Mesostructural Mechanism of Hydraulic Gradient Fluctuations Affecting Internal Instability Using Transparent Soil

For internally unstable soil, fine particles might migrate in the pore channels of its coarse matrix because of seepage flow, and fluctuations in the hydraulic gradient might promote this migration. To reveal the mechanism for this behavior, simulation testing using transparent soil was employed to visualize the soil mesostructure and particle migration process. Three gap-graded transparent soils with internal stability ranging from unstable to marginally stable and stable were made by mixing different amounts of fine particles. First, comparative seepage tests were conducted on the three soils under steady and fluctuating hydraulic conditions. The effect of hydraulic fluctuations depends on the internal stability of the soil. For the internally unstable or marginally stable soils, hydraulic fluctuations promoted internal instability, whereas for the internally stable soil, small applied hydraulic fluctuations had negligible effects on the measured hydraulic characteristics. Particle bridging during the migration process was used to explain the different behaviors. To verify this concept, a two-stage seepage test was performed, and a three-dimensional digital mesostructural model of the specimen was reconstructed based on continuously scanned images of cross sections. The comparison of the digital models at different stages provided visual evidence of particle bridging. It clearly showed that fine particles bridged at some pore constrictions, and the bridges remained intact during the first steady seepage stage and blocked particle migration paths. During the subsequent fluctuating seepage stage, these particle bridges disappeared, and particle migration resumed.