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West Elk Mine Roc alZ - As discussed in Section 2.04.6, Geology Description, there are exposed rock faces on steep slopes where the potential for rockfall exists within the permit area. Theoretically, mining- induced changes in stress and strain and fracturing could trigger additional rockfall from the many sandstone cliffs. Areas of rockfall potential, such as the flanks of West Flatiron within the Apache Rocks and Box Canyon permit revision azeas, will be monitored for rockfall-whether it is naturally occurring or mining induced. As with landslides, due to the lack of structures and limited human activity within the permit area, present and future land use wilt not be significantty impacted. Predicted Subsidence-Related Phenomena and Material Damage Which Would Occur as a Resutt of Subsidence - 2.05.6 (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 azea 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 "worse possible consequence" to these structures, as well as mitigation commitmenu. There are no buildings located in the Apache Rocks and Box Canyon permit revision areas. There are 13 stock ponds in or neaz the Apache Rocks permit revision area and only one stock pond in the Box Canyon permit revision azea. The stock pond embankments aze not expected to be impacted, however, the ponds will be monitored and any subsidence impacts mitigated by MCC per the USFS agreement letter in Extubit 19C. The most significant surface impacts are expected to occur along the precipitous slopes and cliffs immediately north of the Minnesota Reservoir, along the flanks of West Flatiron, and in those azeas within the influence of longwall mining where the overburden thickness is less than 400 feet. The areas with overburden less than 400 feet include reaches of Gribble Gulch and Horse Gulch. The minimum overburden thickness above muting in the Box Canyon permit revision area is 500 feet. ]n all of these azeas, the most severe hydrologic scenarios are as follows: As discussed in Section 2.05.6, Depth of Surface (Tension) Cracks, development of cracks as much as 200 feet deep above the chain and barrier pillars could divert intermittent surface and/or spring flow imo the more impermeable rocks in the overburden. The probability of such surface cracks occumng is very small. For example, based upon MCC mining of longwall panels to date, only a few surface cracks have been observed that aze 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. ' • 2.05^ 1(]7 Revised Jun. 1995 PR0611/96 RN03; RevisedMay 19997889; Revised Jan. 1998 PR08