Overturning progressive collapse mechanism and control methods of excavations retained by cantilever piles

Abstract

Deep excavation accidents occur frequently, including overturning progressive collapse (OPC) caused by local over-excavation or other reasons. However, limited research has been presented on the mechanism, evaluation index and control methods of such progressive failure. Therefore, based on an overturning collapse accident of an excavation retained by double-row piles, this paper uses the finite difference method (FDM) to study the OPC mechanism. The influence of local over-excavation on the internal force, displacement and stability of double-row piles was analyzed. To evaluate the risk of OPC, the anti-overturning stability value (ASV) and modified overturning stability index (MOSI) were proposed as evaluation indexes of overturning displacement and overturning stability. The results show that overturning collapse in the local over-excavation area (LOA) developed to adjacent retaining structures through the loading effect of the soil arch in the active zone, the unloading effect of the soil in the passive zone and the load transfer effect of the capping beam. The ASV can be used to predict the scale of retaining piles that will experience additional displacement caused by local over-excavation. Compared with the traditional overturning stability index (OSI) and the ASV, the MOSI can better reflect the occurrence of OPC and its scale. A capping beam cannot reduce the risk of progressive failure but will increase the risk and degree of OPC, which is different from the progressive bending collapse of cantilever excavations. Therefore, when calculating the overturning stability of retaining structures without considering the load transfer effect of the capping beam, the scale of the OPC outside the LOA will be severely underestimated. Extending the retaining piles and reinforcing the soil in the passive zone in the LOA can control the development of OPC, but the two measures have their own limitations and should be selected according to specific project conditions.