Listed nonmetallic pipe and fittings installed in park drainage systems shall be installed in accordance with their listing and applicable standards. When installed under roadways, minimum depth of cover for nonmetallic drain pipe shall be thirty-six (36) inches. The pipe shall be bedded on a minimum of three (3) inches of clean sand and shall be backfilled with a minimum cover depth of six (6) inches of clean sand, granulated earth or similar material. The trench shall then be backfilled in thin layers to a minimum of twelve (12) inches above the top of the nonmetallic pipe with clean earth, which shall not contain stones, boulders or other materials, which would damage or break the pipe.