Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment Dams1950-1970. Most of these sites had concrete conduits that would slowly release thetemporarily impounded floodwaters. The design criterion used for most of thoseembankment dams arbitrarily required that the seepage path through the saturatedportion of the embankment be increased by 15 percent by adding antiseep collars.This requirement did not vary with the soil type in the embankment dam. Usually,collars were spaced along the conduit every 20 to 25 feet through the earth core ofzoned embankment dams or through the central portion of homogeneous dams.Figure A-1 shows a conduit with antiseep collars with hand compacting of earthfillnext to the conduit. Figure A-2 shows a failure of a conduit with antiseep collarsconstructed around the conduit.A.4 Changes in philosophyFrom 1960 to 1980, a number of small embankment dam failures occurred, eventhough antiseep collars were carefully installed in well constructed earthenembankments. Several of the failures were at structures designed by the NaturalResources Conservation Service. Sherard (1972) reported on a study of thosefailures. The study showed that intergranular seepage and associated backwarderosion piping was not the mode of failure for these embankment dams. Thefailures usually occurred almost exclusively when the completed embankments werefirst subjected to a pool, long before a phreatic surface had time to develop throughthe compacted earthfill. Other studies by Sherard (1973) on larger earthenembankment dams attributed failures and near-failures to internal erosion of claycores through hydraulic fractures in the embankment zones.The reasons why antiseep collars were ineffective in preventing failures near conduitsmay be summarized as follows:• The antiseep collars only influenced water flowing in the immediate vicinity ofthe conduit. The collars did not significantly affect the remainder of thesurrounding earthfill. Most of the failures were found to have occurred notimmediately along the conduit, but in compacted fill outside the zone of theantiseep collars.• The antiseep collars were designed to increase head loss in intergranularseepage, but most failures occurred long before steady seepage conditionsoccurred. Studies showed the failures occurred from water flowing alongcracks within the earthfill, and not through the soil immediately next to theconduit.Sherard, Decker, and Ryker (1972) discuss the mechanisms by which failures canoccur in the following quote from the reference. In the quote, the term piping isA-4
Appendix A—History of Antiseep CollarsFigure A-1.—Hand tamping embankment material next to an antiseepcollar.Figure A-2.—Internal erosion failure along a conduit with antiseep collars.A-5
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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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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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Page 435 and 436: Glossarymalfunction or abnormality
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Page 441 and 442: GlossaryGate chamber: An outlet wor
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Page 445 and 446: GlossaryLeakage (FEMA, 2004): Uncon
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Page 455 and 456: GlossaryStandard Proctor compaction
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Page 459 and 460: GlossaryReferencesAmerican Concrete
Page 461 and 462: IndexAAbandonment of conduits, 225,
Page 463 and 464: IndexCompaction of backfill, 6, 10,
Page 465 and 466: IndexDDamembankment, 113-130, 333-3
Page 467 and 468: IndexExisting conduit, 23, 31, 45-5
Page 469 and 470: IndexHydraulic fracture, 4-11, 17,
Page 471 and 472: IndexMechanical caliper, 246Metal c
Page 473 and 474: IndexPressure flow, see Conduit, pr
Page 475 and 476: IndexSliplining, 39, 45-47, 51, 164
Page 477 and 478: IndexUUltrasonic pulse-echo, 245, 2
Page 479 and 480: Appendix AHistory of Antiseep Colla
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Page 487 and 488: Appendix BCase HistoriesIndexDam Lo
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Appendix B—Case HistoriesProject
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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 HistoriesFigure B
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Appendix B—Case Historiesarea, a
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Appendix B—Case HistoriesLessons
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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 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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