Whenever it is necessary to carry a trench containing a sewer connection under any stone, asphalt or cement sidewalk, no portion of the surface of the sidewalk shall be disturbed or removed. The trench shall be carried underneath, with the underportion of the walk supported in such a manner as to render it safe for public use. All supporting material under the walk shall be placed to prevent the sagging or falling of the walk or damage thereto. Where the City's inspector is of the opinion that it is not possible to comply with the requirement, the City may, in writing, permit the surface of the walk to be removed. In such case, the surface of the walk shall be relaid by the permittee within 10 days from the date of permit.