Evolution mechanism of fracture structure of loaded coal under different unloading rates

In view of the fact that the fracture development data in the unloading process of the loaded coal body was difficult to capture,a pseudo threeaxis seepage experiment was designed to complete the dynamic monitoring of stress-strain-permeability in the loading and unloading process of the loaded coal body.At the same time,based on the conservation of energy,the energy distribution states at different unloading rates during the unloading process were compared,and the effect of unloading rate on fracture development and permeability evolution was explained,and verified by coal sample fracture monitoring and comparison data.The results showed that the fracture structure was affected by unloading rates.The higher the unloading rate of confining pressure was,the more Class I and Class II fractures were developed (the ratio increased).Compared with the original number,the number of Class I fractures increased even more than one time,and the number of Class II fractures increased by 74.13%.During unloading,the permeability curve can be divided into three sections:linear section,parabolic section and stable section,and the distribution characteristics of those three curves were determined by the elastic modulus,loading strength and unloading rate of experimental coal samples.With the increase of unloading rate,coal permeability enhanced,the slope of linear section and curvature radius of parabolic section increased gradually,the slope of the linear segment gradually increased from 0.054 8 to 0.091 9,0.138 8 and 0.216 4,and the radius of curvature of apexes reached 1.232,1.334,1.863 and 2014,respectively.With the increase of unloading rate,the critical stress point of permeability stability decreased from 15.26 MPa to 14.08,13.14 and 10.88 MPa.Under the same experimental conditions,there was a linear relationship between the increment of permeability and the increment of fracture length.The statistical data showed that the higher the confining pressure unloading rate was,the lower the elastic energy conversion rate was,and the higher the energy consumption of fracture development was.The fracture energy consumption of high speed unloading (loading unloading ratio 1∶8) was more than 80%,while that of low speed unloading (loading unloading ratio 1∶1) was only 25.62%.According to comparative studies,it was found that the main body of energy release of coal body was consumed by the expansion events of original fracture group,rather than the initiation of new fracture group during coal mass high speed unloading,and there was a logarithmic relationship between energy consumption for fracture development and unloading rate,which provided data support for the quantitative study on energy consumption and fractures growth