1987-12-01_PERMIT FILE - C1981017 (284)

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- 20 - longwall 101 headgate has been calculated at 3 .370, again • well within the assumed area average angle of draw. See Figure 24 . MC-3 : Located above 7 North No. 1 mine, and 4 North No. 2 mine, this point is apparently in the 7 North subsidence zone reactivated by longwall 101 mining in 1977-78 and not yet stabilized; 7 North was pillared in mid-1973 . While directly over 4 North in No. 2 mine, there is no past or present secondary extraction in the M bed anywhere near MC-3 . Calculating with reference to longwall panel 101, an angle of� 15 . 280 was determined (Figure 25) . Thus the actual angle of draw in this area can be assumed to exceed this amount. MC-5 : This point is located over 1� North No. 3 mine, pil- lared in 1970-71, and 150 feet from the edge of the 2 North pillar area mined in 1979-80 . It is 725 feet from the start- line tailgate corner of longwall 202 , No. 2 mine, where mining began in early 1983 . This, point subsided 1 .73 feet between 1981 and 7/3/84 , but has been stable since the latter date. Recorded subsidence is thus assumed to be attributable entirely to the 2 North No. 3 mine activitg. The angle from longwall panel 202 was calculated as 29 .14 , well above the apparent area average angle of draw as determined above. See • Figure 26 . From these various calculations, it can be determined that the actual angle of draw in the Coal Basin area, while assumed to be in fact somewhat variable depending on surface slope angle (topography) , depth of cover and local stratigraphic o and structural variations, is greater than a minimum of 15 . 28 and less than a maximum of 20 . 99° Given the low subsidence .ratios found, for example at points TP-4 and MC-13 (both 0. 02) at low angles (13 .020 and 14 .800 respectively) , it would seem logical to assume that an areal average would be closer to the minimum than the maximum. Based primarily on the calculations for the first two sets of points given above, 180 would appear to be a sufficiently conservative estimate, according to data obtained to date. While the data from many points are complicated by apparent reactivation of subsidence due to recent mining over or adja- cent to areas mined years ago and probably stabilized, subsid- ence ratios are uniformly much lower than the previously- assumed "worst case" average of 0. 55 . In an area within the subsidence zone of workings in both beds, assuming subsidence from both, a ratio of 0.13 has been calculated; even if lower bed subsidence is assumed to have stabilized before this point, MC-4 , was installed in 1981 , and all movement is attributed to M bed extraction, the resulting ratio is still only 0.26 , the highest value determined for any point regardless of the