The size, slope, alignment, materials of construction of a building water line and the methods to be used in excavating, placing of the pipe, jointing, testing and backfilling the trench shall all conform to the requirements hereinafter specified: Building water service lines shall be of three-quarter-inch minimum diameter. Materials shall conform to those hereinafter specified. Water lines shall be laid in a straight line horizontally and vertically. The excavation for building water shall be carried at least 48 inches below grade; if the forty-eight-inch depth cannot be achieved then insulation equivalent to the required depth shall be required. The trench shall be backfilled with sand or fine-graded gravel, compacted to the flow line of the pipe. The pipe shall then be laid to line and grade on the prepared bed and hand backfilled and tamped with select material free from lumps, clods, stone, etc., to a point one foot over the top of pipe. The remainder of the trench shall be backfilled to the satisfaction of the Superintendent, except that excavations in streets, driveways and sidewalks located in the public thoroughfare shall be backfilled with good quality run of bank gravel compacted in one-foot layers.