Time-dependent mechanical behavior and support timing of surrounding rock governed by crack closure ratio

The long-term deformation characteristics of rock are crucial for ensuring the stability and safety of underground engineering projects. This study examines the time-dependent damage behavior of granite samples from the Shuangjiangkou underground powerhouse under creep loading, with a focus on microcrack propagation and the associated damage mechanisms. A novel approach is proposed, which includes a modified method for calculating instantaneous strain during creep and an advanced creep constitutive model that accounts for crack-induced damage mechanisms in the rock material. The crack closure ratio, a key parameter in the model, is used to track the evolution of both axial and radial crack strains over time. The results reveal that the radial long-term strength is significantly lower than the axial strength, which leads to different failure characteristics in the surrounding rock mass. Based on this, the post-excavation damage of the surrounding rock is assessed, and the surrounding rock is classified into five distinct damage zones using the crack closure ratio. This new classification method differs from traditional elastoplastic zoning approaches. Finally, the study provides recommendations on optimal support timing, which can prevent the surrounding rock from reaching highly damaged zones (HDZ) and construction damage zones (CDZ), thereby maximizing the rock’s bearing capacity and improving engineering safety.