Infiltration of water into unsaturated soils : Suction and hydraulic conductivity models

Water pressures in a dike under over-topping wave conditions can be predicted by finite element calculations. However, these computations are complex due to the non-linear suction and hydraulic conductivity models that capture the unsaturated flow behavior of soils and these calculations require large computation times. As a result, inaccurate results are sometimes used for the prediction of dike stability. Inaccurate prediction may also result from the lag of knowledge with respect to the material behavior on a large scale. Nowadays, the Van Genuchten material model is mostly used in combination with the parameter set that is given in the Staring series. This model and the parameter set was derived for agricultural applications and the laboratory test that support the Van Genuchten model and Staring series are conducted on small samples. This scale does not fit to most geotechnical applications due to heterogeneity of the soils, cracks and structure formation.

This report addresses the mathematical formulation that captures the infiltration process of water into unsaturated soils and presents three alternative suction and hydraulic conductivity models. The numerical formulation for simulating infiltration of water into unsaturated soil is improved by the introduction of the moisture capacity. This modification reduces the inaccuracy of the predictions. The moisture capacity is given by the analytic expression of the pressure derivative of the saturation function. The behavior of the models is investigated by simulating unsaturated groundwater flow in one-dimensional soil columns where water infiltrates at the top into an initially unsaturated soil. Results show that the linear model and the exponential model provide more robust results and are easier to use than the Van Genuchten model.

The material behavior at field scale is investigated at the test sites ’Oijen’ and ’Westervoort’. At the Oijen test site volumetric water content and unsaturated water pressure were measured at the crest of the dike. At the Westervoort test site volumetric water content, unsaturated and saturated water pressures were measured below the inner berm of the dike. The capability of the Van Genuchten model to reproduce field measurement at these test sites was addressed. Simulations of groundwater heads and water pressures were performed for one-dimensional soil columns located at the crest and inner berm of the dike. For these simulations special boundary conditions were needed to capture the two-dimensional flow behavior through the dike.

A climate module was used to simulate evaporation and precipitation based on atmospheric data collected by a KNMI weather station nearby. Although the simulated groundwater pressures match the measured water pressures quite well, no conclusions could be drawn about the applicability of this model, due to the uncertainty in the two-dimensional flow behavior and the complexity of weather conditions that drive the infiltration process.

Dit is een rapport van Kennis voor Keringen (Rijkswaterstaat/Deltares).
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