Laserfiche WebLink
Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <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, 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 <br />tend to close again when the longwall face moves out of their 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 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 <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 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 <br />east drainage of Sylvester Gulch. Rockfalls and landslides in this area located above mined <br />longwall panel 22 are categorized as having a very high rockfall potential. Rockfall and <br />landslide activity were notably accelerated during the mining of longwall pane122. <br />A new, active rockfall area was observed on West Flatiron. The area at location 3 (Map 1) is <br />located in one of the most narrow parts of West Flatiron ridge at a site where probable gravity- <br />induced cracks were observed during the spring 2007 field visit. However, this area is now <br />actively cracking and slabbing above the mined part of longwall pane121. Cumulative extension <br />of the surface cracks observed on September 26, 2007 is on the order of 3 feet. According to <br />Mountain Coal Company (MCC) personnel, cumulative extension of the cracks is three times <br />what it was one week prior to the most recent subsidence monitoring (September 26 through 28). <br />Also, the crack that cuts through the approximate center of the methane drainage wells at the site <br />831-032.780 <br />November 2007 <br />Wright Water Engineers, Inc. <br />Page 2 <br />