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Is there a reason why you favor 500’ below ground level? My naive initial reaction is that seems like tremendous overkill in 99% of applications. Presumably in
by nocoiner 3y ago
Is there a reason why you favor 500’ below ground level? My naive initial reaction is that seems like tremendous overkill in 99% of applications. Presumably in the vast majority of urban areas, 50’ would be more than enough to avoid any and all obstructions.
I suppose once you decide to do something more than shallow trenching, then the marginal cost of additional vertical feet is fairly minor, but even in, say, NYC it seems like 500’ is way, way more buffer than you’d need in the worst case scenario (I have a vague recollection that the very deepest subway line is around 200’ underground, and that’s primarily due to a specific quirk of geography - a hill surrounded by two valleys).
- sidewndr46 3y agoHave you ever walked by a building under construction? It isn't uncommon to see foundation work going down >70 ft. The pilings themselves can be driven in even farther.
- nocoiner 3y agoWhat 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
- 3y ago
- 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.
- voxelghost 3y agoYes, now I don't know which metro-area OP was talking about but in NYC bedrock varies between a few feet to about 150 feet below the surface. http://www.classichistory.net/archives/nyc-bedrock http://www.classichistory.net/archives/nyc-bedrock (see graph. note; unit is meter)
- WarOnPrivacy 3y ago> Is there a reason why you favor 500’ below ground level? Florida aquifer drills are >400' to reach 65° (year round) water. Temps get really static at that level.
- ChuckMcM 3y agoThree reasons, temperature stability, propensity to have rock vs sediment, and in the bay area, somewhat better earthquake resistance. When I was doing ops at Blekko we had a dark fiber run between our office and the data center to get our own "high speed" connection and part of that was a geo-technical survey. Of much of the SF Bay Area peninsula. The geologist suggested if we wanted to be subterranean and avoid both infrastructure and superfund sites, that we'd have to go down 500'. He expressed doubt that anyone would do that given the expense. And there are still some parts of the Santa Clara aquifer you can hit if you don't aim well.
- nocoiner 3y agoSuper interesting, thanks for the response!
- jxf 3y ago> Presumably in the vast majority of urban areas, 50’ would be more than enough to avoid any and all obstructions. This would definitely not be enough in, say, NYC. Grand Central Station is 150 feet below street level, for example. The Oculus is about 80 feet below street level. Things are deeper than they might first appear!