Overburden movement characteristics of top-coal caving mining in multi-seam areas
The fully mechanized top-coal caving mining method was used to mine the 15 m thick No. 3-5 seam at a depth of 450 m in Tongxin mine, located in the Datong area, China. The seam was overlain with strong strata that are hard to cave and four partially mined coal seams. Excessive mine roadway closure and severe ground conditions at the longwall face and the tailgate were induced by periodic weighting and remnant coal pillars left in the four mined coal seams above. These ground conditions were studied using physical modelling and microseismic monitoring. The results indicated that hard rock strata above the coal seam did not cave readily, causing minor and major face loading events. The ground conditions deteriorated when mining was undertaken below the remnant pillars left in the four upper Jurassic seams. In addition, the physical model showed overlying longwall goaf collapse reaching the upper goafs and causing global movement in the strata. The data from the physical model and the underground microseismic survey presented here agreed closely with the visual observations and reported events that occurred at the underground longwall face in Tongxin coal mine. Overall, this study demonstrated the mechanisms of the overburden strata movement, longwall weighting occurrences and deteriorating ground conditions along the longwall face and the tailgate roadway when mining was undertaken under the overlapped Jurassic remnant coal pillars. Methods to minimize these bad ground conditions were suggested, using stress relief hydraulic fracturing methods and blasting, such as pre-splitting and softening of coal pillars and roof strata in key places. A design of small coal pillars < 3-5 min size were recommended for overlying seams to minimize stress concentrations in seams below.