Investigation into the evolution of wetting-induced collapse damage in unsaturated loess using real-time CT scanning

Hydraulic-induced structural damage in unsaturated loess represents a key factor contributing to collapse. Previous studies rarely used CT scanning to reveal the spatio-temporal relationship between the collapsibility deformation and the microstructure evolution of loess through the dynamic process of water infiltration. In this paper, the undisturbed and remolded Q₃ unsaturated loess are taken as the research subjects. Utilizing the self-developed micro-CT-triaxial apparatus, the isotropic pressure loading and water immersion tests with a constant confining pressure (50-300 kPa) are carried out. The real-time dynamic CT scanning of the longitudinal large sections of the specimens is achieved, revealing the cross-scale interaction mechanism of water infiltration - structural damage - collapse deformation during the wetting process of unsaturated loess. The results show that the collapse volumetric strain of undisturbed loess is higher than that of remolded loess, with a difference of 20.5% to 90.4% under the same confining pressure. Additionally, an increase in confining pressure reduces the immersion amount of undisturbed loess by 8% to 22% and shortens the response time of collapse. CT images demonstrate that the wetting front of undisturbed loess migrates along the dominant path of macropores, and the macropores collapse rapidly after flooding and soften the cement-rich area, resulting in the increase of collapsibility deformation. Due to the homogeneous micro-pore structure, the wetting front is uniformly diffused and the collapse response is gentle. Confining pressure influences the collapse behavior via a dual route of "short-term inhibition - long-term promotion": in the short term, it compresses pores to retard water infiltration and causes a delay in the collapse response; in the long term, it aggravates pore collapse and ultimately magnifies the volumetric strain of the collapse. Based on the CT number, a structural parameter is defined, and a damage evolution equation for loess collapse deformation is proposed, taking into account the effects of confining pressure and saturation increment on collapse damage. The research results clarify the dominant mechanism of "preferential seepage - cementation deterioration" in the structural collapsibility of loess, providing a beneficial foundation for enriching and developing the research on the structure and collapsibility mechanism of loess.