Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsFigure B-40.—Deepening of the initial breach channel.B-40). However, the control section, where the flow transitions from subcritical tosupercritical flow, was located in the breach channel, where backcutting could occurand cause a large uncontrolled release. The reservoir basin upstream of the breachwould have to be excavated in order to move the control section back into thereservoir. The inmates donned hip-waders and began the arduous task of excavatingunderwater, and after an hour’s effort, the control section was safely located severalfeet back into the reservoir. Empire Dam was completely breached by July 2, andthe reservoir drained in a very efficient and cost-effective manner.Lessons learnedThis event demonstrated that with a relatively small reservoir and small embankmentdam height, the reservoir can be released in a controlled manner by performing acontrolled breach of the embankment dam. The event also demonstrated that heavyequipment is not always needed to perform such a task. Care should be employedwhen attempting to release water from the reservoir by means of a controlled breachof the embankment dam.ReferenceColorado Division of Water Resources, Dam Safety Engineers Inspection ReportFiles—Incident Report for Empire Dam, State Engineers Office, Dam Safety Branch,1997.B-44
Appendix B—Case HistoriesProject name: Hernandez DamLocation: CaliforniaSummary: Conduit constructed over a cutoff trenchHernandez Dam is a 124-foot high embankment dam located in San Benito County,50 miles southeast of the town of Hollister, on the San Benito River. Theembankment dam was built in 1961. Hernandez Dam is a zoned earthfill withballast berms on both slopes. Slopes are about 3.5H:1V on the upstream side and3H:1V on the downstream side. The embankment dam has a large central clay corewith outside slopes of 3 /4 H:1V. The upstream shell is composed of a pit run sandand gravel zone. The downstream shell zone is clay to sandy clay from the spillwayexcavation. An 18-inch thick layer of river run sand and gravel filter was installed totwo-thirds of the embankment dam height in the abutments, and a blanket drain wasused in the center part of the fill. There is no chimney drain. An idealized crosssection is shown in figure B-41.The outlet conduit of Hernandez Dam is a 48-inch diameter steel pipe encased inreinforced concrete. The pipe is 3 /16 inch thick, and the concrete is 12 inches thick.There are six concrete antiseep collars along the outlet conduit in the clay coresection. The discharge is controlled on the upstream end of the outlet conduit bytwo hydraulically operated 30-inch butterfly valves. There is no downstream valve.The maximum discharge capacity is 400 ft 3 /s with the reservoir water level at thespillway crest. The conduit crosses the cutoff trench in the core section of theembankment dam as shown on figure B-41. The antiseep collars are separated fromthe concrete encasement by 3 /4-inch asphaltic expansion filler material. Steelreinforcement in the reinforced concrete encasement varies with the loadingconditions. The longitudinal rebar is continuous across the construction joints. Thejoints have 1 5 /8-inch shear keys and 6-inch dumbbell-type rubber waterstops.The outlet conduit was installed on variable thicknesses of compressible material.The first 50 feet of the outlet conduit at the upstream toe of the embankment dam ischert bedrock. From this point to the upstream edge of the core trench, a layer ofgravel backfill above chert bedrock serves as foundation for the outlet conduit. Thegravel thickness varies from 1 foot at its upstream end to 3 feet at the edge of thecore trench. In the core section, the outlet conduit bridges above 20 feet ofcompacted clay backfill, which is in the underlying cutoff trench. Downstream fromthe core, the bedrock dips steeply, and the outlet conduit is founded on streambedgravels in the upper zones, grading to gravelly clays in the lower levels abovebedrock. The depth of the alluvium to bedrock was 24 feet prior to theembankment dam foundation excavation. The outlet conduit was constructed in theB-45
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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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Page 487 and 488: Appendix BCase HistoriesIndexDam Lo
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Appendix B—Case HistoriesFigure B
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Appendix B—Case HistoriesProject
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Appendix B—Case HistoriesLessons
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Appendix B—Case Historiesof sever
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Appendix B—Case HistoriesLessons
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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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