1 / 44

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.

dyami
Télécharger la présentation

Chapter 2 Seepage in Soil

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

  35. 38

  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

  38. 41

  39. 42

  40. 43

  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

More Related