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1 <br /> ! The on-site natural till soils in portions of the Vashon advance and pre-Vashon undifferentiated <br /> I sedimenu contain a high percentage of fine-¢rain material which makes them moisture-sensitive <br /> ar.d subject to disturbance when wet. The contractor must use care during site preparation and <br /> � excavadon operations so that the underlying soils are not softened. If disturbance occurs, the <br /> � sofrened soils should be removed and the area brought to grade with structural fill. <br /> l1_5.2 Perm nent lonec <br /> ( Permanent slopes should not exceed a slope of 2H:1 V. Vegetation should be re-established on <br /> these slopes as soon as possible to reduce erosion and instability. <br /> l 16 0 STRUCT 1RA <br /> iSt�uctural fill may be necEssary to establish desired grades. rVl references to structural fill in this <br /> report refer to subgrade prepazation, fill type, placement and compaction of materiais as discussrd <br /> in this section. If a percentage of compacaon is specified under another section of this report, the � <br /> � value given in that section should be used. � <br /> I If fill is to be placed on slopes steeper than SH:1V, the base of the fill should be tied to firm, <br /> s[able subsoil by appropriate keying and benching which would be established in the field to suit <br /> the particulaz soil condidons at the time of grading. The key will act to embed the tce of the new <br /> ( fill into the hillside. Generally, the keyway for hillside fills should be at least 8 feet wide and cut <br /> into the medium dense to ve..ry dense natural sandy soils or hard silt. Level be�hes would then <br /> be cut hurizontaliy across th� tull, following the contours of the slope. No specific width is <br /> � required for the benches, although they aze usually a few feet wider than the dozer being used ro ;. <br /> cut them. All fills proposed over a slope should be reviewed by our o�ce prior to construction. i <br /> ' After overexcavation/stripping has been performed to the satisfaction of the geotechnical <br /> engineer/engineering geologist, the upper 12 inches of exposed ground should be recompacted to <br /> at least 90 percent of the modified Proctor maximum density using ASTM;D 1557 as the standarti. <br /> IIf the subgrade contains too much moisture, adequate recompacdon may be difficult or impossible <br /> to obtain and should probably not be attempted. In lieu uf recompaction, the area to receive fill <br /> should be blanketed with washed roc:k or quarry spalls to act as a capillary break between the new <br /> I fill and the wet subgrade. �Vhere the exposed ground remains soft and further overezcavation is <br /> impractical, placement of an engineering stabilization fabric may be necessary to prevent <br /> ( contamination of the free-draining layer by silt migration from below. ' <br /> Aher recompaction of the exposed ground is tested and approved, or a free-draining rock course <br /> � is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non- j <br /> organic soil, acceptable to the geotechnical engineer, placed in maximum 8 inch loose lifts with <br /> each lift being compacted to at least 95 percent of the modified Proctor maximum den:..ry using ; <br /> 1 AST:�f:D 1557 as the standud. In the case of roadway and utility tre�h filling, the bacldill � <br /> should be placed and compacted in accordance with current local or county codes and standards. <br /> The top of the compacted fill should eztend horizontally outward a minimum distance of 3 feet <br /> I" <br /> 15 <br /> � <br />