The critical mechanics of the initiation of loess flow failure and implications for landslides

An essential hypothesis is that the flow failure landslides occurring in the Chinese Loess Plateau may initiate in the high-moisture-content loess in the capillary zone rather than in the saturated zone within the slope. Two effective stress paths, namely, the monotonically increasing loading and wetting-based constant loading under undrained conditions, are performed on intact and mechanically compacted loess, aiming at specifying their hydromechanical trajectories when subjected to an increasing water content. The results indicate that static liquefaction is initiated when the soil moisture reaches a threshold during the wetting process after exhibiting a sharp increase in pore water pressure accompanied by strain-softening behavior. Accordingly, we define this state as the initiation of flow failure under the unsaturated framework to distinguish it from static liquefaction, and first propose its corresponding critical mechanics and criterion. Its validity and physical justification are mainly evidenced by the following observations: i) Significant difference in the pore characteristics of the test samples before and after flow failure initiation; ii) Approximate equivalence between the measured and predicted hydromechanical thresholds at the flow failure initiation state; iii) Existence of the strength envelope corresponding to the flow failure initiation state; and iv) The uniqueness of the modified critical state line in the full suction range. This paper provides a general approach to accurately identify the flow failure potential for unsaturated loess, especially in the capillarity zone within a slope, which is of vital importance to the early identification and risk mitigation of loess landslides with a fluidization pattern.