2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 26 Tetra Tech geologist during a site inspection on September 20, 2007. These are located just above the Deep Creek Ditch. 10.1.1 Effects of Subsidence and Mine - induced Seismic Activity on Landslides Some of the most important information regarding mine subsidence and mine - induced seismicity was obtained from observations of active landslides on Jumbo Mountain above B -seam 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 are 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 areas. It is important to note that no earth tremors (seismic activity) were felt by Dunrud in all the annual traverses and observations made above the longwall mining areas in the Jumbo Mountain, Apache Rocks, and Box Canyon mining areas during ten years (1996 -2005 inclusive). For example, no tremors were felt during the annual traverse above B -seam 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. S In contrast to room - and - pillar 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 ten years, the major fording 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 near the landslides. Based on the above - mentioned historical evidence from the annual observations, the landslide areas located north and south of Minnesota Reservoir are not expected to be impacted by mine - induced subsidence and seismic activity when longwall panel E9 is mined. 10.2 Rockfalls Rockfalls are 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 are stopped by an obstruction or lose potential energy and stop naturally. 0 831 - 032.810 Wright Water Engineers, Inc.