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r• <br /> Everett General Hospital & Medical Center W-5557 <br /> 29 March 1988 Page 8 <br /> designed with an equivalent fluid unit weight of 50 pcf. Lateral deflection at <br /> the top of the wall on the order of 0.1 percent of the wall height is considered <br /> sufficient to mobilize the active lateral pressures. The above lateral earth <br /> pressures are based on the assumption of a uniform, level granular backfill and <br /> no buildup of hydrostatic pressure behind the walls. To minimize lateral earth <br /> pressures and prevent the buildup of hydrostatic pressures, the wall backfill <br /> should consist of clean, free-draining granular materials with drainage provisions <br /> as discussed subsequently in this report. The wall backfill should generally meet <br /> the recommended gradation for below-slab drainage material presented previously <br /> in this report. If pea gravel or washed rock is used for wall backfill, the gravel <br /> should be separated from adjacent site soils or structural fill with an appropriate <br /> permeable filter fabric. <br /> Surcharges due to sloping ground, adjacent footings, vehicles, construction <br /> equipment, etc., must be added to these values. The above equivalent fluid weight <br /> assumes that the backfill is compacted to approximately 90 percent of the modified <br /> Proctor maximum dry density. Additional compaction immediately adjacent to the <br /> wall will increase the lateral pressures, while a lesser degree of compaction may <br /> permit post-construction settlements. Small, hand-operated compaction equipment <br /> s'iould be used within 3 feet of the wall . It should be noted that limitations <br /> placed on compaction may allow some minor settlement of wall backfill with time. <br /> J Typically, this is not a problem except for settlement-sensitive surfaces or <br /> structures built above the backfill, and for utilities passing through the backfill <br /> into basement walls. <br /> 1 <br /> Lateral forces on the base of the wali can be resisted by a combination of passive <br /> pressure and base friction. A recommended allowable design passive resistance of <br /> 250 pcf (pounds per cubic foot) may be assumed for each foot of penetration below <br /> the ground surface, neglecting the upper 1 foot. An allowable base friction <br /> coefficient of 0.4 is recommended. These passive earth pressure and base friction <br /> values incorporate a factor of safety of at least 1.5. <br /> 1 <br /> I <br />