Rockburst failure characteristics of sandstone under true triaxial double-sided rapid unloading conditions under high pressure

To explore the evolution process and failure characteristics of rockburst in rock masses with a double free surface structure, we conducted a comparative study between true triaxial double-face rapid unloading rockburst tests and single-face rapid unloading rockburst tests, using a true triaxial experimental system capable of rapid unloading from multiple faces. The main conclusions are as follows: the maximum ejection velocity of double-free surface rockburst fragments is higher than that of single-free surface rockburst (20%-40%), and the rockburst ejection duration is generally longer than that of single-free surface rockburst; the peak stress of double-free surface rockburst is smaller than that of single-free surface rockburst, but there will be a more obvious unloading platform stage during unloading. Compared with single-surface unloading rockburst, double-surface unloading is more likely to cause significant brittle failure and violent rockburst; the AE events of single-free surface rockburst are concentrated near one free surface, while the AE events of double-free surface experiments are distributed near two free surfaces. In the later stage of both rockburst experiments, the number of shear cracks will exceed that of tensile cracks. The extra free surface of double-free surface rockburst will enhance crack extension, especially shear cracks, resulting in more serious rock damage. • Dual free surface scenarios exhibit higher peak ejection velocities, prolonged dynamic durations, and anisotropic ejection velocities between surfaces compared to single-surface events. • Despite lower peak stress, dual-surface rockbursts display a unique unloading plateau stage during stress release, promoting intensified brittle failure and more violent energy release. • Dual free surfaces trigger earlier microcrack activity, distribute acoustic emission (AE) events across both surfaces, and induce shear crack predominance in later stages, exacerbating structural damage.