To analyze the fractal characteristics of coal rock failure under unloading conditions, triaxial unloading confining pressure tests were carried out on coal and sandstone rock samples under different unloading rates and initial confining pressures. We examined the distribution of the surface cracks and fragmentation of the coal and sandstone samples that failed under different triaxial unloading confining pressure tests. The results showed that the fractal dimension of the surface cracks in coal and sandstone decreased as the initial unloading confining pressure increased. Thus, shear failure is more obvious in coal or sandstone with high-stress conditions caused by unloading confining pressure than in coal or sandstone with low-stress conditions. However, the fractal dimension of the surface cracks increased with the unloading rates. Additionally, the fractal dimension of the fragmentation in the coal and sandstone samples had a negative correlation with the initial unloading confining pressure. When the initial confining pressure was relatively low, the samples underwent splitting and shear failure; when the initial confining pressure was higher, the failure mode was mostly shear failure and the fragmentation of the samples was less homogeneous. In contrast, the fractal dimension of the fragmentation in the coal and sandstone increased with higher unloading rates. The lithology had a significant effect on the fractal dimension of the surface cracks and on the fragmentation. Samples with more internal fissures had more surface cracks and the fragmentation was more homogeneous when the rock failed compared with samples with less fissures under the same experimental conditions.

The acoustic emission (AE) test of step-loaded samples of coal with different moisture contents was conducted using an AG-X250KN rock servo test system and an AE21C AE-monitoring system to study the influence of moisture content on the creep characteristics of coal. The results show that the moisture content influences the creep characteristics of coal samples, in that the greater the moisture content, the greater the creep deformation of the coal and the lower the creep threshold, creep strength, and creep coefficient values. The improved Burgers model can better describe the mechanical properties of creep in coal. In the transient creep stage, the coal micropores and fractures are gradually expanded, and AE activity gradually increased. In the stable creep stage, the creep deformation rate of coal tended to be stable, while new fractures and AE events and energy were all decreased. In the accelerated creep stage, a large number of fractures were generated in the coal, which gradually developed into unstable macrofractures. The stored energy was released rapidly in a relatively short time span, and the number and energy of AE events reached their maximum value. Therefore, the AE characteristics of coal reflect the evolutionary process of damage during creep.

No 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.

In order to understand the influence of unloading path on the mechanical properties of coal, triaxial unloading confining pressure tests with different initial confining pressure and different unloading rate were carried out. The test results show that the triaxial unloading strength of coal samples under different test conditions is lower than conventional triaxial tests, but the brittleness characteristics are more obvious. This result indicates that the coal samples are easily damaged under unloading conditions. In the axial loading stage of the confinement unloading tests, the axial strain plays a leading role. However, during the confining pressure unloading stage, the circumferential deformation is large, which is the main deformation in this stage. Higher unloading rates of confining pressure are associated with shorter times between the peak stress position and sample complete failure. This shows that samples are more easily destroyed under higher unloading rates and the samples are more difficultly destroyed under lower unloading rates. In addition, with increasing unloading rate, the peak principal stress difference and confining pressure at failure decrease gradually, whereas the confining pressure difference at failure increases gradually. Compared with conventional triaxial compression tests, the cohesion of coal is reduced and the internal friction angle is increased under the condition of triaxial unloading test.