Seepage erosion can lead to the loss of particles in the soil with water flow

thereby changing the mechanical and hydraulic properties of the soil

and then causing the deformation and even damage of earth-rock dams in engineering. In this paper

the coupled CFD-DEM method is used to study the seepage erosion characteristics of gap-graded sandy soil

which is widely found in dam foundation

filter-base soil systems in earth-rock dams and gravel packing for sand control in oil and gas production

after reasonable simplification of its gradation. The effects of hydraulic gradient

confining pressure and fines content on seepage erosion in gap-graded sand soils were investigated through 8 groups of upward seepage simulation. Macroscopic phenomena such as the mass of fines loss

fines loss rate

soil surface settlement and microstructural changes were monitored in the tests and verified against previous indoor tests. The results showed that the effect of fines content on the mass of fines loss

peak fines loss rate and soil surface displacement from the numerical simulation was more significant than the effect of the confining pressure and hydraulic gradient in the tests. In addition

the analysis of microstructure demonstrated that the conversion of the soil force chain transfer structure occurred near the threshold of 25% fines content. The triaxial results indicated that the loss of fines also led to a decrease in peak strength and E50 modulu of soil.