Rock bridge of jointed rock plays a key role in its instability failure and has great influence on underground engineering. In this paper, the direct shear tests of jointed rocks with different rock bridge lengths are carried out, and the mechanical properties, acoustic emission (AE) characteristics, and fracture mechanism during the failure of rock bridge are studied. The results show that the shear strength increases with the increase of normal stress. When the rock bridge is long, cohesion c plays a leading role to control the failure of jointed rock; on the contrary, the internal friction angle φ plays a leading role. AE count rate, cumulative AE counts, and peak AE energy rate increase with the increase of rock bridge length. With the increase of rock bridge length, peak AE energy rate and AE count rate occur earlier. Considering the influence of rock bridge length, the fracture criterion and crack growth angle of shear fracture are derived based on linear elastic fracture mechanics. The theoretically calculated result is consistent with the experimental result. The research in this paper can provide relevant theoretical basis and useful reference for the instability and failure analysis of jointed rock.
Rock failure generally leads to serious consequences, and it is significant to obtain the precursor information prior to failure using associated techniques. Thus, it is essential to acquire and probe the relevant precursor information. In this study, true triaxial tests are performed on red sandstone specimens under varying intermediate principal stress conditions. The thermal infrared image evolution and the temperature-induced change characteristics of rock failure are also analyzed using infrared thermal imaging technology. In addition, with the assistance of a high-speed photography technique, these characteristics during the true triaxial compression and unloading processes are systematically investigated to determine how the intermediate principal impacts on thermal image, temperature, and fracture propagation. Finally, the evolution mechanism of the specimens is summarized, and a non-contact thermal infrared rock failure precursor indicator is proposed, which can give significant advance notice of rock collapse before the abnormal temperature change. The results show that there exist thermal infrared temperature precursors, thermal image precursors, and rapid development of rock macroscopic cracks before rock failure. Abnormal thermal images are prior to the abnormal temperature changes. As the intermediate principal stress increases, thermal abnormalities will change accordingly. Both temperature changes and thermal image anomalous patches can be utilized as precursor information of rock collapse, and the mechanism and specific information of thermal infrared failure precursors can be preliminarily determined in time and space. Our results can function as a significant frame of reference for the analysis and prevention of rock failure due to sudden instability.
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