Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsEl. 485'2 21 12.75151Probable path of main leaks. Gradientis ≅ 10-15 feet/740 feet, or 1:60Figure B-81.—Cross section of Wister Dam showing probable path forinternal erosion through embankment. The length of the flow path wasabout 740 feet, and the head on the entrance tunnels was less than15 feet.extensive grouting, a steel sheet pile wall, and additional upstream and downstreamberms and drains. After completion of the remedial work, the embankment dam hasbeen in operation continuously with little trouble. A major renovation program wasfinished in 1990. The renovation included a slurry panel wall installed for the fullheight of the embankment.Sherard concluded that the cause of the leakage path in the embankment dam couldonly be attributed to hydraulic fracture in the embankment. The flow pathdeveloped immediately above the closure section on the embankment dam, also justabove the old stream channel. Aggravating this condition was the fact that the rightbank of the stream channel (viewed downstream) consisted of a bedrock shelf thatcontributed to differential settlement. A plan view of the area of the leak is shown infigure B-82. A longitudinal profile of the area where the leak developed is shown asfigure B-83. Note that this view is looking upstream.The embankment dam was compacted to a relatively high dry density correspondingto about 97 percent of the maximum Standard Proctor dry density, and it wascompacted at about optimum water content. The soils were silty, relatively low inplasticity, and highly dispersive. Compaction at these conditions probably resulted ina somewhat brittle fill likely to crack when subjected to differential settlement.Lessons learnedThe incident demonstrated that even well constructed embankment dams built bywhat was then state-of-the-art technology are susceptible to hydraulic fracture andinternal erosion if they are not protected by internal chimney filters. Modernembankment dam design concepts include protective filters for all significant andhigh hazard embankment dams, and even low hazard dams if constructed ofB-118