Abstract: The permeability of debris flow deposit, if served as a foundation of check dam, is vital to forming uplift pressure on dam base. Current researches on soil permeability are focused on cohesionless coarse-grained soils and few on wide graded gravelly soils, such as debris flow deposits. The upper limit particle size of the fine particle affecting the permeability of wide graded gravelly soil is determined by the permeability test on the debris flow deposit in Beichuan county, Sichuan Province of China. The relationships between fine-grained soil content and permeability coefficient are also obtained. The results show the coarse particles only play a skeleton role, and the fine particles dominate the permeability of debris flow deposits. The upper limit particle size of 0.1 mm has significant influence on the permeability of wide graded gravelly soil. The content of fine particles (<0.1 mm) has a negative exponential relationship with the permeability coefficient, and the permeability of debris flow deposits tends to be stable when the fine particle content is more than 20%.

Key words: debris flow, wide graded gravelly soil, fine-grained soil, permeability coefficient