Experimental Investigation of Unfrozen Water Content, Pore Structure, and Mechanical Properties of Remolded Warm Frozen Soil from the Ili River Valley

The Ili River Valley is a typical seasonally frozen region in which slope instability frequently occurs during the warm frozen-soil stage, generally at temperatures ranging from approximately −1.5 to 0 °C. In this context, changes in unfrozen water content play an important role in controlling the pore structure and mechanical behavior of warm frozen soil, yet the links among these factors remain insufficiently understood. This study investigates warm frozen soil from the Ili River Valley, with particular emphasis on the role of unfrozen water content in regulating pore-structure characteristics and mechanical response under low-temperature conditions. Low-field nuclear magnetic resonance (NMR), low-temperature triaxial shear tests, scanning electron microscopy (SEM), and quantitative image analysis were employed to examine the relationships between unfrozen water content, pore structure, and macroscopic mechanical properties under different temperatures, initial water contents, and confining pressures. The results show that unfrozen water content decreases markedly with decreasing temperature, especially within the range of −1.5 to −5 °C, and increases with increasing initial water content. These changes are accompanied by significant variations in porosity, pore abundance, and pore fractal dimension, reflecting freezing-induced reorganization of the pore system. Lower temperatures and higher initial water contents promote ice-crystal growth and the formation of larger ice-cemented aggregates, thereby modifying the pore framework. Meanwhile, peak strength and cohesion increase with decreasing temperature and increasing initial water content, whereas the internal friction angle shows a decreasing trend. In addition, porosity, pore abundance, and pore fractal dimension are closely correlated with peak strength and cohesion. The results indicate that unfrozen water content governs the freezing-induced reorganization of pore structure, which in turn controls the strength evolution of warm frozen soil. These findings improve understanding of the role of unfrozen water in low-temperature soil structure and strength evolution and provide a basis for evaluating slope instability in the Ili River Valley.