This study investigates the mechanical response mechanism and temporary support method of surrounding rock in coal mines under varying unsupported roof distances, with 9103 transportation roadway of Wangzhuang coal mine taken as a case study. We established a mechanical model of shallow surrounding rock in the unsupported roof area of coal mines by drawing on the thin slabs and universal falling arch theories, where an expression of the maximum unsupported roof distance was derived. Numerical simulation was conducted using FLAC 3D software to simulate the spatiotemporal evolution of deformation, stress, and plastic zone of surrounding rock in roadway with varying top spacing. Results show that: identified roadway width, the tensile strength of the roof, the load on the roof, and the thickness of the shallow separated rock mass as main controlling factors affecting the empty roof distance. As the unsupported roof distance increases from 0.5 m to 1.0 m, 1.5 m, and 2.0 m, the increments in roof subsidence are 4.82 mm, 3.67 mm, and 8.31 mm, respectively, showing an initial steady subsidence followed by an accelerated trend. The minimum vertical stress at the center of the cross-section along the excavation direction decreases from 7.85 MPa to 1.24 MPa. When the unsupported roof distance increases to 2.5 m and 3.0 m, internal damage in the surrounding rock intensifies, leading to surface deformation instability. The critical value for a reasonable unsupported roof distance is determined to be 2.0 m. To address the limitations of the temporary support device of the coal roadway, we proposed a new type of machine-mounted temporary support device, and its sufficient supporting capacity is verified using Ansys software. The 9103 transportation roadway adopts this support device, the excavation speed is increased by 33.3 % by increasing the cyclic feed of the roadheader and through the parallel operation of knocking on the top and temporary support.

The compaction and re-crushing characteristics of broken rock samples (BRS) in the goaf caving zone directly influence the seepage characteristics in the goaf, mining pressure on the longwall face, and surface subsidence. To study the compaction and re-crushing characteristics of BRS with different particle sizes, the experiment of BRS compaction considering particle size was carried out with the help of acoustic emission (AE) monitoring system. The experimental results indicate that BRS with larger particle sizes release more energy and have a higher number of re-crushing when subjected to the same loading stress. But the smaller the corresponding stress under the same strain conditions of the larger particle sizes. On the basis, a stress-strain model for BRS compaction considering particle size was developed. The cumulative count and energy of AE from BRS continue to increase during the broken stage. The larger the particle size, the more AE events occur. Re-crushing of BRS is mainly characterized by tensile failure. As stress increases, the proportion of shear failure continues to decrease. However, larger particle sizes show a higher proportion of shear failure. The b-value of AE indicates that the BRS undergoes a process of stable crack development during re-crushing. The acoustic emission localization experiment shows that the re-crushing of particles occurs as layered breakage in the vertical direction and uniform breakage in the horizontal direction. Horizontally, breakage starts in the boundary area and spreads evenly to the central area. Vertically, the upper layer breaks first, followed by the middle and lower layers. The research results have certain guiding significance for the overlying rock strata and surface movement, control of mining pressure manifestation, and safety management of goaf.

Research on the dissolution effect and the water purification mechanism of fractured coal and rock mass in the goaf area of underground reservoir is the key to realizing the safe and efficient operation of underground reservoir in mine.This paper takes the underground water reservoir formed by the mining of 31409 working face in Jinjie coal mine as example, and selects the broken coal rock mass in the mining area and deionized water for the experimental study of pollutant release law.We analyse the law of dissolution action of broken coal rock mass in the mining area, and explore the mechanism of the influence broken coal rock mass exerts on the characteristics of the water body.We discover the law of pollutant release of broken coal rock mass under different temperature and weathering degrees, and analyse precipitation and dissolution that occurs during the water purification process in underground water reservoir.The water-rock interaction of dissolution, the adsorption and precipitation of clay mineral surfaces and soluble organic matter in the rock body constitute the water purification characteristics of the groundwater reservoir, with dissolution, adsorption and precipitation each playing a dominant role in influencing mine water quality at different time scales.

As the main bearing structure of the reservoir in the goaf of the mine, the damage instability mechanism under the influence of multi-field coupling of water immersion directly restricts the long-term safe and stable operation of the reservoir in the goaf. In this paper, nuclear magnetic resonance (NMR)was used to study the T2 spectrum, pore throat, porosity change and nuclear magnetic image evolution of coal samples under different times of cyclic immersion under'unilateral' immersion conditions. With the increase of cyclic immersion times, the number of pores, pore throat ratio and porosity of coal samples maintained an increasing trend, which increased by 67.18%, 3.48% and 3.49% respectively. The nuclear magnetic resonance imaging further obtained the permeability and pore change law of water molecules in different times of unilateral cyclic immersion of coal samples: the water molecules in unilateral cyclic immersion of coal samples gradually flow from the immersion side to the internal of coal samples, and finally expand to the whole coal samples, further resulting in an increase in porosity. With the increase of cyclic soaking times, the uniaxial compressive strength and residual strength of coal samples decreased gradually. The average peak strength of coal samples decreased from 15.74 MPa to 11.76 MPa, 9.65 MPa and 8.41 MPa, respectively. The average uniaxial compressive strength of coal samples decreased by 46.56% compared with that of initial coal samples. The average residual strength decreased from 5.55 MPa to 3.08, 2.44 and 0 MPa, respectively. Long-term cyclic immersion has a significant softening effect on coal samples. Based on the experimental results, the evolution law of internal pores and the morphological characteristics of uniaxial compression failure of coal samples under the increasing number of unilateral cyclic immersion were analyzed, and the mechanism of water immersion damage and failure of coal samples was revealed. The experimental results provide a scientific basis for the stability control of reservoir spatial structure in goaf.