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movements during that period were found to be neglic~ible with <br />a maxinnmi positive adjustment of 0.22 feet offset by <br />subsequent negative adjustments of -0.13 and -0.20 feet. <br />Station 18: Station 18 overlies the 1st North, 8 East submain development <br />advanced in January 1981. Overburden totals approximately <br />1100 feet at this site and surveys, initiated in August 1982 <br />and wntinued biannually thrwgh July 1984 detected virtually <br />no subsidence impacts. Maximum annual elevation adjustment <br />was measured at -0.08 feet. <br />Station 19: Station 19 is located adjacent to the 1st North, Et Fast, 5 <br />North, 1 West panel over approximately 1050 feet of <br />overburden. Retreat minux3 of this panel oocur-red in January <br />and February of 1986. Surveys were initiated in August 1982 <br />and continued biannually thrcxx~r July 1984. Biannua:L surveys <br />were reinitiated in September 1985 arcl continued thxux)fi <br />November 1987. Annual surveys followed in 1988 and 1989. <br />Horizontal and vertical movement trends at this site have been <br />erratic. Displacements were typically less than G.50 feet <br />annually since pillaring occurred with the maximwn positive <br />displacement measured as 0.50 feet arri the maxiimmi negative <br />displacement maac~ired as -0.28 feet. Ccanbined displ.ao~mnts <br />indicate an overall gain in elevation of 0.85 feet at this <br />site. <br />• Station 27: Station 27 overlies approximately 1600 feet of overburden in <br />an area that has not been undermined to date. The 1st North, <br />9 East, 1 North, 1 Fast panel approximately 600 feet north of <br />this site was retreat mined during February through Jtuie 1984. <br />?he 1st North, 9 Fast, 1 North, panel approximately 500 feet <br />west of this site was retreat mined during February 1985. <br />Annual surveys were conducted in 1982, 1985, 1986, 19E;8, 1989, <br />arxi 1990 with biannual surveys during 1987. Elevation <br />adjustments of -0.34 and -0.28 feet were noted in September <br />1985 and August 1986 respectively. Subsequent adjustments <br />were erratic but generally minimal with the exception of a <br />1.45 feet change measured in January of 1991. Control station <br />coordinates for this site had been redefined during each of <br />the past two years and it appears likely that survey error <br />contributes to the uncharacteristic movement subsequently <br />detected. <br />Subsidence effects at this site would not normally be <br />anticipated to be detectable at the surface given the <br />overburden depth in the vicinity. A potential explanation for <br />the movement detected may lie in the geologic naturf=_ of the <br />area. 'I2re area is found to be mapped as a Quaternary Glacial <br />Deposit consisting of unsorted, poorly consolidated detritus <br />which may be modified by mass wasting processes. `These <br />deposits typically overlie Quaternary Landslide deposits <br />• formed by downward and outward mass movement of material. It <br />is possible that the elevation adjustments detected ane the <br />result of such natural occurrences. <br />5 <br />