Since the problem was initially identified in 2016, San Francisco’s Millennium Tower has been sinking and slowly but surely tilting westward toward the adjacent Mission Street. While Handel Architects, DeSimone Consulting Engineers, and developer Millennium Partners argue that the project’s execution and design were sound, with current structural issues stemming from the construction of the equally flawed Transbay Transit Center nearby, one fact remains clear: the tower needs to be stabilized to stop its sinking. Now, the developers and structural engineering firm Simpson Gumpertz & Heger have submitted a $100 million proposal to retrofit the tower with a new system of piles reaching into bedrock.
Currently, the tower sits on a 10-foot-thick concrete foundation connected by nearly 1,000 reinforced concrete piles driven nearly 90 feet into a layer of soft clay that extends throughout the Bay Area. While this practice is not uncommon in downtown San Francisco, the sheer mass of the city’s fourth tallest tower puts considerable pressure on the soil below.
The perimeter pile upgrade includes drilling 52 new steel and concrete piles along two elevations. (Courtesy of the Millennium Towers Homeowners Association)
The stabilization plan, led by senior principal Ronald Hamburger of Simpson Gumpertz & Heger, consists of about 50 new steel and concrete piles drilled 250 feet into the bedrock. “Each pile is 24 inches in diameter and weighs 140,000 pounds,” according to the San Francisco Chronicle, “and it would take three to four days to dig” and “an inner reinforced concrete pile would be inserted in each steel shaft.”
How will this new ring of piles remedy the tower’s sagging? According to the Millennium Towers Homeowners Association, the “perimeter pile leveling” will reduce compression on the clay soils along the north and west elevations, while promoting secondary compression and two to three inches of settlement on the east and south elevations. This plan will effectively reduce, but not eliminate, the 14-inch tilt of the building while prospectively stabilizing the overall foundation.
The Perimeter plan is “an effective and practical approach to settlement and tilt issues,” Hamburger said, that “preserves and improves the safety of the building.” Construction could be completed in 18 months if the city approves the permit application.