Laserfiche WebLink
West Elk 3Tne <br />• the mining face was located directly beneath one of the subsidence observation points. This <br />point was located approximately 1,200 feet vertically above the active mining face, and 2,800 <br />feet north of Minnesota Reservoir. <br />In contrast to room-and-pillaz mining, longwall mining is a uniform extraction procedure that <br />basically involves 1) the uniform cutting of a coal face, 2) the caving of the roof behind the <br />moving coal face, and 3) the recompression of the caved material behind the support system, <br />This system therefore causes only a minimum amount of very low magnitude seismic activity <br />(below the threshold of feeling at the ground surface), particularly where the overburden depth to <br />the coal being mined is less than about 1,500 feet. <br />Based on field observations during the past nine yeazs (1996 through 2004), the major finding is <br />that landslide movement occurs in response to moisture and ground saturation, and is not <br />noticeably affected by subsidence or any mine-related seismic activity caused by longwall <br />mining beneath or near the landslides. <br />Records of seismic events in the immediate azea of the West Elk Mine provided by the NEIC <br />indicate that since 1983, the lazgest event registered 3.60 on the Richter scale and occurred on <br />June 20, 2002. No evidence was observed that this event resulted in new or renewed movement <br />of landslides in the mine azea or damage to Monument Dam or Minnesota Reservoir. A coal <br />bounce measuring 3.3 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 area have <br />been recorded in the range of <2.0 to 3.3. None of these events appeaz to have impacted the area <br />• landslides, and in particular, 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 are 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 />Rock ails <br />Rockfalls are the free falling movement of rocks, which have become detached from cliffs or other <br />steep slopes, and move under the influence of gravity and the underlying ground surface. The <br />detached rocks roll and/or bounce downhill, depending on the slope (configuration of the ground <br />surface). Their movement continues until they are stopped by an obstruction or lose potential <br />energy and stop naturally. <br />A low to medium potential exists for rockfalls in the South of Divide mining azea. Analysis of <br />the terrain in the South of Divide mining azea reveals slopes that range from 30 to 80 percent <br />along Minnesota Creek, the Dry Fork and its tributaries, and in local areas along the main fork of <br />Lick Creek. Vertical displacement, tilt, and strain produced by mining may locally trigger <br />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 potential <br />for rockfalls, aze located either in azeas with local access roads, which have only limited travel, <br />2.05-147 Revised June 1005 PRt 0, Rev. March 2006; May 2006 PRIO <br />