Mechanism And Prevention Of Support Crushing Disaster While Mining Out Of The Upper Coal Pillar In Close Distance Shallow Seams

Shendong mining area, the typical occurrence region of close distance shallow seams,.has turned into the mining of the second coal seam step by step. Due to the coal pillars leftin the upper first coal seam, many support crushing accidences happened in the mining ofthe lower second seam while the working face out of the upper coal pillar in the area5m inand out of the coal pillar boundary. Violent weighing phenomenon occurred in theaccidents, such as sharp and huge shrinkage length of support plunger in1~2minutes.According to incomplete statistics,7similar cases had happened since2007, which hadcaused nearly one million yuan of economic loses and been threatening the safe andefficient production of the coal mines in the mining area. So, how to insure the safetyduring mining out of the upper coal pillar is a significant technical problem. Based on themeans of field measurement, theoretical analysis and simulation experiment, themechanism, influencing factors, occurred conditions and the preventions of the supportcrushing disaster in close distance shallow seams are deeply researched.With the analysis of the key blocks’ breaking movement above the coal pillar, themechanism of the support crushing disaster while mining out of the upper coal pillar andgoaf-side coal pillar in close distance shallow seams was revealed respectively. Whenmining out of the upper coal pillar, the breaking block of the key stratum (KS) above thepillar and its broken block in the goaf side would form an arched three hinged structure.Due to that the structure could not reach a stable bearing state all along, all the load of theoverburden strata would be applied on the supports of the working face by the relativerotary motion of the key block structure, which finally caused the support crushing. Whenmining out of the upper goaf-side coal pillar, there would be two situations on the supportcrushing affected by the decorate position of the cut-hole under the goaf-side coal pillar. Ifthe distance between the cut-hole and the coal pillar boundary was further than the firstbreaking step of the KS above the coal pillar, the KS would present periodic breaking statewhile mining out of the pillar, and the mechanism of the support crushing was the same tothat in the mining out of the upper coal pillar. If the distance was between the periodic andfirst breaking step of the KS, the KS would present the cantilevered state with longbreaking span when mining out of the pillar, and the support crushing was caused by theexcessive load from the rotary motion of the KS structure due to that the supports wereunder the swing hinge joint of the structure.According to the analysis of the influencing key factors on the key blocks’ rotary motion, the occurred conditions of the support crushing disaster while mining out of theupper pillar had been mastered. The deeper the depth of the upper pillar, the easier to leadthe instability in coal pillar boundary occurring ahead, thus which would induce thereverted turning in upper key blocks and play a significant inhibition on the occurrence ofsupport crushing disasters. On the other hand, the further the distance between the seams,the longer the breaking position of the key block lag behind the working face by theinfluencing that the overburden strata always broke upward with a certain angle. As the lagdistance met a certain condition, the load from the rotary motion of the key blocks wouldonly act on the gangue in the goaf which can also restrain the support crushing. From theseabove, the discrimination criterion and method of the support crushing occurring whilemining out of the upper pillar in specific conditions had been formed. According to thecriterion, it was concluded that the support crushing disaster would generally exist in themining condition of close distance shallow seams in Shendong mining area.The prevention measures of the support crushing disaster while mining out of theupper coal pillar in close distance shallow seams had been proposed in the paper. Base onthe mechanism of the disaster caused by the rotary motion of the key blocks, from theaspects of prompting the key blocks to spin ahead of time, preventing or destroying itsrotation, the measure of the pre-blasting in the upper pillar boundary, the filling in the goafnear pillar boundary and the pre-blasting in the key block structure above the boundary toprevent the support crushing disaster were put forward. With the prevention measuresabove, the support crushing disaster during the mining out of the upper coal pillar inBulianta coal mine and Shigetai coal mine of Shendong mining area were preventedsuccessfully, which achieved remarkable application effect.