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DID L. NELSON AND ASSOCIATES, INC. <br />consultinf Enilnee,ing Geology <br />July 13, 1990 _ <br />Project No. 9490 <br />Page 6 <br />ground or to saturate foundation areas. Surface drains should be maintained <br />separately, and should not be interconnected with foundation or wall drains. <br />structural Fill <br />All fill placed beneath buildings, pavements, or other settlement sensitive <br />features should be placed as structural fill. Structural fill, by definition, <br />is placed in accordance with prescribed methods and standards, and is <br />monitored by an experienced geotechnical professional or soils technician. <br />Field monitoring procedures would include the performance of a representative <br />number of in -place density tests to document the attainment of the desired <br />ill the site <br />degree of relative compaction. Prior to placement of structural t <br />should be stripped of all vegetation and topsoil materials. The area should <br />then be thoroughly compacted to a dense non -yielding state. Slope areas with <br />surface grades in excess of 20% which are to recieve structural fill should be <br />benched horizontally on 3 foot vertical intervals, prior to surface <br />compaction. Areas to receive structural fill, structural fill materials AM <br />structural fill placement should be approved by a geotechnical consultant. <br />Where foundation or structural fill areas have becor..e softened by moisture, <br />some overexcavation and replacement may be necessary., If dewatering is not <br />complete or if the subgrade will be exposed to water again, we recommend using <br />a 1 to 2 inch rock (railroad ballast) for structural fill. The rock will <br />provide confinement of the underlying soils and will not disturb when exposed <br />to water. The depth of overexcavation and thickness of the rock should be <br />determined in the field. <br />Foundations <br />In our opinion, the use of shallow spread footings founded on the native, <br />undisturbed glacial soils or Whidbey Formation deposits, or on approved <br />structural fill or crushed rock extending to these soils, should prove <br />suitable for support of the proposed dwellings. For planning purposes, an <br />allowable soil bearing pressure of 2000 pounds per square foot (psf) should <br />ba used for design of shallow footings founded into the medium dense to dense <br />