Study on the creep properties and crack propagation behavior of single-fissure sandstone based on the damage bond model

As geotechnical engineering expands to deeper formations, the creep behavior of surrounding rock becomes more significant. To better understand the creep deformation and creep propagation of fissured rocks, this study conducts triaxial compression tests and triaxial creep tests of sandstone samples containing a single fissure. The experimental results show that fissured sandstone has a time-dependent failure mode, and creep loading not only causes the crack that dominates the failure of the fissured sandstone to change from a wing crack to an anti-wing crack but also produces more secondary cracks, which are obvious in samples with a low-angle fissure. The steady-creep rate of fissured samples is higher than that of intact samples and first increases and then decreases with increasing fissure angle. The sample with a 45 degrees fissure produces the maximum axial deformation before entering the accelerated creep stage. The theory of the damage bond model (DBM) applied in Particle Flow Code software is proposed, combining the Kachanov damage theory and linear parallel bond model. The DBM effectively simulates the creep deformation and crack propagation process of sandstone in all creep stages, and the failure mode is consistent with the test results. During the initial and steady-state creep stages, the number of microcracks slowly increases, generating a dispersed distribution that rarely exhibits coalescence. Then, the microcracks propagate unstably and coalesce macroscopic cracks, and the accelerated creep stage starts, which is both stress-and time-dependent. Under the same deviatoric stress, the creep simulation samples generate more microcracks than the triaxial compression samples and exhibit a greater reduction in the width of the preexisting fissure after failure. Creep damage accumulation with time significantly affects the evolution of contact forces and displacement vectors. A stress concentration is more likely to occur along the propagation path of an anti-wing crack in creep samples, and the anti-wing cracks present a trend of shear failure. This study plays an important role in predicting the deformation of fissured surrounding rock in underground engineering and explaining the mechanisms of catastrophic events due to the evolution of damage.