Critical expansion points: Mechanical signs of surrounding rock instability

The stability of surrounding rock in deep underground engineering is often affected by the post‐peak zone, where damage characteristics remain insufficiently understood. A thorough investigation into post‐peak rock behavior is essential for accurately characterizing the bearing capacity of deep surrounding rock. This study examines the mechanical behavior of surrounding rock in deep‐buried tunnels, considering it a plane strain problem. The evolution of microcrack characteristics in rock specimens subjected to plane strain compression tests (PSCT) is analyzed at the macroscopic level. The Rock Bearing‐Expansion Model (RockBEM) is developed at the macroscopic level, incorporating critical expansion points (CEPs) to describe post‐peak damage behavior. The findings indicated that CEPs effectively determine the stages of rock mass damage. The hysteresis of CEPs enables the assessment of rock mass‐bearing performance at different time points, while CEPs also quantify the relative duration of damage stages, providing insight into the severity of post‐peak failure. This study enhances the understanding of post‐peak damage mechanisms in deep engineering rock masses, providing new perspectives on the stability of deep surrounding rock.