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• all of the topsoil materials should be removed prior to placement of the underslab gravels or fill <br />materials. <br />4) The floor slabs should be provided with control joints placed a maximum of 12 feet on center in <br />each direction to help control shrinkage cracking. The location of the joints should be carefully <br />checked to assure that the natural, unavoidable cracking will be controlled. <br />5) The underslab soils should be kept as close as possible to then in-situ moisture content. Excessive <br />wetting or drying of these soils prior to placement of the floor slab could resuh in differential <br />movement after the slabs are constructed. <br />6) If fills are required to bring the undetslab soils to the desved grade, the fill should consist of non- <br />expansive, granulaz materials. The fill should be uniformly placed and compacted in 6 to 8 inch <br />lifts to at least 95% of the maximum standard Proctor density at or near the optimum moisture <br />content, as determined by ASTM D698. <br />The above precautions and recommendations will not prevent floor slab movement in the event the soils <br />beneath the floor slabs undergo moisture changes. However, they should reduce the amount of damage if <br />such movement occurs. The only way to eliminate the risk of all floor slab movement is to construct a <br />structural floor over swell-vented crawl space. <br />• Foundation Walls and Retainine Structures: Foundation walls and retaining structures, which are <br />laterally supported and can be expected to undergo only a moderate amount of deflection, may be designed <br />for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of 45 pcf for imported, <br />free draining granular backfill and 55 pcf for the on-site materials. <br />Cantilevered retaining structures on the site can be expected to deflect sufficiently to mobilize the full active <br />earth pressure condition. Therefore, cantilevered structures may be designed for a lateral earth pressure <br />computed on the basis of an equivalent fluid unit weight of 35 pcf for imported, free draining granulaz <br />backfill and 45 pcf for the on-site materials. <br />The foundation walls and retaining structures should be designed for appropriate hydrostatic and surcharge <br />pressures such as adjacent buildings, traffic and construction materials. An upward sloping backfill and/or <br />natural slope will also increase the earth pressures on foundation walls and retaining structures. <br />The lateral resistance of retaining wall foundations placed on undisturbed natural soils at the site will be a <br />combination of the sliding resistance of the footings on the foundation materials and the passive pressure <br />against the sides of the Footings. Sliding friction can be taken as 0.4 times the vertical dead load. Passive <br />pressure against the sides of the footing can be calculated using an equivalent fluid pressure of 250 pcf. <br />The fill placed against the sides of the footings to resist lateral loads should be compacted to at least 95% of <br />the maximum standard Proctor density, neaz the optimum moisture content. <br />. We recommend imported granular soils for backfilling foundation walls and retaining structures because <br />their use results in lower lateral earth pressures. The imported granular materials should be placed to within <br />Job Number: 03.5838 NWCC, Inc. Page 7 <br />