Study on the instability mechanism of key strata structure in repeating mining of shallow close distance seams

With the successive exploration of Shendong mining area,the exploration of the 2nd main mineable coal seam had been started in some area where the 1st seam had been explored.Due to the close distance of the two seams,the specific condition of repeating mining in close distance shallow seams had formed,specifically in some particular area,the strata behaviors of the lower seam are intense,and this can be seen from the multiple cases of instant descending of support active column and supports crushing,which has severely threatened the mine safety and its efficient production.Further research on instability mechanism of key stratum structure in close distance shallow seams repeating mining were studied by using theoretical analysis,numerical simulation,physical simulation,and field test etc. Key stratum of close distance shallow seams were classified into three kinds or four forms for the first time,that is the single key stratum structure including the thick,hard single key stratum structure and composed single key stratum structure,no typical key stratum structure,and multiple key stratum structure while the upper seam had excavated;instability characteristics of single key stratum in shallow seams while the upper was been excavated and its occurring condition was analyzed.Uphill mechanical model was established,through which the influence of horizontal side force on the stability of single key stratum in shallow seams while the upper had been excavated and the main key stratum had been eroded was studied,the instability mechanism of single key stratum while the upper had been excavated and the surface was gullied was revealed,influencing factors of dynamic strata behaviors were summarized.The key stratum structure mechanical model while crossing the pillar in close distance shallow seams was established,through which the influence of movement of key stratum block of upper seam on the key stratum stability of lower seam was analyzed,the instability mechanism of single key stratum while the upper seam had been excavated and crossing the pillar was revealed as well. Based on the study,the suitable support resistance in Huojitu adit of Daliuta Mine under the condition of single key stratum while the upper had been excavated was determined.Consequently,the precaution measures of instability of key stratum in close distance shallow seams repeating mining of Daliuta Mine were proposed,which ensured the save working of face 21305 and face 21306,through the prediction of the dangerous area and location of periodic weighting as well as the monitoring of face support.