Pore structure evolution and damage creep model of shale subjected to freeze-thaw treatment

To solve the long-term instability problem of engineering project in the seasonal frozen region, we carried out the freeze-thaw (F-T) cycle experiments of shale under different F-T temperatures and numbers of cycles, nuclear magnetic resonance (NMR), and triaxial compression creep experiments to study the mesoscopic damage and creep evolution characteristics of rock. The experimental results show that with the increasing number of F-T cycles and decreasing freezing temperature, the creep deformation and steady-state creep rate of shale increased gradually, while the long-term strength decreased greatly. It was found that with increasing axial stress levels, the influence of F-T temperature difference on axial creep deformation and the creep rate of shale was relatively small, while the influence of the number of cycles was relatively notable. Based on the experimental results, the creep damage variables of F-T shale were proposed considering both freeze-thaw damage and creep damage. And then a nonlinear fractional damage creep model considering the number of F-T cycles and temperature difference was established. The reliability and applicability of the model are verified through the experimental results, which can serve as a theoretical basis for the long-term strength determination, engineering stability analysis, and construction in frozen soil areas.