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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />July 12-15, 2004 <br />4.4.4 Analysis of the Effects of Longwall Mining in Areas with a High to Very <br />High Rockfall Potential <br />In general, the effects of longwall mining beneath the cliffs and over-steepened slopes of the <br />Apache Rocks and Box Canyon mining areas-areas with a moderate to high rockfall potential- <br />have been small to negligible. For example, no new rockfalls (compared to last year) were <br />observed in the cliffs and over-steepened slopes in the West Flatiron area above mined longwall <br />panels 14 and 15. In general, the mckfall potential is high in the cliff areas around West Flatiron. <br />However, rockfalls were observed in the landslide and rockfall azea-an azea with a very high <br />rockfall potential-located above mined longwall pane] 22. These fresh-looking rockfall areas <br />were not observed last yeaz prior to mining longwall panel 22 (Map 2, Figures 22 and 23). The <br />azea is located in the upper part of the first east-trending draw of Sylvester Gulch. The dynamic <br />tilt and strain caused by the moving longwall face apparently was enough to cause rocks near the <br />cliff-forming head scarp (some of which were likely already loose from gravity-induced, lateral <br />stresses) to topple, fall, and roll downslope. Future mining of longwall panels 21 and 21 may <br />accelerate additional rockfalls in this steep cliff area because the area will be within the azea of <br />mining influence of these two panels. <br />Small-magnitude earth tremors may be generated by stress-strain readjustments in the coal and <br />rocks near the longwall mine level, particularly where the overburden depth is greater than about <br />1,500 feet (the overburden depth beneath West Flatiron is as much as 2,250 feet). This seismic <br />activity is believed to have only a minor effect on the rockfall area above mined longwall panel <br />22, compared to the effects of tilt and strain produced during subsidence. This appraisal is based <br />on the fact that no rockfalls were observed in the over-steepened slopes and cliffs in the Hubbazd <br />Creek area when the magnitude 5.2 Rulison nuclear shot was detonated in 1969. The vertical <br />and horizontal ground motion caused by the shot was very noticeable to the author in Hubbard <br />Creek. In the nine years of annual observations in the West Elk mining azea, no ground motion <br />has been detected by the author above any of the longwall mine workings. <br />837-032.640 Wright Water Engineers, Inc. Page 44 <br />September 2004 <br />