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V � UNSTABLE EXCAVATIONS SHORING REMOVAL • An unstable excavation is any of the following: • After the backfill is properly placed completely - Muck, bog, peat, swamp, quick sand, flowing water, around the tank(s): landfill type areas or any other situations where the - Vibrate shoring and pull slowly to top of backfill. soil is inherently unstable. - Fill any voids created by the pulled shoring with - Soil less than 750 lbs./sq. ft. cohesion per ASTM approved backfill. D2166(Test Method for Unconfined Compressive . All shoring should be removed and all voids must Strength of Cohesive Soil), or have an ultimate filled using a long handled probe before continuing bearing capacity less than 3,500 lbs./sq.ft. installation. • If shoring is left in place,the shoring must not degrade over the life of the installation. If the soil is unstable,the tank owner should consult a professional engineer to ensure a proper installation TANK LOCATION-NEARBY STRUCTURES and avoid the potential for tank or property damage. • The tank owner or the tank owner's representative is responsible for determining the proper location of a • A soils consultant can provide additional tank excavation. recommendations, including when a reinforced • The location of a tank can be affected by the location pad is needed under the tank. of nearby structures.When selecting a tank site, care • When a bottom pad is used,it should extend must be taken to avoid undermining the foundations to the excavation walls. of new or existing structures. • Unstable excavations may also require geotextile • Ensure that downward forces from loads carried by fabric(see Section F). the foundations and supports of nearby structures • When the soil is unstable,with or without shoring, (constructed before or after tank installation)are use the minimum spacings(per Figure E-3 and not transmitted to the tanks. (Refer to NFPA 30 for Table E-4). additional details). e Figure E3 Geotextile fabric(filter fabric)allows the passage of B A water but prevents the migration of approved backfill into the native soil and native soil into the approved backfill. Migration may compromise the backfill support of the tank. GEOTEXTILE FABRICS ARE REQUIRED FOR ANY OF THE FOLLOWING INSTALLATIONS LUnstableSoil B • Areas subject to tidal conditions. • Areas subject to frequently changing ground water levels. • Water conditions with silty soil. TABLE E-4 • Muck, bog, peat, swamp, landfill type areas or any other situations where the soil is inherently unstable. 4',6,8'Diameter Tanks • Soils with less than 250 lbs./sq.ft cohesion or with Minimum w/12"x 12"CSI Deadmen ultimate bearing capacity of less than 500 lbs./sq.ft. A 18"(457mm) 24"(61omm) • Geotextile fabrics may also be used in alternate B '/4 Tank Dia. %Tank Dia. installation methods(see Supplemental Documents, 10'Diameter Tanks Section R). Minimum w/18"x8"CSI Deadmen The tank owner or tank owner's representative may also specify the use of geotextile fabrics. A 18"(457mm) 36"(914mm) Non-degradable geotextile fabric can be either woven B %Tank Dia. Y2 Tank Dia. or non-woven and should have a minimum grab tensile 12'Diameter Tanks strength of 120lbsAnch(ASTM D4595)and a maximum Minimum w/18"x8"CSI Deadmen apparent opening size of#50 US sieve(0.300mm) (ASTM D4751). A 24"(610mm) 36"(914mm) Do not use plastic, or any other material that may tear B %Tank Dia. %Tank Dia. or degrade over time, as a replacement for geotextile fabric. 14