Performance of a stiff excavation support system in soft clay and the response of an adjacent building

Excavation for the Chicago and State Subway Renovation Project was undertaken withinl.3 m of the shallow foundations of the Frances Xavier Warde School. The excavation extended down to a depth of 12.2 m. The excavation support system consisted of a 0.9-m thick secant pile wall, one level of cross-lot bracing and two levels of tiebacks. The support system performance data suggests that excavation-related settlement behind a support wall can be estimated reasonably well from inclinometer data. It was further found; that distortions obtained from inclinometer data can be used to estimate distortions in an adjacent structure when settlement and tiltmeter data are not available. The building response data showed that less than 50 percent of the building settlement was due to excavation-induced stress relief and the settlements extended beyond the secant pile wall a distance approximately equal to the depth of the secant pile wall. Damage was first observed in the non-load bearing portions of the building when angular distortions reached 1/920. The data suggest that angular distortions less than 1/1000 in the sagging regions would preclude any damage to the school.; A detailed finite element model of the Warde School showed cracking in the infill wall panels was a function of pre-existing strains induced in the building during the initial self-weight settlement. The analyses demonstrated that the shear strain component governed cracking in the wall panels and that no cracking occurred in wall panels in which the shear strain component obtained from the combined self-weight and imposed support displacement strains was between the range of −0.00035 and 0.00035.