By David E. Hubler
Contributor, EDM Digest
San Francisco’s Millennium Tower may not last a century, no less a millennium if an unconventional engineering solution does not stop the building from tilting.
Engineers have proposed drilling piles down to the bedrock below the building to stabilize one side of the 58-story condominium. They would then let the other side continue to sink until the building straightens itself, the San Francisco Chronicle reports.
“The engineers are very confident this [repair] is going to stabilize the building,” said Vision Winter, a partner at the law firm hired by the Millennium Tower’s homeowners association to obtain funding for the mega-repair. Repairs are estimated to cost between $200 million and $500 million.
Millennium Tower Has Sunk Over 1.5 Feet Since It Opened in 2009
The Millennium Tower opened in 2009 at a cost $350 million to build. Since then, the building has sunk about 17 inches and tilted 14 inches to the west toward Fremont Street.
The high-rise condo has also sunk six inches to the north, toward Mission Street. It’s continuing to sink and tilt.
The building sits on a 10-foot-thick mat foundation, held in place by 950 reinforced concrete piles sunk 60 to 90 feet deep into clay and mud. However, the piles do not reach bedrock.
To retrofit the tower, engineers are proposing to bore 275 to 300 steel and concrete “micro piles,” each almost 14 inches in diameter, down to bedrock. But they will only place the micro piles on one side of the building at a time.
Soil tests for the proposed retrofit are already under way.
“I’m no engineer, but this building is not made of rubber — it’s made of concrete,” resident Pamela Buttery told the Chronicle. Her two-bedroom unit on the 57th floor has sloping floors. “And no one has proved to me that the concrete pad is stable enough to withstand 300 micro piles going into it,” she said.
According to the plan, the first step is for crews to work inside the building and drill about half the new piles on the building’s west side.
Once the west side is stabilized, workers would allow the east side of the building to sink long enough for the building to straighten itself. Only then would the rest of the piles be drilled down to bedrock on the east side to keep the building from sinking further. The entire process could take two to five years.
Winter said he’s hopeful the engineers have found a solution. But others aren’t so sure.
Condo Owners Lawyer Says at Best, Solution Will Stop Further Tilting
“This is not a fix,” said attorney Frank Pitre, who represents 85 Millennium condo owners, a group that is separate from the homeowners association. “At best, this is going to arrest further tilting of the building.”
David Casselman, the Los Angeles lawyer representing 265 owners who are not part of Pitre’s group, said the only real fix would be to tear down the building and rebuild it. “But that probably would cost a couple billion dollars,” he said.
Casselman is suing the Transbay Joint Powers Authority. This public organization is finishing construction on the $2.2 billion Transbay Transit Center, a huge bus and rail hub located next to the Millennium Tower.
The Los Angeles attorney said he has proof that Transbay Joint Powers Authority ignored its legal obligation to prevent the residential high-rise from sinking when it began excavation work on the Transbay Transit Center in early 2012. He said he expects the organization to pay for the repairs.
Both Pitre and Casselman are in agreement, however, in their concern about the disruption and inconvenience to homeowners while the proposed repairs take place.
“They’re telling people that there won’t be any issues of noise or vibration, and I found that astounding given the magnitude of the work they’re taking on,” Pitre said. “Construction management has told us they could have 300 trucks — dump trucks and cement trucks — over time going through here.”
Preliminary work for the repairs has already begun. Crews have torn out a portion of concrete sidewalk and the trees along the street.