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30 <br /> the net static bearing pressure beneath the tanks can be computed in accordance <br /> with the following equations: <br /> DDC improved soil: <br /> q = 140B + 1600 <br /> a <br /> Native soil excavated to at least 8-ft depth and replaced with compacted <br /> fill: <br /> q = 90B + 1050 <br /> a <br /> in which: qa = allowable bearing pressure (psf) <br /> B = tank diameter (ft) <br /> These equations include a factor of safety in excess of 2.5. A 33 percent <br /> increase in allowable bearing pressure can be used for seismic design. <br /> If compacted structural fill is reinforced with geotextiles in accordance <br /> with previous discussions, the allowable bearing pressure can be increased. <br /> Provided the reinforcing extends down from the bottom of the tank to a depth <br /> equal to at least one-fourth the tank diameter, the allowable and maximum <br /> allowable bearing pressures for seismic loads can be increased by 500 psf. <br /> Ringwall Recommendations. We recommend that cylindrical steel storage tanks <br /> at this site have ringwalls. The ringwalls should have a minimum width of 1.5 ft <br /> and a minimum embedment of 1.5 to 2 ft. If possible, the ringwalls should be <br /> proportioned so that the net applied bearing pressure at the base of the ringwall <br /> approximates the net pressure applied by the typical or maximum fluid level in <br /> the tank. <br /> The ringwalls should be proportioned so that the maximum gross contact <br /> pressure under dead, live, and transient loads (including wind and seismic loads) <br /> does not exceed the gross allowable bearing pressure. Based on the anticipated <br /> properties of improved soil or compacted structural fill and the standard bearing <br /> capacity equations, the net bearing pressure can be computed in accordance with <br /> the followincl equations: <br /> 1144A/CC-30 :'McClelland <br />