Study on the Mechanical Properties and Mechanical Response of Coal Mining at 1000 m or Deeper

Abstract

As shallow coal resources are gradually depleted, resource exploitation extends from the shallow into the deep, where the mechanical properties of the coal rocks change significantly. To study the mechanical properties and mining-induced response characteristics of deep coal rocks. On a laboratory scale, laboratory tests and mining mechanics simulations were conducted on coal samples recovered from 1000 m or deeper using a rock mechanics testing system called MTS815 Flex Test GT. On the engineering scale, considering the roadway Ji-14-31050, buried in Pingdingshan Coal Mine No. 12 as the research base, four parameters—anchor bolt stress, borehole stress, roof displacement, and roadway convergence distortion—were monitored to study the mining-induced mechanical response characteristics of the coal rocks. The laboratory-scale study showed that the tensile strength and deformation of the deep coal rocks were generally small when destroyed; the tensile strength was in the range of 0.07–0.15 MPa, indicating low strength and high brittleness; the average compression strength of the coal rocks at 1000 m or deeper was 111.7 MPa, which was significantly greater than that of coal rocks at shallower depths. The axial strain and volumetric strain of the deep coal rocks were also greater than those of the shallow coal rocks, indicating significant plasticity. Under the conditions of pillarless mining, the axial deformation, lateral deformation, and volume deformation of deep coal samples all show a large deformation platform near the peak stress, corresponding to the area in which the volumetric deformation showed a trend of expansion; furthermore, the peak stress was significantly lower in this area. The study on the engineering scale showed the coal mining-affected area (approximately 70 m) along the mining direction of the Ji-14-31050 coal mining face with a depth of over 1000 m in the Pingdingshan No. 12 mine was obviously larger than that of the shallow coal seams. As the mining face advanced, the anchor bolt stress, the roof separation, and the roadway section deformation showed similar patterns of increasing variation. In an area 30-m away from the mining face, the supporting pressure peaked, and the anchoring stress, roof separation, and tunnel cross-sectional deformation all changed significantly, displaying the surging phenomenon. At the same time, the roadway sidewall deformation was significantly greater than the deformation between the roof and floor. Clearly, as the mining depth extended deeper, the mining-induced stress field became increasingly more intense, and the coal mining-affected area increased noticeably. Meanwhile, the surrounding rock deformation and roof separation increased significantly, making it more difficult to control the stability of the rocks surrounding the roadway. The results of this study can provide guidance for roadway support, engineering design and mining technology optimization when mining at 1000 m or deeper.