Characteristics and quantification of fine particle loss in internally unstable sandy gravels induced by seepage flow

For internally unstable sandy gravels, fine sand particles might pass through the constrictions of the coarse gravel particles and flow out of the soil with seepage flow. This phenomenon is referred to as internal erosion, which induces fine particle loss and changes particle size distribution, thereby significantly affecting the soil's hydraulic and mechanical properties. To investigate the characteristics of the fine particle loss process in sandy gravels induced by seepage flow, as well as the corresponding permeability change and skeleton deformation, systematic large-scale internal erosion tests were conducted on two kinds of internally unstable sandy gravels under two hydraulic loading paths (constant and multi-stage hydraulic gradients) and three surcharge pressures (0.5, 1.0 and 2.0 MPa). The initiation and progression of internal erosion under different hydraulic loading paths and stress conditions were summarized. An incremental erosion model, which can describe the fine particle loss process under different hydraulic loading paths and incorporate the effect of surcharge pressure, was established based on the volumetric exchange rate of the deposited fines to the fluidized fines. The model was validated not only with the test results of sandy gravels in this study but also with other test results under various hydraulic and stress conditions to demonstrate its versatility.