Key strata movement monitoring during underground coal mining and its 5 stage movement law inversion：A case study in Hongqinghe Mine

The law of strata movement caused by coal mining has always been a difficult “black box” problem in mining engineering research area. To reveal its internal mechanism and evolution process is the theoretical basis for scientifically solving many mining engineering problems. Based on the key stratum (KS) theory of strata control, and using the ground drilling in situ monitoring method, the actual measurement and inversion study of the internal movement law of the overlying strata are carried out under the conditions of large depth and extra large mining height in Hongqinghe coal mine. The results show that the process of overburden movement from bottom to top presents five typical stages, including the advanced compression deformation stage of the overburden, the upward breaking movement stage by layer, the rapid settlement stage of the overburden as a whole, the downward pressure of the upper strata and the overall stable settlement stage of the overburden. Based on the five stage strata movement characteristics, it is considered that the boundary of the overlying strata advanced movement is similar to exponential curve, the leading influence range is 340 m on the surface and 99 m on the coal seam. The angle that the KS break upwards by layer is about 83°, and the KSs below the buried depth of 320 m break advance to the face, and the above break behind the face. Meanwhile, the specific distribution of the “transverse three zones” of the overlying strata is divided. Along the strike profile, the area between the “similar exponential” strata movement boundary line and the KS fracture line is the advanced compression zone (or named coal wall support zone), corresponding to the strata movement stage 1. The area after the break line of the KS until 410 m on the surface and 190 m on the coal seam behind the face is the separation zone, which corresponds to the strata movement stage 2 to stage 4. The goaf at the rear is the re compaction zone, corresponding to stage 5 of strata movement. The movement of the KS exhibits significant control over the surface subsidence. As it moves upwardly by failure layer by layer, the activity of the surface subsidence continues to rise. When the primary key stratum (PKS) ends its forward rotation and begins to reverse, the surface subsidence begins to enter a peak state, and the continuation of this peak corresponds to the rapid subsidence of the overlying strata as a whole. When the upper strata gradually compacts the lower strata, the activity of surface subsidence begins to decrease. Until the movement of the PKS is stable, the active period of surface subsidence ends, beginning to enter a period of decline. This indicates that the final form of the surface subsidence basin is directly controlled by the movement of the PKS.