超应力卸载作用下煤样冲击破坏试验研究

Experimental study on rock burst of coal samples under overstress and triaxial unloading

煤矿采深进入千米以后,采掘工作面围岩应力普遍超过煤体单轴抗压强度,呈现围岩应力超过煤体强度的超应力现象。基于调研分析,得到煤层单轴抗压强度的分布特征以及开采深度、原岩应力与煤层单轴抗压强度之间的关系,提出了超应力集中系数的概念。采用声发射和被动CT成像技术相结合的研究方法,开展了煤样真三轴超应力卸载作用下冲击破坏试验研究,从而探究声发射波速演化与煤样宏、微观破裂的关系,揭示深地围岩对煤层的超应力加载作用及方式。试验结果表明:(1)不同的应力卸载路径下煤样冲击破坏具有显著的时间延迟效应,应力路径变化越大,其时间延迟越短;(2)三轴卸载状态下煤样的破坏形式复杂多变,多为剪切、拉伸等耦合破坏形式;总体破坏模式表现为首先沿着与轴压方向分布的主裂隙进行扩张破坏,其次在试样表面分布着许多沿轴压方向的小张拉裂隙;(3)在加载初期,煤样内部波速变化范围较小,出现少量高、低波速区;随着载荷初步增加,煤样内高波速区转移与扩展,同时波速异常区明显扩大;当载荷进一步增加,煤样内出现大面积低波速带,波速极小值不断降低,高波速区、波速异常区迅速变化转移;(4)试样宏观破裂面和波速异常丰富区、微观裂隙演化和低波速贯通区形成了较好的对应。

When the coal mining depth is over 1 000 m,the surrounding rock stress around the excavation and mining coalface exceeds the uniaxial compressive strength of coal mass,which is defined as overstress effect.Based on the field investigation and analysis,the distribution characteristics of uniaxial compressive strength of coal samples,and the relationship between mining depth,vertical stress and the uniaxial compressive strength of coal samples were obtained and then the concept of the overstress coefficient was proposed.In this study,acoustic emission（ AE） and CT imaging techniques were combined to perform the CT inversing calculation for coal samples under overstress and triaxial unloading path in a triaxial rock burst system.The relations between velocity distribution and micro-and micro-fractures of coalsamples were studied,revealing the overstress loading effect and model in deep underground mining. Experimental results show：（1） There exists significant time-delay effect under different unloading stress paths.The more changeable the stress paths are,the shorter the time-delay will be.（2） The failure models under triaxial unoading are more complex and changeable,and the main failure model is shear and tensile coupling failure,which embodies as the main cracks along the axial direction expand firstly,then many small tensile cracks along the axial direction distribute on the surface of coal samples.（3） In initial loading phase,the velocity variation in coal smaples is relatively smaller and a few high and low velocity regions appear. When loading increases,the high velocity regions transfer and extend,and the velocity anomaly regions are enlarging continuously. When loading further increases,large areas of low velocity regions occur,while the minimun value of wave velocity constantly decreases,and the high velocity and velocity anomaly regions transfer and change quickly.（4） The wave velocity inversion results indicate that good correspondence were found between the macroscopic fracture surface of specimen and affluent velocity anomaly regions,as well as between microscopic fracture evolution and low wave velocity zones.