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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 24 • cracks, but no mining has been done in the area. The landslides are located in or near the northern boundary of the Box Canyon permit revision area. The southern limit of the landslides is located at least 650 feet north of the nearest planned longwall panel--well outside of a conservative angle of draw. 9.1 Impact of Highway Construcfion Based on a review of aerial photographs taken before highway construction and on communication with Bob Barrett of CDOT, two apparently dormant landslides were located east and west of the mouth of Box Canyon. Now (November 1996), however, this area is covered by extensive slides with vertical, horizontal, and rotational movements. These extensive slides began when the new highway was under construction on the south side of [he North Fork valley. The lower portions of colluvial deposits and toes of the dormant landslides on these steep slopes were removed during construction, which produced steeper, less stable slopes. The oversteepened ground soon became unstable and began to slide. This area was also subject to landslides in the Spring of 1995. Some of the slides show movement as recently as a week to a few months old (November 1996). 9.2 Importance of Baseline Landslide Data • Baseline information on landslides in the Box Canyon permit revision azea needs to be carefully documented poor to mining. Some of the landslide features (cracks and grabens) closely resemble subsidence features; however, no mining has occurred in this area. However, it is the author's opinion that coal mine subsidence could not cause landslides nearly as severe as those observed from the highway construction. Subsidence causes horizontal and vertical displacement of the ground with attendant tilt, curvature, and strain, which may increase landslide activity on slopes that are already unstable. But subsidence does not remove material from the lower parts of deposits. Subsidence processes therefore do not affect the mass distribution of the deposit as did the highway construction near the northern boundary of the Box Canyon permit revision area. 9.3 Highway 133 Landslides in Relation to Angle of Draw Projections The northernmost longwall panel of the Box Canyon mining area, as currently planned, is located about 650 to ?,700 feet south of the landslides on the south side of State Highway 133. Projecting to the surface an angle of draw of 21 degrees, which is the maximum angle measured in the Somerset-West Elk mining area, the closest State Highway 133 landslides are located approximately 600 feet north of the mining area of influence and, therefore should not be affected by longwall mining. • 831-032.181 Wright Water Engineers, Inc.