Experimental design and practice of triaxial stress loading on the gas-containing coal

To reflect a more similar original stress environment and to make the research on the influence of gas on the mechanical properties of coal bodies in the coal mining process more convincing, a triaxial stress loading experiment on gas-containing coal is introduced. On the one hand, the experimental teaching scheme for studying the strength and deformation characteristics of coal based on the triaxial stress loading experiment can be concretized. On the other hand, it can be helpful for students to understand the mechanical characteristics and adsorption deformation characteristics of gas-containing coal, which lays a foundation for subsequent in-depth research.

In this study, with the help of a coupling instrument to measure the adsorption, mechanics, and seepage characteristics of coal, the actual stress environment of coal is simulated by adjusting the loading module and the triaxial stress loading experiment on coal under different confining stress and gas pressure conditions is carried out. Among them, the magnitude of the axial stress and confining stress applied to the coal body can be achieved accurately by adjusting the hydraulic pump, and the required gas pressure can be achieved by adjusting the valve of the gas cylinder. First, the coal samples are sealed in the equipment, and the confining stress and gas pressure are adjusted to the target values. Second, the axial stress is continuously loaded until the coal body is damaged. Finally, according to the automatically recorded full stress–strain curves of coal samples, the specific values of peak stress, axial strain, radial strain, and volumetric strain of coal under different initial conditions can be obtained. Then, the maximum and minimum principal stress values under different stress conditions are plotted in a Cartesian coordinate system and fitted using the Mohr–Coulomb formula, and the cohesive force and internal friction angle of coal samples can be calculated. In addition, the weakening law of the free gas and adsorbed gas on the strength of the coal body can be obtained.

Notably, the free and adsorbed gases have a certain degree of weakening on the strength of the coal body, and the effect of free gas is more obvious. In addition, the change of the confining stress can also affect the weakening ability of gas. When the confining stress increases, the weakening rate of the coal strength by gas slightly decreases. Furthermore, with the help of a theoretical calculation method and a laboratory test method, the adsorption volumetric strain of the coal body is obtained. The result of the theoretical calculation method is smaller than that of the laboratory test method because the calculated bulk strain value arising from the effective stress is slightly lower than the actual value.

The application of the triaxial stress loading experiment on gas-containing coal enriches the research process of coal body mechanical properties. In addition, by introducing the coupled adsorption–mechanics–seepage characterization system into the experimental teaching in the mine safety engineering course, a platform combining practice and theory has been set up, which not only facilitates the investigation of the mechanical properties and adsorption deformation of gas-containing coal but also provides an interactive platform for students to cultivate their innovative thinking.