Abstract: Damage deterioration of rock by freeze-thaw cycles seriously affects the long-term service performance of tunnel projects in cold regions. In order to obtain the fine-scale characteristics of freeze-thaw damage and post-freeze-thaw loading mechanical properties of fissured rocks, tests on the mechanical properties of water-filled fissured sandstones under different levels of freeze-thaw cycles were carried out. Equations were derived to describe the relationship between the volume increment of frozen water, recharge, and the number of freeze-thaw cycles throughout the freeze-thaw process. The particle flow method is used to construct a rock freeze-thaw deterioration model based on the theory of water ice phase transition volume expansion, and the reliability of the model is verified by indoor tests. It is shown that with the increase of the number of freeze-thaw cycles N, the mechanical properties of the specimens show linear attenuation, the degree of attenuation is positively correlated with N. And the mechanical properties of the specimens are the weakest when the fissure inclination angle is 30°. The degradation of fissure specimens due to freezing and thawing is more pronounced than in other parts of the sample. In addition to surface particle flaking, microcracks develop at the ends of the fissures. This results in freeze-thaw damage accumulating both on the surface layer and internally within the specimen. The fine microcracks produced by the freeze-thaw process of granular flow are mainly tensile cracks, and the number of cracks is positively correlated with N. The freeze-thaw microcracks are gradually extended and developed from the surface layer of the specimen to the inner layer, and the number of microcracks and their distribution characteristics affect the load damage morphology of the specimen, and they are mostly manifested as the secondary cracks distributed in the vicinity of the main rupture zone of the load damage rupture zone. This study provides new ideas and references for the damage degradation process of rock under freeze-thaw action, and also helps to promote the prediction of catastrophic changes and long-term service performance assessment of tunnel engineering in cold regions.

Key words: freeze-thaw cycles, prefabricated fissures, discrete element method, damage attenuation, fracture evolution