Every such vessel shall be set on a 6-inch thick bedding of water-washed sand (beach sand shall not be used) free from soil, rock, or gravel. Before any back filling is done, the position of the vessel and the sand bedding shall be approved by the Chief. After such approval, the excavation shall be back filled with the same quality water-washed sand in such a manner that the sand back fill on the sides, ends, and the top of such vessel shall be at least 6 inches thick.