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What percentage of the average urban area is covered by buildings with 70’ foundations? I assume, perhaps incorrectly, that this is something that’s only requir
by nocoiner 3y ago
What percentage of the average urban area is covered by buildings with 70’ foundations? I assume, perhaps incorrectly, that this is something that’s only required for skyscrapers or other similar large commercial buildings? And the average CBD is, probably, what, a low single digit percentage point of the corresponding urban area?
I could certainly be wrong in my assumptions (hence my question!). But it still seems to me like it would be way more efficient to drill much shallower conduits in (what I assume is) the 99.999% of geography not covered by skyscrapers (and to either go deep or continue to go targeted in the remaining fraction).
- sambeau 3y agoI once sat and watched piles being driven for a small housing block, 4 stories high. These ones in fact: https://www.google.com/maps/@56.4618615,-2.9545922,3a,75y,26.85h,91.14t/data=!3m6!1e1!3m4!1sN_TqTjjAPLdiU6Bv4dfD5A!2e0!7i16384!8i8192?entry=ttu https://www.google.com/maps/@56.4618615,-2.9545922,3a,75y,26... Each pile was as about as high as the building. They pushed one in first and it went in like it was going through butter — literally seconds. They then stuck a second one directly on top which went in relatively easily. They then spent a thumping eternity putting a third one in on top of the other two. They repeated this process for days. So, that little building is resting on piles around 2-3 times its height.
- jrk 3y agoThat's a wonderfully evocative image, and I am a total non-expert so this sort of depth may be common, but I do note that that map pin is in what looks like harbor landfill. It may require an unusually deep foundation to reach bedrock there.
- nocoiner 3y agoFascinating, thanks. Do you happen to know what material they used for the piles? I’ve always assumed some kind of concrete column, but that seems possibly incompatible with the process you saw.
- jacquesm 3y agoSteel, concrete, wood. If steel they can use hollow tubes and then later fill them with concrete as well, and they can be driven in using vibration.
- nocoiner 3y agoToo interesting. I just saw your other comment about the longevity of properly maintained wood pilings. Never would have thought that preventing rot would require them remaining wet.
- sambeau 3y agoThey were fat, hollow, metal tubes
- jacquesm 3y agoThere is 'drive down to specified depth', 'drive down until you meet a particular layer' and 'drive down until you have a specified resistance'. All of this depends on the various strata underneath where you want to make your construction. In old days in 'bad ground' (such as pretty much all of NL that doesn't have sand immediately beneath it) those pilings would be made of wood, and if kept properly wet they can last a very long time (100's of years). But if the water table isn't stable they can rot out pretty quickly.
- Steltek 3y agoBoston's Back Bay has a similar problem of keeping old wood pilings wet. I'm sure many old cities have similar areas.
- jacquesm 3y agoIn Amsterdam this is now a serious problem. The pilings there are getting really old and the water table has been changing, sometimes quite dramatically because of deep construction and the waterships's decisions to either purposefully lower it and then later reversing that decision again. This has caused pilings to become exposed, and then to be dried out, re-exposed and dried out again, with significant chances of rot as a result. Lots of historical buildings have had to have their foundations re-done which is super hard given the limited access. Some very ingenious machinery has been built, including a pile driver that is small enough to take under an existing building that drives new piles in situ. The whole thing can be disassembled and then re-assembled under the building. http://funderingstechniek.com/foto/carre3.jpg http://funderingstechniek.com/foto/carre3.jpg
- gonzo41 3y agoVenice has this problem. The whole city was built atop wood piles driven into marsh. The Sea has been trying to take the city back ever since.
- jacquesm 3y agoEventually it will succeed.
- munificent 3y agoI think the answer varies widely based on the underlying geology and what you're building. Ancedote: My neighborhood in Lousiana was a land-filled swamp. Building a single story house there required driving dozens of pylons in to keep the house from sinking into the dirt. When we put a pool into the backyard, it had to be partially filled with water while the concrete was still hardening because otherwise the surrounding water table (something like two feet below the surface) would have popped it out of the ground. This is probably less of a concern on, say, the Colorado Plateau.
- nocoiner 3y agoSounds like your geography is very similar to mine, fellow gulf coaster/swamp dweller. I know my house sits on a bunch of piles as well, though I don’t know how deep they go. I would guess it’s likely less than 50’, though maybe not.
- 8n4vidtmkvmk 3y agoWhat happens if you need to empty and refill the pool?
- atdrummond 3y agoYou can do that once the concrete has hardened and set. Just not before.
- munificent 3y agoNo, as far as I know you can't. The pool is essentially a boat floating on the water table. If you remove too much weight from it, it will float and pop up. It may be that the surrounding concrete decking is strong enough to hold it down, but I'm sure none of that was designed to hold a pool down against an upward force. I think you probably just can't safely fully drain it.