Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment Damsdevice is necessary to make sure that the earthfill is compacted tightly againstthe structure and no voids are present. The soil’s moisture content should bechecked, so that it matches the originally intended water content.• Closure section.—Conduits are often located in closure sections withinembankment dams. Section 5.2 discusses several important designconsiderations for closure sections. Another important factor for the closuresection is the potential for exposed surfaces to become desiccated before theclosure section is filled. Before adding compacted soil to an area that has beenexposed, the soil should be carefully inspected for evidence of desiccationcracking. Soil with desiccation cracks must be removed, moistened, andrecompacted before allowing subsequent earthfill operations to resume. Poorlybonded lifts will result from placing compacted soil on a surface that has beenallowed to dry. Hydraulic fracture can create a pathway for internal erosion inthis zone of the earthfill. Often a few feet in depth of the existing embankmentsurface is moistened and reworked.5.3.3 Dispersive clay backfillEmbankment dams usually contain a zone of lower permeability soil to reduce theseepage through the embankment. In small embankment dams, the entire dam maybe constructed of the same soil, termed a homogeneous construction. If this zone oflow permeability, clayey soil develops cracks, particularly transverse cracks, fromhydraulic fracture, desiccation, or other causes, the integrity of the embankment dammay be compromised. Water flowing through a crack in any soil will erode andenlarge the crack, unless the crack is able to swell closed before erosion occurs. Ifthe crack continues to erode, this can lead to a breaching of the embankment dam.Figures 84 and 85 show failures known to be associated with highly dispersive clayembankments. Figure 84 shows a small embankment dam that failed when waterflowed along a transverse crack in the dam. The transverse crack was caused byhydraulic fracture of the earthfill associated with differential settlement near theconduit. Failures of embankment dams constructed of dispersive clays withoutappropriate defensive design measures have been common (figure 85).These lower permeability zones are intended primarily to reduce seepage inembankment dams. They may be successful in this regard, but if they developcracks, they can still perform unsatisfactorily. The erosion resistance of these zonesdepends on several factors, including the gradation, degree of compaction andcompacted water content, plasticity and electrochemical composition. The mosterosion-resistant zones are high in plasticity with an electrochemical compositionthat results in strong interparticle attraction, compacted to a high percent saturationto reduce their permeability. The least erosion-resistant soils are termed “dispersiveclays.”124
Chapter 5—Foundation and Embankment DamFigure 84.—An embankment dam in Mississippi constructed of dispersiveclay soil, failed on first filling with low reservoir level. Failure was likelycaused by cracks from differential settlement, hydraulic fracture, or poorcompaction about the outlet works conduit. This embankment dam had nofilter.Figure 85.—This embankment dam constructed with dispersive clays failedon first filling of the dam.125
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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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Page 111 and 112: Chapter 3—Hydraulic Design of Con
Page 113 and 114: Chapter 4Structural Design of Condu
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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 195 and 196: 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 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 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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