The engineer trying to stabilize the Millennium Tower, a luxury residential skyscraper in San Francisco that is sinking into the ground and now leaning over two feet off of center, said the building is now tilting three inches per year.
Structural engineer Ronald O. Hamburger made the comments Thursday at a city hearing in which he pitched an updated fix for the building’s foundation, NBC Bay Area reported.
The 58-story, 645-foot tall tower — opened to residents in 2009 — is now tilting 26 inches north and west at Fremont and Mission Streets in the heart of San Francisco’s financial district, the NBC News affiliate reported.
Oct. 22, 201602:32
Residents were informed that the building is settling unevenly and more than anticipated in 2016. The tower sits beside the Salesforce Transit Center, a bus terminal and potential future rail terminus for California’s high speed rail network currently under construction.
But efforts to stabilize the sinking and leaning skyscraper seemed to worsen matters. Engineers halted construction on the fix in summer 2021 so they could “determine why increased foundation movement was occurring and how this could be mitigated.”
To relaunch the stabilization, Hamburger on Thursday proposed slashing the number of support piles beneath the tower from 52 to 18 to “minimize additional building settlement.”
A letter to the Millennium Tower’s general manager last month said the new, quicker fix was needed after engineers identified two potential causes for apparent worsening of the building’s settlement: “vibration of the soils associated with pile installation activity, and unintentional removal of excessive soil as the piles were installed.”
Hamburger said the 18 steel piles will be anchored into bedrock 250 feet under the tower, cutting through rapidly compressing clay and sand soil that the building’s foundation sits upon today.
In a question and answer document, the engineer said if more than 18 piles are installed, “the construction schedule will be extended, and the building will settle and tilt a little more during this period.”
“We judge that the 18-pile solution offers an optimal solution between additional settlement and benefit gained.”
In an exclusive investigation, NBC Bay Area reported Wednesday that an expert review of the perimeter pile upgrade plan found there was a one- to four-day-long delay in summer 2021 between excavating the soil for the existing six pilings and injecting grout to minimize soil collapse.
That gap between excavating and injecting grout went against protocol and “could very well explain the comparatively rapid settlement and tilting that occurred during pile installation in August.”
In an email, Douglas Elmets, spokesperson for the Millennium Tower Homeowners Association, said the perimeter pile plan won’t stop the building’s sinking “until the piles are driven into bedrock and attached to the foundation, which will occur later this year.”
In a Thursday letter to the Millennium Tower Association, the San Francisco Department of Building Inspection said it had approved the new 18-pile plan, writing that it is “satisfied that the associated settlement and tilt remain within safe ranges and support [Hamburger’s] proposal to continue the retrofit using the modified installation procedures.”
SFDBI said they would inspect between each pile installation to ensure work was proceeding as expected.
NBC News reported in 2016 via a public records request that the Millennium Tower was previously expected by its builders to settle a maximum of 5.5 inches by the year 2028.
Source: | This article originally belongs to Nbcnews.com
