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

•Refined analysis of the deformation and damage processes of coal and rock bodies around boreholes using stress threshold.•The grouting material can better inhibit the propagation of the perimeter cracks.•The surface deformation localization zone of SH-CF is similar to that of “SH-complete + SH-hole”.•The strain energy density under load shows a clear localization pattern. 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.