Millennium Fix engineers want new test stack installed – NBC Bay Area

Millennium Tower repair engineers want homeowners to approve a plan to install a new test pile 200 feet to bedrock to find out what triggered a sudden settlement during their previous work to strengthen the tower already tilted.

The plan comes after engineers ruled out a previously suggested cause – the possibility that large amounts of soil had lifted and sucked into the bottom of three-foot-wide tubes used to allow the installation of piles.

In a letter to the Millennium Tower Homeowners Association, repair engineer Ron Hamburger said recent tests have rejected this theory. He says engineers are focusing on two other scenarios.

One is that vibrations when drilling 33 holes to install piles caused the soil to settle under the foundation of the building. The other is that the holes drilled during the pile installation process were too big – depleting tons of surrounding soil that had previously supported the foundation.

Hamburger says the test pile project is needed “to understand whether additional piles can be installed without causing unacceptable settlement and tilting.”
But two experts tell us the test pile program will likely prove what they already suspect – that too much soil was lost when drilling oversized holes, with the lost soil undermining the already fragile foundation. Still, they say, the testing effort may have some value.

“If they can demonstrate that they can do this without causing the accelerated settlement rates to reactivate, I guess that’s the sole purpose of doing these tests,” said David Williams, an expert in deep foundations.

But Williams said that even if the test results warrant the installation of many more piles, engineers should be “ready to shut down immediately” given the risks involved.
“I’m skeptical of the value of the tests,” said geotechnical engineer Robert Pyke, who previously questioned the viability of the fix when he reviewed the plan in 2019.

Pyke said, in his experience, that the surprise shipwreck during construction was clearly preventable if crews simply drilled the correct size holes during installation.

“It’s Drill 101,” Pyke said. “They should have known that from the start – the biggest lesson to be learned is that the design team, to varying degrees, is not competent for this kind of work.”

Pyke said he hopes the testing will allow engineers to improve the fix without further issues. Hamburger told residents that construction of the test pile, once fully authorized, is expected to begin as early as next week.

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