Technical Manual: Conduits through Embankment Dams (FEMA 484)

Introductionwhere conduits were associated with failures and accidents. The case histories inappendix B include a variety of situations where defects in the conduit and poordesign or construction decisions contributed to the failures. Several modes of failureare discussed in this document related to conduits, which include both the backwarderosion piping and internal erosion modes of failure. Appendix B includes very fewcase histories that involve backward erosion piping associated with conduits. This isbecause soils that are highly susceptible to backward erosion piping have seldombeen used to backfill around conduits. Most case histories of failures and accidentsinvolving conduits are related to one of the internal erosion modes of failure. Boththis Introduction and chapter 7 provide more discussion on modes of failure.In 1998, a survey of State dam safety programs was conducted for the InteragencyCommittee on Dam Safety (ICODS) Seminar No. 6 on piping associated withconduits through embankment dams (Evans, 1999, p. 1). Fourteen states providedresponses to the survey. The respondents indicated that 1,115 embankment damswith conduits would likely need repair within the next 10 years. Of these 1,115embankment dams, 53 percent had conduits constructed with corrugated metal pipe(CMP), 23 percent were constructed with steel pipe, and 20 percent were constructedwith concrete pipe.Conduits within embankment dams are often designed using standards notspecifically intended for penetrations through dams. For example, certain pressurepipe standards (e.g., those from the American Water Works Association) may not beapplicable (without design and construction modifications) for use in pressurizedconduits through embankment dams. The purpose and performance characteristicsof conduits through embankment dams differ from those required for water supplypipelines. The use of certain types of manufactured pipe for conduits throughembankment dams is a concern, since these materials were developed andstandardized for applications other than embankment dams. The uniqueperformance requirements for conduits in embankment dams include:• Service life.—Most embankment dams are designed assuming a minimum100-year service life with minimal maintenance. Manufactured pipe needs to bedurable in the expected wet, dry, and freeze/thaw environments found within aconduit.• Accessibility.—As the height of the embankment dam increases, the practicalityof accessing the conduit for repairs decreases. Manufacturing and installationquality control needs to be high to ensure dependable installations.• Strength.—The structural loading on manufactured pipes can be very high due topositive projecting, rather than trench loading conditions, and very highembankments. The pipe needs to be structurally designed for all possibleloading conditions for applications within embankment dams.21