Abstract: Suffusion will occur in the internally unstable sandy soil because of the groundwater seepage. Soil failure caused by the suffusion has adverse effects on the building structures or foundations. In this paper, a model that can calculate the force acted on fine particles in seepage field was established by considering the effective stress of soil and the stress reduction of fine particles. According to the equilibrium state of ultimate stress, the formula for calculating the startup critical hydraulic gradient of fine particles migration in sandy soil during the suffusion process was obtained. A DEM-CFD coupled method and the existing experimental data were used to validate the proposed model. The results showed that the fine particles in the sandy soil started in rolling mode in the beginning, and the startup critical hydraulic gradient is found to be affected by the seepage flow, soil characteristics, and the properties of the particles. The startup critical hydraulic gradient of fine particles on the surface of sandy soil was found greatly affected by the buried depth. The difference between the highest and lowest startup critical hydraulic gradient of the fine particles buried in 1 cm depth was 10.169%, and the difference was reduced to 1.061% when the buried depth was 10 cm. The maximum standard error of the calculation method was 6.038% while compared to the numerical simulation results, the maximum standard error compared to the seepage test results was 11.211%. Therefore, the proposed model can accurately predict the startup critical hydraulic gradient of sandy soil fine particles.

Key words: seepage, suffusion, fine particle loss, onset of migration of fine particles, critical hydraulic gradient, DEM-CFD coupling