Erodibility and Mechanical Behaviour of Gap-graded Soils Close to the Borderline of Geometric Stability: Influence of Critical Parameters

Suffusion is a critical form of internal erosion where fine particles are selectively washed out through the pore network, threatening the stability of water-retaining structures. This study investigates how relative density and confining pressure affect the erosion susceptibility and post-erosion behaviour of a gap-graded soil close to the borderline of internal stability. Pre- and post-erosion drained triaxial tests were conducted at a fine content of 25%. Results show that relative density has a stronger influence on erosion than confinement. Higher densities reduced pore sizes and enhanced the role of fines in the load-bearing skeleton, lowering erodibility and improving strength, stiffness, and dilation. Erosion led to strength reduction, decreased friction angle, and shifts in the critical state line, indicating greater compressibility and weaker post-erosion particle arrangement and contact structure, especially at low densities. While 25% fines content reflected an underfilled structure, increasing it to 30% caused a significant drop in strength, suggesting a fabric transition. These findings highlight the coupled effects of density, confinement, and fines content on suffusion and post-erosion response, offering valuable insights for evaluating the internal stability of granular soils.