All excavations shall be open trench work unless otherwise authorized by the Superintendent or his representative. The foundation in the trench shall be formed to prevent any subsequent settling of the pipes. If the foundation is of firm earth, the earth shall be pared or molded to give a full support to the lower quadrant of each pipe. Bell holes shall be dug to provide ample space for pouring of joints. Care must be exercised in backfilling below the center line of the pipe in order to give it proper support. Backfilling shall be placed in layers and solidly tamped one foot (1') above the pipe.