竖直恒载循环作用下煤层钻孔失稳演化研究

Study on failure evolution of in-seam boreholes under cycle constant vertical loadings

为研究采动影响下煤层钻孔的长时失效机制,采用自主研制的双向加载试验装置开展竖直应力多次循环下钻孔失稳的模型试验。结果表明,恒压阶段钻孔的收缩具有显著的时效性,而升压阶段加剧了钻孔收缩,引入的钻孔收缩模型较好的描述了孔径变化规律;多次循环虽然促使钻孔收缩,但钻孔收缩速率随之衰减。成孔初期围岩损伤剧烈、裂纹发育,但孔壁处裂纹不明显,损伤效应主要表现为钻孔收缩;在第1次循环的恒压段,围岩持续损伤并趋稳,孔壁损伤加强;升压阶段破坏了原有的稳定,加剧了孔壁的损伤并扩大了损伤范围,提高了煤岩剥落的风险,但升压对围岩的损伤程度随循环次数的增多而下降;多次循环后,围岩损伤积累,裂纹进一步扩展、贯通,导致严重的失稳。

To illuminate the long-time failure mechanisms for in-seam boreholes influenced by mining disturbances,the model experiment tests under a cyclic vertical stress were carried out by employing a independently-developed bi-direction loading device.Results show that the shrinkage of the borehole has obvious time effect at the constant stress stage and increases at the boost stress stage,which can be described well by the introduced borehole shrinkage model.Cycle loading makes the borehole shrink but the shrinkage rate decreases with increasing the cycle number.At the initial stage after drilling,the surrounding rock damages intensely with tiny fractures accompanied by no obvious cracks on the hole wall,which mainly causes borehole shrinkage.At the constant stress stage at the first cycle,the damage of the surrounding rock continues and tends to be stable,and the damage on the hole wall is enhanced.At the boost stress stage,the damage on the hole wall increases further and the damage range is enlarged,which raises the possibility of rock peeling from the hole wall.Meanwhile,the influence of stress boosting to the damage of the surrounding rock decreases with increasing the cycle number of loading.After repeatedly loading cycles,the damage of the surrounding rock accumulates and fractures propagate and link,inducing serious instability of the hole.