Investigation of the crack evolution characteristics of coal and rock bodies around boreholes during progressive damage based on stress threshold values

The stress threshold is a key stress indicator of crack evolution of coal and rock bodies around boreholes during progressive damage. To investigate the crack evolution characteristics of coal and rock bodies around boreholes, digital speckle correlation method tests were conducted on intact specimens, specimens with boreholes, and specimens with grouted boreholes, and the characteristic parameters, such as the stress threshold, surface deformation field, crack opening and shear, and strain energy density, were quantitatively analyzed. The results show that (1) the distribution characteristics of surface cracks on the specimen are affected by the borehole structure and grouting; (2) the crack displacement evolution of coal and rock specimens remain basically stable at the initial stage of loading. In the middle and later stages, the cracks are continuously opening and shearing, which directly leads to the damage of the specimen; (3) the energy accumulation and release law of coal and rock bodies is related to the evolution of localization zone. In different areas outside the localization zone, the strain energy density shows different characteristics; (4) the tensile crack is the main reason for the destruction of coal and rock bodies around boreholes. During the progressive damage, the tensile cracks at the upper and lower ends of specimen experience the process of opening, closing and reopening. The presence of cohesion g(x) reduces the crack tensile stress intensity factor, so that the crack is in a balanced state before penetration, which is conducive to the connection and penetration of other cracks. The results provide theoretical guidance for the optimization design of gas extraction boreholes construction and grouting.

Automated summary

The crack evolution characteristics of coal and rock bodies around boreholes were studied using the digital speckle correlation method. Various parameters such as stress threshold, surface deformation field, crack opening and shear, and strain energy density were quantitatively analyzed. The results revealed that the distribution of surface cracks is influenced by the borehole structure and grouting. In the later stages, cracks continue to open and shear, leading to specimen damage. The presence of cohesion reduces the tensile stress intensity factor of cracks, facilitating their connection and penetration.