Effect of Gradation on the Strength and Stress-Dilatancy of Coarse-Grained Soils: A Comparison of Monotonic Direct Simple Shear and Triaxial Tests

A broad spectrum of well-graded, coarse-grained soils are commonly present in natural deposits, though characterization of these materials has been approximated using sand-based engineering methods in liquefaction evaluations. Through combined results of 31 constant stress direct simple shear and drained triaxial compression tests, this study experimentally investigates the effect of mean grain size (D-50) and gradation (C-u) on the drained monotonic strength and stress-dilatancy of poorly- to well-graded, coarse-grained soils. Coarse-grained mixtures of varying D-50 and gradations were prepared to relative densities of 20%-75% and tested under a range of overburden stresses. Results are analyzed in terms of the frictional resistance and dilative contributions to the shear strength of soils with varying gradations, as compared to clean sands, using different shearing modes. It is shown that (1) increased gradation of soils increases the peak shear strength and frictional resistance due to a greater initial rate of dilation exhibited in well-graded, coarse-grained soils; and (2) current stress-dilatancy relationships underestimate the dilative behavior of well-graded test materials.