Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiesarea, a key part of the remediation program wouldbe continued monitoring of instruments installedduring the investigation/assessment phase of theproject. Target “safe” gradients were identified.Based on estimated gradients from instrumentmeasurements, the water in the tailwater poolwould be increased if necessary to reduce the exitgradients to “safe” levels.Lessons learned• The location and extent of internal erosionor backward erosion piping featuresdeveloping within a embankment dam ordam foundation is difficult to find evenwhen direct evidence is available on theapproximate location of such features. Conservative judgment is required in theassessment of piping and erosion problemsand development of appropriate correctiveactions. In many instances, “multiple linesof defense” will be required to adequatelyresolve the deficiency and provideappropriate risk reduction.Figure B-45.—Grouting operationswithin the conduit.• Periodic inspection of an outlet works conduit is required to make a completeassessment of its current condition and safety. While inspections cansometimes be difficult and expensive to perform, they are the only way toobserve certain conditions. Likewise, explorations and instrumentation aroundthe exterior of a conduit may also be the only way to detect developingconditions that warrant further investigations or corrective actions and tomonitor the effect of corrective actions, once completed, on the overall safetyof the embankment dam and its foundation.ReferencesFerguson, K.A., “Rehabilitation of the Lake Darling Dam Outlet Works, NorthDakota,” Proceedings of the Fourteenth Annual USCOLD Lecture Series, Phoenix, Arizona,June 1994.Ferguson, K.A., J.R. Talbot, S.J. Poulos, and A. Arzua, Advanced Seepage Short Course,prepared for the Association of State Dam Safety Officials and presented at MIT(July 2003), University of Colorado at Boulder (October 2003), University ofCalifornia at Davis (July 2004), and Georgia Tech (November 2004).B-53