Experimental study on suffusion and contact erosion in double-layered alluvial foundation with a cut-off wall

Suffusion is defined as fine soil particles' detachment by hydraulic forces and their subsequent transport through pre-existing pores between the coarse particles. Contact erosion would occur at the interface between two soil layers if the coarse–fine soil combinations were submitted to a flow parallel to the interface. In some foundations with complex soil structures, suffusion and contact erosion can interact simultaneously, especially in double-layered or multilayered soil foundations with a cut-off wall. To describe the progression of suffusion and contact erosion, investigate the interaction between them, and explore the influence of retention ratio (Dc35/d85SA) between soil layers, nine double-layered internal erosion tests were performed in this study. The results reveal that the effect of the retention ratio on suffusion and contact erosion in double-layered alluvial foundation with a cut-off wall exhibits a consistent trend. Specifically, at both the onset of erosion and blowout, when Dc35/d85SA < 1, the critical hydraulic gradients increased at almost a constant value of Dc35/d85SA, in contrast, as Dc35/d85SA exceeds 1, a corresponding decrease in the critical hydraulic gradients was observed. Besides, when Dc35/d85SA < 1, the initial mode of erosion was contact erosion at the downstream interface; when Dc35/d85SA > 1, the initial mode of failure was either suffusion at the tip of the cut-off wall or simultaneous suffusion and contact erosion.