Whenever it is necessary to break through existing pavement for excavation purposes and where trenches are to be 4 feet or over in depth, the pavement in the base shall be removed to at least 6 inches beyond the outer limits of the subgrade that is to be disturbed in order to prevent settlement, and a 6 inch shoulder of undisturbed material shall be provided in each side of the excavated trench. The face of the remaining pavement shall be approximately vertical. A power-driven concrete saw shall be used so as to permit complete breakage of concrete pavement or base without ragged edges. Asphalt paving shall be scored or otherwise cut in a straight line. No pile driver may be used in breaking up the pavement.