The paper reports on our effort to extend the well-known Hardin\u27s equation by the influence of the grain size distribution curve. The study is motivated by the fact that Hardin\u27s equation with its commonly used constants can significantly over-estimate the small strain shear modulus Gmax of well-graded sands. Approximately 350 resonant column (RC) tests with additional P-wave measurements have been performed on 33 specially mixed grain size distribution curves of a quartz sand with different mean grain sizes d50, coefficients of uniformity Cu = d60/d10 and fines contents FC. The experiments show that for constant values of void ratio and pressure, the shear modulus Gmax and the small-strain constrained elastic modulus Mmax are independent of the mean grain size, but strongly decrease with increasing coefficient of uniformity. A fines content further reduces the small-strain stiffness. In order to improve the estimation of Gmax and Mmax, the parameters of Hardin\u27s equation have been correlated with Cu and FC. A correlation of Gmax and Mmax with relative density Dr is less accurate. For a certain shear strain amplitude γ, the modulus degradation factor G(γ)/Gmax is smaller for higher Cu-values but does not depend on the fines content. An extension of an empirical formula for the modulus degradation factor is presented
