Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment Damsused to backfill a sinkhole, this system of filters may be placed in reverse order, withthe coarser gradations placed in the bottom of the sinkhole, and progressively finerfilters used to backfill the sinkhole. The intent of this system is to block additionalmovement of soils above the sinkhole into the feature. Ultimately, no remedialmeasure would be considered safe without repairing the conduit, because thehydraulic heads at the discharge point would be excessive for granular filter/drainagezones to control.Filter diaphragms or collars that are limited in size are seldom sufficient to controlthis type of failure. Emergency action consisting of placing an inverted filter withrock cover over the discharge point of water or the face of the embankment dammay be appropriate. Rarely should this type of measure be considered a long termsolution. If internal erosion rather than backward erosion piping is the cause of theproblem, a filter blanket over the discharge area may become plugged, and flow willseek an alternative exit.7.3 Failure Mode No. 3: Backward erosion piping or internal erosion of soilsalong the outside of a conduit caused by hydraulic forces from the reservoirFor this failure mode, water flows along the interface between the conduit throughan embankment dam and the surrounding soil. This failure mode is usuallyassociated with embankment seepage through the soils surrounding the conduit.The seepage along the interface between the conduit and surrounding soil may beconcentrated enough to result in backward erosion piping, if the soils are susceptible.This failure mode is very similar to Failure Mode No. 2. The only difference in thesetwo modes of failure is the source of the water. In Failure Mode No. 2, the sourceof the water causing internal erosion of the soils is a defect in a conduit. In FailureMode No. 3, the source of the water is seepage from the reservoir that concentratesat the interface between the conduits and surrounding soil. The sequence of failureis described as follows and is illustrated in figure 101.1681. Seepage forces and concentrated flow develop along the contact between aconduit and surrounding soil.2. Backward erosion piping can occur if the seepage exits downstream through anunfiltered face or into an overly coarse zone of the embankment dam and thesoils surrounding the conduit are susceptible to backward erosion piping.Continued flow can result in the formation of a tunnel connected to thereservoir that will potentially result in a breach of the embankment dam.3. If soils surrounding the conduit are resistant to backward erosion piping, butcracks or preferential flow paths occur from poor compaction techniques orlater develop from hydraulic fracture, continued flow through the preferential
Chapter 7—Potential Failure Modes Associated with ConduitsFigure 101.—Failure Mode No. 3.—Backward erosion piping or internalerosion of soils along a conduit at the interface between the conduit andsurrounding soils.flow paths will result in internal erosion. An erosion feature similar to thatcaused by backward erosion piping can then develop.4. If the erosion process continues, it can lead to a breaching failure of theembankment dam. As previously discussed, Failure Mode No. 3 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. Figures 8 and9 illustrate failure mechanisms resulting from internal erosion along a conduit.The Tin Cup Dam case history described by Luehring, Bezanson, and Grant (1999)is an example where this failure mode probably contributed to the development ofextensive sinkholes and other distress symptoms at an embankment dam. Asdiscussed previously in this chapter, it seems likely that multiple failure modes169
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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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Chapter 5Foundation and Embankment
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Chapter 5—Foundation and Embankme
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Page 169 and 170: Chapter 6Filter ZonesZones of desig
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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 caus
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Appendix B—Case Historiesof sever
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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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