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SENT AY: GAEVSTONE ENVIROtdMENTA~; 303 6A9 0191; <br />l= <br />letter to John W. Rozelle ~ <br />•. <br />8127/2003 <br />Page 2 of 2 <br />27.03 4:19PM; <br />~: <br />As a general conclusion, our analysisrtdicates t~1at excavation can occur to <br />within 30 to 50 feet of a structure (setback from the pit crest) for bench heights of <br />20 to 30 feet, respectively, to maintairj a slope stAbility factor of safety in excess <br />of 1.5. As a reasonable guide, the an®fysis conf7~ms that a setback of 1.5 times <br />the bench height is safe. <br />:~ <br />Regarding the 5uttle Ditch (and other structures) should excavation occur closer <br />than 1.5 times the bench height (or approximately 40 feet fora 27-foot bench), <br />there is an increased risk that slope failure could damage the ditch. This analysis <br />does not change the result of Greystona's January Suttle Ditch analysis, which <br />used similar parameters to confirm that a reconstructed 3.OH:1.OV slope is <br />expected to be stable, and a setback Qi3tance of mare than ten feet from the <br />crest should be sufficient to protect thg Qitch from slope failure. This setback <br />recommendation applies to other structures as well. <br />Thank you for this opportunity to <br />please call. <br />serve~t.afarge. Should you <br />have questions, <br />Sincere~~, , ~ <br />Mike Kolin, P. E. <br />Engineering Manager <br /> <br />