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West Elk Mine <br />• and tension cracks occur between the scarps. The tension cracks could easily be confused with <br />subsidence cracks, but no mining has been done in the area The landslides aze located in or neaz <br />the northern boundary of the Box Canyon pemut revision azea The southern limit of the landslides <br />is located at least 650 feet north of the nearest planned longwall panel-well outside of the <br />conservative angle of draw. <br />Impad ofHighway Construction <br />Based on a review of aerial photographs taken before highway constmction and on communication <br />with Bob Barrett of CDOT, two appazently dormant landslides were located east and west of the <br />mouth of Box Canyon. In November 1996, however this area is covered by extensive slides with <br />vertical, horizontal, and rotational movements. These extensive slides began when the new <br />highway was under construction on the south side of the North Fork valley. The lower portions of <br />colluvial deposits and toes of the dormant landslides on these steep slopes were removed during <br />construction, which produced steeper, less stable slopes. The oversteepened ground soon became <br />unstable and began to slide. Some of the slides show movement as recently as a week to a few <br />months old as of November 1996. <br />Importance of Baseline Landslide Data <br />Baseline information on landslides in the Box Canyon pemut revision azea needs to be carefully <br />documented prior to mining. MCC will plan to take aerial photographs of the Box Canyon Lease <br />area and field verify any landslides seen in the photograph, prior to mining. Some of the landslide <br />features (cracks and grabens) closely resemble subsidence features. However, it is Mr. Dunrud's <br />opinion that coal mine subsidence could not cause landslides neazly as severe as those observed <br />from the highway construction. Subsidence causes horizontal and vertical displacement of the <br />ground with attendant tilt, curvature, and strain, which may increase landslide activity on slopes that <br />are already unstable. But subsidence does not remove material from the lower parts of deposits. <br />Subsidence processes therefore do not affect the mass distribution of the deposit as did the highway <br />construction neaz the northern boundary of the Box Canyon permit revision area. <br />Highway 133 Landslides in Relation to Angle of Draw Protections <br />The northernmost longwall panel of the Box Canyon mining azea, as currently planned, is located <br />about 650 to 2,700 feet south of the landslides on the south side of State Highway 133. Projecting <br />to the surface an angle of draw of 21 degrees, which is the maximum angle measured in the <br />Somerset-West Elk mining azea, the closest State Highway 133 landslides aze located <br />approximately 600 feet north of the mining area of influence, and therefore should not be affected <br />by longwall mining. <br />South of Divide Mining Area -An extensive landslide area is located above the northern part <br />of longwall panel E9 in the South of Divide mining area (N :, Sec 32, T 13 S, R 90 V~ <br />(Dunrud, 1989). Overburden depth to the E Seam in that area varies from 500 to 550 feet. <br />The landslide surface contains cracks, bulges, and depressions. Movement likely has <br />occurred during the last decade or so, but began many centuries ago. <br />2.05-130 Revised November 1004 PRIO <br />