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Contact erosion, or interfacial erosion, often occurs in layered foundations composed of coarse and fine layers, which refers to the phenomenon that the particles of the fine layer are detached by the flow parallel to the interface and transported through the pore channels of the coarse layer. In order to visualize the initiation and progression of contact erosion at the internal interface of soil strata, the transparent soil model tests were employed to simulate the real soil, and a series of contact erosion tests were conducted on double-layer foundations composed of different coarse and fine layers. Based on the test results, the commonly used geometric and hydraulic criteria for contact erosion were examined. It shows that the geometric criteria are reliable, while hydraulic criteria give much diverse results. For the same fine soil layer, the critical hydraulic gradient varies greatly with the coarse soil layer, while the critical flow velocity varies in a relatively small range since flow velocity has an intrinsic relationship with the dragging force imposed on particles by seepage flow. It is suggested that more efforts should be paid to the criteria based on critical velocity in further studies for contact erosion.

For an internally unstable soil, fine particles can move in the pore channels between coarse particles along with seepage flow; this process is termed internal erosion. To evaluate the internal stability and internal erosion behavior of sandy gravel alluvium beneath the suspended cutoff wall in an embankment dam foundation, a series of horizontal seepage tests were carried out on the four representative gradations of the alluvium layer using a large-scale high-pressure erosion apparatus. The evolutionary trends of hydraulic conductivity, the erosion ratio of fine particles, and volumetric strain under stepwise increasing hydraulic loading were obtained. The results showed that the specimens of different gradations exhibited distinct properties in permeability, particle loss, and deformation, depending on the gradation continuity and fine particle content, which can be attributed to the difference in the composition of the soil skeleton and the arrangement of coarse and fine particles. For the specimens with continuous gradations or relatively high fine particle content, the surcharge pressure can significantly improve their internal stability. By contrast, in the situations of gap-graded gradations or low fine particle content, no considerable improvement was found because the stress was mainly borne by the coarse skeleton. The practical implications of the experimental results were demonstrated by evaluating the seepage safety of the zone beneath the suspended wall in the dam foundation.

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Internal erosion refers to the seepage-induced fine particle migration phenomenon in soil. Deep alluviums in valleys usually contain cohesionless gap-graded sandy gravels with poor internal stability. The construction of embankment dams on such alluviums could pose a high risk of internal erosion. This study systematically investigated the internal erosion of cohesionless gap-graded sandy gravels with an emphasis on the effects of gradation characteristics and particle morphology. A series of large-scale internal erosion tests were conducted on gap-graded sandy gravels with different gap ratios, fines contents, and coarse particle morphologies under the surcharge pressure of 1 MPa. The internal erosion characteristics, including soil permeability, eroded soil mass, and soil deformation during the erosion process were comparatively analyzed in combination with a meso-mechanism interpretation. The results show that the increase of the gap ratio can reduce the internal stability of soil and promote the mechanical instability. Fines content affected the permeability and internal stability of soil by altering the filling state of inter-granular pores and the constraints on fine particles. Coarse particles with higher roundness, sphericity, and smoothness can facilitate the movement of fine particles and promote the mechanical instability of the soil matrix.