Changing anisotropy of G₀ in Hostun sand during drained monotonic and cyclic loading

The inherent and stress-induced anisotropy of saturated Hostun sand has been examined by means of shear wave velocity measurements under drained monotonic and cyclic triaxial conditions. Tests were carried out on samples prepared by moist tamping for a wide range of initial densities in a fully instrumented stress-path triaxial apparatus equipped with bender element instrumentation in three directions. Data have been normalised to account for the variations in void ratio and mean effective stress. The evolution of shear stiffness under anisotropic stress conditions has been explored by performing both constant mean effective stress tests and constant radial stress tests in compression and extension. Some samples were subsequently subjected to multiple drained deviatoric loading cycles, and the evolution of shear stiffness was examined. The tests showed that the initial inherent anisotropy was small, namely, lower than that previously observed in dry pluviated samples of the same sand in a cubical cell apparatus. Changing stress levels in compression or extension resulted in stress-induced anisotropy even when there were no changes in the values for p′, indicating that the widely-used empirical expressions for G₀ under isotropic stress states are not strictly applicable under anisotropic stress conditions. The comparison between stress-induced anisotropy in compression and in extension is discussed, and the evolution of shear stiffness during drained cyclic loading is examined. It is concluded that the changes in inherent anisotropy during these tests were small, but that new constitutive models should account for such changes in state with appropriate normalization.