Hydraulic Anisotropy Of Unsaturated Soils And Its Effect On Stability Of Rainfall Landslides

Soils are often anisotropic due to deposition or compaction.As an important property of soil,the anisotropy plays an important role in many geotechnical engineering problems.Hydraulic anisotropy is generally characterized by permeability anisotropy coefficient,which refers to the ratio of permeability coefficient when water migrates in parallel to and vertical to the compacted surface of soil.The permeability coefficient of unsaturated soil is a function of suction,so the permeability anisotropy coefficient of unsaturated soil may also be a function of suction.At present,there is a lack of systematic experimental analysis of unsaturated soil hydraulic anisotropy.In this paper,horizontal and vertical soil samples were prepared by sampling in parallel and vertical directions along the compacted surface of soil,and their unsaturated hydraulic characteristics were measured mainly by simplified evaporation test.At the same time,the influence of clay content and compaction degree on hydraulic anisotropy was analyzed.According to the measurements,with the help of Comsol finite element software,the stability of shallow homogeneous slope considering unsaturated hydraulic anisotropy under the condition of rainfall infiltration is analyzed by finite element numerical simulation.The experimental results show that the saturated permeability anisotropy increases with the increase of compaction degree,and increases first and then decreases with the increase of clay content.The suction direction of horizontal soil sample and vertical soil sample is different,but the soil-water characteristic curves of the two samples are almost the same.When clay content is 30% and 70%,the permeability anisotropy coefficient of sand-clay mixture decreases with the increase of suction,and the permeability anisotropy coefficient tends to 1.When the clay content is 50% and 100%,the permeability anisotropy coefficient of soil has little change with the increase of suction,but fluctuates around the saturated permeability anisotropy coefficient.It can be considered that the permeability anisotropy coefficient is independent of suction.Compaction degree has no effect on the relationship between permeability anisotropy coefficient and suction.The numerical analysis results show that when the vertical permeability coefficient of slope remains unchanged,the greater the horizontal permeability coefficient,the smaller the suction and safety coefficient at the same rainfall time and at the same height.The coefficient of permeability anisotropy decreases with the increase of suction,and the pore water pressure and safety factor are between the slope with isotropic permeability and the slope with constant permeability anisotropy coefficient.When the permeability anisotropy coefficient increases with the increase of suction,the suction and safety coefficient at the same rainfall time and at the same height are smaller than those of the slope with the constant permeability anisotropy coefficient.At this time,neglecting the permeability anisotropy coefficient with the change of suction will overestimate the safety factor of the slope.When the rainfall intensity is less than the saturated permeability coefficient of soil,the permeability anisotropy has little influence on the landslide stability.