Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsEventually, the tunnel erosion feature reaches the reservoir. Outflow will thenincrease substantially, leading to direct erosion of the embankment dam andcomplete breach or draining of the reservoir through the tunnel that develops.The resulting failure often completely destroys the embankment dam, leavingfew traces of the original piping tunnel. The failure of this embankment damwas attributed to piping of foundation sands. Photo courtesy of NationalOceanic and Atmospheric Administration.Figure 10 (cont’d).—The backward erosion piping process associated with intergranularseepage and the subsequent backward erosion of soil particles.16
Introductionbe observable at the surface of the embankment or foundation. Visual inspection,seepage and turbidity measurements, and pore pressure readings are useful indetecting whether problems like these may be developing in an embankment dam.Chapter 9 discusses inspection techniques in detail.Design and construction inadequacies are often to blame for internal erosion andbackward erosion piping incidents. Designers must understand which designmeasures are effective in preventing these mechanisms of failure.In nonpressurized conduits, water seeping through the embankment dam can enterthe conduit through defects. If the surrounding soils are susceptible to backwarderosion piping, cavities can develop in the embankment and foundation of theconduit. This problem is discussed in more detail in section 7.1.In pressure flow conduits, water under pressure can escape through defects anddamage the surrounding embankment and foundation. This problem is discussed inmore detail in section 7.2.In nonpressurized or pressurized conduits, water seeping along the interface betweenthe conduit and surrounding soil may be concentrated enough to result in backwarderosion piping, if the soils are susceptible. If the soils are resistant to backwarderosion piping, but a crack or potential flow path develops near the conduit, internalerosion can result. This problem is discussed in more detail in section 7.3.If the soils surrounding the conduit are resistant to backward erosion piping,hydraulic fracture may occur. The hydraulic fracture created can then erode and leadto a failure tunnel that is similar to that which develops in soils that are susceptible tobackward erosion piping. This problem is discussed in more detail in section 7.4.Internal erosion and backward erosion piping incidents are often associated withconduits through embankment dams. The following factors increase the likelihoodof these problems developing at a given site:• Conduits constructed across abruptly changing foundation conditions (i.e., aconcrete core wall or bedrock with a quickly changing profile) are more likely toexperience differential settlement. See section 1.2 for more discussion onfactors in locating conduits in the most favorable conditions.• Circular conduits constructed without concrete bedding or cradles are morelikely to experience problems than conduits in more favorable shapes (i.e.,horseshoe). See section 4.1 for more discussion on conduit shapes.• Conduits with an excessive number of joints are more likely to develop defectsthat can lead to problems. See section 4.3 for discussion on joints in conduits.17
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Technical Manual:Conduits throughEm
Page 5 and 6: PrefaceTens of thousands of conduit
Page 7 and 8: Prefacecondition. The hazard classi
Page 9: PrefaceLakeLine MagazineMaryland Da
Page 16: 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
Page 29 and 30: ContentsB-33 Aerial view of Como Da
Page 31 and 32: ContentsB-81 Cross section of Wiste
Page 33 and 34: ContentsMASW, multichannel analysis
Page 35 and 36: Symbolsc, cohesionE', modulus of so
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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
Page 47 and 48: A vortex may form at the location w
Page 49 and 50: IntroductionWater from the reservoi
Page 51 and 52: IntroductionFlowing water from the
Page 53: IntroductionThe process of dislodgi
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 and 70: Chapter 1GeneralConduits have been
Page 71 and 72: Chapter 1—GeneralFigure 14.—Ant
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Page 75 and 76: Chapter 1—Generalmeans the condui
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Page 79 and 80: Chapter 2Conduit MaterialsVarious m
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Page 91 and 92: Chapter 2—Conduit Materials2.3.2
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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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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 HistoriesADownstr
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Technical Manual: Conduits through Embankment Dams (FEMA 484)

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