Performance of Mixed Bracing System in Deep Excavation of Stiff Soil

Abstract

A braced excavation, 23 m deep x 213 m long, for Civic Center Station on the Los Angeles Metropolitan Transit Redline was nearing completion when high strut loads were measured at mid-depth. About four months after the excavation bottomed out and the base slab was cast, four strut-to-wale connections failed. The contractor was directed to install a sister level of braces and later sought compensation from the owner. An investigation into the contractor's claim used the finite element method and determined that the high loads could not have been anticipated from conventional theory, and that they were due both to mixing tiebacks and struts in the same excavation and to preloading the struts. The method and results of the investigation are summarized.