The fracture field and permeability distribution model of comprehensive working face goaf was integrated upon the theoretical examination to investigate the fracture field distribution law of goaf and gas migration and accumulation characteristics, and this model has been applied to the mathematical model of gas migration and accumulation in goaf. The ANSYS FLUENT numerical simulation software was used to obtain the characteristics of gas migration and accumulation in goaf and its influencing factors and analyze the applicability of solving the features of gas migration and proliferation using the porosity model of layer division in goaf. The results were as follows: the porosity around the caving zone was a little big, whereas the porosity in the middle was a little small. The porosity was almost equal along the inclination and strike in a symmetrical distribution. The porosity occurred at the fracture zone with an “O” shape. As the gob layer height increased, the porosity tended to be small. The maximum value of the porosity of the goaf would shrink to the middle of the goaf with the increase of gob layer height. The gas mass fraction along the goaf inclination showed the growth characteristics of “exponential function”, the gas mass fraction along the goaf strike on the air inlet side showed the growth characteristics of “Boltzmann function”, and the gas mass fraction along the goaf strike on the air outlet roadway side manifested the growth characteristics of “linear function”. The main influencing factors were air leakage speed, negative pressure, and porosity distribution. The distribution model of porosity and permeability of different layers of gob can more accurately simulate the characteristics of gas migration and storage.

In multiple seams mining, the seam with relatively low gas content (protective seam) is often extracted prior to mining its overlying and/or underlying seams of high gas content and low permeability to minimize the risk of high gas emission and outbursts of coal and gas. A key to success with this mining sequence is to gain a detailed understanding of the movement and fracture evolution of the overlying and underlying strata after the protective seam in extracted. In Zhuji mine, the No. 11-2 seam is extracted as a protective seam with the pillarless mining method by retaining goaf-side roadways prior to its overlying No. 13-1 seam. An investigation has been undertaken in the panel 1111(1) of Zhuji mine to physically simulate the movement and fracture evolution of the overlying strata after the No. 11-2 seam is extracted. In the physical simulation, the displacement, strain, and deformation and failure process of the model for simulation were acquired with various means such as grating displacement meter, strain gauges, and digital photography. The simulation result shows that: (1) Initial caving interval of the immediate roof was 21.6 m, the first weighting interval was 23.5–37.3 m with the average interval of 33.5 m, and the periodic weighting interval of the main roof was in a range of 8.2–20.55 m and averaged at 15.2 m. (2) The maximum height of the caving zone after the extraction of No. 11-2 seam was 8.0 m, which was 4 times of the seam mining height and the internal strata of the caving zone collapsed irregularly. The mining-induced fractures developed 8–30 m above the mined No. 11-2 seam, which was 7.525 times of the seam mining height, the fracture zone was about 65° upward from the seam open-off cut toward the goaf, the height of longitudinal joint growth was 4–20 times of the mining seam height, and the height of lateral joint growth was 20–25 times of the mining seam height. (3) The “arch-in-arch” mechanical structure of the internal goaf was bounded by an expansion angle of broken strata in the lateral direction of the retained goaf-side roadway. The spatial and temporal evolution regularities of overburden’s displacement field and stress field, dynamic development process and distribution of fracture field were analyzed. Based on the simulation results, it is recommended that several goaf drainage methods, i.e. gas drainage with buried pipes in goaf, surface goaf gas drainage, and cross-measure boreholes, should be implemented to ensure the safe mining of the panel 1111(1).

With the depletion of shallow coal mine resources, the mining depth of coal mines is deepening year by year, therefore, gas explosion and other disasters occur from time to time. Gas drainage is the main measure to prevent gas disasters, and the permeability of coal is one of the main factors affecting gas drainage. In order to explore the seepage characteristics of coal under different confining pressure loading and unloading conditions and different gas pressure, the seepage test of raw coal samples was carried out by using the coal rock triaxial loading seepage test device. The results show that the permeability of coal samples decreases with the increase of confining pressure in the loading stage, and increases with the unloading of confining pressure in the unloading stage; Through calculation, it is found that the permeability loss of coal body in the loading stage decreases with the increase of loading times, and during unloading, this value also decreases with the increase of unloading times, and reaches the maximum value at the first loading and unloading. With the increase of loading and unloading cycles, the permeability loss of coal samples decreases, and the first loading and unloading is the main stage of permeability loss of coal samples. At the same time, it is found that when the confining pressure increases, the permeability loss of coal samples decreases, and the initial permeability of coal samples maintains a good linear growth relationship with the increase of gas pressure.

In this paper, the uniaxial compression tests of coal under different loading rates are carried out by using the MTS rock mechanics loading system and the DS5 acoustic emission instrument, and the mechanical parameters and damage characteristics of coal samples under five loading rates are studied. The conclusions are as follows: The uniaxial compressive strength and the elastic modulus of the sample increase with the increase in the loading rate, whereas the strain at the peak point of stress decreases with the increase in the loading rate; at a low loading rate, the AE ring count of the coal samples is widely distributed, but the maximum AE ring count value is small. At a high loading rate, the AE ring count of the coal samples decreases, whereas the maximum AE ring count value increases significantly. With the increase in the loading rate, the maximum AE ring count and cumulative AE ring count of the sample increase. According to the damage variable curves of the coal samples under different loading rates, the damage evolution process of the samples before the stress peak is roughly divided into three stages: initial damage stage, stable damage development stage and rapid damage development stage. It is found that when the loading rate is low, the deformation time of the coal sample is relatively sufficient, whereas when the loading rate is high, the micro cracks have no time to develop and expand, the sample is destroyed instantly and the destruction of the coal sample is sudden. Therefore, with the increase in the loading rate, the damage variable value at the peak stress of the coal sample generally shows an increasing trend.