Performance of a stiff support system in soft clay

The performance of an excavation support system for a subway station renovation project in Chicago and its effects on an adjacent, shallow-foundation supported building are presented. The 13-m-deep excavation was made through soft to medium stiff clays and was supported by a 900-mm-thick secant pile wall, one level of cross-lot bracing, and two levels of tiebacks. Design considerations are discussed and construction procedures are summarized. Field performance data were collected, including lateral soil movements at five locations, building settlements along the exterior wall and interior columns, support system loads, and observations of building damage. As planned in the design, minor damage occurred to nonload bearing portions of the building. Of the 38 mm of maximum lateral movement adjacent to the building, 9 mm occurred during wall installation, 16 mm developed as the soil was excavated, and 13 mm occurred during tunnel demolition and station renovation as a result of soil creep and reduction of wall stiffness. Settlements extended beyond the secant pile wall a distance approximately equal to the depth of the secant pile wall. The effect of excavation was to cause larger settlements within the affected zone, but not to expand the width of the settlement trough. When distortions exceeded approximately 1/960, damage began to manifest itself in the nonload bearing portions of the building.