Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsconfiguration as the original (although a second riser was added at the upstream endof the spillway). However, a chimney filter was installed which ran axially along theembankment dam. A sand filter diaphragm was also constructed around thedownstream portion of the pipe to control seepage and prevent internal erosion ofembankment material along the sides of the conduit. The side slopes of theexcavation through the remaining embankment dam were designed and constructedas 3H:1V to minimize problems with bridging of the fill. Compaction of theembankment material was carefully monitored and tested during the repairs.Powdered bentonite was added to the backfill under the pipe, because the bentonitewould presumably swell to eliminate any voids.In the years that followed the repair of the embankment dam, Maryland began toexperience problems with other dams constructed with large diameter CMPspillways. In particular, it was noted that many of the joints between sections of thepipe were not watertight. This deficiency is primarily the result of deflection of thepipe, (the design standard allowed 5 percent of the pipe diameter), but poorconstruction techniques and manufacturing tolerances also contributed to theproblem (figure B-52). Embankment dam owners were advised to carefully monitortheir spillway conduits.Accordingly, the owner of Loveton Farms Dam (a local government agency)scheduled inspections of the structure twice per year to document the condition ofB-62Figure B-52.—The failed embankment dam was repaired with a new 78-indiameter CMP spillway. However, soon after reconstruction, this failedjoint was discovered at the upstream end of the pipe near the riser. Alarge void was also observed in the adjacent embankment fill. Note theo-ring gasket has been displaced from the joint.