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SHANNON F~WILSON, INC. <br />li <br />USGS data indicates that the peak ground acceleration with a 10 percent probability of <br />exceedance in 50 years is about O.OSg (Reference 22). Our evaluation of the anticipated seismic <br />conditions was performed using pseudo-static analysis. This analysis indicates that, when <br />horizontal pseudo-static accelerations of O.OSg and O.lOg are applied, the FS decreases to 1.5 and <br />1.3, respectively. <br />In summary, the results of our pazametric stability analysis indicated that the static and psudo- <br />static FS for the proposed excess spoil fills is greater than the minimum FS required by the <br />CDMG Regulations (Reference 1). <br />7.0 CONCLUSIONS AND RECOMMENDATIONS <br />The results of our stability analysis as described above indicate that excess spoil valley fills <br />constructed with the geometries shown on Figures 2 and 6 has calculated Factors of Safety in <br />excess of that required by CDMG Regulations (FS > 1.5, Reference 1). Our stability analyses <br />were completed using several key assumptions. The following paragraphs provide our design <br />and construction recommendations to address these issues. <br />Surface drainage should be designed per regulations, appropriate erosion control should be <br />provided. Benches are not required for stability, but can be considered to control erosion. <br />We recommend that benches and the top of the east fill be sloped or crowned to promote positive <br />drainage from the face of the slope. Because settlement of 1 to 2 percent of the fill height may <br />occur, the grading should be performed such that positive drainage will exist following <br />settlement. It is our opinion that 2 to 5 percent slopes will provide sufficient allowance for <br />settlement. <br />CDMG Regulations require a drain system below valley fills (Reference t). The drain should be <br />constructed along the existing valley bottom and extend from the toe to the head of the fill. <br />Lateral drains should be constructed in locations where seeps or springs are present and should <br />tie into the main drain system at the valley bottom. The size and approximate locations of the <br />main valley drain and the lateral drains are shown on Figure 5. The exact locations of the drains <br />should be determined in the field. <br />• <br />23-1 Al l OS-200-Rl.doc <br />23-1-01105-200 <br />10 <br />