International Journal of Mining Science and Technology最新文献

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.

This study aims to investigate the mechanical response and acoustic emission (AE) characteristic of pre-flawed sandstone under both monotonic and multilevel constant-amplitude cyclic loads. Specifically, we explored how coplanar flaw angle and load type impact the strength and deformation behavior and microscopic damage mechanism. Results indicated that being fluctuated before rising with increasing fissure angle under monotonic loading, the peak strength of the specimen first increased slowly and then steeply under cyclic loading. The effect of multilevel cyclic loading on the mechanical parameters was more significant. For a single fatigue stage, the specimen underwent greater deformation in early cycles, which subsequently stabilized. Similar variation pattern was also reflected by AE count/energy/b-value. Crack behaviors were dominated by the fissure angle and load type and medium-scale crack accounted for 74.83%–86.44% of total crack. Compared with monotonic loading, crack distribution of specimen under cyclic loading was more complicated. Meanwhile, a simple model was proposed to describe the damage evolution of sandstone under cyclic loading. Finally, SEM images revealed that the microstructures at the fracture were mainly composed of intergranular fracture, and percentage of transgranular fracture jumped under cyclic loading due to the rapid release of elastic energy caused by high loading rate.

This study used the stable and convergent Dufort-Frankel method to differentially discretize the diffusion equation of the ground-well transient electromagnetic secondary field. The absorption boundary condition of complex frequency-shifted perfectly matched layer (CFS-PML) was used for truncation so that the low-frequency electromagnetic wave can be better absorbed at the model boundary. A typical three-dimensional (3D) homogeneous half-space model was established and a low-resistivity cube model was analyzed under the half-space condition. The response patterns and drivers of the low-resistivity cube model were discussed under the influence of a low-resistivity overburden. The absorption boundary conditions of CFS-PML significantly affected the low-frequency electromagnetic waves. For a low-resistivity cube around the borehole, its response curve exhibited a single-peak, and the extreme point of the curve corresponded to the center of the low-resistivity body. When the low-resistivity cube was directly below the borehole, the response curve showed three extreme values (two high and one low), with the low corresponding to the center of the low-resistivity body. The total field response of the low-resistivity overburden was stronger than that of the uniform half-space model due to the low-resistivity shielding effect of electromagnetic waves. When the receiving-transmitting distance gradually increased, the effect of the low-resistivity overburden was gradually weakened, and the response of the low-resistivity cube was strengthened. It was affected by the ratio of the overburden resistivity to the resistivity of the low-resistivity body.

Faced with the continuous occurrence of coal and gas outburst (hereinafter referred to as “outburst”) disasters, as a main controlling factor in the evolution process of an outburst, for gas pressure, it is still unclear about the phased characteristics of the coupling process with in situ stress, which induce coal damage and instability. Therefore, in the work based on the mining stress paths induced by typical outburst accidents, the gradual and sudden change of three-dimensional stress is taken as the background for the mechanical reconstruction of the disaster process. Then the true triaxial physical experiments are conducted on the damage and instability of coal containing gas under multiple stress paths. Finally, the response characterization between coal damage and gas pressure has been clarified, revealing the mechanism of action of gas pressure during the initial failure of coals. And the main controlling mechanism during the outburst process is elucidated in the coupling process of in situ stress with gas pressure. The results show that during the process of stress loading and unloading, the original gas pressure enters the processes of strengthening and weakening the action ability successively. And the strengthening effect continues to the period of large-scale destruction of coals. The mechanical process of gas pressure during the initial failure of coals can be divided into three stages: the enhancement of strengthening action ability, the decrease of strengthening action ability, and the weakening action ability. The entire process is implemented by changing the dominant action of in situ stress into the dominant action of gas pressure. The failure strength of coals is not only affected by its original mechanical strength, but also by the stress loading and unloading paths, showing a particularly significant effect. Three stages can be divided during outburst inoculation process. That is, firstly, the coals suffer from initial damage through the dominant action of in situ stress with synergy of gas pressure; secondly, the coals with spallation of structural division are generated through the dominant action of gas pressure with synergy of in situ stress, accompanied by further fragmentation; and finally, the fractured coals suffer from fragmentation and pulverization with the gas pressure action. Accordingly, the final broken coals are ejected out with the gas action, initiating an outburst. The research results can provide a new perspective for deepening the understanding of coal and gas outburst mechanism, laying a theoretical foundation for the innovation of outburst prevention and control technologies.

The lag in quantitative methods and detection techniques for geologic information has resulted in time-consuming and human-experienced geologic analysis in tunnels. Geochemical indicators of rocks can be used to identify adverse geology and to explain the intrinsic causes of damage to normal rocks. This study proposes a method to identify adverse geology by extracting and imaging the indicator elements. The mapping relationship between rock components and geologic bodies is quickly determined by indicator element extraction based on factor analysis, and then the data are gridded for image output. The location and size of the target adverse geology are visually identified through the distribution images of the indicator elements, thus reducing data dimensions and analysis time. A non-destructive, in-situ and fast element detection technique in tunnels was adopted to speed up the process of geology identification. The accuracy of the detection was validated by comparing field and laboratory test results. This study further confirms and refines the previous research, and the results provide references for geological, mining and underground projects.