Crews began installing a 100-foot long steel casing deep into the ground on Tuesday, a test that engineers working on the troubled fix of San Francisco’s Millennium Tower hope will prove that modified construction methods will prevent more sinking and leaning at the troubled high rise.
The building had sunk an inch at the corner and leaned 5.5 inches more at the northwest corner when work on the so-called fix was suspended in August after just three months. As it stands, the high rise is leaning 22.5 inches toward Fremont Street and 9 inches toward Mission Street.
The installation of the new test casing on Mission Street is designed to examine two possible causes for the new sinking and tilting. One is vibrations during that earlier drilling for pile installation. The other is removal of supportive soil during earlier installation of the 33, three-foot wide pile casings over the summer.
Before the work began on Tuesday, crews modified a concrete “guidewall” that was built at the site to keep the casings upright during installation.
Concerned that the entire wall was vibrating during the casing installation process, fix engineers ordered that crews assure there was sufficient space between the casings and the concrete wall around them to stop the entire structure from vibrating during construction.
“It’s surprising that at this stage in the game they are they are still doing trial and error about how to install the piles without doing more harm than good,” said David Williams, an engineer and expert in deep foundations.
Williams said that while the new tests have some value, they also risk more sinking and tilting of the tower. Fix engineers say they will stop work if the building suddenly settles more than a quarter of an inch at the northwest corner.
Williams said that much settlement is unlikely, but he says the tests fail to evaluate the integrity of foundation of the high-rise.
“There are bigger issues to be resolved,” Williams said. “The many structural questions that haven’t been answered.”
Work is expected to last through Wednesday. After the results are in, fix engineers will have to go back to the city for permission to do a second test, which involves sinking a pile down to bedrock.