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respectively) of this report. <br />• regarding these observations can be found in Sections 5.4 and 5.8 (Traverses K -K and K4 -K4', <br />• <br />Spring 2011 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />The length of time between the formation and healing of cracks (crack duration) is a function of <br />their location with respect to the mine geometry, the type of material in which they form, crack <br />width 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 <br />tend to close again when the longwall face moves out of the area of mining influence. Crack <br />duration, in areas of permanent tension, such as above solid coal boundaries or rigid chain pillars, <br />is summarized (from earlier annual observation reports) as follows: <br />1. Cracks in colluvium commonly heal and revegetate in about one to three years. <br />2. Cracks in soft, friable bedrock, such as the soft sandstone above mined B -seam Longwall <br />Panel 13 (Apache Rocks mining area), are no longer visible in roughly three to six years. <br />III 3. Cracks in hard, durable bedrock, such as the sandstone outcrop at Apache Rocks, will <br />likely be visible for many decades. <br />No mining effects on rockfalls or landslides were observed in the Apache Rocks mining area. <br />However, sporadic rockfalls and landslides have been observed in the Box Canyon mining area <br />since 2006 where the steep, upper reaches of Sylvester Gulch and Box Canyon abut West <br />Flatiron. Rockfalls and landslides in this area and above mined B -seam Longwall Panels 18 to <br />22 and 25 are categorized as having a very high rockfall potential. Rockfall and landslide <br />activity were notably accelerated during the mining of Longwall Panels 19 to 22 and 25. <br />However, since 2009, the area has stabilized considerably. <br />A new rockfall /landslide area was observed on the east side of Sylvester Gulch during WWE's <br />fall 2008 field visit (this area is too small to plot on Map 3). This high rockfall hazard area <br />became active as a result of the mining of B -seam Longwall Panel 25 which underlies that <br />portion of Sylvester Gulch. Observations during WWE's spring 2011 visit found there to be <br />little, if any, change since the previous visit. <br />831- 032.793 <br />September 2011 <br />Wright Water Engineers, Inc. Page 2 <br />