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| Experimental study on unloading mechanical properties of deep coal under in-situ stresses |
| WANG Lei,ZOU Peng,XIE Guangxiang,FAN Hao,CHEN Lipeng,ZHANG Yu |
(State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mine,
Anhui University of Science and Technology,Huainan,Anhui 232001,China) |
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Abstract In order to research the mechanical properties of deep coal under in-situ unloading stresses,triaxial loading and unloading tests with different unloading rates were carried out on coal under in-situ stresses by using the MTS816 rock mechanics test system and compared with conventional unloading tests. The stress-strain curve was used to analyze the mechanical properties and deformation characteristics of unloaded coal. 3D reconstruction technology by CT was applied to study the failure characteristics of coal. With this prerequisite, the applicable strength criterion of in-situ coal was discussed. The results show that:(1) The peak strengths of coal with two schemes are inversely proportional to the unloading rate. However,the peak strength of in-situ coal is higher than that of conventional coal,and this trend gradually weakens during low-speed unloading. (2) After in-situ stress recovery, as the unloading rate increases, the peak axial strain of coal first decreases and then tends to stabilize,the peak lateral strain remains unchanged,and the peak volumetric strain first decreases and then increases. Under unloading conditions,the elastic modulus of in-situ coal under unloading condition is more stable,making the strain hardening modulus enhanced. (3) The Mogi-Coulomb strength criterion can better reflect the failure strength characteristics of in-situ unloading coal. The cohesion c of in-situ coal increases 39.23% and the internal friction angle decreases 11.92% compared with conventional coal,indicating that the main control factor of failure resistance of in-situ unloading coal is cohesion.
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