Bought his house this summer. Didn't like that pool liner attempted to float in a few areas during a 2 day ice storm back in October with 3+ inches of rain. Back corner around shed became a lake with water seeping into shed. I'm thinking of installing a sump. Might put a drain hose down this hole if it's just drainage however my gutters just drain onto concrete and I don't see any outlets for drainage in street. This pipe goes down about 4 feet and appears to have been installed at the same time as the concrete about 10-15 yrs ago. 4" pvc/no lid.
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It looks like a regular drain that's missing the grated top. – JACK Nov 27 '20 at 19:07
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Air passage for suctioning air out during liner installation. Guessing – Kris Nov 27 '20 at 21:12
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Deadman line for pumping water out from under pool liner? Safety relief port in case someone gets trapped on the bottom drain? – Kris Nov 27 '20 at 21:22
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air passage makes most sense but still unsure. Pumping water out from the liner might be a stretch since the water table should have come up at least a couple feet inside the pipe during the rain storm but water level didn't rise inside pipe. "no flowing water". Not an umbrella hole. Drain? that's my hope but how do I find out? Maybe I should put a robot down there and see where it goes. haha. – Dan G Nov 28 '20 at 22:03
Probably a place to insert a umbrella pole
It's randomly located so likely not a drain. Any water that near the pool would likely drain into the pool.
Edited to add this picture as an example of one, and a link to the seller
https://www.poolsupplyunlimited.com/pool/aquastar-us105-umbrella-stands/5841p1