2008-03-21_GENERAL DOCUMENTS - C1980007 (3)

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To promote more predictable subsidence, MCC may design pillars to yield and crush after mining (in order to minimize humps in the subsidence profile), and mine at a rapid uniform rate. MCC predicts subsidence cracks may form in areas that are located over or next to underground workings. (See preceding description of predicted crack depths.) Subsidence cracks could dewater streams, ponds, or wetlands; cause local cracking or downcutting in streams; damage roads, fences, and the buildings at the Cow Camp; dewater aquifers or cause cross-stratal flow of ground water or methane. MCC predicts mining may cause local temporary pooling and temporary accelerated erosion in the main channel and east fork of Sylvester Gulch, and in parts of the Deep Creek Ditch. Channel avulsion could occur in the Deep Creek Ditch. To mitigate subsidence impacts, MCC has committed to: filling surface cracks; redirecting flow into the original channels of streams or ditches; installing temporary culverts to bridge surface cracks; repairing subsidence- damaged roads, fences, and buildings; and augmenting any water supply losses, including any mining-caused losses from Minnesota Reservoir or the Deep Creek Ditch (as set forth in the Augmentation Plan contained in Exhibit 52 of the permit application). To detect subsidence impact, MCC has commited to monitoring the following items: wetland vegetative health in the South of Divide area; subsidence magnitudes of Panel El, Minnesota Creek (as set forth in Exhibit 32 of the permit application), and the Deep Creek Ditch (Section 2.05.6(6)(f)(iii)(C)(I- V); surface and ground water flow and quality (as set forth in Exhibit 71 for the South of Divide and Dry Fork lease areas, and in Section 2.05.4 of the permit application for other areas). MCC has committed to performing periodic visual inspections of subsidence impacts to structures and conducting annual traverses recently mined areas . The results of monitoring and inspections are to be submitted to the Division in periodic reports as required by the Rules and the permit application. MCC predicts rock falls or landslides may occur as a result ofmining-induced seismicity. To mitigate these impacts, MCC has committed to placing warning signs where appropriate, conducting visual inspections of possible rock fall and landslide areas, and removing any blockages of roads or drainages caused by mining-caused rockfall or landslides. 2. Section 2.05 and Map 53 of the permit application provide information on the perennial portion of Sylvester Gulch. The information was submitted in Technical Revision 25. The information includes: depth of mining, height of mining, lithologic description of overburden, and a map of the workings. Based 50