Influence of particle size and gradation on shear strength dilation relation of granular materials

Upon close scrutiny of data reported in the literature, taking into account particle-scale characteristics to optimize the precision of the well-known empirical Bolton's equations and imposing particle-size limits on them is recommended. The present paper examines the potential influence of particle size and grading on the shear strength-dilation relation of granular materials from the results of 276 symmetrical direct shear tests. The applicability of physical symmetrical direct shear tests to interpret the plane strain frictional shearing resistance of granular materials has been widely discussed using the discrete element method (DEM) computer code SiGran. Sixteen different grain-size distribution curves of three different materials were tested at different normal pressures and initial relative densities. It is demonstrated that while the contribution of dilatancy to shear strength is not influenced by the variation in the coefficient of uniformity,C-u, in the investigated range, it significantly decreases with increasing mean particle size, D-50. The coefficients of Bolton's equations have been, therefore, adjusted to account for D-50. A comparison of the predictions by the proposed empirical formulas with plane strain friction angle,phi(ps), and dilation angle,psi, data from the literature shows that accounting for the grain size yields more accurate results.