Study on the mechanical model of macro-mecro creep under high seepage pressure in brittle rocks

High seepage pressure has a great significance on creep behaviors of brittle rocks under deep underground engineering. However, the macro-mecro relationship between mecrocrack growth and macroscopic deformation under high seepage pressure during the decelerated, steady-state, accelerated creep has been rarely studied. In this study, based on the stress intensity model of crack tips with the influence of initial cracks and new wing cracks, the mechanical relationship between seepage pressure and initial cracks and new wing cracks is introduced, and the stress intensity model model of crack tips considering seepage pressure is established. Then combined with the subcritical crack law and crack-strain damage model, a macro-mecro mechanical model is proposed to explain the relationship between creep crack growth and macroscopic deformation of brittle rock considering the influence of seepage pressure. The rock behaves elasticity when the applied axial stress is smaller than the crack initiation stress, while behaves plasticity when the axial stress exceeds the crack initiation stress but is less than the rock peak strength. The theoretical creep curves subjected to step axial loading are studied under different seepage pressures, and the rationality of the proposed model is verified by experimental results. Furthermore, the creep evolution of crack length, crack growth velocity, axial strain, and axial strain rate under constant or step loading seepage pressure are discussed. The new model provides a significant theoretical basis for the evaluation of the stability of surrounding rocks in deep underground engineering under high seepage pressure.