Coal Deformation Research Articles

Exploring the ultramicropore structure evolution and the methane adsorption of tectonically deformed coals in molecular terms

The mechanical deformation of coals occurring extensively during the geological period (tectonically deformed coals) can directly alter their pore structures and then the storage of coalbed methane. This study in-situ investigated the effects of different mechanical deformations on the ultramicropore structure and the methane adsorption of coal molecules using molecular simulations. The results show that the shear deformation (< 0.23 GPa) of coals was much easier than the compression (~ 20 GPa). Further, the shear deformation can increase the void fraction (200%) and the surface area (30%) of coal molecules, comparing to the reduction of them by the compressive deformation. Accordingly, compression is not benefited to the methane storage (only remaining 14-22% adsorption amount). While, the shear deformation of coals can increase the methane adsorption amount (reaching 42–50 mmol/g). The ~ 7.5 Å is a key pore size to evaluate the effect of the shear deformation on the methane adsorption amount. Also, the adsorption sites for methane depends on the deformation mode of coals (compression: heteroatoms; shear: C atoms). Overall, the strained Wiser (bituminous, medium-rank) coal shows relatively superiority in the methane storage, while the methane adsorption of Wender (lignite, low-rank) coal is much more sensitive to the mechanical strain.

Study on the evolution rules of coal deformation and failure characteristics caused by multiple mining disturbances

During coal mining operations, the coal will be deformed and damaged due to multiple mining disturbances (MMD), often resulting in disasters, like rock burst. To understand the evolution rules of coal deformation under MMD and its final fracture characteristics after impact dynamic load loading, reduce the adverse effects of mining disturbances, and improve disaster prevention and control capabilities, quasi-static uniaxial cyclic loading-unloading (L-U) and dynamic axial compression tests were conducted on large-sized coal-like samples. During the tests, three-dimensional (3D) laser scanning and acoustic emission (AE) monitoring technology were utilized to accurately capture the full-field deformation and AE response data, facilitating a systematic analysis of deformation and fracture characteristics. The results show that: (1) Under the cyclic L-U effect induced by MMD, each loading cycle causes compression deformation with partial recovery during unloading, presenting an overall “wavy” variation trend. (2) The maximum load is the most critical factor affecting the damaged coal deformation, with smaller load resulting in less overall sample deformation. (3) After the impact dynamic loading, the damaged samples suffered large-scale impact splitting failure, with the compressive-shear layer failure mainly occurred inside the holes. (4) Lower loading during cyclic L-U process correlate with reduced damage degree, and smaller debris particles with a higher fractal dimension when impact failure occurs, indicating a more severe impact failure. (5) With multiple cycles of L-U, the cracks inside the sample gradually extend and expand from around the hole to the outside. The greater the load and the number of cycles, the more serious the crack damage will be. (6) In the practical mining process, it is crucial to reinforce roadway interiors while minimizing low-loading cyclic disturbances induced by MMD. The study has obtained the deformation evolution rules and failure characteristics of coal under MMD, providing a theoretical basis for the prevention and control of corresponding engineering disasters.

The effect of cutting speed on the generation of bituminous coal dust: Experimental and theoretical discussion

To address the issue of an unclear mechanism for dust generation, an experiment was conducted to investigate the dust generation and migration under varying cutting speeds of a single cutting tooth. The findings indicate that coal dust production involves a rapid physical transformation process comprising four stages: coal body deformation, formation of the crushing zone, stress release, and collapse of fractured material. Increasing the cutting speed results in a continuous rise in both the total dust production rate and the respiratory dust rate. At a cutting speed of 10 mm/s, the RF coal sample exhibited a total dust production rate of 32.41 g/t, while the HP coal sample had a total dust production rate of 93.10 g/t. The moisture content of the coal samples exhibited a negative correlation with the total dust production rate, whereas the fixed carbon content and porosity showed a positive correlation with this rate. However, the size of the porosity had no significant impact on the production rates of respirable dust and PM2.5 dust. These research findings enhance the understanding of the dust production mechanism during coal and rock cutting, providing guidance for dust control at the origin.

Technology and application of through layer borehole ultra-high pressure hydraulic slotting in soft coal seam to pressure relief and permeability enhancement

ABSTRACT It is a vital method that controls outburst disasters through gas drainage to reduce gas pressure. But the permeability of coal seam in China is low. Therefore, technologies that can decrease pressure and increase permeability such as ultra-high pressure hydraulic slotting (UPHS) are widely applied. Based on the technology of UPHS, the principle of coal breaking and mechanism of pressure relief and permeability enhancement (PRPE) were first analyzed. The effects of PRPE between contrast borehole and UPHS were compared through numerical simulation. Finally, combined with the field test, the parameters of UPHS were determined, and difference in gas drainage effect between contrast borehole and UPHS was investigated. It is found that the exposed area of coal increased due to macroscopic slotting formed by hydraulic slotting, which provide space for coal deformation. The stress of coal relieved the permeability enhanced, resulting in the gas drainage efficiency increasing. The simulated results showed the pressure relief area and plastic deformation caused by UPHS are significantly increased compared with contrast borehole. The average concentration and total of gas drainage in UPHS are 1.49 and 3.13 times than that of contrast borehole. And radius of gas drainage of UPHS 1.77 times than that of contrast borehole under reduce the 46% engineering quantities.

Study on influencing factors of controllable mechanical behavior of rock-like porous materials.

Due to the discrete and non-homogeneous of similar materials and the inability to realize large-size and original scale modeling, it is difficult to restore the structure and stress state of underground coal and rock mass in similar simulation tests. To solve this problem, a lightweight and suitable for large-scale modeling similar material, rock-like porous material has been developed. The quasi-static uniaxial compression experiment was carried out by using the large tonnage multi-module electronic control test system. And the influencing factors of controllable mechanical behavior of rock-like porous materials were studied. The results showed that, under uniaxial compression conditions, the material stress-strain curve exhibits three phases: elastic stage, failure stage, and platform stage. The uniaxial compressive strength, elasticity modulus, stress drop, and softening modulus of rock-like porous materials basically increase with the increase of density. The stress after peak strength changes from a slow decrease to a "stepped" or even "cliff like" downward trend. Polypropylene fibers have the effect of enhancing the uniaxial compressive strength, elasticity modulus, stress drop, softening modulus, shear deformation, and residual strength stability of rock-like porous materials. The rock-like porous material has a critical loading velocity, and it increases with density. At the critical loading velocity, the material shows obvious shear failure, and the shear inclination angle is the largest, and so is the uniaxial compressive strength. Through the experimental research, the influence laws of density, polypropylene fiber, and loading velocity on the failure mode, mechanical parameters, and mechanical behavior of the material are clarified, and the quantitative relationship between density and each mechanical parameter is obtained. The research is helpful to realize the accurate control of mechanical behavior of rock-like porous materials and further inverts the deformation and failure mechanism of underground coal and rock structures through indoor similar simulation tests.

Heat transfer-deformation characteristics and fracture damage analysis during LN2 freeze-thaw process in different rank coals

Exploring the heat transfer and deformation characteristics of coal bodies of different coal ranks during the freeze-thaw process is of significant importance for analyzing the fracture mechanism under the effect of liquid nitrogen (LN2). This experiment targets lignite, bituminite, and anthracite under both saturated and dry conditions. A real-time temperature-strain monitoring system was employed to observe the heat transfer and deformation characteristics of coal samples with different ranks throughout the freeze-thaw cycle. Additionally, a nuclear magnetic resonance system was utilized to examine the characteristics of pore damage before and after fracturing. The findings reveal: (1) During the freeze-thaw process, the absolute value of the temperature evolution rate for dry coal samples shows a negative correlation with coal rank, indicating a close link between temperature diffusion and intrinsic coal properties like oxygen content and porosity. (2) For saturated coal samples, the absolute value of the temperature change rate during freezing decreases as the coal rank increases, with the opposite trend observed during thawing. The phase change effect of water in fractures during freezing can enhance internal temperature diffusion in the coal body, while it acts as an inhibitor during thawing. (3) Based on the trend of strain fluctuations, the coal body deformation process during the freeze-thaw cycle can be segmented into seven stages, summarizing the general mechanisms of deformation failure. (4) Under saturated conditions, the amplitude of elastic deformation for each sample is negatively correlated with coal rank, with the sequence for dry coal samples being bituminite > anthracite > lignite. (5) The formation of a sealed space at the beginning of freezing is identified as a necessary condition for deformation during the freeze-thaw process, with the formation and strength of the sealed space depending on the temperature diffusion rate, moisture content, and inherent properties of the coal sample.

Feasibility of carbon dioxide geological storage in abandoned coal mine: A fully coupled model with validated multi-physical interactions

It has gained wide attention that Carbon dioxide (CO2) is to be injected into abandoned coal mines for geological storage of CO2-enhanced coalbed methane recovery. Although abundantly evidences in literature indicate that the injection of CO2 will cause lots of interactions among the mechanical characteristics of coal and the properties of CO2 flow, further studies on these multi-physical interactions are still necessary. In this work, a series of laboratory experiments to elucidate the multi-physical interactions of CO2 adsorption, softening effect of coal and the non-Darcy gas flow were conducted. Based on the experimental results, theoretical and empirical models to describe these coal-CO2 interactions were meticulously proposed and validated, the results turned out to be satisfactory. Consequently, the compressive strength and elasticity modulus of coal decrease exponentially with the increased injection CO2 pressure. The gas flow in coal obeys the Izbash non-Darcy model, and coal permeability can be well modified by the volumetric stress. By taking these coal-CO2 interactions into account, this study established of a fully coupled model for coal deformation and CO2 conservation. The model was then implemented into the numerical simulations of CO2 storage in abandoned coal mine by using the finite element method. A series of scenario-based numerical simulations was conducted to investigate the feasibility and limitation of CO2 storage in abandoned coal mine. The conducted experiments, models and numerical simulation will offer implications on the multi-physical interactions between coal and gas especially in CO2 storage in abandoned coal mines.

The mechanism and prevention of rockburst induced by the instability of the composite hard-roof coal structure and roof fractures

Despite the implementation of prevention and control measures, rockburst still occur. The characteristics of composite coal were analyzed by combining monitoring and early warning means. Two types of composite coal in the hard roof were distinguished based on their distinct initiation and destruction processes despite experiencing the same rockburst event. Rockburst caused by structural instability was studied in different mining stages, and the following conclusions were considered. 1) The mechanical model of the instability of composite coal in hard roof was established for different mining stages to reveal rockburst mechanisms. The high-speed advance of working faces changed the fractured structure of upper hard rocks in the pressure-relief protection range. The sudden fracture of long cantilever structures caused dynamic load disturbance to high-stress coal, which resulted in rockburst in the insufficient mining stage. The failure and deformation of coal under high-stress static loads created conditions for the instability and fractures of roof. The disturbance of low hard rock strata aggravated the deformations of damaged coal. Therefore, rockburst appeared in the full mining stage. 2) Structural instability and roof fracture caused by different factors were the main causes of rockburst for composite coal in hard roof in different mining stages. Relevant prevention and control measures were formulated to ensure the safety of working faces, which provided references for the prevention and control of rockburst in working faces under similar conditions.

Elastoplastic analysis on deformation and failure characteristics of surrounding rock of soft-coal roadway based on true triaxial loading and unloading tests

Since accidents such as roof caving, rock fragmentation, and severe deformation are particularly likely to occur during roadway excavation in soft and thick coal seams, grasping the range and distribution of deformation and fracturing of surrounding rock is of crucial for evaluating roadway stability and optimizing support design in such coal seams. In this study, based on the stress paths encountered during roadway excavation, true triaxial loading and unloading tests were carried out on soft coal, and the deformation and strength evolutions of soft coal under different intermediate principal stress conditions were analyzed. The test results show that the stress–strain relationship in the pre-peak plasticity-strengthening and post-peak plasticity-weakening stages follows a quadratic function, and the strengeth evolution conforms to the Mogi–Coulomb criterion. Moreover, analytical solutions for the displacement of surrounding rock, the radius of the broken zone, and the radius of the plastic zone of soft-coal roadways under excavation stress paths were derived after taking the nonlinear hardening and softening characteristics of the strain of soft coal, the Mogi–Coulomb criterion, the intermediate principal stress, and the dilatancy characteristics of surrounding rock into comprehensive consideration. Finally, in accordance with a practical engineering case, the influences of the intermediate principal stress coefficient, the lateral pressure coefficient, and the support force on the deformation and failure characteristics of the soft-coal roadway were analyzed. The analysis reveals that an increase in intermediate principal stress aggravates the deformation of surrounding rock and enlarges the plastic and broken zones; variations in the lateral pressure coefficient alter the shape of the broken zone and the distribution of surface displacement; and an increase in the support force effectively reduces the plastic zone, broken zone, and surface displacement of the roadway. The research results can provide valuable theoretical basis for the stability evaluation and support design of soft-coal roadways.

Evolution characteristics of an induced electric charge signal and identification method of charge for instability failure in loaded coal

A coal-loaded charge induction monitoring system is developed to effectively forecast the dynamic disasters caused by coal failure. Specifically, a digital finite impulse response (FIR) filter is designed to denoise and filter the signal, and the time-frequency domain evolution of induced charge signals is analyzed during coal failure experiments. The quantitative relationships between the induced electric charge and stress-strain energy, and ultimately, between induced electric charge and coal deformation/failure, are revealed. Ultimately, the electric charge sensor exhibits high signal collection frequency and high sensitivity, and the FIR low-pass filter constructed in MATLAB effectively denoises and filters induced charge signals. The main frequency range of the white noise is 50–500 Hz, and the main frequency of the charge signal induced by coal deformation and failure is concentrated in the range of 0–50 Hz. The optimal distances for monitoring cubic and cylindrical raw coal samples using this sensor are 9 mm and 11 mm, respectively. Notably, strain energy is released faster when it can dissipate more readily, and induced charge pulses become denser when more intense signals produce large fluctuations. A method is proposed to identify coal deformation and failure based on changes in the induced electric charge. This study provides a new means of monitoring the early warning signs of dynamic coal mine disasters. Based on our experimental results and conclusions, a new method is proposed to identify coal deformation and failure based on changes in the induced electric charge. The precursor to the moment of coal failure can be identified by monitoring the amplitude of the induced charge, the dynamic trend of fluctuation, and the cumulative number of induced electric charge pulses during the process of coal deformation.

Molecular simulation of free CO2 injection on the coal containing CH4 structure and gas replacement

Understanding the replacement process of pre-adsorbed CH4 by injecting CO2 into the coal slit pores is crucial for CO2 storage and coalbed methane (CBM) extraction. We constructed the model of free CO2 injection into coal containing CH4, and used molecular simulation methods to analyze the replacement and diffusion of gas in coal as well as the coal structure deformation at different CO2 injection pressures. Results indicated that the CO2 adsorption following injection aligned with Langmuir law, and CO2 replaced CH4 more efficiently at 6 MPa injection pressure; After CO2 injection, the coal slit pore volume increased, the coal matrix aromatic structure destroyed, the coal matrix micropores volume increased and expanded radially and compressed longitudinally. Among them, the coal matrix deformation facilitated CO2 adsorption whilelittle impacting the CH4 desorption within the micropores. Higher CO2 injection pressure reduced the diffusion coefficients of adsorbed and free CH4 and CO2, but increased the CO2 adsorption depth in the coal matrix and enhanced the CO2 adsorption stability. The results provided valuable insights into the potential mechanisms of CO2-ECBM.

Coal deformation characteristics during methane displacement by pulsed nitrogen injection: New method of using pulsed energy to improve coal permeability

The displacement of methane by injecting nitrogen is an effective method for improving extraction efficiency in low-permeability coal seams. By conducting the experiments on uniform/pulsed pressure nitrogen injection for methane displacement (UPNI-MD/PPNI-MD), the coal deformation characteristics are quantitatively characterized, and the coal deformation mechanism is discussed. The results show that the technology of methane displacement by PPNI-MD is more effective in enhancing the permeability of low-permeability coal seams. The pressure of nitrogen injection and the amount of methane/nitrogen adsorption are crucial factors affecting the coal deformation during the process of UPNI-MD/PPNI-MD. The erosive effect of methane adsorption on the mechanical properties of coal decreases, while the erosive effect of nitrogen adsorption on the mechanical properties of coal shows a “wave-like” variation in the case of PPNI-MD. The application of PPNI-MD technology can effectively decrease the coal expansion deformation while undergoing the methane displacement process. Nitrogen injection to displace methane breaks the system energy balance of coal, resulting in deformation. And the coal deformation during nitrogen injection for methane displacement depends on the energy carried by the injected nitrogen, the energy change during gas competitive adsorption and the heat exchange between nitrogen and coal. The effective stress of coal depends on the change in methane/nitrogen pressure. The mechanical properties of coal depend on the amount of methane/nitrogen adsorption and the pore pressure. High-energy nitrogen damage to coal during the process of PPNI-MD is manifested in coal deformation caused by methane/nitrogen pressure, coal deformation caused by methane/nitrogen adsorption, and damage to coal mechanical properties caused by methane/nitrogen adsorption.

Effectiveness Spatiotemporal of Pressure Unloading and Gas Extraction in Remote Upper Protected Seam Mining.

Severe gas hazards are increasing in deep mining areas, and there exists an enormous traditional challenge for protected seam mining. In this work, we conducted a multifaceted investigation into the spatiotemporal effectiveness exerted by pressure unloading from a distant safeguarded stratum in concert with the application of gas extraction methodologies. The stress and displacement evolution in the protected coal seam (PCS) are analyzed by applying Flac3D, and then positives are verified by the measurement of coal seam deformation and investigation of the unloading boundary. The results show that diminution coefficient of 0.62, and record an expansive deformation rate of 5.83%. The opportune temporal window for effective gas drainage is discerned, with its time window spans from 58 to 67 days as the expanse between 350 and 400 m progresses at the working face. Continuous and effective gas extraction occurs when transverse cracks in the PCS reach an optimal state of pressure relief. The final cumulative gas extraction was 320,300 m3 with an extraction efficiency of 34.3%. A residual gas volume of 3.68 m3/t and a residual gas pressure of 0.5 MPa reflect the efficient gas extraction.

Research on coal wall failure and stability control technology of large coal seams with a soft and thick seam

AbstractTo address the issues of coal rib instability and high occurrence of roof falls in high extraction height fully mechanized coal mining of soft coal seams, this study comprehensively employs theoretical analysis, numerical calculations, and field industrial experiments. By analyzing the distribution characteristics of coal rib displacement and deformation instability zones under different mechanical parameters of soft coal seams, the instability mechanism of coal ribs in high extraction height mining of thick and soft coal seams is revealed, and corresponding stability control techniques for high extraction height coal ribs and supports are developed. The study shows that the stability of coal ribs in high extraction height mining fields of soft and thick coal seams decreases as the values of mechanical parameters decrease. In other words, the smaller the cohesion and deformation parameters of the coal body, the more prone the coal rib is to roof falls. By utilizing advanced deep‐hole static water infusion technology, the frequency of roof falls in extremely soft high extraction height working faces is reduced by 80%, the average depth of roof falls is decreased by 35%, and the average length of roof falls is reduced by 50%. The stability of the coal ribs is effectively improved.

Phase Transition and Vanadium Leaching Mechanism during Roasting of Vanadium‐Bearing Rock Coals

The phase transformation and vanadium leaching mechanism of stone coal during two blank calcined stages are systematically studied. Composition, structure, and morphology of the materials are analyzed by ICP‐AES, BPMA, XRD, FTIR, and SEM. The results show that the main components of stone coal are SiO2, C, aluminosilicate and FeS2, V2O5 is 0.70%. Two‐stage roasting is a process of deformation, melting, and reconstruction of stone coal, the oxidation of pyrite, decomposition of aluminosilicate, sulfation of calcite, combustion of carbon and the formation of calcium vanadate occur in the first stage, the oxidation of low‐price vanadium occurs in the second stage. At −0.074 mm ≥90%, liquid solid ratio 1.5:1, 10% sulfuric acid reaction at room temperature for 3 h, the vanadium leaching rate in each material is in the order of roasting slag&gt;decarbonized slag&gt;stone coal.

Study on temporal and spatial response of pressure stimulated current induced by coal deformation and rupture

The pressure stimulated current (PSC) signal emitted during the deformation and fracture processes of coal and rock materials holds significant importance for monitoring dynamic disasters in coal mines. However, the spatial response characteristics remain inadequately explored, posing significant challenges to the identification of risk-prone areas for underground dynamic disasters. To study the temporal and spatial response characteristics of PSC signal, the PSC response experiment in the failure process of raw coal under load was carried out, PSC signals from various spatial positions of coal were collected throughout the loading process. The results show that the sudden increase of axial PSC and the sudden decrease of transverse PSC appear with the sudden increase of acoustic emission (AE) hit rates, which is the main characteristic of the aggravation of coal internal fracture degree. The PSCs are closely related to the damage characteristics of the sample in terms of its corresponding position and drop time, which can better reflect the fracture concentration location of the coal sample. Building upon this foundation, the paper explained the internal factors contributing to variations in PSC spatial response characteristics of coal samples during load failure and emphasizes the inhibitory effect of crack development on PSC changes.

Micromechanical property evolution and damage mechanism of coal subjected to ScCO2 treatment

Carbon dioxide (CO2) injection into deep coal seams holds considerable significance in both carbon mitigation and enhanced recovery of clean coalbed methane (CBM). The CO2-coal interaction leads to changes in the physicochemical properties, potentially impacting the stability of the target sequestration coal reservoir. This study applied the nanoindentation, scanning probe microscopy (SPM), X-ray diffraction (XRD), and electron probe X-ray Micro-Analyzer (EPMA) techniques to probe the micromechanical property change and damage mechanism of coal subjected to ScCO2 injection. The result shows that the load-displacement behavior of the tested coal changes with continuous ScCO2 treatment, and the mechanical strength significantly weakens after 4 days of ScCO2 treatment. The nanoindentation test indicates that the peak and creep displacements of tested coal increased by 168.79 % and 1046.32 % respectively with 4-day ScCO2 treatment, inferring that the coal deformation increases and changes from elastic to plastic after ScCO2 injection. As the ScCO2 treatment time increases, Young's modulus and hardness of coal exhibit an exponential decay trend and rapidly decrease by 77.77 %∼89.76 % and 68.37 %∼81.63 % respectively after the initial 3-day treatment, followed by a slow decrease with the ScCO2 duration prolonging. The XRD and EPMA results show that the carbonate minerals reacted preferentially with ScCO2, and silicate minerals experienced gradual dissolution, while the amorphous carbon fluctuated almost unchanged in the tested coal. Carbonate minerals in tested coal have decreased by 70.68 % within the first 1-day ScCO2 treatment, making the most significant contribution to the initial rapid weakening of coal mechanics. The mineral distribution is closely related to the mechanical anisotropy of coal, which is confirmed by the phenomenon that the coal anisotropy gradually weakens with the mineral reaction process. It is inferred that the mineral component and distribution of coal seams are important indicators for evaluating the intensity of physical and chemical reactions in ScCO2-injected reservoirs, which directly determines the mechanical damage behavior of sequestration reservoirs. This research provides basic support for site selection and safety assessment of carbon sequestration in deep coal seams.

Investigation on coal permeability evolution considering the internal differential strain and its effects on CO2 sequestration capacity in deep coal seams

The gas adsorption/desorption-induced coal deformation effect is a significant factor governing the evolution of coalbed permeability. Current theoretical investigations typically coal bulk and fracture deformation induced by gas are equivalent, neglecting the matrix-fracture interactions. Based on internal adsorption stress, this paper proposes Internal Differential Strain Coefficient (IDSC) to quantitatively characterize the relationship between coal bulk and fracture strain under equilibrium conditions. Coupling this coefficient constructs a binary gas permeability evolution model considering matrix-fracture interactions. Through numerical simulations of CO2-ECBM processes under various internal differential strain circumstances using this model, dynamic evolution patterns of diverse parameters are obtained. The research findings indicate that along the direction of CO2 injection, matrix-fracture interactions exhibit a complex trend of initially increasing, then decreasing and then increasing, and the increase in internal differential strain levels results in a downward trend in permeability peak. Additionally, the evolutionary characteristics of CH4 recovery and cumulative CO2 storage rising with increasing internal differential strain levels were obtained on time scales using a fixed-point monitoring methodology. Inspired by the aforementioned laws, this paper discusses the macroscopic influence of burial depth on the effects of internal differential strain, providing new theoretical support for CO2 sequestration injection methods in deep coal seams.

Analysis of mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole

It is essential to elucidate the mechanical mechanism of water injection and large-diameter drilling pressure relief measures on coal rock deformation and failure. This helps coal mines adopt more precise and effective dynamic disaster prevention and control measures. To obtain the mechanical response mechanism and energy evolution characteristics of saturated coal with a single pre-existing hole, uniaxial compression tests was carried out, and the deformation and failure process of coal samples were monitored by high-speed camera and acoustic emission monitoring systems. It is found that water saturation, single pre-existing hole measures, and their coupling effect resulted in a deterioration in the coal sample's strength by approximately 23.49%, 9.47%, and 47.95% respectively. The final failure mode of coal samples with different measures is shear failure. The law of fracture development and expansion and energy accumulation and release in different coal samples is roughly the same, the speed of energy accumulation and release in different stages is different. Water saturation and single pre-existing hole measures can affect the impact risk of coal samples, and water saturation can significantly reduce the impact tendency of coal samples, and reduce the impact energy index of complete coal samples by 56.59%. The impact tendency index of natural coal sample is slightly increased by single pre-existing hole measure, and the impact energy index of intact coal sample is reduced by 46.78% by the coupling of the two measures. These findings can provide technical support for coal mine power disaster prevention and control.

Coherence analysis of the crack strain field in coal rock with borehole-crack composite defects

Cracks in the coal mass surrounding boreholes for gas extraction will significantly impact the efficiency of gas extraction. The primary objective of this research is to explore the effects of original cracks and borehole grouting on cracks propagation in the coal mass around the boreholes. Accordingly, this research employs coal mass with borehole, borehole-cracks composite coal mass, and borehole-cracks-grouting coal mass as experimental subjects. The digital image correlation (DIC) platform of the coal deformation and damage was used to carry out the damage experiment of the coal mass around the boreholes. The Matlab GUI crack detection system was used to extract the crack skeleton, and time domain characteristics of strain and strain rate coherence coefficients in the area near the cracks before and after grouting were obtained according to the Pearson product-moment coherence analysis method. The research findings obtained in this article are as follows: (1) The tips of the prefabricated fractures act as stress concentration zones, where the further extension of wing cracks severely degrades the continuity and uniformity of the coal mass. The injection of ultrafine cement significantly enhances the overall deformation resistance of the coal mass. However, the local stiffness under specific stress conditions shows limited improvement due to the presence of micro-damage around the prefabricated cracks. (2) The expansion of the cracks around the borehole and the wing cracks at the tips of the prefabricated cracks is primarily due to tensile forces. As the cracks extend to the top of the specimen, they transition to a mixed mode of tensile-shear, with the extension of Type II wing cracks being faster and more unstable compared to Type I. (3) The correlation coefficient between multiple crack fields and their highly significant characteristics indicate that crack propagation influences each other. This influence diminishes as the specimen progressively damage. (4) The mechanical properties of the specimen show a strong correlation with the average interference coefficient R̅ of the cracks. Better mechanical properties of the coal rock are associated with higher geometric similarity in the strain and strain rate curves across multiple strain fields, the cracks tend to propagate synchronously.