Development and evolution of Loess vertical joints on the Chinese Loess Plateau at different spatiotemporal scales

The vertical joints of aeolian loess are ubiquitous on the Chinese Loess Plateau (CLP), the distribution variability of which mainly depends on the differences in structural characteristics of loess, water drivers (e.g., rainfall or irrigation), loess strata (e.g., sedimentary depth), and microtopography. This study conducted extensive field investigation and experiments to enhance the understanding of morphologies, cross-scale transformation mechanism, and structural bases of loess vertical joints (LVJs). Moreover, the development and evolution processes of LVJs were refined by considering loess sedimentary dynamics. Results show that: (1) LVJs are the product of diagenesis in loess strata during loess historical sedimentary process; (2) the pore concentration zones and vertical tubular channels in loess are the structural bases of forming original vertical joints (OVJs) at the micro-mesoscale, which is supported by the results from the three-dimensional computing tomography (CT) scanning and scanning electron microscopy (SEM) tests; and (3) the morphologies of LVJs vary with the microtopography at different spatiotemporal scales, and is characterized by the cross-scale transformation mechanism, i.e., that they could transform up (i.e., an expansion behavior) or down (i.e., a degradation behavior) at the micro-, meso-, and sub-macro scales under the influences of dry-wet cycles, loess pile thickening, and exogenic forces. Besides, we also propose the development and evolution mechanism of OVJs in loess from the viewpoint of unsaturated loess, which would help understand the loess anisotropic genesis, structural evolution, preferential flow, and the occurrence of geohazards such as landslides and soil erosion.