San Francisco building officials have formally signed off on a scaled-down plan to fix the sinking and tilting Millennium Tower, a key hurdle that fix officials say will allow engineers to do the work needed to support one corner of the building to bedrock by the end of this year.
The revised plan relies on just 18 piles, instead of the original 52, to anchor the high-rise to bedrock on the two sides where it leans and tilts the most, on Mission and Fremont streets.
Fix engineers billed the original 52-pile plan as not only preventing more sinking, but also reversing the tilt.
But as soon as work started to install piles to bedrock, the building began to lean even further. That prompted fix officials to scrap their original 52-pile plan and submit a new proposal to rely on the 18 support piles already in the ground. They say the reduced number of piles will work just as well.
As part of that work, crews need to extend the existing foundation. Work has started on digging a 25-foot deep trench at the northwest corner of the building to make room for that extended foundation. The plan is to shift some of that load from the leaning building onto some of those bedrock-supported piles.
An expert who has criticized the fix even before it was approved warned that the plan will not likely work the way the fix engineers hope.
“The load transfer from the existing foundation to the new perimeter piles is very complex,” says veteran geotechnical engineer, Bob Pyke, a longtime critic of the fix.
He says that by design, the new support piles are not rigid. They function more like adjustable cushions to keep from damaging the existing concrete slab foundation when load is shifted onto them. But Pyke worries by scaling down to just 18, the piles won’t function as well as they would if there were 52.
“You don’t stop continuing settlement,” he said. “So that’s a basic flaw in the design.”
Pyke has argued that is just one reason city officials should halt the project. Another, he says, is the risk from all the new digging needed at the corner of Mission and Fremont Streets. He fears that will rob supportive ground where it’s needed most. “The last thing that you want to do to a tower — a building that is tilting in one direction — is dig 25 feet deep excavations along the sides of the building that are settling and tilting,” he said.
But now that the city has approved the scaled-down plan, fix engineers say the work at the corner of the tilting tower should be done by year’s end. While an outside expert questioned the assumptions of the computer models used for the fix, its engineers stand by their assertions that the 18 piles are enough to stop the sinking and reverse some of the tilt. The building currently leans about 29 inches at the northwest corner. Fix officials say the building’s elevator and life-safety systems would start to fail should the building reach 40 inches of tilt.
Source: NBC Bay Area