Coal Specimens Research Articles

Uniaxial compressive strength (UCS) and SEM study of liquid nitrogen for waterless hydraulic fracturing in coalbed methane reservoirs of Karaganda Basin in Kazakhstan

Four different cryo-fracturing liquid nitrogen (LN2) treatment processes are compared for stimulation of coal. Namely, peak compression strength, acoustic emission, and energy absorption tests, plus scanning electron microscope (SEM) observations for fracture evolution were conducted for freezing time (FT), freezing-thawing cycle (FTC), negative freezing-thawing cycle (NFTC) and positive freezing-thawing cycle (PFTC) LN2 exposure processes for both dried and water-saturated specimens. The positive and negative freezing-thawing cycle experimental processes are novel variations on multi-contact cryogenic stimulation reported for the first time. In all freezing-thawing processes, water-saturated experiments exhibited lower Uniaxial Compression Strength (UCS) peaks compared to dry coal samples, presumably due to the additional frost forces degrading the overall sample integrity. For all LN2 exposure processes using dry coal specimens, the compaction line decreased and the elastic line decreased in acoustic emission (AE) tests, concurrent with a reduction in compressive strength, as LN2 treatment time increased. This trend reversed in water-saturated samples, highlighting the importance of sample water content on fracturing potential. Energy absorption analyses clearly showed that the strength of coal samples decreased with increasing the freezing time and freezing-thawing cycles, as the LN2 treatment produced more cracks and damage to coal. Furthermore, SEM analysis and image processing confirmed that all LN2 treatment processes resulted in the generation of fractures evident in fracture trace statistic changes on the surface of coal samples. FTC and PFTC processes showed the greatest promise as stimulation methods due to the extent of the network of fractures created. Engineering results were augmented with the inclusion of the geological setting, important for the transportability of these outcomes to coals of similar origin.

A three-dimensional feature extraction-based method for coal cleat characterization using X-ray μCT and its application to a Bowen Basin coal specimen

Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal. Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining industry. Discrete fracture networks (DFNs) are increasingly used in engineering analyses to spatially model fractures at various scales. The reliability of coal DFNs largely depends on the confidence in the input cleat statistics. Estimates of these parameters can be made from image-based three-dimensional (3D) characterization of coal cleats using X-ray micro-computed tomography (μCT). One key step in this process, after cleat extraction, is the separation of individual cleats, without which the cleats are a connected network and statistics for different cleat sets cannot be measured. In this paper, a feature extraction-based image processing method is introduced to identify and separate distinct cleat groups from 3D X-ray μCT images. Kernels (filters) representing explicit cleat features of coal are built and cleat separation is successfully achieved by convolutional operations on 3D coal images. The new method is applied to a coal specimen with 80 mm in diameter and 100 mm in length acquired from an Anglo American Steelmaking Coal mine in the Bowen Basin, Queensland, Australia. It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation. Bedding-parallel fractures are also identified and separated, which has historically been challenging to delineate and rarely reported. A variety of cleat/fracture statistics is measured which not only can quantitatively characterize the cleat/fracture system but also can be used for DFN modeling. Finally, variability and heterogeneity with respect to the core axis are investigated. Significant heterogeneity is observed and suggests that the representative elementary volume (REV) of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.

Dynamic characteristics and crack evolution laws of coal and rock under split Hopkinson pressure bar impact loading

The dynamic mechanical properties and crack evolution characteristics of coal and rock during split Hopkinson pressure bar (SHPB) impact failure are important contents for analysis. In previous studies, the coal and rock specimens used have usually been independent and not closely correlated. In addition, quantitative characterization and analysis methods for coal and rock cracks are immature, and more information has not been fully revealed. The aims of this paper are to comprehensively explore both the dynamic mechanical properties and crack evolution characteristics of coal and rock during impact failure. First, experimental specimens are prepared from coal seam, direct roof rock strata and direct floor rock strata in the same area to highlight the correlations between test pieces. Second, a dynamic strain gauge and high-speed (HS) camera are adopted to reflect the stress wave signal and crack evolution. Then, based on digital image correlation (DIC) technology and the mass screening method, the evolution laws of surface cracks during crushing and the distribution characteristics of sample fragments after crushing are studied from the perspective of fractal, and finally compared with those of the simulation analysis. The results are as follows. (1) The coal and rock samples from the same area have both consistency and differences. The dynamic mechanical properties of coal and rock are affected by the impact velocity and the physical properties of the specimen. Higher impact speeds and densities lead to the more obvious brittleness of the specimen when destroyed. Conversely, the sample shows more plasticity and ductile yield. (2) The self-similarity is significantly manifested in the evolution of surface cracks during impact and the distribution characteristics of fragments after impact. The box dimension and quality screening dimension are applicable to quantitatively characterize the evolution process and results of coal and rock fractures. (3) The simulation results based on the Holmquist–Johnson–Cook (HJC) and Riedel–Hiermaier–Thoma (RHT) constitutive models agree well with the experimental results, and the RHT constitutive model is more consistent. This study may contribute to a more comprehensive understanding of the dynamic characteristics and crack evolution laws of coal and rock under impact loading and provide references for further research and discussion.

Effect of Pore Pressure on Strain Rate-Dependency of Coal

Viscoelastic strain rate-dependent behaviour of coal is critical in several subsurface engineering applications especially coal seams gas production. Such rate dependency is controlled by the interaction between coal bulk and gas sorption (a sorbing gas) or gas pressure (a non-sorbing gas). Despite the research conducted to date, the gas pressure effect (non-sorbing) on the viscous behaviour of sediments in particular coal remains unexplored. We, therefore, investigate the strain rate-dependent mechanical behaviour of coal under isotropic loading to specifically explore the effect of gas pressure (Helium) on its rate dependency eliminating the sorption effect. We perform a set of triaxial experiments on coal specimens at dry and pressurised gas (Helium) conditions under different strain rates under isotropic loading. The experimental results show that all coal specimens have viscoelastic strain rate dependency at a dry condition where viscous effect increases with strain rate. As a result, the bulk modulus of the specimens increases with the increase in strain rates. This strain rate dependency response, however, reduces with an increase in pore pressure and vanishes at a certain pore pressure under the same effective stress to that of dry specimens. We further employ X-ray micro-Computed Tomography (XRCT) to 3D scan a coal specimen saturated with Krypton gas undergoing different loading rates to shed light on the micro-mechanisms of gas pressure effect on specimens’ rate dependency. The XRCT results show that gas can be trapped in small-scale fractures and pores during the loading process leading to a localised undrained response that can stiffen the specimen and reduce its ability to show viscous rate dependency. The obtained results are significant in optimizing coal seam gas production and coal seam gas drainage applications.

The role of porosity in the dynamic disturbance resistance of water-saturated coal

Porosity significantly affects the dynamic response of water-saturated coal and the safety of coal mine engineering in water-rich areas. For mastering the dynamic response of water-saturated coal to the porosity, this study used the coal with four levels of the porosity for dynamic tests. In this process, the dynamic compressive tests were operated on these coal specimen in both of water saturation and dry states, and the reduction factor of dynamic mechanical parameters of the coal in water-saturated state to these in dry state were focused to eliminate the effect of self dynamic properties of the coal. The results indicate that the dynamic mechanical properties non-linear positively correlate with the porosity, and its sensitivity to porosity gradually weakens as with the increase of the porosity. This effect is intuitively expressed in the dominant fracture propagation features recorded by a high-speed camera, and microscopically attributed by the size and number of microfractures around the dominant fracture. The dynamic stress intensity factor model of the dominant fracture was further established with consideration of total cohesive force for the equivalent particle on the fracture surface. It well reveals the effect mechanism of porosity on the meso-deterioration and macroscopic mechanical properties of the coal around the roadway. On this basis, the applicable conditions and suggested methods were put forward to improve the dynamic stability of water-saturated coal with various porosity.

Sulfate diffusion in coal pillar: experimental data and prediction model

The stability of coal pillar dams is crucial for the long-term service of underground reservoirs storing water or heat. Chemical damage of coal dams induced by ions-attacking in coal is one of the main reasons for the premature failure of coal dams. However, the diffusion process of harmful ions in coal is far from clear, limiting the reliability and durability of coal dam designs. This paper investigates sulfate diffusion in coal pillar through experimental and analytical methods. Coal specimens are prepared and exposed to sulfate solutions with different concentrations. The sulfate concentrations at different locations and time are measured. Based on experimental data and Fick’s law, the time-dependent surface concentration of sulfate and diffusion coefficient are determined and formulated. Further, an analytical model for predicting sulfate diffusion in coal pillar is developed by considering dual time-dependent characteristics and Laplace transformations. Through comparisons with experimental data, the accuracy of the analytical model for predicting sulfate diffusion is verified. Further, sulfate diffusions in coal dams for different concentrations of sulfate in mine water are investigated. It has been found that the sulfate concentration of exposure surface and diffusion coefficient in coal are both time-dependent and increase with time. Conventional Fick’s law is not able to predict the sulfate diffusion in coal pillar due to the dual time-dependent characteristics. The sulfate attacking makes the coal dam a typical heterogeneous gradient structure. For sulfate concentrations 0.01–0.20 mol/L in mine water, it takes almost 1.5 and 4 years for sulfate ions to diffuse 9.46 and 18.92 m, respectively. The experimental data and developed model provide a practical method for predicting sulfate diffusion in coal pillar, which helps the service life design of coal dams.

Energy Behaviour of Coal Failure under Uniaxial Cyclic Loading/Unloading

Coal failure is often the precursor of dynamic disaster. The energy evolution behaviour at different stress values was analysed under the gradation of equal amplitude cyclic loading/unloading. Based on the energy dissipation behaviour, the energy evolution model of the coal specimen was established. The multi-parameter energy behaviour predicting model was proposed. Then, the energy storage factor criterion, the energy tangent factor criterion, the energy dissipation growth factor criterion and the energy damage factor criterion of the coal specimen were proposed during the coal fracture process. The energy density and the energy storage status showed different evolution patterns under cyclic loading/unloading. The energy behaviours and status were different in fracture stages of coal specimens, and the dissipated energy behaviour had a sudden response during the failure process. The energy storage and energy dissipation mechanism were related to their respective limits. The energy storage mechanism showed a growth pattern of “low energy promotion, high energy suppression and dissipation promotion, cumulative suppression”. The damage evolution equation and the energy behaviour evolution criterion were established under the cyclic loading/unloading.

Fracturing Behaviors and Mechanism of Serial Coal Pillar Specimens with Different Strength

The fracturing behaviors of serial coal pillars is significant for understanding their failure mechanism. To reveal this, the bearing stress, acoustic emission, electrical resistivity, local strain, force chain distribution, and cracks evolution of serial coal pillars under uniaxial compression were evaluated by experiment and numerical simulation. The results show that four bearing stages are observed during the fracturing process (i.e., nonlinear growth, linear growth, yielding growth, and weakening stages). The acoustic emission features, electrical resistivity responses, strain develops, force chain distributions, cracks evolutions, and local displacement are highly consistent to illustrate the fracturing behaviors. System fracturing of serial coal pillar specimens is appeared along with the collapse of lower uniaxial compressive strength coal pillar specimen. The limit bearing capacity of serial coal pillar specimens is almost equal to the strength of lower uniaxial compressive strength coal pillar specimen. The unbalanced deformation characteristics of serial coal pillar specimens are presented due to the strength differences. The evolution of the key deformation element is the rooted reason for the overall fracturing mechanism of serial coal pillar specimens. For serial coal pillar specimens with different strengths, the critical condition of system fracturing is that the sum of secant modulus of upper and bottom coal pillars is zero, which is expected to predict the system fracturing of serial pillars in the underground coal mining.

Pore-scale hydro-mechanical modeling of gas transport in coal matrix

We present a 3D model coupling a discrete element model and a pore network model specifically developed to describe the different diffusion mechanisms at stake in coal matrix as well as the associated adsorption induced deformations. The material is assumed to be saturated with gas and diffusion occurs through the combination of Knudsen diffusion within the pore space, surface diffusion at the solid surface, and adsorption–desorption at the pore-solid interface. The model is hydro-mechanically coupled in the sense that changes in pore pressure produce hydrostatic forces that deform the solid skeleton, while deformation of the pore space induces pore pressure changes that promote interpore flow. Sorption induced deformations are taken into account by considering an additional pressure term related to the concentration of gas within the medium (the so-called solvation pressure). The implemented transport models are verified against analytical solutions describing diffusion in porous media with and without sorption–desorption, and a comparison is made with a swelling experiment performed on a coal specimen to illustrate the relevance of the proposed approach for describing adsorption induced deformation. As a result, this new pore-scale model offers a precise way to assess coal matrix sorption induced deformation and contributes to the knowledge of CBM storage and transport processes.

Piston mechanism of interaction of non-linear geomechanical and physicochemical gas exchange and mass transfer processes in coal-bearing rocks

The article focuses on a theoretical and experimental framework for the quantification of interaction between nonlinear geomechnical and physicochemical processes in high-stress coal-bearing rock mass during mining under high seismic risk due to large-scale blasting and earthquakes, as well as because of structural and temperature effects. The tests were aimed to examine and study comprehensively the piston mechanism of gas exchange and mass transfer processes, revealed recently at the Institute of Mining, SB RAS, as well as to explain the fact that the earthquake-induced low-velocity (quasi-meter range) pendulum waves (velocity to 1 ​m/s and frequency of 0.5–5 ​Hz) could stimulate an increase in the gas content in coal mines. In order to perform laboratory investigation at the Institute of Mining SB RAS, special-purpose stand for analyzing gas exchange and mass transfer processes in coal-bearing geomaterials under various thermodynamic conditions (P, V, T) and gas composition was constructed in cooperation with the Institute of Semiconductors Physics SB RAS. Matching of air flow rate with compression pressures allowed to obtain relations showing that air flow rate increases at the uncertain time interval under the increasing of the compression pressure. The same measurements was carried out with another gases such as Hydrogen H2, Helium He, methane CH4, carbon dioxide CO2 and carbon oxide CO. The laboratory tests aimed to detailed investigation of the previously revealed “piston mechanism” of gas exchange and mass transfer processes in the coal specimens and their quantitative description in terms of theory of the pendulum waves were carried in the first time. Consequently, there are some arguments for the testing of the opportunity of quantitative description of the “piston mechanism” related to gas exchange and mass transfer processes in the scale of coal mines. It is relevant when pendulum waves induced by powerful earthquakes and technical blasting reaches the mine.

A new digital surface crack image monitoring technology for coal failure

AbstractThe experimental study on the variation law of coal fracture and stress was carried out in the laboratory and engineering fields, respectively. A multiparameter monitoring system including electromagnetic radiation and a high‐speed camera was built, and three stress paths, uniaxial compression, cyclic loading, and graded loading, were used to monitor the dynamic expansion process of surface cracks during uniaxial compression failure of coal specimens. Through the quantitative analysis of the electromagnetic radiation signal in the crack propagation process, it is found that the electromagnetic radiation and the stress change trend are consistent, and the electromagnetic radiation signal is ahead of the failure of coal‐generating rock mass 1–2 s. The surface crack changes after the peak value of electromagnetic radiation and presents a stepwise growth trend, crack length changes on the millisecond time scale, and the crack propagation speed is about 2000 mm/s. The surface cracks appear when the stress reaches a certain degree, and the propagation of the principal cracks is consistent with the failure of the specimen. The electromagnetic radiation value of the coal mass from static period to dynamic is analyzed by using electromagnetic radiation at the engineering site, and it was found that the electromagnetic radiation is consistent with the stress distribution of the mining face. Through normalization, the variation rule of electromagnetic radiation in laboratory and engineering sites is similar, but the peak value of electromagnetic radiation in engineering sites is more significant. Therefore, electromagnetic radiation has a good monitoring effect on the stress distribution and cracks propagation of underground coal mining working faces, which could guide the layout of underground drilling.

Research on Particle Size and Energy Consumption Law of Hard Coal Crushing under Impact Load Based on SHPB Test

To study the particle size distribution and energy variation law of hard coal under a load, an impact compression test of hard coal specimens under different impact loading conditions was carried out using a Φ50 mm diameter Separate Hopkinson Press Bar (SHPB) test system. We implemented the theory of dynamic impact energy of rock to establish the calculation expression of hard coal impact crushing energy dissipation, and we established the Weibull distribution model of a crushing body to analyze the impact velocity in relation to the particle size distribution of hard coal crushing and crushing energy consumption. The results demonstrate that due to the different original states of the specimens, the damage to the specimens under static action is in the mode of conjugate plane shear damage, single bevel shear damage, and tensile damage. The damage process of the specimen under impact load loading is divided into three stages: elastic deformation, elastic–plastic deformation, and plastic softening, while the increase in the strain rate caused the peak stress of the specimen to increase. The Weibull distribution can characterize the impact crushing size distribution of hard coal specimens very well. The parameter of coal rock crushing degree is a power function that is influenced by the impact velocity; the greater the impact velocity, the higher the coal rock crushing degree, but the characteristic index of coal rock crushing fluctuates with the increase in impact velocity. As the impact velocity increases, the incident energy and reflected energy increase linearly, while the transmitted energy increases first and then decreases. The dissipation energy of coal rock crushing also increases linearly with the impact velocity. There is no obvious regular change between the energy dissipation rate of coal rock and impact velocity during impact damage, and the dissipated energy of macroscopic crushing only accounts for 10~20% of the incident energy; most of the energy is used for damping loss and damage loss.

Investigation on the Fragmentation and Outburst Mechanism of Coal Sample with Pore Gas Using CDEM

In this paper, using the continuum-discontinuum element method (CDEM), the fragmentation and outburst process of coal specimen are simulated, and the main factors affecting coal breaking and outburst are explored. The results show that after the coal seam is uncovered, coal generates obvious failure and outburst trend. Near coal-free surface, the fracture coal blocks generate significant displacement, resulting in larger opening widths of coal cracks. Coal deep generates the cracks without an obvious opening width. The crack density of coal with pore gas is larger than those of coal without gas, and it is larger than those of coal without pores. However, in the early stage of coal failure, the obvious separation and outburst ranges of coal with gas are smaller than those of coal without gas, and are smaller than those of coal without pores. The numbers of fracture coal blocks show an increase with the growth of in situ stress, pore ratio and gas pressure. The effect of in situ stress on fracture coal block number (517–10,203) is larger than the effect (7589–15,170) of pore ratio and is larger than the effect (5803–6836) of gas pressure. The effect of in situ stress on a maximum size (0.0387–0.138 m) of fracture blocks is larger than the effect (0.0342–0.0733 m) of pore ratio and is larger than the effect (0.0454–0.0578 m) of gas pressure. The coal outburst velocity and range show an increase with the growth of gas pressure and in situ stress (3.77–5.65 m/s); however, the coal outburst shows a slow decrease with a growth of pore ratio. The effect of gas pressure on the coal outburst velocity (11.51–21.9 m/s) is larger than the effect (3.77–5.65 m/s) of in situ stress and is larger than the effect (4.52–5.23 m/s) of pore ratio. This investigation is beneficial to understand the mechanisms of coal–gas outburst in coal mining and roadway excavation.

Stress Sensitivity Analysis of Seepage Pores in Coal Briquettes Based on Compressed Exhaust and Pressure-Transient Tests

Seepage pores are the main space for methane transport in the coal seam. To further investigate variation laws of seepage pores with stress, this study established a new method based on the compressed exhaust and pressure-transient tests to characterize the seepage pore size distribution under different effective stress conditions. The reconstituted coal briquette and nonabsorbent helium were adopted as the experimental samples. The results showed that the deformation of coal mass mainly originates from seepage pores. With the increase of confining stress, the volume reduction of seepage pores presents a logarithmic growth. The initial seepage porosity of the tectonic coal specimen, i.e., the seepage porosity when effective stress is zero, was estimated to be 5.28%, accounting for 36.24% of the total porosity obtained by helium saturation. When effective stress is in the range of 13–48 MPa, the size of seepage pores generally ranges from 0.1 to 1.0 μm. The volume of seepage pores with sizes ranging from 0.1 to 0.5 μm always accounts for more than 80% of the volume of total seepage pores, and this proportion is not affected by effective stress. The inverse proportional coefficient between the threshold pressure gradient of helium and seepage pore diameter was finally determined to be 0.55 ± 0.19 mPa.

Drilled-hole number effects on energy and acoustic emission characteristics of brittle coal

Energy accumulation and release are the essence of rock deformation and damage. Thus, investigating the damage mode and energy evolution law of coal specimens containing holes to elucidate the mechanism of borehole pressure relief is important. To examine the effect of the number of drilled holes on the energy and acoustic emission (AE) evolution characteristics of brittle coal, uniaxial compression AE tests were performed using four types of drilled coal specimen. The results revealed that the peak strengths and elastic moduli of the specimens exhibited nonlinear and linear decreasing trends, respectively, with the increase in hole number; however, the peak strain initially decreased and subsequently increased. The failure mode changed from shear splitting to penetration crack damage between rock bridges, accompanied by a hole collapse phenomenon. As the hole number increased, the total and elastic energy exhibited a decreasing trend, whereas the ratios of the elastic and dissipative energy to total energy exhibited a decreasing and an increasing trend, respectively. The AEs of different types of drilled coal specimens had similar evolutionary processes, but their activity exhibited an overall increasing trend with an increase in hole number. The AE fractal characteristics of a drilled coal specimen were calculated based on the G-P algorithm. The AE signal of the specimen exhibited fractal characteristics, and the sudden drop in fractal dimension was considered the precursor of specimen failure. The findings provide a reference for understanding the energy evolution mechanism and advance warning for coal pressure relief.

Strength Damage and Acoustic Emission Characteristics of Water-Bearing Coal Pillar Dam Samples from Shangwan Mine, China

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam’s standing sustainability. In the paper, the coal pillar dam body of the 22 layers of coal in the Shangwan Coal Mine is studied (22 represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions.

Experimental Study on the Dynamic Tensile Characteristics of Coal Samples.

As coal mines continue deep mining, the frequency of coal and rock dynamic disasters has also gradually increased. In this paper, dynamic tensile strength deformation, energy evolution, and crack development under an impact test were studied on Brazilian coal samples, using a split Hopkinson pressure bar (SHPB) test device. A high-speed camera was adopted to capture the failure process of the coal specimens. The research results demonstrate that when the impact velocity is greater than 4.75 m/s, the dynamic tensile strength of the vertical bedding direction is higher than that of the parallel bedding direction of the coal samples. With the increase in the impact velocity, the dynamic strain and ultimate strain rate of two types of coal samples are increased, and the average value of the first and second dynamic deformation moduli of coal samples shows decreasing characteristics. As the incident energy increases, the sum of reflected and transmitted energy increases, and the absorbed energy also increases in the two types of coal samples. The two types of Brazilian disc coal samples mainly showed tensile and shear failure characteristics. The dynamic tensile deformation characteristics of the two types of coal specimens are less affected by the impact angles. However, the crack propagation of coal samples was mainly influenced by the impact angles. The test results can be used for the prediction of coal and rock outburst in deep underground coal excavation.

Study on the influence of confining pressure and unloading damage on the bursting liability characteristics of coal

The research on the bursting liability of coal under confining pressure and unloading damage is critical in creating prevention mechanisms for coal mass rock bursts in deep underground mines. Cyclic loading and unloading tests of variable stress with a lower limit were performed under multistage confining pressure and different amplitude unloading to explore their influence on the impact tendency of the coal bodies. Meanwhile, the characteristic parameter analysis of acoustic emissions was used to evaluate the failure. The results revealed that the accumulated number and energy of acoustic emission events gradually decreased with increasing the confining pressure. The coal specimen became denser, and the failure mode gradually transitioned from brittle to ductile. With the increase in unloading amplitude, the cumulative number of acoustic emission events in the coal specimens decreases, the damage degree to the coal body increases, the peak load decreases, and the failure mode transitions from ductile to brittle. The increase in confining pressure results in an increase in the input energy and the elastic strain energy, while the increase in the unloading range of the coal body leads to a decrease in the input energy and elastic strain energy. In addition, after the confining pressures of 3 MPa, 6 MPa, and 9 MPa, the residual elastic energy index of the coal specimens increases by 21.76%, 42.92%, and 71.69%, respectively, compared with the room pressure conditions. The residual elastic energy index decreases by 21.11% and 55.38% for the unloading amplitude of 3 MPa and 6 MPa, respectively, compared with the unloaded coal specimen, indicating that the impact tendency of the coal body is enhanced by the confining pressure conditions.

Development and application of gas adsorption model for coal based on particle flow code

This study investigated the deformation characteristics and engineering properties of gas-bearing coal. Based on the particle flow discrete element method, the gas was regarded as a flexible solid particle, and the relationship between the radius, elastic modulus and gas pressure of the flexible gas particle was derived through the ideal gas equation. Then, a gas adsorption model for coal based on the Langmuir equation was developed by coupling gas particles with rigid coal particles and applied to the coal and gas outburst evolution process. The results revealed that the flexible gas particles can effectively simulate the flow and deformation of gas, and the gas adsorption model exhibits adsorption deformation characteristics consistent with those of real coal specimens. Moreover, the adsorption model can indicate the adsorption saturation of coal particles at any moment, and the distribution characteristics of gas pressure within the microscopic pores, which is helpful to study the microscopic mechanism of gas adsorption process. Finally, the gas adsorption model for coal was applied to the coal and gas outburst and good simulation results were obtained, which further validated the reasonableness of the gas adsorption model. These studies promote the application of the particle method in the direction of gas-solid coupling research, and also provide a theoretical and numerical basis for the prevention and control of gas disasters.

Numerical Investigation into the Mechanical Behaviours and Energy Characteristics of Hard Coal Subjected to Coupled Static-Dynamic Loads

In practical engineering, coal burst is usually caused by the combination of high geo-stress and dynamic loading. To study the dynamic response of coal in geo-stress conditions, numerical models of a coupled static–dynamic split Hopkinson pressure bar (SHPB) test system were established, based on which impact tests for coal specimens at different impact speeds and static pre-stress levels were conducted. The mechanical properties, energy characteristics and failure patterns of coal specimens under coupled static and dynamic loads were analyzed. The results show that when the pre-stress is constant, peak stress, the maximum strain energy and the maximum kinetic energy increase significantly with impact speed. Nevertheless, they are less affected by the static pre-stress, increasing linearly with a pre-stress level under lower impact speeds but becoming stable under higher impact speeds. In addition, weak dynamic loads may trigger the instability of the coal specimen in a high pre-stress condition. Overall, both the impact speed and static pre-stress have influence on the mechanical behavior and energy characteristics of coal specimens under coupled static and dynamic loads, but the influence of the impact speed outweighs that of the static pre-stress.