Ground cracks development and characteristics of strata movement under fast excavation: a case study at Bulianta coal mine, China

Field examples of fast exploitation from underground coal seams in Bulianta mines, China, show that unexpected developmental rules of settlement and cracks can occur. In situ observation and a physical model consisting of sand, plaster, mica, and calcium carbonate were jointly employed to study the movement of strata, as well as the developmental characteristics of surface cracks. The physical model was observed with a high-precision industrial photogrammetric system. The results indicate that ground cracks are caused by strata deformation, but the formation of ground cracks can, in turn, promote the deformation of strata. Moreover, by contrast with coal mining at a speed of about 2 m/day, we found that the ground does not achieve full subsidence until the advancing distance exceeds 2.2 times the mining depth under rapid excavation (approximately 12 m/day), which would cause large errors for surface settlement prediction. OCF (opening and closing fractures) above gobs are self-closing, but the duration of the closing phase is 3.6 times that of the opening phase, different from the symmetric distribution caused by mining with slower speed. The whole developmental cycle is inversely proportional to mining speed, and fast excavation can shorten development time of OCF. However, the horizontal tension deformation is the most stable factor for predicting ground cracks regardless of excavation speed. The research results can provide theoretical basis for dynamic prediction of ground subsidence and cracks development caused by underground coal mining.