During the winter period, all trenches shall be backfilled with stone dust, screening or other suitable pervious material. If the opening is in bituminous concrete, asphalt or in pavement of equal quality, a base course of five inches of two-and-one-half-inch stone, dust bound, and a surface course of two inches of 3/4 bituminous concrete shall be placed in the trenches. No shale, rock or frozen dirt shall be placed in the trenches.