Laserfiche WebLink
SPRING 2007 Subsidence and Geologic Field Observations <br />Box Canyon, antl South of Divide Mining Areas <br />duration, in areas of permanent tension-such as above solid coal boundaries or rigid chain <br />pillars, is 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 <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 rockfalUlandslide 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 panel 22. <br />2.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE <br />FEATURES IN THE WEST ELK MINE AREA <br />Four different types of cracks are observed in the West Elk Mine area: 1) subsidence cracks and <br />bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension cracks. <br />Gravity-induced tension cracks can be distinguished easily in some cases (e.g., where no mining <br />has occurred in the area). In other areas they may be difficult to distinguish, such as in areas that <br />have been mined and conditions are also favorable for construction, desiccation, or gravity- <br />induced tension cracks. A more detailed discussion of each of these crack types is provided <br />below. <br />2.1 Subsidence Cracks and Compression Features <br />Subsidence cracks are open cracks that most likely occur in areas where the ground surface has <br />undergone extension during subsidence processes. Cracks as much as 4 inches wide, for <br />' 831-032.780 Wright Water Engineers, Inc. Page 2 <br />July 2007 <br />