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Response to DRMS Adequacy Review — Cotter LP -21 Mine Reclamation Plan Amendment <br />Geotechnical Stability Exhibit <br />13. This exhibit states that the waste rock will be added to the waste pile in lifts and then <br />compacted Please inform the division of the thickness of the lifts and the compaction <br />method(s) to be used <br />The waste rock pile at LP -21 will be constructed in lifts approximately one to three <br />feet thick. Each lift will be wheel rolled for compaction. <br />14. Please if form the Division of the necessary amount of compaction for the waste rock pile, <br />and the operators plan for insuring that necessary compaction of the waste rock is achieved <br />Compaction is only one of the eight parameters used in calculating the angle of <br />internal friction for the soil stability of the waste rock pile. The factor of safety is a <br />function of the angle of internal friction for the soil and the slope angle relative to <br />the horizontal. The factor of safety for the waste rock pile at LP -21 was calculated <br />to be 2.518 which is well above the recommended 1.4 parameter for the safety factor <br />of permanent slopes. <br />This was a conservative calculation and it is unlikely, even if the degree of <br />compaction is not optimal, that the slope would be unstable at 3H:1 V. The method <br />of plug dumping does, however, result in a fair amount of compaction which will <br />assure a stable slope based on the factor of safety calculation. <br />The method for insuring proper construction and compaction of the waste dump <br />will be visual inspection of the maximum lift thickness and observation of wheel <br />rolling each lift by the operator. <br />15. Please it form the Division of the maximum anticipated total thickness of the waste rock pile. <br />The current height or thickness of the waste rock pile is 43 feet on the south side and <br />18 feet on the north side. The current slope is 4H:1 V. Additional dumping could <br />increase the slope to 311:1 V. There is also still adequate area around the waste rock <br />pile, particularly on the northern side to dump material. It is anticipated that the <br />thickness of the waste rock pile will not exceed 60 feet. <br />6 <br />