Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 2—Conduit MaterialsFigure 23.—Precast concrete pipe being unloaded from delivery truck.Some design agencies, such as Reclamation, do not permit use of pressurized ornonpressurized precast concrete conduits through embankment dams due toconcerns with watertightness, the lack of longitudinal reinforcement extending acrossconduit joints, and the difficulty of adequately compacting earthfill against theconduit below its springline.Other design agencies, such as NRCS, use precast concrete pressure pipe (AmericanWater Works Association [AWWA] C300 [2004a], 301 [1999b], and 302 [2004b])extensively for all embankment dams other than low hazard dams. The typicalNRCS application is a pressure rated pipe in a nonpressurized conduit situationwhere the entrance structure is an ungated riser or tower, and the terminal structureis an ungated plunge pool or stilling basin. Earth Dams and Reservoirs (1990) containsNRCS design guidance for conduits in embankment dams.2.2 PlasticPlastic pipe is often used in the renovation of conduits (e.g., sliplining or lining ofexisting conduits). Plastic pipe that is used in the construction of new, significantand high hazard embankment dams should always be encased in reinforcedcast-in-place concrete to assure quality compaction against the conduit. Use ofplastic pipe in new, low hazard embankment dams is generally limited to smalldiameters (less than 12 inches). Plastic pipe used in low hazard embankment dams isoften not encased in reinforced cast-in-place concrete for economic andconstruction-related reasons. However, use of a filter diaphragm or collar is avaluable defensive design measure, even for low hazard classification sites withfavorable conditions. Some designs may not employ a filter diaphragm around theconduit, but eliminating this valuable feature should be carefully considered andjustified, based on extremely favorable soil conditions, good conduit constructionmaterials and methods, reliable construction practices, and favorable foundationconditions. Plastic pipe is generally considered to have a shorter service life45