Experimental study on the mechanical properties and crack evolution patterns of sandstone with prefabricated double fractures in sub-instability stage

As coal mining in China gradually shifts from shallow to deep layers, the impact of mining disturbances increases, making it more difficult to control the development of surrounding rock fractures. This study aims to investigate the effect of rock bridge length on the sub-instability stage of sandstone with prefabricated double fractures through uniaxial compression tests on sandstone samples with prefabricated double fractures. DIC technology was used to monitor surface displacements in real time. We analyzed the mechanical properties and surface crack propagation characteristics of different sandstone samples in the sub-instability state. Results show that: Compared with intact rock, fissured sandstone exhibits stress drop, entering the sub-instability stage after the first stress drop. As the rock bridge length increases, the ratio between the sub-instability stage duration and the total failure cycle time shows gradual decrease. The ratio between the stress change during the sub-instability stage and the peak stress gradually decreases. Rock bridge length mainly affects the propagation of secondary cracks on the surface during the sub-instability stage. Increasing rock bridge length leads to local disintegration in secondary cracks, and the disintegration area gradually increases. This study offers references to the monitoring and early warning of instability and failure of surrounding rock in coal mines.