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Although the grain size effect on rock’s physical–mechanical properties has been widely studied, little attention has been paid to the reasons for this phenomenon. In this paper, an attempt is made to interpret the effect of grain size on uniaxial compressive strength (UCS) from the point of view of mechanics. Based on the microscopic images of rock with different mineral grain size, actual mineral types and grain sizes were identified and characterized using digital image processing techniques and microstructure analysis. By mapping this real rock microstructure into a well-established rock failure process analysis code, numerical specimens were built to model the grain size effect on the UCS. The results showed that considering only mineral type and grain size, the numerical method can reveal the grain size effect on UCS. Further, an ideal conceptual model is proposed to clearly interpret and discuss the grain size effect on strength. It was found that grain size results in different stress concentrations within rock. Under identical loading conditions, the bigger the grain size is, the stronger the stress concentration is. This different mechanical response can eventually affects the UCS of the rock, resulting in the grain size effect on rock strength. Moreover, fractal geometry is also introduced to quantify rock microstructure with different grain sizes. The results show that there exists a good relationship between the fractal dimension and the grain size, which can describe the grain size effect on rock strength both quantitatively and effectively.

The introduction of nitrogen heteroatoms into carbon materials is a facile and efficient strategy to regulate their reactivities and facilitate their potential applications in energy conversion and storage. However, most of nitrogen heteroatoms are doped into the bulk phase of carbon without site selectivity, which significantly reduces the contacts of feedstocks with the active dopants in a conductive scaffold. Herein we proposed the chemical vapor deposition of a nitrogen-doped graphene skin on the 3D porous graphene framework and donated the carbon/carbon composite as surface N-doped grapheme (SNG). In contrast with routine N-doped graphene framework (NGF) with bulk distribution of N heteroatoms, the SNG renders a high surface N content of 1.81 at%, enhanced electrical conductivity of 31 S cm−1, a large surface area of 1531 m2g−1, a low defect density with a low ID/IGratio of 1.55 calculated from Raman spectrum, and a high oxidation peak of 532.7 °C in oxygen atmosphere. The selective distribution of N heteroatoms on the surface of SNG affords the effective exposure of active sites at the interfaces of the electrode/electrolyte, so that more N heteroatoms are able to contact with oxygen feedstocks in oxygen reduction reaction or serve as polysulfide anchoring sites to retard the shuttle of polysulfides in a lithium–sulfur battery. This work opens a fresh viewpoint on the manipulation of active site distribution in a conductive scaffolds for multi-electron redox reaction based energy conversion and storage.

Gas diffusion and seepage characteristics can reflect the transport capacity of coal mass pores and fracture channels and are also important parameters to determine the mechanism of methane migration. Currently, research on the contribution of diffusion and seepage to methane migration is carried out mainly by fixing one parameter and discussing the effect of the other. However, few studies are focused on the dynamic combined effect of diffusion and seepage on methane migration under in situ stress conditions, which is mainly attributed to difficulties in obtaining the parameters of diffusion characteristics of coal mass under stress. This paper presents a study on the dynamic combined effect of coal mass diffusion and seepage on methane migration under different in situ stress conditions. First, the physical structure model of coal mass under high and low stresses was constructed based on the variations of coal mass porosity, permeability, and computed tomography under different stresses. Meanwhile, based on the methane desorption and diffusion curves of coal mass under different stresses, the diffusion parameters of coal mass were obtained according to the corresponding diffusion model. Furthermore, the coupled multifield model was used to calculate the methane migration in coal mass during the process of borehole extraction with different stresses. The results suggest that the difficulty in methane extraction is directly proportional to the stress, but a huge discrepancy exists in methane migration under high and low stresses principally due to the change of dominant factors in methane migration of coal mass under different stresses. Additionally, given the essential factors controlling the overall methane migration of coal mass under high and low stresses, a joint technical method for promoting methane flow rate under multicoal seam conditions in the same mining area was proposed, i.e., hydraulic fracturing and sand injection (increasing permeability) in coal seams under low stress and simultaneous application of hydraulic fracturing and sand injection and multistage cavitation pressure relief (increasing diffusion) in coal seams under high stress. The research results obtained in this paper provide theoretical support for understanding the methane migration mechanism in coal seams under in situ stress and stimulated gas production.

To further study the load transfer mechanism of roof–multi-pillar–floor system during cascading pillar failure (CPF), numerical simulation and theoretical analysis were carried out to study the three CPF modes according to the previous experimental study on treble-pillar specimens, e.g. successive failure mode (SFM), domino failure mode (DFM) and compound failure mode (CFM). Based on the finite element code rock failure process analysis (RFPA2D), numerical models of treble-pillar specimen with different mechanical properties were established to reproduce and verify the experimental results of the three CPF modes. Numerical results show that the elastic rebound of roof–floor system induced by pillar instability causes dynamic disturbance to adjacent pillars, resulting in sudden load increases and sudden jump displacement of adjacent pillars. The phenomena of load transfer in the roof–multi-pillar–floor system, as well as the induced accelerated damage behavior in adjacent pillars, were discovered and studied. In addition, based on the catastrophe theory and the proposed mechanical model of treble-pillar specimen–disc spring group system, a potential function that characterizes the evolution characteristics of roof–multi-pillar–floor system was established. The analytical expressions of sudden jump and energy release of treble-pillar specimen–disc spring group system of the three CPF modes were derived according to the potential function. The numerical and theoretical results show good agreement with the experimental results. This study further reveals the physical essence of load transfer during CPF of roof–multi-pillar–floor system, which provides references for mine design, construction and disaster prevention.

With the excavation of underground opening, the roof rock block may fall under external dynamic disturbance. In this paper, the roof rock mass was simplified as a rock block system composed of trapezoidal rock blocks. A series of experiments and numerical simulations were performed to study the sliding process of the key block under the horizontal static clamping load and vertical impact disturbance. The propagation of stress wave in the block system were captured and analyzed by using high-speed camera and digital image correlation technique. The results reveal that a pendulum-type wave was generated due to the propagation and superposition of stress waves in the block system. Therefore, the governing mechanism of the sliding displacement of the key block was clarified based on the propagation of incident stress waves or pendulum-type waves. Meanwhile, it is found that the sliding distance of the key block decreases in a power function with the increasing friction coefficient, or decreases in a parabolic function with the increasing trapezoid internal angle. Finally, a case study on the roof block sliding of the roadway at a gold mine was conducted, and it is concluded that the sliding of the key block resulted from the coupled effects of “shear driving” and “low friction” driven by stress wave propagation, regardless of a single or multi-layer rock block system. These results may provide technical guideline for preventing rock-falling accidents caused by blasting disturbances in underground mining.