The effort to shore up the foundation of Millennium Tower in downtown San Francisco, which began in May, may be contributing to an accelerated rate of settlement that engineers are observing, causing alarm.

We learned last week that the $100 million project to dig down to bedrock and add new concrete pilings around Millennium Tower — which will in turn be attached to the building's existing foundations and ultimately stop it from sinking and tilting — had been halted. The reason for the pause is that the 58-story tower had sunk a full inch since the new digging began in May and tilted an extra five inches toward Fremont Street, for a total of 22 inches of tilt at the top.

As NBC Bay Area reports, the exact cause of the accelerated sinking has not been announced, but it is clearly well beyond the expected rate set by engineers of one inch per year, or 5.25 inches total by mid-2026.

And it's logical to surmise that digging around the building's foundation has caused it to sink faster. Over the past several months, workers have been drilling three-foot-wide holes along the Fremont Street side where the new pilings will go, and NBC Bay Area reports that construction documents suggest that these construction activities have led to the accelerated settlement 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," says Rune Storesund, an engineer and executive director of the UC Berkeley Center for Catastrophic Risk Management, who has been monitoring the building's sinking. "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."

Storesund tells NBC Bay Area that the city ought to now be seeking a second opinion about what is happening — getting expert advice outside of the engineering design review panel tasked with approving the fix that's underway, which they did back in 2019.

"The fact that we are seeing, time and time again, performance outside the bounds of what you expected... it's 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," Storesund tells NBC.

The $100 million project, which is expected to significantly slow and ultimately stop the tower from sinking and fix its tilt, is being partially paid for by San Francisco taxpayers, because it was funded in part by the Transbay Joint Powers Authority (TJPA), a city entity. That is due to a settlement with the tower's developer that the TJPA said "minimized the risk to the public from pending claims" in the future.

Prior to the construction of towers that reached the heights of Millennium Tower and Salesforce Tower, it was common practice in this part of the city not to drill all the way down to bedrock. But the developers of Salesforce Tower did, and that construction began just a few years after Millennium Tower's 2009 completion.

The developer, New York-based Millennium Partners, is currently trying to build another skyscraper in Los Angeles, and as the Los Angeles Times reports, that project has faced pushback from state geologists who point to seismic concerns from an active fault that runs under the site.

A spokesperson for the homeowners' association of Millennium Tower, Doug Elmets, said in a letter to residents last month that there "has been no material harm to the building and it remains fully safe."

Previously: Millennium Tower's Accelerated Sinking Halts $100 Million Effort to Stop It

Photo: Justin Sullivan/Getty Images