If the county engineer deems it necessary the county engineer shall appoint, with the approval of the board, a competent inspector to inspect the construction of any bridge or culvert work. The county engineer shall be in direct charge of all construction work and may remove such inspector at any time for cause, except that when federal aid is granted for the construction of any bridge or culvert work such construction shall be under the direct supervision of the secretary of transportation. The fact that the county has an inspector on the work shall not release the contractor from the entire fulfillment of the terms of such contractor’s contract.