67. Every underground tank shall be set on a layer at least 30 cm thick, composed of,
(1) in the case of a steel tank, sifted or natural stoneless sand, compacted mechanically on site; or
(2) in the case of a fibreglass tank, crushed stone or pea gravel.
The tank shall be covered with a layer of the same materials which shall not be more than 30 cm thick measured from the natural ground surface.