The interface between two soils is a weak place where the seepage failure occurs easily. In order to investigate the seepage and seepage resistance properties of the interface between sandy gravel and sand

based on the seepage failure tests for the sandy gravel

the impact factors of the seepages and seepage failure

the occurrence and evolution of contact scouring under the conditions of vertical and horizontal seepages are studied by using large-scale vertical permeameter and self-built annular radial horizontal permeameter. It is concluded from the test results that the typical contact scouring process can be divided into three stages

i.e.

stable seepage

transitional and failure stages. The three stages can be divided by two characteristic hydraulic gradients. The hydraulic gradient corresponding to the limit of the stable seepage stage is called starting gradient and that of the transitional stage is called failure gradient. The permeability of the combined sample of sandy gravel and sand decreases with increasing of the dry density

or decreasing of the particle size of the sandy gravel. But the seepage resistance of contact scouring increases with increasing of the dry density

or decreasing of the particle size of the sandy gravel. On the condition of horizontal seepage

the permeability of the sample is larger but the seepage resistance of contact scouring is less than that of vertical seepage.