Water Retention Characteristics of Aggregate and Granular Materials

Soil water retention is important for characterizing the storage within and the rate at which water moves through a granular material under both saturated and unsaturated conditions. This study characterized the drying curves of 18 compacted aggregate base and granular subbase materials used in pavement construction in Minnesota. In general, drying curves at matric suction greater than 30 cm were within a narrow range of water contents for most materials. This is expected considering that particle size distribution that contributes to water retention (<2 mm) was nearly similar for most samples. Suctions less than 30 cm represented the air entry values of many materials. Pedo-transfer functions showed that water retention properties of roadbed materials could be explained by differences in sand content and dry bulk density of the samples. Correlation coefficients between the water retention function parameters of van Genuchten, Brook and Corey, or Fredlund and Xing and the particle size distribution were good. However, the correlation coefficients of above function parameters versus percent particles passing #200, D10, D60, or the Minnesota's grading number were poor suggesting that complete water retention curve can not be described by a single parameter of the particle size distribution. Existing empirical and physico-empirical models also did not predict well the water retention properties of the roadbed materials because these materials were highly compacted (up to 2.20 Mg m^–3) and were low in clay content. Since aggregate base and subbase materials contain large aggregates that do not contribute to water retention but strongly affect saturated hydraulic conductivities, we point out the difficulties of using water retention characteristics along with saturated hydraulic conductivity to predict unsaturated hydraulic conductivities of these materials.