Behavior of Coarse Widely Graded Soils under Low Confining Pressures

AbstractColluvial soils are usually coarse and widely graded. Shallow-seated failures occur frequently in colluvial soil deposits during rainfall infiltration. This paper investigates the behavior of coarse, widely graded soils under very low confining pressures of 5–25 kPa encountered in shallow-seated failures. Isotropic consolidation tests, drained triaxial tests, and undrained triaxial tests were conducted on several widely graded soils with different coarse contents but with the same void ratio of 0.62. With increasing coarse content, the soil microstructure changes from a fines-controlled structure to a coarse-controlled structure after a critical coarse content of approximately 70%. Silty sand with gravel with a coarse content close to the critical value exhibits the highest compressibility because of the presence of large interaggregate pores. Even under very low confining pressures, such soil still shows strong contractive behavior during drained loading, and generates large positive pore-water pressures during undrained loading. This explains why shallow-seated failures occur frequently in colluvial soil deposits caused by rainfall infiltration. Soils with lower or higher coarse contents than the critical value may show dilative behavior under the same low confining pressures. The critical state friction angle increases with the coarse content.