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West Elk Mine <br />• June 20, 2002. No evidence was observed that this event resulted in new or renewed movement <br />of landslides in the mine area or damage to Monument Dam or Minnesota Reservoir. A coal <br />bounce mea~~ring 33 on the Richter scale occurred in the mine area on October 10, 2004 and no <br />impacts to surface features or structures was noted. Historically, coal bounces in the azea have <br />been recorded in the range of <l.0 to 3.3. None of these events appear to have impacted the area <br />landslides, and in particulaz, Monument Dam or Minnesota Reservoir. <br />Based on the above-mentioned historical evidence from the annual observations, the landslide <br />azeas located north and south of Minnesota Reservoir aze not expected to be impacted by mine- <br />induced subsidence and seismic activity when longwall panel E9 is mined. However, in order to <br />verify predictions, based on the historical evidence, monitoring is detailed in the section "Effects <br />Of Subsidence And Mine-Induced Seismic Activity On Man-Made Structures And Renewable <br />Resources". <br />Rockfalls <br />Rockfalls are the free falling movement of rocks, which Gave become detached from cliffs or <br />other steep slopes, and move under the influence of gravity and the underlying ground <br />surface. The detached rocks roll and/or bounce downhill, depending on the slope <br />(configuration of the ground surface). Their movement continues until they are stopped by an <br />obstruction or lose potential energy and stop naturally. <br />A low to medium potential exists for rockfalls in the South of Divide mining area. Analysis <br />• of the terrain in the South of Divide mining area reveals slopes that range from 30 to 80 <br />percent along Minnesota Creek, the Dry Fork and its tributaries, and in local areas along <br />the main fork of Lick Creek. Vertical displacement, tilt, and strain produced by mining <br />may -ocally trigger already unstable rocks to fall during, or shortly after mining. <br />The areas with steep slopes in the South of Divide mining area, which have the greater <br />potential for rockfalls, are located either is areas with local access roads, which have only <br />limited travel, or are is areas remote from any access roads or other man-made features. <br />Based on a review of aerial photographs and analysis of the USGS 7.5 minute quadrangles, <br />there are seven areas with slopes ranging from 30 to 80 percent that contain local cliffs and <br />ledges (small cliffs 5 to 10 feet high). The areas listed below (listed in an east-to-west, <br />north-to-south direction) have a low to medium rockfall potential (see Map 1 of Exhibit <br />60): <br />1. Steep slopes (with an estimated rockfall potential ranging from moderate to high) <br />located north of Dry Fork and west of Minnesota Reservoir. However, no mining is <br />planned in this area, so this rockfall area will not be affected. <br />2. Two steep ridges with cliffs and ledges, located above the northern part of longwall <br />panel E9 east of the landslide area (mostly in the SE`/e, Sec 32, T 13 S, R 90 V~. There <br />are no roads or man-made structures in the area. <br />. 3. The south end of a steep ridge containing cliffs and ledges located north of the <br />confluence of Deer Creek and Dry Fork above the western edge (within the area of <br />1.05-l32 Revised Navem6er 2004 PR/0 <br />