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I have been doing a bunch of research online into how to build a basement that is below the water table. The common wisdom is to "not waste the money doing that" but as someone who doesn't like to be told no and always likes to fully understand their options, I am curious how I could go about building a basement that is below the water table.

I know it is possible to build a waterproof basement because submarines exist, marinas often build underwater sections of buildings, pools are a thing, etc. The ideal would be to have the primary structure for the basement be poured concrete with no windows or other holes in the walls (plumbing/electricity/air would come in through the ceiling from the higher floors).

For simplicity, lets assume that I don't need sewer out of the basement (perhaps by using an up-flush system to pump sewage/water out). Lets also imagine that the basement is on a flood plain so I can build it while it is dry, but it will be completely submersed in water later in the year.

Finally, again for simplicity lets assume that I am building in a location that has no building codes I need to abide by (so any option is on the table), but I do care about the structure being sound for a long time (e.g., 100 years).

What are the techniques I could use to water PROOF the basement so that it can be fully submerged under water for extended periods of time without leaking?

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  • $\begingroup$ You're doomed to fail (or spend more on the basement than on several houses) if you try to isolate against the water. Just do rudimentary isolation to stop most of the leakage, then add water drainage channels, collection area and an automatic pump. $\endgroup$ – SF. Jan 16 '19 at 9:13
  • $\begingroup$ Check out building swimming pools and reverse the wall structure... $\endgroup$ – Solar Mike Jan 16 '19 at 11:18
  • $\begingroup$ @SolarMike: Nobody minds a swimming pool leaking into the ground around slightly... $\endgroup$ – SF. Jan 16 '19 at 15:13
  • $\begingroup$ @SF. Is chlorine good for worms? $\endgroup$ – Solar Mike Jan 16 '19 at 15:22
  • $\begingroup$ @SolarMike Nobody cares about a small number of worms. $\endgroup$ – SF. Jan 16 '19 at 15:27
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IT's certainly possible to build a cellar below the GW table. You need to ensure two things:

  • water proof
  • countering buoancy due to the groundwater

As for point one, water proof concretes exist, it's a matter of concrete recipe and care in execution. A bit of information on this can be found here. Coating, e.g. water proof PE liners that can be applied to the concrete also exist. Again, this requires care in execution.

As for point 2, the buoancy can be countered by the weight of the building, the weight of the contents (water tanks don't tend to swim when full, oil tanks may be an issue!), by building a larger baseplate (so a bit of ground would have to be lifted as well) or even by friction with the ground. This depends on size, dpeth, and shape of your cellar and needs to be designed together with a structural engineer.

During construction you need to manage the groundwater some way.

Buildings below ground water level are built all the time when unavoidable, usually when you need the depth for some reason.

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  • $\begingroup$ The third thing is the basement should have the floor designed such way that water which accidentally gets in flows in manageable way and is gathered to a manageable reservoir from where it can be pumped away. It will leak sooner or later. You will have to repair it sooner or later. Prepare to that in advance. $\endgroup$ – sharptooth Jan 16 '19 at 12:40
  • $\begingroup$ @sharptooth so your comment is a repeat of the one made by SF above... $\endgroup$ – Solar Mike Jan 16 '19 at 13:08
  • $\begingroup$ @SolarMike Not really, he says it's impossible to prevent leaks, I say it is possible but only for some period of time. $\endgroup$ – sharptooth Jan 16 '19 at 13:11
  • $\begingroup$ @sharptooth No, the obvious that drainage channels collection and an automatic pump - but you missed the automatic bit... $\endgroup$ – Solar Mike Jan 16 '19 at 13:26
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you need to build your basement with concrete, on deep foundation that penetrates into competent layer of soil and is below the level of under ground water.

Assuming your structure is built strong enough to withstand all the hydrostatic and soil pressure, we need to insulate and water proof it.

There are waterproof plastic sheets that are applied over a waterproof rolled over polyurethane film.

This insulation covers all around and from top to the bottom of the exterior of concrete wall, leading on the bottom to a trench sloped to carry any water to a sump pump.

The wall is then covered with 12 inches of gravel built over a 4 inch diameter perforated PVC pipe running on the trench behind the wall, to collect the water and take it to the said sump pump. then backfilled with the dirt from excavation and compacted.

This is standard practice in areas with aquifer. The packages are readily available at the big box building material stores.

They come with manuals and have link to instruction youtube videos. here is a sketch,wall detail

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  • $\begingroup$ I believe these instructions are for building a water resilient basement, not a waterproof one? Imagine if the described structure is underwater, no matter how much you pump out, you'll always have more to pump out because there is no end to the water that is pressing up against the exterior of the building. $\endgroup$ – Micah Zoltu Jan 22 '19 at 4:03
  • $\begingroup$ @Micah Zoltu, there are different approaches depending on the geology of the location. In cases where the subterranean water level is pemanently above the foundation of the basement, then they build a big heavy waterproof structure anchored by deep pylons and equipped with powerful sump pumps. This structure which looks like a dry dock will house the conventional foundation and super structure. The entire west coast of the US and Canada have many buildings like that. Many cities require this as part of basement or subterranean parking. $\endgroup$ – kamran Jan 22 '19 at 5:46
  • $\begingroup$ Ah, interesting. So they build an outer wall and then keep the space between the outer wall and the main wall dry by pumping water out? In such a scenario I'm guessing that you need a pump big enough to pump out as much water as can seep through the concrete per unit of time, so a bigger surface area with more porous concrete would need more water pumped out? IIUC, you are saying that in such a scenario the drain is outside the building walls, but inside the outer walls? $\endgroup$ – Micah Zoltu Jan 23 '19 at 6:09
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    $\begingroup$ No the wall and floor of the dry dock are insulated with positive no leak waterproofing, such as membrane of polyethylene. the default is no leak condition. Sump pump is just for emergency, such as plumbing leaks, or in CA for siesmic event bigger than system design, etc. In major buildings there are automatic lock gates and safety measures such as flood exits. Some of these parkings go deep 4-5 levels. They are built with high tech safety measures. $\endgroup$ – kamran Jan 23 '19 at 6:20

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