The microscopic fabric characteristics of biaxial compression test of typical argillaceous siltstone in Badong Formation

Based on the indoor biaxial compression test of argillaceous siltstone, a numerical model of PFC_2D particle flow was established. The model considered the particle shape of the argillaceous siltstone samples. Five typical particle shapes were selected according to the scanning electron microscope, and the circular particles were selected. A stable numerical sample of a given porosity was randomly generated together. The parallel bond contact model was selected between the particles, and the elastic modulus, Poisson's ratio and peak stress were selected to calibrate the meso-parameters of the saturated and natural argillaceous siltstone samples under different confining pressures, and then the biaxial compression test was carried out. The simulation analysis of the distribution characteristics and evolution of the meso-structure parameters such as particle normal contact force, tangential contact force, coordination number and porosity during the biaxial compression test were analyzed. The test results show that the meso-parameter calibration of the biaxial compression test can ignore the influence of the shear strength indicators c and ϕ. Before and after the failure, the normal contact force and the tangential contact force existed between the particles in the statistical range of the sample in all directions. The presence of confining pressure affects the initial coordination number of the sample, the porosity and the stable coordination number and porosity after the sample is destroyed, and has little effect on the rate of change of coordination number and porosity. The evolution of the spatial porosity of the sample reflects the change of the internal structure when the sample is destroyed to a certain extent, which can reflect the failure mode of the sample more intuitively.