A couple of things: with tile, the recommended slope is 1/4" per foot, but with a pebble floor, you probably want to nearly double that to account for the potential high spots...you really want it to drain. Then, because there is a lot of grout, you might want to consider using a surface waterproofing membrane, either a fabric or paint on the setting bed to limit how far moisture can penetrate. It would be expensive, but you might consider using an epoxy grout, which is pretty much impervious, won't absorb moisture, and therefore doesn't support moisture. You might want to check out
www.johnbridge.com for some thoughts as well.