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Brief

Journal of China Coal Society, the 1st in the field of mining engineering, promotes the development of coal theory and practice, as well as academic exchanges at home and abroad to accelerate the transformation from scientific research to real productive forces. It is supervised by China Association for Science and Technology and sponsored by China Coal Society. Launched in 1964, it aims at providing key topics of coal scientific research and mining development. Its scope covers coal geology, geochemistry, geophysics, mineralogy and coal seam formation and evolution; petrophysics of coal; organic matter-rich shales, including mineralogy, formation, transport and storage of gases in coal and shales; unconventional energy systems (e.g. oil shales, shale gas, and other carbon-based fuels); ore deposits of materials and rare metals in coal and coal-bearing strata; and groundwater management. The journal is included in CA, JST, EI, CSCD.
Editorial Board

Editor-in-Chief
Liu Feng

Associate Editor
Fan Baoying, Xu Shengyang, Zhu Shuangcheng

Analysis of stress distribution characteristics of fully anchored bolt based on actual surrounding rock deformation

LI Yingming;ZHAO Chengxing;CONG Li;MENG Xiangrui;DONG Chunliang

Journal of China Coal Society,2019,Vol 44,No. 10

During the supporting process of a fully anchored bolt, the interaction between the bolt and the surrounding rock will cause the stress distribution of the bolt to change. In order to explore the stress distribution of the bolt in the normal supporting process and critical failed supporting process, the interaction model of the anchored bolt–surrounding rock is established based on the deformation of surrounding rock. The analytical expressions of axial force and shear stress along the length of the bolt during the normal supporting process and critical failed supporting process are derived. Furthermore, the distribution curves of axial force and shear stress along the length of the bolt are obtained. Also, the effects of rock conditions, anchor length, and tray anchoring force on the stress distribution of a fully anchored bolt are analyzed under the consideration of anchorage parameters such as the moduli and cross-sectional areas of the bolt and the anchoring agent. The results show that during the normal supporting of the fully anchored bolt, the stress distribution of the bolt body conforms to the neutral point theory. The factors affecting the axial force and shear stress distribution of the anchor include surrounding rock conditions, anchorage parameters and tray anchoring force. During the normal supporting process, the stress distribution of the fully anchored bolt body conforms to the neutral point theory. As the surrounding rock becomes softer, the neutral point moves to the orifice, and the axial force and shear stress increase. Tray anchoring force affects the stress distribution of the bolt body. The greater the anchoring force, the more uneven the axial-force distribution. In addition, the shear stress from the orifice to the neutral point becomes lower, while that from the neutral point to the bolt end becomes higher. Therefore, in the actual project, the tray anchoring force can be monitored in real time. The theoretical basis of this paper can reveal the force characteristics during the bolt supporting process. In the critical failure of bolt supporting, the harder the surrounding rock, the higher the shear stress and the axial force and the more concentrated the stress distribution. The length of the bolt affects the supporting performance of a fully anchored bolt, but the increase in the bolt length after the bolt length exceeds a certain range does not significantly improve the anchoring effect of the anchor bolt.

Characteristics and evolution mechanism of acoustic emission time-frequency signal during coal failure process

DING Xin;XIAO Xiaochun;LV Xiangfeng;ZHAO Tongbin;YIN Yanchun;SONG Yimin;YANG Xiaobin;PAN Yishan

Journal of China Coal Society,2019,Vol 44,No. 10

The time-frequency domain characteristics of an acoustic emission signal and their essential relationship with mechanical properties of coal are the basis for predicting and warning the instability of coal. On the basis of acoustic emission monitoring tests for the compressive failure of coal with different partings and original cracks, the time-frequency domain evolution of an acoustic emission signal is discussed in detail combined with the digital signal theory, rock mechanics and other related approaches, and the wavelet transform method is introduced. The mechanical expressions of amplitude and frequency of a stress wave are established, which is induced by the elastic energy released from crack growth. The results showed that with the increase in weak partings or cracks in coal, its strength and elastic modulus decrease. The softening characteristic is obvious in the post-peak phase, and there is a signal surge point at which a signal transforms from a low-amplitude oscillation to a high-amplitude pulse of acoustic emission. The higher strength of coal can lead to the higher amplitude of an energy signal, the more cumulative total energy, the larger amplitude of waveform, the longer interval between the two adjacent peaks of signal waveform, and the fewer small oscillations mixed in the signal. The wavelet basis functions of db5 and sym2 have the highest similarity with the time-domain waveforms at the surge point and peak point respectively, which is more suitable for the study of acoustic emission signals of coal. The main frequency band of signal is 0–70 kHz. The lower loading stress level indicates the wider signal frequency distribution, and the signal band distribution gradually moves to the main frequency with the increase in stress. The variation range of stress-wave amplitude is determined by the elastic modulus and the crack propagation rate. The crack size determines the variation trend of amplitude and frequency, and the crack propagation rate is a key parameter to determining the stress wave frequency. Then three parameters affect the time-frequency characteristics of acoustic emission signals. Based on the experimental results, the qualitative description of the frequency and amplitude of AE signals with crack characterization parameters is established, which provides a theoretical basis for improving the accuracy of the AE monitoring method. Therefore, the quantitative application of the theory is the focus in the future research.