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

摘要

For internally unstable soil, fine particles might migrate in the pore channels of its coarse matrixbecause 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 wasemployed to visualize the soil mesostructure and particle migration process. Three gap-gradedtransparent soils with internal stability ranging from unstable to marginally stable and stablewere made by mixing different amounts of fine particles. First, comparative seepage tests wereconducted on the three soils under steady and fluctuating hydraulic conditions. The effect ofhydraulic fluctuations depends on the internal stability of the soil. For the internally unstable ormarginally stable soils, hydraulic fluctuations promoted internal instability, whereas for theinternally stable soil, small applied hydraulic fluctuations had negligible effects on the measuredhydraulic characteristics. Particle bridging during the migration process was used to explainthe different behaviors. To verify this concept, a two-stage seepage test was performed,and a three-dimensional digital mesostructural model of the specimen was reconstructedbased on continuously scanned images of cross sections. The comparison of the digital modelsat different stages provided visual evidence of particle bridging. It clearly showed that fineparticles bridged at some pore constrictions, and the bridges remained intact during the firststeady seepage stage and blocked particle migration paths. During the subsequent fluctuatingseepage stage, these particle bridges disappeared, and particle migration resumed.