Evolution of the Unfrozen Water Content for Partially-Saturated Sandstones and the Critical Degree of Saturation

The frost damage behaviors of rock mass largely depend on its water content. Therefore, it is of great practical significance to investigate the effects of the saturation degree on the evolution of unfrozen water content and the critical degree of saturation of rock mass. In this paper, sandstone samples were diamond-cored and prepared to hold different degrees of saturation. The samples were frozen to a minimum temperature of - 25 degrees C, during which the nuclear magnetic resonance (NMR) measurements were repeatedly performed to explore the changes in the transverse relaxation time (T-2) distribution curves and the unfrozen water contents. The results indicate that most of the pore water at room temperature exists in the relatively smaller pores when the degree of saturation is lower than 60%. Upon freezing, the sandstone holding a lower degree of saturation exhibits a lower freezing speed and a higher residual unfrozen water content. Furthermore, the amounts of the unfrozen bound water of the 100% and 60% saturation samples slightly increase as the temperature drops to - 1 degrees C, whereas the unfrozen bound water content for the 30% saturation sample monotonically decreases during the freezing test. Based on the mechanical tests and the microscopic observation of the rock surfaces after cyclic freeze-thaw weathering, the critical degree of saturation for the studied sandstone was derived as 50%. The findings in this study provide meaningful guidance on the freeze-thaw damage prevention of the rock mass in cold regions.