Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsAlthough all previous man-entry inspections had noted that the seepage into theCMP was clear, the accumulation of soil in the conduit indicated that material wasbeing transported into the conduit. Until remedial repairs could be made, theengineer made frequent man-entry inspections of the conduit in order to takeemergency action, if dam failure was imminent. A decision was made to slipline theexisting CMP with a 48-inch outside-diameter HDPE pipe and then grout theannulus between the pipes. The HDPE pipe joints were fusion welded in the field.A filter diaphragm was constructed downstream around the downstream end of theexisting conduit to control seepage along the outside of the conduit. The work wascompleted in 1996 at a total cost of about $140,000.Lessons learned• Internal erosion or backward erosion piping of embankment material mayoccur in an embankment dam, even if seepage appears clear during infrequentinspections.• CMP is a poor choice for use in outlet works conduits through an embankmentdam.• Sliplining a 60-inch CMP with 48-inch outside-diameter HDPE was costeffective.ReferencesNewhouse, Scott, and Dan McGill, This Old Dam: Must It Have Outlet Works?, 1997ASDSO Annual Conference Proceedings, Pittsburgh, Pennsylvania, 1997.Clevenger, Charles, Dalewood Shores Dam Conduit Repair, FEMA/ICODS Dam SafetySeminar No. 6, Piping Associated with Conduits through Embankment Dams, 1999.B-40