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Permit Revision Adequacy Comment Response November 3Q 2004 Page 19 of 24 . failure of the dam structure and abutments or landslides about the reservoir pool would identify failure threshold(s) for seismic induced threats. Analyses that seismic impacts would be below the threshold(s) would then demonstrate, with applicable factors of safety, that there would be no probable impact to the dam embankment structure, abutments, or the reservoir pool from seismic phenomenon. To validate the predictions of (no) probable impacts and to provide positive monitoring, the Division requests that a line of monuments be placed on a 100-foot spacing across the crest of the Monument Dam, and then at 500-foot centers along the north shoreline of the maximum pool elevation to Horse Gulch, thence north across the two panels north of Minnesota Reservoir in section 29. In addition, a line of monuments is recommended on 500-foot centers from the south abutment of the Monument Dam to the NW corner of panel E-9. The purpose of the most northern monuments is to validate the model used to predict subsidence as mining advances to the south toward the Minnesota Reservoir. The purpose of monitoring the panel immediately north of the Minnesota Reservoir would be to validate the prediction that ground movement did not reach the Minnesota Reservoir impoundment or the Monument Dam embankment. Monuments across the Monument Dam validate the prediction of no movement on the structure. Monuments to the south serve to validate a prediction of no ground movement reaching the Minnesota Reservoir or Monument Dam from mining in panel E-9. It is further recommended that geophones be installed to monitor at both the north and south abutment of the Monument Dam. Such monitoring would begin as soon as mining starts in the 2"d panel to the north, continue through the first, and commence again and end with mining in panel E-9. The purpose of this monitoring is to demonstrate that the seismic values used to determine stability of the Monument Dam are not exceeded as mining approached the structure. The monuments across the dam and the seismic geophones should be established before and monitored during mining of the panels in sections 33 and 34 to establish the local response. A subsidence report on subsidence phenomena associated with the northern panel should be provided to the Division before longwall extraction of the panel closer to the Minnesota Reservoir that demonstrates the accuracy of predictions. The Division further recommends that the centerline of the Monument Dam embankment be described in accordance with Rule 2.05.6(6)(c)(i)(C). MCC Response: MCC is proposing continued monitoring of Monument Dam and Reservoir. 103. The application contains good descriptions of ground movement, slides and rockfalls and some information on seismic impacts. That section (pages 2.05-100, 101) would be a good place to add information on seismic impacts. The size of seismic events, including the maximum probable event, from a subsidence within the mine could be defined. From any such epicenter, isoseismic lines could be drawn based on the lithology of the area for any panel closest to a target feature. MCC Response: New sections added.