Mining Publication: Comparisons Between Cross-Measure Boreholes and Surface Gob Holes
The Bureau of Mines conducted studies to compare the effectiveness of the cross-measure borehole system with that of surface gob holes on two successively mined retreating longwalls in the Lower Kittanning coalbed. Only the cross-measure system was in operation on panel A; on panel b, both systems were in operation. In addition, the cross-measure boreholes on panel B were drilled parallel to the face, whereas those on panel A were drilled at a 45 degrees angle with respect to the longwall axis. The studies show that the cross-measure system captured 71 pct of the methane generated by longwall mining on panel A. On panel B, the surface gob hole dominated over the cross-measure system for about 700 ft (213 m) after it was intercepted. Beyond that point, the cross- measure system dominated. About 93 pct of the methane generated on panel b was captured by the two systems. Cross-measure boreholes that were drilled parallel to the face were as effective as boreholes oriented 45 degrees with respect to the longwall axis. In addition, they were shorter and required less drilling time. Comparison between observed gas pressure differentials in boreholes and calculated values indicate that boreholes can be shortened further to 140 ft (43 m). Additional studies are needed to verify these calculations.