A $100 million fix to the tilting Millennium Tower in San Francisco has cleared reviews and approvals and construction work is scheduled to start in November, published reports say.
The tower is built on piles driven through soft, compressible clay soils deposited by San Francisco Bay and extending into a dense sand layer over ancient marine deposits of clays, silts, and sands, engineering firm Simpson Gumpertz & Heger (SGH) says.
“The city and state had a plethora of permit requirements for us,” SGH senior principal engineer Ronald O. Hamburger was quoted as saying in SFGate.
Construction contracts are reportedly complete and the prime contractor is in the process of securing a insurance policy, but Hamburger did not reveal the contractor’s name.
The tower has settled more than 17 inches, tilting four inches across its footprint — after the structure’s weight and nearby construction projects caused soil consolidation and lateral displacement.
SGH says it:
- Developed detailed nonlinear models of the structure, its foundations, and underlying soil to simulate the effects of settlement and tilting;
- Conducted detailed nonlinear analyses of the structure’s response to earthquake shaking; and
- Designed an underpinning retrofit for the structure, consisting of jacking approximately 20 percent of the building’s weight onto 52 new foundation piles, extended to rock along the structure’s north and west sides.