Last year was our 2nd year with a 18'x36' in ground vinyl liner pool. During the summer when heavy rains occurred there would be water behind the liner and after a few weeks of no rain it would go away. After I closed the pool for the winter we received heavy rains which I though filled my pool because the cover had water on it. I removed a corner of the cover and saw that the wall had bulged out with water behind them, we removed the water from behind but it continued to rain ( not heavy ). When I removed the same corner this spring there was water again but not nearly as bad, I removed roughly 500 gallon from behind the liner. From my understanding there is no drain tile around the pool. ( Steel walls ) I also have a considerable amount concrete around the pool. My question is this: Could all of that water be left over from fall and from the thaw of spring? Could this be surface water finding its way to the outside of the pool? The back yard is ALL clay with sod on top. I plan on putting drain tile around the perimeter of the pool roughly 3' deep or until I hit clay. I have also considered adding a Hydrostatic Relief Valve to my main drains. Has anyone ever had to deal with this before? The first year we closed the pool the rain wasn't nearly as bad and we did not have this problem. Any suggestions would be fantastic and welcomed.