Experimental investigation of the critical water head and failure mechanisms leading to uprush hazards in foundation pits

Uprush hazards represent a significant risk in foundation pit engineering, and understanding the failure mechanisms is critical for mitigating environmental impacts. Laboratory tests were conducted to investigate the failure mechanisms of uprush hazards, with a focus on the development process of surface cracks in clay. These tests examined the variations in the thickness of the clay layer underlying the excavation surface (T) and the ratio of the length to the width of the foundation pit (R). The results indicate that the anti-uprush capacity of the pit improves, and surface cracks are more likely to form as T increases. The water head for surface cracks appearance (H_C) increases as T increases and same as the critical water head for uprush hazards occurrence (H_U). The maximum vertical deformation of the clay at the central point inside the pit during the test process (D_m1) is constrained by comprehensive effect of the shorter and longer walls of pit, and all peak values of D_m1 were observed in tests where R equaled to 2 among all test types. Pit with this shape is advantageous for preventing uprush hazards, as it allows early warning in the process of surface crack development. Two distinct failure modes were identified based on the degree of deformation and the distribution of cracks. Failure Mode-1, characterized by non-fully developed deformation of the clay, represents a localized seepage failure without cracks, culminating in uprush or inrush hazards. In contrast, failure Mode-2 is a general failure accompanied by fully developed deformation and prominent cracks. Mode-2 was further subdivided into Mode-2a, in which surface cracks were concentrated near the central position, and Mode-2b, in which cracks were distributed along the wall sides. These findings provide potential failure modes of uprush hazards for actual engineering under complex conditions, accompanied with qualitive laws between H_C, H_U and R, which is beneficial for preventing the occurrence of uprush hazards.