Stress-Pore-Permeation Characteristics of Coal Specimens with Different Strengths Based on Acoustic Emission Fractals

For mastering the characteristics of coal and rock seepage weakening under total stress-strain, a total stress-strain-permeability test was carried out on four groups of coal specimens with different strengths of 23.450-44.098 MPa in this paper. The development of internal pores in the specimens was represented by acoustic emission (AE) fractal dimension, and internal pore development and the permeability change laws during total stress-strain were analyzed. The correlation characteristics among volume strain, AE fractal dimension, and permeability during the entire stress-strain process were obtained. The results showed that pore development was consistent with acoustic emission ringing in the total stress-strain seepage test and that the acoustic emission fractal dimension could express the pore development characteristics. The relationship of AE fractal dimensional-volume strain, volume strain-permeability, and AE fractal dimensional-permeability showed obviously two stages of volume compression and volume expansion, and the critical point showed the minimum permeability. In the volume compression stage, volume strain increased positively, the AE fractal dimension decreased, and permeability decreased first rapidly and then slowly. In the volume expansion stage, volume strain increased inversely, the acoustic emission signal decreased but burst in the stress concentration area, and permeability increased from slowly to a sudden increase (maximum permeability). The total stress-strain AE fractal-permeability model showed obvious two-stage characteristics of volume compression and volume expansion, with the minimum permeability point as the critical point. The correlation between the model calculation and the test data was greater than 0.90, and the model correlation was significant.