Geoelectrical and Electromagnetic Induction Delineation of Seep Origin
at a US Army Earthen Dam
Abstract
Earthen dam failures of high hazard dams are expected to cause
downstream human life loss and economic and environment loss. Of the
three most prevalent failure modes for earthen dams, seepage (or piping)
failures account for approximately 40 percent. All earthen dams seep and
it is not uncommon to observe some seepage on the downstream toe, but
the flow rate of uncontrolled seepage should be negligible. In general,
seepage occurs through the dam and/or its foundation. The cross section
of the dam is designed to force the waterside head (pressure) to drop as
the water flows through the cross section to the downstream toe.
Uncontrolled or unexpected seepage can promote soil movement (i.e.
piping) and cause a pipe or a cavity to develop within the earthen
structure which can lead to a complete failure of the embankment. At
present there are over 84,000 earthen dams throughout the United States
many of which depend on the US Army Corps of Engineers for inspection,
monitoring, maintenance, and repair. Recently a multi-method geophysical
investigation, was performed at a site where a suspected seep outflow
had increased ten-fold, warranting further investigation. Because the
seep presented in a historically dry stream channel in the karst bedrock
adjacent to the dam abutment there was some question as to whether the
origin of the seep was indeed a result of piping or due to a naturally
occurring seasonal spring. Electromagnetic induction measurements of
terrain conductivity, as well as electrical resistivity tomography
and mise-a-la-masse surveys were performed in an attempt to delineate
the source water of the seep. The geophysical methods identified
potential flow paths from the impoundment to the seep; however, a
geochemical analysis of the seep outflow was compared to the same
analysis of the impoundment water on the upstream side of the dam and
found to be statistically different.