Initiation mechanism of irrigation-induced retrogressive loess landslides on the South Jingyang Tableland, China

Repeated sliding disturbs the slope structure and degrades soil mechanical properties, promoting the progressive development of retrogressive loess landslides on the South Jingyang Plateau, China. These landslides progressively encroach upon the plateau surface and have caused substantial casualties and property losses. Field investigations revealed that two subsequent retrogressive failures caused the landslide crown to retreat by approximately 8 and 7 m, respectively, suggesting that the near-crown zone formed after the initial failure underwent sustained disturbance and structural readjustment. Accordingly, undisturbed loess samples were collected from a sliding-disturbed zone approximately 5 m from the current crown (No.1) and a relatively undisturbed zone approximately 50 m from the crown (No.2). Consolidated undrained triaxial tests, nuclear magnetic resonance (NMR) infiltration tests, and scanning electron microscopy (SEM) analyses were conducted to systematically compare the strength, water-migration characteristics, and microstructural damage of the soils from the two zones, thereby elucidating the triggering mechanism of retrogressive landslides. The results indicate that: (1) The soil in the sliding-disturbed zone exhibited pronounced mechanical weakening. Under natural conditions, the average shear strength of No. 1 was approximately 20% lower than that of No. 2; under saturated conditions, its shear strength was only 71.9–90.0% of that in the relatively undisturbed zone. The disturbed soil was characterized by a higher proportion of large pores, weaker interparticle bonding, and more irregular pore geometries, making its mechanical behavior more sensitive to variations in water content. (2) The sliding-disturbed zone exhibited more pronounced water migration. The average signal intensity of the second peak in the NMR T₂ spectrum was approximately 1.97 times that of the relatively undisturbed zone, indicating a stronger water-filling response within the larger-pore domain. During water infiltration, pore-structure readjustment and changes in pore morphology occurred in the disturbed zone, further promoting structural weakening of the soil. (3) The subsequent evolution of retrogressive landslides can be summarized as follows: the initial failure disturbs the slope and alters its stress state and pore structure; sustained water infiltration subsequently promotes further structural readjustment and degradation of the soil’s mechanical properties, thereby reducing slope stability and increasing the likelihood of renewed failure. This process ultimately produces a cascading “sliding–disturbance–re-sliding” evolution pattern.