New plan seeking to control tower’s sinking
The saga of sinking, tilting Millennium Tower will continue this week when repair crews test a new method after months of work caused the skyscraper to sink further and efforts were halted.
The latest effort to fix the 60-story luxury tower, which leans to the west, is scheduled to begin Tuesday and involves installing a 3-footwide steel sleeve 104 feet into the ground on the Fremont Street side of the skyscraper.
Since it opened in 2009, the tower sank 19 inches and leaned at least 14 inches, prompting property values to sink as well and resulting in a legal settlement that included a $100 million repair job.
The test should help engineers determine whether a different repair method can reduce vibration and the amount of soil removed, both believed to be factors in causing the building to sink during repairs, Ron Hamburger, chief engineer for the repair job, said in a letter to the city. Sensors will also be installed on previously installed pilings and monitored during the test.
“We plan to use the data obtained from this pilot program to predict the amount of additional settlement likely to occur during construction completion,” he wrote, “and to provide assurance that settlement will not exceed tolerable levels.”
NBC Bay Area first reported news of the test of the new repair method. It reported that a second part of the test, installing a 2-foot-wide pile into the steel casing, has not yet been approved.
The $100 million repair job on the building was halted after the discovery that the work was causing the building to sink further. Crews started working on the original plan — to sink 52 piles to bedrock — in May. Unlike many buildings in the area, the Millennium Tower’s original piles did not reach bedrock.
Engineers on the project stopped work Aug. 25 to help determine why repair work was causing the building to continue to sink an additional inch. In September, city officials ordered repair crews not to resume their work until they had a new approach.
The test of the new method, which has been approved by city building officials, could cause the building to sink no more than an eighth of in inch, within the tolerance for the repair work, Hamburger wrote.
The tower at 301 Mission St. at the corner of Fremont Street was the most expensive residential project in the city’s history when it opened and drew highprofile residents such as former 49ers quarterback Joe Montana. But five years later, the building became notorious when engineers monitoring its settlement discovered it was sinking and tilting.