Mechanism of progressive collapse induced by partial failure of cantilever contiguous retaining piles

Although there are many progressive collapse cases of deep excavations and the scale of progressive collapse caused by the initial partial failure of the retaining structures and its consequences are becoming larger and larger, the researches on the mechanism how the partial failure evolves to large-scale progressive collapse are still limited, and the control methods and design theories of progressive collapse haven't been developed. In this study, the long strip excavation with cantilever contiguous retaining piles under partial failure is simulated by the explicit finite difference method, discrete element method and model tests. The redistribution of soil pressures and change of internal forces of the retaining structures are analyzed and compared using the simulated results, and then the transfer mechanism of progressive collapse in the longitudinal direction of the excavation is preliminarily revealed. The concept of load transfer coefficient is raised, which equals to the increment ratio of the internal force of the piles adjacent to the partial failure, and it is found that the load transfer coefficient is a crucial factor which can determine whether or not the progressive collapse can happen. For a retaining structure, the higher the soil strength is, the larger the load transfer coefficient is. When partial failure occurs, soil pressures and internal forces of adjacent intact retaining structures increase rapidly. The continuous top-beam installed on the top of piles can lower the load transfer coefficient, and plays an important role in increasing the capacity of retaining structures to resist progressive collapse.