Numerical simulation of biaxial undrained shear test based on fluid–solid coupling DEM method

The undrained shear test plays a crucial role in comprehending the deformation and failure mechanisms inherent in rock and soil. While the test offers a macroscopic perspective, numerical simulations can delve into the microscopic aspects of sample behavior, thereby compensating for this limitation. Drawing inspiration from the measurement sphere fluid–solid coupling method and the discrete element method, this paper proposes a tailored numerical simulation approach for the biaxial undrained shear test. This approach is implemented in the high-performance discrete element software MatDEM. In this study, a rapid division method for the fluid network is presented, along with a detailed explanation of the control equation governing excess pore water pressure and fluid–solid interaction during shear. Subsequently, A custom flexible boundary with a relatively smooth surface is introduced to simulate the constraint of the rubber membrane, which helps reduce simulation errors. Furthermore, the specific process of the numerical simulation is introduced, and an effective method for generating samples with high void ratios is proposed. Finally, three different cases are presented, involving varying confining pressures, different void ratios, and specific shear paths, respectively. The results indicate that the proposed method can effectively simulate the process of undrained shear tests and accurately capture the excess pore water pressure responses, and lateral deformation of the samples during shearing.