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Rock and gas outbursts occur most frequently in coal mining. Gas-induced geodynamic events in dolomite are a new and unexplored phenomenon. In the last time, two outbursts of gas and dolomite have occurred in the Legnica-Głogów Copper Region in southwestern Poland, which is one of the world's largest copper ore mining regions. Gas-induced geodynamic events are dependent on the co-occurrence of two factors: rock porosity, and the presence of gas under significant pressure in the pores and crevices of the rock. This paper presents a condensed energy balance for the phenomenon of outbursts of dolomites and gases. The quantity of energy depends on the type of transformation taking place during decompression. The work produced enables first the disintegration of the rock, and then its transportation along the mine working space. This paper has focused on the estimation of the energy required to disintegrate the rock. It has been estimated that 0.38 MJ is needed for the size reduction of 1 m3 of ejected mass. Highlights: In the case of a dolomite outburst, the energy is accumulated primarily in the form of compressed gas present in the porous structure of the rock and causes the rock to disintegrate and its transport along the mine working space. Gas decompression during a gas-induced geodynamic phenomena can be between isothermal and adiabatic. Existing disintegration theories: Rittinger's, Kick's and Bond's are based consists chiefly of crushing, compression or abrasion, caused by the action of external factors. Author's theory of disintegration of research on tensile stress and is based on Brazilian tests. The disintegration of 1 m3 of rock requires the production of energy of approximately 0.38 MJ. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a type of coal mine dynamic disaster with complex causes and serious consequences. In China, the number and scale of outburst disasters are among the highest in the world. It is an inevitable stage for outburst that the mechanical instability of the combination of intact coal and tectonic coal (CICTC) in strata. The dip angle of the interface has an important influence on the instability of CICTC-containing gas. To further understand the instability process of CICTC, the uncoordinated deformation of CICTC was defined. According to this, three strain conditions were delimited. The variations of stress under three strain conditions were analyzed, the expressions of additional interfacial stress were established. Then, based on the Mohr–Coulomb criterion, the strength characteristics and failure forms were analyzed. It was found that the dip angle deflects the principal stress in the interface, and the influences of additional stresses on the value of principal stress are opposite. These two reasons make the principal stress state and instability process of the combination complicated. Finally, combined with actual parameters, the instability processes of CICTC under different stress paths and buried depths were studied. It can be found that the failures of CICTC under different stress paths were concentrated appearing in the tectonic coal body, intact coal interface, and tectonic coal interface. With the increase of buried depth, the initial maximum principal stress increases and the reduction from additional stresses to the strength of intact coal interface enhances, which makes the combination damage easily and the failure form change. The research results in this paper can provide a theoretical basis to prevent and control the outburst of the combination containing gas considering the dip angle of the interface. Highlights: The uncoordinated deformation of combination considering the dip angle was first defined. The expressions of additional interfacial stress of combination considering the dip angle were established The influences of different stress paths and buried depths for the instability of combination were discussed quantificationally. [ABSTRACT FROM AUTHOR]

Rock and gas outburst is one of the most significant natural hazards faced by the mining industry. In the last decade, this issue has appeared in firm rock mining (including copper ore mines) in addition to coal mining. A number of methods have been developed for coal mining to assess the hazard posed by gasogeodynamic phenomena, but in the case of copper ore mines, no studies have led to forecasts of this risk. To face the challenge, the authors decided to propose their own concept of risk assessment principles based on a threshold analysis of the phenomenon and a continuous analysis using fuzzy logic. A series of tests were performed to support the paper, investigating selected parameters which directly affect the risk. The evaluation principles for gasogeodynamic phenomena presented herein were the result of experimental investigations carried out by the authors. [ABSTRACT FROM AUTHOR]

A probabilistic risk assessment framework was developed to mathematically represent the complex engineering phenomena of rock bursts and gas outbursts for a heterogeneous coal seam. An innovative object-based non-conditional simulation approach was used to distribute lithological heterogeneity present in the coal seam to respect their geological origin. The changing mining conditions during longwall top coal caving mining (LTCC) were extracted from a coupled numerical model to provide statistically sufficient data for probabilistic analysis. The complex interdependencies among abutment stress, pore pressure, the volume of total gas emission and incremental energy release rate, their stochastic variations and uncertainty were realistically implemented in the GoldSim software, and 100,000 equally likely scenarios were simulated using the Monte Carlo method to determine the probability of rock bursts and gas outbursts. The results obtained from the analysis incorporate the variability in mechanical, elastic and reservoir properties of coal due to lithological heterogeneity and result in the probability of the occurrence of rock bursts, coal and gas outbursts, and safe mining conditions. The framework realistically represents the complex mining environment, is resilient and results are reliable. The framework is generic and can be suitably modified to be used in different underground mining scenarios, overcoming the limitations of earlier empirical indices used. Highlights: Parameters causing rockbursts and gas outbursts were linked along with their influences and interdependencies into a probabilistic risk assessment framework. Dynamically updated system feedback from the numerical model was fed into the framework to represent the current stress state in retreating mining and estimate the probability of the occurrence of rockbursts and gas outbursts. Statistically significant data were used to quantify the probability of rockbursts and gas outbursts using Monte Carlo simulation. [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]

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]

It is very important and effective to characterize the failure mechanism of coal and rock materials from the perspective of energy. Energy dissipation and release are important causes of rock mass failure. To qualitatively and quantitatively characterize the energy evolution process of gas-bearing coal and the energy evolution mechanisms of coal and gas outburst, experimental studies were conducted on gas-bearing coal under different stress paths. The research results demonstrate that the energy evolution of gas-bearing coal has nonlinear characteristics and that the energy density is a quadratic function of stress. The total energy density is mainly stored as elastic energy density and is unrelated to loading paths. According to the distribution characteristics of abutment pressures in a stope, the energy distribution can be divided into an energy dissipation and release zone, an increasing-energy zone, and a stable energy storage zone. During coal mining, abutment pressure in front of the coal wall is an important factor causing energy accumulation, and the stress concentration area is a key area to prevent and control dynamic disasters of coal and rock masses. Furthermore, based on the principle of energy conservation, this study established an energy balance model for coal and gas outburst and derived energy criteria for coal and gas outburst. When Υ > 1 , there is a risk of coal seams have outburst. If Υ = 1 , the system is in a state of limit equilibrium, and mining disturbances could trigger outburst. [ABSTRACT FROM AUTHOR]

Outburst energy is a major factor influencing coal and gas outbursts, albeit its estimation is difficult owing to the lack of amenable means for quantification of gas desorption. In the past decades, determining the mechanism of outbursts is one of the most challenging issues in rock mechanics. In this study, a triaxial coal and gas outburst simulation system was employed to perform simulated experiments using He (to rule out the influence from gas ad-desorption), N2, and CO2. This facilitated understanding of the energy principle underlying the said outbursts and evaluation of the effects of gas desorption on outburst development. Results of this study indicate that outburst energy and energy consumption are influenced by several factors, including outburst pressure, outburst intensity, ejection distance, and particle size of ejected coal. Among these, gas desorption demonstrates the greatest influence when performing controlled tests (using He). Considering the effects of gas desorption, the total outburst energy can be increased by 1.35–2.95 times, thereby causing an enormous increase in the destructive potential of outbursts. Additionally, values of the coal crushing and transport energies can be enhanced by the order of 118.9–206.6% and 157.8–406.6%, respectively, thereby resulting in a stronger conveying capacity of outburst coal–gas flow along with severe coal fragmentation. A further analysis of the energy distribution indicated that in the development stage, gas desorbed from coal acts as the force driving coal transport, whereas free gas energy is mainly consumed during coal crushing. Findings of this study highlight the importance of quantifying contributions of coal gas towards effective interpretation of outburst-causing mechanisms. [ABSTRACT FROM AUTHOR]

The influence of tectonic, geostatic, and mining stresses on coal and gas outburst (CGO) becomes more obvious with increasing mining depth. The limitations of general experimental methods and test systems hinder the direct observation and characterization of the dynamic responses of the multi-physical parameters associated with CGO. In this study, a custom-made large-scale physical simulation test system was used to study the characteristics of outburst evolution under different geo-stress levels, to overcome dynamic disasters in coal mines. The intensity and initial velocity of the outburst coal flow increased with increasing geo-stress. The geo-stress changes and gas pressure released during the outburst process mainly manifested in the relief and abutment stress zones, while the release of the elastic strain energy manifested in the abutment stress zone. This showed that the sphere of influence of the change on the physical parameters, such as the geo-stress, gas pressure, and elastic strain energy, is important for setting the drilling depth in measures to eliminate outburst. Interestingly, there seems to be a mechanism that suppresses the excess energy released during the outburst process, such that the solid–gas ratio of the two-phase outburst flow does not increase with increasing initial geo-stress. The dynamic outburst phenomenon is not completely positively correlated with increasing geo-stress. The mechanism causes cyclical fluctuations in the geo-stress, elastic strain energy, and gas pressure during the CGO process and pulse characteristics of the outburst coal flow simultaneously. [ABSTRACT FROM AUTHOR]

Coal and gas outbursts are small-scale geological disasters controlled by tectonic movement, and tectonic coal is widespread in outburst zones. In this study, we compare tectonic and intact coal specimens to examine the basic properties of tectonic coal. We estimate the different energies and limits of the crushing work ratio of coal from five typical outburst cases using on-site outburst data, and discuss the relationship between outbursts and tectonic coal. The results show that tectonic coal is a product of tectonic movement and its original primary structure is destroyed during the tectonic process. Compared with intact coal, tectonic coal shows low strength properties and a crushing work ratio of 22.11 J/m2. The specific surface area and total pore volume of the minipores, mesopores, and macropores of the coal strongly increase under conditions of intense tectonism, which indicates that tectonic coal has a very high capacity for rapid initial gas desorption. An adequate supply of gas is required to transport outburst coal, such that the existence of coal particles smaller than the critical diameter is important. Our calculations indicate that the crushing work ratio of coal from the five outburst case ranges from 22.19 to 78.67 J/m2. Only the crushing work ratio of tectonic coal satisfies the requirement for these cases. Therefore, the properties of the tectonic coal and crushing work ratio for the five cases indicate that the widespread occurrence of tectonic coal plays a crucial role in outbursts. [ABSTRACT FROM AUTHOR]

A coal and gas outburst can lead to a catastrophic failure in a coal mine. These outbursts usually occur where the distribution of coal seam gas is abnormal, commonly in tectonic belts. To study the effects of the abnormal distribution of this gas on outbursts, an experimental apparatus to collect data on simulated coal seam outbursts was constructed. Experiments on specimens containing discrete gas-enriched areas were run to induce artificial gas outbursts and further study of these outbursts using data from the experiment was conducted. The results suggest that more gas and outburst energy are contained in gas-enriched areas and this permits these areas to cause an outburst easily, even though the gas pressure in them is lower. During mining, the disappearance of the sealing effect of a coal pillar establishes the occurrence conditions for an outburst. When the enriched gas and outburst energy in the gas-enriched area is released suddenly, a reverse unloading wave and a high gas pressure gradient are formed, which have tension effects on the coal. Under these effects, the fragmentation degree of the coal intensifies and the intensity of the outburst increases. Because a high gas pressure gradient is maintained near the exposed surface and the enriched energy release reduces the coal strength, the existence of a gas-enriched area in coal leads to a faster outburst and the average thickness of the spall is smaller than where is no gas-enriched area. [ABSTRACT FROM AUTHOR]

The article describes the design principles, structures and technologies used in developing a large-scale coal and gas outburst simulation experimental apparatus. The apparatus is composed of a coal sample cell, fast coal uncovering component. load carrying frame, loading system, rotation mechanism, main bracket and attachment. The results of the performance test on the apparatus are also discussed.

The article discusses the proposed identification model for the outburst of coal and gas. Topics include the experiments carried to obtain the threshold of the initial expansion energy of released gas (IEERG) indicator, the mechanical conditions which must be met by occurrence of coal and gas outburst, and the potential for the gas in soft coal to transport through the cracks of the barrier layer.

The article discusses the study on the key factors that affects the initiation of coal and gas outbursts. It details how the study was conducted which involved a numerical analysis and an outburst simulator that was developed by connecting a reservoir simulator with a geomechanical analysis code. The results reportedly revealed that the outburst risk of coal mining depends on depositional and mining conditions.