Effect of Vegetation on Slope Stability

Effect of phytology on cant over Stability5.1 IntroductionIncorporating the plant life effect in angle stability has been utilize for many years in geotechnical engineering. The plant effect on careen stability usually ignored in conventional slope analysis and it is considered as a minor effects. Although the flora 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.There are about remedial techniques for tarnish stabilizations in civil engineering practice such as geosynthetic backing or landed estate 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 discolouration bioengineering in civil engineering app lications. This method reduces the cost and local labour force and it is environmental friendly method.The vegetation cover, the bag hightail it out wet from soil slopes through evapo-transpitation leads to shrinking and swelling in soil. After prolonged wet and run dry period, it is possible to foam cracks at dry period due to reduction of moisture content from vegetation covers.5.2 Influence of vegetationThe 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 part 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 starting time responsible for physical interact ion with soil structure5.2.1 Hydrological effectsThe 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 add 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 vegetations 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 effectsThe vegetations root matrix system with high tensile strength can summation the soil confining stress. The soils root reinforcement is described with roots 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 gluiness of the soil.The slope contain large trees need to consider the weight of the tree. The additional surcharge to the slope whitethorn give from large 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 position on soil slope. If the trees primed(p) slope toenailnail, the slope stability will be improved due to additional vertical load. On the other hand, if the trees located at top(prenominal) surface of the slope , hence overall stability reduced due to vertical down slope forceFurthermore, 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 botany effects on soil slope numerical studyIn this parametric study, the effect of vegetation on the stability of slope has been investigated using the list/W software tool. In this study only consider the parameter root cohesiveness known as apparent root cohesion (CR). This coefficient coordinated with Mohr-Coulomb equation.5.3.1 Model geometry20 m10 m20 m10 m20 m physical body 5. 1 Slope geometry20 kN/m3c = 15 kPa20In this parametric study 10 m height 21 homogenous slope (26.57) is used to investigate the vegetation effect on stability an alysis, as shown in Figure 5.1. The soil properties are as follows5.3.2 Vegetation covers arrangement for the numerical model topicSlope geometryDescription01No vegetation cover021 m height vegetation cover-entire ground surfacecohesion 1 kPa to 5 kPa032 m height vegetation cover-entire ground surfacecohesion 1 kPa to 5 kPa043 m height vegetation cover-entire ground surfacecohesion 1 kPa to 5 kPa05vegetation cover only at the slope surface06vegetation cover only at the slope surface and upper surfaceFigure 5. 2 Vegetation covers arrangement for the numerical model5.3.3 The root cohesion values from previous researchersSourceVegetation, soil type and reparationRoot cohesion c v (kN/m2)Grass and ShrubsWu (1984)Sphagnum moss (Sphagnum cymbifolium), Alaska, USA3.5 7.0Barker in HewlettBoulder clay fill (dam embankment) under grass in concrete block reinforced3.0 5.0et al. (1987)cellular spillways, Jackhouse Reservoir, UKBuchanan Savigny * (1990)Understorey vegetation (Alnus, Tsuga, Carex, Polystichum), glacial till soils, Washington, USA1.6 2.1 white-haired(a) (1995)Reed fiber (Phragmites communis) in uniform sands, laboratory40.7Tobias (1995)genus Alopecurus geniculatus, forage meadow, Zurich, Switzerland9.0Tobias (1995)Agrostis stolonifera, forage meadow, Zurich, Switzerland4.8 5.2Tobias (1995)Mixed pioneer grasses (Festuca pratensis, Festuca rubra, Poa pratensis), alpine, Reschenpass, Switzerland13.4Tobias (1995)Poa pratensis (monoculture), Switzerland7.5Tobias (1995)Mixed grasses (Lolium multiflorum, Agrostis stolonifera, Poa annua), forage meadow, Zurich, Switzerland-0.6 2.9Cazzuffi et al. (2006)Elygrass (Elytrigia elongata), Eragrass (Eragrostis curvala), Pangrass (Panicum virgatum), Vetiver (Vetiveria zizanioides), clayey-sandy soil of Plio-Pleistocene age, Altomonto, S. Italy10.0, 2.0, 4.0, 15.0Norris (2005b)Mixed grasses on London Clay embankment, M25, England10.0van Beek et al. Natural understory vegetation (Ulex parviflorus, Crataegus monogyna ,0.5 6.3(2005)Brachypodium var.) on hill slopes, Almudaina, Spainvan Beek et al. (2005)Vetiveria zizanoides, terraced hill slope, Almudaina, Spain7.5Deciduous and Coniferous treesEndo Tsuruta (1969) OLoughlin Ziemer (1982) Riestenberg Sovonick-Dunford * (1983) Schmidt et al. (2001) Swanston* (1970) OLoughlin* (1974)Ziemer Swanston (1977)Burroughs Thomas* (1977) Wu et al. (1979)Ziemer (1981) Waldron Dakessian*(1981) Gray Megahan (1981) OLoughlin 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, JapanBeech (Fagus sp.), forest-soil, New ZealandBouldery, silty clay colluvium under sugar maple (Acer saccharum) forest, Ohio, USAIndustrial deciduous forest, colluvial soil (sandy loam), Oregon, USAMountain till soils under hemlock tree (Tsuga mertensiana) and spruce (Picea sitchensis), Alaska, USAMountain till soils under conifers (Pseudotsuga menziesii), British Columbia, CanadaSitka spruce (Picea sitchensis) western hemlock (Tsuga heterophylla), Alaska, USAMountain and hill soils under coastal Douglas-fir and Rocky Mountain Douglas-fir (Pseudotsuga menziesii), West Oregon and Idaho, USAMountain till soils under cedar (Thuja plicata), hemlock (Tsuga mertensiana) and spruce (Picea sitchensis), Alaska, USALodgepole pine (genus Pinus contorta), coastal sands, California, USAYellow pine (Pinus ponderosa) seedlings grown in small containers of clay loam.Sandy loam soils under ponderosa pine pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii) and Engelmann spruce (Picea engelmannii), Idaho,USAShallow stony loam till soils under mixed evergreen forests, New ZealandYellow pine (Pinus ponderosa) (54 months), laboratoryHemlock (Tsuga sp.), Sitka spruce (Picea sitchensis) and discolour cedar (Thuja occidentalis), Alaska, USACryptomeria japonica (sugi) on loamy sand (Kanto loam), Ibaraki Prefecture, JapanHem lock (Tsuga sp.), Douglas fir (Pseudotsuga), cedar (Thuja), glacial till soils, Washington, USAPinus contorta on coastal sandNatural coniferous forest, colluvial soil (sandy loam), OregonPinus halepensis, hill slopes, Almudaina, Spain2.0 12.06.65.76.8 23.23.4 4.41.0 3.03.5 6.03.0 17.55.93.0 21.05.0 10.33.33.7 6.45.6 12.61.0 5.02.5 3.02.325.6 94.3-0.4 18.2* Back analysis and root density information. In situ require 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 contumacious by field, laboratory tests, and mathematical modelsIn this parametric study apparent root cohesion (CR) was varied over the following range1 CR 5 kPa CR 1 kPa, 2 kPa, 3 kPa , 4 kPa , 5 kPa Three vegetation root depth zones (hR) were used namelyhR 1 m, 2 m, 3 mACBThe soil slope assumed as homogeneous slope. The case 1 soil slope (no vegetation cover on it) compared with the soil slope wi th vegetation cover on it.Figure 5. 3 Slope failure scan through slope realm5.3.4 Vegetation layer entire surfaceThe 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)FOSCase 115001.568Case 215111.57115211.57515311.57915411.58215511.586Case 315121.57515221.58315321.59115421.59915521.605Case 415131.58015231.59315331.60515431.61815531.630Table 5. 2 Slope compend 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 calculati ons. The Table 5.2 shows the factor of preventive analysis results for different root cohesion for different depths.Figure 5. 4 Different root cohesion (CR ) values for factor of safety for different root depthsThe 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 slopes factor of safety.5.3.4 Vegetation layer only at slope surface and upper surfaceC (kPa)CR (kPa)hR (kPa)FOSFOSCase 6Case 515111.5711.56915211.5751.57215311.5791.57415411.5821.57615511.5861.57815121.5751.57215221.5831.57715321.5911.58115421.5981.58615521.6051.591Table 5. 3 Slope depth psychology results for Case 6 and case 5The 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 slopes upper surface vegetation has considerable influence in slope stability.5.3.4 Vegetation layer only at toeC (kPa)CR (kPa)hR (kPa)FOSVegetation layer only at toe15111.56815211.56815311.56815411.56815511.56815121.56815221.56815321.56815421.56815521.568Table 5. 4 Slope Analysis results for Vegetation layer only at toeThe SLOPE/W analysis shows (Table 5.5) for vegetation at toe Figure 5.1 section C. e ither 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 thither 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 toeCBAFigure 5. 5 Slope failure plane through toeCR (kPa)Vegetation at toehR (kPa)FOS111.619211.624311.628411.632511.636121.621221.626321.632421.637521.642Table 5. 5 Slope Analysis results for failure plane through toe region, Vegetation layer only at toeThis slope analysis failure surface was set through slope toe using inlet 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 se ction B C Figure 5.1. So the vegetation layer influent with the slope failure surface.