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MOORE & TABER NORTHERN CALIFORNIA <br /> CONSULTING ENGINEERS AND GEOLOGISTS <br /> -6- <br /> or equivalent. Preparation for compacted fill should include <br /> stripping and disposal of all debris and organic material and <br /> subexcavating any disturbed soils/fill identified during grading <br /> to full depth and to +5 ft outside building lines. The exposed <br /> surface should be field-reviewed by the soils engineer with <br /> respect to exposure of undisturbed original ground then scarified <br /> and compacted to 85-90% relative compaction (ASTM D 1557 ) . <br /> Native soils are highly expansive but could be considered <br /> acceptable for use as compacted fill for nominal grade raises <br /> with acceptance of potential for slab cracking. Significant, <br /> but limited , reduction in slab cracking could be associated <br /> with moisture-conditioning soils to +18-24 inches under slab. <br /> A higher level of assurance against post-construction slab <br /> cracking could inclu4e use of select low expansive import soils <br /> (say, 18-36 inches) under slabs--or possibly lime-treating <br /> native soils to reduce expansive effects. Native soils may <br /> be placed to 35-90% relative compaction (ASTM D 1557) with <br /> 3-5% over-optimum moisture. Any imported fill should be approved <br /> by the soils engineer and , in building area, be of low expansion <br /> potential (say, Expansion Index less than 25) and placed to <br /> 90% relative compaction. <br />