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', 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 movementras 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 permit revision area needs to be carefully <br />documented prior to mining. MCC will plan to take aerial photographs of the Box Canyon Lease <br />azea 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 nearly 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 distribufion of the deposit as did the highway <br />construction near the northern boundary of the Box Canyon permit revision area. <br />Hiehwav 133 Landslides in Relation to An21e olDraw Proiections <br />The northernmost longwall panel of the Box Canyon mining azeal 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 mhring area, the closest State Highway 133 landslides are located <br />approximately 600 feet north of the mining area of influence, andl therefore should not be affected <br />by longwall mining. <br />Impacts Beneath the Mined Coa[ Bed <br />Based on mapping and observations by Mr. Dunrud, in the B-Seam of the Somerset Mine, impacts <br />to the coal and rocks below the mined coal bed are expected toi be limited to about one mining <br />thickness. There is no expected mining impact to the Rollins Sandstone because its top (or <br />upper tongue) lies 20 to 50 feet below the base of the B-Seam. Furthermore, impacts in the floors of <br />the mine workings are expected to be limited to the chain pillars, Because the floors of the longwall <br />panels are loaded with caved roof rocks and overlying strata before deformation in the floor can <br />occur. <br />Floor heaving, pillar punching (the pillaz punches into the floor and roof rocks), and squeezing <br />(plastic flowage, see Dunrud 1976 for more details) are the only expected deformation in the <br />immediate mine floor, which consists of impure coal, shale, and claystone. Deformation in the <br />floors of the chain pillars is expected to occur after the longwall panel is mined and the pillars begin <br />fo crush out under the increased overburden load. <br />• <br />2.05-131 March 1005PR1 / <br />~ ~~,eh <br />