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<br />Angle of Draw <br />The draw, or limit angle (phi angle, from a vertical reference) in~ the Somerset azea ranges from <br />about 8 to 21 degrees. The angle of draw measured for F-Seam! room-and-pillar mining ranged <br />from 11.3 to 16.1 degrees and averaged 14.4 degrees. The angle of draw for B-Seam longwall <br />mining at West Elk ranges from about 15 to 17 degrees after accounting for F-Seam mining <br />influence. Based on this information, the anticipated draw angle will likely be closer to the I S to 17 <br />degree range than the upper limit of 21 degrees for longwall mining in both the Apache Rocks and <br />the Box Canyon mining azeas. In the western panel area of the Apache Rocks mining area, the azea <br />affected by mining should be determined by projecting the draw line from the lowest seam mined <br />(i.e., the B-Seam when it is mined). <br />Break AnQ[e <br />The break angle, the angle (B, from a vertical reference) of a straight line projected from the zone of <br />maximum horizontal tensile strain at the ground surface to the boundary of the mine workings, is <br />more important than the draw angle for hydrologic analyses. The break angle provides a means of <br />determining zones, in relation to underground mine workings where surface water most likely may <br />be impacted. The break angle reportedly averages 10 degrees less than the corresponding draw <br />• angle (Peng and Geng 1982). <br />The break angle ranges from -9 to 3 degrees in the current West Elk Mine subsidence monitoring <br />network. 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 (caused <br />by subsidence) is opposite to the direction to the slope of the ground surface (42 percent slope), but <br />is -9 degrees where the tilt direction is in the same direction as the slope of the ground surface (32 <br />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 the <br />break line from panel boundaries to the zone of high tensile strain); At panel boundaries with solid <br />coal, subsidence data from the West Elk Mine monitoring network shows that the break angle for <br />subcritical mining panels ranges from -9 to 3 degrees with an average expected value of about 0 <br />degrees. <br />Information from the West Elk Mine subsidence monitoring network also indicates that the zone of <br />increased horizontal tensile strain ranges from 100 to 150 feet wide above mine boundaries and <br />from 100 to 250 feet wide above the chain pillazs. This zone is located approximately above the <br />edges of the panels or slightly outside the panel boundaries and above the center of the chain pillars, <br />unless adown-slope component of movement occurs on steep slopes in addition to the differential <br />tilt component. Cracks tend to be more common and more permanent in zones above mine <br />• boundaries, barrier pillars, and unyielding chain pillazs. Any surface or neaz-surface water that <br />2.05-121 March 2005PR11 <br />.~,i~~ <br />