5806.4.3 Depth, Cover and Fill
The tank shall be buried such that the top of the vacuum jacket is covered with not less than 1 foot (305 mm) of earth and with concrete not less than 4 inches (102 mm) thick placed over the earthen cover. The concrete shall extend not less than 1 foot (305 mm) horizontally beyond the footprint of the tank in all directions. Underground tanks shall be set on firm foundations constructed in accordance with the California Building Code and surrounded with not less than 6 inches (152 mm) of noncorrosive inert material, such as sand.
Exception: The vertical extension of the vacuum jacket as required for service connections.