (Lolium multiflorum, Agrostis stolonifera, Poa annua), forage meadow, Zurich, Switzerland
-0.6 – 2.9
Cazzuffi et al. § (2006)
Elygrass (Elytrigia elongata), Eragrass (Eragrostis curvala), Pangrass (Panicum virgatum), Vetiver (Vetiveria zizanioides), clayey-sandy soil of Plio-Pleistocene age, Altomonto, S. Italy
10.0, 2.0, 4.0, 15.0
Norris† (2005b)
Mixed grasses on London Clay embankment, M25, England
~10.0
van Beek et al. †
Natural understory vegetation (Ulex parviflorus, Crataegus monogyna,
0.5 – 6.3
(2005)
Brachypodium var.) on hill slopes, Almudaina, Spain
van Beek et al. † (2005)
Vetiveria zizanoides, terraced hill slope, Almudaina, Spain
7.5
Deciduous and Coniferous trees
Endo & Tsuruta † (1969) O’Loughlin & Ziemer † (1982) Riestenberg & Sovonick-Dunford * (1983) Schmidt et al. ‡ (2001) Swanston* (1970) O’Loughlin* (1974)
Ziemer & Swanston ठ(1977)
Burroughs & Thomas* (1977) Wu et al. ‡ (1979)
Ziemer † (1981) Waldron & Dakessian*(1981) Gray & Megahan‡ (1981) O’Loughlin et al. † (1982)
Waldron et al. † (1983)
Wu ‡ (1984)
Abe & Iwamoto † (1986)
Buchanan & Savigny * (1990) Gray § (1995)
Schmidt et al. ‡ (2001)
van Beak et al. † (2005)
Silt loam soils under alder (Alnus), nursery, Japan
Beech (Fagus sp.), forest-soil, New Zealand
Bouldery, silty clay colluvium under sugar maple (Acer saccharum) forest, Ohio, USA
Industrial deciduous forest, colluvial soil (sandy loam), Oregon, USA
Mountain till soils under hemlock (Tsuga mertensiana) and spruce (Picea sitchensis), Alaska, USA
Mountain till soils under conifers (Pseudotsuga menziesii), British Columbia, Canada
Sitka spruce (Picea sitchensis) – western hemlock (Tsuga heterophylla), Alaska, USA
Mountain and hill soils under coastal Douglas-fir and Rocky Mountain Douglas-fir (Pseudotsuga menziesii), West Oregon and Idaho, USA
Mountain till soils under cedar (Thuja plicata), hemlock (Tsuga mertensiana) and spruce (Picea sitchensis), Alaska, USA
Lodgepole pine (Pinus contorta), coastal sands, California, USA
Yellow pine (Pinus ponderosa) seedlings grown in small containers of clay loam.
Sandy loam soils under Ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii) and Engelmann spruce (Picea engelmannii), Idaho,USA
Shallow stony loam till soils under mixed evergreen forests, New Zealand
Yellow pine (Pinus ponderosa) (54 months), laboratory
Hemlock (Tsuga sp.), Sitka spruce (Picea sitchensis) and yellow cedar (Thuja occidentalis), Alaska, USA
Cryptomeria japonica (sugi) on loamy sand (Kanto loam), Ibaraki Prefecture, Japan
Hemlock (Tsuga sp.), Douglas fir (Pseudotsuga), cedar (Thuja), glacial till soils, Washington, USA
Pinus contorta on coastal sand
Natural coniferous forest, colluvial soil (sandy loam), Oregon
Pinus halepensis, hill slopes, Almudaina, Spain
2.0 – 12.0
6.6
5.7
6.8 – 23.2
3.4 – 4.4
1.0 – 3.0
3.5 – 6.0
3.0 – 17.5
5.9
3.0 – 21.0
5.0
~ 10.3
3.3
3.7 – 6.4
5.6 – 12.6
1.0 – 5.0
2.5 – 3.0
2.3
25.6 – 94.3
-0.4 – 18.2
* Back analysis and root density information. † In situ direct shear tests. ‡ Root density information and vertical root model equations. & Laboratory shear tests.
Table 5. 1 Values of Cv for grasses, shrubs and trees as determined by field, laboratory tests, and mathematical models
In this parametric study apparent root cohesion (CR) was varied over the following range:
1 ≤ CR ≤ 5 kPa ; CR ∈ {1 kPa, 2 kPa, 3 kPa , 4 kPa , 5 kPa }
Three vegetation root depth zones (hR) were used namely:
hR ∈ {1 m, 2 m, 3 m}
A
C
BThe soil slope assumed as homogeneous slope. The case 1 soil slope (no vegetation cover on it) compared with the soil slope with vegetation cover on it.
Figure 5. 3 Slope failure plane through slope region
5.3.4 Vegetation layer entire surface
The case 2 condition applied the vegetation cover entire surface, the vegetation depth (hR) were 1 m and root cohesion were 1 kPa to 5 kPa. The same root cohesion applied to the case 3 and case 4 conditions.
C (kPa)
CR (kPa)
hR (kPa)
FOS
Case 1
15
0
0
1.568
Case 2
15
1
1
1.571
15
2
1
1.575
15
3
1
1.579
15
4
1
1.582
15
5
1
1.586
Case 3
15
1
2
1.575
15
2
2
1.583
15
3
2
1.591
15
4
2
1.599
15
5
2
1.605
Case 4
15
1
3
1.580
15
2
3
1.593
15
3
3
1.605
15
4
3
1.618
15
5
3
1.630
Table 5. 2 Slope Analysis results for Case 1, Case 2, Case 3 and Case 4.
Vegetation cover plays a significant role in slope stability analysis. The root cohesion experiments from various researchers over the past three decades results are shown in Table 5.1. In this research only consider the grass and shrubs root reinforcement. The apparent root cohesion range is 1 kPa to 5 kPa. If we consider the bigger trees in slopes need to consider its weight for slope stability calculations. The Table 5.2 shows the factor of safety analysis results for different root cohesion for different depths.
Figure 5. 4 Different root cohesion (CR ) values for factor of safety for different root depths
The analysis carried out with the software tool SLOPE/W. The graph shows the influence of vegetation cover i.e. root cohesion (CR) and its root depth (hR). The soil slope without any vegetation cover (CR = 0 kPa), the factor of safety is 1.570. This result shows the vegetation cover applied entire surface. The factor of safety linearly increase when increase with the root cohesion and root depth. The root cohesion and root depth has linear relationship with slope’s factor of safety.
5.3.4 Vegetation layer only at slope surface and upper surface
C (kPa)
CR (kPa)
hR (kPa)
FOS
FOS
Case 6
Case 5
15
1
1
1.571
1.569
15
2
1
1.575
1.572
15
3
1
1.579
1.574
15
4
1
1.582
1.576
15
5
1
1.586
1.578
15
1
2
1.575
1.572
15
2
2
1.583
1.577
15
3
2
1.591
1.581
15
4
2
1.598
1.586
15
5
2
1.605
1.591
Table 5. 3 Slope Analysis results for Case 6 and case 5
The vegetation layer only considered at slope surface and upper surface, analysis carried out with SLOPE/W tool. The case 6 analysis results same as the case 2 and case 3. The results not affect with toe vegetation (section C at Figure 5.3) because failure plane only at section A and B section at Figure 5.3. So only influence with slope vegetation layer and upper surface vegetation layer in this slope analysis.
The vegetation layer only at slope surface analysis results (case 6) compared with vegetation only at slope condition (case 5) shows lesser factor of safety values. The slope’s upper surface vegetation has considerable influence in slope stability.
5.3.4 Vegetation layer only at toe
C (kPa)
CR (kPa)
hR (kPa)
FOS
Vegetation layer only at toe
15
1
1
1.568
15
2
1
1.568
15
3
1
1.568
15
4
1
1.568
15
5
1
1.568
15
1
2
1.568
15
2
2
1.568
15
3
2
1.568
15
4
2
1.568
15
5
2
1.568
Table 5. 4 Slope Analysis results for Vegetation layer only at toe
The SLOPE/W analysis shows (Table 5.5) for vegetation at toe Figure 5.1 section C. All the results for different depths and different root cohesion values are the same. The failure plane of this analysis only at section A & B. So there is no influence with the toe vegetation. If the failure plane goes to section only toe vegetation influence in slope stabilization.
5.3.5 Slope failure plane through toe
C
B
A
Figure 5. 5 Slope failure plane through toe
CR (kPa)
Vegetation at toe
hR (kPa)
FOS
1
1
1.619
2
1
1.624
3
1
1.628
4
1
1.632
5
1
1.636
1
2
1.621
2
2
1.626
3
2
1.632
4
2
1.637
5
2
1.642
Table 5. 5 Slope Analysis results for failure plane through toe region, Vegetation layer only at toe
This slope analysis failure surface was set through slope toe using entry and exit method. The Figure 5.5 shows clearly the failure plane, the failure region cover the entire region (A, B & C). The vegetation layer applied at toe region for this analysis. The FOS increase with the increasing root cohesion and root depth, but there is no changes observed from the previous analysis, which is the failure plane only at section B & C Figure 5.1. So the vegetation layer influent with the slope failure surface.