Located at 301 Mission Street, the 650-foot-tall Millennium Tower was designed to be San Francisco’s premier residential address. The project geotechnical report predicted 4 to 6 inches of settlement over the project’s life; however, as construction neared completion in 2009, the settlement had already reached 10 inches. As development occurred on adjacent parcels, accompanied by continuous dewatering of the surrounding soils, settlement continued. By 2014, instrumentation installed to monitor the effect of adjacent construction recorded that the mat had dished, settled nearly 14 inches and that the roof had tilted to the northwest a similar amount. As the City of San Francisco threatened to red tag the building, counsel for the developer, Mission Street Development (MSD), retained Simpson Gumpertz & Heger (SGH) to determine if the settlement had damaged the structure and created a safety issue. Although SGH found that settlement had not appreciably affected the structure’s adequacy, in 2015, with settlement exceeding 16 inches, litigation ensued between the homeowners’ association, MSD, the City of San Francisco, and the development teams for adjacent projects. Under the terms of a negotiated settlement, SGH designed a foundation stabilization upgrade that formed the basis for dispute resolution and is currently under construction. This article focuses on the structural aspects of the problem and the upgrade.
Stabilizing San Francisco’s Leaning Tower