Confining stress effects on global and local responses of internal erosion in gap-graded cohesionless soils

Internal erosion may affect soil hydraulic properties, inducing localized deterioration and even failure of hydraulic geo-structures. Soil is always subjected to stress in engineering practice; however, the effect of stress on the internal erosion-induced change in soil hydraulic properties along seepage path has not been thoroughly investigated in the literature. In this study, downward seepage-induced internal erosion tests on gap-graded cohesionless soil were conducted using a triaxial permeameter with measurable local hydraulic gradients and deformations. Effects of confining stress on global and local responses of internal erosion were investigated regarding global and local hydraulic gradients and soil deformations as well as eroded fine particles. The results show that the development of seepage velocity during internal erosion exhibits a two-stage linear progression with increasing applied hydraulic gradient under lower confining stress, while it exhibits a single-stage linear progression under higher confining stress. As confining stress increases, both initiation and failure hydraulic gradients are significantly increased, and the onset of the nonuniform distribution of local hydraulic gradients is delayed. Higher confining stress leads to a lower cumulative loss of fine particles and a smaller volume shrinkage, concurrently decreasing the degree of the nonuniform distribution of local hydraulic gradients. Spatial nonuniformity of local hydraulic conductivity and deformations indicates that specimens become more heterogeneous due to the migration of fine particles. Furthermore, although specimens subjected to confining stress exhibit volume shrinkage during internal erosion, the post-erosion void ratio of specimens increases due to the dominant effect of fine particles loss on soil pores.