Discrete element simulation of triaxial tests is an important tool for exploring the deformation and failure mechanisms of geotechnical materials such as sands. A crucial aspect of this simulation is the accurate representation of lateral boundaries. Using a coupled finite difference method (FDM)-discrete element method (DEM) approach

numerical simulations of consolidated-drained and consolidated-undrained triaxial tests were conducted under flexible lateral boundary conditions. These results were then compared with those of corresponding triaxial tests using rigid lateral boundaries. The results indicate that

compared to the rigid lateral boundary

the triaxial test using the FDM-DEM coupled flexible lateral boundary better captures both the macroscopic mechanical response and the microscopic particle kinematics of laboratory triaxial specimens. In the consolidated-drained triaxial tests

the strain softening and shear dilatancy of the specimen with the flexible lateral boundary are significantly weaker after reaching peak strength than those of the specimen with the rigid lateral boundary. In the consolidated-undrained triaxial tests

when the axial strain is large

the specimen with the flexible lateral boundary exhibits both a lower deviator stress and a smaller absolute value of negative excess pore pressure. Furthermore

in the consolidated-undrained triaxial tests

as the axial strain increases

the flexible lateral boundary provides weaker lateral constraint and support to the specimen compared to the rigid lateral boundary. Consequently

the stability of the force chains in the specimen with the flexible lateral boundary is lower

leading to more buckling events of force chains within the shear band. As a result

both the anisotropy and the deviator stress are reduced.