Possible leaking Hydrostatic Relief Plug

[Kiwipool]

Good evening guys,

Can you please help me, I can't find anyone locally here in New Zealand who can give me advice.

I have a concrete (indoor) swimming pool with a significant leak. I have been in the house for 10 years, but the pool was built in the 70's I believe, so I don't know it's history. I have tracked the leak to a clay pipe that sits under a cover in the base of the deep end of the pool. Under the cover, there are two pipes, one small PVC one which I believe used to go to the pump but has since been capped off, just under where the pipe from the skimmer goes in to the pump. The other pipe which I believe has the leak, is a large clay terracotta earthenware pipe that goes down about 15cm, then has what feels like concrete or something blocking it off.

The water level when the pool is filled goes down at an alarming rate (The equivalent of the garden hose being on for about 1 hour per day), so last summer I drained the pool, checked all the skimmer and return pipes, removed all the tiles around the pool interior, had the surface concrete ground smooth, sealed and coated with a thick epoxy pool paint, essentially tanking it. However when I filled it up, the leak remained, so the only possible place it should be leaking from is the two pipes at the bottom. With the pool empty and the pipes filled to their necks with water, I left it for a few days and the small PVC pipe was still completely full, but the clay pipe slowly drained dry (although with the pool filled with water, I guess the weight of the water was causing it to drain much quicker then it did with the pool empty.

From my research I think the clay pipe is a Hydrostatic Relief Plug, but there is no thread or anything in it, so I don't know how to stop the water slowly draining through the pipe in to the ground below it. Before I found out what the pipe was, I bought a product that seals leaks yet remains flexible and I am told by the manufacturer that it sticks to pretty much everything, so I was thinking this could be a solution, but since finding out what a Hydrostatic Relief Plug is, I now think perhaps I shouldn't be putting it in the pipe.

Has anyone ever seen a clay Hydrostatic Relief Plug? How would I cap it to stop the leak?

Cheers, Kevin

Here are some pictures for reference, thanks in advance

[Denniswiseman]

That clay pipe looks like it's recessed. You could get a hydrostatic relief valve concreted into the area with some sheet metal with a hole on it to prevent concrete going down the clay pipe
If you have drained down and not had any water come up through the clay pipe do you need a HRV

[Kiwipool]

You could get a hydrostatic relief valve concreted into the area with some sheet metal with a hole on it to prevent concrete going down the clay pipe

Thanks Dennis, I’m not sure I follow about the sheet metal part. Are you saying concrete a smaller HRV inside the mouth of the clay pipe? There is definitely no water coming up through the pipe, as we are on the side of a hill and the land surrounding the pool is lower on two sides, so it would surprise me if it would require one, but I’m no expert by any means.

[Denniswiseman]

What I meant was the HRV would be to small to fit over the hole and stop concrete falling down so you would need some thing thin to prevent it
It may have been installed as a precaution any way