Adequate precaution shall be taken to insure proper compactness of the backfill around sewer piping without damage to the piping. Trenches shall be backfilled in thin layers to 12 inches above the top of the pipe with clean earth containing no stones, boulders, cinders, or other material which would damage or break the piping or cause corrosive action. Mechanical devices such as bulldozers and graders, may then be used to complete backfill to grade. Fill shall be properly compacted. Suitable precautions shall be taken to insure permanent stability for pipe laid in filled or made ground.
('69 Code, § 20-92)