Swimming pools are normally constructed empty. They were withstanding surrounding soil before they were filled, and concrete strength increases with age (for about 90 days, typically). On the other hand, a sunken structure like a pool that is roofed over, becomes a "confined space". Unlike a typical structure, heavier-than-air gases cannot escape from the pool. Such gases could originate from the drain system or flow from leakage outside the pool area. For examples, leaking propane or various gases from sewer lines in the vicinity. A sunken greenhouse would almost certainly be a building code violation for that reason. If you build it, ventilate it by means both active and passive and do not enter if you can't verify that ventilation is working.
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That's a good consideration and something I haven't seen brought up with regards to walipinis (didn't think of it either, though I had read up on the risk awhile back for a postapoclyptic story I was working on about a guy who hunts for old bunkers and safe houses). Nature abhors a vacuum and it likes to fill them with water, mold, and heavier-than-air gasses. Luckily, greenhouses require a certain amount of ventilation, but I doubt most are configured properly for heavier-than-air stuff. Thanks!
Hmm, it would seem to me that pools that break when empty should be rather rare as that that is not a sound construction in general. Maybe in costal areas with raising groundwater table due to sea-level rise it might happen though.
Edit: the lifting issue is somewhat common with underground cisterns and septic tanks though, which are typically expected to be always filled with water.
If the pool has a deeper section it could be used as a pond that acts as additional thermal buffer and a sump pump with float switch could make sure the pool doesn't get flooded during heavy rainfall.
Yeah I kind of wonder if a lot of the stuff about them breaking when left empty is that they won't be suitable as a swimming pool anymore, rather than that they become a death trap. And if you don't value the idea of a swimming pool, or it's already so broken it would need expensive overhauls or replacement, then a walipini with a cracked foundation isn't really a big deal.
But it comes up in every discussion I've found about empty pools, so I'd want to identify any precautions or mitigations to include before I start the sketch
Hmm, I am not really an expert on the topic to be honest and here in Europe private pools tend to be only build in southern Europe in places which I guess rarely have high groundwater tables. But I can imagine it being more of an issue in Florida for example.
as i understand it, if you leave a pool empty too long it will need to be resurfaced. in the 1980s recessions empty swimming pools were commonly used as skateboard ramps. they dont structurally collapse, but their surfaces flake away.
If the soil is dry enough to avoid liquefication, lifting should not happen. A ditch around the pool at some distance would likely solve the problem even if the soil is problematic.
If the concrete is reinforced with rebar, it can withstand sideways load by the soil. If not, the issue of soil pressure would need closer consideration.
Disclaimer: I'm not an engineer