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NOTE: Numbers in brackets [ ] refer to 12' tanks. Tanks not subjected to traffic loads must have a minimum cover depth of 24 [481 inches of backfill, or 12 [36] inches backfill plus 4 inches of reinforced concrete or 6 inches of asphalt. NOTE: Numbers in brackets ( ] refer to 12' tanks. Tanks in high water areas not subjected to traffic loads must have a minimum of 36 [48] inches of backfill covering. WARNING: In a non traffic installation, insure that the areas above the tanks will never be subject to traffic loads. NOTE: These depths of cover are MINIMUMS in all cases. NOTE: Asphalt and concrete pads must extend a minimum of 12 inches beyond tank in all directions. NOTE: Maximum burial depth is 7 feet of cover over top of tanks. NOTE: Unattached manway riser (if present) must not transmit load from the slab to the tank. There must be a minimum 6 inch space between the bottom of the riser and the top of the tank. TANK SPACING NOTE: These are minimum spacings and must be increased as needed to accommodate deadmen or anchor slabs. Refer to Section 7 on tank anchoring. Always allow sufficient clearance to allow the deadmen to be set outside of the tank shadow. STABLE SOIL CONDITIONS Where shoring is not required to maintain a vertical wall from,top to bottom of the excavation, the minimum spacing between the sidewall or endcap of the tank and side of excavation must be 18 inches (24 inches for 12' tanks). If more than one tank is to be installed in the same hole, there must be at least 18 inches between the tanks. UNSTABLE SOIL CONDITIONS If the soil has less than 750 lbs./sq. ft. cohesion as calculated from an unconfined compression test; or in soils having an ultimate bearing capacity of less than 3500 lbs./sq. ft.; or where soil will not maintain a vertical wall, the excavation must allow a minimum space equal to 1/2 the diameter of the tank between the side and endcap of the tank and the excavation wall. The spacing between adjacent tanks, 18 inches remains the same. FILTER FABRIC The tank owner or his technical representative is responsible for determining whether filter fabric or an alternate filtering technique is appropriate for a specific installation. Filter fabric allows the passage of water but prevents the migration and mixing of native soil and backfill material. It preserves the integrity of the backfill envelope which supports the tank. Polyethylene film is not considered an effective material to prevent migration of pea gravel and native soil; it may tear or degrade while in service. Xerxas recommends that filter fabric be used when the tank may be installed in: • areas with frequently changing ground water conditions or areas subject to tidal fluctuations, • unstable soils such as bog, swamp, landfill or mud, • water conditions with silty soil.