3.4 One⁃Strut Failure Analysis

In Singapore，braced retaining wall systems are commonly used to construct cut and cover tunnels／stations for Mass Rapid Transit projects as well as for deep basements for shopping malls.The excavation sides are normally supported by concrete diaphragm walls or secant bored pile walls with two or more levels of struts.The purpose of excavation support system is to provide rigid lateral support for soil surrounding excavation and to limit the surrounding soils movement.One concern in the design of these cut and cover excavation projects is the consequence of the failure of one or two struts（due to accidental damage during construction or incidental design／quality problems）in bracing system，and whether it would lead to progressive failure and eventual total collapse of bracing systems and the surrounding grounds（Endicott，2013；Saleem，2015）.This part aims to investigate how loads from the failed strut are transferred to the adjacent struts and the whole support system.

Unfortunately，in the case of braced strut systems，there is no reported case history detailing the load transfer mechanism due to one⁃strut failure.Since struts provide passive resistance to wall movement，while anchors rely on stresses in the ground being mobilized to retain the wall，it may not be feasible to compare or equalize the redistribution of forces for anchors and struts.In Singapore，part of the design requirement requires the braced retaining wall system to be structurally safe，robust and has sufficient redundancy to avoid catastrophic collapse.In the conventional approach for one⁃strut failure using 2D analysis，the entire level of the failing strut is removed and thus the forces can only be distributed vertically.This generally leads to a more conservative design with heavier strut sections.Thus，3D analysis of one⁃strut failure is essential to provide more realistic understanding of the force／stress transfer behavior of the braced excavation system.This part describes the use of both 2D and 3D FE analyses to assess the impact of the failure of one strut on the remaining struts.