San Francisco’s ill-fated Millennium Tower is making headlines once again for problems taking place beneath its turbulent foundation.
This time, the 58-story skyscraper is sinking even further on its vertical axis as it settles to the north and west, alarming some building scientists who feel the movement could result in significant corrosive damage despite some recent positive corrections.
Engineers say a fix that has been previously made to anchor its foundation into bedrock on the north and west sides has mostly failed to produce the 4-inch reversal their advanced data models had predicted, instead posting a correction of just a quarter-inch a year and, as outside experts surmised, causing the new directional shift that's happening in an unexpected area.
Now, the building is sinking from underneath the center of its foundation mat at a rate of one-tenth of an inch per year. The phenomenon is known as "dishing." A local geothermal engineer who is monitoring the issue told NBC Bay Area this might lead to a cycle of corrosion that would affect the concrete foundation's steel supports.
"The overall situation is very complex," Robert Pyke told the outlet. "It’s highly likely that there's been an increase in cracking for this kind of fix. That should have been obvious that there was a possibility of further increasing cracking and water intrusion into the foundation."
Ron Hamburger, the engineer in charge of the team, who had been at work correcting the tilting problem, says the soil at the site is to blame and that the remedies will nonetheless continue to yield reversals at the northwest corner. (He did not say how long or at which rate they would occur.)
The saga has played out continuously since the issue was made public by the developer in May of 2016. The retrofit enacted has since been valued at $100 million. Currently, tilting, which was first recorded at 16 inches, has advanced to slightly under 29 inches at the northwest corner.
Residents, meanwhile, are being asked to foot the $6.8 million in additional repair work that has reportedly run $20 million over budget.
6 Comments
Geez, they need to reduce the mass by 50% to safely stop the sinking. It is going to require reducing the building in height in half to 1/3 of its current height. Which is the height it should have been in the first place at that location. There isn't a solution to keep this building at its current height without it sinking AND then overturn and damage swath of other properties due to the differential soil conditions. No solid rock bedrock to tie into. The piles would need to be nearly as deep as the building is above ground to keep this from sinking or overturning. They are just pissing money down a toilet on this disaster.
the solution was end bearing piles... instead they ended up explaining to the judge why they didn't do that - the lawsuit totaled more than the initial cost to build the building
It would likely need to be something more like socketed piles. Not just end bearing on the surface of the bedrock but socketed into the bedrock to a certain amount of depth into the bedrock, but yeah, basically agree. It is nuanced differences but has engineering importance but we need not quibble on those nuance. They are too late to install such (end-bearing/socket piles) properly. Without end-bearing/socket piles, it can't support the mass. The mass has to be reduced so the piles don't continue to sink or even overturn as any soil differentials can result in pile shifting and bending. Bottom line, they f---ed up and pissing money down this toilet. I wouldn't say I am an expert on piles so much so that I wouldn't involve a geotechnical and structural engineering consultants even for a house.
The science and engineering of piles involves a lot and alot can go wrong be it a house or a high rise or anywhere in between. Personally, whenever possible, end-bearing or socketing the pile into competent bedrock is best. Simplifies some of the issues versus the issues of relying solely on friction.
I lived and worked in the USA for more than 10 years as an employed architect on the Eat coast. I then realized that the US systems and standards of construction were quite safe and improved especially when erecting high rises buildings. Now if one ship is sinking they should start evacuating it asap. And meanwhile evacuate the architects, engineers, contractors and any other liable persons of expertise who didn't foresee that disaster. I guess the insurance companies might be quite agitated by now.
I am not sure they didn't foresee the disaster. It is a mess now.
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