605 ILCS 5/5-403
When any highway construction projects by a County are to be performed under the supervision and approval of the Department the procedure shall be as follows:
The county board shall, by one or more resolutions, specify the particular section or sections of highway to be constructed and the amount or amounts to be used for such construction. The resolution or resolutions shall be submitted to the Department for its approval. One resolution may be submitted for more than one project. When the resolution or resolutions have been approved by the Department, the county shall cause surveys, plans, specifications and estimates of such construction to be made and submitted to the Department for approval.
Upon receiving such approval, the county may advertise for bids and let contracts for such construction to the lowest responsible bidder; or with the approval of the Department, do the work itself through its officers, agents and employees. No contract shall be let without the approval of the Department. The Department shall have general supervision of such construction whether done by the county or by contract. Upon completion of the construction, if it is found by the Department that such construction has been in accordance with the specifications, plans, surveys, and contracts (if the construction was by contract), the Department shall so certify to the county.
605 ILCS 5/5-403