329 79TH PL SW 2016-01-01 MF Import

The exposed subgrade should be pre-rolled as necessary such that the top 12 inches attains at least 95 percent density, using ASTM D: 1557 as the standard. If the subgrade is more than a few percent from its optimum moisture content, adequate compaction will be difficult or impossible to achieve. It may be necessary to scarify the upper layer, allow it to dry, and then compact. If during stfipping, localized areas of soft subgrade, pumping subgrade, or organic-fich soils are encountered, thes^ areas should be over e%cavated and backfilled with s,ructural fill as recommended below. Structural Fill All fill placed in building areas, beneath dfiveways and roads, should be placed in accordance with recommendations herein for structural fill. Pfior to placement of structural fill, the surfaces to receive soil should be pepared as previously discussed. Structural fill should be placed in loose horizontal lifts and mechanically compacted to a uniform density of at least 95 percent of the modified proctor dry density(ASTM: D 1557). When using compaction equipment weighing 20,000 pounds or less, individual loose lifts should not exceed 8 to 10 inches. In some instances, thicker loose lifts may be placed provided larger compaction equipment is used. Placement of thicker lifts should be evaluated on a case by case basis, taking into consideration rhe numberous vafiables involved (i.e., size and configuration of equipment, soil conditions, weather conditions, structural load tearing requirements, etc.) When constructing a structural fill using smooth drum vibratory compaction equipment on slopes exceeding 10 percent, the individual lifts should be keyed into the native soils at least 3 feet horizontally to form an integrated fill/native soil interface. The structural fill would not require keying if a sheepsfoot roller or similar compaction equipment is used, and the fill/native soil interface is well mixed. A representative of a certified testing company(e.g., Cascade Testing Laboratory, Inc.) should be present during placement of structural fill to perform a representative number of in-place density tests. In this way, the adequacy of the earthwork may be evaluated as grading progresses. The geotechnical engineer should be notified to conduct a site visit prior to structure! fill placement to confirm that site preparation has been satisfactorally accomplished. Similarily, at least one additional site visit should be conducted by the geotechnical engineer dufing placement of the structural fill. The suitability of the soils for structural fill use depends primarily on the gradation and moisture content of the of the soil when it is placed. As the amount of fines (that portion passing the US N0. 200 sieve) increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. Soils, such as those present at this site, which contain more than about 5 percent fines by weight cannot be readily compacted to a firm, non-yielding condition when the moisture content is more than a few percent above or below optimum. gcotcchVtcritage doc 2 9/ 18/95