Laserfiche WebLink
Fall 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />- Additional details regarding these observations can be found in Section 5.19 (Traverse M-M') of <br />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, active rockfall/landslide area was observed on the east side of Sylvester Gulch during our <br />fall 2008 field visit (this area is too small to plot on Map 3). This active area is above Longwall <br />Panel 25 which is currently being mined. The source area for the rockfall is a series of sandstone <br />outcrops which are part of the Barren Member of Mesaverde Formation. This outcrop area has <br />been previously noted as having a high rockfall hazard potential due to the over-steepened slopes <br />831-032.790 Wright Water Engineers, Inc. Page 2 <br />December 2008