Experimental study on migration-deposition characteristics of fine particles in inverse-graded deposits from high-speed long-runout landslides

Rainfall-induced migration and deposition of fine particles within inverse grading deposits can disrupt internal structural stability and increase the risk of secondary geological hazards such as landslides and debris flows. To investigate the influence of fine particle migration and deposition characteristics on the stability of inverse grading deposits, this study conducted laboratory-scale permeability tests, comparing the effects of varying particle concentrations, particle sizes, and seepage velocities on the migration-deposition behavior of fine particles in inverse grading deposits. The experimental results indicate that both particle size and seepage velocity significantly influence the migration and deposition of fine particles. When the median particle sizes were 4 μm and 18 μm, the turbidity of the outflow increased with higher seepage velocities. In contrast, for a median particle size of 34 μm, turbidity significantly decreased, indicating that larger particles exhibit weaker migration capacity under the same conditions. Additionally, the non-uniform distribution of porosity strongly influenced deposition behavior: coarse porosity in the upper layer limited deposition, while smaller pores in the middle layer promoted particle interception and accumulation. This study systematically identifies the migration and deposition characteristics of fine particles in inverse grading deposits under rainfall-driven conditions and highlights the critical influencing factors. The findings provide theoretical support for further research on rainfall-induced landslides and debris flows in inverse grading deposits.