The provisions of this article shall apply to contracts by Separation of Grade Districts, as provided for in Part 2 (commencing with Section 8100) of Division 9 of the Streets and Highways Code.

The commission shall, pursuant to an order entered in the minutes, advertise for bids for the construction of the project either as a whole or in sections, as it may deem fit, in accordance with the plan theretofore adopted and filed, by publishing a notice calling for bids at least once each week for two successive weeks in a newspaper of general circulation published within the district. The notice shall refer to the order and plan for further particulars. If the commission elects to receive separate bids for the construction of sections of the project, the order shall describe the sections for which separate bids are desired. The commission may advertise at the same time and in the same notice both for bids for the construction of the project as a whole and for bids for the construction of separate sections.

All bids for construction work shall be presented under sealed cover and shall be accompanied by one of the following forms of bidders security:

Upon an award to the lowest bidder, the security of an unsuccessful bidder shall be returned in a reasonable period of time, but in no event shall that security be held by the commission beyond 60 days from the time the award is made.

Every contract for the construction of the project or for any section, if it is to be constructed in sections, shall be let, after advertisement, to the lowest responsible bidder, or the commission may reject all bids for the construction of the project or for any section, and may readvertise for bids for doing the work in the same manner and for the same length of time as provided for a notice calling for bids for construction.

The commission may hire all necessary engineers, inspectors and superintendents to supervise the performance of contracts entered into by the commission, or to have charge of doing all work done without contract.

As to any project involving a state highway where the plans have been approved by the Department of Transportation, Division of Highways, the commission and the department may enter into and carry out contracts pursuant to Section 130 of the Streets and Highways Code.