Fundamentals of Engineering Design
5.3.9.2 Soil Data Sources
During the design of many hydraulic structures by the Vicksburg District U.S. Army Corps of
Engineers, it is standard practice to perform a geotechnical analysis. Soil characteristics are obtained by
extracting soil borings and analyzing the soil in the boring. In most instances, soil borings are stratified
consisting of several layers. Each layer, or strata, of soil has unique physical properties. Laboratory tests
are conducted on each layer of soil to determine a variety of physical properties including; moisture content,
percent organic, gradation, internal friction angle, cohesion, and dry unit weight. In addition to the soil
boring, a geotechnical analysis usually includes local physical properties like the average bank height.
Within the Yazoo River Drainage basin in northern Mississippi there are six sub-basins, to include:
Batupan Bogue, Black Creek, Coldwater River, Hickahala Creek, Hotopha Creek, and Long Creek.
Design plans and as-built drawings for hydraulic structures, scattered throughout these six sub-basins, were
reviewed in 1997 to obtain soil-boring information. Design plans, as-built drawings and soil boring results
were primarily obtained from the Agricultural Research Service in Oxford, Mississippi. The U.S. Army
Corps of Engineers Waterways Experiment Station in Vicksburg, Mississippi provided the remaining soil
data.
5.3.9.3 Soil Data Evaluation
Currently, slope stability estimations are based upon single values of internal friction angle,
cohesion, and unit weight. Sixty-five soil borings were obtained within the Yazoo River Drainage basin.
The bank averaged soil properties, for each soil boring, were obtained by:
determining the bank height to be used in the averaging process;
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determining the percentage of bank in each strata;
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multiplying the percent of bank by the internal friction angle, cohesion, and unit weight; and
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sum the percentages to obtain the bank-averaged values.
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Table 5.10 is an example of the soil averaging process. Soil-boring data was limited for the Long Creek
sub-basin. However, Thorne (1988) conducted field investigations in the Long Creek sub-basin to obtain
soil properties. Thorne (1988) reported using the same averaging procedure for the determination of soil
properties. Averaged soil properties from each boring where collected and then combined on a sub-basin
level basis to obtain sub-basin average properties.
Significant variability in soil properties even within sub-basins was observed. Despite this
variability, with no site specific soil data the sub-basin averaged soil properties are the most logical values
to be used in slope stability. Table 5.11 summarizes the maximum, average, and minimum values of internal
friction angle, cohesion, and unit weight for the six sub-basins.
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