A Laboratory Simulation Experiment to Assess Permeability and Shear Strength of a Gravel Soil Colluvium

Landslides are caused by rainfall as one of the main factors. In order to study the effect of rainfall on the physical and mechanical parameters of landslides, a physical model of the colluvium landslide is created in laboratory conditions with silty clay, river sand, and gravel, taking Shuping landslide in the Three Gorges Reservoir area as the prototype. The artificial rainfall is applied to the accumulation model, which is steady for 60 h, and then the gravel soil is taken out along the different elevations of the colluvium for the permeability test and direct shear test, and the evolution law of changes in porosity, the permeability coefficient, and the shear strength parameters along the elevation are studied. Combined with XRF and NMR tests, the spatial variation of the permeability coefficient and shear strength parameters is discussed from the perspective of chemical elements, minerals content, and porosity, and the stability analysis of a colluvium landslide is carried out considering the influence of parameters along the elevation. The results show that under the action of rainfall seepage, the fine particles of clay are transported from upslope to downslope, resulting in more and more fine particles of clay at the toe slope. The original pores are gradually filled, the cementation between particles is stronger, the corresponding cohesion is increased, and the permeability coefficient is reduced. Due to the loss of fine particles at the upslope, the relative content of coarse particles increases, leading to an increase in the internal friction angle. The variability of the slope’s physical and mechanical parameters is a result of the spatial transport of clay particles in the colluvium caused by the rainfall seepage above. Specifically, the permeability coefficient and internal friction angle from upslope to downslope decrease linearly under the action of rainfall, but the law of the cohesion increases linearly. The upslope’s permeability coefficient and internal friction angle decrease by 11% and 8% compared to those of the downslope, while the cohesion increases by 168%. The results of FLAC^3D numerical calculation of Shuping landslide show that the maximum deformation in the X direction of 145 m and 175 m water level increases by 12% and 42%, and the safety factor decreases by 0.63% and 5% under the combined action of rainfall and the reservoir water level, that is, when considering the variation of parameters along the elevation of the landslide. The research findings provide a better understanding of the spatial parameters in similar colluvium bodies under rainfall action.