I just started my 18X36 DIY inground project. I got the hole dug and everything was great.
The next morning I found a few inches of water in the deep end. No rain, I pumped it out, next day same issue. Seems like a few inches a day of water basically fill in the hole.
There are some springs on my property, but I do feel they are lower than the bottom of the pool.
I have read all the floating liner horror stories, and so I think the solution is an over dig with gravel. Through in a PVC with holes, and pipe that out during construction. What I'm concerened about, is if what I have is a spring, does that mean I need some sort of pump on that pipe all the time?