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.~ ~ • <br />Memo to Tom Schreiner 2 Aril 23, 1999 <br />applicant elects to conduct such an evaluation, they should confer with the Division regarding <br />the type and amount of information that would be required. <br />c) Establish a setback of less than 200 feet but greater than 50 feet using assumed conservative <br />geotechnical parameters input to an analysis similar to those provided in the application, but <br />with a progressive failure analysis included. <br />d) Commit to mining at a shallow slope when inside 200 feet of the towers and provide appropriate <br />stability analyses to demonstrate that shallow slope mining will be protective of the stmctures. <br />Also provide a description of the mining method, in terms of the mechanics of earth moving that <br />would be employed to maintain the maximum mined slope angle that is established and <br />validated by the stability analyses. <br />e) Obtain an agreement with the owner of the potentially affected structures that the mine operator <br />will compensate for damage to the structure caused by mining. <br />Unless the applicant elects to commit to a 200 foot setback or obtains a compensation agreement with <br />Public Service Company, they should provide ajustification or rationale for [he magnitude of the point <br />load applied by the transmission towers that was input to the stability analyses included in the <br />application. <br />2. General Setback from the Permit Boundarv. Various Structures Located near the Outside of <br />the Permit Boundarv and from the Proposed Slurry Wall. <br />The applicant proposes a general 35-foot setback from the permit boundary and mining to a vertical 30- <br />foot highwall. There are a variety of manmade structures located at or near the permit boundary. The <br />application also includes a proposal to install a slurry wall 10 feet inside the permit boundary and a <br />commitment to install the slurry wall prior to mining those portions of the permit area that fall inside <br />the proposed slurry wall perimeter. Stability analysis included with the application indicate that the <br />proposed setback and highwall configuration will be protective of offsite structures and of the integrity <br />of the slurry wall with a minimum safety factor against slope failure of 2.5. However, based on the <br />Division's review it appears that all of the potential failure surfaces analyzed by the applicant pass <br />through the bedrock underlying the deposit to be mined, and that the high shear strength assumed to be <br />inherent to the bedrock yields the high safety factors obtained by the analyses. The analysis conducted <br />by the Division and attached to this memo employs the same critical cross section and soil strength <br />parameters suggested by the applicant and demonstrates that a failure may occur through the slurry wall <br />and outside the permit boundary based on the minimum safety factor of 1.5 established in the <br />application. In order to establish an acceptable mining setback from the permit boundary the applicant <br />may consider the options presented in items l.a. through d. above. <br />3. Mining Setback from Irrigation Ditches <br />The applicant proposes a 50-foot setback from the top outside bank of the Fulton Ditch. The Division <br />should require that the setback be established from a point more easily identifiable in the field. For <br />example, the top of the ditch bank on the mined side of the ditch as illustrated on one of the cross <br />