Section 8. The department, when about to construct a state highway, shall advertise in two or more newspapers published in each county in which the highway lies, and in three or more daily newspapers published in Boston, for sealed proposals for the construction of such highway, stating the time and place for opening such proposals, and reserving the right to reject any and all proposals. If a proposal is satisfactory, the department, with the approval of the governor and council, shall make a contract in writing on behalf of the commonwealth for such construction. After the proposals have been accepted or rejected they shall be kept by the department, and shall be open to public inspection for three years, and may then be destroyed by the department. The department may, in the same manner and under the same conditions, contract for the grading of a state highway or for furnishing labor, materials or any other element in its construction. The construction of all state highways shall be under the supervision and subject to the approval of the department and in accordance with plans and specifications furnished by it, and shall be fairly apportioned by the department among the different counties.