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 42 • 16.0 OVERVIEW OF SUBSIDENCE OBSERVATIONS IN ACTIVE MINING AREAS MCC retained WWE to observe and report on the surface effects from subsidence resulting from longwall mining at the West Elk Mine. These observations have been made annually since 1996 and semi - annually since 2007. Excerpts from the WWE Spring 2010 report are as follows: "Based on field observations from 1996 to spring 2010, the effects of longwall mining in the Apache Rocks and Box Canyon mining areas have been less than were initially projected as reported in Exhibit 60 (Dunrud 1998 rev.). The maximum depth of tension cracks was projected to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum depth of cracks at Apache Rocks was estimated to be 30 feet. The maximum depth of cracks near cliffs on West Flatiron was estimated to be between 40 and 60 feet. The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining area by Richard Dunrud in Exhibit 60 was 0.3 to 1.3 percent (0.003 to 0.013). The maximum values of horizontal strain measured at Apache Rocks since 2000 (0.48 percent) was below the median horizontal tensile strain values (0.65 percent) predicted in Exhibit 60. During the spring 2010 field traverses, no subsidence tension cracks or compression bulges were observed in the surficial deposits (colluvium and local alluvium) above mined B -seam longwall panels 14 to 17 and 19 to 25 in the Apache Rocks, and Box Canyon mining areas. Previously observed cracking along the Box Canyon access road beneath Longwall Panel 18 was found to . be healed. Subsidence tension cracks were noted in the recently undermined drill pads for methane drainage wells (MDWs) E1 -19, E1 -15, E1 -11.5 and E1 -8 as well as the Horse Gulch Road above E -seam longwall panel E1. The length of time between the formation and healing of cracks (crack duration) is a function of their location with respect to the mine geometry, the type of material in which they form, crack width and depth, and annual precipitation. Healing occurs as a result of erosion, mass wasting, deposition, infilling, and revegetation. Cracks that form above moving longwall mining faces tend to close again when the longwall face moves out of the area of mining influence. Crack duration, in areas of permanent tension, such as above solid coal boundaries or rigid chain pillars, is summarized (from earlier annual observation reports) as follows: 1. Cracks in colluvium commonly heal and revegetate in about one to three years. 2. Cracks in soft, friable bedrock, such as the soft sandstone above mined B -seam longwall panel 13 (Apache Rocks mining area), are no longer visible in roughly three to six years. Cracks in hard, durable bedrock, such as the sandstone outcrop at Apache Rocks, will likely be visible for many decades. No mining effects on rockfalls or landslides were observed in the Apache Rocks mining area. However, since 2006, sporadic rockfalls and landslides have been observed in the Box Canyon mining area, where the steep upper reaches of Sylvester Gulch and the Box Canyon abut West Flatiron. Rockfalls and landslides in this area and above mined B -seam longwall panels 18 to 22 831 - 032.810 Wright Water Engineers, Inc.