Determining the Shapes of Yield Curves for Unsaturated Soils by Modified State Surface Approach

Attention is increasingly paid to the elastoplastic behavior of unsaturated soils. In the development of an elastoplastic framework for unsaturated soils, it is necessary to determine the initial shape of the yield curve and its evolution with yielding. Accordingly, correct determination of shapes of yield curves is of significant importance. Existing methods rely on use of a series of specimens with "identical" stress history to determine the initial shape of yield curve. Preparation of such specimens requires thoughtful preparation, careful instrumentation, and lengthy equilibrium time, which makes identical specimens very difficult to obtain. As a result, the yield curve obtained through the existing methods could be misleading. Hence, this paper presents a simple method to correctly and rapidly determine the shapes of the yield curves and their evolution during yielding even if the soil specimens do no have identical stress histories. In this new method, a modified state surface approach, recently proposed to model the elastoplastic behavior of unsaturated soils under isotropic conditions, was applied. It overcomes the limitations in the existing methods, and allows correct and rapid determination of the elastic and plastic hardening surfaces, and then shapes of yield curves without additional laboratory work. An example was used to demonstrate the application of the proposed method. The comparison between the proposed method and other methods was discussed from which the capability and effectiveness of the proposed method were evaluated.