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• -30- <br />blocks along bedding planes. This analysis is depicted on Fig. B -6. The <br />analysis was performed for 55 degree and 34 degree cut slopes and is sum- <br />marized on Fig.. 12. The analysis indicates a factor of safety of 1.0 provided <br />water infiltrates joints no closer than 16 feet from the cut crest at a 55 <br />degree slope and no closer than 20 feet for a 34 degree slope. A factor of <br />safety of 1.5 is not achieved unless the "dry" zone its about 60 feet from the <br />crest for a 55 degree cut slope and 45 feet for a 34 degree cut. <br />The analysis indicates that an interceptor ditch must be provided above <br />the crest of the rock cut a significant distance to intercept water beneath <br />the spoil and to preclude failure of the cut. In our opinion, construction of <br />the interceptor ditch beneath the spoil toe would be impractical requiring <br />excavation and backfilling of massive amounts of spoil. The only practical <br />• solution may be to construct a lined interceptor ditch at the toe of the spoil <br />and provide a bench between the spoil toe and crest of the cut. <br />The relative lost space required by necessary bench widths and cut slope <br />angles appears to be almost equal. If a 34 degree (1.5 horizontal to 1 vertical) <br />cut is used, analysis indicates that at least a 45 -foot wide bench should <br />be used. A minimum 60 -foot wide bench is indicated for a 55 degree cut slope <br />It is our opinion, that selection of the cut slope should be made based upon <br />factors concerning construction and your planned mine operation. We believe <br />that a 34 degree cut is the safer configuration. <br />We analyzed the stability of a 55 degree cut slope with dry conditions <br />using Patton's equation. That analysis indicates a factor of safety of about <br />1.8 at a sliding surface inclination angle of 35 degrees. This factor of <br />safety is based upon a friction angle of 27 degrees and no cohesion. We have <br />