Particle-scale study on backward erosion of multilayer erodible medium under converging flow: experimental tests and numerical modelling

Leakage at the bottom of an underground structure can compel ground water to form a converging flow, under which soil erosion may occur, thereby threatening the safety of the structure. Previous studies have shown that converging flow may induce backward erosion, and unravelling the mechanism behind this type of erosion was pivotal to mitigating disasters in practical engineering. Existing studies on this topic have been limited to monolayer erodible medium, while the mechanism behind the backward erosion of a multilayer erodible medium under converging flow remains unclear. In this study, both experimental tests and numerical simulations based on the validated computational fluid dynamics and the discrete element coupling method (CFD–DEM) were conducted to investigate the backward erosion of a multilayer sample under converging flow. The results demonstrated that the multilayer sample was eroded layer-by-layer, whereby residual layers could be observed at the bottom of the sample. The eroded regions in different layers were similar in shape but smaller in size for lower layers. In addition, particle exchange occurred among different layers during the erosion process. In general, lower-layer particles could directly ascend to upper layers in the eroded regions, whereas upper-layer particles mainly settled on the periphery of the eroded regions.