Response of granular material under combined principal stress value and orientation change in 3D space
Laboratory tests on soil adopt simplified stress paths compared to real world counterparts due to mechanical limitations. This study investigates the deformation of granular material under combined principal stress value and orientation change in full 3D space using the discrete element method. Such stress paths are achieved by applying a 3D force line boundary condition on spherical granular material samples. Continuous cyclic tests with stress paths restricted in a fixed plane and in full 3D space, simulating a bidirectional seismic stress path, are both conducted. The importance of taking both principal stress value and orientation change into consideration is highlighted. In the tests, the greatest deformation is observed under pure stress orientation change, while the smallest deformation is observed when the principal stress axes are fixed. The change of stress value and orientation in 3D is also shown to result in deformations different to those within a fixed plane. The origins of these differences are found to be associated with difference in shear modulus, dilatancy, and non-coaxiality at the macroscale, and particle contact and fabric anisotropy at the microscale.