On the effects of multi-directional initial anisotropy on the monotonic constant volume response of a silica sand

The constant volume behavior of sands is substantially influenced by the initial stress anisotropy. This research aims to investigate the stress anisotropy effect by conducting a series of bidirectional direct simple shear tests that can apply initial shear stress on a sample in different directions during the consolidation stage. The experimental program provides insights into the impacts of the initial shear stress and the subsequent principal stress rotation (PSR) on some critical aspects of soil behavior, including the onset of instability, brittleness index, phase transformation, and the critical state line. The findings show that the onset of instability and the brittleness indices are significantly dependent on the initial stress anisotropy. In contrast, the critical state and phase transformation lines are not influenced by the initial anisotropy of the stress state, even in the presence of PSR. Furthermore, the study gives particular attention to the non-coaxiality between the major principal stress and strain rates to illustrate how the non-coaxiality decreases with increasing shear strain. The research also explores the non-coaxiality between the resultant shear stress and shear strain rate and suggests a predictive flow rule accordingly. The results reveal that a substantial level of non-coaxiality may exist between the resultant shear stress and the shear strain rate.