Experimental investigation on through-wall erosion failure of braced excavation in sandy gravel under extreme rainfall

Deep excavations in sandy gravel faced a high risk of seepage failure due to the increasing frequency of extreme rainfall events. Structural deficiencies in retaining walls, particularly leakage defects, exacerbated these failures by triggering through-wall leakage and subsurface erosion, resulting in hazardous ground movements that threaten nearby built environments. However, the influence of rainfall patterns, wall defects, and soil properties on erosion-induced instability remained insufficiently understood. This study conducted twelve reduced-scale 1\(\:\text{g}\) physical model tests to evaluate how defect dimensions, rainfall patterns, and gravel contents affect soil movements and structural performance under extreme rainfall. A semi-empirical ellipsoid-paraboloid model was developed to characterize the spatial and temporal evolution of erosion-induced failure zones. Key findings include: (1) the depth where defect was located was the dominant factor controlling erosion-induced soil movements, with defects at shallower depths accelerating erosion and surface subsidence due to reduced thickness of overlying soils; (2) rainfall patterns indirectly influenced erosion onset and surface instability timing by modulating groundwater responses, with the peak-advanced pattern intensifying hydraulic gradients and accelerating subsurface erosion zone development; and (3) the proposed model effectively described the transition from a confined ellipsoidal erosion cavity to an open paraboloidal failure zone in retained soil, driven by sapping erosion near the wall defect, matric suction loss, and rainwater scouring. These findings emphasized the critical role of defect geometry, hydraulic conditions, and soil gradings in erosion-induced excavation failures. The proposed framework offered a reference for predicting through-wall erosion failures and assessing instability in sandy gravel, providing insights that may contribute to climate change adaptation strategies and geotechnical risk mitigation in urban excavation projects.