Mechanism of overturning progressive collapse of excavations retained by double-row piles induced by over-excavation

Collapse accidents of excavations occur occasionally, including some overturning progressive collapses (OPC) induced by local over-excavation. However, the mechanism and evaluation index of OPC lack studies. Based on a case study on OPC of an excavation retained by double-row piles, the effects of local over-excavation on the internal force, displacement and stability of the cantilever retaining system are investigated, and an evaluation index is proposed, i.e., the anti-overturning status value (ASV), which is the ratio of anti-overturning moment to overturning moment of a pile at a certain moment, to evaluate the status of the pile. The results show that after a local over-excavation, OPC in the zone of over-excavation extends to the adjacent piles owing to the arching effect and capping beam, and ASV of the piles decreases suddenly. The minimum value of ASV during the over-excavation process determines the possibility and the range of OPC. When the retaining system does not include the capping beam, the range of the OPC outside the over-excavation zone will be underestimated. The capping beam cannot decrease the risk of the OPC, because it will enlarge its range, which is different from that in the bending-failure type progressive collapse of cantilever piles. The harmful conditions during the construction process (e.g., overload and disturbance of the vibrating loads) may intensify the level and range of the OPC.