Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsProject name: Bohemia Mill DamLocation: MarylandSummary: Undermining and failure of a new spillway conduit constructed on pilesThe Bohemia Mill Dam is a very old structure, constructed in the early 1900s with atimber spillway structure supported on piles. The embankment dam is about 15 feethigh with a two-lane county road on the crest. In the 1990s, deterioration of thetimber bridge over the spillway led the county to enact weight restrictions andrestrict traffic to a single lane until a new structure could be designed and built.Geotechnical investigations of the embankment dam and foundation revealed thatthe underlying soils are very soft, and a decision was made to replace the spillwaywith a reinforced cast-in-place box culvert supported on 60-foot long steel piles.Seepage control along the culvert was to be provided by a bentonite slurry wall nearthe center of the embankment dam and by a filter drain at the downstream end.Construction of the new spillway, slurry cutoff wall, filters, and new lake drain wascompleted in 2002 (figures B-24 and B-25).In early 2004, an engineer inspecting the bridge notified the owner that he observedclear seepage from under the downstream end of the spillway conduit, but there wasno indication of internal erosion or backward erosion piping of the embankment orfoundation soils.However, within a few months, a sinkhole was noted in the pavement on theembankment dam crest (figure B-26). The seepage flow had significantly increased,and soil particles were observed moving downstream. The condition rapidlyworsened, and the size of the sinkhole increased (figure B-27). Particles of thebentonite slurry cutoff wall were observed to be washing downstream from an areathat appeared to be boiling (figure B-28). Attempts to create a sandbag weir aroundthe boil to reduce leakage under the spillway conduit were unsuccessful, and the lakewas drained.Currently, the structure has not been repaired. Repair options under considerationinclude construction of a jet grout slurry wall along the upstream side of theembankment dam and spillway or installation of sheetpile cutoff wall along theupstream side of the dam and culvert.B-26
Appendix B—Case HistoriesFigure B-24.—Because of soft foundation soils, numerous 60-foot long pipepiles were installed in winter 2001 to support a new reinforced cast-inplaceconcrete spillway structure.Figure B-25.—This the downstream end of the spillway at the end ofconstruction in 2002.B-27
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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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Page 471 and 472: IndexMechanical caliper, 246Metal c
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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 HistoriesFigure B
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Appendix B—Case HistoriesLessons
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Appendix B—Case HistoriesThe caus
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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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