基于CT扫描的煤岩裂隙特征及其对破坏形态的影响

Characteristics of coal-rock fractures based on CT scanning and its influence on failure modes

为了研究煤岩原始裂隙结构对其宏观破坏特征的影响,以河南城郊煤矿21304工作面的煤岩为研究对象,利用nanoVoxel-4000系统对不同围压作用下加载前后的煤样进行了CT扫描试验。基于图像处理软件对裂隙结构进行提取和三维重建,结合等效球体模型定量分析了煤岩内不同尺度原始裂隙在空间的分布特征。通过计算平面内原始裂隙与破裂主裂隙的裂隙组构张量,以及空间内原始主裂隙面与新产生主裂隙面的倾角,对比分析了不同围压下原始裂隙分布方向与破坏时新生成主裂隙方向之间的关系。研究结果表明:煤样试件内含有大量不同尺度的裂隙,等效直径小于1 000μm的裂隙数量多,体积小;等效直径大于5 000μm的裂隙数量少,体积大;当等效直径小于5 000μm时,随着裂隙等效直径的增加,裂隙的数目逐渐减少;在等效直径小于300μm时,裂隙体积分数随等效直径的增加而增大;等效直径在3 00~5 000μm的裂隙的体积分数整体呈现下降趋势,且等效直径在大于1 500μm以后波动较大。利用2阶和4阶裂隙组构张量计算得到的椭圆形分布规律,可以有效的描述裂隙的优势方向和劣势方向,且相比2阶张量,4阶对于平面裂隙分布规律的描述更全面。不同围压下平面内原始裂隙的2阶和4阶裂隙张量计算的优势方向与破坏主裂隙的发展方向一致,三维空间新产生的破坏主裂隙面会沿着原有较大主裂隙面方向扩展,形成了近似平行的裂隙面。不同围压作用下,煤岩试件内部损伤破裂面的三维分形维数不同,随着围压的增大裂隙面的分形维数减小,二者具有较好的相关性。

To study the influence of original fracture of coal rock on its macroscopic failure characteristics,the coal at the No.21304 working face of the Chengjiao Coal Mine in Henan Province has been investigated.The CT scanning test was carried out before and after loading coal samples under different confining pressures by using nanoVoxel-4000 system.The fracture structure was extracted and reconstructed based on the image processing software.Combining with the equivalent sphere model,the original fractures of different scales were quantitatively analyzed.By calculating the fracture fabric tensor of the original fracture and the failure master fracture in the plane,and the dip angle of original fracture surface and the new main fracture surface in the space,the relationship between the distribution direction of the original fracture and the direction of newly generated fracture under different confining pressures was analyzed.The results show that the coal sample contains a large number of fractures of different scales.The number of fractures with equivalent diameter less than 1 000 μm is large,and the volume is small and the number of fractures with equivalent diameter greater than 5 000 μm is small and the volume is large.When the equivalent diameter is less than 5 000 μm,with the increase of the equivalent diameter,the number of fractures decreases.When the equivalent diameter is less than 300 μm,with the increase of the equivalent diameter,the fracture volume fraction increases.The equivalent diameter ranges from 300 μm to 5 000 μm,the volume fraction of fracture is generally decreased,and the fluctuation is obvious when the equivalent diameter is more than 1 500 μm.The elliptic distribution rule calculated by the second-order and fourth-order fracture fabric tensor can effectively describe the dominant and inferior directions of the plane fractures,and the fourth-order tensor is more comprehensive than the second-order.The dominant direction of the second and fourth-order fracture tensor calculations is consistent with the development direction of the new main fractures under different confining pressures.The new failure main fracture surface will expand along the direction of the original larger main fracture surface,and forming a nearly parallel fracture surface in three-dimensional space.Under different confining pressures,the three-dimensional fractal dimension of the internal fracture surface of coal specimens is different,with the increase of confining pressure,the fractal dimension of fracture surface decreases,and there is a good correlation between them.