Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case Historiespipe. During an inspection in 1994, large voids (3 feet diameter and 20 feet long)were noted in the embankment around the upstream end of the pipe, and the firstjoint downstream of the riser had suddenly separated by about 0.1 m.The embankment dam was determined to be unsafe, and the riser portion of thespillway was removed to minimize impounding of water until a more detailedinspection could be conducted. A more thorough investigation of the embankmentdam utilizing seismic tomography revealed that nearly all of the embankment fillaround the conduit was of low density. Since the soils are frost susceptible (siltysands and sandy silts of low plasticity), it is quite possible that freezing damaged thesoils adjacent to the conduit. The melting of ice lenses that may have formed in thebackfill would leave voids through which internal erosion could occur. Also, theformation of ice lenses can create forces large enough to deform the thin steel pipe,causing the joints between pipe sections to open.The embankment dam was deemed to be unsafe and is scheduled to be removed.Lessons learnedUse of large diameter CMP conduits in embankment dams should be avoided.Vertical trenches transverse to the embankment dam are never permissible, unlessthey are in rock and backfilled with concrete. Sloping the sides of excavations to nosteeper than 2H:1V is always recommended.ReferenceSchaub, W., Investigation of the Loveton Farms StormWater Management Pond, prepared forBaltimore County Bureau of Engineering and Construction, June 1996.B-63