Rock Unloading Failure Precursor Based on Acoustic Emission Parametric Fractal Characteristics

Abstract The mechanical response characteristics and occurrence mechanism of coal and rock under unloading conditions are key to evaluating the stability and control of engineering rock excavation. Triaxial unloading confining pressure tests of coal and rock based on different unloading rates and different initial confining pressure conditions were conducted with fractal theory, research into the fractal characteristics of coal and rock acoustic emission time series under an unloading confining pressure, and the correlations with the unloading failure of coal and rock. The correlation dimension of the acoustic emission signal showed a variation law of a sudden increase, followed by a decrease, increase, and continuous decrease during the experiment. The average time differences between the time when the correlation dimension of coal and sandstone decreased and the actual fracture time of the rock sample were 13.6 s and 16.7 s, respectively. The change in correlation dimension showed the change in internal damage and fracture of the rock sample during the test. The HURST exponent of the acoustic emission time series was calculated at the beginning of the loading. The HURST index showed an overall stable trend and fluctuated around 0.5. A sudden drop in the HURST index resulted when the axial load reached 85% of the peak stress of the rock sample. When the HURST index dropped suddenly (minimum less than 0.2) and then increased rapidly (maximum above 0.8), the rock sample was close to rupture. A HURST index of 0.8 can be used as an index of rock sample unloading and fracture.