Teams began drilling new test holes around the base of San Francisco’s Millennium Tower on Friday as part of a city-mandated effort to investigate what was wrong with the planned solution. the tilting structure. The city also wants to prevent the problem from escalating when the troubled $ 100 million project resumes.
The NBC Bay Area Investigation Unit was the first to report last month that the repair of the 58-story tilting structure – which relies on driving new piles into the bedrock to support the building that s collapsed on two sides – had caused the building to sink another inch and tilt five more inches in the space of three months. It currently leans over 22 inches toward Fremont Street at the top of the building.
Last week, the city’s building inspectorate called for a complete halt to allow time for a thorough investigation. In a notice to residents on Friday, tower management said the goal of the testing effort is to “better understand the condition of the soil immediately adjacent to the piles that have already been installed.”
The drill samples will be analyzed in a lab and teams will install sensors at the bottom of the holes to measure vibration levels when installing new piles, the advisory said.
In communications with city officials and the expert group, repair engineers blame the vibrations during pile digging to cause sudden densification and greater settling of soil layers under the building.
An outside expert – who had questioned the viability of the fix long before work began – now questions both that theory and the timing of the testing effort.
“If it’s needed now, why wasn’t it done before they started this $ 100 million fix?” Asked veteran geotechnical engineer Robert Pyke, who expressed concerns about the viability of the patch in 2019.
While Pyke says he supports testing now, he says a more plausible explanation for vibration is the removal of soil while drilling for the pile installation that began in May along Fremont Streets and Mission. Removing all that soil, he said, left less ground support for the foundation.
In documents released by the city, repair engineer Ronald Hamburger calculated that, on average, 20 cubic feet of soil – weighing about a ton – had been removed for each of the 35 steel pile casings installed to date.
Pyke says the “loss of land” – especially the 35 tons of soil removed in three months – was just too much for the already troubled foundation. He says crews could have used more precise and controlled methods to avoid the problem.
“In that case, they should have taken whatever steps were necessary to ensure that there was no loss of land – anything greater than zero is unacceptable loss of land,” Pyke said in a statement. interview Friday.
The notice says the work is expected to last all weekend. It is not known, however, how quickly the pile installation work could resume on the project, which was supposed to last two years.