Small-strain stiffness anisotropy of soils subjected to internal erosion

The behaviours of soil subjected to internal erosion have been widely investigated, yet the evolution of soil fabric anisotropy during erosion and the corresponding changes in post-erosion stiffness degradation curves have never been explored. This study developed a back-pressure-controlled, bender-equipped triaxial permeameter to measure the internal erosion behaviour of soils under different stress states and its consequences on the anisotropic mechanical behaviour of eroded soil. Evolutions of fabric anisotropy during erosion were evaluated by measuring the shear wave velocity at various wave propagation and polarisation directions under the isotropic loading condition. Soil samples with and without erosion were then sheared following a path of constant mean effective stress to determine the stiffness degradation curves. Results show that the losses of fines and the rearrangement of soil particles due to erosion increased the fabric anisotropy (i.e. increased horizontal alignments of soil particles) by 10% at all confining pressures. Such increase resulted in eroded specimens that were stronger than their intact counterparts upon triaxial compression, but the opposite trend was observed upon extension. The eroded specimens had a higher volumetric threshold shear strain than the intact ones, thereby suggesting that higher strains were required to substantially change the soil structure and reduce the small-strain shear stiffness.