Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsFigure B-55.—When the pool level of this embankment dam was only about2 to 3 feet deep, flow along the outside of the CMP resulted in loss of theadjacent soils.The spillway structure was removed and replaced and has performed satisfactorily.Lessons learnedThis significant hazard embankment dam was improperly designed and constructedas a “temporary sediment basin” (which has less rigid construction requirements) anddid not have proper inspection during construction.ReferenceMaryland Dam Safety Division, Dam file No. 318.B-68
Appendix B—Case HistoriesProject name: Olufson DamLocation: WashingtonSummary: Outlet works conduit failureOlufson Dam was a privately owned embankment dam located in Pierce County,near Gig Harbor, Washington that experienced an outlet works conduit failure. Theembankment dam was 18 feet high, with a storage capacity of 15 acre-feet and21 acre-feet at the top of the dam. The principal spillway consisted of a 2-footsquare, concrete, drop inlet conduit. An open channel in the abutment served as anemergency spillway. The embankment dam was constructed in the 1960s withoutthe benefit of engineering plans. The owner did all the work himself, includingplacing earthfill and mixing his own concrete onsite. Conditions exposed by thefailure suggest that the elements of the construction that required skill weresubstandard. In particular, the concrete work suffered from inadequate cementcontent, poor overall mix gradation, and improper reinforcing. Thick steel cable wassubstituted, in part, for conventional reinforcing steel. Likewise, these cables wereimproperly positioned in the conduit section thus minimizing its enhancement of thetensile load capacity of the conduit. To limit concrete volumes, it appeared theowner had embedded bricks, rocks and concrete rubble into the walls as a fillerduring concrete pours. This practice, termed cyclopean concrete construction, hasbeen successfully used in large gravity structures, but was inappropriate for thinwalled, concrete box conduits.On December 11, 1996, a sinkhole 20 feet in diameter and 17 feet deep opened upin the crest of the embankment dam (figure B-56). At the time the sinkholedeveloped, the property on which the embankment dam sat was uninhabited due tothe recent death of the property owner. The sinkhole was discovered by neighborswalking the streambed to investigate the cause of muddy streamflows. This wasfortuitous in that the sinkhole was discovered before it lead to an embankmentfailure. The sinkhole appeared to have resulted from a collapse in the top section ofthe cast-in-place box culvert that served as the principal reservoir outlet. The failedsegment of the conduit allowed overlying masses of embankment soil, over time, torepeatedly drop into the conduit, where flows then flushed the soil downstream.This sequence of events was supported by the record of stream flows in adownstream gauging station. The gauge record shows normal flows interrupted by aseries of near zero creek flows immediately followed by short, abnormally highchannel discharges. The zero flows are interpreted as incidences of soil massesfalling into and plugging the conduit. The following anomalous high flows representa blowing out of the plug and a release of backwater in the conduit and inlet towerupstream of the plug.B-69
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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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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 Historiesproject,
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Appendix B—Case Historieswatersto
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Appendix B—Case HistoriesADownstr
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Technical Manual: Conduits through Embankment Dams (FEMA 484)

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