Testing and evaluation of unsaturated coarse-grained soils

Tests of the soil moisture characteristic of coarse-grained soils have been conducted at the University of New Mexico using the hanging column and pressure plate apparatus. Because these soils have a rapid decrease in the unsaturated hydraulic conductivity with decreasing moisture content, these tests have been evaluated using the HYDRUS-1D finite element computer program that considers the soil hydraulic conductivity. For the hanging column tests this evaluation allowed for verification of equilibrium conditions, and computation of the unsaturated hydraulic conductivity for a series of non-equilibrium measurements. For the pressure plate tests, soil samples seldom reached equilibrium. Measurement of evaporation related moisture loss showed that multiple cycles of depressurization should not be used. Revised pressure plate test procedures that provide thinner soil samples, single pressure settings and computer model prediction of equilibrium time are recommended.;Test data from this study and test data from other studies were used to derive a method that portrays the changes in the soil moisture to matric suction relation caused by changes in the dry bulk density or porosity. This procedure was applied to compute changes in the soil moisture characteristic, using equations first presented by van Genuchten (1980) or Brooks and Corey (1964). The effect of compaction on the saturated hydraulic conductivity was evaluated from test data using a modified equation first proposed by Carman (1939). This equation originally derived for clay soils, and has been re-evaluated with this study for granular soils. The change in saturated hydraulic conductivity with compaction is predicted by considering that a portion of the pore water is immobile. The moisture characteristic parameters of the unsaturated soil are used to estimate the amount of immobile water and the saturated hydraulic conductivity.