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Abstract After underground coal mining, rocks are often subjected to tensile damage by the interaction of dynamic and static loadings. The process of rock fracture development under dynamic and static loadings will be released in the form of acoustic energy to form an acoustic signal. In addition, the acoustic signals in dynamic loading differ from that in static loading. Therefore, this study conducted three-point bending experiments with continuous dynamic loading and dynamic–static coupling loading on semi-circular red sandstone specimens. The acoustic signals during red sandstone specimens’ tensile damage were monitored in real-time. The results show that red sandstone’s tensile strength and deformation are enhanced under dynamic–static coupling loading. The red sandstone has a more effective acoustic emission hit rate, energy rate, and r during tensile damage under continuous dynamic loading. In dynamic loading, macroscopic fractures are developed in red sandstone, which has few acoustic emission events but releases strong acoustic signals. In static loading, the pores inside the red sandstone are compacted, the rock particles are rearranged, and the tiny fractures are closed, and its acoustic emission events are many but low in energy. In addition, the rock particles in the front area of the static loading fracture are tightly cemented, which increases the difficulty of separating the rock particles in the front area of the fracture under dynamic loading. Then weakening the red sandstone fracture development process and suppressing its acoustic signals. The research results provide more insight into the differences in tensile damage processes in red sandstone under the interaction of dynamic and static loadings.

With the accelerated development of surface well cementing technology, the engineering site has higher requirements for the early fluidity and coagulation performance of surface well cementing water slurry. In order to improve the top-off efficiency of cement injection operation in cementing engineering and improve the application of cementing engineering field operation, the method of nanoalumina particles modified cementing cement was proposed, and the NMR technique was used to study the change characteristics of the hydration reaction of nanoalumina-modified cement slurries (with the water-cement ratio of 0.44) with the mass fractions of 0.00% wt, 0.05% wt, 0.10% wt, 0.15% wt, and 0.20% wt at different hydration ages were investigated to characterize the changes in hydration reactions, and to probe the aqueous phase distribution, changes in the total signal amount of physically bound water, peak shape index, and migration rate of the nanoalumina-modified cements. The results showed that the T2 patterns of nano-alumina-modified cement pastes at the hydration stage would show three relaxation peaks, corresponding to the flocculated structure-filled water (0.1−10 ms), capillary water (10 ms), and free water (800−1000 ms), respectively; and the total signal of the physically bound water decreased gradually with the increase of the mass fraction of nanoalumina, in which the nanoalumina-modified cement pastes with the mass fraction of 0.20%wt. Alumina-modified cement paste has the fastest reduction rate of signal; the peak shape index shows the trend of increasing and then decreasing, when the hydration reaction is carried out to 600 min, the modified cement paste with mass fraction of 0.20%wt has the fastest rate of change, and the rate of the relaxation peak moving to the short relaxation direction is accelerated; the combination of the migration rate of the nanoalumina-modified cementing cement hydration reaction stage division, and the hydration mechanism of its hydration mechanism respectively acted in the four Hydration period: ① initial hydration period (5−60 min), the hydration rate did not change significantly; ② accelerated hydration period (60−600 min), nano-alumina to make the physical binding water in the cement paste into chemical binding water in a shorter period of time, the hydration process is accelerated to achieve the effect of promoting the hydration of the cement; ③ stabilized hydration period (600−1200 min), the cement system tends to stabilize; ④ Delayed hydration period (1200−1800 min), the rate of conversion of physically bonded water to chemically bonded water slows down, and the hydration process of cement slows down. It was concluded that the hydration period of nanoalumina-modified cement paste can be divided into four stages, and nanoalumina plays the roles of physical filling, accelerating, stabilizing, and retarding for the cement paste, respectively. In the accelerating period, nanoalumina promotes the early flow of cement, which is conducive to the pumping and flow of cement paste, and in the retarding period, nanoalumina retards the further hydration process of cement, which is conducive to the development of early strength. Among them, the nano-alumina modified cement paste with a mass fraction of 0.20% wt has the greatest influence on the cement hydration process, promotes the conversion of filler water to bound water, and accelerates the hydration reaction of cement paste to the greatest extent during the accelerated hydration period, which serves as the basis for providing guidance for the preparation and application of cementing cements in the field.

The three-field (stress, displacement and fracture field) evolution laws of the inclined coal seam group are complex, which are important for the transport and storage of pressure-relief gas. In order to investigate the three-field evolution law of inclined coal seam group, the study carried out a similar simulation experiment of multiple mining in inclined coal seam group with the 1930 coal mine in Xinjiang as an object. The collapse pattern of the overlying strata was analyzed, the stress evolution characteristics of overlying strata was obtained, the overlying strata displacement distribution and movement direction characteristics were analyzed, and the characteristics of mining-induced fracture distribution were elucidated. The influence of the three-field evolution law on gas migration was further investigated, and a directional borehole gas extraction field test was carried out for veri-fication. The study results shown that, the rectangular ladder platform of mining-induced fracture shown obvious asymmetric characteristics under multiple mining of inclined coal seam group. The overlying strata stress of the lower side was more variable, and the pressure-relief effect was more obvious with increasing mining frequency, while the overlying strata stress of the higher side was less variable. Combined with the gravity-dip effect, the overlying strata of the higher side was more prone to be damaged, and the collapse order was preferred with asymmetric characteristics. The displacement distribution of overlying strata was asymmetric, with significant displacement on the high side and large changes in the movement direction. The frequency of mining-induced fractures in the high side fractured area was significantly higher than that in the low side. The high side fracture area had more fracture distribution and fracture aperture was bigger. The mining-induced fractures shown the asymmetric characteristics of “high expansion-low compression”. The multiple mining made the asymmetric characteristics of three-field more significant. In addition, there was a “slow decreasing-fast decreasing” in the overlying strata penetration. Based on the relationship between three-field evolution characteristics and gas mi-gration, the mechanism of preferential gas extraction targeting was revealed. Combined with the experimental results, the process of determining the target area for gas extraction in the fractured zone based on the three-field evolution law was constructed. The gas extraction effect in the field was great, which ensured the safe and efficient recovery of the working face. The results of this study provide a theoretical reference for the accurate extraction of pressure-relief gas in the inclined coal seam group, aiming to improve the gas extraction from the inclined coal seam group, prevent gas overlimit in the upper corner, and achieve safe and efficient mining of the inclined coal seam group.

Abstract During the construction of coalbed methane extraction wells, cementing cement sheath is crucial for the stability and sealing of surface wells. One effective method to enhance these properties is the addition of lignosulfonate. However, the mechanism of the effect of calcium lignosulfonate on the whole process of cement hydration is still unclear. In this paper, the water distribution and variation characteristics of calcium lignosulfonate modified cement paste were revealed by low-field nuclear magnetic resonance technology, and the hydration ion experiment of modified cement was carried out to obtain the variation characteristics of hydration ions of modified cementing cement. Finally, the formation mechanism of hydration products was clarified by analyzing the phase change of modified cement stone. The results indicate that the cement paste’s hydration process can be divided into four stages: dissolution, crystallization, acceleration, and decline. During the dissolution stage, calcium lignosulfonate’s air entraining effect maintains the cement paste in a stable suspension state. In the crystallization stage, calcium lignosulfonate’s electro-repulsion delays the formation of hydration products and the hydration process. During the acceleration stage, the addition of calcium lignosulfonate reduces bound water formation in the cement slurry’s flocculation structure, and the released filled water participates more in the hydration reaction, reducing the total relaxation signal’s increasing trend. In the decline stage, the cement paste has solidified, and the system’s water is primarily in the porous medium. The research results have practical guiding significance for the addition of calcium lignosulfonate in cementing operations.

In coal mining, rock strata are fractured under cyclic loading and unloading to form fracture channels. Fracture channels are the main flow narrows for gas. Therefore, expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata. In this regard, graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels. Then, the three-dimensional models for fracture channels in sandstones were established. Finally, the fracture channel percentages were used to reflect the flow conductivity of fracture channels. The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels. It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels. The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels. When the proportion of fracture channels varies gently, fluids flow evenly through those channels. However, if the proportion of fracture channels varies significantly, it can greatly affect the flow rate of fluids. The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.

Abstract Gas drainage is an effective technology for gas control in coal mines. A high borehole-sealing quality is the fundamental precondition for efficient gas drainage. The expansibilities of cement pastes used in borehole-sealing processes are critical for the borehole-sealing effect. Nanosized magnesia expansive agents are used to improve the expansibilities of cement pastes and improve the borehole-sealing effect. Nuclear magnetic resonance spectrometry and scanning electron microscopy were adopted to study the effects of nanosized magnesia on the hydration of borehole-sealing cements used with different preparation methods. The results showed that an increase in the mass fraction of the nanosized magnesia promoted cement hydration, and the mass fraction was positively correlated with the promotion effect. The use of different preparation methods did not change the water-phase distribution in the cement. When using the wet-mixing preparation method, nanosized magnesia promoted the induction, acceleration, and deceleration periods of hydration; when using the dry-mixing preparation method, the nanosized magnesia promoted the induction period of cement hydration, and the promotion effect was less obvious than that seen when using the wet-mixing method. When using the wet-mixing preparation method, the nanosized magnesia was uniformly dispersed, thus enlarging the surface area of the reaction, which provided more nucleation sites for the hydration products of the cement and therefore accelerated the hydration reaction. When using the dry-mixing preparation method, the nanosized magnesia powders were dispersed nonuniformly and aggregated. Under these conditions, only a few nanosized magnesia particles on the surfaces of the aggregated clusters took part in hydration, so only a small number of nucleation sites were provided for the hydration products of cement. This led to inconsistent hydration of cement pastes prepared using the dry-mixing method. The surface porosity of the cement prepared with the wet-mixing preparation method first decreased and then increased with increases in the mass fraction of the nanosized magnesia. The cement surface exhibited compact hydration products and few pores, and the surface was relatively smooth. In comparison, the surface porosity of the cement prepared using the dry-mixing method fluctuated with increasing mass fraction of the nanosized magnesia, resulting in a rough cement surface and microfractures on some surfaces. The two preparation methods both reduced the surface porosity of the cement. The wet-mixing preparation was more effective and consistent in improving the compactness of the cement than the dry-mixing preparation. These results provide important guidance on the addition of nanosized magnesia in borehole-sealing engineering and the selection of cement preparation methods, and they also lay a solid foundation for realizing safe and efficient gas drainage.

China’s coal seams are characterized by high ground stress, low permeability, high gas pressure, and high gas content, and their gas outburst hazards are serious. Therefore, some effective gas extraction measures must be taken for coal seams. However, only ground gas extraction technology is prone to emerge extraction gaps, resulting in poor extraction effects. Moreover, it is a challenge to achieve coal seam abatement in a short period because of the large work and extended period using underground gas extraction technology only. Combining ground and underground gas extraction technologies to achieve a joint ground and underground gas outburst prevention is one of the key technologies to ensure a safe coal mining. In the coal seam group mines, different modes of joint ground and underground gas outburst prevention should be adopted according to the distance between coal seams in order to rapidly eliminate gas outburst risk in the coal seam group. Based on the advantages of combined ground and underground gas extraction technology, the connotation of time and space conversion and complementary advantages of combined ground and underground gas outburst prevention technology is clarified. Long-time and comprehensive coverage extraction of gas from ground wells achieves a coal seam gas reduction in exchange for underground operation space. Underground drilling in the underground space accurately extracts gas from the target area, shortens the gas outburst prevention time and enhances the effect of coal seam gas extraction. Because of the characteristics of close distance coal seam groups, such as significant influence of interlayer mining, this paper puts forward a typical prevention model of gas outburst in close distance coal seam group “boreholes group coverage - cooperative extraction”, which realizes advance planning, advance construction and overall prevention of gas outburst. Given the characteristics of long distance coal seams with small mining pressure relief area, the typical model of “enhanced reflection improvement and progressive extraction” for the gas outburst prevention mode of joint ground and underground technology is proposed, which realizes the strengthening pressure relief, mining follow-up and progressive key gas outburst prevention in the management of long distance coal seams. In addition, the authors have independently developed the gas outburst prevention technology of joint ground and underground technology for close distance coal seams and long distance coal seams to further improve the gas outburst prevention effectiveness of joint ground and underground technology and analyzed the application effect by using the Shaqu No.1 Mine and Zhuji West Mine as typical cases. The application results show that the amount of coal mined in the Shaqu No.1 Mine in the close distance coal seam group is about 800 thousand tonnes, and the amount of gas efficiently extracted is about 4.2 million m3. In the long distance coal seam group of the Zhujixi Mine, the underground drilling quantity is reduced by about 60%, and the gas extraction standard time is shortened by about 40%.

Drilling gas drainage boreholes in coal mines is a major measure to prevent gas disasters in coal mines. However, coal mining disruptions can affect the stability and shape of gas drainage boreholes. Therefore, multi-level cyclic loading and unloading tests with a constant lower limit and constant-amplitude multi-level cyclic loading and unloading tests were conducted on hole-containing specimens. The results show that cyclic loading and unloading enlarges the deformation modulus of specimens with holes. The stress change during cyclic loading affects the deformation modulus of the specimens, rendering them more brittle and vulnerable to damage under high stress-induced disturbances. Additionally, the dissipated energy per unit volume of the specimens containing holes increases exponentially during multi-level cyclic loading and unloading. The cumulative dissipated energy per unit volume of the hole-containing specimens exhibits logarithmic growth during constant-amplitude multi-level cyclic loading and unloading. Furthermore, a discontinuity in the curves is observed as the stress level rises, which promotes brittle failure around the hole. The research results expound the deformation and energy evolution characteristics of hole-containing specimens during cyclic loading and unloading, and provide a reference for clarifying the failure modes of boreholes in the gas extraction process.

In the coalbed methane extraction of surface wells in the mining area, the mining of coal seam in working face will cause a significant disturbance to overlying rock layer, which in turn will lead to the deformation and damage of surface wells. The coalbed methane extraction of surface wells in the mining-disturbed areas cannot be effectively performed. Cementing technology can effectively elongate the life span of mining-disturbed coalbed methane surface wells. As a critical factor of the cementing quality of surface wells, the early strength of the cement needs to meet the high cementing requirements. Cement stone is formed after cement consolidation. The mechanical properties of cement stone are of great importance to maintain the stability of mining-disturbed coalbed methane surface wells. It is an effective way to improve the cementing effect by adding dispersants to enhance the mechanical properties of cement stone. There are few researches on the deformation and failure characteristics of calcium lignosulfonate modified cement under the influence of mining disturbance. Uniaxial compression tests of calcium lignosulfonate modified cement with different mass fractions under cyclic loading were carried out to investigate the effect of calcium lignosulfonate on the deformation and failure characteristics of cement stone in mining-disturbed coalbed methane surface wells and its modification mechanism. The results show that the P-wave velocity and peak stress of cement stone increase first and then decrease with the increase of calcium lignosulfonate mass fraction, while the total acoustic emission ringing counts of cement decreases first and then increases with the increase of calcium lignosulfonate mass fraction. With the increase of the uniaxial cycle steps, the deformation modulus of cement stone shows a strengthening phenomenon, and the initial loading and unloading cycle has the most significant strengthening effect on the deformation modulus. With the addition of calcium lignosulfonate, the modified cement stone shows a tensile-dominated → shear-dominated → tensile-dominated combined damage modes. Besides, the fractal dimension of modified cement stone shows a trend of decrease before increase, indicating that the addition of the appropriate amount of calcium lignosulfonate can effectively improve the damage resistance of cement stone under uniaxial cyclic loading. With the increase of the mass fraction of calcium lignosulfonate, the pores between cement hydration products show a trend of decrease first and then increase. The addition of an appropriate amount of calcium lignosulfonate can promote the formation of abundant C−S−H gel and ettringite in cement, and the precipitates interweave on the surface of cement particles, which can significantly improve the peak stress of cement stone, and play a positive role in improving the mechanical properties of cement stone. Additionally, the porosity of cement stone decreases, which leads to the increase in the longitudinal wave velocity of cement stone and the decrease of cumulative acoustic emission ringing counts of the cement stone during cyclic loading. However, when the calcium lignosulfonate is excessively added, the air entraining and electrical repulsion of calcium lignosulfonate play a dominant role in the hydration process, which will introduce more bubbles, resulting in the occurrence of the gap between the cement particles, and the inhibition of the early formation of C−S−H gel and ettringite, which has a negative impact on the mechanical properties of cement stone. Furthermore, the porosity of cement stone increases, which leads to the decrease of the longitudinal wave velocity of cement stone and the increase of the cumulative acoustic emission ringing count of cement stone in the process of cyclic loading. Therefore, the influence of calcium lignosulfonate on the mechanical properties of cement stone has a double effect.

Surface wells are prone to deformation and even instability under the mining action caused by coal seam mining. This is also a key issue that restricts the application of gas drainage technology in surface wells in mining pressure relief areas. Based on the mining of high-incline coal seam group in Xinjiang 1930 Coal Mine, this paper carried out similar simulation experiments of surface well deformation under high-incline coal seam group mining, by monitoring overlying rock movement, coal seamstress changes, and the axial and circumferential directions of different positions of surface well pipelines. Deformation reveals the law of deformation of surface wells under the mining of large-dip coal seams. Research shows that: first, surface wells are in a state of sheer, compression, and tension during the mining process. Most of the time, the shear deformation in the axial direction is greater than the tensile deformation in the axial direction, that is, the shearing action on the surface well is dominant. At the same time, during the mining process, the shear deformation in the circumferential direction and the axial expansion and contraction deformation of the surface well is negative to a certain extent. The correlation, to a certain extent, shows that the shearing and tensioning, and compressing effects of surface wells are mutually restrictive; secondly, the axial deformation of surface wells shows an increasing trend as a whole, but the phenomenon of tension and shrinkage alternates in the process, and there is a big difference in the constant increase in the repeated mining of the near-level coal seam, and the changing trend of the circumferential shear deformation is similar to that of the near-level coal seam. The direction of displacement is always the inclination direction of the coal seam; the final study also found that the surface well defamation law in the upper part of the main key layer showed an overall “increase-decrease” alternating three times, and “increase-decrease” in the main key layer and lower part. Repeat four times alternately. The research results can provide certain theoretical support for the engineering application of surface wells in high-dip multiple mining areas.

The dams formed by rock pillars around underground reservoirs are frequently subjected to drying-wetting cycles, so that rock pillars may be deformed, broken and develop a large number of fractures. This further causes a massive loss of water and heavily affects the stability of the dam formed by rock pillars. To explore the damage evolution characteristics and the deterioration mechanism of mechanical properties of sandstone under drying-wetting cycles, mechanical tests were carried out on sandstone samples subjected to different numbers of drying-wetting cycles. Acoustic emission technology was used to real-timely monitor the failure process of sandstone under loading. At the same time, damaged rock samples were observed under a scanning electron microscope. On this basis, the development characteristics of fractures in the sandstone were analyzed, damage evolution of sandstone in different stress loading stages was expounded. Research results show that the more the drying-wetting cycles are, the more obvious the compaction stage and the more significant the deterioration of the peak strength and elastic modulus, accompanied by the gradual decrease of the strain energy and cumulative acoustic emission count. According to scanning electron microscope observations, fractures inside the sandstone increase in quantity and begin to coalesce after 10 drying-wetting cycles, so that loose particles and fracture degree gradually rise, and accumulation of damage causes mechanical properties of sandstone to gradually deteriorate. The research results provide a solid theoretical basis for stability evaluation and protection of dams formed by rock pillars around underground reservoirs in goaf.

Cementing technology can effectively extend the life of coalbed methane surface drilling. The fluidity of the cement slurry during cementing is critical to the cementing quality. It is an effective way to improve the cementing quality by increasing the fluidity and pumping efficiency of cement paste by adding dispersants. In this paper, the effect of naphthalene dispersant on the hydration kinetics of G-grade cement was investigated by nuclear magnetic resonance. The results show that the increase of naphthalene dispersant mass fraction can effectively slow down the hydration of cement. The first relaxation peak of cement is correlated with its mobility, and the calculation of its peak index, area size, area share and combined action factor can be used to evaluate the retarding and dispersing effect of naphthalene dispersant on cement slurry more accurately. The main effects of naphthalene dispersant retarding and dispersing are: naphthalene dispersant causes an increase in ξ potential of cement particles, which makes the potential barrier overcome by cement particle coalescence rise; the sulfonic acid group of naphthalene dispersant can form a more stable complex with Ca2+, resulting in a reduced hydration rate. Naphthalene dispersant can decompose in cement into anions adsorbed on the surface of cement particles, forming a solvation film and producing a lubricating and dragging effect. The obtained research results provide theoretical support for the preparation and pumping of on-site cementing slurry from three aspects: the fluidity of the naphthalene dispersant modified cement slurry, the retarding effect and how to choose the pumping period.

Coal and coalbed methane (CBM) coordinated exploitation is a key technology for the safe exploitation of both resources. However, existing studies lack the quantification and evaluation of the degree of coordination between coal mining and coalbed methane extraction. In this study, the concept of coal and coalbed methane coupling coordinated exploitation was proposed, and the corresponding evaluation model was established using the Bayesian principle. On this basis, the objective function of coal and coalbed methane coordinated exploitation deployment was established, and the optimal deployment was determined through a cuckoo search. The results show that clarifying the coupling coordinated level of coal and coalbed methane resource exploitation in coal mines is conducive to adjusting the deployment plan in advance. The case study results show that the evaluation and intelligent deployment method proposed in this paper can effectively evaluate the coupling coordinated level of coal and coalbed methane resource exploitation and intelligently optimize the deployment of coal mine operations. The optimization results demonstrate that the safe and efficient exploitation of coal and CBM resources is promoted, and coal mining and coalbed methane extraction processes show greater cooperation. The observations and findings of this study provide a critical reference for coal mine resource exploitation in the future.

The purpose of the study is to explore the mechanism of coordinated slag disposal in the hydraulic cutting process, ensure the safety implementation of the hydraulic cutting operation and increase the success rate of hydraulic cutting. In the ultra-high pressure hydraulic cutting technique, the method for determining the cutting pressure is ambiguous, the coordination mechanism of various factors (including cutting pressure and coal-dropping speed) lingers unclarified; the slag disposal mechanism during hydraulic cutting is inexplicit. Aiming at these problems, a model for coordinated slag disposal during hydraulic cutting based on coal‐water two-phase flow was established. The critical flow velocity in the moving laminar flow regime is taken as that during the coordinated slag disposal from boreholes. The relationship curve between the coal-dropping speed and cutting pressure under different Protodyakonov coefficients of coal seams was obtained. Hence, the model for coordinated slag disposal during hydraulic cutting was established; the selection interval of reasonable pressure for coordinated slag disposal during hydraulic cutting was determined. The reasonable cutting pressure for slag disposal in coal seams with a Protodyakonov coefficient of 0.48 was determined as about 80 MPa. During the cutting test, the average net gas extraction from the boreholes for hydraulic cutting was 4.5 times larger than that from the conventional boreholes. Furthermore, the gas permeability coefficient of the boreholes for hydraulic cutting increased by 25 times; the effective extraction radius was more than doubled. It indicated that the model for reasonably selecting the cutting pressure based on the coordinated slag disposal theory can effectively guide the selection of the cutting pressure on site. While solving various problems occurring in the hydraulic cutting process on site, the model can be used to improve the cutting effect, which provides a theoretical basis for reasonably selecting the pressure during ultra-high pressure hydraulic cutting.

Abstract Energy is an important research parameter in rock mechanics. To explore the law of energy evolution of coal, a one‐time loading and unloading test under uniaxial compression was conducted on coal taken from four different coal mines. By utilizing the area integral method, the total input, elastic, and dissipated energy densities in coal at different unloading levels were calculated. The correlation between various energy densities and the evolution law of energy density with different unloading levels was attained. The peak strengths of the coal slowly declined with an increasing unloading level, which conforms to the relation of a linearly decreasing function. The energy dissipation rate has nonlinear characteristics, and the shape of the dissipation rate fitting curve changed from an upper concave to a downward concave with increasing strength. In all the coal samples, the energy density grew nonlinearly with the unloading level. Moreover, the growth rate of the total energy density was the highest, followed by the growth rates of the elastic and dissipated energy densities. All ratios of the elastic and dissipated energy densities to the total input energy density and the ratio of the dissipated energy density to the elastic energy density were constant. As the strength increased, the input energy and the elastic energy density increased at a faster rate, and they observed the same law. There is an insignificant relationship between the degree of destruction of the coal and the level of unloading. Energy is a major factor that drives the failure of a test piece, but this is not the main factor that determines the degree of damage to the test piece.

Abstract The revelation of the distribution characteristics of stress and fractures in the floor strata is of great significance to improve the regional gas drainage efficiency and to eliminate or reduce the risk of coal and gas outburst in adjacent coal seams during multi‐seam mining. In this paper, the stress distribution in the floor under mining condition was derived, and the characteristics of stress zones in the floor were elaborated while mining the 22201 working face in Shaqu coal mine in Shanxi province, China. Besides, the propagation laws of fractures in different stress zones were theoretically analyzed, and the distribution law of fracture angles in the floor strata under different mining‐induced stresses was obtained. The research results demonstrate that the compression and transition zones show high vertical stress and less developed fractures and are dominated by fractures with a large angle (>60°), which is unfavorable for gas drainage. The expansion zone in the floor within 2‐40 m in the rear of the working face is characterized by high unloading degree of vertical stress and developed fractures. Moreover, fractures with a small angle (

Abstract Long‐term mechanical property is critical for underground energy storage in a salt cavern. Multistage loading has been widely used in creep tests to save test time and cost. If the number of loading stages is too small or the loading time of each stage is short, the long‐term strength of salt rock is difficult to determine. For this reason, this paper carried out a ten‐stage loading creep test on the salt rock. The loading time of each stage is about 14 days, and the total creep time is more than 5 months. The stress‐strain curve and the stress‐steady‐state creep rate curve obtained from the test results can be well described by an exponential function. Based on the two‐stage power function model, a new method for determining the long‐term strength is proposed. The method is very similar to the results determined by the isochronous stress‐strain curve method. Moreover, considering that the rock is damaged during the accelerated creep phase, the stress will no longer be a constant value during the accelerated creep phase, but a slow loading process. In this paper, the accelerated creep phase is a coupled process of loading and creep. The total strain in the accelerated creep phase is equal to the sum of the loaded strain and the creep strain, and a new nonlinear creep damage constitutive equation is derived. The model was verified by the creep test results. The results show that the model can well describe the whole process of salt rock creep. It can theoretically explain the reason why the accelerated creep phase of salt rock exhibits nonlinear acceleration characteristics.

Abstract Coal‐like sampling obtained through compression molding is an important application of powder compression molding technology in mining engineering. To obtain ideal coal‐like samples for the revelation of the seepage property of low‐permeability soft coals, gas seepage studies, which utilized the Taguchi method, were performed on coal‐like materials with different particle sizes, activated carbon weight, Portland cement weight, and forming pressure. The effect of a single factor on the fluid‐solid coupling property of coal‐like materials was analyzed. The results indicate that the permeability and axial stress curves that correlated with strain in the conventional triaxial tests can be divided into three clear phases, and that layered damage appears in all tested specimens. The stress‐permeability relationship model of coal‐like materials is proposed. The influence of process parameters on the strength and permeability of coal briquettes during gas seepage tests was experimentally investigated. The Taguchi method and gray correlation analysis were integrated to determine the best combination of input factors through the key indicator of the gray relational grade, which is required to satisfy multiple quality goals in gas seepage coal‐like materials. The contribution percentage of the input factors to the outputs was determined using analysis of variance; it indicated that coal particle size was the prominent influencing parameter followed by activated carbon, forming pressure, and Portland cement.

Abstract Broken coal and rock masses are the major part of the goaf. The compaction characteristics of coal and rock masses and the breakage law of whose particles during compaction exert an important influence on various aspects including control of strata motion, prediction of surface subsidence, and backfill mining. In this paper, the triaxial compaction experiment on broken coal‐rock masses with different mixing ratios was carried out. The test results showed that with the increase of stresses, the strain of coal‐rock masses gradually rose while the porosity, bulking factor, and degree of compaction gradually declined. During the compaction of coal‐rock masses, the fitting curves of the strain, porosity, bulking factor, and degree of compaction with stresses of coal samples all appeared as a cubic function of stresses. The breakage behavior of coal particles underwent three stages: structure re‐arrangement and breakage of particles, particle breakage, and compression‐induced deformation of particles. With increasing stress, the crushing amount of particles gradually grew while the increase rate of the crushed particles gradually decreased and the larger the particle strength was, the lower the increase rate of the crushing amount. Additionally, in the compaction process of samples, particle breakage mainly appeared before the stress reached to 8 MPa while the coal and rock particles were hardly crushed after the stress was larger than 8 MPa. With increasing stresses, the particle size gradation of samples gradually became reasonable and the lower the particle strength of samples was, the more reasonable the particle size gradation of compacted samples. The particle size gradation of various compacted and crushed samples showed a favorable fractal characteristic. In the stage with a low stress, the value of fractal dimension D rapidly grew and the fractal dimensions D of various samples tended to be stabilized after the stress reached to a high level.

Abstract The increase in extraction height will increase the mining‐induced overlying strata failure height. In this scenario, the strata pressure behavior is strong in a fully mechanized face with large mining height (FMFLMH), which frequently causes coal wall falls, roof falls, and hydraulic support failure accidents (e.g., support closure and hydraulic column damage). The key to solving these issues is to determine support's working resistance of the FMFLMH. In this paper, comprehensive theoretical analysis, numerical simulation, and field observation were applied to determine the support's working resistance in the FMFLMH based on movement type of the first subordinate key stratum (SKS 1). First, six kinds of movement types of SKS 1 in the FMFLMH are found and defined by theoretical analysis and numerical simulation, which are the direct caving movement type of cantilever structure (direct caving), the double‐sided rotation movement type of cantilever structure (double‐side rotation), the quadratic rotation movement type of cantilever structure (quadratic rotation), the alternative movement type of cantilever structure hinged structure (alternate hinged), the voussoir beam structure movement type (voussoir), and the short voussoir beam structure movement type (short voussoir), respectively. Besides, based on this, the support load calculation model of each movement type was established, and a formula for the support working resistance of each movement type was obtained. Finally, the correctness of the formulae for the support working resistance under six types of movement of SKS 1 were verified using measurement data from four FMFLMHs in China. These research results have important guiding significance for reasonable selection of support and ensuring safe mining of the FMFLMH.

Abstract Based on the Langmuir adsorption model, the adsorption constants of eighteen coal samples from the same coal mine with outburst risk, weak outburst risk, and non‐outburst risk were tested by high‐pressure capacity method under different temperature conditions. The results show that the adsorption constants a and b monotonically decrease with the increase in temperature. The relationship between a and temperature shows three stages: accelerated decreasing stage, decelerated decreasing stage, and stable stage. The b decreases with the increase in temperature also shows three stages, namely relative stability, slow decrease, and accelerated decrease. The a × b for the outburst coal seam decreases with increasing temperature, which is similar to b. The a × b for the weak and non‐outburst coal seams decreases with increasing temperature, which is similar to a. Furthermore, the a of the outburst coal seam decreases with the increase in temperature, and the decrease of b is the largest. The a of the weak and non‐outburst coal seams decreases with the increase in temperature, and the magnitude is relatively large. The b decreases slightly, with a smaller magnitude. The a × b of weak outburst seam is smaller than that of non‐outburst seam and is larger than that of outburst seam. The achievements can provide guiding significance for coal and gas outburst prevention and control.

In-situ stress estimation is of great significance in deep coal mining. The vertical stress of the underground rock mass is generally estimated by the product of the upper cover average specific gravity and its depth, and it is difficult to estimate the horizontal stress. In this study, based on the Mohr-Coulomb criterion, the ultimate stable equilibrium conditions of the underground rock mass were applied to estimate the horizontal stress range. The estimated values agreed strongly with the in-situ measurement results, demonstrating the validity and feasibility of this estimation method. Meanwhile, this approach can explain the phenomenon whereby the ratio of the horizontal to vertical stress is distributed in a wide range of the shallow cruse zone but in a narrow range of the deep area. Furthermore, the influences of different parameters on the Mohr-Coulomb envelope were analyzed. Keywords: Rock mechanics, Horizontal stress, Mohr-Coulomb criterion, Deep rock

Coal mines in China are characterized by low permeability, and hydraulic slotting is an effective technique to increase the gas permeability of coal. However, the slot depth cannot be accurately determined, which results in the layout parameters of the slotted borehole being empirically designed. In this paper, based on the law of momentum conservation, the velocity distribution law at the cross section of the water jet was derived. Based on the linear relationship between water jet impact power and the erosion volume, the law of pressure attenuation of water jet and Mohr–Coulomb criterion, changes of the slot depth with time and the influence range of water jet slot is confirmed. The maximum slot depth prediction model is also established. The results of field test show that the maximum error between the theoretical calculation value and the measured value is less than 10%, which better validates the prediction model of the slot depth. Keywords: Gas drainage, Hydraulic slotting, Impact power, Slot depth

The aim of this study is to explore the coupling relationship between fractures in overlying strata and gas seepage fields for pressure relief during mining of outburst-prone coal seam groups and to quantitatively characterize the distribution characteristics of favorable areas for coalbed methane (CBM) drainage in mining-induced fractures of overlying strata. For these purposes, by taking Shaqu No. 1 Coal Mine (Lvliang City, Shanxi Province, China) as a research object, this research studied migration and caving characteristics of overlying strata by combining physical similar material simulation, numerical simulation, and field measurement. Moreover, this study analyzed spatial distribution patterns of mining-induced fractures in overlaying strata, quantitatively characterized distribution parameters of asymmetric-oblique-quadrilateral fracture development zones in overlying strata, and precisely divided areas favorable for CBM drainage. On this basis, evolution laws of an asymmetric fracture-seepage community in overlying strata in outburst-prone coal seam groups were obtained, thus optimizing design parameters for directional drilling in the fracture zone in overlying strata. The research results demonstrate that, due to mining-induced influences, strata present different migration and rupture patterns along rupture lines on both sides. Because of different rupture angles α and β, mining-induced fractures in overlying strata are distributed as an asymmetric-oblique-quadrilateral fracture development body in space. Furthermore, based on the coupling relationship between fracture development states and pressure-relief gas seepage in the fracture development zone in overlying strata of the 4305 rear working face in the mine, this research obtained rupture angles α ∈ [74, 90) and ß ∈ (70, 82] on both sides of the fracture development zone. Moreover, the intervals favorable for CBM drainage for pressure relief on both sides of the fracture development zone in overlying strata of 4305 rear working face in the mine were separately determined as xaj ≤ 22.68 m and 24 m ≤ xβj ≤ 37.8 m. If the gas drainage system is designed in this zone, it is of great significance to precise and efficient pressure-relief CBM drainage in the fracture zone and ensure production safety in the mining space.

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.

Tectonic coals in coal seams may affect the process of enhanced coalbed methane recovery with CO2 sequestration (CO2-ECBM). The main objective of this study was to investigate the differences between supercritical CO2 (ScCO2) and intact and tectonic coals to determine how the ScCO2 changes the coal’s properties. More specifically, the changes in the tectonic coal’s pore structures and its gas desorption behavior were of particular interest. In this work, mercury intrusion porosimetry, N2 (77 K) adsorption, and methane desorption experiments were used to identify the difference in pore structures and gas desorption properties between and intact and tectonic coals after ScCO2 treatment. The experimental results indicate that the total pore volume, specific surface area, and pore connectivity of tectonic coal increased more than intact coal after ScCO2 treatment, indicating that ScCO2 had the greatest influence on the pore structure of the tectonic coal. Additionally, ScCO2 treatment enhanced the diffusivity of tectonic coal more than that of intact coal. This verified the pore structure experimental results. A simplified illustration of the methane migration before and after ScCO2 treatment was proposed to analyze the influence of ScCO2 on the tectonic coal reservoir’s CBM. Hence, the results of this study may provide new insights into CO2-ECBM in tectonic coal reservoirs.

The integration of hydraulic slotting and gas drainage techniques has become a mainstream technique for enhancing permeability in coal seams with low permeability. However, the mechanism of action of this process is unclear. In this paper, field experiment and laboratory tests are described that aim at elucidating this process. Given the sensitivity and accuracy of test methods and their corresponding determination principles, a combination of mercury intrusion porosimetry and nitrogen gas adsorption was proposed as a complementary technique and the pore-size distribution (PSD) was obtained. It is shown that the proportion of minipores decreases remarkably, whereas that of the macropores gradually increases with the decrease in the distance from the slotted borehole. By contrast, the mesopores and micropores present insignificant changes. Meanwhile, the adsorption pore and the seepage pore show a similar variation in tendency with the minipores and macropores, respectively. Moreover, the specific surface area decreases substantially with the decrease in borehole distances. The integration of hydraulic slotting and gas drainage can lower the gas-adsorption properties and enhance the gas-seepage capacity within the disturbed zone significantly. The paper highlights the guiding factors for improving the enhanced coal bed methane recovery.