Junbiao Ma

To understand the change in overburden structure after coal seam group mining, we investigated the overburden characteristics and bearing capacity of abandoned coal mines in a coal seam group. We provide a theoretical basis for the construction and utilization of a coal mining subsidence area under a complex geological environment. This paper takes the construction project of Zhongtie Huizhi Square in Zhangqiu District, Jinan City, Shandong Province as the engineering background. According to the occurrence conditions of the study area, theoretical analysis, similar simulation, numerical simulation, and engineering practice verification are used. The overburden structure characteristics of abandoned mines in a shallow-buried coal seam group were studied. The results show that the development height of the water-carrying fractured zone after the mining of the 3#, 4#, and 9# coal seams is 17 m, 19.5 m, and 27.1 m, respectively, which shows that the height of water flow in the fractured zone is proportional to the buried depth of the coal seam after coal seam mining. After the model is set aside for three months, the degree of development of the residual fracture in the goaf is analyzed, and the distribution law of residual porosity in the longwall old goaf of a shallow-buried multiple coal seam is obtained. The development rate of residual fissures on both sides of the goaf is between 20.31% and 42.31%. The residual fracture development rate in the middle is relatively small, being between 8.21% and 18.53%. We comprehensively analyzed the characteristics of overlying strata in the abandoned mine under actual stratum conditions, and compared the empirical calculation results, theoretical research, similar simulation, and numerical simulation results in the specification with the engineering practice to prove the reliability of the research.

To explore the failure mechanism of rock with holes and fissures, uniaxial compression tests of sandstone samples with combined double hole and double fissure defects were carried out using Particle Flow Code 2D (PFC2D) numerical simulation software. The failure behaviour and mechanical properties of the sandstone samples with combined double hole and double fissure defects at different angles were analysed, and the evolution results of the stress field and crack propagation were studied. The results show that with a decrease in fissure angle, the crack initiation stress, damage stress, elastic modulus and peak stress of the defective rock decrease, while the peak strain increases, and the brittleness of the rock is weakened. Rocks with combined double hole and double fissure defects at different angles lead to different failure modes, crack initiation positions and crack development directions. After uniaxial compression, both compressive stress and tensile stress concentration areas are produced in the defective rock, but the compressive stress concentration is of primary importance. The concentration area is mainly distributed around the holes and fissures and the defect connecting line, and the stress concentration area decreases with the decreasing fissure angle. This study can correctly predict the mechanical properties of rock with combined double hole and double fissure defects at different angles and provide a reference for actual rock engineering.

Gob-side entry retaining, also termed as non-pillar mining, plays an important role in saving coal resources, high production and efficiency, extending the service life of mine and improving the investment benefit. Herein, a gob-side entry retaining method involving the use of bag filling material for wall construction is proposed based on the thin seam mining characteristics. First, a gob-side entry retaining mechanical model is established, and the side support resistance of the 8101 working face is calculated. The mechanical properties of the bag material are investigated through experiments, and the construction technology of the gob-side entry retaining approach involving the use of bag filling material for wall construction is introduced. The deformation on the two sides, the roof and floor of the roadway, are simulated via numerical methods and monitored during field tests. The results show a small control range for the deformations and a good roadway retention effect, thereby proving the feasibility of the bag filling material for wall construction. This study provides a reference for the development of gob-side entry retaining mining for thin coal seams.