Effect of Vegetation on Slope Stability

Effect of phytology on tend Stability5.1 IntroductionIncorporating the phytology effect in set up perceptual constancy has been used for legion(predicate) years in geotechnical engineering. The botany effect on tip stability usually ignored in conventional angle abbreviation and it is considered as a minor effectuate. Although the vegetation effect on slopes qualitatively appreciated after the pioneer quantitative research. The vegetation c everywhere is recognized in urban environs and it is generally utilized along transportation corridors such as highways and railway, river channels, canals, mine waste slopes and artificially make sloping ground.There ar some remedial techniques for smear stabilizations in civil engineering put 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 orders such as using the vegetation allot or soil bioengineering in civil eng ineering applications. This method reduces the cost and local labour force and it is environmental friendly method.The vegetation cover, the solutions draw forbidden moisture from soil slopes through with(predicate) evapo-transpitation leads to shrinking and swelling in soil. After prolonged wet and change period, it is possible to foam cracks at dry period due to reduction of moisture sate from vegetation covers.5.2 Influence of vegetationThe vegetation effect influence on soil slopes, generally classified into deuce 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 matriculation suction. Water is removed from the soil realm in several ways, either evaporation from the ground line up 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 a ncestry responsible for physical interaction with soil structure5.2.1 Hydrological effectsThe influence of vegetation cover in soil moisture content in different ways. The rain water evaporates spinal column to atmosphere ultimately reduce the amount of water infiltrate into the soil slope. The vegetation roots unpack 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 development the matrix in unsaturated soil or return 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 cistron, it is generally considered in vegetation effects on slope compend, 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 downstairs different vegetation covers. This result shows the of import effects of vegetation hydrological effects are soil structure.5.2.2 Mechanical effectsThe vegetations root matrix arranging with high tensile strength can increase the soil confining stress. The soils root reinforcement is depict with roots tensile test and adhesional properties. The additional shear strength of soil is turnn by the shew root bound together with the soil mass by providing additional apparent ro fadess 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 undecomposed or adverse condition depending of the location on soil slope. If the trees located slope toenail, the slope stability will be improved due to additional vertical load. On the other hand, if th e trees located at speed prove of the slope, hence overall stability reduced due to vertical down slope forceFurthermore, the wind freight rate 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 solitary(prenominal) larger trees.5.3 flora effects on soil slope numeric translateIn this parametric study, the effect of vegetation on the stability of slope has been investigated using the SLOPE/W software tool. In this study further consider the parameter root glueyness known as apparent root cohesiveness (CR). This coefficient incorporated with Mohr-Coulomb equation.5.3.1 Model geometry20 m10 m20 m10 m20 m foresee 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 analysis, as shown in enrol 5.1. The soil properties are as follows5.3.2 Vegetation covers arrangement for the numerical model casingSlope geometryDescription01No vegetation cover021 m height vegetation cover- inviolate 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 old researchersSourceVegetation, soil type and location commencement cohesion c v (kN/m2)Grass and ShrubsWu (1984)Sphagnum moss (Sphagnum cymbifolium), Alaska, the States3.5 7.0Barker in HewlettBoulder remains 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), arctic till soils, Washington, ground forces1.6 2.1Gray (1995)Reed fiber (Phragmites communis) in uniform sands, laboratory40.7Tobias (1995)Alopecurus geniculatus, smoke meadow, Zurich, Switzerland9.0Tobias (1995)Agrostis stolonifera, forage meadow, Zurich, Switzerland4.8 5.2Tobias (1995)Mixed pioneer grasses (Festuca pratensis, Festuca rubra, genus Poa pratensis), al long, 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, England 10.0 caravan Beek et al. Natural understory vegetation (Ulex parviflorus, genus 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, USA visual modality till soils under wint er fern (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 (Pinus contorta), coastal sands, California, USAYellow 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,USAShallow stony loam till soils under mixed evergreen forests, New ZealandYellow pine (Pinus ponderosa) (54 months), laboratoryHemlock (Tsuga sp.), Sitka spruce (Picea sitchensis) and yellow cedar (Thuja occidentalis), Alaska, USACryptomeria japonica (sugi) on loamy sand (Kanto loam), Ibaraki Prefecture, JapanHemlock (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 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 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 lineament 1 soil slope (no vegetation cover on it) compared with the soil slope with vegetation cover on it.Figure 5. 3 Slope misadventure plane through slope orbit5.3.4 Vegetation layer entire surfaceThe theme 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 nerve 115001.568 bailiwick 215111.57115211.57515311.57915411.58215511.586Case 315121.57515221.58315321.59115421.59915521.605Case 415131.58015231.59315331.60515431.61815531.630Table 5. 2 Slope compendium 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 t o 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 device 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 515111.5711.56915211.5751.57215311.5791.57415411.5821.57615511.5861.57815121.5751.5721 5221.5831.57715321.5911.58115421.5981.58615521.6051.591Table 5. 3 Slope Analysis 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 ample 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. 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 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 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 ther e 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.