1125 80TH ST SW 2016-01-01 MF Import

Cornerstone Columbia Dei, ,lopment Co. W-6984 5 July 1990 Page 6 compacted to a uniform density of at least 90 percent of the modified Proctor maximum dry density (ASTM:D 1557). As a continuation of our geotechnical engineering effort, we recommend that a representative from our firm be present on a full time basis during placement of structural fill to observe the work and perform a representative number of in-place density tests. In this way, the adequacy of the earth work may be evaluated as construction progresses. The suitability of soil for structural fill use depends primarily on the gradation and moisture in the soil when it is placed. As the amount of fines (that portion passing the U.S. No. 200 Sieve) increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. Soil containing more than about 5 percent fines by weight, such as the bulk of the site soils, cannot be consistently compacted to a firm, dense, non-yielding condition if the moisture is more than a few percent above the optimum moisture content. Structural fill should consist of a predominantly granular soil, free of organics and other deleterious material, with a maximum rock size of about 6 inches. The native soils enc)untered in the test pits contained relatively large amounts of fines, and as such may require considerable drying and moisture conditioning prior to use as structural fill. If grading must take place when the onsite materials are too wet to compact, the soils should be permitted to dry, prior to further fill placement. This may require scarification and aeration of the material that can be completed dependably only during extended periods of dry weather. If owing to moisture or weather conditions, the site soils cannot be adequately dried back, they should be removed and wasted. In order to expedite filling in wet site conditions, if the onsite silty materials cannot be used, imported fill should contain no more than 5 percent by weight passing the No. 200 Sieve, when measured on the minus No. 4 Sieve fraction. Soils of this gradation may be successfully compacted under a wider range of weather conditions. However, even with a material of this type, delays in grading due to inclement weather can still occur. 4.3 Shallow Foundations It is our opinion that shallow, isolated or cont;nuous spread footings founded within undisturbed native sods and/or structural fill placed above the medium dense native