The sums required by this chapter to be paid to the city shall be used for the replacement of the surface of paved or improved streets, alleys or sidewalks in accordance with the plans and specifications of the city therefor. All such work shall be done by employees of the city under the direction of the director of public works; provided, that the director of public works may require that contractors shall backfill, with suitable well-compacted material, all excavations in dirt, gravel, sealed earth and sidewalk locations and if the same is not done to the satisfaction of the director of public works, the completion may be done by the city and the cost for the same shall be paid by the contractors. (Prior code § 14-205)