The deformation of expansive soil in seasonally frozen regions caused by freeze-thaw cycles has severely affected the long-term performance of engineering applications. The alteration of expansive soil microstructure has resulted in many geotechnical engineering failures, such as soil cracking and settlement. Consequently, the micropore contraction and expansion mechanisms of expansive soil have drawn extensive attention. Nuclear Magnetic Resonance (NMR) is widely used as a rapid, non-destructive detection technique for moisture monitoring and microstructure evolution characterization in porous media. In addition, Magnetic Resonance Imaging (MRI) can visualize the migration pattern of pore water under different numbers of freeze-thaw cycles. SEM is the most effective and direct method to reveal the structure of particle and micropore arrangement. This paper investigates the pore size evolution and pore structure distribution characteristics of saturated expansive soil via 6 freeze-thaw cycle tests using NMR and SEM techniques. The evolution law of saturated expansive soil under freeze-thaw cycles is obtained. The results show that pore water migrates from the center to the periphery under freeze-thaw cycles. The pore size decreases as the number of freeze-thaw cycles increases and small particles increase significantly. During the freeze-thaw cycle, the arrangement pattern changed from surface-surface contact to stacking.

In recent years, many researchers have evaluated the sustainable use of waste tire rubber as an aggregate in soil. Its effectiveness has been widely acknowledged. The main objective of this work is to study the influence of rubber fibers on shear strength and pore structure characteristics in relation to expansive soil. In this context, we conducted a series of experiments that were carried out on reinforced expansive soil with rubber fiber contents of 0, 5, 10, 15, and 20%. The results show that the shear strength and maximum dilatation angle increase gradually with rubber fiber content. Due to the pore water pressure and creep effects, the deviator stress and effective cohesion of the samples under the consolidated drained conditions were higher than those under the undrained conditions. The converse was true for the internal angle. The addition of an appropriate amount of (5-10%) rubber fiber can effectively inhibit the development of soil cracks and reduce the porosity of the samples. The results obtained can highlight the beneficial effects of rubber fiber, which is highly desirable in many backfill applications.