Experimental study on the longitudinal evolution of the overtopping breaching of concrete-face sand-gravel dams

Longitudinal erosion is the main mechanism of the overtopping breaching of dams. Although this type of erosion has been studied by conducting flume model tests, there are no experimental studies on the longitudinal erosion of concrete-face sand-gravel dams (CFSGDs). In this study, flume model tests were conducted to analyze the overtopping breaching of a CFSGD. Based on monitoring data recorded by pore water pressure sensors, measuring rods, and high-definition cameras, the longitudinal erosion process of the CFSGD was analyzed, and the influence of joint water stop failure on the longitudinal erosion process was studied. The results showed that the scour pit and the central mound area were the most evident erosion characteristics, similar to that observed in an actual breaching process. A longitudinal evolution model was proposed, which reflects the erosion characteristics of the CFSGD at different stages. The longitudinal erosion rate and undercutting erosion rate were found to first increase and then decrease. The joint water stop failure did not change the longitudinal erosion characteristics; however, the seepage reduced the shear strength of the soil, and the dam breaching duration was 1.53 times that observed in the intact water stop case. Through a force analysis of the soil particles on the seepage surface, the critical angle (θ_cr) for erosion failure on the seepage surface was determined, and a formula for the critical shear stress (τ_c) of the soil particles under the action of seepage and overflow was derived.