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West Elk one <br />• colluvium less than one foot to a few feet thick, no cracks were observed in colluvium more than <br />about ten feet thick. <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 B-Seam. <br />Angle ofDraw <br />The draw, or limit angle (~, from a vertical reference) in the Somerset area ranges from about 8 <br />to 21 degrees. The angle of draw measured for F Seam room-and-pillar mining at West Elk <br />Mine, which has overburden rock lithology similaz to the E Seam, ranged from 11.3 to 16.1 <br />degrees and averaged 14.4 degrees. The angle of draw for B Seam longwall mining at West Elk <br />Mine ranges from about 15 to 17 degrees after accounting for F Seam mining influence. Based <br />on this information, the anticipated draw angle will likely be closer to the 15 to 17 degree range <br />than the upper limit of 20 degrees for longwall mining in the current and South of Divide mining <br />azeas. <br />Break Angle <br />The break angle, the angle (B, from a vertical reference) of a straight line projected from the <br />zone of maximum horizontal tensile strain at the ground surface to the boundary of the mine <br />workings, is more important than the draw angle for hydrologic analyses. The break angle <br />• provides a means of determining zones, in relation to underground mine workings, where surface <br />water most likely may be impacted. The break angle generally averages 10 degrees less than the <br />corresponding draw angle (Peng and Geng 1982). <br />The break angle ranges from 9 to 3 degrees in the West Elk Mine subsidence monitoring network <br />azea. Topography appears to control the location of the zone of maximum tensile strain and <br />consequently the break angle. For example, the break angle is 3 degrees where tilt direction <br />(caused by subsidence) is opposite to the direction of the slope of the ground surface (42 percent <br />slope), but is 9 degrees where the tilt direction is in the same direction as the slope of the ground <br />surface (32 percent slope). <br />Tensile strain caused by subsidence commonly reaches a maximum value in lineaz zones above <br />mining panels. The location of these zones can be determined by the break angle (the angle of <br />the break line from panel boundaries to the zone of high tensile strain). At panel boundaries with <br />solid coal, subsidence data from the West Elk Mine monitoring network shows that the break <br />angle for subcritical mining panels ranges from 9 to 3 degrees with an average expected value of <br />about 0 degrees. <br />Information from the West Elk Mine subsidence monitoring network also indicates that the zone <br />of increased horizontal tensile strain ranges from 100 to 150 feet wide above mine boundazies <br />and from 100 to 250 feet wide above the chain pillazs. This zone, is located approximately <br />above the edges of the panels or slightly outside the panel boundaries and above the center of the <br />chain pillazs, unless adown-slope component of movement occurs on steep slopes in addition to <br />• the differential tilt component. Cracks tend to be more common and more permanent in zones <br />above mine boundaries, barrier pillazs, and unyielding chain pillars. Any surface or neaz-surface <br />1.05-/34 RevisedJ~me 2005 PRIO, Rev. March 1006; May 2006 PRlO <br />