A Drainage Column Test for Determining Unsaturated Properties of Coarse Materials

A column test was developed to define the water retention curve and the unsaturated permeability function of coarse materials during drainage. The test includes five steps. First, the material is placed in the column at a constant density, using vacuum and deaired water to reach close to 100 % saturation, which is checked using a mass and volume method. Second, the saturated hydraulic conductivity is determined by a constant head test. Third, a gravity drainage test is performed and the volume of drained water is monitored versus time. Fourth, after full drainage, which can take several weeks, the material is removed from the top of the column, to determine the water content versus elevation and thus the water retention curve. Fifth, the water retention data and selected models are used to predict the drainage flow rate and compare the predictions to the experimental data. This helps to define the best hydraulic functions for the material. Examples are provided to illustrate the key elements of the test.