Modelling water retention characteristic of unsaturated soils

The water retention characteristic or Water Retention Curve (WRC) is an important constitutive feature of soils. Previous experiments have indicated that specific surface area has effects on the WRC. It has also been observed that a linear relationship generally exists between the air-water interface area and the pore saturation in unsaturated soils. However it seems that no study on their internal linkage with the WRC has been reported yet. This paper tries to explain the water retention curve according to the physical and chemical behaviours of the phases involved in unsaturated soils. Using the capillary, interfacial surface theories and averaging theorem, a deterministic formula which represents the water retention characteristic is derived. This formula demonstrates the internal linkage of the WRC to the specific surface area of porosities. It shows agreement with experimental observations. Based on this formula, a fitting model is proposed for the WRC of soils. Finally, this model is tested to fit the WRCs of a wide range of soils, and compared with other main models.