San Francisco skyscraper tilts 3 inches per year

San Francisco's swanky Millennium Tower is slowly sinking -- and, as a result, the 58-story skyscraper tilted up to 3 inches to both its north and west last year, according to the engineer tasked with fixing it.

Opened in 2009, the estimated $350-million project comprises two buildings, the larger of which is home to 419 luxury apartments, including a lavish $13-million, 5,500-square-foot penthouse.

The tallest residential structure in the city, the tower's uneven settling has caused cracks in the surrounding sidewalk and the basement walls of its smaller, 12-story sister building next door, reports CNN.

Though engineers are now working to stabilize the skyscraper, a city hearing last week revealed that the tilting will continue for several more months. Experts have blamed nearby construction projects and a process known as "dewatering" for weakening the soil beneath the tower.

The $100-million fix, announced in October 2020 after years of lawsuits and hearings, is expected to be completed later this year. According to the stabilization project's chief engineer Ronald Hamburger, who spoke at the hearing, the building has continued to tilt at a rate of 2.5 to 3 inches north and 0.75 to 3 inches west during 2021, he confirmed in an email to CNN.

"We start this new year 2022 as we ended last year and many other years, with the Millennium Tower continuing to sink and tilt," city supervisor Aaron Peskin said at last week's hearing, according to NBC.

In recent years, the rate of tilting has lessened as the building settled, according to Hamburger. But fluctuations have occurred, including increased leaning from September to December 2021, part of which coincided with a two-month pause in construction over concerns that stabilization efforts may be worsening the problem, according to local media outlets. As of December 31, the tower had leaned a total of 24 inches to the west and 7.9 inches to the north, Hamburger said. It has settled around 18 inches deep into the ground.

But the tilting is expected to worsen through September of this year, Hamburger said, as engineers install new support piles -- concrete columns encased in steel -- that will reach more than 250 feet into the bedrock below to anchor the building.

"Once these piles are installed... using hydraulic jacks, some of the building's weight will be transferred to these new piles," Hamburger explained. "This will reduce the excessive pressure on the Old Bay Clay materials that caused the original settlement, stop the settlement and allow the building to begin recovering some of the tilt that has occurred.

"The retrofit is not going to 'stop the sinking' until the piles are driven into bedrock and attached to the foundation," he added. "Because the stabilization is still under construction, it has not yet worked to help the building."