The contractor shall excavate all trenches to provide a minimum cover of five and one-half (5.5) feet above the top of the water service line. Excavation may be carried on by any reasonable method preferred by the contractor which shall insure systematic progress without danger to the completed work or the private or public property adjacent to the work. Whenever the contractor shall excavate below the grade of the water service line, they shall backfill the trench to grade with selected sand tamped in place.