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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area Page 20 • d. Landslide located near the southwest corner of un-mined longwall panel E8 in the N'/: of Section 8 and the S''h of Section 5, Township 14 South, Range 90 West. 10.1.1 Effects of Subsidence and Mine-induced Seismic Activity on Landslides Some of the most important information regazding mine subsidence and mine-induced seismicity was obtained from observations of active landslides on Jumbo Mountain above longwall panels 8 and 9, which were mined during the mid 1990s. Landslide movement occurred during unusually wet periods before mining, during mining, and after mining and subsidence was complete. The landslides located north and south of Minnesota Reservoir aze similar to those on Jumbo Mountain. Both occur in surficial material (rocks, gravel, sand, silt, clay, and soil) and local outcrops of bedrock that have slumped and flowed downhill during periods of increased saturation. Cracks, bulges, and depressions or troughs, and springs were locally observed in both landslide azeas. It is important to note that no earth tremors (seismic activity) were felt by the author in all the annual traverses and observations made above the longwall mining azeas in the Jumbo Mountain, Apache Rocks, and Box Canyon mining azeas during the last 9 years (1996-2004 inclusive). For example, no tremors were felt during the annual traverse above longwall panel 13 in 1999, when the mining face was located directly beneath one of the subsidence observation points. This point was located approximately 1,200 feet vertically above the active mining face, and 2,800 • feet north of Minnesota Reservoir. In contrast to room-and-pillaz mining, longwall mining is a uniform extraction procedure that basically involves 1) the uniform cutting of a coal face, 2) the caving of the roof behind the moving coal face, and 3) the recompression of the caved material behind the support system. This system therefore causes only a minimum amount of very low magnitude seismic activity (below the threshold of feeling at the ground surface), particularly where the overburden depth to the coal being mined is less than about 1,500 feet. Based on field observations during the past nine years, the major fmding is that landslide movement occurs in response to moisture and ground saturation, and is not noticeably affected by subsidence or any mine-related seismic activity caused by longwall mining beneath or neaz the landslides. Based on the above-mentioned historical evidence from the annual observations, the landslide areas located north and south of Minnesota Reservoir aze not expected to be impacted by mine- induced subsidence and seismic activity when longwall panel E9 is mined. 10.2 Rockfalls Rockfalls aze the free falling movement of rocks, which have become detached from cliffs or other steep slopes, and move under the influence of gravity and the underlying ground surface. The detached rocks roll and/or bounce downhill, depending on the slope (configuration of the ground surface). Their movement continues until they aze stopped by an obstruction or lose • potential energy and stop naturally. 831-032.620 Wright Water Engineers, Inc.