Freezing characteristics and microstructural damage evolution of granular materials in cold regions under freezing-thawing cycles

Granular materials in high-latitude cold and high-altitude areas are susceptible to damage and deterioration by freezing and thawing, which is directly caused by a change in microscopic pore structure, and moisture is the key factor. To investigate the unfrozen water change and storage characteristics of granular materials under freeze-thaw action, samples based on the soil-rock content were prepared, and freeze-thaw cycle tests were conducted. The evolution law of the unfrozen rate under freeze-thaw cycles was studied by low-field nuclear magnetic resonance (LF-NMR), and the evolution of pore and fissure spatial structure in granular samples was studied by three-dimensional X-ray microscopy (3D-XRM). The results show that nuclear magnetic signal intensity in the positive temperature region satisfies the linear fitting relation, and the average decrease in the unfrozen rate in the negative temperature region from - 1 to - 4 degrees C is more than 15%/degrees C. The freezing characteristic curve shows an exponentially decreasing trend and an overall left-bottom shift of the curve with increasing number of freeze-thaw cycles, implying that the pore space is more developed. The freeze-thaw cycle intensifies the hysteresis phenomenon. The slope of the unfrozen rate curve of the freeze-thaw process in the temperature range of 0 to - 10 degrees C is significantly greater than that in the temperature range of - 10 to - 35 degrees C and reaches a maximum in the temperature range of 0 to - 4 degrees C. The degree of freeze-thaw hysteresis increases significantly after the first five freeze-thaw treatments. The pore parameters of the dense sections decrease first and then increase, and the pore and fracture sections show a consistent trend with respect to the variation in the number of pore fractures, pore-fracture area, equivalent diameter, and surface porosity. The parameter values of the fracture sections are slightly higher than those of the pore sections under the same number of freezing-thawing cycles. The pore expansion effect is significant in the early freeze-thaw period; the average and maximum pore volumes increase first and then decrease slightly, and new micropores are mainly formed in the late freeze-thaw period. The freeze-thaw cycle effect destroys the weak surface of the internal structure through water-ice phase change frost heave and water migration erosion, causing granular accumulation instability.

Automated summary

Granular materials in high-latitude cold and high-altitude areas can be damaged by freezing and thawing, due to changes in microscopic pore structure and moisture content. This study investigated the unfrozen water change and storage characteristics of granular materials under freeze-thaw cycles. The results showed that the freezing characteristic curve exhibited an exponential decrease and a left-bottom shift with an increasing number of cycles, indicating a more developed pore space. Freeze-thaw cycles intensified the hysteresis phenomenon and caused pore expansion in the early freeze-thaw period. The study also observed the destruction of weak surface structures through frost heave and water migration erosion, leading to granular accumulation instability.