2018-09-04_PERMIT FILE - C1980007A (44)

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West Elk Mine Potentiallv Unstable Slopes In an environment such as in this azea, any steep colluvial slope has the potential to become unstable if it becomes saturated and/or is cut by steep-sloped excavations. Because of the highly varying slopes and the vazying physical properties of the colluvium or bedrock in the azea, a detailed and exact prediction of the future location or behavior of these features is quite difficult if not impossible. Again, however, when the current and post-mining land uses are considered, these features provide a minimal and acceptable hazazd. Rock all Extensive rockfall research by the Colorado Geologic Survey, the Colorado Department of Transportation, the Oregon Department of Transportation, and others, provides considerable insight into the problem. Rockfall requires two conditions. First, cliffs or boulders must exist to provide a source. Second, the slope must be steep enough for the rocks to fall, bound or roll down the slope. Even a cursory evaluation of the azea or the geologic and slope maps indicates lazge areas of potential rockfall. Two conditions provide limits to this occurrence. Extensive reseazch indicates that rockfall will not be initiated on slopes less than 36 degrees (70 percent) (Rick Andrew, Colorado Department of Transportation, and formerly Colorado Geological Survey, personal communication, December 1994). Secondly, the extensive thick stands of • Gambel's oak greatly inhibit rockfall. Although one tree will not stop a boulder, each one in the oakbrush stand reduces its energy and a thick mature oakbrush stand will easily trap boulders up to three feet in diameter or lazger. Rock fall is triggered or caused by the freeze/thaw cycle in the cracks of rocks and underlying soil, erosional undercutting of shales below sandstone cliffs, wetting or erosion of the soil that is supporting individual boulders, or by vibrations from earthquakes, nearby blasting, or traffic vibrations. Determining azeas of potential hazazd can be carried out quite accurately by evaluating potential rock sources, slope steepness, and by cazefully noting field evidence of past rockfall. However, determining the exact location, severity or timing of future falls is neazly impossible. Changes in surface stress conditions along cliff faces may dislodge some additional rockfall. It would be extremely difficult to determine if future rockfall is caused by mining or an acceleration of the natural and normal causes previously identified. A third condition must be present for rockfall to become a hazazd. That factor is the presence of structures or people in the rockfall zone. Again, because of the lack of planned structures and the minimal visitation by people to the area, rockfall provides a minimal and acceptable geologic hazard. Limited areas of rockfalls aze present within the South of the Divide permit area. Twenty to forty foot cliffs formed by the sandstone of the upper Barren Member and the lower Ohio Creek Member of the Mesaverde Formation exist along the west facing slopes of Coal Creek Mesa. Rockfalls have been naturally occurring over time in this area as evidenced by boulders and debris deposits on the downslope from the cliffs. Mining induced subsidence may accelerate 2.04-03 Revised Nwem6er 2004 PRIO