Abstract: Large-scale construction is being conducted along the Qiantang River. However, the foundation pits along the river are threatened by floods and strong tides. Typical geological cross sections of the soil seawalls and concrete seawalls along the river are selected for this study, and the dilatant characteristics of silt are taken into account. Numerical simulations based on the hardening-soil model are carried out to study the effects of the river tidal level, pit-river distance, insertion depth of enclosure walls and pit excavation depth on the safety of the alluvial silt foundation pits. The results demonstrate that under high tidal levels the pits with cross sections of soil seawalls will probably display a mode in which the destruction of seawalls occurs before the destruction of the pits. Moreover, the support and protection structures will fall down into the pit as a whole when the pit evacuation depth is small. When the pit evacuation depth is large, however, the support and protection structures will display a kicking destruction mode. Additionally, the flowing-soil destruction is found to occur most easily when the pit bottom is within half the insertion depth of enclosure walls.