68. Every underground tank liable to be subjected to overhead vehicular traffic shall be sunk(1) to a depth of at least 1 m below ground level, be covered with a layer not less than 90 cm thick of materials referred to in section 67 and a layer of bituminous concrete not less than 10 cm thick; or
(2) to a depth of at least 45 cm, be covered with a layer of at least 30 cm thick of materials referred to in section 67 and a slab of reinforced concrete not less than 15 cm thick. The slab of reinforced concrete shall extend at least 30 cm beyond the perimeter of the tank.