Post-construction verification of hydraulic conductivity of soil in an earthen dam
Earthen dams are used throughout the country to hold back flood waters from cities, towns, and farmlands, make reservoirs to supply drinking water, and create lakes for recreational purposes. The hydraulic conductivity of the soil in the dam is the critical parameter in assessing whether the dam will retain water. It is a function of the type of soil and placement conditions including unit weight and water content. Hydraulic conductivity is difficult and expensive to measure in situ and is typically measured in a laboratory. Correlations between the compacted soil's hydraulic conductivity, unit weight, and water content are used to confirm the soil in the earth dam meets the specifications. Traditional engineering practice, specifies a minimum dry unit weight and a range of water content that are acceptable. However, it is possible to develop a 'zone-of-acceptance' (ZOA) (dry unit weights and water contents) for which a compacted soil meets the hydraulic conductivity specification for the dam. After constructing an earth dam in southern Boone County Missouri, samples of the soil were compacted in the laboratory and their hydraulic conductivity was measured. A ZOA was developed. More than 400 in situ unit weight and moisture content tests using a nuclear density gauge were measured on the soil during construction of the dam. The data were superimposed on the laboratory generated ZOA to determine the expected hydraulic conductivity of the soil in the dam. All of the points indicate the in-situ hydraulic conductivity to be less than 1 x10-7 cm/sec. Ninety-nine percent of the data points indicated hydraulic conductivity less than 1 x10-8 cm/sec. A seepage analysis using the hydraulic conductivities indicate that less than five gallons of water per day will seep through the 1,530 feet long dam. This seepage rate is well within the acceptable performance criteria for an earth dam of 300 gallons per day. The dam is expected to perform adequately with the given hydraulic conductivity and seepage rate. The ZOA should be established prior to construction in order to properly evaluate the condition of the dam while under construction along with quality control and assurance measures.
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