San Francisco

Millennium Tower Granted Permission to Resume Limited Work on Fix Thursday

The “fix” relies on sinking piles to bedrock on two sides of the building to stop and even reverse the sinking of the 58-story tower.

NBC Universal, Inc.

San Francisco building officials are allowing some work to resume on the so-called fix of the leaning Millennium Tower after a key test of modified installation techniques earlier this month showed “minimal overall” additional settlement and tilt where the tower is leaning the most.

The “fix” relies on sinking piles to bedrock on two sides of the building to stop and even reverse the sinking of the 58-story tower. But in the first three months of fix work, the tower sank more than an inch at the northwest corner and tilted 5 ½ inches west toward Fremont Street, forcing a halt to work in August.

Fix engineers scrambled to find ways to limit vibration and soil loss during installation – two of the suspected causes of the accelerated settlement since May.

A first key test involved installing a single three-foot-wide, 100-foot-long casing at the northwest corner of the building, where it’s tilting and sinking the most. Fix designer Ron Hamburger declared it “successful” soon after the test was done, citing real-time data showing settlement of about one hundredth of an inch over the two day test.

Data for the entire week, however, reflected settlement of 1/20th of an inch, with a lean of a quarter of an inch west toward Fremont Street. That level was just under the maximum tilt that would have been allowed to happen during the test itself.

The weekly rate also approached the level of sinking and tilting that had been occurring during the earlier pile installation over the summer, data shows.

Robert Pyke, a veteran geotechnical engineer, said one weeks’ worth of data is far more indicative of what is going on than two days. In his view, the data showed the modifications were not as effective as Hamburger had concluded.

“The settlement and tilting resulting from any operation is not instantaneous, it develops over time,” he said. “It troubles me that the design team still doesn’t understand the causes of the sinking and tilting.”

The plan had called for a second phase of installing a pile down to bedrock. But the equipment to do that work is not available, so Hamburger sought permission to install more casings.

The now approved alternative is to push forward and install five casings along Mission Street, according to a letter by the acting head of the DBI, Patrick O’Riordan.

“Our position is based on the minimal overall settlement and tilt produced by the 36-inch pilot casing” test, O’Riordan told homeowners in a letter Wednesday.

The work is conditioned on real time settlement monitoring, the supervision of a drilling expert, and a requirement that work stop if the tower settles more than a quarter of an inch at the northwest corner.

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