Hydraulic flushing gas extraction technology is gradually being applied in coal mines because it can effectively improve the gas extraction efficiency. This study aimed to explore the effect of strain-softening on permeability and gas pressure, due to the deficiency of previous studies on hydraulic flushing that did not consider the strain-softening of surrounding rocks. First, we analyzed the essence of strain-softening based on previous studies and proved the existence of strain-softening in surrounding rocks of the hydraulic flushing borehole by a field example. Subsequently, we established gas extraction models for different borehole diameters with and without considering strain-softening through the permeability evolution equation, the gas migration equation, and the strain-softening equation. We found that strain-softening significantly effects the permeability and gas pressure of the surrounding rocks of the borehole: the larger the diameter, the greater the effect on strain-softening. Compared with strain-softening not considered, the permeability, the permeability increase range, and the gas extraction standard range in surrounding rocks of hydraulic flushing boreholes will increase under the same borehole diameter when strain-softening is considered. The borehole diameter increases from 0.113 to 0.7 m, and the increases in permeability are 592.76 × 10-12, 614.25 × 10-12, 615.13 × 10-12, and 655.29 × 10-12 m2, respectively, the increases in permeability increase radius are 0.050, 0.121, 0.193, and 0.255 m, respectively, and the increase in gas extraction standard range radius under the two indicators are 0.119, 0.124, 0.190, 0.253 m and 0.052, 0.102, 0.150, 0.191 m, respectively. The gas pressure distribution in surrounding rocks of the borehole will also be changed by considering strain-softening, resulting in a slowly increasing gas pressure zone near the borehole: the larger the diameter, the greater the zone. Our research results proved that it is necessary to consider the strain-softening of surrounding rocks when studying gas extraction through hydraulic flushing boreholes. Moreover, we provide a novel explanation for the mechanism of gas extraction through hydraulic flushing boreholes based on the study's results. The finding of this study can help better understand the mechanism of enhanced gas extraction through hydraulic flushing boreholes.

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.

To reduce the filling cost of high-water backfilling material (HWBM) in mining backfill and improve the recycling utilization of the industrial waste such as the coal fly ash. The ultra-fine fly ash (UFA) was added to the HWBM as a partial replacement in this work. Therefore, a series of experiments were performed to investigate the effect of UFA on the mechanical properties of the HWBM at the different curing conditions, then the hydration mechanism of the HWBM blended with UFA was analyzed by XRD and SEM method. The result indicates that the strength of the HWBM decreased with the increasing of UFA dosages, but the addition of UFA can improve the residual strength of the initial HWBM. Additionally, when the HWBM was cured at the laboratory air condition, its carbonation process was restrained obviously as the UFA dosages were less than 15% at the ages of 28 days, which indicates the UFA can improve the weathering resistance of the HWBM with the curing ages increasing effectively. The XRD and SEM results also shows that the degree of crystallinity of the HWBM increased when UFA dosages were less than 15% effectively, while there were few obvious changes on types of hydration products. It indicates that the main affects of UFA on the performance of HWBM is filler and dilution, which reduced the contact area between hydration products of HWBM and CO2 in the air, further improved the carbonation resistance of HWBM.

This paper proposes a new montmorillonite-type multiple network composite gel for the prevention of coal spontaneous combustion. The first network is formed by the cross-linking of polyvinyl alcohol (PVA) and boric acid under alkaline conditions. The second network is formed as a result of intermolecular hydrogen-bonding interactions between polyacrylamide (PAM) and polyvinyl alcohol. Montmorillonite (MMT) is designed as the backbone material in the preparation of composite gels. The optimal ratios of the reactants of the composite gel were determined through orthogonal experiments. The experimental results showed that PVA had the greatest influence on the gelation time, whereas the PAM concentration had the strongest influence on the gel permeability. The optimal blending ratio was 4% MMT + 2.5% PVA + 1.5% PAM. The chemical performances of the composite colloids, such as inhibition rate, reactive functional groups, and kinetics, were investigated. Results showed that multiple network composite gels could effectively inhibit the coal spontaneous combustion reaction. Based on the principle of coal spontaneous combustion and the cross-linking network structure of the composite gel, the flame-retardant and fire-extinguishing mechanisms were also explored in terms of both physical and chemical inhibition pathways.

In this study, a sump in the Xingdong coal mine (buried at a depth of over 1200 m) was used to analyze the surrounding rock's stability and control technology. Under the combined influences of various complex conditions, such as the burial depth of over 1200 m, ultra-high ground stress, and location under the goaf, the sump support became extremely difficult, severely restricting the efficient production of the mine. The overall pressure-relief mechanisms and degree of the sump surrounding the rock environment under the goaf were studied, and the rationality of the sump location was verified through numerical simulations and field tests. A more effective support scheme was proposed based on the deformation characteristics and failure mechanisms of the temporary sump-surrounding rock under the supporting conditions. The combined control technology employed the lengthened strong anchor bolts (cables), full-section concrete-filled steel tubular supports, and pouring full-section reinforced concrete and full-section long-hole grouting reinforcement. The field test results showed that after adopting the new support scheme, the sump-surrounding rock tended to be stable after three months. The sump roof subsidence amount, floor heave amount, and convergence of the two sidewalls of the sump were 17.2–19.2 mm, 13.9–16.5 mm, and 23.2–27.9 mm, respectively, thus satisfying the application requirements. This study provides an essential reference for deep-mine roadway support under a complex high-ground-stress environment.

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.

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.

AbstractThis paper studies the width of narrow coal pillars, mining‐induced failure characteristics, and surrounding rock control effect of gob‐side entry driving (GED) adjacent to 2‐1208 filling working face with an approximately 900 m depth. Laboratory experiments, numerical simulations, loosening circle tests, and engineering practices are conducted. The mechanical properties of the filling body, the distribution and evolution law of the second invariant deviatoric stress (J2), and the variation in the plastic zone of the surrounding rock in GED are studied. The conditions of various coal pillar widths and the gob backfilled or not of the adjacent working face are also considered. The results of roadway driving along the filling working face in the deep coal mine are that: (1) The evolution law of J2 and the plastic zone features of the surrounding rock in GED have changed significantly. Thus, it is unreasonable to still adopt the customary theory of narrow coal pillar determination and roadway support design method of GED adjacent to nonfilling working face. (2) The plastic zone of GED has typically asymmetric distribution characteristics during driving and retreating, which is mainly concentrated at the upper corner of the virgin coal rib. And the elastic zone changes slightly by the influence of mining‐induced pressure on the working face, indicating that support in this area is easier. (3) During the stable period of roadway driving, there includes a “ring‐shaped” J2 depression zone at the upper corner of the coal pillar rib. While the peak zone distribution of J2 in the virgin coal rib of GED is approximately “crescent‐shaped,” and the peak zone is inclined to the upper corner of the virgin coal rib, which implies that this area is a crucial control region. (4) During the retreating period of panel 2‐1210, the direction of the maximum principal stress of GED gradually deflects from the direction of the gob to that perpendicular to the roof and floor, which represents that the key support area of GED has changed. Therefore, the advanced support of GED needs to be reinforced by single hydraulic props. The width of the coal pillar was determined to be 5 m, and a targeted truss anchor cable support method with bidirectional resistance function is proposed for the roof and both ribs, which realized the stability control of GED during deep filling mining.

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.