Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment Damsmost concern. Once a crack forms and water enters the crack, three possibilities canresult (ASDSO, 2003):1. Water penetrates soil adjacent to the crack and the soil begins swelling. If thecrack is small and not continuous through the embankment dam, it can swellshut and not develop into a problem.2. Water runs through the crack, but the crack is small, so the velocities are low,and the soil is resistant to erosion. A wet spot may appear, but no internalerosion takes place.3. The soil in the embankment dam, such as dispersive clay, is erosive. Internalerosion begins, and a concentrated leak develops. More and more internalerosion occurs, and the embankment dam fails from the breach that is formed.Cracks in embankment dams caused by hydraulic fracture may provide a pathway forinternal erosion. If soils in the pathway of the crack are highly erodible, the crackwill enlarge quickly, leading to a breaching type of failure. Hydraulic fracture iscommon near conduits, because the conduits create differential strains insurrounding embankment soils.Hydraulic fracture of embankment dams can occur when the piezometric head ofwater within the dam is greater than the lateral effective stress on the earthfill.Sherard (1986, pp. 905-927) discusses hydraulic fracturing in detail. Figure 81illustrates how on first filling, a wetting front moves through the embankment dam.Figure 82 shows an example of hydraulic fracture in an embankment dam. For anexample of a project that experienced hydraulic fracture near a conduit, see the casehistory for Piketburg Dam in appendix B.Problems often occur on first filling of the reservoir. About 42 percent of allembankment dam failures due to internal erosion or backward erosion piping occuron first filling (Foster, Fell, and Spannagle, 2000, p. 1025). As discussed in section9.1, filling the embankment dam’s reservoir for the first time requires caution. Slowfilling of the reservoir is important to allow the wetting front to slowly penetrate intothe embankment dam. This will allow the soils to swell and deform, which helpsprevent hydraulic fracture. Typical filling rates are in the range of 0.5 to 2 feet perday. The designer should consider the rate of reservoir rise when determining thehydraulic capacity of the conduit. For guidance on the hydraulic design of conduits,see chapter 3.Excavations for conduits increase the potential for differential settlement, andspecial care is recommended for any excavations used near and under conduits. Theexcavation should be wide enough to accommodate motorized compaction116
Chapter 5—Foundation and Embankment DamAveragewetting frontActualwetting frontFigure 81.—On first filling, a high hydraulic gradient exists in theembankment dam as a wetting front moves through the dam. The wettingfront will not be smooth. Projections will exist due to the differentpermeability of the embankment dam. The water pressure pushing againstthe soil can easily be greater than the lateral stress, and hydraulic fracturecan result. Figure courtesy of ASDSO.Figure 82.—The failure of this embankment dam located in South Carolinawas attributed to hydraulic fracture. The eroded seam located to the rightof the conduit may have been the hydraulic fracture that formed andallowed internal erosion and failure of the embankment dam. Theembankment was composed of dispersive clays.117
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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 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
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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 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 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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