Underground wall could be tied to SF’s Millennium Tower repair plan – NBC Bay Area

A nearly forgotten underground wall could prove a serious impediment to reversing the tilt of the Millennium Tower, a major goal of the $100 million fix for the ailing San Francisco skyscraper, officials said. experts at the NBC Bay Area Investigation Unit.

The perimeter piling upgrade – which began a year ago this month – relies on installing piles on bedrock and supporting the sinking and tilting building over two sides along Mission and Fremont streets.

The idea is to both stop the settlement at this corner and shift some of the tower’s weight to the opposite side of the foundation. In theory, this shifted weight should trigger further settlement on opposite sides and cause a partial tilt reversal. New data shows that the tower currently leans 28 inches to the northwest, measured from the roof. Much of this leanness occurred last summer, after crews began digging to install piles.

During a hearing before the board of supervisors this year, Millennium repair engineer Ron Hamburger stressed the need to complete the beleaguered repair effort.

“While the building remains safe, we believe the project needs to resume construction and complete this construction quickly,” Hamburger said at the January meeting of the government audit and oversight committee. “To allow us to recover the tilt as originally planned.”

But some experts say there could be a hitch – a big one – to this plan. It is a three-foot-thick, 90-foot-high steel and cement wall built directly below the eastern edge of the 10-foot-thick slab foundation.

“It just creates a lot more uncertainty as to how it will react when you implement…the fix,” said deep foundation expert David Williams.

The wall was put in place early in the project to act as a buffer between the tower foundation and the five-level deep parking garage being constructed adjacent to the east side of the tower.

This is the side, according to Hamburger’s plan, that must stabilize and sink once the weight is shifted in order to right the leaning tower. The wall is shown in the early construction photos, which also show that just above the wall is a space filled with about a foot of dirt. The gap was designed to allow the building to settle the 6 inches that engineers estimated would occur after construction. But the building has sunk so much that surveys in 2018 indicate it is now almost above that wall. Williams says the wall could support that side of the building indefinitely and in doing so negate any chance of reversing the tilt.

“So there’s a lot of concern that it could be snagged on the shoring wall,” Williams said.

Asked by a panel of city-appointed experts whether the wall could prevent the building from straightening, Hamburger assured them that when the weight of the building shifted east, the entire wall below the foundation would sink. in the clay below, and this will allow the tower to begin to straighten.

But veteran geotechnical engineer Bob Pyke remains skeptical.

“I’m inclined to think the wall won’t come down very easily,” Pyke said. “It is possible that after or if the upgrading of the 18-pile perimeter piles is completed, that practically nothing will happen, that it will remain there.”

Pyke says the main unknown is whether the wall is strong enough to support the weight without sinking. If the wall holds, the solution may stop the sinking at the northwest corner, but the tilt – which is expected to reach 30 inches by the end of the project – could be permanent.

A nearly forgotten underground wall could prove a serious impediment to reversing the tilt of the Millennium Tower, a major goal of the $100 million fix for the ailing San Francisco skyscraper. Raj Mathai talks to Jaxon Van Derbeken about it.

Bonny J. Streater