An increasing number of catastrophic progressive collapses of deep excavations have occurred throughout the world. However, the research on progressive collapse mechanisms is limited. In this paper, two categories of model tests were conducted to investigate the mechanism of partial collapse (sudden failures of certain retaining piles) and progressive collapse. The model test results show that partial collapse can cause a sudden increase in the bending moments of adjacent piles via an arching effect. The load-transfer coefficients are defined to be equal to the peak increase ratios of the maximum bending moments in adjacent piles (peak moments caused by collapse over the values before the collapse). When the maximum load-transfer coefficient is larger than the bearing capacity safety factor of the piles, the partial failure will lead to progressive collapse. The influential factors of the progressive collapse mechanism, such as the partial collapse extent, excavation depth, and capping beam, were also investigated. During progressive collapse, the previous failed pile could cause new stress arching; simultaneously, the soil behind certain nearest intact piles could become loosened and destroy the arch springing of the stress arching, causing the progressive collapse to cease gradually.

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Multibench retaining systems can be used in large area excavations for the purpose of eliminating horizontal struts. However, there is no design method for this retaining system. Based on the mobilizable strength design (MSD) concept, a design procedure for a two-bench retaining system considering the interaction of the first and second retaining structures was proposed and tested. Based on an admissible strain field for a two-bench retained excavation in undrained condition, the shear strain in the superimposed strain and the lateral earth pressure distribution acting on the retaining structures can be determined. Then, the mobilized shear strength corresponding to the strain field could be calculated by the equations of force and moment equilibrium. Further, the crest displacements, earth pressures, and bending moment in a two-bench retained excavation can be calculated. The calculated results using MSD were verified by the finite difference analysis.