Triaxial creep characteristics and damage model for red sandstone subjected to freeze-thaw cycles under different water contents

In view of the influences of the natural environment in cold regions on the long-term stability of rock mass, the red sandstone samples along Sichuan-Tibet railway are taken as the research object, and the nuclear magnetic resonance and triaxial creep experiments are conducted on the samples subjected to freeze-thaw cycles under different water contents. The experimental results show that the increase of water content promotes the development and expansion of pores in red sandstone during freeze-thaw cycles, and the development degree of mesopore in red sandstone after freeze-thaw cycles is the highest. The time-dependent deformation characteristics of red sandstone after freeze-thaw cycles present obvious softening trend, and the creep strain and rate of red sandstone subjected to freeze-thaw cycles increase with the increase of water content, while the load capacity and the long-term strength decrease significantly, and the fracture surface evolves from single shear to tension splitting gradually. According to the experimental results, a modified Nishihara model considering freeze-thaw damage and water deterioration is proposed based on the principle of damage mechanics, which is further extended to a three-dimensional creep constitutive equation based on the elastoplastic mechanics. The parameters of the model are identified by using the universal global algorithm with the creep data, and the variation trend of the parameters and deterioration coefficients with the number of freeze-thaw cycles and water content is revealed. Finally, the model is written into FLAC^3D to realize the numerical simulation of triaxial creep experiments of rock, and it is verified that the model can well reflect the creep characteristics at all creep stages of red sandstone subjected to freeze-thaw cycles under different water contents. The research results can provide scientific basis and technical reference for the long-term stability of geotechnical engineering in cold regions along Sichuan-Tibet highway.