The Millennium Tower is sinking at a faster rate than the limit that had been set by engineers working on the fix for the troubled building, an analysis of monitoring data shows.
“We are outside the limits in an alarming fashion,” says Rune Storesund, an engineer who is the executive director of the UC Berkeley Center for Catastrophic Risk Management.
Storesund has been monitoring the sinking and tilting of the tower for years. Since work started on the “perimeter pile upgrade,” the building has leaned five more inches toward Fremont Street, for a total of 22 inches.
The upgrade – approved by a city-appointed engineering design review board in 2019 – involves extending 52 piles 200 feet down to bedrock and tying them to an extended foundation to prevent more sinking on two sides. Modeling submitted as part of the fix proposal suggested the tower would partially shift back the other way as a result, partly righting itself over time.
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But when work began with drilling three foot wide holes to install piles along Fremont in May, the problem got worse, not better. Work was halted last week, the same day city officials released data showing accelerating tilting and sinking despite a partial work shut down.
Documents show that fix designers have so far blamed the new settlement on construction activities, including crews having drilled holes for the piles extending to bedrock that were slightly oversized. Experts say oversized holes can cause soil to shift that had been supporting the existing foundation.
In a chart summarizing monitoring data as of mid-August, fix engineers posit a “limit” of sinking. That limit, the chart specifies, is an inch of settlement per year. At that rate, the chart indicates, the building would reach the maximum level of 5.25 inches in mid-2026.
The notes on the chart suggest that the settlement would then halt and remain 5.25 inches at the base in the following years. City building officials did not respond to questions as to how the fix-engineers determined either the one-inch-per-year “limit” or the 5.25 inch threshold for maximum settlement.
However the fix engineers established the limit, the accompanying data indicated that the building’s northwest corner had, in fact, settled about one inch between mid-May and mid-August alone and was headed at steeper that the specified limit rate toward the 5.25 inch maximum.
“That’s a big red flag,” Storesund said. “It’s tilting and leaning way more than they predicted.”
Documents submitted to the city as part of the fix include an analysis of expected settlement accompanying the project. It predicts as much as four inches settlement – but at the core of the building. The sides would experience less than two inches of settlement over a 50 year span, the analysis indicates. But soon after construction, the building began to sink still more along Fremont Street and engineers have predicted more settlement would occur on the Mission Street side of the building.
“Clearly, there is a deviation or a skew between what you thought was going to happen and what’s actually happening out there,” Storesund said. “This is a non-routine, non-standard project….and it’s a heavily congested city environment, so the consequences of if you don’t get it right, are pretty severe….”
Given the gravity of the issues involved, Storesund urges the city to seek guidance beyond that provided by the city’s engineering design review panel, the same panel that approved the troubled fix in 2019. He says the city could use the advice of scientists with no prior involvement.
“The fact that we are seeing, time and time again, performance outside the bounds of what you expected,” Storesund said, makes it “time for a fresh look, a fresh set of eyes -- relying on the same team to provide that fresh perspective, I think would be challenging.”
So far, documents show the engineering design review panel expressing full confidence in the fix team as of last month. City Department of Building Inspection officials said Tuesday they are awaiting more data about the settlement to determine next steps.