Effects of gravel content on shear resistance of gravelly soils

• The intergrain granular void ratio is a more representative parameter for gravelly sand.
• Gravel content reduces the drained shear strength less than 20% of that of pure sand.
• Strength parameters of sand-like gravelly soils can be estimated from the sand matrix.
• A preliminary procedure for evaluating colluvium strength parameters is proposed.

Shallow landslides in colluvium, which generally consists of wide particle gradation in a loose state, occur in steep-slope terrains. To perform stability analyses, shear strength parameters that represent soil constituents, in-situ stress state, and shearing mode are needed. This experimental study investigates the effects of gravel content on the shearing characteristics of gravelly soils based on the intergrain state concept. A series of drained simple shear tests was performed on reconstituted specimens of gap- and well-graded sands with various gravel content levels. The results reveal that: (1) the shear strength of gravelly soils depends on the packing condition of dominant particles, with the intergrain granular void ratio being a more representative parameter for the soil packing condition than the global void ratio; (2) the drained shear strength of loose, gravelly sand is dominated by sand matrix properties, with the inclusion of gravel content reducing the strength by no more than 20% compared to that of pure sand; (3) the strength parameters of sand-like gravelly soils can be estimated from the sand matrix with proper reduction.