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.~ <br />Longwall subsidence and slope monitoring points were <br />installed and surveyed in the fourth quarter of 1984. The <br />location of these points and the advancement of the first <br />and second longwall panel are shown on the enclosed map <br />entitled 8519-LS. The monuments have been =surveyed on a <br />quarterly basis since longwall production began in May, <br />1985. The results from the first three surveys were <br />submitted and reviewed in January, 1986. The results from <br />the fourth and fifth surveys were submitted and reviewed in <br />August, 1986. The results from the sixth and seventh <br />surveys were submitted and reviewed in February, 1987. The <br />results of the data obtained from the last two surveys are <br />included in this report (first half 1987). <br />Results from the these reports are as follows: <br />1. June, 1985 showed no detectable subsidence took place. <br />2. Dec, 1985 surveys showed detectable subsidlence as a <br />result of the first longwall panel being advanced 2,750 <br />feet. <br />3. June, 1986 surveys showed a decrease in subsidence in <br />all rows except R as a result of the longwall advancing <br />1,250 ft in the second panel and a cantilever effect <br />over the first panel. <br />4. Dec, 1986 surveys showed that in all rows, except S and <br />V, ground elevations had decreased in the vertical <br />direction when compared to prior data. This was due to <br />the longwall being mined 2,900 ft in the second panel <br />causing the strata to bend down as the entry pillars <br />between the panels failed, thus, overriding the <br />cantilever effect. <br />Currently the longwall is in the third panel. 7'he June <br />survey shows all rows have subsided as compared t:o prior <br />data. Subsidence over the first two panels was small and <br />over the third panel up to six feet. The maximum <br />subsidence for the affected rows are shown in table 1. <br />The survey shows no increases in ground elevations in the <br />second panel as the third panel is being mined. This was <br />observed in prior surveys between the first and second <br />panels. The difference in the two surveys is due to the <br />twenty mile sandstone outcropping over the first panel end <br />remaining intact over the other panels (see figures 1). <br />This created a weight imbalance in the strategraphic layers <br />over the first and second panels, causing the ground to <br />teeter-totter over the entry pillars, thereby, raising <br />ground elevations in the first panel. As the longwall <br />retreated in the second panel, the entry pillars broke <br />