Experimental study on progressive collapse of tied-back retaining system of excavations induced by partial over-excavation or surcharge loading

Collapse accidents of tied-back excavations caused by over-excavation or overloading occasionally occur. However, the studies on the mechanism of progressive collapse under these conditions are still lacking, which limits the targeted prevention and control of such accidents. Based on two cases of excavation collapse, the model tests on the progressive failure of tied-back excavations are designed to investigate the influences of partial over-excavation or overloading on the deformation, earth pressure and internal forces of the anchors, piles and capping beam. The results show that after partial over-excavation, the soil arching effects generated outside the excavation and the load transfer effects of the capping beam cause a significant increase in the internal forces of the adjacent piles and anchors. Under this condition, the two effects are exacerbated by partial failure of anchors in the over-excavation area, leading to the progressive failure of the adjacent anchors. If their embedment depth is small, the bending moment of the piles decreases after the failure of the anchors, and finally the overturning failure occur due to the lack of constraint on the pile top. On the contrary, the maximum bending moments will increase and eventually leads to bending failure and collapse. The deeper the normal excavation depth, the stronger the soil arching effects caused by over-excavation and anchor failure, and the worse the capability to resist progressive failure of excavations. Therefore, the reinforcement of the anchors should be given priority to prevent progressive failure in the partial component strengthening method. The excessive surcharge load will cause progressive failure of the anchors from the center of the overloading area. Different anchor placement heights lead to different surcharge loads required to trigger the progressive failure of the anchors, the progressive failure path and the components that need to be specially checked against potential surcharge loading risks may also be different. When the anchors are set on the waler beam, the load transfer coefficient of the anchors is greater than that of the piles under surcharge loading, and the priority needs to be given to the design of local reinforcement of the anchors. When the anchors are set on the capping beam, a greater surcharge load is needed to trigger the progressive failure, and the load transfer coefficient of the piles is greater than that of the anchors, and the priority should be given to the design of the piles.