There is substantial information in the literature on soil water retention characteristics but most of this information is for relatively loose agricultural soils. In this paper, we present the wetting and drying curves of 18 compacted aggregate and granular materials used for pavement construction in Minnesota. In terms of traditional sand, silt, and clay contents, all these base and subbase materials were nearly similar. Also, the drying curves of these materials were within a narrow range of water contents. The main differences among these curves were in the inflection points (air entry values) and in the water contents either near saturation or at 15,300 cm of suction. This is expected considering that particle size distribution of most samples were nearly similar. We also tested the empirical and physico-empirical models in the literature for predicting water retention of roadbed materials. In general, these models did not predict well the water retention properties of roadbed materials because of high densities (up to 1.95 Mg m-3) or low clay content. We also present the Pedo-transfer function models that predict water retention properties of roadbed materials from easily measurable properties such sand percent and dry bulk density. Since there are substantial quantities of larger size aggregates in pavement materials that do not contribute to water retentions, we point out the difficulties of using water retention characteristics of aggregate materials to predict its hydraulic conductivity functions.
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