Millennium Tower Fix Monitor sidelined from analysis role – NBC Bay Area
After generating monitoring reports that highlighted issues in the Millennium Tower test pile installation program, an outside drilling consultant hired by Millennium owners will no longer perform on-site monitoring for San Francisco building officials, NBC Bay Area News learned.
The consulting firm, Dan Brown and Associates, had been hired by the owners to notify the city of the testing effort, which triggered an accelerated tilt and sink until it was shut down in August. The building now leans more than two feet at the top toward Fremont Street, after sinking about two inches at the northwest corner.
Since resuming installation work on a test basis in October, the company has produced regular technical assessments, including a report last month that indicated that a key strategy to prevent loss of support soil during l installation was not working as well as expected.
When the first of a series of 24-inch-wide test piles were sunk into bedrock for four days in mid-November, the company generated another report. In a tech note to a city construction official on November 23, obtained by NBC Bay Area, the company revealed that more than five tons of soil had been inadvertently removed during the installation of the pile. test. The hope was to avoid losing ground under and around the foundation, as experts say this is a key factor in triggering the building sag.
Seven days after that memo, patch designer Ron Hamburger informed city officials – in a November 30 letter – that “settlement monitoring indicates that no further settling or tilting has occurred as a result of this 24-inch pilot plant “. He informed the city that the Dan Brown firm would no longer be present during the next tests. While acknowledging that this amounted to an exception to the agreed test program provisions, Hamburger said the city’s own design review committee “doesn’t think it’s necessary.”
Shortly after this letter, however, the city released data showing that during the four-day period in which the first 24-inch support pile was installed, the building had in fact tilted by. an extra quarter of an inch.
While it does not mention any monitoring data, the Dan Brown firm’s Nov. 23 report suggests an explanation. By drilling more than 250 feet deep, near bedrock, crews inadvertently “mined” or sucked up more than five tonnes of material. This loose layer of sand was then sucked up and removed during the drilling process. The report indicated that crews had to pump additional trucks of concrete grout to fill the void created during the installation of the piles. The report estimates that it took at least 5 cubic meters of additional grout to fill the void, which equates to around five tonnes of lost ground.
The report cites data showing that additional grout was pumped twice during installation in August. It was the month repair work was halted as a precaution due to accelerated sinking and tipping.
Neither the latest tilt data nor Dan Brown’s November 23 memo had been made public when the city authorized the drilling business to be kept off-site and limited its role to data analysis. daily boreholes provided to them.
“This is not a proper review,” said Bob Pyke, a geotechnical engineer and repair effort critic. “For a situation like this, the examiner has to be on site and have full access to what’s going on.”
Indeed, a second test stack was installed at the beginning of December without the Dan Brown firm carrying out on-site monitoring. Data later released by the city shows the building tilted another quarter of an inch after the piles were installed.
Supervisor Aaron Peskin – who first pushed for oversight by outside experts – wonders what lies behind the decision to decrease the company’s oversight role.
“So at the end of the day it’s not right,” he said of the decision. “We wanted a third pair of eyes and ears. The building inspection department came to the supervisory board and said it was there for the duration of the installation.
Peskin said that when asked about the decision, building officials said the Brown Company “will always review daily drilling data to analyze, advise and provide guidance to minimize settlement during installation. pilots “.
Ben Turner, the firm’s representative on the project, did not respond to requests for comment. Peskin said he would address the issue at a hearing on Jan.6.