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Fall 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• Longwall Panel E1. Additional details regarding these observations can be found in Section 5.23 <br />Traverse M-M') of this report. <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 Longwall Panel <br />13 (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 <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, fresh-looking cracks and scarps were observed during field visits since 2006 <br />suggesting that local, sporadic falls and slides continue in the rockfall/landslide area near the <br />head scarp of the first east drainage of Sylvester Gulch in the Box Canyon mining area. <br />Rockfalls and landslides in this area and above mined Longwall Panels 18 to 22 and 25 are <br />categorized as having a very high rockfall potential. Rockfall and landslide activity were notably <br />accelerated during the mining of Longwall Panels 19 to 22 and 25. <br />A new rockfall/landslide area was observed on the east side of Sylvester Gulch during our fall <br />2008 field visit (this area is too small to plot on Map 3). This high rockfall hazard area became <br />active as a result of the mining of Longwall Panel 25 which underlies that portion of Sylvester <br />Gulch. Observations during our fall 2009 visit found there to be relatively little change since our <br />• <br />831-032.791 Wright Water Engineers, Inc. Page 2 <br />April 2010