San Francisco’s Millennium Tower is on the move again.
The building, known for years for a pronounced tilt, moved another quarter inch during the four days it took to install a test piling, NBC Bay Area reported. The tower has settled almost two inches at the northwest corner and tilts just more than two feet at the same edge since work to secure its north and west sides began in May.
Bob Pyke
The latest data, which tracks settlement, tilting and water pressure levels under the building, shows that it also settled another tenth of an inch during the installation.
Geotechnical engineer Bob Pyke said the movement is small yet significant. “This is a large drop — you can’t see the scale on this plot — but this is a pretty dramatic effect,” he told NBC.
Engineers also noticed a fluctuation in water pressure more than 100 feet below the foundation ,where there is a large layer of clay, on the Mission Street side of the property.
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Efforts to stop San Francisco's Millennium Tower from further sinking, leaning resuming
Pyke said that while the data shows pressure quickly returned, the loss would probably generate some settlement due to the drilling method.
“It’s no different from sucking a straw into a milkshake,” Pyke said. The removal method uses suction to vacuum water and debris from the bottom of the shaft that’s being drilled and Pyke said the process is most likely causing stress to the clay under the foundation.
Other local experts say the drop in pressure is actually evidence that the method may not be working as well as intended.
Rune Storesund
“You can accidentally remove soil that you want to stay in place,” Rune Storesund, a geotechnical engineer who runs UC Berkeley’s Center for Catastrophic Risk Management, told NBC. “You’re always going to get settlement — obviously you want that to be as low as possible.”
Ron Hamburger, who designed the fix, has assured city officials that the latest settlement is within expected levels. He has been granted permission to install two more test piles to help determine how many twill be needed to fully secure the structure.
[NBCBA] — Victoria Pruitt