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In order to explore the relationship between IEERG and outburst intensity and verify the feasibility of the former in predicting coal and gas outburst, a series tests with different gases and different gas pressures were conducted on the basis of self-developed coal and gas outburst simulation system and IEERG measuring instrument. The results show that with the increase of gas pressure, the IEERG increases gradually. Under the same gas pressure, the coal has the strongest adsorption capacity for CO2, followed by CH4 and N2. When the IEERG is less than 24.40 mJ·g−1, no outburst will occur. When the IEERG is greater than 24.40 mJ·g−1, weak outburst will occur. When the IEERG is greater than 34.72 mJ·g−1, strong outburst will occur. This shows that the magnitude of IEERG is closely related to the outburst. The larger the IEERG, the greater the possibility of outburst and the greater the intensity of outburst. It is feasible to predict the risk of outburst using IEERG, and it can be quantified. [ABSTRACT FROM AUTHOR]

The stress wave generated by impact or dynamic load will produce significant reflection and transmission at the rock coal or rock interface during the propagation process. This will produce dynamic effects such as dynamic tensile, stress superposition and mutation. These dynamic effects will lead to obvious vibration at the interfaces, which is a key factor leading to dynamic damage and the failure of coal and rock mass. In the process of underground engineering excavation, the dynamic damage of a series of layered rock masses is one of the important factors causing geological disasters. Based on the two–dimensional similar material simulation experiment, the coal and rock mass combined of five layers of fine sandstone, medium sandstone, coal, coarse sandstone and mudstone was taken as the research object, and single and multi-point excitation (synchronous/step-by-step) were used to test the time–history vibration curves of rock–coal and rock–rock interfaces under impact load. It was concluded that the change of extreme value of vibration amplitude presented two stages: first increase, and then attenuation. Most of them required 2.25 cycles to reach the peak value, and the dynamic attenuation of amplitude conformed to the law of exponential. Based on Fast Fourier transform (FFT), the spectrum structures of the amplitude–frequency of interface vibration were studied, and the two predominant frequencies were 48.9~53.7 Hz and 92.4 Hz, respectively. Based on the Hilbert-Huang transform and energy equation, 5~7 vibration modes (IMF) were obtained by decomposing the time–history curves. The three modes, IMF1, IMF2, and IMF3, contained high energy and were effective vibration modes. IMF2 accounted for the highest proportion and was the main vibration mode whose predominant frequencies were concentrated in 45.6~50.2 Hz. Therefore, IMF2 played a decisive role in the whole vibration process and had an important impact on the dynamic response, damage and failure of coal and rock mass. In real conditions, the actual predominant frequencies can be converted according to the size and mechanical properties of the coal and rock mass, and the vibration response characteristics of the interfaces between coal and rock mass under impact load were preliminarily revealed. This study can provide reference for monitoring and early warning of coal and rock dynamic disasters, prevention and control of coal and gas outburst and technical development. [ABSTRACT FROM AUTHOR]

The 110 mining method is a high-efficiency entry-retaining technology without coal pillars or filling materials. At present, there is no precedent for its application in the Huaneng Group. In order to introduce this technology, it is planned to carry out experimental application research in the Yuwang NO.1 Coal Mine of the Huaneng Diandong mining area. The Yuwang NO.1 Coal Mine is a typical coal and gas outburst mine with a coal seam group. In view of the introduction of the 110 mining method under these conditions, first, the geological conditions of the Yuwang NO.1 Coal Mine in the Huaneng Diandong mining area are analyzed, the geological characteristics of the test mining face are summarized, and the practical feasibility of the 110 mining method is analyzed according to the geological characteristics of "one soft, one low, two high, and two complex". Then, according to the engineering experience, calculations, and analysis, the key parameters of roof cutting of the test mining face in the Yuwang NO.1 Coal Mine are obtained, and with the help of a numerical simulation, the roof-cutting height, the roof-cutting angle, and the blasting parameters are numerically simulated and analyzed. The roof-cutting key parameters of the test face are obtained as follows: the roof-cutting depth is 7 m, the roof-cutting angle is 15°, and the blasting method is continuous hole blasting with 500 mm spacing. After that, according to the coal and gas outburst and the occurrence conditions of coal seams in the Yuwang NO.1 Coal Mine, a variety of gob closure design schemes and gob gas drainage design schemes are put forward, and the field effect investigation scheme is given. After the tunnels and open-off cut of the test coal mine's first mining face are connected, under the guidance of the above research results, the field blasting test and the 110 mining method entry-retaining test are further carried out to verify the rationality of the design of the roof-cutting parameters and the feasibility of the gangue wall closure and gas drainage design. Furthermore, in the process of the field practice, continuous research is carried out on the stope pressure law and the adjacent layer gas drainage technology under the 110 mining method, and finally, the 110 mining method application technology system in the Diandong Mining Area is formed. [ABSTRACT FROM AUTHOR]

• Application problems of the Hodot's energy criterion were found. • The characterization methods of coal deformation energy and gas expansion energy were determined. • The influence mechanism of coal deformation energy on outburst was discussed. • A spatiotemporal evolution model of the release of outburst energy was proposed. Coal deformation energy and gas expansion energy are the main power sources for coal and gas outbursts (called outbursts for short). Although the combined effect hypothesis of the outbursts has been widely recognized, the contribution of the coal deformation energy to the outbursts is always despised compared with the gas expansion energy. In this paper, the famous energy criterion for outburst trigger proposed by Hodot was reviewed systematically. It was found that Hodot's energy criterion pays more attention to coal deformation energy and almost ignores gas expansion energy, different from the views in most literatures related to outburst energy. The characterization methods of coal deformation energy and gas expansion energy were summarized. Experimental results show that the deformation energy of tectonic coal with high outburst risk has a power function relationship with stress and the power exponent is between 1 and 2, indicating that the deformation properties of tectonic coal is between soil and elastomer. The releasing mode of the deformation energy of tectonic coal is different from that of intact coal, that is, the deformation energy of intact coal and tectonic coal mainly releases outward and dissipates inward respectively after they are damaged. The coal deformation energy was proved to have the same order of magnitude with the gas expansion energy, about 102 to 103 kJ/t. A new energy release model for whole process of outbursts was established to discuss the role of coal deformation energy in actual outbursts. It was finally determined that the pre-release of deformation energy of tectonic coal is the necessary condition for the accumulation of the free gas expansion energy and outburst trigger. [ABSTRACT FROM AUTHOR]

Aiming at the identification of coal and gas outburst risk, using the advantages of the clone selection algorithm (CSA), such as self-adaptation and robustness, and the characteristics of fast convergence of particle swarm optimization (PSO) algorithm, the complex decoding problem, and mutation process brought by CSA binary coding are used. It is difficult to control the problem. Using PSO optimization, the problem of abnormal detection and identification in coal and gas outburst monitoring is developed and studied, and a CSA coal and gas outburst risk anomaly detection and identification model based on PSO optimization variation is established. The model uses the coal and gas outburst index data as a collection of antigen-stimulated antibodies to achieve abnormal detection and identification of measured data. With the help of the measured data, the verification results show that the model can effectively detect and identify the risk of coal and gas outburst, and the identification results are consistent with the risk of coal and gas outburst in the field. It can be used as an effective risk identification model to guide coal mining work. [ABSTRACT FROM AUTHOR]

As the depth of coal mining is gradually increased in China, the intensity of related coal and gas outbursts will become ever-greater. Accurate forecasting of coal and gas outburst is the key to preventing and controlling such incidents. This study uses gray relational analysis (GRA) to identify the four main geological and environmental indicators of coal and gas outburst risk based on the characteristics of the factors influencing coal and gas outburst and their relationships with outburst strength. A Fuzzy SPA (F-SPA) model was constructed using set pair analysis and the coupled weighting method, and coal and gas outburst intensity level was predicted for 25 locations in coal mine No. 8 in the Pingdingshan mining area using the F-SPA model and applying a confidence criterion. The forecast results were compared with those derived using three other methods as well as against actual outbursts that have occurred at those locations. The results of this comparison show that the method proposed in this study has a forecast accuracy of up to 96%. The reliability of the prediction model is verified by error analysis. This case study demonstrates the reliability of the proposed method and provides a new tool for the prevention and control of coal and gas outburst hazards. [ABSTRACT FROM AUTHOR]

We present a very high-resolution bathy-morphological map of the offshore sector around the Scoglio d’Affrica islet (northern Tyrrhenian Sea, Italy). The study area covers a sector of 45 km2, between 3 and 85 m depth. Its central part, i.e. the apex of the Ridge, is characterised by a flat or gently sloping seafloor, where three mud volcanoes, and 250 pockmarks are recognised. Differently, the western and eastern Ridge flanks are steeper and characterised by 60 quasi-rectilinear escarpments and small ridges, more than 20 morphological highs, and elongated channels occasionally floored by bedforms. The seafloor shallower than 40 m is covered by Posidonia oceanica, forming compact and continuous or fragmented meadows intermingled with sandy patches. The main map represents the bathy-morphological setting of the area, which is largely affected by fluid seepage, providing insights for habitat mapping and preliminary marine geohazard assessment due to the violent gas outburst from mud volcanoes. [ABSTRACT FROM AUTHOR]

Coal and gas outbursts in underground mines are the result of dynamic rupture propagation along coal seams, starting from localized instability caused by the sudden release of gas and strain energies. Although physical simulations have facilitated quantitative studies of outbursts, and rupture processes have been investigated both experimentally and theoretically, the underlying mechanisms responsible for triggering outbursts remain poorly understood. This lack of understanding seriously limits our ability to prevent gas hazards. Here we systematically analyze the essential characteristics and energy principles of outburst experiments over the past 70 years and explore the onset of natural outbursts. The results show that although laboratory experiments and natural events are dynamically similar, their triggering mechanisms are fundamentally different. Specifically, actual outbursts depend highly on stress conditions and gas desorption; in contrast, experiments can be performed under stress-free conditions or even entirely dominated by free gas, often with low threshold pressures (< 0.7 MPa). Energy analysis indicated that the high porosity of the experimental briquette resulted in much higher free gas energy than in the actual coal seam (up to ~ 40 times), enabling the simulated outbursts to overcome mechanical barriers easily and, in turn, to be stress independent. Furthermore, our findings suggest that the triggering process of a natural outburst is unlikely to be transient. Instead, the development of fractures and the consequent enlargement of free gas energy are essential processes for initiating ruptures and generating potential precursors. Our results highlight that understanding the role of gas and stress is crucial for accurately interpreting outburst mechanisms. Highlights: The essential characteristics and energy principles of outburst experiments were systematically analyzed. Outburst experiments are fundamentally different from natural events in triggering mechanisms. Outburst simulations are essentially free gas-driven dynamics. Fracture development and increased free gas energy are necessary processes to initiate ruptures. [ABSTRACT FROM AUTHOR]

When building tunnels in karst areas, the hidden high-pressure water-bearing karst caves are prone to cause geological disasters such as water inrush and mud gushing. In order to study the disaster process of water and mud resistant rock mass with the quasi-masonry structure and the accompanying catastrophe information of water inrush in karst tunnels by the drilling and blasting method, a new method based on the DEM and fictitious joint technology is adopted to simulate the evolution process of water inrush and study the evolutionary characteristics of catastrophe information like seepage pressure and displacement under the conditions of different joint inclination angles, tunnel depths, and thickness of the water and mud resistant rock mass. The research results show that (1) the seepage pressure and displacement in the water and mud resistant rock mass decrease and increase respectively along the water and mud resistant rock mass from top to bottom in the process of water inrush. (2) The displacement and seepage pressure in the water and mud-resistant rock mass increases with the increase of the joint inclination angle. The greater the tunnel depth and thickness of the water and mud-resistant rock mass, the faster the propagation speed of the fissure zone. (3) With the increase of the water and mud-resistant rock mass thickness, the failure mode of the tunnel outburst layer structure gradually changes from the overall collapse to the partial collapse of the vault to form a relatively stable slump arch. The research results can provide some guidance for tunnel construction in the karst tunnel. [ABSTRACT FROM AUTHOR]

Pore structure and methane adsorption of coal reservoir are closely correlated to the coalbed gas occurrence and outburst. Full-scale pore structure and its fractal heterogeneity of coal samples were quantitatively characterized using low-pressure N2 gas adsorption (LP-N2GA) and high-pressure mercury intrusion porosimetry (HP-MIP). Fractal pore structure and adsorption capacities between outburst and nonoutburst coals were compared, and their geological significance to gas occurrence and outburst was discussed. The results show that pore volume (PV) is mainly contributed by macropores (>1000 nm) and mesopores (100–1000 nm), while specific surface area (SSA) is dominated by micropores (<10 nm) and transition pores (10–100 nm). On average, the PV and SSA of outburst coal samples are 4.56 times and 5.77 times those of nonoutburst coal samples, respectively, which provide sufficient place for gas adsorption and storage. The pore shape is dominated by semiclosed pores in the nonoutburst coal, whereas open pores and inkbottle pores are prevailing in the outburst coal. The pore size is widely distributed in the outburst coal, in which not only micropores are dominant, but also, transition pores and mesopores are developed to a certain extent. Based on the data from HP-MIP and LP-N2GA, pore spatial structure and surface are of fractal characteristics with fractal dimensions D m 1 (2.81–2.97) and D n (2.50–2.73) calculated by Menger model and Frenkel–Halsey–Hill (FHH) model, respectively. The pore structure in the outburst coal is more heterogeneous as its D n and D m 1 are generally larger than those of the nonoutburst coal. The maximum methane adsorption capacities ( V L : 15.34–20.86 cm3/g) of the outburst coal are larger than those of the nonoutburst coal ( V L : 9.97–13.51cm3/g). The adsorptivity of coal samples is governed by the micropores, transition pores, and D n because they are positively correlated with the SSA. The outburst coal belongs to tectonically deformed coal (TDC) characterized by weak strength, rich microporosity, complex pore structure, strong adsorption capacity, but poor pore connectivity because of inkbottle pores. Therefore, the area of TDC is at high risk for gas outburst as there is a high-pressure gas sealing zone with abundant gas enrichment but limited gas migration and extraction. [ABSTRACT FROM AUTHOR]

Gassing, outbursts of high-rate gas flows through the taphole, is a phenomenon that sometimes occurs during tapping of silicon furnaces. In addition to being an HSE challenge, this might also affect the tapping, while the warm gas might harm the equipment in the tapping area. The pressure above the metal bath, influencing the gassing, is affected by permeability, and the gas flow through the charge materials in the furnace. Pressure drop and permeability have been measured for different gas velocities through particle beds of different charge mixtures, with the raw materials and particle sizes used in industrial silicon production. The experimental results have been upscaled to conditions in industrial furnaces through modeling flow in a packed bed. The findings are compared with results from measurements of pressure drop in industrial furnaces. [ABSTRACT FROM AUTHOR]

The 2021 eruption within the Fagradalsfjall volcanic system in Iceland provided a rare opportunity to record acoustic data generated by a basaltic fissure. Eruptive activity in May 2021 was defined by a sequence of repetitive lava fountaining activity. Here we describe key observations and analysis conducted on acoustic data recorded by a four-element infrasound microphone array near the eruption site. Detailed inspection of acoustic waveforms and comparisons with seismic data and lava fountain height measurements revealed a complex eruptive sequence during each lava fountain event: acoustic tremor during peak lava fountaining was followed by a transition to Strombolian-style activity with distinct high-amplitude impulsive waveforms. Quantitative comparisons to jet noise spectra find complex turbulence acoustics during each event, with evidence of variations in the wavefield centred on peak lava fountain heights. Strombolian explosions could mostly be modelled by oscillations of bursting gas slugs at the top of the magma column, with a minor number of events exhibiting Helmholtz resonance behaviour instead. We find an increase in bubble radii between early and late May, suggesting a widening of the upper conduit during the lava fountain sequence. Finally, we propose that higher acoustic amplitudes, in addition to a wider conduit in late May, indicate higher gas flux through the conduit culminating in shorter lava fountain events. This study highlights the value of deploying acoustic sensors for providing additional constraints on eruption dynamics and source parameters during effusive fissure eruptions in Iceland and elsewhere. [ABSTRACT FROM AUTHOR]

A gas outburst risk level prediction method, based on the Whale Optimization Algorithm (WOA) Improved Extreme Learning Machine (ELM), is proposed to predict the coal and gas outburst hazard level more accurately. Based on this method, recommendations are given according to the gas outburst risk level with the help of the Case-Based Reasoning (CBR) method. Firstly, we analyze the accident reports of gas outburst accidents, select the gas outburst risk prediction index, and construct the gas outburst risk prediction index system by combining the gas outburst prevention and control process. The WOA-ELM model was used to predict the gas outburst risk level by selecting data from 150 accident reports from 2008 to 2021. Again, based on the coal and gas outburst risk level, CBR is used to match the cases and give corresponding suggestions for different levels of gas outburst risk conditions to help reduce the gas outburst risk. The results show that the WOA-ELM algorithm has better performance and faster convergence than the ELM algorithm, when compared in terms of accuracy and the error of gas outburst hazard prediction. The use of CBR to manage prediction results can be helpful for decision-makers. [ABSTRACT FROM AUTHOR]

This study optimized the laminate structure of a composite cylinder under the constraint of minimum layup thickness. Based on the progressive damage theory, a finite element model of the cylinder was established, and the NOL ring tensile test was used to verify the accuracy of the damage theory. The winding angle, the number of layers, and the helical/hoop ratio (the stacking sequence) were selected as the optimization factors, and the burst pressure value was used to evaluate the quality of the laminate structure. Then the orthogonal experiments were designed by RSM. Combined with FEA, the function model of the burst pressure of the gas cylinder and each optimization factor was established to obtain the optimal layering scheme, satisfying the minimum burst pressure. In addition, finite element analysis was used to verify the optimal scheme, demonstrating that the error of the burst pressure predicted by the quadratic model established by the response surface design was not more than 5%. This study provides a faster and more efficient optimization method for the optimization design of composite gas cylinder layers. [ABSTRACT FROM AUTHOR]

Mechanics-seepage synchronous tests on gas-bearing coal under three different stress paths were designed and implemented to evaluate how load path affected the mechanical strength and permeability of deep mining-disturbed coal. The cracks-count evolution of coal specimens during instability was observed through DEM numerical simulation. The results showed significant stress-strain and strength variations under different paths. At the time of failure, the specimen deformation and peak strength were Test 1 > Test 2 > Test 3, while the permeability was Test 3 > Test 2 > Test 1, with specimen permeability in Test 3 rising prominently. From numerical simulation, the cracks count was Test 2 > Test 3 > Test 1, with tensile cracks taking the largest proportion in Test 2 and shear cracks taking the largest proportion in Test 3. Our findings shed some light on the research and disaster prevention regarding coal and gas outburst. [ABSTRACT FROM AUTHOR]

Composite coal seam is a kind of special coal seam that is formed after complex geological structure action, including a tectonic coal sublayer. Gas can't be drained easily from this coal seam, which cause coal and gas outburst accidents (CGOA) occur frequently. A new hydraulic flushing technology was proposed in this paper to improve gas drainage. Accordingly, a new set of mechanical equipment, technology process, roadway layout system and gas extraction monitoring system are provided by drawing lessons from the Guhanshan coal mine. The stress variation and damage failure were investigated by numerical simulation on the basis of flushed-out coal and borehole geometry. The gas drainage effect, coal seam permeability and outburst risk-sensitive indexes of the special coal seam were obtained by field measurements. According to the results, hydraulic flushing can enlarge the stress unloading and plastic damaged zone around boreholes dramatically, and the coal seam permeability can be increased largely. • A new technology was proposed to enhance gas drainage in the special low-permeability composite coal seam with a tectonic coal sublayer. • The stress damage and permeability increase mechanism of hydraulic flushing were revealed by theoretical analysis and numerical modeling. • Abundant field experiments were carried to investigate and verify the gas drainage effect in the coal seam after hydraulic flushing. [ABSTRACT FROM AUTHOR]

Coal seam gas drainage is not only the effective measure for gas outbursts control, but also has the great significance for the use of gas energy and reduction of greenhouse-gas emissions. Coal permeability is the key parameter of gas drainage, and the permeability model of mining-damaged coal has the important theoretical guidance value for gas drainage in low permeability coal seams. In order to better understand the permeability model of damaged coal, the key points during the establishment of permeability model, which are the selection of coal structure, selection of boundary conditions and influencing factors, were first analyzed. Then, the expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. According to the contribution of new fractures, the permeability models of mining-damaged coal were divided into the empirical permeability models (EPM), volumetric strain permeability models (VSPM), plastic strain permeability models (PSPM) and damage variable permeability models (DVPM). After that, the quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. Finally, the prioritization of 18 indexes affecting the permeability were determined, and the potential research directions of permeability models of mining-damaged coal were pointed out. The results of this paper can provide the theoretical guidance for the improvement of permeability model of mining-damage coal in the future. • The expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. • The quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. • The prioritization of 18 indexes of factors affecting the permeability were determined. [ABSTRACT FROM AUTHOR]

• The effect of SiO 2 nanofluid with different concentrations on water adsorption capacity of coal is obtained. • The wettability-enhancing effect of SiO 2 nanofluid with different concentrations is investigated by contact angle tests. • The effect of nanofluid concentration on the wettability enhancement of coal is revealed. Water-based SiO 2 nanofluid, a wetting agent, has been proved to be feasible in improving the control effect of coal seam water injection against coal and gas outburst. However, the effect of nanofluid concentration on the control effect is not clear. Therefore, it is necessary to investigate the wettability-enhancing effect of nanofluid with different concentrations. In this paper, firstly, the effect of SiO 2 nanofluid with different concentrations on water adsorption capacity of coal was obtained. Then, the wettability-enhancing effect of SiO 2 nanofluid with different concentrations was investigated by contact angle tests. Finally, the effect of nanofluid concentration on the wettability enhancement of coal was revealed. The results showed that increasing the nanofluid concentration can enhance the water adsorption capacity and wettability of coal. In addition, the wettability-enhancing effect of nanofluid gradually got worse with time, while the stability of wettability-enhancing effect gradually got better. Furthermore, the 0.5 wt% nanofluid had the best sustainability and stability of the wettability-enhancing effect, while 1.0 wt% had the worst sustainability and stability. The research achievements could provide important basis for the selection of nanofluid concentrations in coal seam water injection. [ABSTRACT FROM AUTHOR]

In this study, triaxial compression and seepage tests were conducted on briquette and raw coal samples using a coal rock mechanics-seepage triaxial test system (TAWD-2000) to obtain the complete stress–strain curves of the two samples under certain conditions. On this basis, the different damage forms of the two coal samples and the effect of their deformation and damage on their permeability were analyzed from the perspective of fine-scale damage mechanics. Moreover, the sensitivity of permeability to external variables and the suddenness of coal and gas outbursts were discussed. The results show that the compressive strength of raw coal is 27.1 MPa and the compressive strength of briquette is 17.3 MPa, the complete stress–strain curves of the two coal samples can be divided into four stages and show a good correspondence to the permeability–axial strain curves. Since briquette and raw coal have different structural properties, they present different damage mechanisms under load, thus showing great diversity in the permeability-axial strain curve, especially in the damage stage. The deformation affects the seepage characteristics of briquette mainly in the latter two stages, while it affects raw coal throughout the test. The four stages of the complete stress–strain seepage test of raw coal can well explain the four stages of coal and gas outburst process, i.e., preparation, initiation, development, and termination. Hence, the law of coal permeability to gas variation can be utilized for the coal and gas outburst prediction and forecast. The research results are valuable for exploring the real law of gas migration in coal seams. [ABSTRACT FROM AUTHOR]

Coal–rock combination is an important structure for understanding the mechanism of coal and gas outburst, rock burst and coal roadway deformation. In the study, theoretical analysis and physical similarity simulation were adopted to investigate the influencing law and mechanism of coal thickness on the compressive strength and prepare five types of coal–rock combination specimens. Uniaxial compression tests were conducted to obtain the characteristics of the final failure state and the variation law of the compressive strength of the specimens. Results showed that the failure pattern and compressive strength of the specimens were quite different from the rock and coal samples. The following final failure state characteristics of the combination appeared as the proportion of coal thickness increased: "H"-type tensile failure, "hyperbola"-type co-shear failure and "X"-type shear failure. The values of the compressive strength and longitudinal wave velocity of the combination were between rock and coal and presented a negative exponential function with increasing coal thickness. The compressive strength of the rock body part weakened because of the interface in the coal–rock combination, whereas the coal body part strengthened. Further, a compressive strength calculation model was established for the combined body considering the dual effects of coal–rock size effect and interfacial effect. This study is helpful to explain the mechanical behavior of coal in mining engineering. [ABSTRACT FROM AUTHOR]

Gas explosions or coal and gas outbursts can cause transient destruction of combined coal–rock, and the dynamic mechanical response of combined coal–rock masses plays a key role in accident failure, but we now know little about the dynamic mechanical responses of combined coal–rock. In this article, we selected three rocks (limestone, shale, sandstone) and two coals (bituminous coal and anthracite coal) to form combined coal–rock, and analyze their dynamic mechanical properties by using the SHPB system. We find that the dynamic compressive strength and elastic modulus of combined rock–coal are lower than the average value of single rock and coal, while the ultimate strain and strain rate of combined coal–rock are higher than the average values of single rock and coal. Compressive strength and elastic modulus of the combined body increase gradually with increasing confining pressure, and the strain decreases accordingly. The dynamic stress–strain curve demonstrates an obvious double-peak at high strain rate (85.55 s−1 and above in the present work), while there is no obvious double-peak of the curve at low strain rate. Dynamic compressive strength of combined coal–rock body increases significantly with increasing confining pressure at low strain rate, but it increases more smoothly at higher strain rate. The elastic modulus also increases with increasing confining pressure, and it seems to be stable as confining pressure increases at low strain rate. The ultimate strain decreases gradually with increasing confining pressure but more gently compared with that at low strain rate. Besides, longitudinal fractures of combined coal–rock bodies include penetrating fractures, partially penetrating fractures, and interrupted fractures stopped at the coal–rock interface. The dynamic mechanical response of combined coal–rock is of guiding significance for maintaining the stability of the roadway and formulating the support measures for the roadway. [ABSTRACT FROM AUTHOR]

Water inrush disaster is one of the major disasters affecting the production safety of coal mines following roof caving, fire, gas outburst, and dust explosion disasters. It is urgent to reveal the water inrush mechanism and take effective measures to prevent the disasters. More than 80% of water inrush accidents occur around geological structural zones such as faults and karst collapse columns (KCCs). The water inrush events from KCCs caused huge economic losses and heavy casualties, and the water inrush process often shows certain hysteresis characteristics. Taking the water inrush disaster from a KCC during roadway excavation in PanEr Coal Mine of Huainan Mining Area as the case study, the delayed inrush mechanism of KCC was analyzed from the aspects of floor failure, KCC activation, seepage transition, and water inrush development characteristics. The results show that the rock mechanical properties and the excavation depth are the main factors affecting the floor failure characteristics. The seepage transformation from pore flow to fracture flow and pipeline flow, with the change in internal composition structure, is the internal mechanism of the delayed water inrush from KCC. The research is of great significance for the prediction and prevention of water inrush disasters from KCCs. [ABSTRACT FROM AUTHOR]

Coal will continue to play an important role in China's economic development and social development in the coming decades. However, due to the complex distribution conditions of coal resources, the mining of coal resources is subject to various restrictions. Coal and gas outburst is an important issue in coal mining, and the threat to the mining of coal resources caused by high gas outburst activity has been receiving more attention. In order to solve the problems related to safe and efficient mining under coal seams with gas outburst, such as mining difficulties, large amounts of work, resource waste, no guaranteed gas treatment time, and low economic efficiency, it is necessary to innovate mining technology and methods for managing gas outburst in coal seams to improve the efficiency of coal mines and to solve the above problems. This study proposes a green mining technical method system known as the "L-H method", which is applicable to the safe and efficient mining of coal seams with gas outburst based on combined theoretical analysis and numerical simulation. The following research results are achieved: (1) The "L-H method" is proposed, and a mining area model of coal seams with gas outburst is established. The specific details of the method and the implementation process are introduced. (2) Examples of H coal mine applications are presented, and the effects of the implementation of the "L-H method" are analyzed and summarized through mine pressure observations from roadways, and it is concluded that the implementation of top-cutting and pressure-relief technology has a good control effect on the roadway-surrounding rock and that gas extraction reaches the national standard of less than 8 m3/t for protrusion prevention; this ensures safety and also achieves efficient mining. This study will provide a good reference for the implementation of green mining methods to similar coal and gas outburst mines. [ABSTRACT FROM AUTHOR]

Fault structures developed in coal seams, which are often associated with roof collapse, water inrush, gas outburst, and other accidents, are common geological hazards in coal exploration and development. The accurate detection of micro-structures such as small faults has always been a research focus to ensure safety in coalfields. Three-dimensional (3D) seismic research is one of the most efficient methods for obtaining the structural characteristics of coal areas and identify small faults in coal seams, but it is difficult for traditional seismic data imaging technologies to meet the high-precision demand of current coal exploration. Aiming at the characteristics of 3D seismic data in coalfields, we calculated the difference coefficients based on the optimized inversion algorithm and proposed a variable-density acoustic equation optimized with a temporal–spatial staggered-grid finite difference forward algorithm. On this basis, by combining normalized cross-correlation imaging conditions and GPU/CPU collaborative parallel processing technology, we developed an efficient and high-precision 3D reverse time migration method suitable for 3D seismic data in coalfields. Numerical tests verified the accuracy and efficiency of the proposed migration method for the imaging of coal-measure strata with small fault structures and could effectively identify 5 m small faults in the coal seam. The migration test of 3D seismic data in real coalfields showed that our 3D reverse time migration method has good practicability for high-precision imaging of 3D seismic data in coalfields and is an effective method for the precise imaging of small faults in coal measures. [ABSTRACT FROM AUTHOR]

In view of the dynamic phenomenon that coal and rock are susceptible to external impact disturbance in the mining process, combined with the rheological hypothesis mechanism of coal and gas outburst, the RLSS-II-type triaxial loading creep test system of gas-containing coal and rock developed by ourselves is used to carry out the conventional triaxial rheological test of gas-containing coal and rock and the rheological disturbance effect test of gas-containing coal and rock under impact disturbance. The strength limit neighborhood of gas-containing coal and rock is determined, and the gas-containing coal and rock entering the strength limit neighborhood are subjected to different impact disturbances. The experimental results show that, (1) under the confining pressure of 0 MPa, 2.5 MPa, and 5 MPa, the longitudinal deformations of gas-bearing coal and rock are 32 mm, 27 mm, and 22 mm, respectively, indicating that the deformation of coal and rock will be affected by confining pressure, and with the increase of confining pressure, the deformation will decrease. (2) In the test, the deformation of coal and rock in the late stage of uniform creep stage can be regarded as a strain threshold. Before this threshold, the strain of coal and rock is not obvious, and the deformation is only 1.1 mm. After exceeding a threshold, the deformation of coal and rock is 9 mm, and the deformation increases significantly. Then, it enters the accelerated creep stage quickly and finally damages. The vicinity of this threshold is called the strength limit neighborhood of coal and rock containing gas. (3) The gas-bearing coal and rock without entering the strength limit neighborhood and entering the strength limit neighborhood are changed by confining pressure and different impact disturbance, respectively. It is found that, whether in the strength limit neighborhood or outside the strength limit neighborhood, the confining pressure has an effect on the strain, but the influence is not large. Under different impact disturbance, the deformation of coal and rock within and outside the strength limit neighborhood is 8 mm and 0.4 mm, respectively, and the deformation changes obviously, indicating that the impact disturbance has a great influence on the deformation of coal and rock within the strength limit neighborhood, and the coal and rock with large impact disturbance are destroyed before the coal and rock with small impact disturbance, indicating that the greater the impact disturbance, the shorter the time required for destruction. [ABSTRACT FROM AUTHOR]

Accidental coal and gas outbursts lead to major disasters in coal-producing countries and are difficult to mitigate. To elucidate the energy dissipation law for coal and gas outburst shock waves in a complex ventilation network of mines, a coal-and-gas-outburst-energy-propagation simulation and parameter determination test device were developed and used to perform physical simulation experiments under different outburst strength conditions. These experiments were combined with numerical simulations to obtain the propagation law of outburst shock waves in a roadway and analyze different outburst intensities according to the gas counterflow criterion, highlighting the hazard characteristics of shock waves in a fresh air tunnel. The results showed that when a shock wave passed through the turning roadway, its intensity and speed were greatly attenuated and that the overpressure value of a shock wave in a straight roadway was greater than that at a corner. Upon encountering a rigid wall, the superposition of the incident and reflected shock waves increased the peak overpressure of the shock wave per unit area. When a fresh air roadway is near the coal uncovering position in rock drift ("Shimen") a corresponding counter-backflow device should be installed at the location of the connecting road near the fresh air roadway, under the condition that normal ventilation is not affected. These numerical simulation results are consistent with general experimental trends, indicating that the analyses conducted in this study are reliable and can provide a theoretical basis for the prevention of secondary disasters due to coal and gas outbursts in mines. [ABSTRACT FROM AUTHOR]

In the gob-side entry retaining by roof cutting (GERRC) technique, pressure is offloaded via directional roof cutting, and a roadway is automatically formed due to the ground pressure and rock-breaking expansion. To improve the application of the theory and technical system of GERRC in the Karst area in Southwest China, this research studies the key technology of GERRC in a high gas outburst coal seam, based on the engineering background of the 39114 working face of the Honglin coal mine. According to the geological conditions of the 39114 working face, by means of formula calculation, UDEC numerical modeling, and on-site drilling peeping, the optimal roof-cutting parameters suitable for the 39114 working face were determined: the roof cutting height was 7 m, the roof cutting angle was 15°, and the spacing of pre-splitting blasting holes was 600 mm. Additionally, the above roof-cutting parameters have achieved good results in the engineering practices of the 39114 transportation roadway, which shows that the technology of GERRC is feasible in high gas outburst mines and achieves the goal of safe and efficient mining. [ABSTRACT FROM AUTHOR]

Accurate determination of coalbed methane (CBM) content can not only prevent and control coal and gas outburst disasters in mines, but also facilitate the prediction of coal seam methane resource reserves, which is significant for injecting carbon dioxide to enhance CBM recovery. The freezing core sampling technique proposed in recent years for accurate determination of the CBM content has faced temperature-pressure coupling effects on gas desorption in coalin a low-variable temperature environment. In this paper, we utilized a self-developed high-low temperature alternating adsorption-desorption experimental device to study the gas desorption process in coal under temperature-pressure coupling conditions in a low-variable temperature environment. The experimental results show that the main promotion and inhibition effects of temperature on the gas adsorption and desorption in coal in a low-temperature environment were consistent with those in an ambient temperature environment, i.e., the decrease of temperature promoted coal gas adsorption, while the increase of the temperature promoted coal gas desorption. It is found that throughout the temperature change process (TCP), there existed a 'pseudo-desorption' phenomenon, and the cumulative desorption amount showed a consecutive rising-declining-rising trend and finally stabilized. The whole process can be regarded as consisting of a main desorption process and a retrograde desorption process. In the main desorption process with the same TCP, the cumulative desorption amount and initial desorption rate increased with the increase of initial equilibrium pressure. In the retrograde desorption process, there existed a peak retrograde desorption rate, and the staged retrograde desorption amount conformed to the linear decrease law. A comprehensive analysis of the temperature-pressure coupling effect on gas desorption throughout the TCP was conducted. Thus, we proposed some considerations and precautions for the configuration and supply monitoring of the cold source in the process of the measurement of CBM content using the freeze-coring method. • Temperature-pressure coupling effect is studied at low/ambient variable temperatures. • Gas desorption amount shows a rising-declining-rising trend. • Pseudo-desorption occurs in the temperature change process due to the coupling. • Linear-decreasing stepwise retrograde desorption process shows a peak desorption rate. [ABSTRACT FROM AUTHOR]

Coal seam water injection is proved an effective method for preventing coal and gas outburst. However, the poor wettability of coal seam has resulted in poor effect of coal seam water injection in preventing coal and gas outburst. In addition, the coal seam is in a state of injected-uninjected cycle due to multiple water injection. Therefore, it is important to study the wettability of coal seam with adding wetting agent under multiple water injection. In this paper, the effect of cyclic water injection on the wettability of coal with different SiO 2 nanofluid treatment times was investigated through contact angle tests. The results showed that increasing the treatment time of SiO 2 nanofluid would enhance the wettability of coal. In addition, the wettability of nanofluid-treated coal samples gradually got worse with the number of cycles increasing. In general, the effect of SiO 2 nanofluid on enhancing the wettability was approximately 2 to 4.5 times better than that of water. However, the difference of wetting enhancement between water and SiO 2 nanofluid became less obvious as the number of cycles increased. Even though, the nanofluid-treated coal samples showed the better wettability than water-treated coal samples after 13th cycle. Furthermore, the potential application of SiO 2 nanofluid in coal seam water injection was proposed, which had important significance for improving the effectiveness of coal seam water injection. [ABSTRACT FROM AUTHOR]

Coal seam water injection is one of the important technical means to prevent mine disasters. It can effectively change the mechanical properties of coal, prevent dust, coal and gas outburst, and reduce gas concentration. In order to expand the range of coal seam water injection, make coal seam water injection more effective in wetting coal mass, and determine the effective type of ionic surfactant required for coal seam water injection, triaxial seepage experiments and spontaneous imbibition experiments are conducted on the long flame coal samples from Houwenjialiang Coal Mine in Ordos City, Inner Mongolia Autonomous Region, China in this paper. Moreover, the effects and mechanisms of different ionic surfactants on the seepage and wetting stages of coal seam water injection are studied, which provides a theoretical basis for on-site coal seam water injection. After analysis and discussion, it is found that the influence of cationic surfactants on the seepage stage of coal seam water injection first decreases and then increases while the influence of anionic surfactants on the seepage stage of coal seam water injection shows a gradually increasing trend. Furthermore, both cationic and anionic surfactants show more significant wetting effects as the concentration of the surfactants increases. In addition, the adsorption of cations to coal molecules is stronger than that of anions, and the water solubility of cations is better than that of anions. [ABSTRACT FROM AUTHOR]

In order to realize dynamic, continuous, and real-time prediction of coal and gas outburst risk in real time in blasting driving face, an outburst risk prediction method based on the characteristics of gas emission after blasting is proposed. In this study, the causes of abnormal gas concentration in blasting driving face are analyzed, and the identification method of abnormal gas concentration based on weighted K-nearest neighbor (weighted KNN) is proposed. The correlation between gas emission characteristics after blasting and K1 value is analyzed, and the prediction model of outburst risk based on convolutional neural networks (CNN) is established and applied in Jinjia coal mine in China. The results show that the causes of abnormal gas concentration mainly include ventilation stop, blasting operation, sensor adjustment, and other abnormalities. The accuracy of the identification method is 86.1%. Especially, the identification accuracy of blasting operation is 92%. There are strong correlations between the growth rate, peak value, and decay rate of gas concentration after blasting and K1 value, and the maximum correlation coefficient is 0.92. Using the prediction model, 28 times of jet holes and 1 small outburst event are predicted successfully, and the efficiency of the prediction model is 76.39%. By this technology, the utilization rate of gas information is improved, and the relationship between the change characteristics of gas concentration after blasting and the risk of coal seam outburst is established, which is of significant for improving the prediction accuracy and risk management ability of coal and gas outburst. [ABSTRACT FROM AUTHOR]

To understand the stability of coal and rock in areas of normal faults disturbed by the dynamic loads of blasting, the damage characteristics of coal and rock within a normal fault are investigated using similarity simulation tests. The mechanism for coal and gas outbursts within a normal fault is analyzed theoretically, and the results indicate that the maximum tensile stress in the vertical direction of the blasting hole in the normal-fault model is 1.17 times that in the no-fault model. The propagation of cracks near the blasting hole produces crushing circles, and more cracks are produced in the normal-fault model, causing severe damage to the coal seam and floor rock adjacent to the upper wall of the normal fault. Meanwhile, coal on the surface of the coal seam falls off. The cracks extend to the roof rock through the footwall of the normal fault. Cracks in the adjacent strata and coal seam interpenetrate those around the blasting hole, which is a potentially dangerous area for coal and gas outbursts. The cumulative damage caused by blasting vibrations increases the extent and scope of the damage to coal and rock, and broken coal and rock provide weak surfaces and gas flow channels that can lead to dynamic gas disasters. The research results will provide a theoretical basis for gas dynamic disasters induced by blasting disturbance in normal fault structures. Based on cases of coal and gas outbursts in the Didaoshenghe Coal Mine in Heilongjiang Province, China, an important reason for such incidents is considered to be the blasting areas of normal faults being disturbed by air-powered coal drilling. [ABSTRACT FROM AUTHOR]

Accurate determination of coalbed methane (CBM) content is of great significance to coalmine methane prediction, methane extraction, and gas outburst prevention. The adsorption time affects the CBM content by affecting the Langmuir adsorption constants when using the indirect method to determine the CBM content. To determine the reasonable adsorption time, representative coal samples (anthracite, lean coal, and gas-fat coal) and different destruction types (soft coal and hard coal) were selected. The experimental methods used are mercury intrusion porosimetry test, N2 adsorption/desorption test, and methane isothermal adsorption test. The results show that the pore volume and the specific surface area obtained by the BJH method and BET model increase with increasing coal rank and destruction type. For anthracite soft and hard coal, lean soft coal, and gas-fat soft coal, the N2 adsorption/desorption hysteresis loop is not closed when the relative pressure is low, indicating the existence of ink-bottle pores in these coal samples. For all tested coal samples, the longer the adsorption time is, the larger of coal methane adsorption capacity, Langmuir constant a , and CBM content are. The Langmuir constant b decreased with increasing the adsorption time. Under the condition that the reliability of the measured value is 85%, the reasonable adsorption times of coal samples are 28 h for anthracite, 18 h for lean coal, and 9 h for gas-fat coal. This study has practical help to improve the determination of CBM content, the prediction of coal seam gas outburst, and the development of CBM. [ABSTRACT FROM AUTHOR]

Coal and gas outburst have always been one of the principal gas energy disasters in coal mines. Pre-extraction of coal-bed methane (CBM) is an effective method to reduce the outburst risk. However, during coal mining and drilling for CBM extraction, coal will inevitably undergo plastic deformation or even failure. This will result in a change in gas's migration behavior, bringing severe challenges to coal mine gas disaster prevention and effective CBM extraction. Therefore, it was considered necessary to study the evolutionary law of permeability characteristics, and the mechanical property response of coal under engineering disturbance. Based on generalized plastic theory, the plastic strain in coal is calculated by using the non-associated flow rule. The damage variable was modified with reference to the stress–strain constitutive relationship, that was introduced into a simplified permeability model to successfully construct an original coupled damage–permeability model based on the elastoplastic mechanics in coal (D–P coupling model). The proposed model has been verified by carrying out a tri-axial compression-seepage experiment (mining simulations) under different confining pressures and a tri-axial seepage experiment (extraction simulations) under different effective stresses and under different pore pressures. The results showed that during the whole stress–strain process, the new model could well reflect the seepage behavior of CBM on whether coal permeability decreased before the yield point or increased sharply after peak failure. Coal permeability decreased with an increase of effective stress and pore pressure, and the new model corresponded well with the experimental results. Finally, the relationships between plastic strain, damage variables and mechanical properties in coal were discussed. The proposed model has provided a theoretical basis for coal mine gas disaster prevention and CBM extraction. Highlights: Tri-axial compression-seepage experiments under different confining pressures (mining simulations) were conducted. Based on the generalized plastic theory, the plastic deformation of coal was quantified. The mathematical model from elastoplastic deformation to damage and seepage of coal was established. The relationships between plastic strain, the damage variable, and mechanical properties of coal were analyzed. [ABSTRACT FROM AUTHOR]

In view of the high gas pressure and low permeability of deep coal seam, it is difficult to control gas, which affects the safety production of coal mine. The technical scheme of hydraulic fracturing to improve the permeability of coal seam is put forward, and the gas drainage technology is used to control the gas emission of coal seam. The fracturing effect under different water pressure, different gradient fracturing times, and in situ stress is analyzed by using 3DEC (3-Dimensional Distinct Element Code) discrete element software. The simulation analysis and field verification results show that the coal seam gas pressure increases linearly with the buried depth. In situ stress characteristics and hydraulic strength are the key factors affecting the effect of hydraulic fracturing. The fracturing radius increases with the increase of flow. When the construction pressure of hydraulic fracturing test is 18 MPa, the distance between fracturing hole and drainage hole is 8.5 m. The actual measurement shows that after hydraulic fracturing and gas drainage, the maximum gas emission is reduced by 51%, and the average gas emission is reduced by 58%. [ABSTRACT FROM AUTHOR]

Methane is one of the most dangerous gases encountered in the mining industry. During mining operations, methane can be broadly classified into three states: mining excavation, stoppage safety check, and abnormal methane concentration, which is usually a precursor to a gas accident, such as a coal and gas outburst. Consequently, it is vital to accurately predict methane concentrations. Herein, we apply three deep learning methods—a recurrent neural network (RNN), long short-term memory (LSTM), and a gated recurrent unit (GRU)—to the problem of methane concentration prediction and evaluate their efficacy. In addition, we propose a novel prediction method that combines classical time series analysis with these deep learning models. The results revealed that GRU has the least root mean square error (RMSE) loss of the three models. The RMSE loss can be further reduced by approximately 35% by using the proposed combined approach, and the models are also less likely to result in overfitting. Therefore, combining deep learning methods with classical time series analysis can provide accurate methane concentration prediction and improve mining safety. [ABSTRACT FROM AUTHOR]

• Release pressure and diameter determined the distribution range of smash district. • Filling level of LCO 2 directly control the distribution range of crack district. • LCO 2 filling parameters can be used to guide LCO 2 -PB field applications. Liquid carbon dioxide phase-transition blasting (LCO 2 -PB) is a promising alternative method for enhanced coalbed methane (ECBM) extraction and prevention of coal and gas outbursts. During the process of the LCO 2 -PB fracturing the coal seam, the improper application of fracturing model will reduce the efficiency of gas drainage and cause gas disasters. To clearly understand the ECBM modes of LCO 2 -PB fracturing coal seam process, this paper employed experiments to investigate the effect of leakage characteristics of the LCO 2 -PB on fracture models of coal cubes, including release pressure (P r), filling level (V), release diameter (D r) and blasting hole diameter (D b). We propose the combination I and II of the LCO 2 -PB leakage parameters and applied two methods to the in-situ tests. The results revealed that the gas flow of the boreholes of the LCO 2 -PB with the combination I is 0.368 m3/min, which is 2.07 times that of the combination II (0.178) in the soft coal seam. Besides, the average methane flow of the boreholes with combination II is 0.796 m3/min, which is 1.78 times the combination I (0.448) in the hard coal seam. The adjustment of leakage parameters of the LCO 2 -PB technology can be helpful for ECBM and reduce the risk of coal and gas outbursts in the soft and hard coal bed. [ABSTRACT FROM AUTHOR]

• The whole safety production accidents of coal mines in Guizhou Province were analyzed. • The time–space dynamic evolution of coal and gas outburst accidents were proposed. • Coal and gas outburst accidents, influence factors and suggestions in Guizhou Province were studied. The safety situation of coal mines in Guizhou has been improving for many years, but coal and gas outburst accidents continue to occur. This paper analyzes the safety situation of coal mines in Guizhou by collecting data on accidents and disasters in the last decade: 2011–2020. The distribution characteristics, time–space dynamic evolution, and factors influencing coal and gas outburst disasters were studied. Countermeasures and safety management suggestions for disaster prevention and control were proposed. Our analysis shows that the primary disasters occurring in coal mines in Guizhou province are gas, roof, and water disasters, which are mainly distributed in the western and northern regions, as well as closely related to the complex geological conditions in the region. Coal and gas outburst is the most serious disaster threatening the safety of coal mines in Guizhou province in the last decade. The occurrence and development of gas outburst accidents presents three stages. The coal and gas outburst disasters mostly occurred in the driving face in the last decade, while their distribution showed obvious regional characteristics. In particular, Liupanshui, Bijie, and southwest Guizhou have a greater risk of coal and gas outbursts. Thus, it is necessary to focus on and formulate strict and effective outburst prevention measures when performing mining operations. Finally, according to hazard analysis, objective factors and management factors underlying coal mine disasters in Guizhou, specific countermeasures, and safety management policy suggestions of government and enterprises are provided. This research has certain significance for the safety supervision by the government and the staff engaged in the prevention and control of coal and gas outbursts. [ABSTRACT FROM AUTHOR]

• A new gas pressure inversion model is established. • The application result of the new model proves the uneven distribution of gas pressure. • The gas pressure value shows an approximate normal distribution law in statistics. • The evolution law of outburst elimination in the coal seam presents the characteristics of non-uniformity. China is one of the countries most seriously affected by coal and gas outburst disasters. Gas pressure is an important parameter to evaluate the risk of coal and gas outburst. In order to solve the problem existing in the traditional gas pressure measurement method, a dynamic inversion model of gas pressure while drilling is established based on the gas emission rule of coal particle and the gas radial migration equation of borehole wall in this paper. Combined with the directional drilling system and the special gas emission measuring device, the dynamic inversion model is applied to measure the gas pressure in the coal seam. The results show that: (1) The theoretical value of gas pressure obtained from the new model has a small error range by comparing with the actual value of gas pressure, which indicates that the new model has good reliability and applicability; (2) The distribution range of gas pressure value is wide, which shows the non-uniformity of gas occurrence in coal seam. The heterogeneity of gas pressure value shows an approximate normal distribution law in statistics; (3) Influenced by the occurrence of original gas in coal seam, the evolution law of outburst elimination in each area of coal seam also presents the characteristics of non-uniformity. The research results have important practical significance to assist the early warning of coal and gas outburst and to achieve coal and gas co-mining with safety mining. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rock outbursts and gas are the most serious natural hazards in underground coal mine. • The outburst sensor detects methane concentration, pressure and acoustic effects. • The dynamic properties of the sensor tested on the measuring bench in laboratory. • Field tests of the outburst sensor were carried out in the experimental mine. • The microphone and the pressure signals appear before of methane concentration. Outbursts of methane and rocks are, similarly to rock bursts, the biggest hazards in deep mines and are equally difficult to predict. The violent process of the outburst itself, along with the scale and range of hazards following the rapid discharge of gas and rocks, require solutions which enable quick and unambiguous detection of the hazard. The paper presents the functions of an integrated outburst detector which was equipped with three measuring and detection elements: a pressure sensor, a microphone and a chamber for constant measurement of methane concentration. A measuring stand for testing the dynamic properties of the integrated methane discharge sensor is described. It has been shown that both the pressure transducer and the microphone react immediately after a change in thermodynamic conditions, and the time constant of the pellistor methane concentration sensor was estimated at a few seconds. Field tests of the integrated sensor were also carried out, the main purpose of which was to verify the dynamic parameters of the sensor in medium scale experimental conditions during methane and coal dust explosions in the testing gallery at an experimental mine. [ABSTRACT FROM AUTHOR]

Gas bubbles bursting at the sea surface produce drops, which contribute to marine aerosols. The contamination or enrichment of water by surface‐active agents, of biological or anthropogenic origin, has long been recognized as affecting the bubble bursting processes and the spray composition. However, despite an improved understanding of the physics of a single bursting event, a quantitative understanding of the role of the physico‐chemical conditions on assemblies of bursting bubbles remains elusive. We present experiments on the drop production by millimetric, collective bursting bubbles, under varying surfactant concentration and bubble density. We demonstrate that the production of supermicron droplets (with radius larger than 35 μm) is non‐monotonic as the surfactant concentration increases. The bursting efficiency is optimal for short‐lived, sparsely distributed and non‐coalescing bubbles. We identify the combined role of contamination on the surface bubble arrangement and the modification of the jet drop production process in the bursting efficiency. Plain Language Summary: Understanding the production of primary ocean spray aerosol and its dependence on meteorological and environmental variables is critical for modeling potentially health‐adverse effects from dispersants used to break down oil spills, as well as radiative processes and cloud microphysical properties. We demonstrate experimentally that the coupling between the chemical properties of contaminated water and the physical mechanism of droplet production when a bubble bursts controls the efficiency of aerosols production during collective bubble bursting, and reveals an optimal production regime at intermediate contamination. This fundamental study paves the way to improved predictions of aerosol production in contaminated water. Key Points: Spray droplet production by collective bursting bubbles is non‐monotonic as surfactant concentration increasesAn optimal drop production is observed at intermediate surfactant concentrations for short‐lived, non‐coalescing surface bubblesThe saturation of the water surface by air bubbles, or formation of a foam, inhibits the drop production by bubble bursting [ABSTRACT FROM AUTHOR]

Hydraulic fracturing can increase the fracture of coal seams, improve the permeability in the coal seam, and reduce the risk of coal and gas outburst. Most of the existing experimental specimens are homogeneous, and the influence of the roof and floor on hydraulic fracture expansion is not considered. Therefore, the hydraulic fracturing test of the simulated combination of the coal seam and the roof and floor under different stress conditions was carried out using the self-developed true triaxial coal mine dynamic disaster large-scale simulation test rig. The results show that (1) under the condition of triaxial unequal pressure, the hydraulic fractures are vertical in the coal seam, and the extension direction of hydraulic fractures in the coal seam will be deflected, with the increase of the ratio of the horizontal maximum principal stress to the horizontal minimum principal stress. The angle between the extension direction of the hydraulic fracture and the horizontal maximum principal stress decreases. (2) Under the condition of triaxial equal confining pressure, the extension of hydraulic fractures in the coal seam are random, and the hydraulic fracture will expand along the dominant fracture surface and form a unilateral expansion fracture when a crack is formed. (3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks. [ABSTRACT FROM AUTHOR]

The thermal effect caused by deep magma intrusion can not only accelerate the metamorphism of coal body, but also bring additional thermal field that changes the mechanical environment of coal seams, thereby affecting the permeability of coal seams. Different from shallow coal resources, deep coal resources are in a mechanical environment characterized by limited stress and strain. Thus, the thermal effect has a more significant influence on the distribution and permeability characteristics of deep coal seams. In this study, the evolution history of highly metamorphic coal seams in Yangquan mining area was analyzed, and the main effect of magmatic activity on coal seams was obtained. Based on the determined vitrinite reflectance data of typical mines in Yangquan mining area, the maximum paleotemperature was calculated by adopting the Barker's method. Furthermore, the paleotemperature distribution in Yangquan mining area was summarized, and its relationship with the metamorphic degree was acquired. Then, a new permeability model considering the thermal strain was proposed to analyze the permeability evolution in deep coal seams at different ground temperatures. Finally, through a combination of the results of gas pressure and outburst number in Sijiazhuang Mine, Yangquan No. 5 Mine and Xinjing Mine, the influence of ground temperature on the gas outburst risk in Yangquan mining area was explored. The following conclusions were drawn: The maximum paleotemperature in Yangquan area can be 303 °C. In addition, the paleotemperature in the south is higher than that in the north of Yangquan mining area. The various temperatures at different depths bring about different degrees of thermal stress to different coal seams, leading to different strains. Under the fixed displacement boundary conditions in the deep, the coal seam folds and bends to varying degrees. Moreover, the difference in the ground temperature raises the a value of coal seams and lowers the permeability, which promotes the formation of gas-rich zones and increases the risk of coal seam outburst. The research results can help mines to make proper gas disaster prevention plan for different zones. [ABSTRACT FROM AUTHOR]

Thin coal seams in mines usually lack gas data. Thus, preventing and controlling gas outbursts of thin coal seams are difficult. In this study, a coal structure index, which is used to express the damage degree of coal, was estimated by logging curve. In accordance with the contour line of the floor of the coal seam, structural curvature was calculated to express the complexity of the coal seam structure quantitatively. Subsequently, relationships among the burial depth, thickness, coal structure index, structural curvature were analyzed on the basis of the gas pressure of coal seam. The gas pressure values of the coal seams of Nos. 22, 24, and 27 in the study area were predicted by multiple linear regression (MLR) and were then verified and analyzed. The deviation rate of the MLR method was 6.5%–19.7%, with an average of 13.0%. The average deviation rate between the predicted value and the measured value was 11.6%, except for the measuring point of No. 2, which had a large deviation. Results show that the prediction accuracy of the aforementioned method is acceptable and has practical value in the prediction of gas pressure in thin coal seams without measured data. The results in the gas pressure prediction provide a basis for evaluating the risk of gas outbursts in thin coal seams. [ABSTRACT FROM AUTHOR]

This study aims to effectively solve the problem of the destruction of the coal roof and floor overlying rock after mining the protective layer, and determines whether the gas in the protected layer can be effectively released. We do this based on the engineering background of the Weng'an Coal Mine; research and analysis of the movement of the roof and floor overlying rocks; the evolution of cracks and the pressure relief characteristics of the protected layer after mining the protective layer; and through theoretical analysis and similar simulation experiments. Through numerical simulation, it was found that the protected layer was depressurized due to the mining of the protective layer, and the decompression rate of the protected layer was 0.2–0.8. In addition, the overall expansion rate of the protected layer was greater than the requirements of the "Detailed Rules for Prevention and Control of Coal and Gas Outbursts" for coal mines, and the investigation of the residual gas pressure and content of the protected layer revealed that the protected coal seam had been mined in the upper protective layer coal face. The gas pressure dropped to 50.7% of the original coal seam gas pressure, the rate of decrease was 49.3%, the residual gas content dropped by 68.67%, and the gas concentration in the return airway was 0.31% on average, meeting the national regulations that require its value to be less than or equal to 1%. The study comprehensively demonstrates that mining of the protective layer is beneficial to the release of gas from the protected layer, and provides a practical reference for coal and gas outbursts in mines. [ABSTRACT FROM AUTHOR]

Based on thermal–fluid–solid coupling law in coal and gas outburst, a multi-physical field numerical analysis model is built for the whole outburst process. The response laws of stress, gas pressure, temperature, and seepage in different areas and different time nodes around coal and rock mass in the coal and gas outburst under high stress condition are discussed. Research results show: Firstly, the stress response law of the coal and rock mass around the burst hole is initial vibration–sudden attenuation–late stability. Secondly, the gas pressure response law in different areas is that the gas pressure response rate decreases gradually with the increase of the distance from the outburst. Thirdly, the adsorbed gas contained in the broken coal near the outburst port is desorbed rapidly and expands to do work, and the temperature changes dramatically after outburst occurs. In contrast, with the increase of stress, the proportion of elastic potential in total coal and gas outburst energy increases, and the proportion of elastic potential is positively correlated with stress. The critical gas pressure under the energy condition of coal and gas outburst decreases with the increase of stress. It illustrates that the lower gas pressure can also meet the energy condition of coal and gas outburst under high stress. [ABSTRACT FROM AUTHOR]

During the process of coal road excavation, various interference signals, induced by environmental noise, drilling, and scraper loader, will affect the risk assessment of coal and gas outburst using acoustic emission (AE) and electromagnetic radiation (EMR) monitoring technology. To distinguish between different interference signals and danger signals, discrete wavelet transform (DWT) was used to decompose and reconstruct signals, and continuous wavelet transform (CWT) was used to obtain the time-frequency plane. The research results show that: (1) interference signals generally exhibit fluctuating changes within small ranges; in comparison, the intensity of AE and EMR signals caused by coal and rock fracture is found to continuously rise for a long period (longer than 2 h). (2) Different interference signals and danger signals differ significantly in their time-frequency plane. (3) Through decomposition and reconstruction of original signal, obvious precursor information can be found in the time-frequency plane of reconstructed signals. [ABSTRACT FROM AUTHOR]

Fully mechanized top-coal caving mining with high mining height, hard roofs and strong mining pressure are popular in the Shendong mining area, China. The occurrence of dynamic disasters, such as rock burst, coal and gas outburst, mine earthquakes and goaf hurricanes during the coal exploitation process under hard roof conditions, pose a threat to the safe production of mines. In this study, the characteristics of overburden fracture in fully mechanized top-coal caving with a hard roof and high mining height are studied, and the technology of advanced weakening by hard roof staged fracturing was proposed. The results show that the hard roof strata collapse in the form of large "cantilever beams", and it is easy to release huge impact kinetic energy, forming impact disasters. After the implementation of advanced hydraulic fracturing, the periodic weighting length decreases by 32.16%, and the length of overhang is reasonably and effectively controlled. Ellipsoidal fracture networks in the mining direction of the vertical working face, horizontal fracture networks perpendicular to the direction of the working face, and near-linear fracture planes dominated by vertical fractures were observed, with the accumulated energy greatly reduced. The effectiveness of innovation technology is validated, and stress transfer, dissipation and dynamic roof disasters were effectively controlled. [ABSTRACT FROM AUTHOR]