Since the utilization of CO concentrations in the Upper Corner as early warning of CSC (Coal spontaneous combustion) is fraught with problems like the complications of gas origin and lacks accuracy, this paper provides a way of early detection of CSC by carbon and oxygen isotope fingerprints.(1)The results show that during the low-temperature oxidation of coal samples, the δ18O value of CO first decreases and then increases with rising coal temperature, while the δ13C value of CO continuously increases with rising coal temperature.(2)By collecting CO concentration, δ13C, and δ18O data from monitoring points in the upper corner and goaf during both maintenance and coal mining periods, the sources of CO in the upper corner and their contribution ratios were determined. During the maintenance period, ambient-temperature oxidation in the goaf contributed 56% of the CO in the upper corner. During the coal mining period, this contribution was 44.7%. The difference in source contributions is mainly because the maintenance period was dominated by oxidation processes in the goaf, whereas the coal mining period was significantly influenced by the rapid oxidation of freshly exposed coal dust produced by the coal cutting process. (3)A coal spontaneous combustion early warning model integrating δ13C and δ18O was established by using a piecewise cubic polynomial (30–150 °C and 150–210 °C) to characterize the relationship between δ18O and coal temperature, and the PsdVoigt1 function to characterize the relationship between δ13C and coal temperature. This model enables accurate identification and graded early warning of spontaneous combustion risks under different operational stages.

In deep coal mining, geological conditions and in-situ stress environments are highly complex. When hydraulic fracturing technology is applied to destress composite hard roofs, the fracturing effectiveness is jointly influenced by multiple factors, including the lithology of overlying and underlying strata, borehole placement horizon, and borehole angle, while the mechanisms of fracture propagation and evolution remain insufficiently understood. To investigate fracture trajectory patterns under different stratigraphic conditions and borehole inclinations, this study takes the 72313 working face of the Xutuan Coal Mine as the research background. The mechanical properties of roof rocks and the pore structure characteristics of the overlying strata were analyzed. Combined with XSite numerical simulation software, the effects of different fracturing horizons and borehole inclinations on hydraulic fracturing performance were examined. Based on synthetic rock mass technology and the distinct lattice method, a coupled model of particle motion in the rock matrix and fluid flow was established.The results show that: Under high confining pressure, fine sandstone exhibits significant microcrack closure, a marked increase in elastic modulus, and a high brittleness response, whereas mudstone, due to its loose structure, high clay content, and strong pore sealing, shows pronounced plastic deformation capacity. Multi-source scanning results reveal that fine sandstone possesses good pore connectivity and a stable seepage network, facilitating fracturing fluid transport and fracture activation; in contrast, mudstone has isolated pores and low permeability, which tend to form high-pressure gradients and induce fluid instability, thereby suppressing fracture volume growth and network complexity. Numerical simulations demonstrate that borehole deviation angle can alter the local stress perturbation structure, inducing directional deflection of fractures; when the deviation angle is 60°, fracture paths align more closely with the principal stress direction, and the branching network develops more fully. Sensitivity analysis indicates that tensile strength, fracture toughness, and compressive strength are the primary sensitive parameters, together contributing more than 70% to fracture network volume, while elastic modulus, Poisson’s ratio, and porosity play relatively minor roles. Field observations further show that prioritizing fracturing slotting in fine sandstone horizons can promote fractures with greater connectivity and extension capacity.The findings provide theoretical support for optimizing hydraulic perforation layout and fracturing effectiveness in deep mining environments, thereby improving fracturing efficiency while ensuring roadway safety.

Based on the engineering background of(1) roof-cutting and retaining roadway working face in Dingji Coal Mine of Huainan, the stress characteristics of the floor and the stress transfer mechanism caused by mining in the process of retaining roadway are studied. The stress in the roadway and the roadway after roof-cutting is relieved, and the floor stress is symmetrically distributed. During the first mining period, the stress at 5 m of the roadway side is concentrated, and the high stress is transmitted to the floor through the coal body. The stress on the solid coal side is restored, and the large-scale pressure relief on the goaf side forms an asymmetric stress field. The vertical stress is transmitted to the floor through the 'short cantilever' structure of the roof, and the 'short cantilever' structure is characterised by high tension and low compression. As the gangue collapses, the bending subsidence of the overburden rock causes the key parts of the roof to rotate and sink, and the high additional load is transmitted to the floor through the coal side and the supporting body in the roadway, resulting in an increase in the stress on the floor of the retained roadway. After the goaf is compacted, the floor fully touches the gangue and shares the load with the key parts of the coal body and the roof.

One side of 7403 fully mechanized top coal caving mining with a large dip angle in Zouzhuang Coal Mine is a gob; the return airway is excavated along the floor, while the haulage roadway is excavated along the roof, and the working face is mined along the coal seam floor. The deformation and failure of the surrounding rock of the roadway show obvious asymmetry. The stability of the surrounding rock of the roadway cannot be guaranteed by using a conventional full-section equal-strength symmetrical support form. Taking the 7403 working face as the engineering background, a research method combining theoretical analysis, numerical simulation, and field measurements was employed to identify the key deformation and failure zones. The asymmetric coupled support technology for roadways was proposed, and a comparative analysis of the support effectiveness of different support schemes was conducted. The results showed that: (a) the main reason for the asymmetric deformation and failure characteristics of the roadway is due to the different layout layers and rock types of the two sides of the roadway. The high side of the haulage roadway and the low side of the return airway are the key parts of the deformation and failure of the surrounding rock in the roadway. (b) On the basis of the original symmetrical support of “anchor belt net cable,” the key parts that cause differential deformation and failure are reinforced with through-layer anchor cables to achieve coupled support of the key parts of deformation and failure of surrounding rocks in a large dip angle coal seam roadway. (c) The control effect of roadway surrounding rocks under different support schemes (no support, symmetric support, asymmetric support) is compared and analyzed. After the roadway adopts asymmetric support technology, the horizontal stress of the two sides of the roadway increases, the shear force distribution is more uniform, the range of the plastic failure zone decreases obviously, the stability of the roadway surrounding rock increases, and the deformation decreases.

Gob-side entry retaining with roof cutting is an important technique for achieving safe and efficient deep coal mining. However, under the combined effects of primary roadway excavation and secondary intensive mining-induced disturbances, severe surrounding rock deformation often occurs, with floor heave becoming a prominent problem. To clarify the stress transfer mechanism between the overlying strata and the floor during gob-side entry retaining, as well as the fracture evolution behavior of floor rock under variable loading rates, similarity simulation tests and uniaxial variable-rate loading experiments were conducted. The floor stress response and the mechanical behavior, acoustic emission (AE) characteristics, and fractal features of sandstone under different loading rates were systematically analyzed. The similarity simulation results indicate that the floor surface experiences instantaneous unloading after roadway excavation. With the advancement of the working face and periodic roof caving, the accumulation and compaction of caved gangue lead to stress recovery in the floor. After the overlying strata structure becomes stable, variations in floor stress gradually diminish, and the system enters a quasi-static equilibrium stage. Based on this stress evolution background, variable-rate loading experiments reveal that the loading rate has a significant influence on the fracture mode and mechanical response of sandstone. Under rapid loading, high strain rates promote concentrated crack initiation and rapid coalescence, accompanied by intense energy release, resulting in typical brittle failure. When quasi-static loading is introduced at an earlier stage, cracks initiate at multiple locations and propagate slowly, the pre-peak nonlinear stage is prolonged, brittleness is reduced, and ductility is enhanced. Under fully quasi-static loading, damage accumulation is most sufficient, and the fracture process exhibits a progressive failure mode. AE results show that rapid loading induces sudden and highly concentrated AE activity, with sharp pre-peak increases in AE counts and energy, leading to an extremely short warning window. As the loading rate decreases, crack propagation is restrained, AE events become continuously active, and high-energy release persists even after peak stress. Multifractal analysis demonstrates that the AE fractal spectra exhibit a bell-shaped distribution, which initially broadens and then converges with increasing stress. Rapid loading or late-stage rate reduction leads to left-skewed spectra with enhanced burst characteristics, whereas lower loading rates or early-stage rate reduction produce more symmetric spectra, indicating progressive crack development. Early-warning indicators constructed from the variance of multiple AE parameters further show that under rapid loading, the variance rises sharply shortly before peak stress, resulting in delayed and short-lived warning signals. With decreasing loading rate, the variance exhibits sustained growth with dense multi-peak features, triggering L1, L2, and L3 warning levels sequentially and significantly extending the warning window. Under fully quasi-static loading, variance fluctuations become more frequent, enabling earlier identification of accelerated crack coalescence. These results indicate that a lower loading rate promotes more complete fracture evolution and enhances the identifiability of failure precursors. The findings provide deeper insight into rock fracture characteristics and offer a theoretical basis for stability monitoring and disaster early warning in deep rock engineering.

In block caving mines, stress evolution induced by undercutting poses a significant threat to the integrity of extraction structures. Focusing on the Pulang Copper Mine, this research analyzes the controlling factors behind the vertical stress concentration ahead of the undercutting line. Numerical and field data revealed that the undercutting span, burial depth, and internal friction angle of the rock mass were the dominant influencing factors. Specifically, vertical stress intensifies with larger spans and greater depths. Conversely, the extent of the failure zone ahead of the face increases with burial depth but decreases with a higher internal friction angle. This study provides both a mechanistic understanding and a practical basis for optimizing undercutting parameters under similar conditions.

To explore the differences in fracturing effects of different levels of ground level wells in unmined coal mines,and to reveal the interaction mechanism of construction effects in various stages of gas control throughout the entire cycle,combined with the construction status of ZJ2-1 and ZJ2-2 wells in Zhujidong Mine,the differences in fracturing effects of each section and level were analyzed from the perspectives of fracturing construction scale,fracturing construction curve,and pressure drop data,and compared and verified with micro-seismic monitoring results;Analyze the impact of drilling and fracturing effects on gas and water production efficiency,as well as the reas-ons for production shutdown,reveal the interaction mechanism between the entire cycle of surface horizontal well drilling,fracturing,and drainage engineering,and propose key issues and suggestions for gas control in surface horizontal wells in unmined coal mines.The res-ults showed that the fracture opening effect of the coal seam in ZJ2-2 well was better than that of the top and bottom plate fracturing in ZJ2-1 well and ZJ2-1 well,while the microseismic results showed that the top and bottom plate fracturing had better fracture opening ef-fect than the coal seam fracturing.This is because the fracturing effect is not only good,but also requires uniform crack opening and high crack persistence.The drilling effect is reflected in two aspects:drilling trajectory and wellbore stability.Drilling trajectory not only dir-ectly affects wellbore stability,but also affects fracturing effect.Fracturing construction will reduce wellbore stability,and wellbore stabil-ity is the foundation for ensuring the smooth progress of drainage projects.The stability of the wellbore cannot be ignored due to factors such as drilling layer,fracturing construction,reservoir rock layers,and long-term immersion.Good fracturing effect is only a necessary condition for good drainage effect,and the quality of drainage effect is largely influenced by the stability of the wellbore and the refine-ment of drainage system.The production stages of both wells can be divided into five stages:backflow,upward production,stable produc-tion,shutdown,and re production of gas.The backflow rate of ZJ2-1 well is relatively high and greater than 1,indicating the invasion of adjacent water sources,resulting in poor gas production of ZJ2-1 well with better fracturing effect.Finally,the key issues in the gas con-trol process of surface horizontal wells in unmined areas of coal mines were summarized from the perspectives of optimal selection of sur-face horizontal well gas control technology,precise control of drilling trajectory,stability control of wellbore,optimization of drainage system,and detection of water storage environment.Corresponding development suggestions were provided in order to provide ideas for gas control of surface horizontal wells in unmined areas of coal mines.

To reveal the anisotropy characteristics of mechanical behavior, energy evolution and fracture morphology of the layered rock mass during triaxial compression tests, MTS 815 mechanical test system is used to perform the relevant experimental studies on layered rock mass specimens with different bedding angles and confining pressures by combining energy analysis and CT scanning. Effects of bedding angle and confining pressure on the energy evolution characteristics of layered rock masses are deeply studied. With increasing bedding angle, the elastic energy, dissipative energy and total energy at the peak strength showed two changing trends: first increase and then decrease and then increase, first decrease and then increase. With increasing confining pressure, the anisotropy coefficient of elastic energy first decreases and then increases, the anisotropy coefficient of dissipative energy first decreases and then increases, and the anisotropy coefficient of total energy first increases and then decreases and then increases. The sensitivity of dissipative energy at peak strength to bedding angle is the highest and the sensitivity of elastic energy is the lowest. Micro-cracks reconstruction and quantitative characterization are first used to reveal the mechanism of confining pressure on the failure modes of layered rock specimens. At bedding angle = 45 degrees, the two-dimensional crack area rate first increases and then decreases along the change of specimen height. When the confining pressure is 10 MPa, the dispersion degree of two-dimensional crack distribution along the specimen height is the largest, and the dispersion degree of two-dimensional crack along the specimen height is the least at 20 MPa.

In the block caving mining method, deformation and collapse can occur in roadways due to vertical stress concentration in front of the undercutting line. Therefore to better understand this behavior, we establish a mechanical model of two neighboring solidly-supported and two neighboring simply-supported stepped bottom space structures based on the theory of elastic plates. From this we derive an approximate analytical formula of the bending deflection function and an expression of the stress distribution using the Rayleigh-Ritz method. This mechanical model is applied to control the stability of perimeter rock near the undercutting advancement line in the Pulang Copper Mine in China, and optimize the positional relationship between adjacent undercutting advancement lines. The results indicate that both a larger misalignment between adjacent advancing lines and a closer spacing of the undercutting roadways significantly reduce the stability of the surrounding rock. The results of this study provide theoretical support and methodology for undercutting planning in block caving.

The seepage effect of leaching solution in the process of in-situ leaching mining may lead to the deterioration of the mechanical properties of the aquifuge, and affect the safety of coal uranium co-mining. Taking the conglomerate aquifuge of Jurassic Zhiluo Formation in Inner Mongolia as the research object, the physicochemical properties and mechanical deterioration mechanism under acid leaching environment were discussed. The mass and metal cation concentration of tometer. The variation characteristics of mineral composition and microstructure conglomerate in sulfuric acid solution were measured by electronic balance and atomic absorption spectropho of conglomerate with immersion time were analyzed by XRD and SEM. Triaxial compression tests were carried out to study the mechanical response of conglomerate under different immersion time. The evolution laws of parameters such as peak stress, elastic modulus, axial strain and dissipation energy density were analyzed. The coupling relationship between mechanical parameters and physicochemical parameters was quantified based on correlation analysis. The results show that the mass of conglomerate first increases and then decreases with the acid etching time. The concentration of metal cations in the solution is consistent with the mass change trend, and the concentration of Ca2+ is the highest, indicating that the chemical reaction between calcite and sulfuric acid is an important factor leading to the deterioration of mechanical properties of conglomerate. Acid corrosion gradually dissolves the conglomerate cement, expands the internal fractures, and increases the pores, resulting in changes in the rock microstructure. The peak stress, elastic modulus and yield stress of the conglomerate increased briefly in the initial stage (1 d) and then gradually decreased. After 20 days of immersion, the peak stress decreased by 26.69% compared with the natural state, and the peak volumetric strain decreased significantly, indicating that the specimen changed from brittle failure to ductile failure mode. The growth rate of dissipative energy density in the post-peak stage presents the characteristics of “slow-fast-slow”, and the failure process of the specimen is more gentle under the action of long-time acid leaching. The peak stress, elastic modulus and total strain energy density have a strong positive correlation (r>0.8), and the metal cation concentration can be used as a characterization parameter for the deterioration of mechanical properties of conglomerate.

This study aims to investigate the performance of hybrid fiber-steel slag synergistically modified conductive mortar. Firstly, statistical models were developed using Response Surface Methodology (RSM) to establish the relationship between variables (including steel slag content, steel fiber content, and carbon fiber content) and response values (including compressive strength, flexural strength, and resistivity). The mechanisms underlying the synergistic modification of response values by the variables were analyzed using scanning electron microscopy (SEM) and computed tomography (CT). Secondly, a Multi-Objective Particle Swarm Optimization (MOPSO) was employed to obtain a Pareto solution set that achieves the optimal performance of the conductive mortar. Finally, conductive concrete slabs were cast based on this solution set, and their electrical heating performance were evaluated. The results indicate that the relationships between the variables and each response value can be described by quadratic polynomial models. Steel slag enhances the density of the matrix and effectively strengthens the interfacial bonding between carbon/steel fiber and the matrix, while carbon fiber suppresses the settling of steel fiber. Furthermore, both steel slag and steel fiber possess the ability to assist carbon fiber in forming conductive pathways. The optimized conductive mortar achieves compressive and flexural strengths of 52.61 MPa and 18.92 MPa, respectively, with a resistivity as low as 0.72 Omega center dot m. Additionally, the conductive concrete slabs cast based on the Pareto solution set maintain a heating rate of 6.0-6.6 degrees C/h after 80 minutes of energizing duration, demonstrating a good electrical heating performance.

With the rapid development of deep underground engineering (e.g., deep mining, geothermal exploitation, and high-level radioactive waste disposal), high-temperature granite in deep environments is often subjected to complex thermo-mechanical coupling effects, thus making its mechanical properties and failure mechanisms critical to engineering safety. Hence, to evaluate these aspects of high-temperature granite in uniaxial compression tests, we developed a new thermo-mechanical coupling model based on cohesive zone model in ABAQUS software. The numerical model not only solves the problem of heat transfer between cohesive elements, but also represents the grain composition of granite, and adhesion and occlusion between grains. We verified the effectiveness of the model by comparing its results with those of laboratory experiments. We used this numerical model to study the effects of mineral-grain size and boundary strength on the mechanical parameters, failure mode, and microfracture characteristics of high-temperature granite. With increasing mineral-grain size, both peak stress and peak strain first decreased, then increased, and finally decreased; the failure mode changed from tensile-shear mixed failure to shear-dominant one at temperatures of 150, 300, and 450 ºC, and the proportion of intergranular cracks decreased. With an increase in the boundary strength, both peak stress and peak strain increased; the main failure cracks of granite became concentrated in the local range; and the proportion of intergranular cracks decreased constantly.

ObjectiveIn abandoned mines, the storage zones of free gas are closely associated with overburden fractures in goaves. To accurately estimate free gas resources in abandoned mines, it is necessary to thoroughly investigate the rupture mechanisms of blocks in main roofs in goaves, as well as the overburden fractures and rupture trace morphologies in goaves. MethodsBased on the masonry beam theory, this study established a mechanical model for the primary cantilever beam of a main roof. The analytical solutions of the stress components in the primary cantilever beam were analyzed using the stress inverse method, and the mathematical expression of periodic rupture traces was derived based on the Mohr-Coulomb failure criterion. Using the general rock mechanical parameters of the main roofs of goaves of various coal seams in the Panyi Mine in the Huainan mining area, Anhui Province, this study analyzed the stresses and strains of the primary cantilever beam, along with the strain energy density of the beam under different elastic moduli and Poisson's ratios. The rupture traces were calculated and plotted using the Maple software, and the influences of the internal friction angle and cohesive force on the rupture traces were analyzed. By constructing the experimental platform for physical simulation of mining face 1252-1 in the Panyi Mine using similar materials, this study analyzed the fracture network in the overburden in the goaf using the ImageJ software. Results and Conclusions The results indicate that the rupture of the primary cantilever beam of a main roof was primarily affected by horizontal and shear stresses, with tensile-shear failure predominating. The primary cantilever beam fracturing occurred initially in the compressed zone on its upper surface and then propagated toward the compressed zone on its lower surface. Substituting the overburden parameters of goaves in the Panyi Mine into the rupture trace expression yielded two parallel L-shaped rupture traces (i.e., rupture traces I and II). The rupture traces extended vertically from the top of the simulated block downward until inflection points and then propagated back downward to the bottom of the block. With an increase in the cohesive force, the number of rupture traces increased from two to three. Fracture traces I and II exhibited an L-shaped pattern, extending to the right side, whereas rupture trace III gradually extended vertically to the bottom of the block. The scanning results of overburden fractures in the goaf obtained using ImageJ software revealed that zones I-1 and I-2 were median-elevation gas enrichment areas, while zones II-1 and II-2 were high-elevation gas enrichment areas. The results of this study provide a scientific basis for designing the locations of gas drainage boreholes in an abandoned mine while also offering a reference for similar mines.

In response to the problem of slow excavation speed caused by long support time in deep roadways, the characteristics of the interaction between surrounding rock and support, the development characteristics of support structure stiffness, and the deformation coordination characteristics were analyzed. Virtual support force was introduced to simulate the constraint effect of excavation face space, and a dynamic mechanical model of the interaction between surrounding rock and support considering the time machine and time effect characteristics of support was established; The rock mechanics behavior, failure mode, acoustic emission, and energy evolution characteristics of “single-sided lateral unloading—three-way five sided force—single-sided lateral support-vertical continuous loading” under different secondary support conditions were studied using a true triaxial rock mechanics test system and acoustic emission system; Based on the principle of energy analysis, the energy evolution law under different secondary support timing conditions was analyzed. The research results indicate that as the secondary support timing after single-sided unloading and initial support is delayed, the stress–strain curve growth during the elastic deformation stage is relatively slow, the peak strength σ1max when the surrounding rock fails is lower, and the corresponding maximum principal strain ε1max is also larger. Moreover, the earlier the larger acoustic emission activity occurs, the faster the release rate of stored elastic energy inside, and the failure mode gradually changes from tensile shear composite failure to shear failure. In addition, the earlier the secondary support timing is applied after single-sided unloading, the significantly increased total energy U, elastic energy Ue, and dissipated energy Ud of the rock at the peak stress point, and the non-linear function relationship between the strain energy conversion rate u before the peak and the timing of secondary support. Based on the coupled support principle of the “beam arch” bearing structure of the roadway roof, a staged joint support idea is proposed. Combined with specific engineering, a staged support scheme for the coal roadway is formulated, and the application effect of staged support is evaluated. On the premise of ensuring the safety of coal roadway construction, it is expected that the excavation speed of coal roadway can be increased from 6 to 15 m/d.

The segmented fracturing and pressure relief gas drainage technology in the horizontal well of the coal seam roof is a key means to guide the efficient gas extraction in deep, high gas, soft and low permeability coal seams.Based on this, a full lifecycle development concept for the segmented fracturing, pressure relief,and gas drainage engineering of the coal seam roof horizontal well is proposed, including three stages: early scientific planning, mid-term engineering construction, and later safety management.This paper summarizes the research progress on the full life cycle development of segmented fracturing and pressure relief gas drainage in coal seam roof horizontal wells, constructs an overall research framework for key scientific issues related to segmented fracturing and pressure relief gas drainage in coal seam roof horizontal wells, and looks forward to its future development direction.The results show that the expansion of segmented fracturing cracks in the horizontal well of the coal seam roof is extremely complex due to geological factors, construction parameters, and physical properties.It is urgent to comprehensively evaluate the primary and secondary relationships under the influence of multiple factors in crack expansion, and reveal the mechanism of coal seam roof fracturing crack expansion under the influence of multiple factors.Exploring the critical relationship between the coupling of stress, water, temperature, and coal factors on the promotion and inhibition of coalbed methane adsorption, desorption, and migration, and establishing an optimal model for coalbed methane adsorption and desorption under multiple critical indicators, is the key to achieving efficient coalbed methane extraction. At present, there is insufficient research on the residual water from coal seam roof fracturing in terms of polluting water sources, inhibiting gas drainage, and dangerous accumulation, and there is a lack of management experience and technical support in a “Chinese style” approach.Guided by the development concept of the entire life cycle of coal seam roof fracturing engineering, this paper analyzes its high water consumption, fracturing fluid selection and potential pollution, reasonable selection of proppants, air pollution and seismic risks, and puts forward corresponding policy and regulatory formulation ideas.Realizing real-time and accurate monitoring of multi crack competition expansion in complex environments, revealing the mechanism of segmented fracturing and cross interface crack expansion of coal seam roof under the influence of multiple factors, constructing an effective radius evaluation model for segmented fracturing crack expansion of coal seam roof horizontal wells, exploring the water and gas migration law under the dual anisotropy conditions of three-dimensional stress and coal body structure, achieving advanced detection of potential danger areas for water and gas aggregation during fracturing of coal seam roof horizontal wells,and establishing a refined evaluation model for the effect of segmented fracturing,pressure relief, and gas drainage in coal seam roof horizontal wells is currently a key scientific problem that urgently needs to be solved.Finally, the development direction of precision,coordination, intelligence, comprehensiveness, and demonstration of segmented fracturing and pressure relief gas drainage technology for coal seam roof horizontal wells was outlined.

To prevent severe damage to the surrounding rock of the roadway under the high ground pressure within deep mining, the stress evolution of the surrounding rock was studied, and the surrounding rock control technology in the II1057 roadway of Zhuxianzhuang Coal Mine before and after passing fault was conducted through theoretical analysis, numerical simulation and engineering practice. The results show that when the roadway is advanced to 20 m away from the fault, the surrounding rock is broken, cracks are generated at 2.56 m above the roof, and the advance abutment pressure of the roadway is 17–20 MPa. Through the borehole detection technology, the development of the internal cracks in the surrounding rock was observed, and the surrounding rock control technology scheme (the advanced grouting in front of the roadway and the active support of rock bolt + anchor cable + M steel belt + steel beam + metal mesh) was proposed. The engineering practice shows that the roof separation, roof-to-floor convergence and sidewall convergence are greatly reduced, and the mechanical properties of the surrounding rock of the roadway are greatly improved, leading to the effective control of the deformation of the roadway surrounding rock.

This study aims to alleviate the serious deformation of surrounding rock (SR) in an extremely soft and fragile fully mechanized caving face roadway (ESFFMCFR, the 8# coal seam, Huaibei mining area) under a conventional support. Laboratory tests of roadway SR were conducted. The results show that in this coal seam, the extremely soft and fragile coal body has a high clay mineral content, so it is of low strength and breaks and softens easily. With reference to the mechanical tests on coal and rock mass around the coal seam and the monitoring results of roadway deformation, the roadway deformation is mainly caused by the development of fractures in the roadway SR, the separation of the support body and SR and the loose supporting structure. Considering the engineering environment and deformation characteristics of SR in the ESFFMCFR (the 8# coal seam, Huaibei mining area), this study proposed a synergistic support system of “lowering, drilling, anchoring, grouting and flatting (LDAGF)” for the ESFFMCFR based on the synergistic mechanism of support and SR under the basic principles of synergetics. Specifically, the synergistic support system of “LDAGF” includes the following measures: floor breaking and side lowering, bolt advance support, anchor cable support, advance water injection and grouting and flat-roof U-shaped steel shed support. Furthermore, this synergistic support system was applied on the ESFFMCFR in the 8# coal seam of Xinhu and Guobei coal mines, Huaibei mining area. The on-site application results reveal that when the synergistic support system is adopted, the maximum subsidence values in the above roadway roofs are 117 mm and 121 mm and the maximum displacement values of the two sides are 66 mm and 74 mm, respectively, which proves an excellent support effect. The synergistic support system, which can effectively control the serious deformation of the SR in ESFFMCFRs and ensure long-term stability and safety of the roadways, is suitable for the support of ESFFMCFRs and is of great guiding significance for roadways of the same type.

Scientific and reasonable roof-cutting parameters are key to ensuring pressure relief of the retained roadway roof. This manuscript takes the 7135 working face of Qidong Coal Mine as the engineering background and uses theoretical analysis, numerical and on-site measurement methods to study the quantitative relationship and pressure relief effect between different roof-cutting parameters of GERRC. We established a fracture criterion based on the tensile strength of the main roof of the uncut joint along the cutting line. We analyzed the quantitative relationship between different main roof thickness, cutting height, cutting angle, and the main roof tensile stress of the uncut joint. We found that within a small range of cutting angles, as the cutting angle decreases, the tensile stress on the main roof of the uncut joint increases. When the cutting angle is 0, the main roof tensile stress of the uncut joint reaches its maximum. As the cutting height increases, the limitation of the cutting angle on the cutting height becomes smaller. Numerical simulation was conducted to study the distribution patterns of maximum and minimum principal stresses along the direction of the roadway roof during the retention period under different roof-cutting heights and angles. Based on this, the optimal unloading effect of the roadway roof and the minimum concentration of mining stress were obtained at a roof-cutting height of 9 m and a roof-cutting angle of 80°. Through on-site measurement of the stress on the reinforcement anchor cable during the retention period, the deformation of the sinking roadway roof, and the pressure relief control effect of the retained roadway roof, the pressure relief effect and scientific rationality of the design of the cutting height and cutting angle were verified.

The important technical process to ensure the success of gob-side entry retaining by roof cutting (GERRC) was the advanced pre-splitting blasting to cut off the mechanical connection between the roadway and working face roof. The whole-cycle roof structure evolution and stress characteristics of GERRC were analyzed. The factors affecting the roof deformation of GERRC were analyzed, and the quantitative relationship between the roof deformation of GERRC and the support stiffness was determined. The results showed that the temporary support stiffness was higher, the support position to the side of the roof cutting was closer, and the roof subsidence deformation of GERRC was smaller. It is proposed to use a single support mass with a high stiffness to control the deformation of the roof, but it also made the support mass and roof elastic potential energy aggregate. To fully utilize the matching of the support stiffness and roof subsidence, improve the stability, and control the subsidence deformation of the roof in GERRC, double-row stacking supports were adopted in the inclination of GERRC, which were used to increase the stiffness of the support system.