Study on strength limit neighborhood range of gas-bearing coal rock under different loading conditions

The occurrence mechanism of coal and gas outburst disasters is still unclear. According to the rheological hypothesis, gas-bearing coal and rock may enter the “strength limit neighborhood” under long-term load, and it is more likely to be destroyed by external impact disturbance. The “strength limit neighborhood” of gas-bearing coal rock under different loading conditions is studied to find the influence law of entering this state, and the corresponding micro-discrimination criteria are established,” using the self-developed triaxial test system of rock rheological disturbance effect and the seepage experimental device of coal rock rheological disturbance effect, the rheological disturbance experiment of gas-bearing coal rock under different confining pressures, different gas pressures and axial pressures was carried out. The coal rock before and after rheological disturbance was analyzed and compared by means of nuclear magnetic resonance, and the T₂ spectrum curve, porosity and pore size distribution under different loading conditions were obtained. The test results show that: ① In the rheological process of coal rock under different loading conditions, there is a critical value for the sensitivity of external disturbance. When the applied force is less than this critical value, after the impact disturbance, the porosity of coal rock decreases and the specimen is compressed closely. When the force is greater than this critical value, after the impact disturbance, the large pore size of coal rock increases obviously, the T₂ spectrum curve shifts to the right, and the peak height increases, It shows that the stress threshold is the key to determine whether the coal rock enters the “strength limit neighborhood”. ② Confining pressure and gas pressure have opposite mechanical properties to coal rock. It is shown in the compressive capacity, damage degree, long-term strength and “strength limit neighborhood”. Among them, the greater the confining pressure, the earlier the coal rock enters the “strength limit neighborhood;” The greater the gas pressure, The greater the gas pressure, the earlier the coal rock enters the neighborhood. ③ the pore size distribution and T₂ spectrum curve of coal rock can directly reflect the internal and external micro-variation characteristics of rock’s “strength limit neighborhood.” ④The constitutive equation of axial pressure, confining pressure and gas pressure determined by the porosity of coal rock can be used to judge the timing of coal rock entering the “strength limit neighborhood” under different loading conditions. As a criterion for entering the “strength limit neighborhood,” the greater the confining pressure, the greater the stress threshold when entering the “strength limit neighborhood,” the greater the gas pressure, and the smaller the stress threshold when entering the “strength limit neighborhood.”