Identify this hole near my bottom drain?

[Earthman Bob]

I just drained our pool to get it ready for plastering tomorrow and saw this port next to the bottom drain. It had a cover on it that was filled with gunite to match the pool. Under that cover it looks like a plastic container, but can't see how deep it goes as there is debris in there. That cover was sealed with an O-ring.

In the first pic below, you can see the bottom drain at the bottom, the other hole in the top-middle, the gunite-filled cover on the right, and a screw-in plug that I found with my pool stuff (that will also screw into that same hole).

So first question is, what is this hole for?

Second question is, could it be the sources of loss of pool water? If so, I'll use the screw-in plug. If it's a sealed chamber, I'll use the gunite cover.

Thanks for your insights--

 

[HermanTX]

Most bottom drains have 2 drain covers - this reduces the suction force across the ports so no one can get entrapped. Could be someone closed off one of the drains? Normally these 2 holes are joined together via pipework (under the gunite). You may want to discuss with your PB before the plaster is installed to clean it out and ask if they plan to put 2 new drain covers on it.

Have you measured the diameter of this hole and compared to the drain?

 

[Rancho Cost-a-Lotta]

It's most likely a hydro-static valve which allows ground water to enter the pool as opposed to lifting to the entire pool structure when emptying the pool. You may have another mystery plug in the shallow end or in the skimmer. If the pit under the valve is dry, it's unlikely this would be the source of your leak.

 

[Earthman Bob]

It's most likely a hydro-static valve which allows ground water to enter the pool as opposed to lifting to the entire pool structure when emptying the pool. You may have another mystery plug in the shallow end or in the skimmer. If the pit under the valve is dry, it's unlikely this would be the source of your leak.

Mike, you got it! Just spoke to the guys doing the plaster, and it is indeed a hydrostatic plug. Mystery solved.

Thanks for the reply--

 

[Earthman Bob]

Most bottom drains have 2 drain covers - this reduces the suction force across the ports so no one can get entrapped. Could be someone closed off one of the drains? Normally these 2 holes are joined together via pipework (under the gunite). You may want to discuss with your PB before the plaster is installed to clean it out and ask if they plan to put 2 new drain covers on it.

Have you measured the diameter of this hole and compared to the drain?

That's what I had originally been thinking it was, before I found out that hydrostatic plugs existed. Learn 10 things new every day, eh?