Simulation and Analysis of Riverbank Slope Stability with "Upper Clay and Lower Sand" Binary Structure

When the water level in the river channel is lower than the groundwater level around it, seepage into the river channel may trigger instability of the riverbank slope, affecting the safe operation of related infrastructure. The binary structure riverbank slope composed of upper clay and lower sand has a more complex stability problem due to the significant differences in mechanical behavior and permeability between the two soil layers. For this type of "upper clay and lower sand" riverbank slope, a discrete-continuous domain coupling and fluid-solid coupling numerical method, combined with the strength reduction method, was applied to simulate the failure process of the riverbank slope under unsaturated seepage, while the longitudinal and lateral scouring effects of the river flow were not simulated. The influence of the thickness of the overlying clay layer and the water level in the river channel on the stability of the riverbank slope was discussed. The following conclusions can be drawn. 1) Near the water level line of the river channel, the groundwater outflow velocity is the largest and the direction is outward, which leads to a decrease in the shear strength of the sand surface; this induces instability of the sand layer below the water level line in a flow sliding mode and the sand layer above the water level line in a retrogressive mode; the clay layer does not slide due to its higher unsaturated strength. 2) Under the same riverbank slope height, a thicker overlying clay layer leads to a lower rate of groundwater outflow from the riverbank slope surface, a smaller drag force on the sand particles, a less loss of shear strength, and thus a higher safety factor. 3) The increase in the water level in the river channel weakens the fluid-solid coupling effect at the riverbank slope surface, increases the potential energy of the sliding mass, and reduces the unsaturated zone range of the sand layer; these three effects jointly cause the safety factor of the riverbank slope to first decrease and then increase with the increase in water level; when the water level exceeds the interface of soil layers, partial instability may occur in the upper clay layer, increasing the volume of the sliding mass.