Study On Unsaturated Shear Strength Of Loess Based On Tensile And Double Side Pure Shear Tests

The loess area has a thick soil layer,a deep groundwater level,and most of the loess is unsaturated.During the formation of unsaturated loess landslides,tensile cracks are first generated on the trailing edge,followed by shear failure of the slip surface.It can be seen that the tensile strength and shear strength of unsaturated loess are of great significance for evaluating the stability of unsaturated loess slopes.For slope stability evaluation,the expression of shear mechanism requires shear strength model and strength parameters.At present,the commonly used model is the Mohr-Coulomb model,which cannot reflect the tensile characteristics,and there are few studies on the theory of joint strength of loess,which can judge both tensile failure and shear failure.This paper relies on the National Natural Science Foundation of China project"Reliability analysis of slopes within the irrigation area in loess plateau and robust decision-making for the failure prevention（41877242）".The L₅ loess and S₅ paleosoil on the south bank slope of the Jing River were used as the research objects.The shear strength under different normal stresses was obtained by double-side pure shear test and slow shear test,the shear strength parameters obtained by different fitting methods were analyzed.The shear strength model of unsaturated loess was analyzed by experimentally obtaining shear strength parameters and matrix suction of the soil sample at the end of shearing under different normal stresses.At the same time,the tensile strength obtained by the loess tensile test was carried out,and the joint strength model of unsaturated loess was established.The following conclusions were mainly obtained.（1）A model for estimating pure cohesion using matrix suction was establishedThe pure cohesion of unsaturated loess was obtained by double-side pure shear test.The slow shear test was used to test the shear strength of different moisture content and different normal stress,and it was found that the cohesion obtained based on Mohr-Coulomb criterion was 7.1%～36.7%lower than that obtained by double-side pure shear,and the lower the moisture content,the greater the gap.The matrix suction after slow shear test under various stresses was tested by filter paper method,the soil-water characteristic curve was determined by Fredlund and Xing model,and a model for calculating pure cohesion by matrix suction was established.（2）Based on the strength theory of Mohr-Coulomb,an unsaturated loess shear strength model considering pure cohesion was establishedFixed cohesion is the pure cohesion obtained by the double-side pure shear test,and the shear strength parameter considering pure cohesion is obtained by the Mohr-Coulomb criterion.Based on the shear strength model of Khalili unsaturated soil,the shear strength model of unsaturated loess considering pure cohesion was established,and compared with the shear strength model of Khalili unsaturated soil,the unsaturated loess shear strength model considering pure cohesion has better applicability to unsaturated loess.The effectiveness of the unsaturated loess shear strength model was verified by unsaturated direct shear test,and the feasibility of unsaturated loess shear strength was determined by combining conventional direct shear experiment and filter paper method to test the matrix suction.（3）A quadratic parabolic joint strength model of loess considering tensile stress was establishedThe tensile strength of unsaturated loess was obtained by loess tensile test,and the tensile strength and shear strength ofτ-σplane were fitted by quadratic parabolic joint strength theory.It is found that the coefficient n to be determined in the model is a power function of the matrix suction under zero stress,and the tensile strength has a linear relationship with the logarithm of the matrix suction under zero stress.Through normalization,a quadratic parabolic joint strength model was established to estimate shear strength and tensile strength by matrix suction.（4）The strength curves of the quadratic parabolic joint strength model and the shear strength model based on Mohr-Coulomb strength theory were compared and analyzedCompared with the shear strength model based on the Mohr-Coulomb strength theory,the quadratic parabolic joint strength model can obtain the tensile and shear strength of unsaturated loess without testing the cohesion and internal friction angle.In the shear failure stage,it is safer to predict the shear strength of the slope in the natural state with the quadratic parabolic joint strength model.For stability analysis of rainfall-induced shallow landslide,it is safer to use the shear strength predicted by the shear strength model based on the Mohr-Coulomb strength theory.