Based on two severe through-wall leaking accidents during deep excavations in water-rich sandy strata, the deformation behaviors of the excavation during leaking were investigated and the major causes incurring the accidents were summarized by field investigation and review of measured data. To reveal the disaster-causing mechanism of leakages, a 3D fluid-solid coupled numerical model was established and several leaking scenarios were analyzed. The results indicate that the leaking accidents occur suddenly during excavation in water-rich sandy strata. When the leakage takes place at 1.075 times the final excavation depth, the total hydraulic gradient and water-earth pressure behind the wall undergo a sudden increment up to 9-12 times and 3-3.6 times those before the incident, respectively, causing a rapid increase in lateral wall displacements. Compared with the case leaking below the excavation surface, a greater increment of ground settlement, a closer distance between the location at which the maximum increment happens and the excavation pit, and a larger affected region will be incurred for the cases where the through-wall leakages are exactly at or above the excavation surface. In addition to the obvious effects of load-bearing and isolating deformation, the dynamic seepage forces during leaking can also be weakened by the isolation piles. However, when the pile length exceeds the optimal value (1.6 times the final excavation depth), the improvement on protective effects of the isolation piles is no longer obvious.