Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiesproject, and served only to document the placement of earthfill and compaction.The local government agency overseeing the project was a highway department,which had little dam construction experience.The embankment dam was completely removed, and a new embankment dam andconcrete spillway conduit were constructed 2 years later in the same location. Whenthe foundation for the new dam was being prepared, the State inspector observedroots and other debris, under the original embankment fill. Since such materialshould have been removed, this reinforces the notion that supervision ofembankment dam construction by qualified engineers is essential. Afterreconstructing the embankment dam, the owner eventually decided not to pursuelegal action to determine fault. The original contractor went bankrupt just before thefailure occurred, the local government may have incurred some liability foroverseeing the construction, and the design was apparently completed in accordancewith the approved standards in place at the time the design was started. (Althoughmore restrictive standards requiring different conduit joints had been developedbefore the original embankment dam was constructed, the design approval wasapparently “grandfathered” under the older standard.)The failure resulted in new requirements that spillway conduits not be installed intonear-vertical trenches excavated into the foundation or partially completedembankment dam. This trench installation technique is common procedure forhighway culverts, because the sides of the trench facilitate “arching” of the backfill,reducing the load on the culvert. However, this results in areas of low soil pressurealong the conduit, facilitating seepage along the conduit. Filters are now routinelyconstructed around the downstream portion of the conduit to intercept this type offlow and prevent internal erosion. In addition, the use of large diameter flexibleconduits for embankment dam spillways has been substantially reduced in the last10 years because of this and other failures related to large deformations, difficulty inobtaining watertight joints, and difficulty placing earthfill under the sides of theconduits (Van Aller, 1993).ReferencesState of Maryland, Dam Safety Division, unpublished investigation notes and filephotographs (MD Dam No. 372). The reports of the forensic investigation were notsubmitted to the State and are not public information.Van Aller, Recent Failures of Large Corrugated Metal Pipe Spillways, ASDSO 1993 AnnualConference, 1993.B-11