Mechanical deterioration of flawed rocks under freeze-thaw and cyclic loading

Evaluating the freeze-thaw (F-T) deterioration characteristics and fatigue mechanical responses of flawed rocks under the combination of F-T treatment and cyclic loading is essential for geotechnical engineering in high-cold mountainous regions. In this context, experimental and numerical investigations are conducted on F-T treated double-flawed sandstone specimens with different flaw angles (30°, 45°, 60° and 75°) under uniaxial cyclic compression loading. The influence of F-T cycle (FTC) number and flaw angle on the frost heave pressure (FHP), mechanical properties and fracture mechanisms of flawed specimens are experimental analyzed using membrane pressure sensors, digital image correlation (DIC) and displacement trend line (DTL) method. The results show that the 60 FTCs reduce the FHP over 75%, and high FTC also degrades the stiffness and energy storage capacity of rocks, leading to higher coupled damage and lower fatigue life. With increasing flaw angle, the FHP shows a monotonic increase trend to about 3 MPa after the first F-T, while the critical irreversible strain, dissipated energy and fatigue life are all featured by overall upward trends except an obvious drop in 60°. The repeated FTCs complicate the fatigue crack network and transfer the failure mode from shear-dominated to tensile-dominated. As the flaw angle increases, the shear failure feature is enhanced at first and then weakened with a turning point of 60°. Furthermore, considering the cryogenic thermo-hydro-mechanical (THM) coupling, a numerical model is established based on finite element method (FEM). Under repeated FTCs, unrecoverable deformation and permanent damage accumulate within double-flawed rocks, and frost heave propagation is further stimulated along the flaw orientation until the final F-T failure. Besides, the F-T damage occurs only in the early freezing process when FHP is generated.