Experimental research on deformation and failure characteristics of coal by staged cyclic loading and unloading

The stress state of the gas drainage borehole under the influence of mining is relatively complicated. As the working face advances, it experiences a complex combination of loading and unloading, which may easily lead to instability and destruction of the drainage boreholes. In order to investigate the deformation and failure characteristics of coal boreholes under the influence of mining, monotonic loading and multistage cyclical loading and unloading tests were carried out to analyse the strength characteristics of the samples under different loading paths. Crack growth and the evolution of the displacement around the borehole during uniaxial compression of specimens were analyzed by using digital scatter technique (XTDIC)and the displacement evolution characteristics of the localized zone of the sampleswere analyzed by arranging the“virtual extensometer”. The results show that the compressive strength of the specimen is reduced by 4.59% under multistage cyclical loading-unloading path compared with monotonically loaded specimen. Under the multistage cyclical loading-unloading paths, the final failure of the specimens containing boreholes is caused by the penetration of the far-field cracks and the shear cracks on both sides of the borehole to form a macroscopic fracture zone.The failure mode of the specimens under different loading paths is tensile-shear composite damage with an“X”shape. The tension and misalignment displacement of the specimen at the top point of loading and the low point of unloading gradually increases with the number of cycles, presenting a fluctuating upward trend, and lagging behind the stress change in time.The higher the level of the displacement measurement point around the hole, the greater the displacement of the specimen loaded at the same time. Under the multistage cyclical loading and unloading path, the displacement of the measured point during the unloading stage of the specimen has a slight recovery, the maximum recovery is 0.149 mm, and the displacement increases with time in a“stepped”incremental phenomenon.