Fractal characteristics of acoustic emission of gas-bearing coal subjected to true triaxial loading

Abstract

As coal mining proceeds deeper, coal and rock are subjected to growing stresses and gas pressures. Consequently, coal and rock gas dynamic disasters pose more and more considerable threats and hazards which should be warned in advance. Coal and rock in the field is actually in the true triaxial stress state. Study on deformation characteristics and precursory information of coal and rock under this state is meaningful for disaster warning. In this paper, experiments on the deformation, failure and acoustic emission (AE) characteristics of gas-bearing coal under true triaxial loading conditions were carried out. Besides, the variation law and fractal characteristics of AE under different gas pressures and confining stresses were analyzed. Furthermore, the effects of gas pressure and confining stress on the deformation, failure and fractal characteristics of coal were discussed. The results show that the process of AE variation under true triaxial loading conditions can be divided into two stages, namely the slow growth stage and the accelerated growth stage. A higher gas pressure corresponds to a shorter duration of slow growth stage, while a higher confining stress corresponds to a longer duration of slow growth stage. AE time series has fractal characteristics, and the correlation dimension can characterize the damage degree of a loaded coal sample. The dynamic changes, i.e., fluctuation-increase-decrease, in correlation dimension can accurately reflect the damage evolution process of a coal sample. In addition, the gradual reduction of correlation dimension can be used as the precursor information of coal sample instability and damage. The research results boast instructive significance for preventing the occurrence of coal and rock gas dynamic disasters and for reducing casualties and property loss in coal mines.