When we had our pool installed, after the sand was down, but before the pool was setup, it rained. I recall that the sand did not drain very well - we had puddles, and then they had to wait a week before setting up the pool as the sand retained the water, and was too wet - would leave footprints under the pool. It was a fine sand. Generally you want a very course sand for the chickens. It will drain better, and the chickens can use it as grit. I get my sand from a local building supply company, and I get the coarsest sand they sell. Drains well, and the chickens enjoy scratching around in it. Fine sand also retains odors and seems to go bad quicker. I think it all comes down to how fast does it dry out after being wet. The faster, the better.