Mechanisms of Rapid Entrainment and Acceleration in Landslide-Initiated Debris Flows: The Role of Static Liquefaction

Introduction

Landslide-initiated debris flows in post-earthquake settings often exhibit explosive volume growth (bulking) and unexpected acceleration, causing devastation extending far beyond the initial failure footprint. Here, we apply the concept of instantaneous base liquefaction to interpret the catastrophic transition from continuum slope failure to fluidized debris flow. We hypothesize that when a collapsing soil mass overrides a loose, saturated basal layer, it imposes rapid, largely undrained loading. This loading triggers static liquefaction (distinct from seismic cyclic liquefaction) in the runout path, effectively creating a low-resistance basal layer that facilitates deep-seated entrainment and rapid acceleration. 

 

Methodology

To test this hypothesis, we re-analyzed data from large-scale rainfall experiments conducted by Sakai et al. (2025). The experiments utilized a 22 m-long (10 m at 30°, 6 m at 10°, and 6 m flat), 3 m-wide, and 1.6 m-deep flume filled with loose sandy soil, designed to simulate the contractive behavior of post-earthquake surficial deposits. We compared two scenarios under a rainfall intensity of 100 mm/h for approximately 2 h, differing only in the initial hydrogeologic condition of the lower gentle slope: Case 1 was initially unsaturated, whereas Case 2 was initially saturated with a high groundwater table established by antecedent rainfall. Internal deformation was visualized using white-sand tracer columns and high-speed imaging.

 

Results

The failure modes differed fundamentally between the two cases. In Case 1, failure was largely confined to the shallow surface layer of the upper slope, with negligible entrainment of the lower layer. In Case 2, however, the arrival of the upper sliding mass triggered near-instantaneous shear deformation across the full depth of the lower gentle slope. High-speed imagery revealed that this deep-seated mobilization occurred within ~1 s of impact. The white-sand tracers in the lower section were not eroded progressively from the surface; instead, they were sheared and mobilized coherently from the base upward, consistent with a rapid loss of basal strength. These observed kinematics are inconsistent with purely traction-driven erosion processes and instead indicate an undrained strength collapse within the basal layer.

 

Conclusion

These results provide physical evidence that the saturated lower layer did not fail solely due to surface shear stress but rather underwent impact-induced base liquefaction. A static liquefaction front likely propagated ahead of the overriding debris mass, effectively reducing basal resistance and enabling massive entrainment of bed material. Our findings suggest that the static liquefaction potential of the runout path can be as critical as source-area stability for hazard assessment in multi-hazard environments characterized by seismic loosening followed by intense rainfall. 

References

• Iverson, R. M., Reid, M. E., & LaHusen, R. G. (1997). Debris-flow mobilization from landslides. Annual Review of Earth and Planetary Sciences, 25, 85-138.
• Steers, L. J., Beddoe, R. A., & Take, W. A. (2024). Propagation velocity of landslide-induced liquefaction and entrainment of overridden loose, saturated sediments. Engineering Geology, 334, 107523.
• Sakai, N., Ishizawa, T., & Danjo, T. (2025). Experimental Research on Rain-Induced Landslide Mechanism Using Large-Scale Rainfall Experimental Facility: Findings and Challenges. In B. Abolmasov et al. (Eds.), Progress in Landslide Research and Technology (Vol. 3, Issue 2). Springer.