Experimental study on instability and failure mechanism of sandstone under freeze-thaw and load.

Freeze-thaw damage is the primary cause of instability in rock slopes in cold regions, while the mechanical properties of rock are significantly influenced by loading rate. This study aims to investigate the evolution of mechanical behavior of sandstone under the coupled effects of loading rate and freeze-thaw cycles. Uniaxial compression tests were conducted on sandstone specimens subjected to different numbers of freeze-thaw cycles (0, 30, 50, 70) and loading rates (0.05, 0.10, 0.15, 0.20 mm/min) to systematically study the deterioration mechanism of its mechanical properties. The experimental setup incorporated a real-time acoustic emission (AE) monitoring system combined with high-speed camera technology to analyze the influence of loading rate and freeze-thaw cycles on the mechanical characteristics, failure modes, and AE features of sandstone. The results demonstrate that the uniaxial compressive strength (UCS) and elastic modulus (E) of sandstone exhibit a negative correlation with the number of freeze-thaw cycles and a positive correlation with loading rate, while the ductility characteristics show an opposite trend. Notably, the attenuation constant λ follows a monotonically decreasing pattern with increasing loading rate. Failure mode analysis reveals that sandstone predominantly exhibits tensile-shear composite failure characteristics at complete failure, with tensile cracks dominating in quantity. As the loading rate increases, the proportion of tensile cracks significantly rises, whereas the increase in freeze-thaw cycles reduces the disparity between the number of shear and tensile cracks. The initial stage of internal crack propagation in sandstone is accompanied by a significant increase in AE events rate and drastic fluctuations in b-value. The absence of low AE events rate and the sharp decline in b-value can serve as crucial precursor indicators for predicting the instability and failure of sandstone. Based on the experimental data, a predictive model for the strength index attenuation of freeze-thaw damaged sandstone considering loading rate effects was established, providing a theoretical basis for stability assessment of rock engineering in cold regions.