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FIGURE 5-4 5.3.3. UNSTABLE IN SITU SOIL CONDITIONS CAUTION Xerxes recommends that the tank owner seek the advice of a local foundation professional engineer if the in situ soil is extremely soft or inherently unstable (for example, peat, quicksand, muck, landfill, very soft or highly expansive clay, underground stream, etc.). 5.3.3.1. 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 3,500 lbs./sq. ft.; or where soil will not maintain a vertical wall, the excavation must allow a minimum space equal to half the diameter of the tank between the excavation wall and both the side and the endcap of the tank, to enhance lateral resistance. (See FIGURE 5-5.) 5.3.3.2. The spacing between adjacent tanks is to be at least 18 inches. (See FIGURE 5-5.) FIGURE 5-5 5.3.3.3. A reinforced concrete slab may be required under the tank as a foundation in the excavation where the bottom is unstable. 5.4. TANK LOCATION — NEARBY STRUCTURES CAUTION Xerxes recommends that the tank owner seek the advice of a local foundation professional engineer to determine the proper placement of a tank excavation near any existing structure(s). Improper placement may result in tank and/or other property damage. 5.4.1. The location of a tank can be affected by the location of nearby structures. When selecting a tank site, care must be taken to avoid undermining the foundations of existing structures or new buildings to be constructed. (See FIGURE 5-6.) 12 11 FIGURE 5-6 5.4.1.1. Ensure that downward forces from loads carried by the foundations and supports of nearby structures (constructed before or after tank installation) are not transmitted to the tanks. 5.4.2. Typically, the way to check the placement of the tank in relationship to a nearby structure is to do the following: 5.4.2.1. Determine the depth of burial needed for the tank. 5.4.2.2. Locate the footing of the structure to be considered. 5.4.2.3. Determine the line that would fall into the ground from a 45 -degree angle drawn downward from the comer(s) of the footing of the foundation that is closest to the tank. 5.4.2.4. The tank must not fall within the "shadow" of the 45 - degree -angle line drawn from the foundation's footing. (See FIGURE 5-6.) 5.4.2.5. If the tank would fall within this "shadow," do one of the following to ensure that the tank does not fall within the "shadow": a.) move the tank away from the existing building, b.) move the foundation of the building to be constructed away from the tank, or c.) deepen the footing of the planned building's foundation. 5.5. GEOTEXTILE 5.5.1. The tank owner or the owner's technical representative is responsible for determining whether a geotextile or an alternate filtering technique is appropriate for a specific installation. Geotextile allows the passage of water, but prevents the migration and mixing of in situ soil and the select backfill material. Geotextile helps preserve the integrity of the select backfill envelope, which surrounds and supports the tank. 5.5.2. Xerxes recommends that geotextile be used when the tank is installed in: • areas with frequently changing groundwater conditions or areas subject to tidal fluctuations, • unstable soils such as cited in SECTION 5.3.3., • water conditions with silty in situ soil. 5.5.3. For further information concerning geotextile specifica- tions and installation procedures, consult the geotextile supplier's installation guidelines or instructions.