Progressive internal erosion during triaxial shearing: an X-ray micro-computed tomography study

Fines removal during internal erosion results in the permeability increase and strength reduction of granular soils. This study examines internal erosion during shearing of granular soils under triaxial loading conditions using a specially designed apparatus and X-ray computed tomography scans. Two gap-graded soil samples with different fines content are tested and scanned multiple times to characterise sample- and pore-scale volume change and fines content using image-processing techniques. Results show that internal erosion increases gradually with axial strain and has a complex impact on permeability increase due to the spatial variation of fines content. Triaxial shearing induces both pore contraction and dilation, destabilises the clogging of fine particles and promotes internal erosion. Also, the erosion of force-supporting fine particles at a high fines content facilitates pore contraction. The progressive internal erosion due to mechanical disturbance leads to the formation of a flow channel from the upstream loading platen throughout the sample, resulting in a dramatic increase in permeability. Overall, this study provides novel insights regarding the interplay between the hydro-mechanical processes during shearing under constant seepage.