The best system I ever saw was this. Dig the area down 18 inches. Fill with 6inches of sand on the bottom and 12 inches of pea gravel. The sand allows for urine or vomit to sink down and be away from the dog. The gravel allows for a cool area for the dogs to lay on and the movement of the gravel is a great shock absorber for movement. If it is feesible for you, I'd suggest doing that.