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.

Small-strain shear stiffness (G0) is an essential parameter to predict deformation characteristics and dynamic properties of granular materials. It is empirically known that G0 increases with decreasing a void ratio (e0) and increasing isotropic stress level (p0 '). Recently, the effect of particle shape on G0 has been studied; however, the mechanism underlying the evolution of G0 is not fully understood. Using the discrete element method (DEM), this contribution quantifies the G0 of granular materials by performing small-strain probing where multi-sphere clumped particles are used to vary particle shape and surface topology systematically. The Hertzian contact theory is applied for each sphere-element contact to capture the stress-dependent contact stiffness. The results reveal that G0 is well correlated with e0 or mean coordination number for a given particle shape; however, G0 is measurably reduced when finer sphere-elements dominate inter-particle contact responses. The present study proposes two contact-scale expressions of G0 for non-spherical particles based on contact area (CA) and micromechanical effective medium theory (EMT) by extending the EMT expression for spherical particles; both can capture the effects of particle shape and p0 ' on G0 under given conditions where particle breakage does not occur.