The department may, with the approval of the State Public Works Board, receive bids for the construction of several public works projects as a single project. Where more than one appropriation has been made for the several projects united as a single project under the provisions of this section, payments for the single project shall be made from the separate appropriations on the proportional basis that may be determined by the department and approved by the Department of Finance. In the case of several projects united as a single project pursuant to this
section, those projects shall be within 100 miles of one another.