Junqi Cui

Previous studies studying hydraulic flushing technology for gas extraction, the surrounding rock is often simplified to an isobaric state, and the failure of coal is also analyzed using the Mohr–Coulomb model, which is inconsistent with the field. To solve this problem, we analyzed the strain-softening model and derived a formula for the distribution of plastic zones in the surrounding rock of boreholes, considering strain-softening. Theoretical analysis was conducted on the influence of the lateral stress coefficient and cohesion in the crushing zone on the plastic zone, and verified it with the numerical model. Furthermore, the distribution law of the plastic zone in the surrounding rock of hydraulic flushing boreholes, considering strain-softening and bidirectional non-axisymmetric stress, was explored by changing the borehole diameter. The research results show that the lateral stress coefficient affects the distribution pattern of the plastic zone, and the cohesion of the crushing zone affects the size of the plastic zone. As the diameter of the borehole increases, the difference between the plastic zone radius calculated by the strain-softening model and the Mohr–Coulomb model gradually increases, indicating that the larger the borehole diameter, the greater the error in the plastic zone obtained by the Mohr–Coulomb model. This indicates that the strain-softening of the surrounding rock should be considered when studying the plastic zone of the surrounding rock in a hydraulic flushing borehole under non-axisymmetric conditions. The relevant research results provide a basis for the subsequent use of numerical simulation methods to study the permeability and gas flow law of the surrounding rock in hydraulic flushing, considering strain softening.

The buried depth of coal seams in China gradually increases as shallow resources decrease. The purpose of this article is to reveal the effect of buried depth on gas extraction boreholes. Firstly, we analyzed the shortcomings of the Mohr–Coulomb (M-C) constitutive model for simulating excavation problems and introduced the strain-softening (S-S) model and its advantages. Subsequently, we constructed the gas extraction models with different buried depths based on the S-S model and combined them with the evolution equations for permeability and the equations for gas migration. Then, we studied the difference between the M-C and S-S models using numerical simulations. We found that the influence of the S-S phenomenon in the surrounding rock of boreholes is more significant as the buried depth increases—that is, the strain-softening has a significant buried depth effect. When the buried depths were 300, 500, and 700 m, the maximum ratios of permeability to the initial permeability obtained from the Mohr–Coulomb model were 1.37, 6.88, and 97.56, respectively; the maximum ratios of permeability to the initial permeability obtained from the strain-softening model were 2.06, 291.23, and 3629.66. The differences in the increase zone of permeability from the two models also increased with the increase in the buried depth. The distribution curves of gas pressure in the surrounding rock of boreholes obtained from the S-S model were below those of the M-C constitutive model at the same buried depth, and the zones with slowly increasing gas pressure with the increase in buried depth were more obvious in the S-S model compared with the M-C constitutive model. The differences in borehole effective extraction radius also increased gradually between the S-S and M-C models as the buried depth increased. Through the analysis of the research results, we found that S-S of the surrounding rock has a lesser influence on the gas extraction boreholes, and the S-S phenomenon in the surrounding rock of boreholes can be ignored when the buried depth is relatively shallow. It is increasingly necessary to consider the S-S phenomenon in the surrounding rock of boreholes with the gradual increase in the buried depth.

This study aimed to solve the problem ignored by previous research on hydraulic flushing gas extraction technology regarding the strain-softening of surrounding rock. Firstly, through the analysis of experimental data, previous studies have proved that the essence of strain-softening is that the internal friction angle remains unchanged while the cohesion decreases. According to the variation law of cohesion of surrounding rocks in a borehole, we theoretically analyzed the stress distribution of the surrounding rock considering strain-softening and determined the theoretical formula of the plastic zone radius. Subsequently, we established a numerical calculation model considering strain-softening using the COMSOL numerical simulation software. We simulated the influence of residual cohesion on the stress and plastic zone of surrounding rocks of the borehole. We found that with the decrease in residual cohesion, the peak stress transferred to the deep and the plastic zone radius gradually increased. The simulation results were compared with the theoretical values, and the errors were found to be within 10%, which verifies the model's accuracy. Subsequently, we studied the influence law of strain-softening on the stress and plastic zone of surrounding rocks of boreholes with different radii. We found that the larger the borehole radius, the greater the influence of strain-softening. The field measurement results show that the stress reduction zone of a hydraulic flushing borehole is about 10 times the borehole radius. The stress reduction zone calculated without considering strain-softening is 1.305 times the borehole radius, and that calculated while considering strain-softening is 6.663 times the borehole radius. Thus, we proved that strain-softening is an essential factor affecting the stress distribution of surrounding rocks of the borehole. When studying the gas extraction through a hydraulic flushing borehole, it is necessary to consider the strain-softening of surrounding rocks.

Previous studies have shown that the influence of deep dynamic pressure on the surrounding rock control of a coal roadway is one of the difficulties in mine roadway support. Based on the investigation of the headgate 11231 in a coal mine, this study analyzes the damage characteristics of coal roadway surrounding rock affected by deep dynamic pressure, expounds on the difficulties of controlling the roadway surrounding rock, and creatively proposes a cooperative control technology of external anchor–internal unloading for regulating large deformation of roadways. The vertical stress distribution and transfer law of surrounding rock with different hole-making depths, spacing, and lengths after roadway excavation were simulated and studied, and an appropriate parameter range of hole-making space in the stage without dynamic pressure influence was obtained. Considering the influence of mining dynamic pressure, the surrounding rock pressure relief effect of each optimized hole-making parameter was analyzed. In addition, the optimal hole-making parameters (the hole-making depth, spacing, and length were 8 m, 3.2 m, and 3 m, respectively) that can effectively reduce the high stress of roadway shallow surrounding rock in two stages (without and with dynamic pressure) and ensure integrity of the shallow surrounding rock were obtained. The actual field application shows that the new technology can reduce the higher rib deformation by approximately 850 mm and achieve a good surrounding rock control effect. The research and practice show that the pressure relief control for soft coal roadways with deep, violent mining and large deformation has achieved success, providing technical support for the maintenance of the same type of roadway.

Gas control in the heading face of a coal roadway is an important and difficult point in coal mining in China. On the basis of analyzing the disadvantages of high gas control cost and long drainage period in the existing mine heading face, a long-distance pre-drainage method of long-distance drilling is proposed to control the gas in the heading face so as to improve the tunneling speed. Applied to the engineering geological conditions of Changcun coal mine, the technology is studied in detail. First, a gas migration model considering permeability changing with time is established, and the model is put into the numerical simulation software to study the variation law of permeability and gas pressure under the conditions of single borehole and multi-borehole drainage. The results show that with the increase of drainage time, the permeability around the borehole increases gradually, the gas pressure decreases gradually, and the permeability at the borehole boundary increases the most, reaching 1.2 times the initial permeability. In the process of multi-borehole drainage, there will be mutual influence between boreholes, but with the increase of borehole spacing, the degree of this influence gradually decreases. Second, according to the results of numerical simulation, a reasonable gas drainage scheme is designed and applied in the field. The field application shows that the technology has a good gas drainage effect, the gas drainage concentration and flow are at a high level for a long time, the drilling cuttings quantity is always lower than the critical value, and the excavation length of roadway increases by more than 50 m per month. These results indicate that this technology is a promising method to realize the safe and rapid excavation of a mine coal roadway.

In order to solve the problem of difficult gas extraction in coal mine, a method of gas extraction from coal seam by interval hydraulic flushing is put forward. Based on the coal seam gas occurrence conditions of 7609 working face in Wuyang Coal Mine, the numerical simulation research on gas drainage by ordinary drilling and hydraulic flushing drilling was carried out by using COMSOL numerical simulation software. The results show that with the increase of hydraulic flushing coal quantity, the effective gas drainage radius also increases. The effective extraction radius of ordinary drilling is 0.5 m, and the effective extraction radius is 1.0 m, 1.2 m and 1.3 m respectively when the coal flushing quantity is 0.5t/m, 1.0t/m and 1.5t/m. As multiple boreholes are drained at the same time, the boreholes will affect each other, which will reduce the gas pressure and increase the effective drainage radius, the spacing between boreholes can be greater than twice the effective drainage radius of a single borehole when arranging boreholes. And the smaller the flushing interval, the more uniform the gas pressure reduction area. According to the numerical simulation results, the ordinary drilling and 1.0t/m interval hydraulic flushing test were carried out in the field. Through observation and analysis, the gas concentration of the interval hydraulic flushing drilling module was increased by 31.2% and the drainage purity was increased by 5.77 times compared with the ordinary drilling module. It shows that the interval hydraulic flushing drilling can effectively improve the gas drainage effect.