Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsFigure 97.—Soil particles being carried into an outletworks conduit through a joint.Figure 98.—Leakage from an unauthorized “field joint”constructed by the contractor about 5 feet downstreamfrom the spillway riser structure. An inspectionrevealed that nearly all of the joints were exhibitingsevere leakage and loss of embankment material.piping could occur from seepage forces surrounding the defect in the conduit if thesoils are susceptible. If the defect is large enough and the reservoir head is highenough, the loss of particles from the surrounding soil body caused by backwarderosion piping could be severe.160
Chapter 7—Potential Failure Modes Associated with ConduitsInternal erosion could occur if a preferential flow path (like a hydraulic fracture)develops that is connected to the conduit defect. If internal erosion occurs in soilssurrounding a conduit defect in this failure mode, the potential for eventual failure ishigh, because all soils when subjected to continued flow along a preferential flowpath are erosive over time. Highly erodible soils, such as nonplastic silts, broadlygraded silty coarse-grained soils, and dispersive clays, could develop erosion featuresmore quickly. The most likely manifestation of erosion in this failure mode is asinkhole that develops on the embankment surface.As previously discussed, Failure Mode No. 1 may involve either a backward erosionpiping or internal erosion mechanism of particle erosion. The Introduction of thisdocument includes extensive discussions of factors that should be evaluated todetermine which of these mechanisms is likely for a specific situation.The sequence in which this failure mode could develop is illustrated in figure 99 anddescribed in the following steps. Note that the following description specificallyinvolves the development of backward erosion piping in a situation where theconduit is surrounded by soils susceptible to this failure mechanism. A similar set ofsteps could be described for a scenario involving internal erosion rather thanbackward erosion piping, when the surrounding soils are not susceptible to backwarderosion piping. For the sake of brevity, this description of similar steps in an internalerosion scenario is not repeated.1. As water is impounded in the reservoir, seepage develops through theembankment dam. The time for this to occur varies with the permeability ofthe embankment zones. A phreatic line develops, and seepage forces are activein the saturated soils around the conduit.2. Seepage can enter any defects in the conduit, if the conduit has an interiorpressure lower than the water in the soil pores. If the seepage discharging intothe nonpressurized conduit has sufficient gradient and soils are susceptible tobackward erosion piping, soil particles may be carried with the water.3. Backward erosion piping of the soils in the embankment dam will cause atunnel to develop for soils that can support a tunnel. If the soils cannotsupport a tunnel, a sinkhole may occur instead. A failure can occur if the defectin the conduit is large enough to allow most of the reservoir water to escape.4. If soils between the reservoir and the defect in the conduit are not susceptibleto backward erosion piping, and no preferential flow paths occur in thesurrounding soils, the defect in the conduit may not result in immediateproblems.161
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Technical Manual:Conduits throughEm
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PrefaceTens of thousands of conduit
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Prefacecondition. The hazard classi
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PrefaceLakeLine MagazineMaryland Da
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Conduits through Embankment Dams11.
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Contents23 Precast concrete pipe be
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Contents81 On first filling, a high
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Contents117 This conduit was severe
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Contents158 An example of a histori
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ContentsFigures in the AppendicesNo
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ContentsB-33 Aerial view of Como Da
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ContentsB-81 Cross section of Wiste
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ContentsMASW, multichannel analysis
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Symbolsc, cohesionE', modulus of so
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ContentsD 3350D 4221D 4253D 4254D 4
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IntroductionConduits convey water f
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IntroductionFigure 3.—A 15-inch d
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Introductionunstable. When seepage
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IntroductionAn embankment dam with
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A vortex may form at the location w
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IntroductionWater from the reservoi
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IntroductionFlowing water from the
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IntroductionThe process of dislodgi
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Introductionbe observable at the su
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IntroductionFigure 11.—Example of
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Introductionwhere conduits were ass
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Introduction• Conduits within lev
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IntroductionFigure 12.—An example
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Introductionmore recent practiced h
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IntroductionThe “best practices
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Chapter 1GeneralConduits have been
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Chapter 1—GeneralFigure 14.—Ant
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Chapter 1—GeneralFigure 17.—Fai
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Chapter 1—Generalmeans the condui
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Chapter 1—Generalfacilitate futur
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Chapter 2Conduit MaterialsVarious m
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Chapter 2—Conduit MaterialsFigure
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Chapter 2—Conduit Materials• Co
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Chapter 2—Conduit MaterialsSteel
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Chapter 2—Conduit Materials2.3.2
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Chapter 3Hydraulic Design of Condui
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Chapter 3—Hydraulic Design of Con
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Chapter 4Structural Design of Condu
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Chapter 4—Structural Design of Co
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Page 151 and 152: Chapter 5Foundation and Embankment
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Page 169 and 170: Chapter 6Filter ZonesZones of desig
Page 171 and 172: Chapter 6—Filter Zonesseepage or
Page 173 and 174: Chapter 6—Filter ZonesFigure 87.
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Page 189 and 190: Chapter 6—Filter Zones6.8 Specifi
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Page 195 and 196: Chapter 7Potential Failure Modes As
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Page 215 and 216: Chapter 8Potential Defects Associat
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Page 231 and 232: Chapter 9Inspection and Assessment
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Chapter 9—Inspection and Assessme
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Chapter 10Evaluation by Geophysical
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Chapter 10—Evaluation by Geophysi
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Chapter 11Appropriate Emergency Act
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Chapter 11—Appropriate Emergency
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Table 11.2.—Potential problems an
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Chapter 12Renovation of ConduitsThe
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Chapter 12—Renovation of Conduits
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Chapter 13Replacement of ConduitsGe
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Chapter 13—Replacement of Conduit
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Chapter 14Repair and Abandonment of
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Chapter 14—Repair and Abandonment
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ReferencesThe following references
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ReferencesBureau of Reclamation, De
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ReferencesCooper, Chuck, E. Wesley
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ReferencesJacobsen, S., “Buckling
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ReferencesNatural Resources Conserv
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ReferencesU.S. Army Corps of Engine
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ReferencesWooten, R. Lee, Peter S.
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Additional ReadingThe following ref
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Additional ReadingCarter, Brent H.,
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Additional ReadingJohnson, Brian, J
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Additional ReadingNatural Resources
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Additional ReadingU.S. Army Corps o
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Additional ReadingVan Aller, Hal, R
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GlossaryThe terms defined in this g
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Glossary• Backward erosion piping
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GlossaryCathodic protection system
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GlossaryConsequences (FEMA, 2004):
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Glossarymalfunction or abnormality
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GlossaryDrawdown (FEMA, 2004): The
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GlossaryFailure mode (FEMA, 2004):
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GlossaryGate chamber: An outlet wor
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GlossaryImpervious: Not permeable;
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GlossaryLeakage (FEMA, 2004): Uncon
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GlossaryOpen cut: An excavation thr
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GlossaryPolyvinyl chloride (PVC): A
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GlossaryResistivity: A measure of t
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GlossarySettlement (FEMA, 2004): Th
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GlossaryStandard Proctor compaction
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GlossaryThermoset (ASTM F 412, 2001
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GlossaryReferencesAmerican Concrete
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IndexAAbandonment of conduits, 225,
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IndexCompaction of backfill, 6, 10,
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IndexDDamembankment, 113-130, 333-3
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IndexExisting conduit, 23, 31, 45-5
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IndexHydraulic fracture, 4-11, 17,
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IndexMechanical caliper, 246Metal c
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IndexPressure flow, see Conduit, pr
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IndexSliplining, 39, 45-47, 51, 164
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IndexUUltrasonic pulse-echo, 245, 2
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Appendix AHistory of Antiseep Colla
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Appendix A—History of Antiseep Co
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Appendix BCase HistoriesIndexDam Lo
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Appendix B—Case HistoriesDam Loca
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Appendix B—Case HistoriesProject
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Appendix B—Case HistoriesFigure B
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Appendix B—Case Historiesproject,
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Appendix B—Case Historieswatersto
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Appendix B—Case HistoriesLessons
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Appendix B—Case HistoriesThe cond
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Appendix B—Case Historiesroadway.
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Appendix B—Case Historiesentire c
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Appendix B—Case Historieshauled i
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Appendix B—Case Historiescoal tar
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Appendix B—Case Historieshistory
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Appendix B—Case Historiesarea, a
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Appendix B—Case Historiespipe. Du
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Appendix B—Case Historiesinvert o
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Appendix B—Case Historiesdownstre
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Appendix B—Case HistoriesThe down
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Appendix B—Case Historieshose and
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Appendix B—Case HistoriesADownstr
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Technical Manual: Conduits through Embankment Dams (FEMA 484)

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