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

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 sys- tem. 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 frac- ture fabric tensor of the original fracture and the failure master fracture in the plane,and the dip angle of original frac- ture surface and the new main fracture surface in the space,the relationship between the distribution direction of the o- riginal fracture and the direction of newly generated fracture under different confining pressures was analyzed. The re- sults show that the coal sample contains a large number of fractures of different scales. The number of fractures with e- quivalent diameter less than 1 000 μm is large,and the volume is small and the number of fractures with equivalent di- ameter 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 diam- eter ranges from 300 μm to 5 000 μm,the volume fraction of fracture is generally decreased,and the fluctuation is ob- vious when the equivalent diameter is more than 1 500 μm. The elliptic distribution rule calculated by the second-or- der 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 sec- ond 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 differ- ent,with the increase of confining pressure,the fractal dimension of fracture surface decreases,and there is a good cor- relation between them.