Study On The Instability Mechanism Of Unsaturated Loess Landslide

For the past few years, due to the unreasonable agricultural irrigation, pipe leakageduring urban construction and so on, there is an increasing number of landslide hazard occursnot only in the rainy season and is of great danger, which has been a typical landslide hazardin loess plateau in China. As a matter of fact, the theory of unsaturated soil mechanics ismore suitable for its stability estimation. The landslide induced by pipe leakage in Dongxiang,Gansu Province was taken as a representative example in this paper, which combined thelandslide invoked by irrigation in Heifangtai, Gansu Province. Moreover, with field survey,physical mechanics experiments, numerical simulation, etc, this paper studied mechanicalcharacteristics of unsaturated loess, variation of landslide seepage field and the slope stability.The main research findings are as follows:（1）The Malan Loess of Dongxiang and Heifangtai, Gansu Province was tested forSoil-Water Characteristic Curve. After analyzing the suction range of loess in study area,fitting analysis was conducted aimed at frequently-used Soil-Water Characteristic Curvemodel. As a result, it is considered that such Soil-Water Characteristic Curve models,including Gardner, Van Genuehten, Fredlund and Xing, were suitable for loess in studyarea. According to the Soil-Water Characteristic Curve model, the unsaturatedconductivity was estimated. Additionally, conductivity of drying and wetting process wascomparatively analyzed for Gardner and Van Genuehten model.（2）As for the unsaturated conductivity of loess, taking No.13landslide of Jiaotouya inHeifangtai, Gansu Province as an example, the groundwater uptrend was simulated.Additionally, unsaturated-saturated seepage field of Dongxiang landslide in GansuProvince was simulated, and we analyzed not only the variation of groundwater level infour decades owing to the irrigation, but also the influence range of one-year infiltrationinside the slope.（3）The direct shear test of matric suction was conducted for Malan Loess in Dongxiang,Gansu Province, where the matric suction was respectively50,100,150and200KPa andthe vertical load was respectively50,100,150and200KPa. As a result, the loess here wassclerosing. After analyzing unsaturated intensity, it was found that the frictional angle was constant with the increasing of matric suction. The impact of matric suction variesbetween50and100KPa. In the meanwhile, after analyzing simplified Bishop, Fredlundand Lu Ning strength theory, it is concluded that Bishop and Fredlund strength theory aremore suitable for the loess in study area（4）Through the deviatoric stress wetting unsaturated triaxial test, when axial pressure andconfining pressure are invariant, the soil was cutting indentation when shearing. The soilmight be destroyed with the decreasing of suction when deviatoric stress was less. Whendeviatoric stress was larger, the soil became strain in saturated condition. What’s more,the variation of strain and dilation were unapparent with the alteration of suction.（5）Combined with the simulation of Dongxiang landslide and Jiaoyatou seepage field inGansu Province, the slope stability was analyzed by traditional soil mechanics theory andunsaturated soil mechanics theory. The theoretical stability coefficient of unsaturated soilstrength is greater than traditional security coefficient of saturated soil, which achieves aconsistent value when the soil is saturated. The difference of conductivity has a certaineffect on slope stability during drying and wetting process in Soil-Water CharacteristicCurve.