Question on ground cover/tarp for pool

Hi,

I've been lurking for a while and I can't quite find what I am looking for.

Every year we have been getting a new Intex pool, usually a 10 or 12 footer. It seems that every year we get the same problem. After a couple of months, the bottom of the liner gets a lot of "stretch marks". Its like the bottom was almost pulled apart in small places. These spots happen all over, so its not necessarily near the seams or near any hardware.

I usually set the pools up on a tarp on top of concrete. We have a small yard so this is the only place we can put it. There are no rocks or grommets from the tarp under it.

Is it just bad liners? Am I using the wrong kind of tarp? What should I put between the pool and the concrete?