Modelling the water retention behaviour of anisotropic soils

Water retention curve (WRC) is an important parameter for unsaturated soils. It is greatly affected by the anisotropy of pore structure, as supported by experimental results in the literature. So far, however, the mechanism and theoretical modelling of anisotropy effects have not been investigated. These two issues were explored in this study based on two-dimensional analysis of soil pores, while were approximated as a series of ellipses for simplicity. According to experimental results in the literature, the pores of anisotropic specimen are more elongated than those of isotropic specimen on average. The elongated pore has a higher water retention ability than the round pore when they have the same area. As a consequence, the water retention ability of anisotropic specimen is higher than that of isotropic specimen. On the basis of this mechanism, a new WRC model was proposed for isotropic and anisotropic soils. To verify the new model, it was applied to simulate the WRCs of three soils with isotropic and anisotropic pore structures. Measured and calculated results were well matched with the coefficient of determination (R²) in the range of 0.89 to 0.99 and the root-mean-square error (RMSE) ranging from 0.009 to 0.073. It is convincingly demonstrated that the new model is able to capture the influence of anisotropy on WRC.