A CFD-DEM investigation into the cyclic degradation behaviors in gap-graded sand under suffusion

The issue of suffusion caused by damage to underground pipelines is becoming increasingly severe. Simultaneously, the cyclic load near the damaged pipeline further exacerbates the degradation of the strength and resilient modulus of the surrounding soil. However, the cyclic degradation characteristics of soil subjected to suffusion under cyclic loading have not been well understood, especially at the micro-level. In this paper, the coupled computational fluid dynamics and discrete element method are used to conduct the suffusion test of gap-graded sand, and the cyclic degradation characteristics of the specimens before and after suffusion are investigated through cyclic triaxial test. The effects of hydraulic gradient, confining pressure and initial fine particle content on the cyclic degradation of eroded specimen are analyzed in detail. The macroscopic differences of cumulative strain and resilient modulus before and after suffusion caused by initial fine particle content are discussed. The mechanism of the cyclic degradation is revealed from a micro-perspective, including mechanical coordination number, cumulative contact contribution and strong contact force chain. The results indicate that suffusion significantly alters the microstructure of gap-graded sand, reducing its resilient modulus and exacerbating cyclic degradation. The loss of fine particles destabilizes the coarse-grained skeleton, leading to increased axial strain and changes in mechanical coordination numbers and contact force distributions. Moreover, higher initial fine particle content induces more pronounced microstructural changes before and after suffusion.