The dwelling requirements of the 2017 Oregon Plumbing Specialty Code (OPSC) Section 313.5 requires that "piping in the ground shall be laid on a firm bed for its entire length." OPSC Section 3l4.4 requires that "adequate precaution shall be taken to ensure proper compactness of backfill around piping without damage to such piping".
Trenches shall be backfilled in thin layers to 12 inches (305 mm) above the top of the piping with clean earth, which shall not contain stones, boulders, cinderfill, frozen earth, construction debris or other materials that would damage or break the piping or cause corrosive action.
Bedding and backfilling material
The inspection Services Division has and will accept the use of 3/4 minus rock or gravel for the purposes of bedding and backfilling such as underground plumbing underground plumbing piping described in OSPC sections 313.5 and 314.4.