Stage-Dependent Freeze–Thaw Deterioration of Weakly Cemented Sandstone Under Different Initial Water Contents: Macroscopic Response and Same-Region Microcrack Evolution

Initial water content influences both the extent and stage-dependent evolution of freeze–thaw deterioration in weakly cemented sandstone. Sandstone from a cold-region open-pit mine was conditioned to five initial water contents (0%, 1.22%, 2.58%, 3.91%, and 5.06%) and subjected to up to 100 freeze–thaw cycles. Effective porosity, P-wave velocity measured at the prescribed moisture states, uniaxial compressive strength (UCS), elastic modulus, macroscopic failure, and same-region scanning electron microscopy were investigated. After 100 cycles, the effective porosity of initially saturated specimens increased from 12.00% to 20.30%, whereas the P-wave velocity decreased from 2050 to 980 m/s; UCS and elastic modulus decreased by 85.99% and 87.18%, respectively. The first 30 cycles accounted for 79.52% of the total porosity increase and 76.64% of the P-wave velocity reduction, but only 32.57% of the total UCS loss. Thus, physical-property changes were concentrated in the early stage, while substantial mechanical degradation continued subsequently. Low-water-content specimens exhibited slower, more nearly uniform porosity evolution. Macroscopic cracking and same-region SEM observations revealed more pronounced crack opening, coalescence, and fragmentation at higher initial water contents. A conceptual framework involving frost-heave-induced cracking, potential grain-contact weakening, and redistribution of existing pore water under sealed conditions is proposed. The findings provide a theoretical basis for freeze–thaw damage assessment and stability analysis of cold-region open-pit mine slopes.