Top 10 for Chimney Filter/Drain Systems (PowerPoint Slides)

TOP 10 FOR CHIMNEYFILTER / DRAIN SYSTEMSJohn W. France, PE, URS CorporationPresented at National Dam SafetyProgram's Technical Seminar No. 19 -Filters, Drains, and Geotextiles in Damsand LeveesFebruary 23, 2012

#1 – Be cautious in consideringnatural soils for filtersRare to find natural soils suitablefor filters‣Not “clean” enough‣Can be gap graded‣Variable gradationsReadily available ASTM C33 fineaggregate is an excellent filter inalmost all cases

Effectiveness of ASTM C33Fine Aggregate as a Filter

#2 – Always mathematicallyregrade base soilRequired in all current filterdesign guidance: NRCS,USACE, USBRProvides retention for critical finerportion of the base soilFailure to mathematically regradecan result in substantiallyinadequate filter

Filter Gradation DesignPoint 1 is to satisfy filter requirements – prevent internal erosionPoint 2 is to satisfy permeability requirementsAll other points are to prevent gap grading and prevent segregation

Importance of Regrading theBase Soil – Example 1

Importance of Regrading theBase Soil – Example 2

#3 – Limit fines content infilters and drainsSuggest

Effect of Fines ContentAfter Barber and Sawyer, 1952

#4 – Do not overestimatepermeability of concrete sandEven with low fines content,permeability of ASTM C33 fineaggregate will be on the order of10 -2 to 10 -3 cm/secCannot convey large seepagequantities from concentratedflowsCannot be first material placedwith flowing water

Effect of Concentrated Seepage

#5 – Consider constructabilityin selection of filter / drainzone dimensionsIn many cases calculatedseepage quantities do not requirelarge filter thicknessesConstructability will most oftencontrol filter dimensions

A Set of SuggestedChimney Filter DimensionsSimultaneous ConstructionSimultaneous ConstructionDownstreamModification

“Christmas Tree” Effectfrom Placement

Trenching Method3 ft min.

#6 – Consider methodspecifications for filter / drainmaterialsOvercompaction of filter anddrain materials is not desirable‣Increases breakdown‣Reduces permeabilityTypical filter and drain materialsare easy to compactField compaction testing can bedifficult for these materials

Typical method specificationsfor filter / drain materials2 to 4 coverages with vibratorycompaction equipment8-inch loose lifts; less for handoperatedequipmentFreely add water duringcompaction of clean filter

Typical CompactionCurves for Clean Sands

#8 – Include full-heightchimney filters in embankmentrepairs or enlargements Provides protection against internalerosion through defects Lowers phreatic surface‣ Preventing breakout of seepage ondownstream face‣ Increasing stability of downstream slope In risk analysis, a chimney typicallylowers probability of an unfiltered exitto 0.001 to 0.0001.

Prevention of Internal ErosionThrough DefectsWithout ChimneyFilterWith ChimneyFilter

Lowering of Phreatic SurfaceWithout Chimney FilterWith Chimney Filter

Internal Erosion Event Tree Reservoir Rises Erosion Initiates Erosion Continuation (lack of filtering) Progression Step 1 (roof forms tosupport a pipe) Progression Step 2 (upstream zonefails to fill crack/pipe) Progression Step 3 (constriction orupstream zone fails to limit flows) Unsuccessful Intervention Catastrophic Breach

Top Elevation for ChimneyFilter Historic practice – top of estimatedphreatic surface Current practice‣ Top of maximum normal pool as aminimum‣ Often top of maximum flood pool

Discussion by R.F. Ripley,May 1982 1• P. 1194. …On the other hand, the writer hasnot found a single case of piping or internalerosion of core fines where the core wasprotected with a filter zone of cleancohesionless sand-rich material for whichcare was taken to prevent segregation duringplacement, and where necessary, the filterzone itself was adequately protected byappropriate downstream zones.1Hermusia F. Soares “Design of Filters for Clay Cores of Dams,”ASCE Journal of Geotechnical Engineering, January, 1982

J.L. Sherard, lecture, 1984 1• P. 5. I believe there is already sufficientevidence from dam behavior, supportedby theory, to require the designer toassume that small concentrated leakscan develop through the impervioussection of most embankment dams,even those without exceptionaldifferential settlement.1“Debatable Trends in Embankment Engineering,” Presentationto the A.S.C.E. National Capital Section GeotechnicalCommittee, Seminar on Lessons Learned from GeotechnicalFailures, February 3, 1984.

Design of Small Dams, ThirdEdition, 1987• P. 190. Recently, to avoid constructiondefects such as loose lifts, poor bondbetween lifts, inadvertent pervious layers,desiccation, and dispersive soils, inclinedfilter drains in combination with ahorizontal drainage blanket have becomealmost standard. Because drainagemodifications to a homogeneous sectionprovide a greatly improved design,the fully homogeneous section shouldseldom be used.

Ronald Hirschfeld, InteragencyCommittee on Dam Safety(ICODS) article, 1995 1• Every dam should have a first line ofdefense against piping, in the form ofa chimney drain and a blanket drainunder the downstream shell, unless thedesigner can determine that there is nounacceptable risk in eliminating one orboth.1“Design, Construction, and Inspection of EmbankmentDams to Reduce the Risk of Piping,” Seepage, Piping andRemedial Measures, March 23-24, 1995

Corps of Engineers EM1110 2 2300 (2004) 1• The primary line of defense against aconcentrated leak through the dam coreis the downstream filter (filter design iscovered in Appendix B). Sinceprevention of cracks cannot be ensured,an adequate downstream filter must beprovided (Sherard 1984).1General Design and Construction Considerations for Earthand Rock-Fill Dams

#8 – Use two-stage filter / drain ifconcentrated flows are possibleProvides capacity to handle largeflowAlso negates effects of filtercontamination

Two-Stage Chimney Filter /Drain System

Prevent Concentrated Flowsfrom Overwhelming Filter

Washakie Dam – OriginalModification

Washakie Dam – TemporaryBerm and Piezometers Readings

Washakie Dam – SecondModification

Negate Effects of FilterContamination

#9 – Carefully select plasticpipes for toe drains andprovide for camera inspectionPerformance of different pipetypes has variedCamera inspection providesverification – short term and longterm

Plastic Pipe MaterialsProduct Type Advantage Disadvantage Recommended?SolidStrong, weldedjoints, flexibility ofperforation sizeand typeHighest cost, specialordered or handdrilled perforationsHighlyHDPESingle -WallEconomicalPoor historicperformance, weakNoCorrugatedDouble -WallEconomical,successfulapplications, largeperforation sizesLow strength, carefulinstallation requiredModeratelyPVCSolidWellScreenDainPipeStrongSmall perforationapertureModeratelyEconomical Weak, brittle NoCorrugatedDouble -WallEconomical Weak, Brittle NoReport DSO-09-01, Physical Properties of Plastic Pipe Used in ReclamationToe Drains, Bureau of Reclamation, September 2009

Camera inspectionNumerous cases of damage duringconstructionCamera inspections during construction‣ After initial burial – 3 to 5 feet‣ After completion of constructionDesign considerations for inspection‣ Pipe diameter‣ Access points‣ Pipe slope

#10 – Use two-stage filter /drain systems for collectorpipesSlotted pipes embedded in filtersand often become pluggedFull pipe capacity is not realized

Clogging of Slotted DrainPipe Embedded in Sand Filter

Two examplesWashakie Dam, WY‣ Flow increased from about 50 gpm toabout 500 gpmAntero Dam, CO‣ Flow increased from about 5 gpm toabout 50 gpm

Recommended CollectorPipe Configuration

More informationMontana Dam Safety TechnicalNote No. 4‣ http://dnrc.mt.gov/wrd/water_op/dam_safety/technical_ref.asp‣Filters for Embankment Dams(NDSP)‣ http://www.fema.gov/library/viewRecord.do?fromSearch=fromsearch&id=4943‣Plastic Pipe Guidelines (NDSP)‣ http://www.fema.gov/library/viewRecord.do?id=3356

Questions?