A yearslong retrofit effort to stop San Francisco’s Millennium Tower from tilting and sinking is in its final stages has been deemed successful in counteracting the 409-unit condo building’s issues.
The Millennium Tower’s Homeowner’s Association announced on Wednesday that an analysis of the tower’s foundation showed recovery of nearly one inch of tilt following a final load transfer that was conducted as the last phase of the $100 million project over the past 10 days.
“We look forward to completing the remaining non-structural elements of the Perimeter Pile Upgrade in the coming months and are confident that the engineering upgrade will restore our building’s reputation and the value of condominiums while putting to rest to any lingering questions about the Tower’s stability,” said Association President Howard Dickstein.
A total of 18 million pounds of the 645-foot building’s weight were transferred onto 18 concrete piles that were installed below ground to anchor it to bedrock. The retrofit project aims to relieve stress on soils that have compressed beneath the tower, which caused its movement.
With the structural work now completed, all that remains is the installation of vaults allowing for maintenance access and the restoration of the muni lines, sidewalk and landscaping on Fremont and Mission streets, according to the HOA. This final phase of the project is expected to wrap up by the end of August.
Ron Hamburger, the tower’s engineer of record, confirmed on Wednesday that the building has “stopped settling” and has “started to recover some of the tilt that has occurred over the years.”
The building at 301 Mission St. had already recovered about three-eighths of an inch of its westward tilt and one-eighth of an inch of its northward tilt in January following an initial load transfer that saw 1.5% of the tower’s weight jacked onto the six piles installed along Mission Street, and was out of the ground by about a tenth of an inch at the time. The project also involved the installation of 12 piles along Fremont Street.
As of this January, Millennium Tower had settled about 18 inches and was tilting to the west about 28 inches and to the north about 12 inches.
“In January we activated the the piles on Mission Street, but not on Fremont Street, and we put a partial load on those piles. That was basically to stabilize the building and to allow the contractor to do the work on Fremont Street without causing additional building movement,” Hamburger said. “That worked very successfully. Since January, they’ve excavated down 25 feet along Fremont Street. They’ve constructed an extension of the existing foundation to encompass the Fremont Street piles. And within the last 10 days, they’ve stressed those piles up.”
The positive news followed reports that the retrofit had actually accelerated the building’s lean and settlement, causing it to be temporarily suspended in the summer of 2021 so that the project’s procedures could be modified. The revised plan decreased the amount of piles that were to be installed as part of the project from 52 to 18.
The tower was completed in 2009 and gained international notoriety in 2016 when it was discovered the structure was sinking about an inch a year and leaning up to 14 inches, sparking a lawsuit by its homeowners. All parties involved eventually reached a legal settlement in 2020 that cleared the way for the retrofit, which began in early 2021.
Hamburger described the retrofit program as the “most challenging project” of his 50-year career.
“The size of the building, the amount of load we were transferring — it’s really the first time that’s something of this magnitude has been done,” he said. “The basic approach, which is called the underpinning, has been done many times, but at much smaller scales. So we really had to invent some things to basically let us deal with the difference in scale of this project.”
The tower will be monitored for the next decade. Hamburger said that initially, surveyors will be “measuring the elevation of the mat and the tilt of the building on a weekly basis.”
“After a period of about six months, assuming things are going well, and we fully expect they will, that’ll drop off to a monthly basis and then finally to a quarterly basis,” he said. ” There will be periodic measurements for the next 10 years. And reports will be filed with the city to assure them that the building’s behaving well.”