I'm sure that one of the guys with pool installation experience will be along shortly, but I can tell you from experience that if you only have sand for the bottom and you have a leak in the shallow end, the water will wash all the sand down to the lowest point, causing trenches under your liner. My pool builder used a combination of sand and concrete but I really don't recommend that either because we have small rocks that occasionally appear.

I think most of the pool builders in this area use foam. We have a very high water table (Louisiana).

Janet