After generating monitoring reports that highlighted problems in Millennium Tower’s test pile installation program, an outside drilling consultant hired by the Millennium homeowners will no longer be performing on-site monitoring for San Francisco building officials, NBC Bay Area News has learned.
The consulting firm, Dan Brown and Associates, had been hired by the homeowners to advise the city about the testing effort, which had triggered accelerated tilting and sinking until it was halted in in August. The building is now leaning more than two feet at the top toward Fremont Street, having sunk about two inches at the northwest corner.
Since installation work resumed on a testing basis in October, the firm has regularly generated technical assessments, including a report last month that indicated a key strategy aimed at preventing the loss of supportive soil during installation was not working as well as hoped.
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When the first of a series of 24-inch wide test piles was sunk to bedrock over four days in mid-November, the firm generated another report. In a technical memo to a city building official on Nov. 23, obtained by NBC Bay Area, the firm revealed that more than five tons of soil had been inadvertently removed during the test pile installation. The hope was to prevent losing ground under and around the foundation, because experts say it is a key factor in triggering building settlement.
Seven days after that memo, fix designer Ron Hamburger notified city officials – in a Nov. 30 letter – that “settlement monitoring indicates that no additional settlement or tilting occurred as a result of this 24-inch pilot installation.” He notified the city that the Dan Brown firm would no longer be present during upcoming testing. Although acknowledging that amounted to an exception to the agreed-to provisions of the testing program, Hamburger said the city’s own appointed design review panel “does not believe this is necessary.”
Soon after that letter, however, the city released data showing that over the four day period that the first 24-inch support pile was installed, the building had in fact tilted an additional quarter of an inch.
Although it does not mention any monitoring data, the Nov. 23 report from the Dan Brown firm suggests an explanation. While drilling more than 250 feet down, near bedrock, crews inadvertently “mined” or sucked out more than five tons of material. That loose layer of sand was then vacuumed up and removed in the drilling process. The report noted that crews had to pump in truckloads of additional concrete grout to fill in the void created during pile installation. The report estimates it took at least 5 cubic yards worth of additional grout to fill the void, which is the equivalent to about five tons of lost ground.
The report cites data showing that additional grout was twice pumped in during installation in August. That was the month when fix work was halted as a precaution because of accelerated sinking and tilting.
Neither the latest tilting data nor the Dan Brown Nov. 23 memo had been made public at the time the city okayed keeping the drilling firm off-site and limited its role to analyzing daily drilling data provided to them.
“This is not a proper review,” said Bob Pyke, a geotechnical engineer and critic of the fix effort. “For a situation like this, the reviewer needs to be on site and have full access to what’s going on.”
In fact, a second test pile was installed in early December without the Dan Brown firm performing on-site monitoring. Data later released by the city shows the building tilted another quarter of an inch following that pile installation.
Supervisor Aaron Peskin – who pushed for the outside expert oversight in the first place -- wonders about what was behind the decision to diminish the firm’s oversight role.
“So, bottom line is it’s not okay,” he said of the decision. “We wanted a third set of eyes and ears. The Department of Building Inspection came before the Board of Supervisors and said that they were there for the duration of the installation.”
Peskin said that when he asked about the decision, building officials said the Brown firm would still “review daily drilling data to analyze, advise and provide guidance to minimize settlement during the pilot pile installations.”
Ben Turner, the firm’s representative on the project, did not respond to requests for comment. Peskin said he’ll take up the issue at a hearing Jan. 6.