Effect of shrinkage on air-entry value of soils

The soil–water characteristic curve (SWCC) is an important property of unsaturated soils. One key parameter of the SWCC is the air-entry value. For a soil that does not shrink as soil suction increases, the air-entry value is the same regardless of whether the gravimetric water content-based SWCC (SWCC-w), the volumetric water content-based SWCC (SWCC-θ) or the degree of saturation-based SWCC (SWCC-S) is used. However, for a soil that shrinks as soil suction increases, the air-entry value depends on the SWCC. The air-entry value determined from the SWCC-w is shown to underestimate the air-entry value for a soil that shows shrinkage as soil suction increases. For such cases, the SWCC-S should be used to determine the air-entry value. The SWCC-S can be constructed using the SWCC-w and the shrinkage curve. The shrinkage curve provides the void ratio and the water content for calculating the degree of saturation which can then be used to transform the SWCC-w to the SWCC-S. The shrinkage curve can be easily constructed from the final volume measurement of a drying soil specimen, as shown in this paper. The sensitivity analyses performed on 40 soils showed that the minimum void ratio of the shrinkage curve (ash) has a very significant effect, while the curvature of the shrinkage curve (csh) has a negligible effect on the SWCC-S, and therefore, on the determination of the AEV. A procedure is proposed for determining the air-entry value of soils exhibiting shrinkage upon drying.