Impact of Plastic Compression on the Small Strain Shear Modulus of Unsaturated Silts

Abstract

This article presents an experimental laboratory study on the effect of plastic compression and stress history on the small strain shear modulus, Gmax, of unsaturated soils. In previous studies, most of the experiments for the measurement of the Gmax values of unsaturated soils were performed along the elastic unloading–reloading curve of the compression curve, and the effect of plastic compression on the measured Gmax was ignored. This research aimed to provide a set of high-quality Gmax data along elastoplastic paths of the compression curve that can be used to further investigate this effect. In this study, a bender element system was used to measure Gmax values under different matric suctions and mean net stresses, the axis translation technique was implemented for suction control, and high values of confining pressure were applied using a high-pressure hydraulic pump. A digital image processing (DIP) method was also used to measure changes in the soil specimen’s volume during loading. Experimental results showed an increase in the small strain shear modulus during loading, with trends that follow transitions in the shape of the curve for void ratio versus mean effective stress. During unloading, the values of unsaturated small strain shear modulus were observed to decrease with a mean net stress decrease while being consistently higher than those measured during loading at the same stress levels. This study also highlighted the effects that hydraulic hysteresis and the initial loading conditions might have on the values of Gmax measured during loading and unloading. Finally, the Gmax values measured in this study were used to evaluate the accuracy of an existing model in predicting the small strain shear modulus of unsaturated soils along elastoplastic paths of the compression curve.