From the news,
"Millennium Tower, the tony but troubled downtown high-rise that made international headlines last year when the secret got out that it’s slowly sinking and tilting, returned to its customary place in the news late Tuesday when NBC Bay Area revealed that the building “has tilted two and half more inches in just the first half of this year, according to new monitoring data.”
Says the affiliate:
The data, compiled by the ARUP engineering firm brought in by officials of the nextdoor Transbay transit terminal project, suggest the structure is tilting twice as fast as it had been in earlier ARUP data.
It is now listing at least 14 inches toward the massive Salesforce building going up nearby on Mission Street. The data also show the building has sunk close to 17 inches at its low point, settling about an inch since the problem emerged last year.
Supervisor Aaron Peskin, who has conducted a series of City Hall inquisitions trying to figure out who dropped the ball on the building’s design, took to Twitter to voice his exasperation.
“Accelerated sinking continues,” tweeted Peskin, then sarcastically referenced Mayor Ed Lee’s efforts last year to reassure U.S. Senator and former Mayor of San Francisco Dianne Feinstein that the city could manage the building’s woes.
In comments to NBC Peskin compared his hearings (which he vowed to continue) as “yelling into the wind.”"
They should have just built it with the flammable cladding. It'd be that much lighter, then when it burned, nobody would care if it was sinking or not- in fact, the more the better in that case.
Here is a link to a cross section of the soil conditions commissioned by Transbay Joint Powers Authority TJPA for the neighboring Transbay Center. Link TJPA are a greater bay area, regional authority, so you can imagine the other parties would love for the Transbay Center to be partly/mostly to blame for the Towers settlement & tilt. Considering the circulation of water in & out of SF Bay, the ground section should largely be indicative of what lies beneath the Millennium Tower project. The Tower takes up the block between Fremont St. & Beale St. Horizontally flip the building section below, to overlay the soil cross section in the above link to get an idea what is going on.
The parking garage basement of the Mid-Rise Tower was a 75 foot deep excavation. The garage foundation looks to be sitting on a lens of clay while the Tower foundation is on sand. The High Rise sits on 950 - 60 to 80 foot piles with a 10ft thick pile cap with a 35ft to 50ft deep excavation. Unknown is if the length of the piles is quoted as from original ground, top of the pile cap or from excavation grade.
Here is a link to a paper on the structural design of the tower. It doesn't mention the extensive use of GFRP. Link You can see the GFRP in the connection of the Steel Link Beam; here Link & again here Link
This link from CRSI gives some good details about the overall project. Link The first photo on the page gives a good idea of the concentration of piles under the tower. The 4th photo in the series is the podium/mid-rise basement/parking garage. The basement/parking garage photo is from around Summer 2007 and by this stage the very heavy Tower was topping out. The basement used an integral waterproofing admixture, so there isn't anything to stop water once the cracks grow too large. Here's my best 'eye-ball gestimate' of where the 10ft thick pile cap is in relation to the garage excavation. Since the building is leaning away from the garage, you have to wonder if this is when the tilting started. The tower foundation is supposed to be designed for 14kpsf. Wouldn't this be a vulnerable time for the tower to take on a slip circle tilt? If 7ft diameter concrete piles 200ft deep & anchored in bedrock on the South side of the property aren't preventing a Northern tilt, then how is a 75ft deep concrete box on the East side of the property, possibly built on top of a clay lens, suppose to prevent the building from tipping to the West?
The Tower was completed in 2009 and the Transbay Authority, TJPA began excavating next to the Tower in 2011. Before doing so & exclusive to the Millennium Tower/TJPA property line, TJPA took the preventative measure of drilling 181 7-foot diameter overlapping concrete piles all the way to bedrock at a cost of $58 million. Link The Transbay Center excavation is 65ft deep and runs 4 blocks, including along the South side of the Millennium property, with both the Transbay excavation & Millennium Tower project ending at Beale St. Link According to Millennium Tower's attorney (HOA?), the Transbay site has dewatered 5 million gallons/month for most of its duration & the water table has dropped 20 feet. As the TJPA states, their hole in the ground is the size of 120 3 meter deep Olympic sized swimming pools. Then again thier dewatering is enough to fill over 79 of those pools to date. Still, the HOA at Millennium Tower should be worried when the TJPA finally stops dewatering because those cracks in their basement garage are going to be fountains when the water table is recharged. If the dewatering is affecting the clay maybe the Tower will rise back up when TJPA stops dewatering?
The tower had sunk 12 of its 16 inches before the Transbay Center excavated next to the building. Refutation from the TJPA: Link As TJPA sees it, the Tower is just too heavy & sinking into the mud. They are probably right. The Old Bay Mud may be behaving like a pseudo-plastic and yielding gradually, while the garage sits on a (fulcrum) lens of another clay deposit and the pile cap of the tower is squishing the water out of a sand pile.
Maybe I don't understand well enough the nomenclature of Geotechnical Engineering but when I started looking for information on the possibility that disturbed hence less consolidated clay might not produce an elastic response but instead display a prolonged visco-elastic or thixotropic phase; there was very little research. Just bits here and there that the phenomenon does exist in some clay and was noted after the Kobe Quake. That might suggest that resistance piles driven through sand in to dense clay might not be as resistant as planned and that after an earthquake when the clay has experienced liquefaction could, under sufficient load, remain unstable. There is a fair amount of information about clay behavior and high initial shear resistance with some elastic recovery as high as 99% but not all clay has this recovery shear strength. Include the high initial shear resistance with a dramatic loss of shear strength and lesser resistance on recovery & the recovery stage starts looking more and more like thixotropic/visco-elastic behavior and that means deformation under load/creep.
Considering that the City of San Francisco has prior experience with the soils in the area, BART & Muni tunnels & underground stations AND both the City & the Developer knew in advance that the Transbay Center was going to be built, why the developer & why the City didn't determine to build a tower that could stand all on its own is hard to fathom. Seems TJPA was the only party think holistically and to act with anticipation or practice any preventative measures. If I was TJPA, I'd tell the HOA, the Developers & the City of San Francisco that when they have all each spent $58 million, then the TJPA will be willing to discuss what part TJPA will thereafter play. Link
Thanks epoxybot...great post. "The first photo on the page gives a good idea of the concentration of piles under the tower. The 4th photo"... they don't mention the type of teaspoon used to place the concrete in photos 3 and 6...
I'm not a highrise expert, only done 30 storeys, but, looking at the soil profile... I wouldn't like to construct anything tall or heavy in that stuff... there may have been a really good reason that everyone else went to bedrock. Also, only done a little seismic stuff and I have no idea of how a building, founded in that 'stuff' would behave during a seismic event...
There are more clever people out there than I would have imagined.
Looks like they terminated the piling before they hit the 'Old Bay Clay'...
One of the arguments that Millennium Partners has issued regarding their liability is that the residents of Millennium Tower, in effect OWN the tower & the tower's problems.
I've said it before, and maybe even on this thread. Condominium projects are litigation magnets. And this one is the perfect storm.
[ul]
[li]Condominium. Check[/li]
[li]Extremely wealthy owners. Check[/li]
[li]A good portion of them are probably lawyers. Check[/li]
[li]Owners heavily inve$ted in the project. Double Check [/li]
[li]And real damages. Check[/li]
[/ul].
This one will be in the news and courts for many years. There will be quadruple digit replies to this thread before it goes away.
Jed... you'd think with all the lawyers that they would be able to 'plot a course' to wind this up... surprised the SEAOC wasn't a little more proactive. I guess no one wants to say anything for fear of it coming back to bite them... Read the ARUP report and thought it was very technical and very well done. A real concern is how this building might lay down in a serious seismic event... and, maybe FEMA will pick up the insurance tab...
Would it be cheaper for everyone to put in re-leveled floors than fix the building? My old house had issues with a door not closing correctly because the ground was shifting. A contractor wanted to fix the foundation, I just adjusted the door hinge.
The expectation is that re-leveling will soon be 90 degrees after the building lays down. Not only is the tilt increasing, the tilt rate is increasing. This is not a situation that ends well. Has anyone make a graph of the two factors or has the data been too poorly collected to make a useful prediction?
... or at least those in the direction of tilt <G>. Is there a site that has a running record of the settlement and tilt and rates of these? You'd think someone would be monitoring this.
According to one observation by a geotechnical engineer, sorry I didn't save the link, one problem is that the Transbay Authority, TJPA did too good of a job with their secant pile wall. The wall does too good of a job of resisting earth movement and without similar resistant walls on the North & West sides of the Tower, North & West is where the ground is shifting.
The Transbay Authority didn't just build a wall, they heavily buttressed the wall. This is why Millennium Partners & Millennium residents focus on dewatering, because the wall works. Millennium residents also accuse the Transbay Authority of attempting to hide the shifting but it may be that the TJPA was unaware of the preexisting condition of the Tower's accelerated settlement when they entered into an easement contract with Millennium Partners and were alarmed at what they were seeing from early monitoring results. My guess is TJPA approached Millennium Partners and Millennium Partners wanted to keep the situation private.
Perhaps the original plan was the tower would be on bedrock by slowly lowering itself thorough the muck, but the asymmetry introduced by the new construction has upset that plan.
What is very surprising is the effect the execution of the easement agreement had on the settling rate.
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