Abstract: To investigate the evolution of the pore structure and the relationship between the pore structure and the macroscopic mechanical properties of the expansive soil in the seasonally frozen area, nuclear magnetic resonance (NMR) tests were carried out on the Jiamusi undisturbed expansive soil. Besides, the scanning electron microscope (SEM) test was also adopted to distinguish the pore structure of the soil. Meanwhile, the influence of the consolidation pressures and the freeze-thaw cycles on the pore structure of the expansive soil were studied, and the relationships between the evolution of the pore structure and mechanical properties of the expansive soil were further explored. The results show that: i) Cracks developed in the Jiamusi expansive soil induce a trimodal characteristic of the T2 time distribution curve. The distribution of the pore size of the undisturbed sample behaves significantly differently with the varied consolidation pressure, including the three stages of the pressure is less than, slightly greater than and much greater than the pre-consolidation pressure. As the consolidation pressure is increased, the adjustment rate of the pore structure decreases. The void ratio of the soil decreases with the increased freeze-thaw cycles. The increased freeze-thaw cycles will lead to the increased proportion of mesopore and the decreased proportion of the macropore, while the micropores are not sensitive to the variation of the freeze thaw cycles. ii) The stress-strain curves vary from strain-softening to strain-stable with the increased freeze-thaw cycles and the failure modes of the expansive soil change from brittle failure mode to plastic failure mode. The unconfined compressive strength decreases exponentially with the increased freeze-thaw cycles. The shrinkage characteristic of the expansive soil is enhanced significantly under the freeze-thaw cycles. iii) The variation of the pore structure exhibit the linear trend with the mechanical characteristic. The effects of freeze-thaw cycles on the pore structure of the expansive sole are consistent with that on the macroscopic behaviors. The results obtained in this study can be used to establish a quantitative relationship between pore structure and engineering performance of the expansive soil.

Key words: pore structure, NMR, unconfined compressive strength, freeze-thaw cycle, expansibility