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变速率加载砂岩力学响应及声发射破裂前兆识别预警

Mechanical response of sandstone under variable loading rates and acoustic emission-based identification and early warning of fracture precursors

  • 摘要: 切顶留巷是深部煤炭安全高效开采的重要技术手段,但受一次掘进与二次强采动影响,围岩变形剧烈,底鼓问题突出。为揭示留巷过程中覆岩−底板的应力传递规律及底板岩石在变速率加载下的破裂演化机制,开展了相似模拟及单轴变速率加载试验,综合分析底板应力响应特征与不同加载速率下岩石力学、声发射与分形行为。相似模拟结果表明:巷道开挖后底板表面瞬时卸荷,随工作面推进与覆岩周期垮落,垮落矸石堆积压实使得底板应力回升;覆岩结构稳定后底板应力变化趋缓,系统进入准静态平衡阶段。在此基础上开展的变速率加载岩石力学试验表明,加载速率显著影响砂岩的破裂模式与力学响应,快速加载下应变速率高、裂纹集中起裂并迅速贯通,能量释放剧烈,表现为典型脆性破坏;较早切入准静态加载时,裂纹呈多点萌生与缓慢贯通特征,峰前非线性阶段延长,脆性减弱、延性增强;全程准静态加载下损伤累积最充分,破裂过程呈渐进性。声发射结果显示,快速加载下AE信号突发集中、计数与能量峰前急剧上升,预警窗口极短;降低加载速率后,裂纹扩展受抑、事件转为连续活跃,峰后仍保持高能量释放。多重分形分析表明,AE分形谱呈“钟形”分布,随应力增加先展宽后收敛。快速加载或晚期切速使谱形左偏、突发性增强;低速或早期切速下谱形更对称,表明裂纹渐进扩展。基于AE多参量方差构建的预警指标进一步表明快速加载下方差在峰前短时急剧抬升,预警信号出现晚、持续时间短;降低加载速率后,方差呈持续上升、多峰密集,L1、L2、L3三级预警依次触发,窗口明显延长;全程准静态加载时方差波动更密集,可提前识别裂纹加速贯通的过程,表明加载速率越低,裂隙演化越充分,破裂前兆越易识别。降低加载速率有利于裂纹的充分演化发展,从而增强岩石破裂前兆特征的可识别性与可预见性,为深部岩体稳定性监测与灾害预警提供理论依据。

     

    Abstract: Gob-side entry retaining with roof cutting is an important technique for achieving safe and efficient deep coal mining. However, under the combined effects of primary roadway excavation and secondary intensive mining-induced disturbances, severe surrounding rock deformation often occurs, with floor heave becoming a prominent problem. To clarify the stress transfer mechanism between the overlying strata and the floor during gob-side entry retaining, as well as the fracture evolution behavior of floor rock under variable loading rates, similarity simulation tests and uniaxial variable-rate loading experiments were conducted. The floor stress response and the mechanical behavior, acoustic emission (AE) characteristics, and fractal features of sandstone under different loading rates were systematically analyzed. The similarity simulation results indicate that the floor surface experiences instantaneous unloading after roadway excavation. With the advancement of the working face and periodic roof caving, the accumulation and compaction of caved gangue lead to stress recovery in the floor. After the overlying strata structure becomes stable, variations in floor stress gradually diminish, and the system enters a quasi-static equilibrium stage. Based on this stress evolution background, variable-rate loading experiments reveal that the loading rate has a significant influence on the fracture mode and mechanical response of sandstone. Under rapid loading, high strain rates promote concentrated crack initiation and rapid coalescence, accompanied by intense energy release, resulting in typical brittle failure. When quasi-static loading is introduced at an earlier stage, cracks initiate at multiple locations and propagate slowly, the pre-peak nonlinear stage is prolonged, brittleness is reduced, and ductility is enhanced. Under fully quasi-static loading, damage accumulation is most sufficient, and the fracture process exhibits a progressive failure mode. AE results show that rapid loading induces sudden and highly concentrated AE activity, with sharp pre-peak increases in AE counts and energy, leading to an extremely short warning window. As the loading rate decreases, crack propagation is restrained, AE events become continuously active, and high-energy release persists even after peak stress. Multifractal analysis demonstrates that the AE fractal spectra exhibit a bell-shaped distribution, which initially broadens and then converges with increasing stress. Rapid loading or late-stage rate reduction leads to left-skewed spectra with enhanced burst characteristics, whereas lower loading rates or early-stage rate reduction produce more symmetric spectra, indicating progressive crack development. Early-warning indicators constructed from the variance of multiple AE parameters further show that under rapid loading, the variance rises sharply shortly before peak stress, resulting in delayed and short-lived warning signals. With decreasing loading rate, the variance exhibits sustained growth with dense multi-peak features, triggering L1, L2, and L3 warning levels sequentially and significantly extending the warning window. Under fully quasi-static loading, variance fluctuations become more frequent, enabling earlier identification of accelerated crack coalescence. These results indicate that a lower loading rate promotes more complete fracture evolution and enhances the identifiability of failure precursors. The findings provide deeper insight into rock fracture characteristics and offer a theoretical basis for stability monitoring and disaster early warning in deep rock engineering.

     

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