Influenced by the superposition of mining on double key strata, the gob-side entry in the fully mechanized top-coal caving mining of extra-thick coal seams is highly susceptible to severe ground pressure phenomena, such as rock bursts and significant roadway deformations. This area represents a critical focus for prevention and control during the mining process. Therefore, this article focuses on the fully mechanized caving mining of thick coal seams at the Caojiatan mine. It analyzes the distribution and migration characteristics of the key stratum in the overlying stratum and establishes a mechanical model of the transverse pressure bearing structure in the mining area. The analysis covers the transition of the transverse pressure bearing structure through four states, from a virtually stable state to an unstable state, filling the gap left by traditional S-R theory in analyzing the transverse pressure bearing structure in the mining area, and revealing the mechanism of strong mining pressure manifestation along the gob-side entry in the ultra-thick coal seam under the influence of double key strata. The results indicate that the main factors affecting the manifestation of strong mining pressure along the goaf are the pressure-boosting effect caused by the instability of the double-key strata, the excessively long cantilever length of the lateral pressure-bearing structure, and the underfilling of the goaf gangue. To this end, a combination of directional energy accumulation blasting and enhanced blasting for roof cutting and pressure relief technology was proposed to reduce the cantilever length of the pressure-bearing structure and increase the filling degree of collapsed gangue, thereby reducing the pressure of the roadway to control deformation. The effect of this technology was comprehensively studied using numerical simulation and on-site experiments, verifying the effectiveness of this technology in controlling the deformation of the roadway surrounding rock. The peak pressure decreased by a maximum of 18.5%, and the average step distance decreased by 41%. The maximum reduction in tunnel deformation is 81.9%. This study provides a scientific basis for the deformation control of the roadway under similar conditions.

The static stress field significantly influences crack initiation and propagation around a borehole induced by a static cracking agent (SCA). Through experiments and theoretical analysis, this study investigates this effect on fine-grained sandstone specimens subjected to SCA under uniaxial, equal biaxial, and unequal biaxial loading. Crack characterisation and full-field strain monitoring reveal that, as the static stress level increases, the total number and propagation length of surface cracks decrease. The macroscopic failure mode correspondingly shifts from multiple cracking to failure dominated by a few principal cracks or even local spalling around the borehole. Consequently, the fractal dimension and fracture density globally decline. Under uniaxial and unequal biaxial loading, cracks preferentially propagate along the direction of the maximum applied stress; under equal biaxial loading, cracks are more prone to random initiation and multi-directional distribution. A higher static stress level also delays the emergence of strain-concentrated zones and reduces their expansion rate. Theoretical analysis shows that confining pressure suppresses crack initiation and propagation by reducing the total circumferential tensile stress around the borehole. Under unequal biaxial loading, however, the non-uniform distribution of circumferential stress imposes directional control on the suppressed cracking. Finally, practical implications for SCA-based fracturing are discussed. It is suggested that, when the surrounding stress constraint is relatively strong, the suppressing effect can be mitigated by optimising borehole parameters and layout. Under anisotropic static stress conditions, the directional crack-propagation advantage can be harnessed to enhance fracture connectivity and quality.

This study investigates the roadway layout optimization of self-formed roadways without coal pillars (SFRCP) in close-distance coal seams. By integrating theoretical analysis, numerical simulation, and field application, the mining influence mechanism in close-distance coal seams was revealed, an optimal design scheme for the roadway layout was proposed, and its effectiveness was validated. First, based on the structural characteristics of the stope in close-distance coal seams, a mechanical model for mining stress transmission was established to elucidate the distribution of floor stress induced by the upper stope. Subsequently, the deformation and failure mechanisms of the surrounding rock for SFRCP were analyzed, identifying the key factors influencing roof deformation and coal rib failure depth. These findings formed the basis for proposing an optimal design scheme of roadway layout. Numerical simulations further compared the mine pressure evolution under different layouts, confirming the rationality of the proposed scheme. Finally, field engineering application based on the optimal roadway layout was conducted. Monitoring results confirmed that the surrounding rock deformation met the required safety standards, and the gob-side entry retaining performance was satisfactory. This research provides valuable insights for safe and efficient mining in close-distance coal seams.

This article carries out the preparation of new solid carbon filling materials. Its function mainly lies in isolating the upper and lower underground mining gobs for sealing the air leakage channel and sequestering carbon in underground. The carbon sequestration effect during the preparation of the material is analysed by microstructure and mechanical properties. The effect of carbon sequestration during the preparation of the material is reflected by the microstructure and mechanical properties. The study results show that the best performance of each grouting material is achieved when the cement dosage is 40%, bentonite dosage is 6%, water-solid ratio is 1:1 and sodium silicate dosage is 3%. Sodium silicate and bentonite stimulate the hydration reaction can promote the material compressive strength enhancement. A numerical simulation of grouting and plugging leakage in mining zones was carried out. The amount of interlayer air leakage quantity was reduced by 40.9% and 32.8% compared with the initial one.

Pillarless mining is a crucial technique for minimizing coal resource waste and promoting green, sustainable mining practices. In ultra-close coal seam geological conditions, roadways in the lower seams frequently encounter severe roof strata damage and intense surrounding rock stress due to disturbances from upper-seam mining and pressure exerted by residual coal pillars. These challenges pose significant safety risks to pillarless mining operations. To address these issues, this study analyzed the stress and damage distribution characteristics in roadway surrounding rock under ultra-close seam conditions through geological exploration and theoretical modeling. Based on the principles of “low-disturbance roof cutting pressure relief” and “roof repair and anchoring”, a novel pillarless mining method for gob-side roadway formation by roof cutting in ultra-close coal seams (PMM-UCS) was proposed. The stress evolution laws within the stope under this method were systematically investigated, and engineering applications were conducted at Longmenta Coal Mine. Results demonstrate that, compared to conventional pillar-retained mining, the PMM-UCS substantially reduces both the magnitude and spatial extent of surrounding rock stress concentrations at the advanced and lagging faces, effectively shifting stress concentrations away from the roadway. This optimized stress distribution enhances stope stability and greatly facilitates the successful formation of roadways in pillarless mining. Field implementation showed favorable roadway retention with minimal deformation, an intact cross-section, and no significant structural failure, thereby satisfying the reuse requirements for subsequent mining operations.

Dynamic phase transition of natural gas hydrates confined within complex pore–throat structures is a key factor impacting the safe and efficient development of hydrate-bearing deposits. In this work, hydrate-bearing samples with varying saturation were first reconstructed with the proposed ice-seeding method using actual marine soil in hydrate-bearing sediments from the South China Sea. Dynamic evolution characteristics of hydrate formation in evolving porous media under different temperature and pressure conditions were analyzed in detail. Combined with high-resolution CT scanning, image processing, pore network extraction, and statistical analysis, the typical microscopic pore–throat structures of hydrate-bearing sediments were revealed, and the presence of nanopores was identified. Furthermore, highly controllable heterogeneous pore–throat structures were constructed for microfluidic chips by integrating stochastic modeling, equivalent modeling, and machine learning approaches. On this basis, a novel microfluidic testing method was developed for investigating the dynamic formation, dissociation, and phase transition characteristics of natural gas hydrates in complex pore structures by controlling the temperature. This study provides reliable data support and theoretical guidance for the productivity prediction of marine hydrate-bearing deposits.

The weak and broken roof, explosive control and other problems seriously restrict the promotion of non coal pillar self-forming roadway technology. In order to solve such problems, a new method of non coal pillar self-forming roadway through non-blasting roof cutting and pressure relief was proposed in this study. A systematic research system of “theoretical analysis-physical experiment-engineering verification” was constructed with the 9103 working face of Longmenta Coal Mine as the research object. Firstly, the theoretical analysis was conducted to reveal the roof cutting mechanics mechanism of rock mass weakened by dense drilling, establish the design criteria for key drilling parameters, and obtain the key design parameters of dense drilling in the test working face. Secondly, the physical model test was conducted to make clear that the dense drilling method can directionally cut off the goaf roof along the set position, reducing the stress and deformation of the roadway surrounding rock. Finally, the field engineering tests were conducted, and monitoring results showed that the pressure relief effect of the dense drilling method was comparable to that of the directional blasting method, achieving non coal pillar self-forming roadway mining under non blasting conditions.

A vinyl acetate ethylene modified cement based thin spray on liner was tested to clarify how curing temperature, relative humidity, and age control tensile behaviour and sandstone interfacial bonding. A single factor design was used. Temperature levels of 20, 30, 40, 50, and 60 ◦C were applied at 30% RH for 7 d. Humidity levels of 30%, 45%, 60%, 75%, and 90% RH were applied at 20 ◦C for 7 d. Ages of 7 to 35 d were examined at 20 ◦C and 30% RH. Tensile responses were measured on dog bone specimens. Interfacial bonding was quantified using pull off type tests on sandstone composites. Tensile strength showed an optimum with temperature, increasing from 1.72 MPa at 20 ◦C to 3.14 MPa at 40 ◦C and then decreasing to 2.26 MPa at 60 ◦C, while failure strain decreased from 7.55% to 6.04%. Peak interfacial bond strength also peaked at 40 ◦C, rising from 1.10 MPa to 1.96 MPa, and bond energy reached 3107.9 N·mm. Humidity caused a threshold type deterioration. Tensile strength dropped to 0.79 MPa at 90% RH, whereas failure strain increased to 16.88%. The interface was more humidity sensitive, with peak bond strength decreasing to 0.33 MPa and bond energy decreasing to 849.4 N·mm at 90% RH. The results highlight that high humidity makes the interface the controlling weakness, while moderate temperature improves early performance when moisture is controlled.

As a typical sedimentary rock, coal possesses a natural bedding structure that results in anisotropic mechanical properties. This study investigates the anisotropic failure behavior of a bedding coal roadway through true triaxial tests and numerical simulations. The mechanical responses of horizontal (0°) and vertical (90°) bedding coal rock under excavation were examined using a true triaxial apparatus. Additionally, the Finite-Discrete Element Method (FDEM) was employed to simulate the failure state of the rock mass following roadway excavation. The results indicate that the bearing capacity curves of both types of coal rock undergo three distinct stages: an elastic rising stage, a stress adjustment stage, and a residual stress stage. The ultimate bearing capacity of the 0° bedding coal rock was found to be higher than that of the 90° sample. Their failure modes also differ significantly. The 0° coal rock exhibited brittle failure, characterized by a sharp drop in bearing capacity during the stress adjustment stage, followed by a recovery to the residual stress level, accompanied by the formation of a single penetrating shear crack. In contrast, the 90° coal rock showed a gradual decline from peak stress to the residual stress level during the stress adjustment stage, eventually breaking into multiple fragments. Correspondingly, the roof and floor of the roadway are more prone to fragmentation, while the two sides are susceptible to brittle failure. To address these anisotropic failure mechanisms, corresponding support strategies are proposed. It is recommended to use flexible rock mesh combined with anchor cables capable of containing fractured rock at the roof and floor, while energy-absorbing bolts/cables are suggested for supporting the roadway sides to mitigate impact failure.

To address failures in sealing gas extraction boreholes in deep coal seams, where traditional cement-based sealing materials lack self-healing capabilities and fail to handle fractures developing around boreholes due to mining stress, this study reports the permeable crystalline self-healing cement (PCSHC) and systematically evaluates its active sealing and self-healing performance. Axial and radial compression tests on coal–PCSHC composites reveal the synergistic match between composite strength and PCSHC’s healing capability. Then, A physical simulation device was used to assess sealing performance in boreholes oriented upward, parallel, and downward. Field tests were conducted at a working face in a Chinese coal mine. Results indicate that CO₂ concentration in parallel boreholes sealed with PCSHC is 2.77 times higher than cement after 600 s, and 2.42 times higher in 30° downward boreholes. CO₂ levels steadily increase in PCSHC-sealed boreholes, unlike the fluctuating or declining trends observed with cement. Gas extraction concentration remains stable at 75%–80%, with average borehole concentration and net extraction volume 2.15 and 2.23 times higher than cement, and a single-borehole attenuation coefficient below 20%. PCSHC effectively mitigates sharp concentration drops caused by mining-induced fractures and demonstrates excellent dynamic sealing performance and engineering applicability, providing support for highly efficient gas extraction.

This study uses the two-dimensional finite-discrete element method (FDEM) to examine how rib spacing and confining stress affect the resin bonding performance of rock bolts. Tensile tests were simulated for bolts with a fixed rib angle of 75° and rib height of 1.5mm, but with rib spacings of 6mm, 12mm, and 18mm, under confining stresses of 2, 10, 20, and 30MPa. Increasing the rib spacing can lower the confining pressure threshold for the transition from parallel shear failure to wedge failure. At 2-20MPa confining stress, the 6mm rib spacing bolt shows the poorest bonding, failing through parallel shear at the bolt–resin interface. The 12mm spacing produces a mix of parallel shear and dilatational failure, yielding better bonding, while the 18mm spacing achieves the best performance by promoting dilatational and wedge-shaped resin failure. At 30MPa, all bolts tend to fail by wedge failure at the rock - resin interface, and their bonding performances become similar, although localized parallel shear still occurs near some ribs of the 6mm bolt. The findings of this study can serve as a reference for the rib profile design of rock bolts at varying excavation depths.

Investigations into the production of gas hydrates from marine sediments have demonstrated that commercial viability necessitates a daily gas production rate of 130,000 to 200,000 m3. However, the second-round trial production in the South China Sea yielded only 28,700 m3/day, falling short of the rule-of-thumb for economic feasibility. Given the coexistence of natural gas hydrates (NGHs) and shallow gas in the subsurface reservoirs of the South China Sea, a co-production strategy (simultaneously exploiting NGHs and shallow gas) was proposed to reduce costs and enhance production efficiency. In this study, a large-scale, three-dimensional, multi-phase, and multi-component model was established based on the NGHs–shallow gas symbiotic system in the Qiongdongnan Basin. A dual horizontal well configuration was designed to extract NGHs from the hydrate-bearing layer and natural gas from the underlying shallow gas layer. Co-production via dual horizontal wells expanded the hydrate dissociation zone from the near-wellbore region to deeper strata, particularly enhanced the dissociation of NGHs in the region between the two horizontal wells. By the 10th year of simulation, the peak and cumulative volume rate of CH4 released from hydrate dissociation increased to 3.52 and 1.45 times under the co-production scenario, resulting in a 2.4-fold improvement in NGH recovery efficiency. Sensitivity analyses of bottom hole pressure and length of the horizontal intervals revealed that reducing bottom hole pressure significantly improved the daily and accumulative gas production from hydrate-bearing reservoirs. The length of horizontal intervals emerged as a critical factor influencing the dissociation of NGHs, whereas it had negligible impact on gas production from shallow gas reservoir with satisfied permeability. This study provides insights into optimizing the development of marine hydrate resources via integrated exploitation strategies.

In the process of extracting naturally-occurring oceanic gas hydrates, the dissociation of hydrates can cause a reduction in soil strength. This reduction has the potential to trigger slope failure and submarine landslides, which present a catastrophic threat to offshore facilities and hydrate production. This research aims to create a robust and accurate machine-learning model that can efficiently predict stability of submarine continental slopes where gas hydrates are widespread. By collecting and analyzing 144 relevant cases, a comprehensive dataset was constructed, incorporating slope basic data, overlying and underlying layer data, and geological parameters of the hydrate layer. After conducting a correlation coefficient analysis between the characteristic parameters of the dataset and the output, the key characteristic parameters were determined. To model the dataset and assess its performance, four machine learning techniques were employed: Random Forest (RF), XGBoost, K-Nearest Neighbors (KNN), and Support Vector Machine (SVM). Formation physical parameters, geotechnical parameters, and NGH parameters were taken as input parameters, and the stability of NGH slopes was taken as prediction indicator. Evaluation metrics such as the ROC and confusion matrix were employed to comprehensively evaluate these models’ classification ability. Among these algorithms, the RF algorithm achieves the best prediction accuracy and AUC value, demonstrating its potential in submarine continental slopes stability prediction of natural gas hydrates. Additionally, sensitive analysis using Gini impurity calculations revealed that hydrate decomposition degree is the most significant factor affecting slope stability, followed by the burial depth and thickness of the hydrate layer. The slope angle, cohesion, and internal friction angle also have significant impacts. This study provides a new perspective for predicting submarine continental slopes stability with NGH and offers a scientific basis for safe and efficient NGH exploitation.

The rapid development of transportation infrastructure in challenging geological regions necessitates innovative tunneling methods that balance efficiency, safety, and cost. This study addresses the critical construction bottleneck of large-span soft rock tunnels under high ground stress, where conventional methods often lead to unacceptable delays. Focusing on a 24.53 m span railway tunnel in southwest China, we present the significant engineering application of a “pilot-tunnel-first” method as a strategic solution to stringent schedule pressures. The core innovation lies not only in the adoption of a large 13.2 m wide pilot tunnel but also in a synergistically enhanced support system, featuring elongated bolts (6 m and 12 m) and strengthened steel arches. Numerical simulations and field validation confirmed that this optimized approach achieves a stability comparable to the traditional double-side drift method while dramatically accelerating progress. The successful implementation shortened the construction period by 1.96 months for a key 123 m section, with a manageable cost increase of approximately Chinese Yuan (CNY) 782,000, thereby ensuring the timely opening of the entire tunnel. The primary significance of this research is to provide a proven and practical technical strategy for overcoming similar soft rock tunneling challenges where project timelines are paramount, offering a substantial value for the design and construction of modern infrastructure under complex constraints.

Repeated mining makes the deformation and failure of roadway surrounding rocks more complex than those under single-seam conditions. To address the large deformation and support failure in the 40,111 roadway of the Dafosi Coal Mine, this study combined theoretical calculations and numerical simulations to analyze the evolution of stress, displacement, and plastic failure under different seam spacings and mining stages. Failure depth h0 of floor was affected by the synergy of included angle θ, internal friction angle φ4, and elevation H. h0 decreased with increasing θ (0.35-0.45 m/°) but increased with higher φ4 (0.36 m/°) and H (1.6 m/m). The coupling effect significantly changed h0 within 14.76-36.37 m. Reduced interlayer spacing intensified roadway deformation, increasing roof displacement from 230 to 394 mm and peak horizontal stress from 25.7 to 29.3 MPa. Based on the simulation and field verification, an optimized support scheme using "5-4-5" roof and "3-4-3" side cables effectively limited roof-floor convergence to 165 mm and side displacement to 125 mm. Besides, the surrounding rocks became stable after approximately 90 m of excavation. The 41,213 return airway employed an asymmetric support system, using left-side anchor cables and right-side anchor bolts, which effectively limited surface displacement to 22 mm at the sides and 15 mm at the roof and floor. After compensating for a 17.9% pretension loss, the cables maintained stable anchorage. Field observations indicated that the mining influence extended from 10 m ahead to 60 m behind the working face, with dilatancy concentrated in the shallow strata and more pronounced deformation on the left side. The results provide theoretical guidance for the stability control and support optimization design of roadways with complex stress.

Natural gas hydrate (NGH) is a clean resource characterized by abundant potential reserves, clean combustion, and high energy density. Although significant progress has been made in the development of NGH resources all around the world, challenges still exist that hinder commercial exploitation, such as a low daily gas production rate and short steady production periods. One significant reason lies in the complex gas–liquid–solid phase transitions occurring within the formation during production, which lead to changes in flow capacity. Understanding the phase change mechanism of NGH reservoirs will help to further reveal the production increase mechanism. To address the phase transitions’ effect on production, this paper establishes a numerical simulation model for the depressurization exploitation of natural gas hydrates in order to investigate phase transition characteristics at the field scale. First, the phase equilibrium calculation method is presented and the phase equilibrium curve is modified by considering the capillary effect, soluble salt, and surface adsorption. Then, the phase transition model is successfully characterized in a simulation and the numerical simulation model is established based on the first test project parameters in the Shenhu area. The production characteristics of different sediment types (montmorillonite, South China Sea sediments, kaolin, and silt) are analyzed under the effects of water content and salinity. It is shown that lower initial water content and higher salinity result in higher gas production. The results provide a better understanding of the effects of phase transition parameters on NGH production at the field scale.

Compared to traditional long-wall mining, the gob-side entry retaining by roof-cutting (GERRC) technology reduces the amount of entry excavation by 50%, and recovers the previously wasted coal pillar resources, which has obvious superiority in mining efficiency and economic benefits. In response to the problem of reasonable entry location of the GERRC in close-distance coal seams, this paper takes the 9105 Working face in Longmenta Coal Mine as the engineering background and conducts an in-depth study by using methods such as theoretical analysis, numerical simulation and field tests. Firstly, the structural characteristics of the overburden rock for GERRC in the close-distance coal seam are analyzed, based on which a method for determining the reasonable location of the GERRC in close-distance coal seams is proposed; secondly, a mechanical model for load transfer and rock mass damage of the upper coal seam floor was established to analyze the floor damage characteristics and stress distribution law, forming the theoretical analysis of the reasonable entry location; Finally, the optimal entry location (22.5 m from the center of the upper remaining coal pillar) was used for field test of GERRC at the 9105 Working face of Longmenta Coal Mine. It has achieved good on-site application results.

With the continuous advancement of the"double carbon"goal,the construction of a new power system with renewable energy as the main body has become the core task of China's energy transformation.The demand for large-scale and highly flexible energy stor-age technology is steadily on the increase,which provides an important development opportunity for the development of closed/aban-doned mine pumped storage power stations.The potential of pumped storage construction in underground space of closed/abandoned mines in Anhui Province is evaluated in order to solve the constraints of site selection and other factors on the construction of traditional pumped storage power stations,explore the innovative path of green transformation of mining areas and coordinated development of new power systems,and promote the high-quality development of pumped storage in Anhui Province.The development planning of pumped storage power stations in recent years is summarized through bibliometrics and policy analysis,and the development trend of the construc-tion of pumped storage power stations in closed/abandoned mines is analyzed.Combined with the pumped storage resource endowments such as the location of closed coal mines,the scale of underground space and the distribution of water resources in Anhui Province,the 60 closed/abandoned mines in the province in recent years are divided into three gradients according to the distance from the main water sys-tem:high potential area within 50 km,medium potential area within 50-100 km and low potential area outside 100 km,and typical mines are selected to carry out fine evaluation and discuss the construction scale of pumped storage power stations.Taking Zhuzhuang Coal Mine in Huainan-Huaibei Coal Mines mining area as an example,the feasibility of building pumped storage power station in Zhuzhuang Coal Mine is analyzed in detail from the aspects of underground available space and effective storage capacity of underground storage reservoir.The installed capacity of the proposed pumped storage power station in Zhuzhuang Coal Mine is designed to reach 17.87 MW.The carbon emission calculation model of closed/abandoned mine pumped storage power station is used to calculate the carbon emission reduction of 11300 t/d compared with traditional thermal power generation.The results show that the use of closed/abandoned mines to build pumped storage power stations can become an effective support for the development of new energy storage construction in Anhui Province,and make a positive contribution to the adjustment of energy structure and carbon neutrality in Anhui Province.

This study investigates the influence of shallow water-bearing sand layers on surface subsidence characteristics in coal mining areas with thick loose strata, with the ultimate goal of contributing to sustainable environmental protection. Firstly, a numerical simulation test was designed to analyze and study the influence of the loose layer thickness, mining height, bedrock slope, and sand inclusion on the surface movement and deformation characteristics. Secondly, the mechanical model of seepage flow in the sand layer was established to study the influence mechanism of the internal stress distribution of the sand layer and the seepage of the water body after mining on the surface subsidence. Finally, by studying the law of surface subsidence corresponding to the mining of 3205 working face in a mine, it was found that mining caused the partial overlying soil layer to move integrally and generate a large displacement difference with the adjacent layer, which verifies the conclusions of numerical simulation and mechanical analysis. The results of the study show that the thickness of the loose layer is the main control factor that causes the surface subsidence range and the building damage to increase; the shallow water-bearing sand-bearing layer has two types of movements: displacement and flow. The critical hydraulic slope has not reached the sand. The layer has a linearly increasing horizontal displacement value in the thickness direction; when the critical hydraulic slope is reached, the sand layer cannot transmit the frictional force, causing the overlying soil layer to slide as a whole. Both forms are prone to tensile damage on the surface. The research results provide a theoretical basis and practical case for surface subsidence reduction and green mining under similar geological conditions.