5706.4.5.7 Depth of underground storage tanks.

The maximum distance between the top and bottom plates of an underground flammable and combustible liquid storage tank shall not exceed 40 feet (12 192 mm).

5706.4.5.8 Underground tank location.

Underground tanks shall be located such that the top plate thereof shall be at least 2 feet (610 mm) below the established grade. No such tank shall be buried within 10 feet (3048 mm) of any building or adjoining property line. Individual underground tanks shall be covered with 2 feet (610 mm) of earth or 4 inches (102 mm) of reinforced concrete extending 10 feet (3048 mm) beyond all the vertical walls of the tank, and the excavation made to receive the tank shall be backfilled with well-compacted clean sand or earth, free of any ash or other corrosive substance, and free from stones larger than will pass through a 1-inch (25-mm) mesh. Underground tanks may be erected in groups of 2 or more, when such tanks are separated by a space of 1 foot (305 mm), and provided the reinforced concrete top cover extends unbroken over the open space between the tanks.

5706.4.5.9 Mounded-over tanks.

Mounded-over tanks shall be considered underground tanks. A mounded-over tank may be erected with its base at any desired elevation not higher than the grade plane of the premises. It shall be enclosed with a steel, reinforced-concrete, or closed-face-concrete cribbing wall extending from the established grade to the top of the top cover, with the exterior face of the wall at least 10 feet (3048 mm) from the exterior face of the wall of the tank, and backfilled between the tank and the enclosing wall with compacted clean earth or sand containing no ash or other corrosive substance. The wall and backfill may be replaced with compacted similar fill, extending from the established grade to the level of the top cover on the tank, at the normal angle of repose of the material so used, with the provision that the width of the sloped material at the level of the top of the top cover shall be at least 10 feet (3048 mm) wide. When two or more mounded-over tanks, either rectangular or vertical-cylindrical, are grouped together, the tanks shall be at least 1 foot (305 mm) apart from each other, and the group of tanks shall be enclosed around the periphery with the same type of wall and backfill described above for one mounded-over tank.

5706.4.5.10 Maximum capacity of flammable liquid underground tanks.

Underground storage tanks used for storage of a flammable liquid shall not exceed 500,000 gallons (1892 500 L), except that the commissioner may approve the installation of underground tanks with a capacity not to exceed 6,000,000 gallons (22710 000 L) where such greater capacity is determined to be necessary because of the capacity of the marine vessel, watercraft, pipeline or tank car delivering such liquid and where such increased capacity does not endanger the public safety.