Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsUntil about the mid-1980s, the most common approaches for controlling seepagewere antiseep collars (also known as cutoff collars) and careful compaction (specialcompaction using small hand held compaction equipment) of backfill aroundconduits. Antiseep collars are impermeable diaphragms, usually of sheet metal orconcrete, constructed at intervals within the zone of saturation along the conduit.They increase the length of the seepage path along the conduit, which theoreticallylowers the hydraulic gradient and reduces the potential for backward erosion piping.Antiseep collars were designed primarily to address intergranular seepage (flowthrough the pore spaces of the intact soil). Antiseep collars did not fully address theoften more serious mechanism of failure (internal erosion), that occurs when waterflows through cracks and erodes the compacted earthfill near the conduit outside thezone of influence of the antiseep collars in the compacted earthfill near the conduits.In the 1980s, major embankment dam design agencies including the U.S. ArmyCorps of Engineers (USACE), and the Bureau of Reclamation (Reclamation)discontinued using antiseep collars on conduits for new dams. Reasons why antiseepcollars were abandoned include:• Antiseep collars impeded compaction of soils around the conduit.• Antiseep collars contributed to differential settlement and created potentialhydraulic fracture zones in the fill.• Designers realized that problems associated with conduits were more likely tobe caused from internal erosion mechanisms than from intergranular seepage.• Designers achieved increased confidence in the capability and reliability offilters to prevent internal erosion failures.• Antiseep collars can form a foundation discontinuity that could result indifferential settlement and cracking of the conduit.The Natural Resources Conservation Service (formerly Soil Conservation Service,SCS) also discontinued using antiseep collars on new embankment dams, butcontinues to allow them on small, low hazard dams and only under certain restrictiveconditions.Figures 14 through 16 show examples of the construction difficulties involved withcompaction around antiseep collars. Appendix A gives a detailed history of thedesign rationale used for antiseep collars and reasons for their being discontinued.Figures 17 and 18 show examples of the ineffectiveness of antiseep collars inpreventing embankment dam failure resulting from internal erosion near conduits.32
Chapter 1—GeneralFigure 14.—Antiseep collars impeded the compaction of soils around theconduit. Hand tampers were used next to the antiseep collars.Figure 15.—Compaction around antiseep collars was difficult using largeequipment.Most embankment dam designers, dam regulators, and dam-building agencies nowrecommend a zone of designed filter material surrounding the penetrating conduit.Some designs use a filter diaphragm located about midway between the centerline ofthe embankment dam and downstream toe. Other designs use a filter collar aroundthe downstream portion of the conduit. Often, a chimney filter serves as adiaphragm to protect the conduit, as well as satisfying other functions of33
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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.
Page 19 and 20: Contents23 Precast concrete pipe be
Page 21 and 22: Contents81 On first filling, a high
Page 23 and 24: Contents117 This conduit was severe
Page 25 and 26: Contents158 An example of a histori
Page 27 and 28: ContentsFigures in the AppendicesNo
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Page 31 and 32: ContentsB-81 Cross section of Wiste
Page 33 and 34: ContentsMASW, multichannel analysis
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Page 39 and 40: IntroductionConduits convey water f
Page 41 and 42: IntroductionFigure 3.—A 15-inch d
Page 43 and 44: Introductionunstable. When seepage
Page 45 and 46: IntroductionAn embankment dam with
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Page 49 and 50: IntroductionWater from the reservoi
Page 51 and 52: IntroductionFlowing water from the
Page 53 and 54: IntroductionThe process of dislodgi
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Page 57 and 58: IntroductionFigure 11.—Example of
Page 59 and 60: Introductionwhere conduits were ass
Page 61 and 62: Introduction• Conduits within lev
Page 63 and 64: IntroductionFigure 12.—An example
Page 65 and 66: Introductionmore recent practiced h
Page 67 and 68: IntroductionThe “best practices
Page 69: Chapter 1GeneralConduits have been
Page 73 and 74: Chapter 1—GeneralFigure 17.—Fai
Page 75 and 76: Chapter 1—Generalmeans the condui
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Page 79 and 80: Chapter 2Conduit MaterialsVarious m
Page 81 and 82: Chapter 2—Conduit MaterialsFigure
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Page 91 and 92: Chapter 2—Conduit Materials2.3.2
Page 95 and 96: Chapter 3Hydraulic Design of Condui
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Page 113 and 114: 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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Chapter 6Filter ZonesZones of desig
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Chapter 6—Filter Zonesseepage or
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Chapter 6—Filter ZonesFigure 87.
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Chapter 6—Filter Zones3. Upstream
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Chapter 6—Filter Zonesbedrock sur
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Chapter 6—Filter Zoneszones, and
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Chapter 6—Filter Zonesfiltering t
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Chapter 6—Filter Zonessoil. This
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Chapter 6—Filter Zones6.8 Specifi
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Chapter 6—Filter ZonesVarious deg
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Chapter 6—Filter Zones• For pre
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Chapter 7Potential Failure Modes As
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Chapter 7—Potential Failure Modes
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Chapter 8Potential Defects Associat
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Chapter 8—Potential Defects Assoc
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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 Historiespipe. Du
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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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