Back-filling up to the first 18 inches above the top of the utility pipes or similar installations shall be done with thin layers. Each layer is to be tamped by manual or mechanical means. Layers that are hand tamped shall not exceed 4 inches in thickness. Layers that are power tamped shall not exceed 6 inches in thickness. The same requirements shall apply to the remainder of the back-filling if tamping is the method used for back-filling. Back-filling of all pipes of over 24 inches in diameter shall be carried up to the spring line of the pipe in three-inch layers, with each layer moistened and thoroughly tamped with suitable mechanical equipment. The back-fill around all pipes 24 inches or less in diameter shall be flooded or tamped as specified above to a depth of 18 inches above the top of the pipe before any additional backfilling is placed thereon. Any deviation from the method set forth above shall have prior written approval of the Public Works Director.