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16.0 OVERVIEW OF 2007 SUBSIDENCE OBSERVATIONS IN ACTIVE <br />MINING AREAS <br />Mountain Coal Company retained Wright Waters Engineers, Inc. to observe and report on the <br />surface effects from subsidence from the West Elk Mine. These observations have been conducted <br />annually since 1996 (11.5 years). Excerpts from this report are as follows (Wright Water Engineers, <br />2006). <br />"Based on field observations during the past 11.5 years (1996 to spring 2007), the effects of <br />longwall mining in the Apache Rocks and Box Canyon mining areas have been less than were <br />initially projected in Exhibit 60 (Dunrud 1998 rev). The maximum depth of tension cracks was <br />projected to be 50 to 100 feet on steep slopes and 75 to 200 feet near cliffs. The maximum depth <br />of cracks at Apache rocks is estimated to be 30 feet. The maximum estimated depth of cracks near <br />cliffs (at Location 5 in this report) is 50 to 75 feet. <br />The range of maximum horizontal tensile strain values predicted for the Apache Rocks mining area <br />by the authors is 0.3 to 1.3 percent (0.003 to 0.013) in Exhibit 60. the maximum values of <br />horizontal strain measured at Apache Rocks in 2000 and april 2007 (0.47 and 0.48 percent, <br />respectively) are below the median horizontal tensile strain values (0.65 percent) predicted in <br />Exhibit 60. <br />During the spring of 2007 field traverses, no subsidence tension cracks or compression bulges <br />were observed in the alluvium above mined longwall panels 14, 15, 16, 17, 22, 23, and 24 in the <br />apache rocks and Box Canyon mining areas. During the past 8.5 years of field observations <br />(including Spring 2007), no cracks were observed in the alluvium in Sylvester Gulch above mined <br />longwall panels 14 through 16, where the overburden depth to the B-Seam ranges from 900 to <br />1,600 feet. <br />The length of time between crack formation and healing (crack duration) is a function of their <br />location with respect to the mine geometry, the type of material in which they form, on crack width <br />and depth, and annual precipitation. Healing occurs as a result of erosion, mass wasting, <br />deposition, infilling, and revegetation. Cracks that form above moving longwall mining faces tend to <br />close again when the longwall face moves out of their area of mining influence. Crack duration, in <br />areas of permanent tension - such as above solid coal boundaries or rigid chain pillars, is <br />summarized (from earlier annual observation reports) as follows: <br />1. Cracks in colluvium commonly heal and re-vegetate in about one to three years. <br />2. Cracks in soft, friable bedrock, such as the soft sandstone above mined longwall panel 13 <br />(Apache Rocks Mining area), are no longer visible in roughly three to six years. <br />3. Cracks in hard, durable bedrock, such as the sandstone outcrop at Apache Rocks, will likely <br />be visible for many decades. <br />No mining effects on rockfalls or landslides were observed in the Apache rocks mining area. <br />However, fresh-looking cracks and scarps have been recently observed, suggesting that local, <br />sporadic falls and slides continue in the rockfall/landslide area near the head scarp of the first east <br />drainage of Sylvester Gulch. Rockfalls and landslides in this area located above mined longwall <br />panel 22 are categorized as having a very high rockfall potential. Rockfall and landslide activity <br />were notably accelerated during the mining of longwall panel 22." <br />• <br />Tetra Tech - 090717IP 34