In order to study the changing rules of displacement filed and stress field of overlying strata on isolated pillar with filling mining method, a self-designed model of isolated pillar with an equivalent mining height and a monitoring system of stress is employed to study the progressive failure of overlying strata and the changing rules of induced displacement and stress, as the equivalent mining height increases. The findings from the trial tests show the following: (1) When the equivalent mining height is small, the overlying strata on the isolated coal pillar only bend and subside, but the overlying strata located on the goafs of two sides remain stable. (2) As the equivalent mining height increases, the degree of stress concentration on section coal pillar will rise and produce plastic failure in the first place near large caving goaf. The strata can subside between isolated working face and adjacent large caving goaf. (3) As the equivalent mining height increases further, new cracks in the roof of isolated working face will unite the cracks of carving goaf on two sides in horizontal direction, leading to a significant rise of the height of cracks. Three goafs will evolve into a large carving goaf, and the vertical cracks on the outer side of the carving goaf intersect with one another to form “fracture band”. The research acquires the key points for prevention in mining the isolated coal pillar with filling method and provides guidelines to implement this technique and to prevent rock burst.

Uniaxial compression tests were performed on coal specimens with five fissure angles to study the mechanical behaviors and acoustic emission (AE) characteristics of fractured coals. AE and video monitoring techniques were used to examine crack propagation in the fractured specimens. The stress–strain curves, mechanical properties, and cracking processes at different fissure angles were analysed. The AE counts and dominant frequency characteristics, during the failure processes of the specimens, were investigated. In addition, five types of AE signals were classified according to the AE spectral frequency analysis, and low-frequency–high-energy signals were used to accurately predict the brittle fracture processes of the fractured specimens. Finally, a comparison with sandstone specimens revealed the influence of primary cracks on the strength of brittle coal specimens containing preexisting fissures under uniaxial compression. The test results are helpful to elucidate the mechanical behavior and failure mechanism in underground engineering, such as hydraulic slotting in mines.

The recovery of top coal in the caving face directly impacts the efficiency of mining coal resources. The geological conditions and mining parameters are well known to be significant influences on the recovery of top coal. This study focused on the 9-301 working face, which is located in a thick coal seam with a large dip angle. The influences of the coal seam’s dip angle, mining direction, and coal caving mode and interval on the recovery were analyzed using PFC2D simulation. Field trials were also carried out. The results of the numerical calculations show that the recovery of top coal is clearly affected by the dip angle, with recovery decreasing as the dip angle is increased. Mining from the top to bottom along the dip of the coal seam is beneficial to improve recovery. The top-coal recovery using the multicycle-sequence coal caving method is higher than when using single-sequence coal caving and single-interval coal caving modes. The top-coal recovery using “one cutting and one caving” (coal caving interval of 0.8 m) was higher than that under two cuttings and one caving (coal caving interval of 1.6 m). During the field trials, the recovery of top coal under different caving intervals and modes was measured. The results show that the recovery of top coal is optimal when using one cutting and one caving with multicycle-sequence coal caving modes. The field measurements are consistent with the simulation results. The results of this study can help guide additional research for optimizing the recovery of top coal from thick coal seams with large dip angles.