The theory and technology of rock bolting are fundamental research topics for strata control in civil and mining engineering. Rebar bolts are commonly used for roadway primary support in underground coal mine. To adapt to deep resource mining, a new left threaded rebar bolt has been developed. Compared to conventional rebar bolts, the result of installation test showed that the new bolt reduced of 41.5% and 57.9% in stirring resistance force and torque, respectively. In laboratory pullout tests, PVC and aluminum sleeves were used to simulate weak and medium strength surrounding rocks. The average peak pullout force, displacement at the peak load and energy absorption increased by 27%, 107% and 108%, respectively, using PVC sleeve; and increased by 113%, 109% and 212%, respectively, using aluminum sleeve. Field tests were conducted under soft coal, hard coal and medium strength rock geo-conditions. Different borehole depths were selected to precisely calculate the average anchorage performance of the new bolt. Results showed that the average peak pullout force of the new bolt increased by 37%, 38% and 28%, respectively, under different surrounding rock conditions. Moreover, based on on-site test results, the pullout curves in field-testing were summarised and classified into 6 different patterns, which were discussed from a viewpoint of causality mechanism. The research findings validate that the newly developed bolt has better anchorage performance compared to conventional rebar bolts, making it a new anchorage material for deep resource mining.

The longwall mining method with gob-side entry retaining via roof cutting is a new underground coal mining method which has the characteristics of a high resource recovery ratio and environmental friendliness. Due to the complexity of this method, the research method of case-based dynamic on-site monitoring, analysis, adjustment, and optimization is usually adopted. Based on a roadway retaining via roof cutting project, in addition to the traditional indirect monitoring method of hydraulic support pressure, this study innovatively establishes a direct monitoring method for roof caving by monitoring the gangue pressure in the goaf, which provides data for the roof cutting effect and offers a new method for studying the overlying strata movement. In the project, a comprehensive monitoring and analysis system was established, including gangue pressure, cable bolt stress, bracket pressure, roadway deformation, and roof separation, which was used to dynamically analyze the effect of roof cutting and optimize the support design. The results show that the pressure of the hydraulic support close to the roof cutting is low, indicating that roof cutting is favorable in the roadway retaining mining method. The roadway deformation in the advanced abutment pressure area of the working face is small. The mining-induced stress caused by the collapse and compaction of the overlying strata in the goaf is the dominant factor affecting the effect of roadway retaining, especially in the 50–100 m range behind the working face, where the dynamic load causes high bearing capacity of the support elements, large roadway convergence, and roof separation. Temporary support and supplementary reinforcement should be added when necessary. The monitoring system presented in this study is highly comprehensive, simple, reliable, and low in cost, providing a reference for roof cutting roadway retaining projects and roof caving-related studies.

Coal contains cracks and has strong heterogeneity, so the data dispersion is large in laboratory tests. In this study, 3D printing technology is used to simulate hard rock and coal, and the rock mechanics test method is used to carry out the coal–rock combination experiment. The deformation characteristics and failure modes of the combination are analyzed and compared with the relevant parameters of the single body. The results show that the uniaxial compressive strength of the composite sample is inversely proportional to the thickness of the weak body and directly proportional to the thickness of the strong body. The Protodyakonov model or ASTM model can be used as a verification method for the results of a uniaxial compressive strength test of coal–rock combination. The elastic modulus of the combination is the equivalent elastic modulus, and the elastic modulus of the combination is between the elastic modulus of the two constituent monomers, which can be analyzed using the Reuss model. The failure of the composite sample occurs in the low-strength material, while the high-strength section is rebounding as an extra load on the low-strength body, which may cause a sharp increase in the strain rate of the weak body. The main failure mode of the sample with a small height–diameter ratio is splitting, and the failure mode of the sample with a large height–diameter ratio is shear fracturing. When the height–diameter ratio is not greater than 1, it shows pure splitting, and when the height–diameter ratio is 1~2, it shows a mixed mode of splitting and shear fracture. The shape has a significant effect on the uniaxial compressive strength of the composite specimen. For the impact propensity, it can be determined that the uniaxial compressive strength of the combination is higher than that of the single body, and the dynamic failure time is lower than that of the single body. It can hardly determine the elastic energy and the impact energy of the composite with the relationship to the weak body. The proposed methodology provides new cutting-edge test technologies in the study of coal and coal-like materials, with an exploration of their mechanical properties under compression.

Acoustic emission (AE) can reflect the dynamic changes in a material’s structure, and it has been widely used in studies regarding coal mechanics, such as those focusing on the influence of loading rate or water content change on the mechanical properties of coal. However, the deformational behavior of coals with various strengths differs due to the variation in microstructure. Hard coal presents brittleness, which is closely related to certain kinds of geological disasters such as coal bursts; soft coal exhibits soft rock properties and large deformation mechanical characteristics. Therefore, conclusions drawn from AE characteristics of a single coal sample have application limitations. This paper studies the deformation patterns and AE characteristics of coals with different strengths. A uniaxial compression experiment was carried out using coal samples with average uniaxial compressive strengths of 30 MPa and 10 MPa; the SAEU2S digital AE system was used to measure the AE counts, dissipation energy, and fracturing point distributions at each deformation stage of the different coals. The results show that the bearing capacity of hard coal is similar to that of the elastic stage and plastic deformation stage, but it may lose its bearing capacity immediately after failure. Soft coal has a relatively distinct stress-softening deformation stage and retains a certain bearing capacity after the peak. The AE counts and dissipation energy of hard coal are significantly higher than those of soft media, with average increases of 49% and 26%, respectively. Via comparative analysis of the distribution and development of internal rupture points within soft coal and hard coal at 15%, 70%, and 80% peak loads, it was observed that hard coal has fewer rupture points in the elastic deformation stage, allowing it to maintain good integrity; however, its rupture points increase rapidly under high stress. Soft coal produces more plastic deformation under low loading conditions, but the development of the fracture is relatively slow in the stress-softening stage. We extracted and summarized the AE characteristics discussed in the literature using one single coal sample, and the results support the conclusions presented in this paper. This study subdivided the deformation process and AE characteristics of soft and hard coals, providing a theoretical guidance and technical support for the application of AE technology in coal with different strengths.

Geotechnical engineering is a key element for all engineering construction that establishes contact with the earth, including foundation engineering, slope engineering, tunnel engineering, mining engineering, etc

Entry retaining via roof cutting is a new longwall mining method that has emerged in recent years, and is characterized by high resource utilization and environmental friendliness. Due to the complexity of this method, a field study is commonly employed for process optimization. Roof blasting is a key operation for retaining the entry, and the current practice involves dynamically adjusting blasting parameters through on-site testing and postblasting monitoring. However, the existing literature lacks detailed descriptions of blasting operations, making it difficult for field engineers to replicate the results. In this study, based on a roof cutting project for entry retaining, a preliminary design of blasting parameters is made based on theories and on-site geological conditions. The on-site test methods and equipment for roof-cutting blasting are described in detail, and the fractural patterns under different blasting parameters are analyzed. After the retreat of the working face, the state of roof caving in the goaf is analyzed based on monitoring data, and the effectiveness of top cutting is evaluated through reverse analysis, leading to dynamic adjustments of the blasting parameters. This research provides a reproducible construction method for roof-cutting operations and establishes the relationship between blasting parameters and post-mining monitoring data. It contributes to the development of fundamental theories and systematic technical systems for entry retaining via roof cutting, offering high-quality case studies for similar geological engineering projects.