Stronger Embankments Start in the Lab
By Ann Perry
March 27, 2008
The safety of earthen embankments, including levees and dams, depends
in large part on how resistant they are to erosion. That resistance can hinge
on the soil materials used in their construction.
J. Hanson and
L. Hunt work at the Agricultural Research Service (ARS)
Engineering Research Unit in Stillwater, Okla. They have refined methods
for estimating the erodibility of large embankment structures with a lab-scale
version of the Jet Erosion Test (JET).
Hanson developed JET to evaluate the condition of streams and dam
embankments. In the field, JET applies stresses to soil beds with a water jet
that can be pumped at various flow rates.
The team studied the roles of compaction effortthe mechanical
force needed to increase soil densityand water content in soil erosion.
They measured compaction effort using standard engineering tests, which involve
dropping a hammer onto soil samples from a specific distance for a specified
number of times. As part of their evaluation of compaction effort, they also
varied the soil water content, which affects soil plasticity, in their samples.
The engineers observed that the erodibility of their lab samples
varied significantly between the two soil types they tested, which were a silty
sand and a silty clay. Both soil types also exhibited a large range of erosion,
depending on compaction effort and water content.
For instance, lab soil samples that were compacted while containing
optimum levels of water showed a significantly stronger resistance to erosion.
Higher compaction efforts also increased erosion resistance, and soil texture
and plasticity influenced erosion resistance as much, or sometimes even more,
than compaction factors. The team compared these results with large-scale field
controls and found that their lab-scale JET tests accurately assessed soil
erodibility in samples as small as 10 centimeters in diameter.
Overall, these results indicate that soil type and compaction factors
can be used to make soil at least 1,000 times more resistant to erosion. These
findings will help engineers factor in soil type and other variables to predict
embankment failure rates when designing flood control structures.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.