The 59-story Millennium Tower in San Francisco was built on a floating pile foundation. Rather than sitting on bedrock, floating piles are supported by the gripping action of friction with the surrounding mud and sand, and some settling is anticipated. In the case of the Millennium Tower, the ground has been unable to adequately support the weight of the tower.
During design of the building, it was projected that the maximum vertical settling of the Millennium Tower would be 5.5 inches (or 15 centimeters) over 20 years from its completion in 2009. In fact, by 2018 the building had sunk by 18 inches (or 46 centimeters) with a lean of 14 inches (or 35.5 centimeters) to the northwest.
The issue is the subject of several legal disputes, with the developer alleging that construction of the neighboring Transbay Transit Center caused the settling, countered by an assertion that the building had already sunk beyond design projections before construction commenced on the Transit Center.
A fix involves underpinning the structure with additional piles along the tower’s north and west sides driven down to the bedrock below. It is expected that this will stabilize the building and substantially correct the tilt over a ten-year period.