New data shows San Francisco’s Millennium Tower tilted an additional quarter of an inch during the week of the first test of the troubled fix -- the maximum level predicted by the designer of the $100 million project.
Since work began on the plan to shore up the tower on two sides in May, the building has sunk more than an inch at the northwest corner, which translates to another 5¾ inches of lean to the west toward Fremont Street. The sudden accelerated settlement led to a work stoppage in August and a test program for new construction strategies designed to stop more sinking and tilting during construction.
The monitoring data covering that first phase of testing released Thursday indicate the tower sank about 1/20th of an inch at its northwest corner, while tilting a quarter of an inch toward the west in the period ending Oct. 18. Those amounts are roughly in line with what fix engineer Ron Hamburger had told residents were the most that could occur -- about 1/16th of an inch of settlement and up to a quarter of an inch in new tilt to the west.
That added settlement, however, nearly matched the largest drop logged since all pile installation work stopped on Aug. 22. The biggest recorded drop – slightly more than 1/20th of an inch – was recorded the period ending Sept. 20 – a timeframe when pile and casing installation had been halted. It also occurred after Hamburger had assured residents that settlement had slowed considerably because of the work stoppage.
Last week, soon after crews installed the 100 foot long, three foot wide steel casing in the first test phase, Hamburger declared the test was “successful,” stressing that building movement over two days was approximately one hundredth of an inch.
“This demonstrates that it is possible to install the remaining casings,” he told homeowners. “A second test will be performed soon to confirm that other aspects of the work can be performed in an acceptable manner. If this second test is successful, we look forward to a resumption of work and completion of the project.”
Bob Pyke, a veteran geotechnical expert, cautioned against overinterpreting the limited available data. “But settling and tilting clearly continue,” he said, adding: “There is no clear explanation why this is happening, which I think is very troubling.”
Meanwhile, Hamburger has been pushing for a second round of testing – which involves sinking a 24-inch wide test pile more than 200 feet down to bedrock. But that phase is not expected until after next week at the soonest, said Patrick Hannan, spokesman for the Department of Building Inspection. He said it had nothing to do with the results recorded to date, however.
"There is issue with a piece of equipment, so they are not going to start on the 24-inch piles," Hannan said, adding that the fix engineer is developing a proposed new test plan for submittal on Friday.