Direct Application of the Soil-Water Characteristic Curve to Estimate the Shear Modulus of Unsaturated Soils

The small-strain shear modulus of soils is a key parameter in the design of geotechnical systems and analysis of the soil-structure response to earth and earth-supported infrastructure. However, the small-strain shear modulus is not unique to a specific soil type. The small-strain shear modulus is a complex function of the state conditions (e.g., void ratio, suction) and stress states (e.g., in situ stresses, preconsolidation stresses). For unsaturated soils, the variations of the state conditions at a given stress state are captured by the soil-water characteristic curve (SWCC). This paper presents a general method to predict the small-strain shear modulus of unsaturated soils based on an inverse relationship between the small-strain shear modulus and the SWCC. Both vary with matric suction, but the two do not exactly mirror each other. Thus, the original SWCC was modified to improve the alignment with the small-strain shear modulus. In addition, a change in the void ratio induced by changing the net normal stress led to a change in the SWCC. Therefore, the modified SWCC was further adjusted for net normal stresses different from the original net normal stress. Finally, a small-strain shear modulus prediction equation was developed in accordance with the modified SWCC. The proposed equation consisted of several fitting parameters that were estimated from the modified SWCC. The developed prediction models were shown to be applicable to the numerous case studies for various net normal stresses and over a large range of matric suctions.