Experimental Study of the Transversely Isotropic Freeze–Thaw Deformation Characteristics of Rock and Its Microscopic Mechanism under Unidirectional Freeze–Thaw Condition

The large temperature fluctuations in cold regions can lead to significant freeze–thaw (FT) deformation and deterioration of rock. Situations involving unidirectional FT action are frequently encountered in rock engineering, such as in tunnels and slopes in cold regions, and may induce severe frost damage. However, most current studies on rock deformation and deterioration have been conducted under uniform FT conditions. The unidirectional FT deformation characteristics of rock and their microscopic mechanism remain unclear. Therefore, multiple cycles of unidirectional FT deformation experiments on sandstone were conducted under varying freezing temperatures and saturation levels. The evolution of microfractures and the distribution of pore diameters were monitored using optical microscopy, scanning electron microscopy, and mercury intrusion testing to reveal the microscopic mechanism underlying macroscopic deformation. Results show that, under unidirectional FT conditions, the frost strain parallel to the FT direction is consistently higher than that perpendicular to it, indicating that the rock exhibits transversely isotropic deformation characteristics. At freezing temperatures of ‒10°C, ‒20°C, and ‒30°C, the stabilized values of parallel frost heave strain were approximately 872, 1,382, and 909 με, respectively, while the stabilized residual strain values were 681, 986, and 944 με. As the freezing temperature decreased, the frost heave strain initially increased and then declined, while the residual strain increased and subsequently tended to stabilize. Under unidirectional FT action, microfractures on rock surfaces were generally perpendicular to the FT direction. Internally, more microfractures tended to align parallel to each other, despite initially being randomly distributed. These observations demonstrate a directional damage effect on the rock microstructure, resulting in macroscopic transversely isotropic deformation. Furthermore, cumulative mercury intrusion in the sandstone increased from 0.0268 to 0.0399 mL/g after repeated FT cycles. This accumulation of pore volume reflects irreversible plastic deformation of the rock’s microstructure, leading to macroscopic residual strain.