Soil-water characteristic curves - Determination, estimation and application

The soil-water characteristic curve (SWCC) is a relationship between water content in a soil and suction. The SWCC was first plotted by Edgar Buckingham, a soil physicist, in 1907 for six soils ranging in texture from sand to clay. It was adopted for use in unsaturated soil mechanics by the geotechnical engineering community. The SWCC is now almost treated as the index property of unsaturated soils. It has been used as a proxy for permeability and shear strength of unsaturated soil. Most soils have a sigmoidal SWCC, otherwise known as a unimodal SWCC as opposed to a bimodal SWCC which has been found for some soils. Although determining the SWCC is easier than determining permeability or shear strength for unsaturated soil, the test is still time-consuming and it is not easy to determine the entire SWCC. Incomplete or insufficient SWCC data may lead to an incorrect SWCC and hence inaccurate determination of permeability and shear strength. Progress has been made to expedite the experimental determination of SWCC as well as estimating the SWCC from basic soil properties using pedotransfer functions. In addition, SWCC has been represented using volumetric water content, gravimetric water content or degree of saturation. Different representations may have dire consequences on its application. Determining the SWCC using volumetric water content or degree of saturation presents challenges in estimating the instantaneous volume during the experiment. In this paper, the current state-of-the-art in determining, representing and estimating SWCC and its applications are described and critically examined.