Laserfiche WebLink
Subsidence Evaluation for the <br /> Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 31 <br /> coal face, and 3)the recompression of the caved material behind the support system. This system <br /> therefore causes only a minimum amount of very low magnitude seismic activity (below the <br /> threshold of feeling at the ground surface), particularly where the overburden depth to the coal <br /> being mined is less than about 1,500 feet. <br /> Based on field observations during the past 25 years,the major finding is that landslide movement <br /> occurs in response to moisture and ground saturation, and is not noticeably affected by subsidence <br /> or any mine-related seismic activity caused by longwall mining beneath or near the landslides. <br /> Based on the above-mentioned historical evidence from the annual observations, the landslide <br /> areas 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. <br /> With regard to the landslide north of Minnesota Reservoir, longwall mining in the B-seam panel <br /> B 13-A did not have any observable effect on the landslide. The proposed E-seam mining in panel <br /> E12 will occur further south and with shallower overburden. As shown in Map 1, the head of the <br /> landslide will be located over the southern edge of panel E12, where subsidence is projected to be <br /> between 4 and 4.5 feet. As a result, the upper portion will be subject to subsidence with the <br /> outcome being a flattening of the slope in the upper reaches of the slide. This flattening of the <br /> upper reaches of the slide will reduce the movement potential and likely focus any movement that <br /> might occur to just the upper reaches. In addition, the overall slope of the mapped slide is less <br /> than in adjacent areas, further suggesting greater stability and a reduced potential for reactivation. <br /> Lastly,Mr.Dunrud has reviewed the conditions associated with the proposed mining activities and <br /> the mapped landslide. It is his opinion that while subsidence might cause minor reactivation of <br /> the landslide near the upper end, it is unlikely that there will be any movement that would reach <br /> or effect Minnesota Reservoir. <br /> 10.2 Rockfalls <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 high potential exists for rockfalls in the Southern Panels and Sunset Trail mining areas. <br /> Analysis of the terrain in these mining areas reveal slopes that range from 30 to 80 percent along <br /> Minnesota Creek,the Dry Fork of Minnesota Creek and its tributaries, and in local areas along the <br /> main fork of Lick Creek. Vertical displacement, tilt, and strain produced by mining may locally <br /> trigger already unstable rocks to fall during, or shortly after mining. <br /> The areas with steep slopes in the Southern Panels and Sunset Trail mining areas, which have the <br /> greater potential for rockfalls, are located either in areas with local access roads, which have only <br /> limited travel, or in areas remote from any access roads or other man-made features. Based on a <br /> review of aerial photographs and analysis of the USGS 7.5-minute quadrangles, there are areas <br /> with slopes ranging from 30 to 80 percent that contain local cliffs and ledges (small cliffs 5 to 10 <br /> 831-032.912 Wright Water Engineers, Inc. <br /> December 2020 <br />