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r <br /> ' August 28, 1991 BALBT&CHANG ASSOCIATES <br /> File No. 1525-69 <br /> TABLE 2 <br /> LATERAL EARTH PRESSURES <br /> Earth Pressure Equivalent Fluid Density. <br /> Active 45 pcf <br /> At Rest 65 pcf <br /> Passive 300 pcf <br /> The above recommended lateral earth pressures are ultimate values and appropriate factor of <br /> safety should be applied by the design engineer. Also, these values are for on-site soils only <br /> and do not include any allowance for hydrostatic pressures. Coefficient of sliding friction of <br /> 0.3 is recommended for use in design between underside of concrete structure and native soil. <br /> Where backfill is placed against retaining structures, it is recommended that granular <br /> materials be used in the zone immediately adjacent the structure. This is to reduce the <br /> probability of hydrostatic pressure build-up from groundwater seepage and infiltration of <br /> surface water. The width of this granular backfill zone should be equal to or greater than <br /> ?,w one-half the embedment depth. Backfills should be compacted to between 85 and 90 percent <br /> of maximum dry density and at or near optimum moisture content as determined by ASTM <br /> designation D1557-78. Over-compaction may induce lateral earth pressures greater than those <br /> recommended above, therefore, should be avoided. <br /> { <br /> G. Underground Storage Tank Installation <br /> No groundwater was encountered within 40 feet below the existing ground surface at the time <br /> of field exploration. Available information also indicates that the first groundwater table is <br /> beyond the depth of 50 feet. Therefore, it is our opinion that tank installation should not <br /> encounter groundwater related construction problems. <br /> i <br /> Since the near surface site soils are mostly cohesive soils, we anticipate that no to slight cave- <br /> in may occur during the tank site construction. For temporary construction slopes, we <br /> recommend the inclination no steeper than 1/2 (horizontal) to 1 (vertical). We also <br /> recommend that the design and construction procedures of the tanks be reviewed with the tank <br /> manufacturer. Shoring, if used, should be designed and installed by experienced engineers <br /> Pate 13 of 17 <br />