Quote:
Originally Posted by horatio_the_hermit
Wow. I had no idea this was a thing. I guess that means we'll start to see above-ground progress made sooner I'd thought.
|
Except it's not true. I think it's funny how there's so much wild speculation without a lot of information on this site. I find this site useful for project news and pictures, but don't come here for technical information. Most of what I read here about engineering and construction methods is flat-out wrong. Drives me crazy. Usually I hold my tongue, but this time only...
This is bottom-up construction. The caissons are done first because it is not possible to excavate such a deep hole without them. The caissons hold back the soil outside of the building allowing the hole to be excavated. If the excavation is deep enough, and depending on the geotechnical conditions, even that might not be good enough - once you get down deep enough temporary bracing may need to be added to keep the caissons in place. You see that everywhere downtown - that's how they built the Millennium tower and 350 Mission across the street. It's also what you can see in the Transbay Terminal excavation.
Then once the hole is fully excavated and braced, the structure construction begins from the bottom up. The temporary bracing is removed as the structure rises from the bottom of the excavation. That's what is going on now in the Transbay Terminal excavation. I haven't been by in a while, but the concrete that is being poured started at the bottom and may even be rising to the surface at some parts of the excavation at this point. When all of the substructure is constructed and temporary bracing is removed, the superstructure will begin.
There is such a thing as top-down construction, but I'm not aware of any projects currently being built that way in SF (or even any in at least my recent memory). With top-down construction (at least one version of it), instead of putting in temporary bracing as the excavation proceeds, the permanent structure would be put in place moving downward. Depending on the details, this might also allow the structure above-grade (if there is one) to proceed simultaneously, saving time. Depending on the details, this can be much more technically challeging and expensive. The tradeoff is time. The decision on which way to go involves as always, technical feasibility and money.
Of course after that rant, I could be wrong and 181 Fremont and the Salesforce Tower might be being built top-down - I don't have any information - but I have never seen it in SF and don't see why the economics of these two sites would be so much different from all of the other projects surrounding it (unless the developers are worried about completing before the crash and are willing to spend the extra $).
And that's all I'll say about that.