The residential high-rise Millennium Tower in San Francisco has settled a total of 16 inches since opening, 2 inches deferentially. A spokesman for the tower owner blames the settlement on the excavation next door for the Transbay Transit Center, a $2.4 billion dollar project being constructed 60 feet underground.
However, the Transbay Joint Powers Authority (Transbay) hired geotechnical firm Arup in 2010. Their initial report indicated that the tower had already settled 10 inches by the time excavation began for the Transit center. Arup seems to have been tasked with design, installation, and perhaps monitoring of a geotechnical monitoring program, some information on the scope is available in a 2012 presentation given by Arup available on the Transbay website.
Transbay responded to the allegations in a press release on August 1, 2016. They note that the building designers knew of the proposed excavation at the time but still elected for piles that did not extend to bedrock. According to SFGate, the (driven?) piles extend approximately 80 feet below grade into dense sands. The building designers predicted 6 inches of settlement over the life of the structure. Transbay spent $58 million for an ‘underground buttressing system’ prior to the start of excavation. The system consists of what sounds like a secant pile wall made up of 7 foot diameter reinforced drilled shafts that are embedded into bedrock.
The condos at the tower sell from $1.6 million to more than $10 million and reportedly are home to some pretty high-profile residents, including former 49ers quarterback and hall of famer Joe Montana. With so much money riding on the line, you can bet there will be a big legal battle. And geotechnical experts will be front and center!