2018-09-04_PERMIT FILE - C1980007A (40)

Home Browse Search

West Elk Mine • Predicted Subsidence Related Phenomena and Material Damage Which Would Occur as a Result of Subsidence - Z05.6 (6)(b)(i)(B&C) Predicted subsidence impacts for the mining area have been described in detail in the following section entitled "Subsidence Prediction" 2.05.6 (6)(e)(i). Given the magnitude of the subsidence projected in the above referenced section, the following outlines the material damage which could result as a consequence of the projected subsidence. Structures in the permit area are described in Section 2.05.6 (6)(a)(i & ii). The discussions in Section 2.05-6(6)(e)(ii)(A-C) and Section 2.05.6(6)(f)(iv)(A-D) include the "worst possible consequence" to these structures, as well as mitigation commitments. There are no buildings located in the Apache Rocks and Box Canyon mining areas and one building in the South of Divide area. One building and a water diversion flume for the Deep Creek Ditch are located within the Dry Fork mining area. Thirteen stock ponds were located in or near the Apache Rocks mining area and only one stock pond in the Box Canyon area. There are 24 stock ponds within the South of Divide mining area. The stock pond embankments are not expected to be impacted, however, the ponds will be monitored and any subsidence impacts mitigated by MCC per the USFS agreement letter in Exhibit 19C. The most significant surface impacts are expected to occur along the precipitous slopes and cliffs immediately north of the Minnesota Reservoir and in those areas within the influence of longwall mining where the overburden thickness is less than 400 feet. The areas with overburden less than 400 feet include approximately 30 acres along the western reaches of Longwall Panels E2 -E5. Accounting for this small area, the minimum overburden thickness above mining in the South of • Divide permit revision area is 375 feet. In all of these areas, the most severe hydrologic scenarios are as follows: As discussed in Section 2.05.6, Maximum Depth of Surface Cracks, development of cracks as much as 100 feet deep above the chain and barrier pillars could divert intermittent surface and/or spring flow into the more impermeable rocks in the overburden. The probability of such surface cracks occurring is very small. For example, based upon MCC mining of longwall panels to date, only a few surface cracks have been observed that are considered to be solely related to B Seam mining. As discussed later, there are many "healing" and "sealing" mechanisms that act to close surface cracks. In the event that a surface crack opens and stays open, surface and spring flows that encounter relatively permeable zones in the overburden will move downgradient and likely reemerge as springs with subsequent discharge into the Dry Fork. WWE and Mr. Dunrud have determined that there is virtually no potential for a surface crack in the permit area to be deep enough to connect with a mine fracture zone. In the extremely unlikely scenario in which this occurs, however, the implications would be minor. If this scenario were to happen in the Dry Fork basin, surface and/or spring flows could be discharged into the mine workings. Waters collected within the mine workings would be treated, if necessary, to comply with the National Pollution Discharge Elimination System (NPDES) permit requirements and pumped through a drill hole back into the Dry Fork basin. Losses within the mine would be minor — ie: less than 5 percent of total inflows. The magnitude of replacement water provided by MCC in the Dry Fork/Minnesota Creek basin is orders of magnitude more than will be required, based upon the subsidence evaluation conducted by WWE with Mr. Dunrud. 2.05-117 Rev. 06/05- PRIO, 03/06- PRIG, O5106- PRIO, 11/060- TRI07, 04/07- TRI08, 09/07- PRI2, 02/08- PR72, 11/10. TR124 Z%14%lr