Abstract: The study on the seepage in wide-grading soils, the main material of colluvial landslides, is the premise and foundation of the research on the mechanism of rainfall-induced colluvial landslides. The seepage in wide-grading soils is a complex process including the transport of water and fine particles. However, the migration of fine particles is usually ignored. Therefore, three soil samples with different D15 /d85 (D15 is the particle size which 15% of the coarse-grained group by weight is finer than; d85 is the particle size which 85% of the fine-grained group by weight is finer than) were tested by self-made large-scale permeameter to study the transport of water and fine particles in wide-grading soils. The results show that the D15 /d85 has a vital influence on the permeability coefficient and the migration of fine particles. The smaller permeability coefficient is, the more difficult migration of fine particles occur in soils of small D15 /d85; the larger permeability coefficient is with the violent change, and significant fine particles movement observed in soils of large D15 /d85. The change of permeability coefficient reflects the movement of fine particles in the soil, and three modes of particle migration are proposed for the wide-grading soils. The research improves the understanding of the permeability characteristics of wide-grading soils and provides a new mechanism of rainfall-induced colluvial landslides.

Key words: wide-grading soils, D15 /d85, permeability, the migration of fine particles