Mechanical properties and energy evolution mechanism of coal prone to ejection
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摘要:
通过开展不同主应力条件下真三轴加卸载试验，研究不同加卸载路径下易弹射煤体的力学特性、破坏特征及能量演化规律，揭示开挖卸荷易弹射巷道围岩弹射机制。研究表明：易弹射煤样在高应力单面卸荷比加载破坏更剧烈，当轴压为峰值的90%时卸荷，易弹射煤样表面出现一条大剪切裂缝，轴向应变率高，其自身破坏程度大，弹射现象明显；当轴压为峰值的80%,70%时卸荷，煤样未发生整体宏观破坏，轴向应变率低，自身的破坏程度小，仅在临空面产生张拉裂纹，弹射现象不明显；随着第二主应力的增大，在一定范围内对易弹射煤样有补强的作用，易弹射煤样内部复合型裂纹先增加后减小，破坏形态由剪切破坏转变为张拉–剪切复合破坏，最后发展为劈裂破坏，弹射剧烈程度呈现出先增加后减小的现象；高应力卸荷破坏过程中，弹性能转化为耗散能瞬间释放，耗散能占比急剧增大，抛离母体的碎块携带能量弹射出去，临空面出现明显的交叉网格裂缝，弹射现象明显；高应力易弹射煤样卸荷后张拉–剪切裂纹迅速扩展贯通，发生张拉–剪切复合破坏，弹射现象显著；卸荷后，易弹射煤样出现张拉–剪切裂纹，RA急剧值增高，AF值持续降低，AE呈现高能、高计、高幅、高频特征。研究结果可为类似地下工程开挖围卸荷岩微冲击现象控制提供借鉴。
Abstract:In this paper, true triaxial loading and unloading tests under different principal stress conditions were conducted to study the mechanical properties, failure characteristics, and energy evolution laws of coal bodies that are prone to ejection under different loading and unloading paths, aiming to reveal the ejection mechanism of the surrounding rock of roadway that is prone to ejection after excavation and unloading. The results are as follows. Firstly, the damage of coal samples that are prone to ejection was more severe under high stress single-sided unloading than under loading; when the axial pressure is 90% of the peak value, a large shear crack appeared on the surface of the coal samples, the axial strain rate is high, its own damage degree is large, and the ejection phenomenon was obvious; when the axial pressure was 80% and 70% of the peak value, respectively, the coal samples did not undergo overall macroscopic damage, the axial strain rate is low, and the degree of damage is small, only generated tensile cracks on the free face, and the ejection phenomenon was not obvious. Secondly, the increase of the second principal stress had a reinforcing effect on coal samples that are prone to ejection within a certain range, the internal composite cracks of coal samples that are prone to ejection first increase and then decreased, and the failure mode changed from shear failure to tension-shear composite failure, and finally developed into splitting failure; the intensity of ejection first increased and then decreased. Thirdly, in the process of high stress unloading failure, elastic energy was converted into dissipated energy and released instantly, and the proportion of elastic energy increased sharply, causing the fragments being ejected from the mother body with energy; there were obvious cross mesh cracks in the free face, and the ejection phenomenon was obvious. Fourthly, for coal samples that are prone to ejection under high stress after unloading, the tension-shear cracks rapidly expanded and connected, resulting in tension-shear composite failure, and the ejection phenomenon was obvious; Fifth, tension-shear cracks appear in ejectable coal samples of after unloading, RA increased sharply and AF decreased continuously, AE showing the characteristics of high energy, high meter, high amplitude and high frequency. The study results can provide reference for the on trol of micro impact phenomena in unloading surrounding rock during excavation of similar underground engineering.
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Keywords:
- coal ejection /
- principal stresses /
- unloading /
- energy /
- rock burst
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图 1 加卸载方式及声发射探头位置
Figure 1. Loading-unloading mode and acoustic emission probe position
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图 5 不同第2主应力加载轴向应力–应变曲线
Figure 5. Stress-strain curves under different second principal stress loading conditions
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图 6 不同第二主应力宏观破坏图
Figure 6. Macroscopic failure diagram of different second principal stress
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图 7 单位体积中的能量耗散和可释放应变能的量值关系
Figure 7. Quantitative relationship of energy release and energy dissipation
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图 16 高应力卸荷能量转化特征
Figure 16. Energy transformation characteristics of high stress unloading
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图 17 不同第二主应力能量转化特征
Figure 17. Energy transformation characteristics of different second principal stresses
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图 20 不同第二主应力计数–能量
Figure 20. Acoustic emission evolution characteristics of different second principal stresses
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图 21 不同第二主应力幅值–峰频
Figure 21. Acoustic emission evolution characteristics of different second principal stresses
表 1 初始应力值
Table 1 Initial stress value MPa
编号 $ {\sigma _{\text{1}}} $ $ {\sigma _{\text{2}}} $ $ {\sigma _{\text{3}}} $ TSX–1 20 10 5 TSX–2 100%$ {\sigma _{\mathrm{F}}} $ 10 5 TSX–3 90%$ {\sigma _{\mathrm{F}}} $ 10 5 TSX–4 80%$ {\sigma _{\mathrm{F}}} $ 10 5 TSX–5 70%$ {\sigma _{\mathrm{F}}} $ 10 5 TSX–6 20 5 5 TSX–7 20 15 5 TSX–8 20 20 5 
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表 2 高应力卸载力学参数
Table 2 High stress unloading mechanical parameters
试样编号 $ {\sigma _{\max }} $/MPa 弹性模量/GPa 应变率/s–1 TSX–2 31.5 1.30 0.052 TSX–3 28.35 2.32 0.045 TSX–4 25.2 2.56 0.026 TSX–5 22.06 2.56 0.020
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表 3 不同第二主应力力学参数
Table 3 Mechanical parameters of different second principal stresses
试样编号 $ {\sigma _{\text{2}}} $/MPa $ {\sigma _{\max }} $/MPa E/GPa 破坏程度 TSX–6 5 30.4 1.88 剧烈 TSX–1 10 31.5 2.00 剧烈 TSX–7 15 38.1 2.94 非常剧烈 TSX–8 20 27.1 2.03 非常剧烈
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表 4 高应力卸载下煤样能量值
Table 4 Energy value of coal sample under high stress unloading
kJ/m3 试样编号 $ U^{{\mathrm{e}}} $ $ U^{{\mathrm{d}}} $ $ U $ TSX–2 404.236 323.922 728.752 TSX–3 151.399 391.750 543.149 TSX–4 92.924 72.222 165.146 TSX–5 69.843 88.283 158.126
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表 5 不同第2主应力加载能量值
Table 5 Different second principal stress loading energy values
kJ/m3 试样编号 $ {U^{\text{e}}} $ $ {U^{\text{d}}} $ $ U $ TSX–5 160.511 260.062 420.573 TSX–1 133.863 423.475 557.338 TSX–7 300.161 353.578 653.739 TSX–8 274.588 325.642 600.230
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