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During and following periods of significant precSpitation the silty <br />native soils would be difficult to handle and impossible to adequately <br />compact. The native soils are suitable for use as fill beneath the <br />building only during the normally drier summer months, when the moisture <br />content in the material can be controlled. If wet weather construction <br />is planned, we recommend that all structural fill consist of imported <br />clean sand or sand and gravel with less than 5 percent fines (by dry <br />weight) passing the No. 200 sieve. All fills placed in the building area <br />should be spread in lifts of 10 inches or less in loose thickness, <br />conditioned to the proper moisture content for compaction, and each lift <br />thoroughly compacted to at least 95 percent of the maximum dry density as <br />established by AASHTO Compaction Test Procedure T-180 (ASTM D1557 equivalent). <br />FOUNDATION SUPPORT <br />We recommend that all footings have a minimum width of 18 inches and <br />that the footings be founded a minimum of 24 inches below the lowest <br />adjacent finished grade. Foundation subgrades should be thoroughly <br />compacted with mechanical vi)�:atory eq'.iipment prior to constructing forms <br />and placing steel. Any soft subgrade areas detected during this compac- <br />tion should be overexcavated to suitable bearing soils. The excavated <br />material should be replaced with imported granular bacY.fill which is <br />compacted in lifts to at least 95 percent of the previously specified <br />standard. If native silty soils are used as fill to raise site grades, <br />the footing subgrades should be protected from disturbance and moisture <br />by placing a thin layer of lean concrete or 3 to 4 inches of crushed rock <br />immediately following excavation. <br />Footings so founded in the compacted fill may be designed for an <br />allowable soil bearing pressure of 3,500 pounds per square foot. This <br />value applies to the sum of all dead plus long-term live loads, exclusive <br />of the weight of the foundation and backfill above the foundation; it may <br />be increased by one-third for transitory loads, such as wind or seismic. <br />Foundation settlements are expected to be small and to occur rapidly <br />as loads are applied. Total settlements are estimated at less than 3/4 <br />-5- <br />RAMC-S O MOORS i <br />