Deep Foundations

San Francisco’s Sinking Tower

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San Francisco’s 58-story Millennium Tower was once praised by the concrete and engineering communities. Today, the city’s tallest residential tower stands as a cautionary tale.

Since its completion nearly eight years ago, the SoMa-located tower has sunk 16 inches and tilted at its base by at least two inches to the northwest.

For perspective, up to six inches of settling during the life of a structure is considered normal.

“It’s not an eminent danger, but is obviously an eyebrow raising amount of settlement,” said Patrick Otellini, chief resilience officer with the city of San Francisco, adding the tower was designed above code minimum.

The massive concrete structure was built on unconsolidated man-made fill that was once submerged, San Francisco shoreline. Because of this, the soil at the site is highly susceptibility to liquefaction and saturation, which leads to instability. Put a heavy structure on unstable soil and various amount of settlement can occur, depending partially on how the building is anchored.

Likely knowing this, the builder owner still opted to anchor the building with 80-foot-deep piles, which went through mud-fill and dense sand, rather than with 200-foot-deep piles into bedrock.

Had the later piles been utilized, some believe the building would not be tilting; however, other concrete towers in SoMa have foundations at the same subterranean level.

Unintended Discovery

The building’s rate of sink and tilt has been known and monitored since 2010. The shifting first came to light during pre-construction of the Transbay Transit Center, which border’s the south side of the tower.

The Transbay Joint Powers Authority (transit authority), the public agency constructing the transit center, hired a consulting firm to determine the effect excavation operations for the project could have on the existing tower.

According to the consultant, two years after its completion the tower had settled 10 inches.

Knowing this, prior to excavation the transit authority spent nearly $58 million on an underground buttressing system, intended to shore up the tower before digging commencement.

Since that point the tower has sunk an additional six inches, and is now sinking at a rate of roughly an inch per year. It’s also been reported that cracks and water intrusion have appeared in the building’s underground garage.

Otellini believes some type of settlement-stopping retrofit is imminent and as significant litigation among the building’s owner, transit authority and the tower’s residents.

Finger Pointing, Value Diminution

Both Millennium Partners, the building’s owner, and the transit authority are currently placing blame on one another.

Millennium claims the excavation for the transit center contributed to the tower’s destabilization, while the transit authority points to construction defects and improper design – along with the fact that the structure had already sunk 10 inches prior to transit center excavation.

In 2008, the transit authority agreed to pay the tower’s owner for any soil settlement or cracking caused by the transit hub’s construction. If found liable for any of the tower’s settling issues, this agreement could cost the transit authority and taxpayers millions in retrofit costs and test the $2-billion-dollar budget of the transit terminal.

While the two parties point fingers at each other, tower residents are taking action.

In early August, four law firms teamed up with one resident to file a class action lawsuit against both Millennium Partners and the transit authority. Citing improper design on the part of the building owner and further destabilization of the tower on the transit authority, the claim states the structure’s sink and tilt are significantly de-valuing the condos.

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