Measurement of in situ stress is critical to understand the deformation and destruction of the underground space surrounding rock, and dynamic disaster of the coal mine. At present, with the increasing depth of mining, in situ stress parameters are more and more important for coal mine. In this paper, a novel method for in situ stress measurement with Kaiser effect was studied and applied in the Baijiao coal mine. First, we presented a comprehensive analysis method for the identification of Kaiser effect point. Then, a calculation method for in situ stress measurement based on the Kaiser effect on acoustic emissions was suggested. After that, the in situ stress test of Baijiao coal mine is taken as the research object, an experiment using acoustic emissions monitoring during uniaxial compression testing was performed to investigate the mechanical properties and acoustic emissions characteristics. Finally, in situ stress of the study area was calculated using the novel calculation method above and calculation results were verified using stress relief method and hydraulic fracturing method. The results showed that the calculation results obtained using the proposed method were valid and credible. Therefore, the calculation method for in situ stress measurement and the proposed comprehensive analysis method using the Kaiser point could be applied for in situ stress testing using the Kaiser effect method.

The present experimental study on permeability characteristics for raw coal under different stress states is implemented by applying the triaxial self-made ‘THM coupled with servo-controlled seepage apparatus for gas-containing coal’; the result indicates that the flow rate of gas in the coal sample gradually decreases with the nonlinear loading of axial pressure and increases with the nonlinear unloading of axial stress and confining pressure. The flow rate, axial stress and confining pressure curves all satisfy the negative exponential function relation. When the sample reaches the peak intensity, the sample will be destroyed and the stress will drop rapidly; then the flow rate of the sample will increase rapidly. At this stage, the flow rate and axial strain show an oblique ‘v' pattern. The flow rate of the coal sample increases nonlinearly with the increase of gas pressure; the relation curve between flow rate and gas pressure satisfies the power function relation. Under the same confining pressure and gas pressure conditions, the larger the axial stress, the smaller the flow rate of the coal sample. Under the same axial stress and gas pressure conditions, the flow rate of the coal sample will first decrease, but then increase as the confining pressure decreases. During the post-peak loading and unloading process, the flow rate of the coal sample will decrease with the loading of confining pressure but increase with the unloading of confining pressure, and there will be an increase in wave shape with the increase in axial strain. The flow rate of each loading and unloading confining pressure is higher than that of the previous loading and unloading confining pressure. At the post-peak stage, the relation curve between the flow rate of the coal sample and the confining pressure satisfies the power function relation in the process of loading and unloading confining pressure.

To better apply nuclear magnetic resonance (NMR) to evaluate the petrophysical characterization of high-rank coal, six anthracite samples from the Baijiao coal reservoir were measured by NMR. The porosity, T ₂ cutoff value, permeability and pore type were analysed using the transverse relaxation time ( T ₂ ) spectrum before and after centrifugation. The results show that the T ₂ spectrum of water-saturated anthracite can be divided into a discontinuous and continuous trimodal distribution. According to the connectivity among pores, three T ₂ spectrum peaks were identified at the relaxation times of 0.01–1.7 ms, 1.7–65 ms and greater than 65 ms, which correspond to the micropores (less than 100 nm), mesopores (100–1000 nm) and macropores (greater than 1000 nm), respectively. Based on the T ₂ cutoff value, we divided the T ₂ spectrum into two parts: bound fluid and free fluid. By comparing two classic permeability models, we proposed a permeability model to calculate the permeability of anthracite. This result demonstrates that NMR has great significance to the exploration of coal reservoirs and to the understanding of the development of coalbed methane.

To further understand the fracture behavior of rock materials containing en echelon flaws and determine precursor information for the final collapse of damaged mineral assemblies under compression, a series of uniaxial compression experiments using a loading system, an acoustic emission system and a video camera was conducted on sandstone specimens containing en echelon flaws. The mechanical properties, energy dissipation characteristics and acoustic emission parameters of compression failure of selected specimens were successively analyzed. The results showed that crack initiation was accompanied by a stress drop, step-like characteristics on the energy consumption curve and increased crackling noises, which were used as early warning signals before the final collapse happened. In addition, we used the histogram statistics method and maximum likelihood method to analyze the distribution of acoustic emission energy and determined that the acoustic emission energy distributions of sandstone specimens containing en echelon flaws followed a power law. With the progress of the experiment, the optimum exponents changed in different stages and gradually decreased as failure was approached, which could also be used as an early warning signal before the final collapse happened. This paper may provide some theoretical basis for monitoring and warning about the collapse and instability of engineering rock masses containing en echelon flaws.