ProceedingsoftheInstitutionofCivilEngineershttp://dx.doi.org/10.1680/geng.9.00086Paper900086Received12/11/2009 Accepted09/02/2011Keywords:computational mechanics/embankments/failuresICEPublishing:All rightsreservedGeotechnicalEngineeringDeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemaDeformation and strength ofembankments on soft Dutch soilj1EvertdenHaanPhDGeotechnical Researcher,Deltares,Delft,TheNetherlandsj2AntoineFeddemaBEngSeniorConsultant/Manager,Deltares,Delft,TheNetherlandsj1j2Embankment anddykestability intheNetherlands has always beenevaluatedbyeffectivestress analysis. Thesubsoil of most of these structures is organic, weak andsoft, but the internal frictionangle of these soils issurprisingly high. Empirical methods are used to obtain acceptable, reduced values of friction angle from triaxial testsfor useinstabilityanalyses. It appears possible, however, todofull justicetothepeculiar combinationof lowstrengthandstiffnessandhighfrictionanglebymeansofthenite-elementmethodusingaviscousversionoftheCam-claymodel.Allparametersofthemodelarefoundfromasingletestinaconstant-rate-of-strainK0oedometer.The approach is illustrated by two case histories, after rst providing insight into the peculiar properties of the Dutchsoils, and the manner in which they are dealt with.NotationCcreep factorc9 cohesione voids ratioh heighth0initial heightK0lateral earth pressure at restK0,ncvirgin compression value ofK0k permeabilityM critical-state strength parameterp9 isotropic effective stressp9c0initial equivalent yield stressp9eqequivalent isotropic effective stressp90initial value of equivalent stressq deviatoric stresssuundrained strengthsu=9pundrained strength ratiot timetageage of deposittcreepcreep durationnatnatural strainvolvolumetric straink* cam-clay swelling factor* cam-clay compressibility factor* creep factor Poisson ratio9axaxial effective stress9pvertical yield stress9radradial effective stress9v0initial vertical effective stress9 internal friction angle1. IntroductionDutch organic soils and peat, although weak and soft, havesurprisinglyhighvaluesof theeffectivestrengthparametersandundrainedstrengthratios. Their internal frictionangles, 9, canfar exceed the 30358 range that is common in sands, andgenerally become higher as the organic content increases. Forexample, peat has9 values of upto908, andinorganicclaysvalues of 40608 arecommon. 9 is animportant parameter intheNetherlands, as theeffectivestress approachis usedalmostexclusivelyinstabilityanalyses. Toobtainreasonablefactors ofsafety, 9isdeterminedat strainsfarbelowfailure, andthetruestrength of the material is not accounted for. For a realisticassessment it isnecessarytocombine(low) stiffness and(high)strength parameters in one analysis, and the nite-elementmethodistheobviousmeanstoachievethis.Good results have recently been obtained with nite-elementcalculations of thedeformationof embankments constructedonsoft Dutchsoils. This will be illustratedbytwocase histories.The constitutive model used was a viscous version of themodied Cam-clay model. The necessary soil stiffness andstrength parameters were determined fromoedometer tests inwhichaconstantrateofstrainwasapplied, andlateralstresswasmeasured.Thepaperstartsoffbygivingthebackgroundtothehighstrength1parametersofDutchorganicsoilsandpeat,andthewayinwhichDutch engineers have dealt with this in the past. Then theconstitutivemodelandtheparameterdeterminationaredescribed.Finallythetwocasehistoriesarepresented.2. Strength of Dutch soft soilsDutch organic soils and peat, although weak and soft, havesurprisinglyhighvaluesof theeffectivestrengthparametersandundrained strength ratios. Figure 1 shows stress paths fromundrainedtriaxial compressiontestsonsamplescoveringawiderange of organic contents. For comparison, test results are alsoshown for a loose and a dense sand. The organic soils weresampledbelowthecrest andthetoeof theLekriver dyke, andwere reconsolidated to the in situ stresses. The heavily com-pressed crest samples are normally consolidated, and Table 1provides the characteristics of these tests. The internal frictionangles 9farexceedthe30358thatiscommoninsands,andarehigher as the soil becomes softer and more organic. Peat, forexample, hasvaluesupto908, andorganicclayfrom408to608.The application of such values in Bishop slip circle analysiswouldyieldunrealisticallyhighfactorsofsafety,butthemethodsof parameter determination developed by Dutch engineers fortheir effectivestress slipanalyses yieldmuchlower values. Thehightriaxial strengths andthepositiveeffect of organiccontentremainedlargelyunnoticeduptothe1990s.Theundrainedstrengthratiossu=9parealsohigh: 0.44and0.40fortheorganicclaysinFigure1, andratherthanfailing, thepeatdilatesandcontinuestoincreaseitsshear resistance. Thelightlystressed toe material also dilates during shear and is clearlyoverconsolidated.Datafor QueenboroughorganicclayfromJardineet al. (2003)are included in Table 1. The values of 9 and su=9paresomewhat smaller thanfor the Dutchorganic soils, but the9valueisalsoquitehigh.Figure2plots 9 of normallyconsolidatedorganic Dutchsoilsagainst their bulk density. Each bar in the gure is based onnumeroustests. Aconsistent trendexists. Bulkdensitycorrelatesn.c.o.c.High-organic clayLow-organic clayDense and loose sandPeat0501001502000p: kPaq: kPaLateral tensionboundary100 50Figure 1. Effective stress paths of Dutch soft soils in undrainedtriaxial compression illustrating the high angle of internal friction.(Reconsolidation to eld stresses. n.c., normally consolidatedmaterial under dyke crest; o.c., overconsolidated material adjacentto dyke)Bulk density: kN/m3Water content: % 9: degrees su=9v0su=9pPeat 10.4 309 83 0.74 0.62aHigh-organic clay 12.8 170 54 0.53 0.44aLow-organic clay 14.7 76 44 0.49 0.40aQueenborough clay 14.6 85 3538 0.53 0.300.33aAssuming OCR 1.2.Table 1. Characteristics of triaxial tests on Dutch (Figure 1) andQueenborough organic soils2GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddematosomeextent withorganiccontent, andindicationsofthelatterareshown.Theveryhigh 9valueofpeatswithorganiccontentsabove20%isduetotheeffectsofbretensioning. Thebresinpeat are orientated predominantly in the horizontal plane, andduring triaxial compression the bres extend and reinforce thematrix of granular and humiedorganic material in which thebres are embedded. Landva and La Rochelle (1983) describethismechanismat length, andthe9 908conditioninpeat hasoften been reported. It occurs when the pore pressures duringshearincreasetoequalthecellpressure.Coutinho and Lacerda (1989) showsimilar relationships as inFigure 2 for Brazilian (Juturnaiba) organic soil. However, theorganic contentbulk density relationship of those soils differsfromthe average Dutch relationship, and points to more pre-compressionoftheformer.Bulkdensityandwatercontent arethemainstaysofcorrelationswith compressibility and strength parameters in Dutch organicsoil practice. These usually sufce, and only rarely are theorganic content, degree of humication or Atterberg limitsdetermined.Mostoftherelativelyuncompressedpeatsarebrousandpseudo-brous,whereasdeep,buriedpeatcanbeamorphous.Apuzzlingfeatureof Figure2isthat, despitetheverydifferentstructures of peat and organic clay, the trend is quite uniformacross such a large range of organic contents. Jardine et al.(2003) in their study of the Queenborough clay explain theincrease of the shear strength parameters by colloidal organicmaterial affecting the surface behaviour of clay particles.Colloidal activityis alsoexpectedinamorphous, humiedpeat,but lesssoinbrouspeat, andsotheuniformtrendinthegureissurprising.ThecurrentDutchtest anddesignmethodsandtheevaluationofstability are briey discussed below, before turning to thealternativemethodusingnite-element calculationswithparam-etersfromaconstantrateofstrain(CRS)K0-oedometer.3. Dutch methodsKeverling Buisman (1934) developed the Dutch cell apparatus(Figure 3) fromearlier somewhat similar devices in use byEhrenberg,andbyTerzaghi(seealsoDeBoer, 2005,fromwhichitappearsthatthersttriaxialtestsever,byvonKarmanin1910onmarbleandsandstone(Vasarhelyi, 2010), went unnoticedbythe soil mechanics pioneers). The Dutch cell differs fromthetriaxial apparatusmainlybythecross-sectionofthepistonbeingequaltothatofthesample.Sampleswere15 cmhighand6.6 cmindiameter, andfreedrainageoccurredthroughporous platens.Radial drainage alsooccurredthroughfolds inthe loose-ttingrubber membrane. The membranes were tailor-made, and in-cludedrubberangesthatwereclampedintotheupperandlowerplates; correctionsweremadefor theuplift onthesamplefromthe annular gap around the piston. Pore pressures were notmeasured,andback-pressure wasnotused.Thetest wasperformedinmultiplestages, ineachof whichtheverticalloadwasincreased,andthecellpressure wasrstallowedtoequilibrateandwas thenloweredbydrainingoff droplets ofwater from the cell. This increased the deviator stress andinducedshear straining, the lateral component of whichcausedthe cell pressure to increase gradually and to arrest the shearstrains. Inolderproceduresthecell pressurewasloweredtonearfailure, but later, when large numbers of tests had to beperformed, a standard of 3 kPa was adopted. A stage wasconsideredcompletewhentherateofverticalstrainhadsubsidedto 10 mper hour. The horizontal strains in the sample werequite limited, andinessencea near-K0conditionwas imposed.The c9 and9 values that werededucedfromfour consecutivestageswerelow, butweresatisfactoryinthesensethat factorsofsafetynear 1were obtainedfor dykes consideredtobe onthevergeoffailure. Effectivestressanalysiswasnearlyalwaysused: degrees20 18 16 14 1235%10%02040608010Bulk density: kN/m3From Figure 1Betuwe railway, AlblasserwaardHigh-speed railway, Rijpwetering60% 20% Organic contentsFigure 2. Internal friction angle 9 of Dutch organic soils asfunction of bulk density Figure 3. The Dutch cell apparatus3GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemain embankment analysis, although a quick cell test procedureexisted in which consolidation was not allowed, and whichbasicallyyieldedundrainedstrength.The cell test has now been replaced by multi-stage triaxialtesting, using isotropic consolidation and undrained shear. Toavoidthehigh 9values,thestagesareterminatedataxialstrainsbetween2%and5%, andthestressenvelopesatthesestrainsaretaken to represent failure. The exact strain value to use hasbecomeamatter of debate, andevenconfusion, andDenHaanandKruse(2006) explainhowthedisturbanceduringprecedingstagesproducesincreasedvaluesofc9andreducedvaluesof9.Theundrainedapproachtostrengthisnowbeinginvestigatedasanalternativeforlimit equilibriummethods. Thepossiblemeritsof thesimpleshear test arealsounder investigation, asthistestcloselymimics failures inpeat, whichtendtofollowhorizontalshearplanes.Theeffectivestressapproachandthereducedfrictionanglesarealso used in nite-element calculations. The soft and viscousnature of organic soils has beenmodelledbyspecialisedcreepmodels, for example in Plaxis by the soft soil creep model(Vermeer and Neher, 1999), and in a similar model that isincludedintheImperial Collegenite-element program(BodasFreitas, 2008). The former has led to poor matches with eldmeasurements of embankment deformations when using thereduced friction angles. An alternative method of parameterdetermination has been developed that seems to give muchbetter matches. Before discussing this, it is necessary rst todescribe briey the creep model used in the nite-elementcalculations.4. The creep model for soft viscous soilThe creep model combines modied Cam-clay (Roscoe andBurland, 1968) with the isotache description of soil compress-ibility(e.g. DenHaan, 1996). Figure4illustrates themodel fortriaxial conditions, andmakes useof theparameters inTable2for Sliedrecht peat. Figure4(a) shows thewell-knownmodiedCam-clayellipsedrawnthroughthepresent (p9, q)stressstateinA, p9beingtheisotropiceffectivestress(9ax=3 29rad=3)andq14 12 10 8 6 4 21000 100 10peq 10: kPa (log scale)1/2x1/2( */*) x1/2( */*) x002040608101Volume strainABCln(10) * ln(10) * 11Stress decreases1/2xCreep ratedecreasespc0Reference isotacheCreep rate */(day) p0(b)024681012140p: kPa(a)q: kPapeqMhvAFigure 4. Illustration of the nite-element creep model: (a) p9qplot with modied Cam-clay ellipse as plastic potential surfacethrough state point (A, present stress state; h, rate of volumetriccreep strain; , rate of shear creep strain); (b) isotaches in p9volspace withp9eqand voldetermining rate of volumetric creepDepth: m-G.L. Bulk density:kN/m3Water content:%k* * * M 9: degrees K0,ncOCR2.9 10.5 500 0.05 0.29 0.027 0.2 2.39 58.6 0.29 2.8Table 2. Characteristics of Sliedrecht peat, Betuwe railway,km 16.74GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemathe deviatoric stress (9ax9rad). However, the ellipse is nowusedas a plastic potential surface rather thanas yieldsurface.The outward normal at any point on the ellipse denes thedirectionoftherateofchangeoftheviscoplasticorcreepstrainvector, with the horizontal component forming the volumetricpart and the vertical component forming the distortional part.The height of the ellipse is determined by the Cam-clay M-parameter, andthecut-off onthep9 axisistheequivalent stressp9eq: The latter is used in the isotache graph (Figure 4(b)) todetermine the magnitude of the volumetric creep strain rate.Isotaches aresimplyacollectionof lines instressstrainspaceonwhichthe rate of strainis constant. The slope of the lines,* vol/ln(p9), is similar to the Cam-clay parameter e/ln(p9). The volumetric creep rate @vol,vp/@t is con-stant oneachline. Theviscoplasticnatureofsoil compressionissuchthat, at constant p9, volumedecreaseswithtime, but at anever-decreasing rate. This is reected in Figure 4(b) by lowerlineshavingalower rateof strain. Thevertical spacingof theselines is determined by the creep parameter * vol/ln(@vol,vp/@t ). This parameter is similar to the well-knownC vol/log(t )creepparameter. Thecreepstrainrateisreadoff at the present values of p9eqand volumetric strain. ElasticstrainsaregivenbytheCam-clayswellingfactor k*andPoissonratio. Elasticstrainratesarecalculatedandaddedtothecreepstrainrates, andbyintegrationintimeandspacethestrainsanddeformationsareobtained.A reference isotache is dened on which the rate of volu-metric creep strain is equal to */(1 day). The initial yieldequivalent stress p9c0is on this line as shown, and an OCRvalue is given by the ratio of p9c0and the initial value of theequivalent stress p90:Aloading increment during a multi-stage test on a laboratorysampleisshownschematicallyintheisotachegraph. TheinitialpositionAisonthereferenceisotache,andbecauseconsolidationis rapid, strains are at rst essentiallyelastic, whichbrings thestate topoint Bonanisotache where rates of strainare high.CreepthenoccursalongBC,rapidlyatrstbutquicklydiminish-ing with time. At any point on BC the rate of creep is byapproximationequalto*/tcreepwheretcreepisthecreepduration.SotherateatBwouldbeinnite(or,rather,veryhigh),andafteronedayCisreached, wheretherateis */(1day). Thisiswhythe reference isotache corresponds to the common one-daylaboratory compression curve, and its cut-off equals the usualpreconsolidation pressure. For the same reason, the use ofOCR 1 in calculations produces unrealistically high rates ofstrain, equal tothoseafter 1dayof loadingonlaboratory-sizedsamples.Insitustressstraindevelopsalonglowerisotachesthancanbemeasuredinthelaboratory, owingtothelarger timescaleandlarger drainagedistances, andthereforeOCRvalues shouldbewellabove1,evenfornormallyconsolidatedsoil.Inthelattercase, OCRcanbederivedfromtheisotacheonwhichcreeprateequals */tage, wheretageis theageof thedeposit under theinsituvalueofp9.5. Parameter determinationAll parameters of this creepmodel canbedeterminedfromtheconstant rateof strain(CRS) K0oedometer test (DenHaanandKamao, 2003). This concernsnot onlythecompressibilitypara-meters k*, *, * and , but also the critical-state strengthparameter M. Figure 5is a schematic diagramof the CRSK0oedometer,whichisplacedinatriaxialcelltomakeuseofpistoncontrol, loggingandback-pressurefacilities. Horizontal stressismeasuredbystraingaugesplacedonthebackofasectionoftheoedometerring,whichhasbeenturneddowntomembranethick-ness.Porepressureis measuredunderneaththesample,anddrain-ageistothetriaxial cell space. Samplediameter is63 mm, andsample height is 2035 mm. Correction for wall friction is possibleby measuring the vertical load at both the top and the bottom.Figure6showstheresult ofsuchatest onpeat fromSliedrecht,therstofthecasestobepresentedfurtheron.Bymeasuringnotonlyvertical stress andstrainbut alsothehorizontal stress, thecompletestressstrainstrain-raterelationshipis known, sothatthecreepmodel canbettedtotheresults. Thetest procedureLoadcellTriaxialcellspaceSampleHorizontalstressLoadcellPore pressureFigure 5. Schematic diagram of constant rate of strain K0oedometer5GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddema150 100 5000204060Time: h(a)Vertical strainUnloadingRelaxation200 100MeasuredCalculated01002003000p: kPa(b)q: kPa(d)1000 100 10 1MeasuredCalculated002040601v:kPaVertical strain h v,: kPavh150 100 50MeasuredCalculated01002003004005000Time: h(c)150 100 50MeasuredCalculated002040608100Time: h(e)K0Figure 6. Test result and t, CRS K0oedometer test on Sliedrechtpeatincludesanunloadreloadlooptoassistindetermining k*and ,andarelaxationphasetoassistindetermining*.Theinteractionofthevariousmodel parametersissuchthat, fornormallyconsolidatedstatesandconstantrateofstrain,M 31 K0,nc 21 2K0,nc 21 K0,nc 1 2 (=k1)1 2K0,nc 1 2 =k1 K0,nc 1 s1:so that Mcan also be obtained using reasonable estimates ofK0,nc, */k*and .Table2providesthecharacteristicsandparametersofthetest onSliedrechtpeat.Thestrainsarequitehigh,upto65%,andinthis6GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemacase natural strains were used in the tting exercise. Naturalstrainisdenedbytheintegrationof innitesimal incrementsofcompressionrelativetothepresentheight,nat hh0dh=h ln h=h0 ln 1 2:Atsmallstrains,thenaturalstrainisverysimilarinmagnitudetothe normal (linear) strain . However, at larger strain levels,naturalstrainisnumericallyincreasinglylargerthanlinearstrain,andthishastheeffect of straighteningtheconcavestressstraincurve, whichoftendevelopsat higher stresseswell inexcessoftheyieldstress(asinFigure6(d)).Thefrictionanglefoundfromthe t is quite high, and the question is whether this valuecoincideswiththetriaxial compressionvalue. Theanswertothisquestion is obtained froma project where numerous CRS K0oedometertestsandtriaxialtestswereperformed. Inthisproject,Ground-breaking methods of dyke safety evaluation (VanDuinen, 2008), which was performed for Rijkswaterstaat (thenational publicworks authority), threedykecross-sections werethoroughlyinvestigated. SampleswereobtainedusingtheBege-mann continuous stocking sampler (Begemann, 1971). Thetriaxial andCRSK0oedometer tests performedfor this projecthavebeenusedtocompareMvalues: seeFigure7. Thegureislimitedtotriaxial testsonmaterial takenfromunderthecrest ofthedyke, andwhichwas reconsolidatedat approximatelytheinsitu stresses. It therefore concerns normally to slightly over-consolidatedbehaviour.ItincludestherelevantdataofFigure 1.The results are plotted against bulk density, and cover a widerangeofsoft Dutchorganicsoils. PlottedareMand9obtainedfromthetriaxial test, andbyttingtheCRSK0oedometer test.MeasuredK0,ncvaluesfromtheCRSK0oedometer test arealsoshown.Thehighvalues of Mand9 areagainconspicuous, just as inFigure 1(sin 9 3M/(6 + M); byapproximation925Mfor0.5 , M , 2.5). Theagreement betweentriaxial MandMfromtheCRSK0oedometertest isreasonable. TheveryhighstrengthofthepeatandthehighlyorganicGorcumlightclay(GL),wheretriaxial compression values tend to M 3 and 9 908, isunderestimated, however, andthis couldbeanindicationof theinuenceoforganicbresonstrength, whichwouldnot befullydevelopedinoedometric conditions. The measuredK0,ncvaluesarealsogiveninthegure. Theirlowvaluesinthepeatswillbenoted; these indicate that partial bre tension develops in thehorizontalplane.Theagreement inFigure7is consideredsufcient toapplytheCRSK0oedometer parameters, includingM, inthecreepmodel.CRSK0oedometer parametersetshavebeendeterminedfortwoembankment constructionprojects, andappliedinnite-elementcalculations. The values of Mand9 fromthe tests are used20 18 16 14 12M, triaxial tests M, fitted to oedometer tests K0,ncmeasured005101520253010Bulk density:kN/m3K0,ncMOrganic clayPeatGLGL GL619254369486900Figure 7. Comparison of M from triaxial compression tests andM obtained from tting CRS K0oedometer tests by the creepmodel, both as function of sample bulk density. Measured K0,ncisalso shown (GL: Gorcum light clay)7GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemawithout anyreduction, andaremuchhigher thanisusual insoftsoil. On the other hand c9 0 is taken. The Cam-clay modelexpectszerocohesioninnormallyconsolidatedconditions,andintriaxialtestsverylittlecohesionismeasured.6. Betuwelijn embankmentThe measureddeformations of the Betuwelijnrailwayembank-ment at km16.7nearSliedrecht havebeenanalysedbymeansofnite-element calculationsintheproject Lateral groundstressesonpiles,whichwasperformedforCUR,theDutchequivalentoftheUKsConstructionIndustryResearchandInformationAsso-ciation (CIRIA). The cross-section used in the nite-elementcalculationwas as showninFigure 8. The nite-element meshconsistedof 99715-nodedelements. Anupdatedmeshanalysiswasused(updatedLagrange)toaccount forthelargedeforma-tions. SevenCRSK0oedometertestswereperformedonsamplesdistributedover the8.5 mdepthof thesoft layers. Figure6andTable2providedataononeofthesetests.Inaccordancewiththeupdatedmeshtechnique, thecompressibilityparametersk*and*aretakenwithrespect tonatural strains. Thebulkdensityofthe samples varied from10.3 to 15 kN/m3, the water contentfrom670%to75%, andtheMvalues foundfromthesoft soilmodel varied from2.4 to 1.7. The correlation of Mand bulkdensityagreedwithFigure7,butnotriaxialtestswereperformedin this project. The nite-element calculation with these para-meters gave goodagreement withthe measuredhorizontal andvertical deformations, as shown in Figure 9. Only the initialdeformations after therst lift areheavilyoverpredicted, whichmay be due to some uncertainty regarding the initial loadingsequence.30 25 20 15 10 5141210864202460Distance from centreline: mElevation: m NAP15432g.w.l.Soft layers(fibrous and pseudo-fibrouspeat and high-organic clay)Dense sandInclinometerSettlement plateFigure 8. Cross-section, Betuwelijn railway embankment, km16.7 near Sliedrecht10000 1000 100 100121Time: daysSettlement: mMeas.Calc.(a)06 05 04 03 02 01(b)Stage 1 (71 days)Stage 5 (372 days)602 daysCalc. Meas.16141210864200Horizontal displacement: mElevation: m NAPFigure 9. Deformations, measured and calculated, Betuwelijnrailway embankment, km 16.7: (a) centreline settlements;(b) lateral deformations at inclinometer location8GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddema7. IJkdijkTheproject MacroStabilityExperiment oftheIJkdijkFounda-tion was instigated to test innovative dyke stability monitoringtechniques(Vanet al., 2009). Adyke, 6 mhighand100 mlong,wasbuiltatBooneschansandbroughttofailure:seeFigure10.Across-sectionof thedykeis giveninFigure11. It consists of asandcore(1) coveredbyclay(2), andthesubsoil consists of athin crust of clay (3), followed by 12 mof peat (4), a thinAllerdsandlayer(5), (sandwithaslight organiccontent)andabaseofstiffPleistocenesand(6).Thedyke wasbroughttofailureby lling the basin behind the dyke and digging a toe ditch(phaseI), thendeepeningandwideningtheditch(phaseII), andnallybypumpingwater intothesandcoreof thedyke(phaseIII). ForthispurposeinltrationtubeswereinstalledasshowninFigure 11,andconnectedtoapumpingsystem.Thedykefailedafewhoursintothepumpingphase. Extrastepswereinplacetoensurefailure(emptyingtheditch, andwater-llingof arowof containers onthecrest), but provedunneces-sary. Thesandcoreandclayshellstructureofthedykeistypicalof river dykes inthe Netherlands, andwater-llingof the coremimics theeffect of highriver levels. Afull descriptionof theproject, includingalimit equilibriumanalysisofthefailure, willbe given in Zwanenburg et al. (2012). Here a nite-elementanalysis will bepresentedof thedeformations occurringduringconstructionanduptothepointof failure.Thepeat layer wasthedominant sourceof deformations. It wasmodelledbythecreepmodel describedearlier. Averageparam-etersweretakenfromsix CRSK0oedometer testsperformedonthis peat. Bulkdensityvariedbetween9.9and11.4 kN/m3, andwater content between 285%and 625%. Table 3 gives theparametersused. TheMvalueis2.6, andisthereforeveryhigh.Small-strain compressibility parameters were used in this case.TheOCRinthepeat wasfoundbycalibratingthecalculationtotheoedometertests:therstconstructionliftwasapplieddrained,andOCRwasadaptedbytrialanderrortoobtainthesamestrainin the peat as the average value in the tests at the calculatedvertical effective stress (vert 18% at 9v 42 kPa). Thisapproachcircumvents therather largevariationinyieldstressesfound in the tests. The permeability of the peat (k) wasdetermined in the CRSK0oedometer tests fromthe hydraulicFigure 10. IJkdijk macro stability experiment dyke at Booneschansdirectly after failure15 10 5 0 5 10 15 20 2512108642024630Distance from toe: mInfiltration tubesPore pressure gauges1 223456Elevation: m NAP IIIIIIIg.w.l.InclinometerSettlement plateFigure 11. Dyke cross-section, IJkdijk macro stability experiment,20089GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemagradient set up fromthe undrained base to the draining upperboundary. It was foundthat kdecreases as strainincreases, andthiswasformulatedask k010vol=Ck3:Theparametersk0andCkwereagaintakenasaveragesfromthesixtestsperformedonthepeat.Anite-element calculationwasperformedusinganemeshof205415-nodedelements.Asmall-strainanalysiswasapplied,andthe construction phase deformations were incorporated in themesh. Figure 12 compares the results of the calculation withmeasurementsof(a) construction-phasesettlementsbelowthecrest(b) porepressuresmeasuredinthesoftsoilsunderthecrestandadjacenttotheditch,duringconstructionandthefailurephases(c) thehorizontaldeformationsnearthetoeduringthefailurephases.Theconstructionphasewasmodelledineight liftsof undrainedloadapplicationandsubsequent consolidation. Theinclinometerwas placed after construction, and horizontal deformations aretakenfromthenullmeasurementoftheinclinometer.Theconstructionphasesettlementsarereasonablywellpredicted,as seeninFigure 12(a). Settlements were not measuredduringthefailurephases.Figure12(b)showsthat theconstructionphaseporepressuresinthepeat underthecrest arepredictedquitewell. Hadit not beenpossibletoletpermeabilitydecreasewithincreasingcompressionasdescribedabove,thetwouldnothavebeenasgood.Theporepressures during the rst lift are overpredicted, because thethicknessofthislift wastakenaslargerthaninreality. Thiswasnecessaryto enable the inltration wells tobe included in thenite-elementmesh.The pore pressures in the top clay under the crest are poorlypredicted, simply because the gauge is located close to theassumed phreatic line. During the consolidation phases thecalculated pore pressures take on the phreatic value. The porepressuresinthepeatneartheditcharepredictedreasonablywell.The horizontal deformations at the inclinometer location inFigure12(c) depart fromthenull measurement 37.35daysafterthe start of construction, at which point the nite-elementcalculationpredictedamaximumof 0.185 m. Phase0 inFigure12(c) showsthehorizontal deformationthat accumulatedduringthe5.8daysbetweenthenull measurement andthebeginningofphaseI.Reasonable agreement is obtained between the measured andpredictedhorizontal deformationsduringphases0, I andII. Thecalculation failed in phase II after applying 33%of the phase(deepening the ditch), earlier than in reality, where failure occurredinphaseIII. Thisisasatisfactoryoutcome, asthenite-elementplane-strainsimulationdoesnot account for theadditional resis-tance provided by the side planes at both ends of the failuresurface. Withthethicknessofthepeatlayerdecreasingto1 matthe northern end of the failure zone, the end effects are expected tobeespeciallystrong. Zwanenburget al. (inreview) calculateanextra 15% of lateral shear resistance from the side planes.Itwasenvisagedthat thevertical loadonthesubsoilinphaseIIIwould be increased bygraduallysaturating the sand core. Theinltrationprocess inphaseIII is showninFigure13. Alowerinltration tube placed in the top clay was operated separately fromthe six inltration tubes in the rst sandll layer. The gure showstheinltrationpressuresmeasurednearthepumpsatthesouthernendof theembankment andtheporepressures measuredinthefailedsectionof thesandcoreapproximatelyat thelevel of theinltrationtubes, andapproximately1 mhigher. Theinltrationsequence was rather complicated, and pressure build-up was higherthanexpected.Variouspumpingpausesfurthercomplicateaffairs.Thedischargedamount ofwater totalledabout 200 m3, whichisexpectedtosaturatethelower approximately1.5 mof thesandcore, and to increase the vertical load by approximately 1 kPa.In Figure 13 the development of the maximum horizontaldeformationofthetoe-lineinclinometerisalsoshown.Thereisaclear correlation between the inltration pressures in the nalphase before failure and the rate of increase of the horizontaldeformations. Theadditional weight of thewater is verysmall,andis not expectedtocontribute signicantlytothe failure. Apost-mortem trial pit constructed through the failure surfacerevealedthat coresandhadpenetratedthesoft layersbelowthecrest.Itisnowpostulatedthatfailurewasacceleratedbyboththehighinltrationpressuresandthepenetratingsand. Thehorizon-talthrustwouldbeincreasedbyboth,especiallyiftheinltrationpressurescouldpropagateintothepenetratedsand.Bulk density:kN/m3k* * * M 9:degreesK0,ncOCR k0: m/s Ck10.5 0.02 0.22 0.02 0.21 2.6 65.1 0.25 3.3 1.15 3 1080.35Table 3. Characteristics of IJkdijk peat, Booneschans10GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddema50 45 40 35 30 25 20 15 10 500102030405060Days after start of construction (13 August 2008 06:00)Settlement under crest: mSettlement plateCalculated(a)50 45 40 35 30 25 20 15 10 5 0020 015 010 005AllerdIn peat under crestIn top clay under crestIn peat near ditchMeas.Calc.III II IInc.32101234567Days after start of construction (13 August 2008 06:00)(b)Total head: m NAP108642020Horizontal displacement, inclinometer no. 53: mDepth: m NAPFull line: measurementSymbols: calculation0: Start of phase II: End of phase I: filling basin, digging ditchII: End of phase II: ditch deeperIII: Phase III: infiltration in sand coreIIIA: During pause in infiltrationIIIB: Infiltration ends; failure imminentIIIB IIIA 0 II IPeatTop clayClayshellPleisto-cenesand(c)Figure 12. Results of nite-element calculation, IJkdijk macro-stability experiment, 2008: (a) construction phase settlements;(b) pore pressures; (c) horizontal deformations during failure phase11GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemaThenite-element calculationfailedbeforetheserather compli-catedprocessesoccurred, after0.33oftheditch-deepeningphasehad been applied. Anite-element calculation with a coarsermeshofsome800six-nodedelements,however,didnotfaileveninphase III, as detailedinanearlier paper (inDutch) byDenHaan and Feddema (2009). There, inltration pressures wereappliedbymeansofwells, andthesandpenetrationintothepeatwasalsomodelled. Afactorofsafety(FS)wasdeterminedattheend of phase III by undrained reduction of the Cam-clay M-parameter, andFS 1.17was found, whereas FS 0.85wouldbeexpectedif the15%sideplaneresistancewasaccountedfor.Thecoarsemeshhasconsiderablyfewerdegreesoffreedomthanthener mesh, andthisappearstooffer additional resistancetofailure. Tobesuremeshnenesswassufcient, averynemeshof429715-nodedelementswasalsoused.Itproducedessentiallythesameresultsasreportedinthispaper.8. DiscussionThetwonite-element calculations presentedinthis paper haveappliedacreepmodelthatisaviscousversionofmodiedCam-clay, to embankments on soft organic clays and peat, withparametersdeterminedfromconstant rateofstrainK0oedometertests. ThelatterincludethestrengthparameterM, whichisquitehighinthesesoils, whilecohesioniszero, andstrain-dependentpermeability. The nite-element calculations with this modelappeartoproduceverysatisfactorytstothemeasureddeforma-tionsandporepressures,andthefailureoftheIJkdijkcaseisalsocoveredsatisfactorily.Thedeterminationofthestrengthparametersofsoftorganicclayand peat has long troubled the geotechnical profession in theNetherlands. The very high 9 value of these soils posesproblems both in laboratory strength testing and in stabilitycalculations. The procedure used in this paper interpretingconstant rateof strainK0oedometer tests withintheframeworkof a viscous version of modied Cam-clay to produce bothstrength and compressibility parameters is possibly a viablealternativeforDutchpractice.The ability to predict vertical and horizontal embankmentdeformationshasabearingonllmaterialconsumption,ontrackor roadmaintenance, andondeformations of foundations, pilesandutilitiesburiednear thetoeof theembankment. Theabilitytofaithfullypredictporepressuressetupduringconstructioncanfurther reduce the occurrence of failures duringconstructionifadequate surveillance and feedback are performed. The mostimportant functionof calculations, however, istopredict failure,andinthisrespecttheadequateindicationof failureoftheIJkdijkembankmentisencouraging.Predicting failure of dykes has become animportant aspect ofgeotechnicalengineeringintheNetherlands. Dykeauthoritiesarerequiredtoevaluatedykesafety every5years,andtherearesome17 000 kmof such dykes in the Netherlands! Evaluation is interms of thefactor of safetyandprobabilityindices determinedfromlimit equilibriumanalyses. Using the nite-element ap-proachdescribedinthispaper, adykecanrst bebuilt up, from15:00:00 13:00:00 11:00:001001020304009:00:00(Pore) pressure: kPa020406080100120140160Max. horizontal deformation: mmLower infiltration tube (A)Infiltration tubes in sandfill (B)Pore pressure at base of sandfill (C)Pore pressure 1 m above base ofsandfill (D)Horizontal deformation27 Sept. 2008ADCB50607080Figure 13. Development of inltration pressures and porepressures in sand core and of maximum lateral inclinometerdeformation, phase III failure stage, IJkdijk macro-stabilityexperiment, 200812GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemahistoryas it were, tothepresent state, takingadvantageof theprolongedcreepcompressionunderthedykeandtheaccompany-ingincreaseinshear resistance. Thenafactor of safetycanbedetermined by one of various methods in which failure issimulated. The most usual approachis the stepwise, undrainedreduction of the Cam-clay strength parameter Muntil failureoccurs,givingthefactorofsafetyastheratiooftheavailableandthereducedvalueofM.However, the applied creep model has several limitations thatneedtobeconsidered.Theseare(a) volumechangesandporepressuressetupbyrotationofthestresstensorduringconstructionandloading(b) anisotropyofcreeprates(c) overconsolidationeffects.More general limitations lie in such matters as not modellinglocalising deformations along shear surfaces during failure,inadequateknowledgeoftheshapeoftheshearstressenvelopeinprincipalstressspace, thenon-uniquenessofsolutionswhennon-associativity is assumed on the shear stress envelope (e.g. byassumingzeroviscoplasticvolumechange), andtheinabilitytomodelsideplaneresistance.Stresstensor rotationoccursduringembankment constructionasa result of load spreading. At the toe, for example, the initialgeostaticstatewithverticalmajorprincipalstressrotatesover908tothepassivestate, andlocationsbetweencrest andtoeundergointermediateamountsofrotation.Insoft soils suchrotations usuallyinducevolumedecreaseandpore pressure increase. These effects are not dealt withbythecreepmodel usedhere. Jardineet al. (1997) describehow, onceconsolidationhas occurredunder therotatedstresses, undrainedloadingwithout further rotationyields highundrainedstrength,closetothat whichisobtainedwithout anyrotation. Thisisdueto the soils fabric gradually adapting to the rotated state ofstresses andstrains. Indyke safetyevaluationthe failure loadsstemmostlyfromrisingwaterlevels, andastheseareessentiallyhorizontal, theywill induce freshrotations of the stress tensor,andappearsoquicklyastobeessentiallyundrained. Thiseffectisprobablysmall, however: theIJkdijknite-element modelwasrunwithextremewater loading(water level inthebasinbehindthe dyke raised quickly to crest level), and it was found thatrotationswerelessthan108inthezonesinwhichshearfailureisexpectedtooccur.ModiedCam-clayis anisotropic model inthe sense that theyield ellipse remains orientated along the isotropic stress axis.Developmentsareunderwayinwhichtheellipserotatesdepend-ingontherelativeamounts of isotropicanddistortional plasticstrains. Suchanisotropicmodelsappear toimprovetstomeas-uredsoil behaviour, andhaverecentlybeenadaptedtoaccountforsoilviscosity, aswellasthespecicbehaviourofpeat(Leoniet al., 2010). This mayprovideanavenuefor further improve-mentoftheapproachpresentedhere.Thecreepmodel doesnot deal adequatelywiththeoverconsoli-datedstate. Onunloading, rates of creepstrainreduce stronglyandbehaviour becomesessentiallyelastic, andonlycritical-statestrengthisused. Embankmentswithasignicant passivezoneofhighlyoverconsolidatedmaterial maythereforebelessamenabletotheapproachdescribedhere.ThesideplaneandmeshsizeeffectsonthemomentoffailureintheIJkdijkcasehavebeennoted,andinanycalculationoffailureitwillbenecessarytotaketheseeffectsintoaccount.Notwithstanding these limitations, and given that due care isexercised, the procedure described in this paper should allowsuccessfulapplicationofthenite-elementmethodtothecalcula-tion of deformations and strength of embankments on softorganicsoils.REFERENCESBegemann HKSPh(1971)Soilsamplerfortakinganundisturbedsample66 mmindiameterandwithamaximumlengthof17metres.Proceedingsofthe4thAsianConferenceoftheInternationalSocietyforSoilMechanicsandFoundationEngineering,Bangkok,Thailand,pp.5457.Bodas Freitas TM(2008)NumericalModelling oftheTimeDependentBehaviour ofClays.PhDthesis,ImperialCollegeLondon,UK.Coutinho RQ and Lacerda WA(1989)StrengthcharacteristicsofJuturnaibaorganicclays.Proceedingsofthe12thInternationalConferenceonSoilMechanicsandFoundationEngineering,RiodeJaneiro,Brazil,vol.3,pp.17311734.De Boer R(2005)TheEngineer andtheScandal.Springer-Verlag,Berlin,Germany.Den Haan EJ(1996)Acompressionmodelfornon-brittlesoftclaysandpeat.Geotechnique46(1):116.Den Haan EJ and Feddema A(2009)Deformatieensterkte vanophogingenendijkenopslappeNederlandsegrond.Geotechniek13(4):5255.Den Haan EJ and Kamao S(2003)ObtainingisotacheparametersfromaCRSK0-Oedometer.SoilsandFoundations43(4):203214.Den Haan EJ and Kruse G(2006)CharacterisationandengineeringpropertiesofDutchpeats.InCharacterisationandEngineeringPropertiesofNaturalSoils(TanTS,PhoonKK,HightDW andLeroueilS(eds)).Swets&Zeitlinger,Lisse,Switerland,vol.3,pp.21012133.Jardine RJ, Zdravkovic L and Porovic E(1997)Anisotropicconsolidationincludingprincipalstressaxisrotation:Experiments,resultsandpracticalimplications.Proceedingsofthe14thInternationalConferenceonSoilMechanicsandGeotechnicalEngineering,Hamburg,Germany,vol.4,pp.21652168.Jardine RJ, Smith PR and Nicholson DP(2003)Propertiesofthe13GeotechnicalEngineering DeformationandstrengthofembankmentsonsoftDutchsoilDenHaanandFeddemasoftHoloceneThamesEstuaryClayfromQueenborough,Kent.InCharacterisationandEngineeringPropertiesofNaturalSoils(TanTSetal.(eds)).Swets&Zeitlinger,Lisse,Switzerland,vol.1,pp.599644.Keverling Buisman AS(1934)Proefondervindelijkebepalingvandegrensvaninwendigevenwichtvaneengrondmassa.DeIngenieur,26June,8388(inDutch).Landva AO and La Rochelle P(1983)CompressibilityandshearcharacteristicsofRadforthpeats.InTesting ofPeatsandOrganicSoils(JarrettPM(ed.)).ASTMInternational,WestConshohocken,PA,USA,ASTMSTP820,pp.157191.Leoni M, Karstunen M and Vermeer PA(2010)ReplytodiscussiononAnisotropiccreepmodelforsoftsoils.Geotechnique60(12):963966.Roscoe KH and Burland JB(1968)Onthegeneralisedstressstrainbehaviourofanidealisedwetclay.InEngineeringPlasticity(HeymanJandLeckieFA(eds)).CambridgeUniversityPress,Cambridge,UK,pp.535609.Van MA, Zwanenburg C, Koelewijn AR and van Lottum H(2009)Evaluationof fullscaleleveestabilitytestsatBooneschans.Proceedingsofthe17thInternationalConferenceonSoilMechanicsandGeotechnicalEngineering,Alexandria,Egypt,vol.3,pp.20482051.Van Duinen TA(2008)Grensverleggendonderzoekmacrostabiliteitbijopdrijven:Fase2.C.Deltares,Delft,TheNetherlands,Report419230-0040(inDutch).Va sa rhelyi B(2010)Tributetothersttriaxialtestperformedin1910.ActaGeodaeticaetGeophysicaHungarica45(2):227230.VermeerPAandNeherH(1999)Asoft soil model thataccountsforcreep. InBeyond2000 inComputationalGeomechanics(BrinkgreveRBJ(ed.)). Balkema, Rotterdam,TheNetherlands, pp. 249261.Zwanenburg C, den Haan EJ, Kruse G and Koelewijn A(2012)FailureofatrialembankmentonpeatinBooneschans,TheNetherlands.Geotechnique,http://dx.doi.org/10.1680/geot.9.P.094.WHAT DO YOU THINK?To discuss this paper, please email up to 500 words to theeditor at journals@ice.org.uk. Your contributionwill beforwardedtotheauthor(s)forareplyand,ifconsideredappropriatebytheeditorialpanel,willbepublishedasadiscussion in a future issue of the journal.Proceedings journals rely entirely on contributions sent inby civil engineering professionals, academics and students.Papersshouldbe20005000wordslong(briengpapersshould be 10002000 words long), with adequate illustra-tions and references. 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