48. In the rock, the fence posts mentioned in section 47 shall be driven in holes adapted to their diameter and at a depth of at least 500 mm (19.7 in); in loose soil, they shall be driven to a depth of at least 1.2 m (3.9 ft) in holes having a diameter of at least 350 mm (13.8 in) at the opening. The space around the poles shall be filled with concrete.