Evaluation of shear strength equations for unsaturated soil

This paper presents the evaluation of ten published shear strength equations of unsaturated soil. The applicability of each shear strength equation to estimate or predict unsaturated shear strength of various soil types on the drying andwetting pathswas studied and the results are presented in this paper. The unsaturated shear strength equations are classified according to the nature of the equation, either it is a fitting type or a prediction type shear strength equations. The evaluation was performed based on the comparison between estimated and measured shear strengths from ten sets of published data that correspond to a wide range of different soil types. The results show that the fitting type equations, Vanapalli et al. (1996a) and Lee et al. (2005) equations provide good agreement with most of the measured drying and wetting shear strengths. The prediction type equation, Garven and Vanapalli (2006) equation, is only able to predict most of the drying shear strengths. It was found that none of the prediction type equation is able to predict both drying and wetting shear strengths reasonably. However, the main advantage of the prediction type equation is the expediency in estimating the unsaturated shear strength. The advantage and limitation of each selected shear strength equation are also highlighted and discussed in this paper.