San Francisco tower is tilting at 3 inches a year

Resident told in 2016 that building built in 2009 was settling into ground more unevenly than anticipated

A San Francisco tower is sinking and tilting at three inches per year as engineers scramble to find a solution.

The Millennium Tower, a luxury residential building, is now two feet off centre. Structural engineer Ronald Hamburger detailed an updated solution to fix the foundation of the building during a city hearing on Thursday, NBC Bay Area reported.

The building has 58 stories and measures 645 feet (197 metres) and opened in 2009. It’s now tilting 26 inches northwest in the middle of the city’s financial district.

In 2016, residents were told that the way the building was settling into the ground was more uneven than had been anticipated. The builders expected that the building would settle at most 5.5 inches by 2028, NBC News reported that year.

The skyscraper is situated beside the Salesforce Transit Center, a bus terminal, and the possible location of the end station for California’s high-speed rail network that’s currently being built.

Attempts to fix the sinking and tilting building appear to have made the issue worse. Engineers stopped their work last summer to “determine why increased foundation movement was occurring and how this could be mitigated”.

Mr Hamburger on Thursday suggested decreasing the number of support piles under the tower from 52 to 18 to “minimize additional building settlement”.

In a letter to the general manager of the Millennium Tower last month, engineers said the new and faster solution was necessary because of the “vibration of the soils associated with pile installation activity and unintentional removal of excessive soil as the piles were installed” made the settlement of the building worse.

Mr Hamburger said the support piles would be installed into bedrock 250 feet (76 metres) under the tower. The piles would be cutting through clay and sand soil that’s compressing quickly.

In a document laying out answers to questions, Mr Hamburger said that if more than 18 piles are put into place, “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,” he added.

There was a delay of between one and four days last summer between removing the soil for the current six piles and the injection of grout to counteract the collapse of the soil, according to a review of the plan to upgrade the piles. The delay went against protocol and “could very well explain the comparatively rapid settlement and tilting that occurred during pile installation in August”.

A spokesperson for the Millennium Tower Homeowners Association, Douglas Elmets, told NBC that the new pile plan won’t halt the sinking of the building “until the piles are driven into bedrock and attached to the foundation, which will occur later this year”.

The San Francisco Department of Building Inspection (SFDBI) told the Millennium Tower Association in a letter on Thursday that the plan to install 18 piles had been approved, saying that they are “satisfied that the associated settlement and tilt remain within safe ranges and support [Mr Hamburger’s] proposal to continue the retrofit using the modified installation procedures”.

The SFDBI said they would perform an inspection following the installation of each pile.