Uh oh, this is not good:
- Millennium Tower Sank More Even After Partial Work Stoppage, Documents Show
on July 30, the homeowners association board agreed to put a two-to-four-week hold on shaft drilling work, documents show.
But by that time, 39 shafts had already been installed. Soon, crews refocused on sinking 24-inch diameter piles along Fremont Street.
But by mid-August, it was apparent from the data that the problem was more complex than fix engineers had hoped. Despite the pause, the building had settled another half inch – possibly due to soil being displaced during installation of the 24-inch piles. That prompted the board to put all pile installation on hold as of Monday.
Data released by the city on Monday shows the tower is currently leaning 22 inches towards Fremont Street – compared to 17 inches when work began.
- Millennium Tower's Accelerated Sinking Halts $100 Million Effort to Stop It
an outside expert, Oakland-based structural engineer David Williams, tells NBC Bay Area that the new data on accelerating sinking is not nothing.
"The trend is the thing that’s very disturbing, the fact that they have reactivated settlement,” he tells the station, adding that the speed of the new sinking is of special concern."
- Repairs of Leaning San Francisco Skyscraper on Hold; Engineering Expert Blasts Plan
Karp says each new piling being driven is causing more issues.
“You never place piles or piers closer than three pier diameters apart. These are 36 feet [sic; I think the piles are 36 *inches* in diameter] so they should be nine feet apart and they’re, what? — five feet or something. As you do one, you’re disturbing the ground, then you go to the next one you’re disturbing the ground there and you go to the next one until you have a whole zone of disturbance that can’t be fixed,” Karp explained.
- San Francisco's Millennium Tower fix halted after further sinking observed
The fix has been likened to putting a bumper jack next to a flat tire, and involves the installation of piles 250 feet deep along the north and west sides of the tower, to be tied beneath the sidewalk to the original foundation.
From last summer, here's a picture and an article with more details on the repair that is being attempted:
- Reviewers OK Fix for San Francisco's Leaning Millennium Tower
The structural upgrade is designed to meet the requirements of the voluntary seismic improvements section of the San Francisco Existing Building Code.
SGH’s Hamburger says his team selected the voluntary seismic upgrade route because it offered the easiest path through the permitting process. “The building code has provisions to allow VSUs without having to bring the building into conformance with the current code,” he says. The primary intent is to arrest settlement, but the 52 piles also will improve seismic performance, he adds.
The existing mat is supported by 950 14-in.-square precast concrete piles. The aim is to remove 20% of the building weight from the underlying clay strata.
The load reduction represents “what I could comfortably transfer from the new piles to the existing mat without major modification of the existing mat,” says Hamburger, whose client is Paul Hastings LLP, a lawyer retained by the tower’s developer, Mission Street Development. MSD is part of Millennium Partners.
The system relies on loading each pile with 400 tons using a permanent hydraulic jack that reacts against a new mat extension. The jacks would be housed in a maintenance access vault above the mat. The scheme calls for a so-called indicator test pile, which is installed to scale.
The weight loss would result in an almost immediate rebound of about 1 in. of the tower’s north and west sides. That would remove about 25% of the tilt, predicts Hamburger. “Over time, we expect another 25% to 50% of the tilt to come out through continued settlement of the south and east sides,” he says.
Under the plan, piles would be drilled 4 ft, 9 in. on center under sidewalks—200 ft on the west side and 100 ft on the north side, just outside the tower footprint. The team has applied for an easement to work under the sidewalks, which meet at the block’s northwest corner.
Piles would extend up through an 8-ft-wide extension of the tower’s 10-ft-thick reinforced concrete mat—4 ft from the old mat. New and old mats would be connected by chipping into the side of the old mat and coupling new and old reinforcing steel.
The plan was to install 52 new piles, and they've drilled the shafts for 39 of them, so they're about 75% done with drilling the shafts.
This troubled construction project is even more troubled now.