Attorneys are preparing a class-action lawsuit on behalf of homeowners in San Francisco's Millennium Tower against the building's developer and a government agency that is constructing a transit center nearby. A complaint filed in the Superior Court of California alleges that "construction failures" by the two parties are the cause of the sinking and tilting of the 58-story residential tower since its completion in 2008.

The Millennium is the tallest residential building in San Francisco and has been ranked by Worth magazine as one of the top 10 residential buildings in the world. However, it has sunk 16 inches and tilted up to 6 inches "because of its defective foundation," the complaint alleges.

The Millennium Tower has sunk 16 inches and tilted up to 6 inches since its completion in 2008. Image credit: Hydrogen Iodide, under a Creative Commons Attribution-Share Alike 3.0 Unported license.According to plaintiff attorney Ronald Foreman, of Foreman & Brasso, the Millennium is currently sinking at the rate of around 1 in. per year and, over time, is likely to sink up to an additional 15 in. into landfill beneath the building. As a result, cracking and buckling, as well as water intrusion, have appeared in both individual units and a common area of the 419-unit building, he says.

The complaint alleges that the Millennium sits on man-made mud fill, which it describes as "highly susceptible to liquefaction" because it is unconsolidated, young and, in the case of the tower, placed in an area that was once submerged bay floor. "Highly liquefiable soil can become unstable if it experiences changes in loading, such as when a large building like the Millennium is erected," the complaint says.

To cut costs, the complaint asserts, the Millennium was anchored using a concrete slab and 80-foot piles in dense sand, rather than in bedrock 200 feet deep. The transit authority, Transbay, allegedly contributed to the subsidence by digging a 60-foot hole next to the tower and removing lateral and subjacent support. A buttressing system built by Transbay to shore up the tower prior to the beginning of excavation in 2010 for a transit center did not stop the tower's sinking and tilting, the complaint says.

Both defendants named in the complaint—which seeks "at least" $500 million in damages—have issued statements denying culpability while blaming each other for the sinking and tilting.

The Millennium Tower's concrete construction, rather than steel, resulted in a heavy building that rests on layers of soft, compressible soil, the TJPA says. "By the time the TJPA started work on its project in 2010, the Millennium Tower had already settled 10 in.—four more inches than Millennium’s engineers predicted over the life of the building."