SF Tower settlement Part II 18

[1503-44]

"Appreciation has dropped to 2%"
Well that's less than inflation, but more than interest rates.

Although as I said, probably nobody bought in for either of those reasons.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[dik]

Looks more interesting, if you cannot get in to fix it. I don't know if the soil is unstable (sometimes a problem with marine clays) It looks like the problem may be a little more difficult than initially assumed. I don't know when it becomes dangerous... California like Florida, only bigger.

East of Winnipeg, a while back, we had a grain elevator fail in Transcona. It was a excellent example of Terzaghi's proposed manner of failure... It happened shortly after he came out with his failure mode. It is likely that Russia knows Winnipeg as being west of Transcona.

I'm sure glad that I wasn't involved.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[AusG]

Someone needs to publish a new engineering correlation between building list angle and rate of capital appreciation, decline in which was the spark for the mitigation project. It will go negative pretty soon if this keeps up.

 

[dik]

You forgot the

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MacGyverS2000]

Or did he... ?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[dik]

It did wonders for Pisa...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[apper.42]

If you want to reduce settlement in the northwest direction, do not drill 36" dia. piles through the unconsolidated marine clays and old bay clays and give a path to release the excess pore water pressure from the 10' thick matt and pile group sitting on them and thereby increase the settlement instead of slowing it down. I'm no geologist but my god you don't have to be to know this was a very bad idea. We know that the direction of natural excess consolidation is in the northwest direction so lets not drill more holes to give a path to relieve any added pore water pressure in both clay layers.

 

[TugboatEng]

The 36 inch parts were the casings. The drove the piles inside of the casings. It seems they had thought of the problem but their solution wasn't adequate. Remember, these are the same engineers that blamed the sinking on the Trans-Bay terminal nextdoor so it appears they have been hyper-focused on issues near the surface but now it becomes obvious that the problem is deeper.

For a tangent, there is a naval architect that seems to have trouble estimating the as constructed weight of their vessels. They had to sponson one after sea trials and another required crane assistance to get out of the drydock. Anyways, in both cases the vessels weighed much more than predicted and I wonder if the same is true for this building.

 

[dik]

Thanks... problem eluded me... My original thoughts were that the pore water was seeping into the shaft, like a 36" dia well, but the report for the Trans Bay Terminal by Arup 'dispelled' the moisture loss. Our clays are so impermeable that I had no problem with their explanation. I also thought that the thick slab wasn't supporting load and it was used to transfer the friction pile loads to the building... maybe with their failure, it has become a mat foundation too, or maybe it was designed that way (I would normally only use the value for the friction piles and not a combination).

This complicates the 'fix' by a whole magnitude. Has there been a real serious look at the actual dead load of the structure... with all the owner fixings?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[apper.42]

The 36" parts were the casings

Just my point. So the casings were open for the pile driving which allowed a perfect path for pore water pressures in the clays to be releasd and allow faster settlement. The driven piles were not carrying any of the load yet but allowed pore pressure to drive water from the clays, particularly the marine clays, and cause more consolidation. Average pressure on the 10' thick slab and pile group was over 11 kips per square foot according to the presentation of the solution when proposed.

 

[dik]

Youda thunk that with all the mimes involved that they would have looked into that type of problem.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TugboatEng]

with all the owner fixings?

That was exactly the problem on the sponsoned vessel I mentioned. On the first the architect let the shipyard choose the fixings. The second vessel the architect chose the fixing and it did not require sponsons despite otherwise identical construction.

 

[apper.42]

Youda thunk

Pore water is not free flowing water it is usually released under excess pressure leading to further consolidation. Pressure on the old clays was in the order of 0.7 x 11000 psf (from pressure bulb under pile group) or about 53.5 psi. Pore pressures were much higher with the building loading than without. All of the pressure was now being released in the direction to increase the differential settlement. I'm betting that they didn't have their piezometers installed yet to monitor pore pressures. Hey I'm just a simple engineer that used Terzagi and Peck 1967 as my soils text book.

 

[dik]

I would have thought all water contained in clay, other than that adhered to the clay 'platlets' was in the form of pore water. Our high plastic clays are nearly impermeablej, but they do consolidate (we have a slightly different issue they are still 'springing' up from the last glaciation).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[apper.42]

Clays are impermeable but excess pore water pressure (pressure above insitu levels) will cause clays to consolidate. The younger marine clays would consolidate more and faster than the older bay clays which are directly under the proposed pressure bulb caused by the building pile group. The degree and unevenness of the consolidation of the clays under the Millennium Tower were underestimated from the start so the rate was higher than estimated and the settlements were different over the longest direction of the building. This is pretty basic stuff and predicted settlements are often not correct. Boreholes originally were pretty well spaced out so that didn't help. If I'm wrong then shoot me, looks as if the original engineers were no better. As the old clays consolidate the 10' slab starts to pick up some load and the marine clays consolidate.

 

[dik]

Yup... missed that... it's just the relaxation and the new 100' shafts were at about the same elevation as the original piles. I have to look back at the earlier postings, but I thought the upper soil was filled material which could increase the pile loads, too as well as increase pore pressures.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[apper.42]

Like I said I'm no fancy geotechnical engineer but this is pretty basic stuff. The overlying soils were there to start with according to the soil report but the 10' concrete slab less any dirt excavated for it and the basment(s) would be surcharge which is estimated at 224000 kips for the entire structure divided by 100' x 200' size of pile group or 11.2 ksf.

 

[dik]

Same here... I wonder why vibrating wire pieziometers, or something of that ilk, weren't installed? I also wonder if they have a real handle on what they are doing?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[apper.42]

Piezometers were to be installed but I'm almost willing to bet they were not installed yet because no loads had yet been transferred to the new piles.

 

[apper.42]

So their original plan might still work but they forgot that upsetting the status quo (pore pressure wise) might result in increased short term settlements; now they need to get a handle on things before they do any more damage. Seal those 36" casings if thy haven't and get their piezometers installed and working at the very least.

 

[SwinnyGG]

I'm out of my depth in this conversation, but I've been following along. Reading the latest news (1" added settlement since the start of installation of reinforcing piles, now 22" of tilt at the top) is scary to a layman, and raises some questions.

Looking at the building structure and foundation as separate elements, with a connection between them (I know this is a vast oversimplification), that connection must have some high level of moment capacity to handle wind and seismic load. With the building now tilted 22", it seems that the side opposite the tilt is now subject to a steady-state moment, to which any wind load or seismic induced moment would be added. I would imagine the moment capacities, especially for seismic loads, are huge - but how far can the building tilt before the added moment applied to the theoretical foundation/structure connection from the tilt puts the building in a situation where the total moment (tilt + wind + seismic) exceeds design capacity? Is this building approaching the point where it will be deemed completely unsafe and unsalvageable?