Gravel and coarse sand make up significant portions of some environmentally important sediments, but the hydraulic properties of the sediments are typically obtained in the laboratory using only the fine fraction (e.g., <2 mm or 4.75 mm) after the gravel are sieved out. Researchers have found that the content of gravel has a significant impact on the hydraulic properties of the bulk soils.
With the funding support from the U.S. Department of Energy’s Remediation and Closure Science Project and the CH2M HILL Plateau Remediation Company’s Remediation Decision Support project, Scientists in the Pacific Northwest National Laboratory conducted laboratory experiments to measure the porosity and the saturated hydraulic conductivity of binary mixtures with different fractions of coarse and fine components. They proposed models to predict these properties of soils containing gravels.
Experiments using five binary mixtures of coarse and fine components showed that the porosity of a mixture linearly decreased to a minimum value and then increased as the gravel fraction increased. The impact of the gravel on the saturated hydraulic conductivity was more significant and more complex than that on porosity. There was a critical gravel content below which gravel blocked flow, and hence, the saturated hydraulic conductivity decreased slightly as the gravel content increased. Once the gravel content was greater than the critical value, the saturated hydraulic conductivity increased sharply by as large as orders of magnitude because of the formation of continuous large pores.
Fred Zhang, the senior author of the study says, “The Hanford Site vadose zone underlying radioactive waste tanks, trenches and cribs in south-central Washington State contains substantial regions characterized as gravelly sands and sandy gravels. Some of the soils contain more than 60% gravel. Correct handling with the gravel impacts on hydraulic properties of the soils is of critical importance for accurate simulating and predicting contaminant transport in order to protect the groundwater.”
from : soils.org
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