Technical Manual: Conduits through Embankment Dams (FEMA 484)

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Conduits through Embankment DamsDuring an inspection of the interior of the conduits in April 2001, it was discoveredthat material had been deposited inside the middle conduit near an opening in aconstruction joint. This was occurring through a hole in the floor of the middleconduit at a construction joint near station 1+30. Approximately 1 ft 3 of silt and finesand were deposited on the floor. However, this deposit was observed during thewinter when no irrigation releases are made. More deposition may have occurredduring irrigation season that was washed downstream and not observed.Consequently, the total volume of material could have been much greater than the1 ft 3 observed in 2001. Reclamation theorized that plugging this opening couldresult in redirecting the erosion through a different hole or crack in the conduit.Also, redirecting the erosion might cause a more dangerous path to develop alongthe foundation contact of the conduits, and a piping exit might develop downstreamof the embankment dam. If the exit point were located within the outlet channel,early detection would be very difficult.Another area of concern was the condition of the north conduit that was reportedlyplugged at each end prior to the final raise of Pablo Dam in 1932, but was neverconfirmed. Therefore, it could be possible that a nearly full reservoir head couldexist at the end of the north conduit, which was less than 100 feet from thedownstream toe of the dam. After much discussion between all involved parties, itwas decided to completely remove and replace the original outlet works.As an interim measure, a temporary patch was installed over the opening to preventadditional material from being eroded into the conduit while allowing for relief ofwater pressures. The patch consisted of filter fabric under a metal screen. DuringMarch 2002, the geotextile portion of the patch ruptured and approximately 0.5 ft 3of silt and fine sand were deposited into the conduit. The patch was repaired soonafter the rupture was discovered. Reservoir level restrictions were implemented inApril 2003 and were to be kept in place until the removal and replacementmodifications could be completed.The construction of a new outlet works began in November 2004 and wascompleted in the by the spring of 2005. The major aspects of the work included:• Construction of a cofferdam to maintain an area free of water duringconstruction.• Clearing, grubbing, and stripping prior to excavation.• Removing existing embankment dam slope protection.• Excavating embankment materials to accommodate construction of the newoutlet works (Slopes transverse to the dam centerline were excavated at 4H:1V).B-74
Appendix B—Case HistoriesFigure B-58.—Pablo Dam nearing completion.• Removal of the existing reinforced concrete intake structure, conduits, retainingwalls, and apron.• Constructing a lean concrete mudslab, on which to found the new outlet works.• Constructing reinforced cast-in-place intake structure, conduit, retaining walls,and apron. The new conduit was double barreled with each barrel having a6-foot 3-inch inside diameter. The exterior surface of the conduit was sloped at1H:10V below springline and was curved above springline to provide a goodsurface to compact earthfill against. Each conduit joint was a treated as controljoint with longitudinal reinforcement extending across the joint and 6-inch PVCwaterstop.• Installing two emergency guard gates and two regulating gates within theupstream intake structure.• Constructing a chimney filter and drain system. The filter extendeddownstream and encased the outlet works conduit. Filter materials encasing theconduit consisted of sand processed to a specified gradation from an approvedoffsite source.• Placing and compacting zoned earthfill in the embankment dam closuresection.B-75
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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 HistoriesLessons
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Technical Manual: Conduits through Embankment Dams (FEMA 484)

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