A bit of elmer's glue will be used to secure the sand and gravel to the edges. I have heavy gravel, light, and sand.
The heaviest gravel is placed first, followed by the medium, and then finally the sand. Not only does this add a touch of realism to these floors, but it strengthens them as well.
I make sure to glue the gravel and sand to the underside of any 'overhangs', where the sculpey sheet is not directly supported by the cork.
The extra firm grey sculpey is a lot stronger than the regular white sculpey, but it is always best to provide that extra bit of strength.