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Chapter 2 Seepage in Soil

Chapter 2 Seepage in Soil. “In engineering practice, difficulties with soils are almost exclusively due not to the soils themselves, but to the water contained in their voids. On a planet without any water there would be no need for soil mechanics”. Karl Terzaghi. Definitions.

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Chapter 2 Seepage in Soil

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  1. Chapter 2 SeepageinSoil “Inengineeringpractice,difficultieswith soilsarealmostexclusivelyduenotto thesoilsthemselves,buttothewater containedintheirvoids.Onaplanet withoutanywatertherewouldbeno needforsoilmechanics”. KarlTerzaghi

  2. Definitions •Groundwatertable(GWT)orPhreaticsurface)–topof groundwaterflow •Phreaticzone–subsurfacebelowGWT •Vadosezone–subsurfaceaboveGWT •Aquifer-ageologicformationthatisunderGWTandis capableofyieldingwaterasawatersupply. –Confined Aquifer-soilorrockbelowthelandsurfacethatissaturated withwater. –Unconfined Aquifer-anaquiferwhoseupperwatersurface(water table)isatatmosphericpressure. •Aquaclude-geologicformationthatcannottransmitwater rapidly. •Artesianwater-groundwaterthatisunderpressure.

  3. Totalhead: ht=hv+he+hp 11.hv~0ingroundwaterflow becauseseepageflowisslow 2.hedependsonthelocationof datum z 3.hpdependsonwaterpressurein soilpores 4.Seepageflowinsoilisalways fromahighertotalheadtoa lowertotalhead.

  4. RelationbetweenthePressure head(hp,inmeter))&Porewater pressure(u,inkPa): uhpw(unit:kPa) u w hp (unit:m)

  5. TotalHeadatPoint“P” hhpz Groundsurface GWT o hp P hpismeasuredvertically downfromGWTorpiezometer hp z z zismeasuredverticallyup fromthedatum o Datum

  6. Example(Codutop.265) ComputetheporewaterpressureatpointsAandB el.(m) 90 89 87 83 80

  7. GroundwaterFlowinSoil Thetotalheaddifference(h)isthedrivingforceof groundwaterflowinsoil. 1mX X P P 1m 1m Impermeablestratum GWflowfromXtoP Impermeablestratum NoGWflow

  8. ImportantReminder •Theabsolutevalueofthetotalheadisnot importantbecauseitdependsonthe selectionofdatum; •Tosolvegroundwaterflowproblems,we selectadatumanduseitasthereferencefor thetotalheadsatallpointsofinterests; •Groundwateralwaysflowsfromahighertotal headtoalowertotalhead,regardlessthe locationofdatum.

  9. Darcy’sLaw h SoilSample A L Darcy(1856)foundthattheflowrateinaporousmedium (e.g.soil)is 1.proportionaltothetotalheaddifferenceh 2.proportionaltothecross-sectionalareaAperpendicular totheflowdirection 3.inverselyproportionaltothelengthofthesoilsampleL

  10. Darcy’ssLaw Q=khiA Q(m3/s)=flowrate kh(m/s)=coefficientofpermeabilityorhydraulicconductivity. i=hydraulicgradient[-] A=cross-sectionalareaperpendiculartodirectionofflow(m2)

  11. Alternatively,Darcy’sLawmaybeexpressedin termsofFlow Velocity q=ki (unit:m/s) where i hydraulicgradient[-] q=Q/A Darcyvelocity(m/s)

  12. HydraulicGradient,i dh dx i Thenegativesignofhydraulicgradient–to ensuretheflowdirectionistowardsthepositive hydraulicgradient Weoftenusei=h/Lin1Dcalculation–but remembertoindicatethedirectionofflow

  13. Darcy’sLawisvalidwhen Theflowislaminar(noturbulence)–thisisvalidfor flowinallsoils; Soilisnearlysaturated–S~100%; Theflowissteady(timeindependent)–knownasthe steadystateseepageflow

  14. HydraulicConductivitykh •Themostreliablewayofdeterminingkhis fromexperiments(laborin-situ) •Empiricalequationshavebeendeveloped underspecificconditions •Everythingelsebeingequal,khisthehighest whenasoilissaturated

  15. TypicalkhValues 10-110-210-310-410-510-610-710-810-910-1010-1110-12 Gravels Sands Silts HomogeneousClays Fissured&WeatheredClays Unit:meter/second,m/s X100,cm/s

  16. Constantheaddevice– forkhmeasurementofgranularsoils

  17. Fallingheaddevice forkhmeasurementoffinegrainedsoils

  18. EmpiricalEstimateofkh Kozeny-CarmanEquation (Readpp.280-282) • For sandy soils only e3 1e kh • khisafunctionof –Grainsize –Shapefactor –Voidratio –Unitweightoffluid –Viscosityoffluid

  19. SeepageVelocity   InDarcy’slaw,q=ik,whereqisthe“apparent”flowvelocity. The“true”flowvelocityisthevelocityofwatermoleculesflowing throughatortuouspathinsoil–seepagevelocityv.  Therelationshipbetweentheseepagevelocity(v)andDarcy velocity(q)is q n v  ThereforetheseepagevelocityisalwayshigherthenDarcy’s velocity

  20. Computing11--D GroundwaterFlow Q=kiA(unit:m3/s) FlowRate q=ki (unit:m/s,orcm/s) FlowVelocity Steps: Calculatethehydraulicgradient,i=h/L; Measurethehydraulicconductivity,kh; Determinetheareaperpendiculartothegroundwaterflow,A. 1. 22. 3.

  21. FlowthroughAnisotropicSoils

  22. FlowthroughAnisotropicSoils H H i kiHi Hi  i i kv kH (k ) kiH Hi kH i

  23. Example:Calculatekhandkvforthelayeredsoil d1=1m d2=1m Layer1 Aquifer Layer2 Aquitar k=k1=10-6m/s k=k2=10-10m/s d1+d2 d1d2 k1k2  k1d1k2d2 d1d2 kV  kH 

  24. Lessonslearnt Whensubsurfacesoilisstratified •Horizontalseepageiscontrolledbyaquifer; •Verticalseepagecontrolledbyaquitar

  25. SeepagePressure Whenwaterflowsthroughasoil,theviscousdragtendstomovesoilgrains andproducesaforce,knownasaseepagepressure. Upwardflow •Liquefaction:Ifanupwardflowingwaterpassingthroughasand,andthe seepagepressureequalstothesubmergedweightofthesand,theinter- granularpressurebecomeszero.Thesandthenisina"quick"condition andisincapabletosupportaloadonitssurface. •Erosion:theupwardflowingwatertendstoremovesomeofthefines,often knownas"piping”. •Bottom heave (blow out):Iftheupwardflowingwaterpassingthrougha clay,andtheseepagepressureequalstothesubmergedweightoftheclay, theclaywillheaveandcrack. Downwardflow Downwardflowingwaterwillgenerateadditionalpressureonsoil,whichis equivalenttoadditionalloadingonsoil,generatingsettlement.

  26. Soilliquefaction/piping •Soilsuddenly suffera transitionfroma solidstatetoa liquefiedstate •Occurinloose tomoderately saturated granularsoils duringcyclic loading

  27. Blowout/BottomHeaveofClaysin Excavation Clay SandAquifer Piezometer

  28. ConditionsforLiquefaction,PipingandBlowout u2 (z=z2,h=h2,u=u2) Elevation (z=z1,h=h1,u=u1) u1 Area=A Plan Asoilelementexperiencingupwardflowofwater

  29. A(u1u2) UpliftForce ForceduetoSoilweightAsat(z2z1) u2 Porewaterpressure u2w(h2z2) u1w(h1z1) u1

  30.  A(u1u2) Asat(z2z1) UpliftForce Forceduetoweight Forpipingtooccur, upliftforce>soilweight A(u2u1)  Asat(z2z1) w(h1h2)w(z1z2) sat(z2z1) w(h1h2) sat(z2z1)w(z2z1) ( h1h2) (z2z1) sat w w 

  31. u2 (z=z2,h=h2) sat w w ( h1h2) (z2z1)  (z=z1,h=h1) u1 ( h1h2) (z2z1) But =i =Hydraulicgradient sotheconditionforpipingmaybewrittenas i>ic

  32. CriticalHydraulicGradientforPiping/Blowout satw w Gs1 1e  ic 1.Whentheupwardhydraulicgradientinsoili>ic, liquefaction/piping/blowoutwilloccurinsoil. 2.Criticalhydraulicgradientisasoilpropertyandisnotrelated tothehydro-geologicconditionofthesite. ic i F.S. 3.FSagainstliquefaction/piping/blowout: 4.FS=2isrequiredindesign

  33. Example:Basalstabilityoflandfill Wastebulkunitweight=7kN/m3 50mx50m z ?m 10m B 11m CL,bulkunitweight=20kN/m3 A Piezometer SP 1. 2. Calculatethemaximumexcavationcanbemadeforthislandfill; Iftheexcavationhastobe6.5mdeeptomeetthedesigncapacity,what wouldyoudo?

  34. Erosionproblemsinearthworks • • Seepagepressurebeloworwithindamshasledtoseveralcatastrophic failures. Itmaybepreventedifwecoverthesurface,wheretheseepageemerges, withcoarsermaterialsthathelptheescapeofthewaterbutpreventthe erosionofthefines.Iftheseepagepressurehasarathergreatupward component,itmaybenecessarytoaddweighttothetopofthefilterto counterbalancetheupwardforces.

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  36. Impact • • • • • Dischargeofamixtureof600-700thousandcubicmetresofredmudand water. Ninepeoplewerekilled,andapprox.120peoplewereinjured. Thespillingredmudflooded800hectaresofsurroundingareas. ThemaincomponentcontainedintheredmudisFe2O3(ironoxide-which givesititscharacteristicredcolour)at40-45%.Othercomponentsare Al2O3,SiO2,CaO,TiO2,andNa2O,accordingtoMAL. Theredmudcontains: –110mg/kgforarsenic, – 1.3mg/kgformercury, –660mg/kgforchromium(ofwhich0.46mg/kgforthehighlytoxichexavalent chromiumCr-VI), –40mg/kgforantimony, –270mg/kgfornickel, –7mg/kgforcadmium. 39

  37. AjkaTailingsDamFailure(2010--10-4) West E W North 40

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  41. Summaryy 1.Thetotalheadgovernstheseepageflowinsoil,whichincludes –elevationhead –pressurehead –velocityhead,whichisnegligibleinsoils 2.Theseepageflowisalwaysfromahighertotalheadtoalower totalhead; 3.Darcy’slawstatesthattheseepageflowdependson –Hydraulicconductivityofsoiland –Hydraulicgradientofthesite 4.ThetrueseepagevelocityinsoilishigherthanDarcy’ssvelocity alongatorturouspath; 5.TheSeepagewillgenerateporewaterpressureinsoil –Upwardpressure–pipingandblowout –Downwardpressure–settlement 6.Theseepageflowinearthworkscanbecontrolledby – – Selectionofsoil Compaction

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