Study on the mechanism and countermeasures of progressive collapse in deep excavation retained by multilayer struts

There is still a lack of research on the resilient design of excavations retained by multilayer struts against progressive collapse. This paper utilizes a discrete element method (DEM) model to investigate the mechanism of the force variation in struts and diaphragm walls during excavation and progressive collapse based on the Nicoll Highway accident in Singapore. This study identifies dangerous time points, which occur after the excavation has reached a certain depth, leaving a layer of soil without a corresponding strut. At these points, the bottommost one or two installed struts and diaphragm wall are critical components that can trigger progressive collapse if they fail. To prevent this, two concepts are proposed and are validated by DEM simulation. The sparse upper and dense lower (SUDL) strut arrangement concept ensures progressive collapse resistance of the retaining structure at the bottom of the excavation. The concept of dynamic redundant struts (DRSs) utilizes redundant struts to provide temporary protection in the vulnerable sections of the retaining system during excavation. Alternating the installation and removal of DRSs creates dynamic redundancy, ensuring progressive collapse resistance throughout the construction process in a relatively economical way. Additionally, the DRSs can effectively reduce the lateral deflection of the retaining wall.