Any excavation made by any sewer contractor in a right-of-way shall be protected and guarded by fencing or covering and properly lighted. The protection of the public from the danger of such excavation shall be the responsibility of the sewer contractor; and the contractor shall be liable for any damage caused by his failure to properly protect and guard such excavation. If the contractor or owner fails to properly protect and guard such excavation, the city may properly protect and guard such excavation and charge the cost thereof to the sewer contractor or owner, who shall, upon receiving written notice of the amount of such charge immediately pay the same to the city.