Small-strain shear stiffness of sand-gravel mixtures

University of Canterbury, Department of Civil and Natural Resources Engineering, Christchurch, New Zealand

^* Corresponding author: gabriele.chiaro@canterbury.ac.nz

Abstract

Small-strain shear stiffness (G₀) is a key parameter for seismic ground response analysis and performance evaluation of various earth structures and foundations. To account for its pressure and density dependency, G0 has been conveniently correlated to mean effective stress (σ_m^′) and void ratio (e). Such an approach is suitable for conventional uniform materials. However as found in this study, it is not always applicable to estimate the G₀ of sand-gravel mixtures (SGMs), because it essentially fails to account for the combined effects of density and gravel content (G_C). In this study, aimed at addressing this issue and developing a theoretical framework and empirical correlations suitable for estimating the G₀ of SGMs, a series of bender element laboratory tests were carried out on selected SGMs. Specifically, SGMs were obtained by mixing two clean sands – namely New Brighton Sand (mean diameter, D₅₀ = 0.2 mm) and Dalton River Washed Sand (D₅₀ = 0.75 mm) – and rounded pea gravel (D₅₀ = 5.5 mm). Shear wave velocity of specimens having G_C = 0, 10, 25, 40 and 60% and prepared at a relative density (D_r) of 20, 30, 45 and 60% was measured at σ_m^′ = 50, 100, 150 and 200 kPa. The laboratory results indicated that G₀ of SGMs increases with increasing both the D_r and σ_m^′, whereas the effect of G_C would be marginal to significant depending on the limiting and threshold sand contents. To correlate G₀ simultaneously with both G_C and D_r, the equivalent void ratio (e_f(eq)) was adopted. It is shown that the use of e_f(_eq) makes it possible to uniquely describe the G₀ of SGMs for any combination of G_C and D_r over the full range of σ_m^′ level applied in this study.