Impact of Grain Size Distribution Curve on the Small Strain Shear Modulus of Unsaturated Clean Sand

Previous studies on saturated and dry clean sand indicated that for a constant void ratio, the small strain shear modulus, G(max), is strongly affected by grain size distribution of the material. The effect of grain size distribution was mostly examined through differentiating two related terms of grain size distribution curve - that is - mean grain size, d(50); and coefficient of uniformity, C-u. In this study, the G(max) results of bender element tests on specimens of three different grain-sized sands were used to evaluate the role of grain size distribution on the behavior of G(max) for clean sand in an unsaturated state. The tests were performed at matric suctions, psi, ranging from 0 to 15 kPa, along the drying path of the soil water retention curve, SWRC, under a constant mean net stress of 70 kPa. Results showed an "up-and-down" behavior for the small strain shear modulus during drying, complying with the shape of the suction stress-matric suction curve. Furthermore, based on the experimental measurements, D-50 increase had an effect of shifting the SWRC to the left, reducing the rate of changes in G(max), while an increase in C-u widened up the SWRC, decreasing the values of G(max) upon drying. Results of this study were also compared with values of G(max) predicted using available approaches for the small strain shear modulus of unsaturated soils, and it was concluded that suction stress is the parameter which is responsible for the behavior of drying G(max) of unsaturated sands with different grain size distribution curves.