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'• Based on this analysis in the current mine azea, 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 slopes and cliffs on the <br />flanks of West Flatiron and north of Dry Fork. Cracks may be as much as 200 feet deep when both <br />the E and B-Seams aze mined. <br />Very little evidence of cracking above the longwall panels due to the downwarping process was <br />observed by Mr. Dunrud. One northeast-trending crack along a bedrock joint was observed above <br />the east chain pillars of 1NW longwall panel'. Mine personnel have observed that this crack has <br />almost completely closed. Three minor subsidence cracks were iobserved above 8NW longwall <br />panel. These cracks were only a few inches wide and occurredi over very shallow overburden. <br />Field observations by MCC personnel indicate that these cracks are healing quickly. Two of the <br />cracks were observed over internal portions of the panel; these were likely the result of dynamic <br />subsidence. <br />Cracks were also observed south of Lone Pine Gulch and north of the Mautz cabin. After extensive <br />analysis by former Colorado State Geologist, John Rold, and Mr. Dunrud, it was concluded that the <br />cracks were the result of both landslide movement and mining activities in the F and B-Seams. <br />During field investigations by Mr. Dunrud and WWE personnel, fit became evident that longwall <br />• mining has caused less surface cracking than did room-and-pillari mining in the nearby Somerset <br />area. <br />Cracks in the overburden more than about 20 coal-extraction thicknesses above the longwall mining <br />panels should extend no further downward from the ground surface than the depth to the neutral <br />surface of the upper downwarped surface (about 80 to 100 feet in ~he current mine monitoring area <br />and perhaps 50 feet to as much as 200 feet in the West Flatiron azea and to perhaps as much as 200 <br />feet in the western panels of the Apache Rocks mining area, where both the E and B-Seams are to <br />be mined). <br />Cracks that occur within the panel area also tend to close, once mining faces move out of the surface <br />area of influence (DeGraff and Romesburg 1981). Any local bed sepazations during active <br />subsidence between rocks of different strengths will likely close once equilibrium conditions occur. <br />However, any cracks present above rigid chain pillars or mine boundaries may remain open where <br />permanent tensile stresses remain after mining is completed due to the convex curvature of the <br />subsidence profile. <br />Maximum horizontal tensile and compressive strain is significantly greater above rigid chain pillazs <br />and lease boundaries between two mines than it is above longwall mining faces. This is because <br />tensile strains caused by mining the two adjacent panels are additive above the common chain <br />pillars or unyielding mine boundary pillars. Cracks tend to be,wider and deeper above barrier <br />pillars or lease boundary pillars than chain pillazs because of theingreater rigidity (for example, the <br />• lazge tension crack on the north side of Lone Pine Gulch). <br />2.05-120 Mnsch 2005PR11 <br />~4.ati <br />