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should consist of non-expansive, granulaz materials. The fill should be uniformly placed and compacted in 6 to 8 inch lifts [o at least 95% of the maximum standard Proctor density at or near the optimum moisture content, as determined by ASTM D-698/AASHTO T-99. The above precautions and recommendations will not prevent floor slab movement in the event the clays or bedrock materials beneath the floor slabs undergo moisture changes. However, they should reduce the amount of damage if such movement occurs. The only way to eliminate the risk of all floor slab movement is to construct a structural floor system over awell-vented crawl space. Perimeter/Underdrain Systems Any levels of the proposed building structures constructed below the existing or finished ground surfaces should be protected by perimeter/underdrain systems to help reduce the problems associated with surface drainage during high runoff periods. Localized perched water or runoff can infiltrate the lower levels of the structures at the foundation level. This water can be one of the primary causes of differential foundation and slab movement. Especially, when expansive soils are encountered. Excessive moisture in crawl space areas or lower level can lead to rotting and mildewing of wooden structural members and the formation of mold and mold spores. The formation of mold and mold spores could have detrimental effects on the air quality in these areas, which in turn can lead to potential adverse health effects. The drains should be located around the entire perimeter of the lower levels and be placed and at least 12 inches below any floor slab or crawl space level and at least 6 inches below the foundation voids and bottom of the foundation walls. We recommend the use of perforated PVC pipe for the drainpipe, which meets or exceeds ASTM D-2729 requirements, to minimize the potential for crushing the pipe during backfill operations. The holes in the drainpipe should be oriented down between 4 o'clock and 8 o clock to promote rapid runoff of the water. The drainpipe should be surrounded with at least 12 inches of free draining gravel and should be protected from contamination by a filter covering of Mirafi 140N subsurface drainage fabric or an equivalent product. The drains should have a minimum slope of 1/8 inch per foot and should be daylighted at positive outfalls protected from freezing, or be led to sumps from which the water can be pumped. Caution should be taken when backfilling so as not to damage or disturb the installed underdrains. We recommend the drainage systems include at leas[ one cleanou[, be protected against intrusion by animals at the outfalls and be tested prior to backfilling. We also recommend that the client retain this firm to observe the underdrain systems during construction to verify that they are being installed in accordance with our recommendations and observe flow tests prior to backfilling the systems. In addition, we recommend that an impervious barrier be constructed to keep water from infiltrating through the voided areas and/or under [he foundation walls. The barrier should be constructed of an impervious material, which is approved by this office and placed below the perimeter drain and up against the sides of the foundation walls. Atypical perimeter/underdrain detail is shown in Figure #12. The placement of the impervious membrane and properly compacted clays in the crawl space areas to the top of the footings or at least 12 inches above the top of the foundation voids or bottom of the foundation walls will help reduce the moisture problems in these areas. lob Number: 07-7442 NWCC, Inc. Page 7