Multiscale evaluation of the structural characteristics of intact loess subjected to wet/dry cycles

The engineering properties of loess are significantly influenced by seasonal wet and dry climates, the effects of which are essentially inseparable from variations of its structure. To realize the multiscale evaluation of the influence of wet/dry (WD) cycles on the intact loess structure, several WD cycles were applied to intact loess samples. The samples were then subjected to X-ray computed tomography, scanning electron microscopy, and nuclear magnetic resonance tests, paired with state-of-the-art image processing techniques to assess the evolution of the mesoscopic and microscopic structure (i.e., fissures, particles, and pores). The results indicate that the loess fissuring is gradual; that is, it commences in the first two cycles, propagates in three to five cycles, and reaches an equilibrium in six to ten cycles. In the equilibrium stage, the average width of the fissures is inhibited owing to the emergence of secondary fissures. The primary cause for the fissuring is uneven shrinkage (generating tensile stresses) inside and on the surface of the soil. Further, as the WD cycles progress, the orientation of the particles near the polar angle exhibits depolarization, the non-uniformity of the particles and pores increases, the particle roundness improves, and the pore curvature decreases. Moreover, the changes in pore structure include mesopore and macropore expansion, along with the generation of new micropores. These findings serve as a basis for understanding the structural variations in loess exposed to WD cycles, which can help explain changes in the engineering properties of loess in semiarid regions.