United States Department of Transportation
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Stormwater Infiltration in Highway Embankments – Saturated Hydraulic Conductivity Estimation for Uncompacted and Compacted Soils
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2017-10-01
[PDF-17.32 MB]
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Edition:Final Research Report
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Abstract:The estimation of hydraulic conductivity (Ksat) is a key step to assess the rate of infiltration, whether that estimate is for an infiltration pond or trench, if it is for a highway embankment, or if it is for natural dispersion in general. The focus of this research is to assess available methods for estimating Ksat, especially with regard to the ability of various methods to assess Ksat in both a loose, uncompacted state as well as in a compacted state for embankments. Ksat prediction for natural soils is also considered. To accomplish this, a series of relatively large diameter (i.e., 6 to 9 inch) saturated hydraulic conductivity tests were conducted both in a loose state and in a compacted state. Existing Ksat prediction equations such as those developed by Hazen (1892), Slichter (1898), Terzaghi (1925), Chapuis (2004), and Massmann (2003) were evaluated and, using the Ksat laboratory measurements gathered in this study, were empirically optimized to improve prediction performance. Those equations that included soil porosity, n, or void ratio, e, were given preference for further development, since n or e were determined to be the best parameters to address the effects of compaction on Ksat. The empirically optimized Slichter, Terzaghi, and Chapuis equations were found to provide the most accurate prediction performance. Since it may be difficult to obtain a measured porosity or void ratio at design time, a method to estimate the soil porosity using grain size parameters plus degree of soil compaction, or for natural soils, degree of over-consolidation, was developed, and could be used in the optimized equations with only minimal reduction in Ksat prediction accuracy. The optimized Slichter Equation was used for several example infiltration facilities as was done by Massmann (2003) to determine what effect the use of this new equation would have on infiltration design and infiltration rate prediction accuracy.
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