Hysteresis and Uncertainty in Soil Water-Retention Curve Parameters: Fid-Bau Portal

Hysteresis and Uncertainty in Soil Water-Retention Curve Parameters

Likos, William J. / Lu, Ning / Godt, Jonathan W.

Abstract

Accurate estimates of soil hydraulic parameters representing wetting and drying paths are required for predicting hydraulic and mechanical responses in a large number of applications. A comprehensive suite of laboratory experiments was conducted to measure hysteretic soil-water characteristic curves (SWCCs) representing a wide range of soil types. Results were used to quantitatively assess differences and uncertainty in three simplifications frequently adopted to estimate wetting-path SWCC parameters from more easily measured drying curves. They are the following: (1) $α^{w} = 2 α^{d}$ , (2) $n^{w} = n^{d}$ , and (3) $θ_{s}^{w} = θ_{s}^{d}$ , where $α$ , $n$ , and $θ_{s}$ are fitting parameters entering van Genuchten’s commonly adopted SWCC model, and the superscripts $w$ and $d$ indicate wetting and drying paths, respectively. The average ratio $α^{w} / α^{d}$ for the data set was $2.24 \pm 1.25$ . Nominally cohesive soils had a lower $α^{w} / α^{d}$ ratio ( $1.73 \pm 0.94$ ) than nominally cohesionless soils ( $3.14 \pm 1.27$ ). The average $n^{w} / n^{d}$ ratio was $1.01 \pm 0.11$ with no significant dependency on soil type, thus confirming the $n^{w} = n^{d}$ simplification for a wider range of soil types than previously available. Water content at zero suction during wetting ( $θ_{s}^{w}$ ) was consistently less than during drying ( $θ_{s}^{d}$ ) owing to air entrapment. The $θ_{s}^{w} / θ_{s}^{d}$ ratio averaged $0.85 \pm 0.10$ and was comparable for nominally cohesive ( $0.87 \pm 0.11$ ) and cohesionless ( $0.81 \pm 0.08$ ) soils. Regression statistics are provided to quantitatively account for uncertainty in estimating hysteretic retention curves. Practical consequences are demonstrated for two case studies.