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Subsidence Evaluation For Exhibr7 608 South ofOivide Mining Area Page 13 pillars, and unyielding chain pillars. Any surface or neaz-surface water that might be present in this zone has a higher probability of being impacted than that occurring in any other areas above the mining panels. 5 6 Angle of Major Influence The angle of major influence, (l, (also called angle of influence of the point of evaluation) is defined by Peng (1992, p. 11) "... as the angle between the horizontal and the line connecting the inflection point and the edge of the radius of major influence." The radius of major influence (r) is therefore the horizontal distance from the vertical projection of the inflection point [o the point of maximum subsidence and the limit of subsidence (Figure 3). The angle of major influence is used for computer modeling by the influence function method. In the B Seam mining at West Elk Mine, the angle of major influence ranges (from a horizontal reference) from about 70 to 80 degrees. The angle of major influence may also be referenced to the vertical, as has been done for the break angle and angle of draw. The angle of major influence (from a vertical reference) is roughly equal to the angle of draw (Figure 3), and is therefore also predicted to range from 10 to 20 degrees. 5.7 Relation Between Dynamic and Final Subsidence Defornrations Maximum dynamic tilt (change of slope) and horizontal tensile and compressive strain are reportedly less above longwall mining panels than are the final tilt and strain values at panel boundaries. Dynamic tilt and strain decrease, relative to final tilt and strain, as the rate of face advance increases. Dynamic tilt and strain reportedly decrease with increasing speed of longw=all coal eattraction (Deng 1992, p. 20-21). Based on observations in a West Virginia coal mine: 1. Maximum dynamic tilt decreased by an average of 42 percent (from 0.0024 to 0.0014) as the mining face rate of movement increased from 10 to 40 feet per day; dynamic tilt therefore decreased by 14 percent as the face rate of movement increased by 30 feet per day. 2. Maximum dynamic tensile strain decreased by an average of 22.5 percent (from 0.0031 to 0.0024) as the mining face velocity increased from 10 to 40 feet per day; dynamic horizontal tensile strain decreased by 7.5 percent as the face increased by 30 feet per day. 3. Maximum dynamic compressive strain decreased by an average of 48 percent (0.0062 to 0.0032) as the face velocity increased from 10 to 40 feet per day; dynamic horizontal compressive strain decreased by 16 percent as the face increased by 30 feet per day. 831-032.690 Wright Water Engineers, Inc.