Newly released Millennium Tower-fix documents appear to bolster a theory about what led the building’s sinking and tilting to get worse when crews began to sink new support piles to bedrock last summer.
The previously undisclosed documents date back to July and August of last year, when the “perimeter pile upgrade” project -- designed to bolster the sinking luxury high-rise on two sides -- was in full swing with the first six piles being installed to bedrock.
The process involves crews drilling a hole more than 250 feet down, slowly inserting a steel support pile before sealing up the installation hole with concrete grout.
But when the crews installed the first pile, No. 8, in July, they had to pump in far more concrete grout than expected -- based on the depth and size of the hole that was dug to bedrock, the documents obtained by NBC Bay Area’s Investigative Unit indicate. Pile No. 8 Logs indicate it took 14 more tons than originally calculated – so much that crews had to summon another truck to deliver the extra grout late in the day.
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Records for subsequent piles showed each required 10 or more tons of extra grout to install. One expert says he thinks he knows why all the extra grout was needed – it was filling in for large amounts of ground being sucked out during the drilling process.
“The ground was already highly stressed because of the 58-story building that it was supporting on piles,” said Harry Poulos, an internationally recognized specialist in tall building foundations.
The pressurized drilling process to sink piles around the tower, he said, would inevitably suck up a large volume material that would otherwise clog the bottom of the hole. That ground being sucked out, he said, could easily trigger accelerated settlement. Making matters worse, the documents show the pile installation effort was dogged by delays of between one and four days before drilling and the grouting – allowing time for the ground to shift and accelerated settlement to occur of the soils above.
“I think anytime you do excavation and delay, that leaves you open to the possibility of loss of strength and deformation settlement, in this case, of the ground,” Poulos said.
The sudden sinking and tilting got so bad in August that city officials ordered a halt to the fix work to allow time for testing and reevaluation.
In an independent assessment of the testing effort, independent drilling expert Ben Turner talked about the potential for drilling to lead to lost ground, calling it “unintentional mining.” He told city officials in his report last November that such mining “could not be ruled out” on the project.
But at a hearing earlier this month, Turner told city supervisors that he now believes the extra grout wasn’t flowing into voids created by drilling, but was being absorbed through naturally occurring gaps in an ancient sandy layer known as the Alameda Formation.
“We actually expect that some grout will penetrate into the formation,” Turner said. “So after extensive discussions with the contractor and among all the players here, we believe that is the most likely explanation.”
But one local geotechnical engineer familiar with the project is not convinced.
“It's more likely to be over-mining,” said Bob Pyke, who questioned the viability of the fix even before work started.
As evidence for the mining theory, Pyke points to an analysis of the first successful indicator test pile installed in the spring of last year. It was during that test that crews first discovered they had to used more grout than expected.
Thermal data taken with instruments embedded in the grout detected the heat signature of an apparent pool of grout as it cured some 240 feet down, in the sandy Alameda Formation.
Pyke agrees with Turner that the Alameda layer sands would be more permeable – and could absorb some grout, but the amount of grout needed to cause a spike in thermal data, he said, would point to a large volume than could be slowly absorbed in a sandy layer.
Another red flag, Pyke says, is that monitoring data from early last year indicates settlement began to accelerate during the indicator pile program – a hint of more settlement to come.
“The indicator pile should have served as a canary in the coal mine -- these were all warning signs,” Pyke concluded, adding that engineers should have paid attention to all the red flags, especially when the building began to sink and tilt more dramatically with each pile installation.
The current plan is to only install 18 piles, instead of the original 52. But the revised proposal has yet to be approved by city officials.