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West Elk Mine <br />compression. Crack depth may be much less, but not more than these maximum values. An <br />unpublished study for the U.S. Bureau of Mines (Engineers International) indicated that surface <br />crack depth rarely is greater than about 50 feet. Also, cracks will likely terminate at boundaries <br />of shales and claystones. <br />Based on this analysis in the current mine area, it is estimated that the maximum depth of the <br />tension cracks would range from 50 feet to as much as 100 feet in the Apache Rocks and the Box <br />Canyon mining areas. Also, because of reduced or lacking lateral constraint, cracks may locally <br />range from 75 feet to perhaps as much as 150 feet deep along the precipitous doges and cliffs on <br />the flanks of West Flatiron and north of Dry Fork. Cracks may be as much as 200 feet deep <br />when both the E and B-Seams are mined. <br />Very little evidence of cracking above the longwall panels due to the downwarping process was <br />observed by Mr. Duttrud. One northeast-trending crack along a bedrock joint was observed <br />above the east chain pillars of 1NW longwall panel. Mine personnel have observed that this <br />crack has almost completely closed. Three minor subsidence cracks were observed above 8NW <br />longwall panel. These cracks were only a few inches wide and occurred over very shallow <br />overburden. Field observations by MCC personnel indicate that these cracks aze healing quickly: <br />Two of the cracks were observed over internal portions of the panel; these were likely the result <br />of dynamic subsidence. <br />Cracks were also observed south of Lone Pine Gulch and north of the Mautz cabin. After <br />extensive analysis by former Colorado State Geologist, John Rold, and Mr. Dunrud, it was <br />concluded that the cracks were the result of both landslide movement and mining activities in the <br />F and B-Seams. <br />Outing field investigations by Mr. Dutuud and WWE personnel, it became evident that longwall <br />mining has caused less surface cracking than did room-and-pillaz mining in the nearby Somerset <br />area. <br />Cracks in the overburden more than about 20 coal-extraction thicknesses above the longwall <br />mining panels should extend no further downward from the ground surface than the depth to the <br />neutral surface of the upper downwarped surface (about 80 to 100 feet in the current mine <br />monitoring area and perhaps 50 feet to as much as 200 feet in the West Flatiron area and to <br />perhaps as much as 200 feet in the western panels of the Apache Rocks mining area, where both <br />the E and B-Seams are to be mined). <br />Cracks that occur within the panel area also tend to close, once mining faces move out of the <br />surface area of influence (DeGraff and Romesburg 1981). Any local bed separations during <br />active subsidence • between rocks of different strengths will likely close once equilibrium <br />conditions occur. However, any cracks present above rigid chain pillars or mine boundaries may <br />remain open where permanent tensile stresses retrain after mining is completed due to the. convex <br />curvature of the subsidence profile. <br /> <br />2.05-120 RevisedJm+uary 1498 PR08 <br />