Designers of the troubled Millennium Tower fix recently blamed the newly accelerated sinking on crews drilling “slightly larger” holes to install steel encased piles down to bedrock, documents show.
Even a slight gap between soil and the casings due to an oversized excavation hole, experts say, could potentially undermine surrounding soil.
In the case of the Millennium Tower, the soil around the new holes is believed to have been providing critical support for the tower’s already sinking foundation.
“It’s loss of ground,” said six-decade veteran geotechnical engineer Larry Karp, who co-wrote an analysis two years before the fix began predicting it would make the tower’s leaning problem worse. “When there’s loss of ground, the (surrounding) ground moves in and everything above it settles to fill that gap.”
Karp, along with structural engineer Josh Kardon, predicted in a July 2019 report to the Board of Supervisors that the loss of both soil and ground water would lead to “irreparable damage” to the building.
Monitoring data released this week showed that, as the local engineers predicted, the tower has tilted some five additional inches toward Fremont Street since work on the fix began in May. Currently, it’s now leaning some 22 inches to the north and west.
The accelerated sinking showed up soon after digging started for the 52 piles on two sides of the tower to buttress it from the side.
The building had already tilted three inches in late July when fix engineers briefed a city-appointed engineering design review panel on the situation.
An email from the head of that panel summarizing the July 30 meeting relay three theories about what led to the new settlement.
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Besides the slight-oversized holes dug for the two-foot diameter piles to be tied to bedrock, the engineers theorized that soil was heaving upwards and had to be removed during drilling of wider, three-foot diameter shafts extending the first 100 feet towards bedrock.
The digging for those bigger shafts, they said, could also be compacting the layer of soil under the existing foundation piles, contributing to causing the sinking.
But Karp says the new sinking was clearly the product of a flawed plan, not bad execution.
“If you look at the whole picture of what they had there, it was a bad design to begin with,” he said.
Meanwhile, city Department of Building Inspection officials went out to check on the life safety systems of the tower Friday and found them to be fully functioning. However, DBI officials said in a statement that inspectors did identify unspecified evidence of new settlement.
