Page1 / 16

10 PLAXIS Bulletin (S) - PLAXISPLAXISPLAXISEditorialN 10-...

This preview shows document page 1. Sign up to view the full document.

View Full Document Right Arrow Icon

10 PLAXIS Bulletin (S) - PLAXISPLAXISPLAXISEditorialN 10-...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: PLAXISPLAXISPLAXISEditorialN 10- MARCH 2001Since the release of the Dynamics module lastyear users have been running dynamicIn April the newly developed 3D Tunnelanalyses. As this is a new module of Plaxis,program will officially be released . Manymany users have to gain experience with it.months were spent in testing the program,Therefore, the Plaxis Users Association (NL)not only at the Delftech park office, butorganised a well attended Dynamics day withalso by a group of Beta testers. The 3Dguest speakers Prof. dr. ir. A. Verruijt and T.K.Tunnel program is specifically intended toMuller from IFCO. After these interestingmodel tunnels such as shield tunnelslectures, hands-on exercises were conducted(second Heinenoord Tunnel, see Newin the afternoon to exchange practicalDevelopments) and NATM tunnels. But theexperience. Last January, during the course3D Tunnel program allows other 3DComputational Geotechnics at Berkeleysituations to be modelled as well, such asUniversity (USA) a one day Dynamics coursethe 3D excavation and installation of awas introduced. Some 30 participants from thisdiaphragm wall (M. de Kant, see Plaxisearthquake sensitive area were present.Practice). A short description of the 3DTunnel program is provided in this bulletin.The analysis of flexible soil retaining walls istaken a step further. From Blums analysis andLWinkler spring type models to an analysis in3D Tunnel program was presented at a usersPlaxis. The approach of using a Finite Elementmethod with an adequate soil model to analysebackground of the 3D Tunnel program, aflexible soil retaining walls shows that somehands-on exercise was given on a simpleinteresting results can be obtained (Columnexample problem. Further more it was shownPlaxis bulletinPlaxis B.V.P.O. Box 5722600 AN DelftThe NetherlandsE-mail:bulletin@plaxis.nlast year, in September, a Beta release of themeeting. Besides a lecture on the (theoretical)Bulletin of thePLAXISUsers Association (NL)Vermeer).that the PLAXIS 3D Tunnel program is certainlycapable of other analyses beyond tunnelling.A case about the indentation of a tractor wheelabout the role of OCR in the model. A practicalexcavation of a slurry wall were presented.Editorialimplemented, questions have been askedin soft soil (University of Wageningen) and theIN THIS ISSUE:Since the Soft Soil Creep model has beenexample is given and preliminary conclusionsColumn Vermeerare drawn in The role of OCR in the SSC modelsee Plaxis Practice.New developmentsImproved userservices supportEditorial staff:Recent activitiesMarco Hutteman, Plaxis Users Association (NL)Martin de Kant, Plaxis Users Association (NL)Peter Brand, Plaxis bvPlaxis PracticeJan Gabe van der Weide, Plaxis bvThe role of OCR12Users forum14Agenda16Scientific Committee:Prof. Pieter Vermeer, Stuttgart UniversityPartial geometry of NATM tunnel1Dr. Ronald Brinkgreve, Plaxis bvPLAXISPLAXISColumn VermeerFE-computations. We will consider a singleanchored wall for three different cases.ON SINGLE ANCHORED RETAINING WALLSWe considered the geometry of Fig. 1, i.e. anThe analysis of flexible soil retaining wallsexcavation depth of 10 m, an embedment ofbecame possible through the work of Blum2.5 m and an anchor at a depth of 2.5 m. Thein the 1930s. Considering single-anchoredanchor force was given a fixed value ofor single-propped sheet-pile walls, he100 kN/m.distinguished between two types ofFor all three different cases (A, B and C) theembedments:following soil properties were adopted:qfree earth supportqfixed earth supportSubmerged soil weight was used, as weree earth support implies a relatively shortFconsider a water table at the soil surface, beingwall with minimum embedment. Fixednot lowered at all, i.e. neither in front norearth support implies a somewhat largerbehind the wall. The excavation was done inembedment. According to Blums definition,three stages of construction:full fixity is achieved when the fixity moment1 Installation of wall and excavation to a depthequals the field moment.of 2.5 m2 Application of anchor load of 100 kN/mBlums design procedures for retaining walls3 Excavation down to final depthwith free or fixed earth support can be foundin most textbooks. In the authors opinion theyHardening soil model: Soil behaviour wasconstitute outstanding contributions to Soilsimulated using the HS-Model of the PlaxisMechanics. However, as Blums analysis involvescode. For virgin oedometer loading, thisneither the wall stiffness nor the soil stiffnessimplies an increasing tangent stiffness modulusit is bound to be inaccurate. As a consequence,according toone is now mostly using Winkler spring typewithmodels. Unfortunately it is difficult to selectappropriate spring constants and I would ratheruse the FE method. To assess the impact ofwherestiffnesses we decided to perform a series ofadopted the exponent m = 0.5. Within the HS-is the major principal stress. WeModel unloading-reloading is described on theFigure 1Single-anchored wallwith free earth supportwith 3 stages ofconstruction: firstexcavation, anchoringand final excavation. Inpractice anchors will beinstalled just above thegroundwater table.basis of Hookes law. Youngs unloadingreloading modulus for increments of stressand strain reads:Table 1 Stiffness parameters.whereis the minor principal stress. For allanalyses, the over-consolidation ratio was takento be OCR = 1.0 and initial stresses werecomputed using Ko = 0.5. The HS-Model alsoallows for a specification of soil stiffness indrained standard triaxial tests. For all analyses,we used2PLAXISPLAXISThe only difference between the stiff soil ofThe simulation of arching behind a flexible wallCases A and B, and the soft soil of Case C relatesmakes the FEM superior to subgrade reactionto the stiffnesses. The stiff soil is simply a factortype models. In the latter case the spring will15 stiffer than the soft soil, but the relationyield plastically as soon aseah is reached andis 1/1/4 for both soils. Moreover,active pressures will never reach smaller valuesboth the stiff and the soft soil are convenientlygiven the same strength parameters.than eah= kah . . Fig. 2 clearly demonstrateszthe significance of arching, as computed activeCase A: Considering the FE-results for theearth pressures are well below the dashed linecombination of a stiff soil and a flexible wall,for eah. It happens for flexible walls in stiff soils.one observes in Fig. 2a considerable wallbending up to about 5 cm. As a consequence,the active earth pressures reduce significantly;stiffness is the fixity of its base. There is aeven below the classical minimum of e ah .Indeed, plots of stresses showed significantFigure 2Single-anchored wallwith free earth support.Another feature of a wall with low relativesignificant fixity moment! Here it should bearching between the anchor and the passivelength using Blums design rules for a wall withpressure below the bottom of the excavation.no fixity at all. Due to the significant amountnoted that we computed the embedmentof arching and the base fixity, computedbending moments are small; approximatelyhalf the ones that would follow from Blumsdesign rule.Case B: Typical Blum-type results are obtainedwhen considering a stiff wall in a stiff soil (CaseB). Below the anchor classical active earthpressures are reached. The passive ones arenot fully mobilised, as we designed the wall fora factor of safety of 1.5 on the passive earthpressure. The base of the wall shows no fixityat all and bending moments agree well to theones that follow from Blums analytical designprocedure. Please note that the same earthpressures and bending moments would havebeen obtained for the combination of a softsoil and a flexible wall. In such a case we wouldhave the same relative wall stiffness as for thestiff-stiff combination of Case B.Case C: I was amazed when consideringcomputational results for a stiff wall in a softsoil. Despite the use of a factor of safety of 1.5,the passive earth pressure is nearly completelymobilised. It appears to be caused by anenlarged active pressure. Apparently, the soilis so deformable that wall displacements ofabout 5 cm are insufficient for a properreduction of pressure on the active side. As aconsequence of the high pressure a bendingmoment of nearly 300 kNm/m occurs. Nodoubt, this is well beyond the values that would3PLAXISPLAXISfollow from Blums design analysis.5 m is considered. Following Blum s designrules this would yield full base fixity, such thatFor a stiff wall in soft soil, I would also doubtthe fixity moment equals the field moment.the results of subgrade reaction typeComputational results for all three differentcalculations, as this method suffers from therelative wall stiffnesses are shown in Fig. 3. Fordifficulty of selecting proper spring constants.comparison, previous data for the shorter wallRealistic values would be required both for theare indicated by dashed lines.active and the passive zone; otherwise it isIt appears from Fig. 3 that bending momentsimpossible to predict the high bendingare only slightly reduced when increasing wallmoments as obtained for a stiff wall in softlength. This is surprising as some textbookssoil.suggest a significant effect on the bendingmoments. Considering present computationalEmbedment: For studying the effect ofFigure 3 Singleanchored wall for fixedearth support. Dashedlines indicate results forfree earth support.data, we conclude that bending moments areembedment, we reconsider the wall of Fig. 1,in general not significantly reduced bybut now the penetration depth is doubled.increasing wall penetrations. Present dataHence, instead of 2.5 m, an embedment ofshow, that the reduction of the field moment,as caused by the fixity moment, is more or lesscompensated by a slight increase of activepressure, as caused by the stiffening of theentire system. Deep penetration is neither ofgreat import when considering displacements.Indeed,asignificantreductionofdisplacements is only achieved for Case A.Conclusions: When considering a stiff wall ina stiff soil (Case B) typical Blum-type results areobtained. In this case classical active earthpressures will occur, at least below the anchor.Obviously, the passive ones will not be fullymobilised, if the wall is designed for a factorof safety equal of 1.5 on the passive earthpressure, as done in the present example.A flexible wall in a stiff soil (Case A) will result inconsiderable wall bending and low bendingmoments. The stiff soil transfers a large partof the active pressure by arching and theflexible wall gets a relative small pressure.A stiff wall in a soft soil (Case C) will result inhigh active pressures and, as a consequence,high bending moments.Finally we conclude that bending moments arein general not significantly reduced byincreasing wall penetrations.P.A. Vermeer, Stuttgart University4PLAXISPLAXISNew Developmentsmonitored.Calculationsofdifferentconstruction phases are performed for theThe Plaxis 3D Tunnel program is about toNorth bank. In a 3D finite element model (onebe released. In the previous Bulletin it wassymmetric half) the sub-soil, the Tunnel Boringexplained why this first 3D Plaxis programMachine (TBM) and a part of the final lining wereis devoted to tunnels. At the moment,modelled according to the Grout pressurequite some engineering and research ismodelling procedure (see Fig. 1). The sub-soilfocused on tunnelling, both NATM andwas schematised by means of 8 layers, withshield or bored tunnelling. Tunnellingtheir location and properties as listed in Tableinvolves three-dimensional aspects that1. All layers were modelled using the Mohr-cannot be analysed with conventionalCoulomb model. The layers located under themethods. Hence, there is a demand for atunnel were given a high unloading stiffness.3D design model for tunnels. Nevertheless,creative users of the Plaxis 3D TunnelThe hydrostatic pore pressure distribution forprogram may find many other applicationsall layers was determined from a phreatic levelin addition to the analysis of tunnels.at +1.0 m.n the past few months, beta-testers haveIThe 3D finite element model consists of 3440used a pre-release of the 3D Tunnel programquadratic volume elements divided over ain practical applications. Some of thesenumber of slices (see Fig. 2). Each slice is 3.0preliminary results are presented in thism in the longitudinal tunnel direction. The TBMwas modelled over 3 slices and composed ofshell (plate) elements, with a flexural rigidityTable 1 Soil layers and parameters used in the Mohr-Coulomb modelLayer TopTypemunsatkN/m3EsatkN/m3 -ckN/m2K0kN/m2EI = 50103kNm2/m, a normal stiffnessEA = 10106 kN/m and a weight w = 38,15kN/m2. The radius of the TBM is 4.25 m and itsMSL17.227.00.0 0.580.35 m thick concrete tunnel lining was6.5 0.47modelled using volume elements with the3.0 0.47following properties:5.0 0.4524.610621.00Drained16.517.20.3439003.03-1.50 Drained20.520.50.30296000.04-5.75 Drained19.019.00.31185000.033.00.027.035.0kN/m2,= 24 kN/m3, E =n = 0.2.5-10.00 Drained19.56-17.25 Drained20.520.50.30444000 0.036.56.5 0.507-20.75 Undrained 20.020.00.32119000 7.031.01.0 0.55The tunnel boring process was modelled8-25.00 Drained21.00.30593000 0.037.57.5 0.56according to the Grout pressure modelling21.0193003.036.5Undrained 16.50.303900centre point is located at -12.3 m MSL. The2.5019.50.340.0 0.581procedure as schematised in Fig. 1.Bulletin. In this article I will shortly present someresults of a 3D calculation for the SecondHeinenoord Tunnel, the first large-scale boredtunnel project under soft soil conditions in theSouth-West of The Netherlands.The situation at the Second Heinenoord Tunnelis described in various publications (seeFigure 1 Modelling aspects in Groutpressure modelling procedure.References). The tunnel is formed by two tubeswith outer diameters of 8.5 m, which wereA front pressure was applied at the bore fontbored under the river Oude Maas. In order toto support the soil. The front pressure is 140gain experience with tunnel boring under softkN/m2 at the top of the TBM and 259 kN/m2soil conditions, the situation was extensivelyat the bottom. The TBM is conical. The tail5PLAXISPLAXISradius is 2 mm smaller than the front radius,m (TBM 3 slices, liquified grout zone 2 slices,which corresponds with a contraction of abouttunnel lining 9 slices). The results of the0.48% (0.16% per slice per phase). Behind thecalculation at the end of Phase 14 areTBM grout is injected in the tail void. It ispresented underneath.assumed that the grout remains liquified over2 slices (6 m), which results in a grout pressureFig. 2 shows the deformed mesh. This ploton the surrounding soil. The grout pressure isclearly shows the settlement trough at thekN/m2at theground surface, with a maximum settlementbottom. Behind the liquified grout zone theof about 22 mm. The results are quite realistictunnel lining is activated and jack forces areand correspond reasonably well to theapplied in backward direction (varying frommeasurements. This statement also applies to125Figure 2 Deformedmesh at the end ofPhase 14 (deformations50 times enlarged).3365at the top and 190kN/m2kN/m2at the top and6731kN/m2at thethe width of the settlement trough.bottom).Calculations using contraction only tend toIn the initial situation, initial stresses areoverestimate the width of the settlementtrough, whereas calculations according to thegrout pressure modelling procedure givemore realistic results. The deformations justabove the tunnel lining are somewhat largerthan at the settlement surface (max. 38 mm).Fig. 3 shows the shadings of total displacements. This plot confirms the above and clearlyshows where the larger displacements occur(just above the lining). In this plot it can also beseen that the buoyancy of the tunnel isrelatively little, since the displacements belowthe tunnel are small.In addition to the displacements, the stressescan be visualised in the full 3D mesh as well asin individual and user-defined cross sections.From such plots the three dimensional archingaround the tunnel can be viewed. There arealso several possibilities to show the forces anddeformations of the TBM and the tunnel lining,both in 3D and per cross section. Themaximum bending moment in the TBM isaround 100 kNm/m and the maximumbending moment in the lining is around 80kNm/m. These values are also quite realistic.From the results it can be concluded that it isFigure 3 Shadings oftotal displacement at theend of Phase 14(displacements 50 timesenlarged).very well possible to calculate the threegenerated by means of the K0-procedure,dimensional effects around bore tunnels andusing K0-values as listed in Table 1. The wholeto accurately predict surface settlements usingcalculation is divided into 14 phases. In Phasethe grout pressure modelling procedure.1 the TBM enters the model in the first sliceThe above analysis took some 6 hours toand the process advances 1 slice in each phase.calculate on a Pentium III 500 Mhz with 768 MBIn the final Phase 14 the TBM has advanced 42RAM.6PLAXISPLAXISImproved UserReferences:[1] Bakker K.J., van Schelt W., Plekkenpol J.W.(1996), Predictions and a monitoringSupport Servicescheme with respect to the boring of theSecondHeinenoordIn:As you may have read in the last PlaxisGeotechnical aspects of undergroundbulletin, the Plaxis company is growing.construction in soft ground, (eds: R.J. MairCurrently the staff consists of 10 people.and R.N. Taylor). Balkema, Rotterdam. pp.This means we can further increase the459-464.quality and the range of products. In April[2] JaarsveldE.P.,Tunnel.PlekkenpolJ.W.,this will first show by the release of the 3DMessemaeckers van de Graaf C.A. (1999),Tunnel program. Obviously these newGround deformations due to the boringproducts and the increasing group of Plaxisof the Second Heinenoord Tunnel. In:users (now over 2000 professionals !) requireGeotechnicalforgood user services. Last year we employedtransportation infrastructure (eds: F.B.J.a full time support engineer in order toBarends, J. Lindenberg, H.J. Luger, L. dekeep up with the foreseen demand onQuelerij, A. Verruijt). Balkema, Rotterdam.profession support.infrastructurepp. 153-159.Tweede Heineneoordtunnel, verslag vanWeen groootschalig praktijkonderzoek naarObviously such projects have a deadline. Hence,geboorde tunnels. Final report COBit is important that users can rely on a goodcommittee K100. CUR / COB, Gouda.adequate HelpDesk. Not only on an operational[3] CUR / COB (1999), Monitoring bij dee realise that most support questionsoccur while working on a project.Validatielevel, but also on a more scientific level. To ensureGroutdrukmodel, Meetveld Noord. Reporturgent questions get the highest priority, we areof COB committee L520. CUR / COB, Gouda.introducing two different levels of support.[4] CUR/COB(1999),3D[5] Bakker K.J. (2000), Soil Retaining Structures;development of models for structuralSupport service level 1:analysis. Dissertation (Delft University ofThis service level is free of charge and providesTechnology). Balkema, Rotterdam.assistance by e-mail within reasonable response[6] Peters R., Safari B. (2000), 2D Modelleringtimes, obviously depending on the supportvan het tunnelboor- en consolidatieproces.demand. No telephone and scientific assistanceInternalis provided within this service level.reportBSRAP-R-00004,Bouwdienst Rijkswaterstaat, Utrecht.[7] CUR / COB (2000), Toetsingsrichtlijn voorContact HelpDesk:e-mail:support@plaxis.nlhet ontwerp van boortunnels voor wegen railinfrastructuur. Final report COBSupport service level 2:committee L500. CUR / COB, Gouda.The second level of support is to users whohave a PLAXIS software support agreement.Ronald Brinkgreve,This agreement provides a professional supportDELFT UNIVERSITY OF TECHNOLOGYservice on operational as well as on a scientific& PLAXIS BVlevel, within 24 hours of normal working days.Contact HelpDesk:e-mail:support@plaxis.nlfax:+31 15 2600 451telephone:+31 15 2600 450For more information on the supportagreement, please contact Plaxis BV.7PLAXISPLAXISRecent Activitiescourses. It is also good to realise that the localcourses are organised and lectured inPlaxis 3D Tunnel programcooperation with different local experts, localAs this bulletin shows, the Plaxis 3D Tunneluniversity professors as well as professionalsprogram is nearly finished. Currently the Plaxisfrom the engineering practice. This formulateam puts the finishing touch to the 3D Tunnelcreates tailor made courses, providingprogram and the final release is expected intheoretical and practical backgrounds on theApril this year. The 3D Tunnel program isuse of Plaxis. The courses in the Netherlandsspecifically intended to model shield, boredreflect the Plaxis philosophy with respect toand NATM type of tunnels. Hence the programlecturing and the usage of the Plaxis program.has special features such as a state-of-the-artThe contents of the courses therefor are usedtunnel designer, realistic grout pressureas the blue-prints of all international course.modelling, staged construction, Jointed rockmodel, etc. The userfriendlyness of the input,During the past course in Noordwijk, thetheandNetherlands, we welcomed 41 participantsrobustness of the numerical procedures are infrom virtually all over the world. Seventeenline with the Plaxis 2D software. This meansdifferent nationalities, including people fromcreating the finite element model andJapan, Jamaica, Mexico, Brazil, Israel, Southprescribing the calculation phases are relativelyAfrica, just to mention a few. This is a clearsimple, especially when compared to theindication of the true International characterefforts it takes using general purpose FE codes.Plaxis is enjoying.automaticmeshgenerationSuch codes are usually command driven andlack advanced options needed for themodelling of soil, structures and soil/structureinteraction. With this new tool, Geotechnical3Dcalculationscanbecomegenerallyapplicable.Figure 2 Participants and lecturers in theNoordwijk course.So far some 10 new courses have beenscheduled for this year. At the upcomingcourses in Noordwijk, new lectures dedicatedto both Dynamics and 3D modelling arepresented (see also Agenda).Plaxis PracticeFigure 1 The Plaxis 3DTunnel ProgramRESEARCH PROGRAM ON THE IMPACT OFDIAPHRAGM WALL INSTALLATION.CoursesVALIDATION OF A 3D-FE MODEL.In the past year 11 courses on ComputationalGeotechnics were lectured at different1. Introductionlocations all around the world. Six in Europe,The North-South metro line in Amsterdam willthree in Asia, one in the US and one in theconnect the northern and southern suburbsMiddle East. From the agenda in this bulletinwith the city centre (De Wit, 1998). For reasonsyou can see we plan to continue lecturing suchof protecting the historic city centre and8PLAXISPLAXISrestricting the disruption of city life, a boredtunnel will be applied that follows the streetqmeasurements of stress in the trench(pizometers on reinforcement cages).pattern as closely as possible and is lowered toa great depth. Consequently the undergroundThe geotechnical profile is comparable withstations are at a great depth as well. Thethe locations of the future stations of thestations will be constructed in a building pitNorth-South line, see table 1.with 40m long braced diaphragm walls. One ofthe most important aspects in the design ofTable 1. Geotechnical profile enthe stations is the impact of construction onsoil-parameters.historical buildings. Because knowledge on theType of soilTop levelNAP (m)impact of diaphragm wall construction onCPT(Mpa)surrounding buildings is only limited, it wasFill (sand)+2,010-15decided to carry out a research project,Clay-1,00,5consisting the following four phases:Peat-3,50,51 prediction of the impact with a 3D FE-Clay, silt-7,00,5-1,0model;Peat-13,01,52 full scale test;Sand-14,0 1)8-303 interpretation of test results;Silty sand-17,01-54 validation of the 3D FE-model.Clay (eemklei)-25,01-5This paper focuses on phase four.Sand-28,010-30Silt-42,032. Full scale test1) =The test was carried out at the construction siteFoundation layer of ancientbuildings on wooden pilesof the Mondriaan Tower, where diaphragm wallpanels are applied as foundation elements as3 FE - validation calculationswell as building pit walls. Figure 1 gives anThe main goal of the validation calculations isimpression of the test site. During the differentdeveloping a model which can be used tostages of construction of 5 panels, a monitoringpredict the soil displacements during theprogram was conducted consisting of:construction of a diaphragm wall. TheFig. 1 Impression ofthe test-sitecalibration was carried out, mainly by changingthe soil parameters within a certain bandwidth.(extensometers, inclinometers);qmeasurements of vertical and horizontaldisplacements in the surrounding soilqThis calibration procedure has resulted in asettlement and bearing capacity tests onbest-fitpiles;calculations were made with the officialcalculation(BF).Additionally,North/South-line parameter-set (NS), whichhas also been used in the 2D tunnel- andbuilding-pit calculations. In both calculations,BF and NS, the hard-soil model is used.Element mesh and modelling procedureFor the 3D-calculations a preliminary 3D-versionof PLAXIS (6.4b) has been used. The mesh isbuild of 3D-wedge-elements with 15 nodesand 6 Gauss-points. For the calculation of onepanel a mesh with approx 5,000 nodes hasbeen used (fig. 2). Only a quarter of the panelis modelled, which means that there are twoplanes of symmetry: in plan view x- and zdirection.9PLAXISPLAXISFig. 2 Finiteelement meshThe construction of a single diaphragm wallpanel is modelled using the following stages:1 Excavate the single trench by switching thesoil elements off and, simultaneously,=d.applying the bentonite pressure (bis hardened.Above the hardened concrete, the wet= d . ) acts on theconcrete pressure (cfaces of the trench (cc= volume weight of wetconcrete). The lateral pressure in step 2 cantherefore be described by a bilinear relation:were hcrit is a critical depth which depends onthe concrete placing rate, cement type,temperature etc. (Lings et al 1994). In case ofa constant rate of concrete placing,tois equalb.During installation the stresses in the soil willinitially decrease (stage 1) and, subsequently,increase due to the wet concrete pressure(stage 2). The stresses decrease exponentialwith increasing distance from the trench.Among these stress-paths the stiffness of thesoil varies strongly. The calculations wheretherefore carried out using non-linear materialmodels: the hard soil model (HSM) and thesoft-soil model (SSM). Only the elementsrepresenting the clay and peat layers aremodelled as undrained.Calculation results for a single panelFigure 3 shows the horizontal effectivestresses, at a depth of NAP-15m, during thedifferent stages (1 and 2). The initial horizontalstress is about 50 kPa. During excavation thestress at the centre of the panel decreases toabout 25 kPa. Due to horizontal load transfer(arching), at the edges of the panel, the stressincreases and exceeds K0 situation. Afterconcreting, the stress at the centre increasessignificantly. At the corner a decrease ofeffective stress occurs.The calculations have shown that load transfernot only acts horizontal but also in verticalFig. 3 Horizontaleffective stresses indifferent stagesb)on the faces of the trench (d = depth,b = volume weight of bentonite).2 Fill the trench with concrete by increasingdirectiontorelativestifflayers.Thedisplacements in layers beneath those stifflayers therefore decrease and are no longer ofthe lateral pressure.influence on displacements of the top layers.Directly after pouring the trench, fromTherefore the influence of the panel-depth inbottom up, the lower side of the concretea layered subsoil with stiff layers is very small10PLAXISPLAXISwhich was an important conclusion of theprediction calculations.The model was calibrated on the displacementsin the concrete-phase (stage 2), becausedisplacements during the excavation phase(stage 1) were to small.Figure 4 shows the horizontal displacementsat the centre of the panel, at approx. 2m fromthe trench. The maximum displacement occursin the Holocene top-layers, particularly the peatlayer at NAP-6m. The BF-calculation is in goodagreement with the test-results. Regarding thetop layers, the NS-calculation is also accurate,Fig. 4 Horizontal displacements,middle of the panelbut the horizontal displacements at lowerlayers are over predicted. The computedvertical maximum displacements of the sandlayer at a level of NAP-15m (foundation layerof wooden piles) are for both the BFcalculation and the NS-calculation in goodagreement with the test-results (fig 5). At thetest-site a settlement bowl occurs with amaximum settlement at a distance of 3m fromthe trench. This is in good agreement with theNZ-calculation, although the calculations showsa much wider settlement bowl.Influence of panel size and installationsequenceAt the test-site 5 panels were monitored, withdifferent sizes and shapes. In contrast to theFig. 5 Vertical displacements onlevel NAP -15mcalculations and literature, the panel width wasof little influence on the displacements. Thereason for this discrepancy has probably beenthe installation sequence of the panels. Thesmall panel (b=2,7m) was excavated in aundisturbed situation whereas the wide panel(b=6,4m) was excavated between two formerinstalled panels. Those adjacent panels causesa reduction of displacements because of:qhardening (due to the installation of theadjacent panels the soil is overconsolidated);qload transfer to adjacent stiff concrete walls.To confirm this interpretation, the describedsituation was computed with the calibratedmodel (NZ-parameters). Figure 6 shows theFig. 6 Influence of panel sizeand sequence of installation onhorizontal displacementshorizontal displacements of undisturbed panelswith different widths, and a wide panel11PLAXISPLAXISbetween to installed small panels. Althoughhardening was not calculated due to theundrained behaviour, the results are inThe role of OCR inthe SSC Modelagreement with the expectation.The Over-Consolidation Ratio (OCR) isConclusionsdefinedThe diaphragm wall research project haspreconsolidation stressenlarged the understanding of the impact ofin-situ stress: OCR =D-wall installation on surrounding soil. Mainparameter that indicates the amount ofconclusions are:overconsolidation of the soil. As long as theastheratiopbetweenthep and the effective/ yy. OCR is a statesoil behaviour is stiff. This applies tothe ground deformations were relativelyunloading as well as to reloading. As soonas the existing preconsolidation stress isdisplacements in the soft top layers;passed by primary loading of the soil, thethe width of a diaphragm wall panel is ofsoilgreat influence on ground displacements.preconsolidation stress further increasesDue to vertical load transfer, the influencealong with the effective stress level. In thatof the panel-depth in a layered subsoil withcase the soil is said to be in a state ofstiff layers, is relatively small;normal consolidation, which would implythe computed displacements with theOCR=1.0. This is exactly what happens incalibrated 3D-FE model are in goodCam-Clay type of models, like the Soft Soilagreement with the test -results.qpreconsolidation stress, i.e. OCR>1.0, thesmall, with exception of the horizontalqeffectivenearby wooden pile foundations;qthe installation of a diaphragm wall in alayered subsoil has no significant impact onqmodel in Plaxis.stressbehaviourisissoft,belowwhilstthetheIngenieursbureau AmsterdamANorth/South Line Design Office,Plaxis. In that respect the meaning of the OCR-Amsterdam, The Netherlandsvalue is similar, although the transition fromsimilar behaviour can be observed whening. M. de Kantusing the Soft Soil Creep (SSC) model inthe stiff reloading behaviour to soft primaryLiteratureloading is more gradual (see Fig. 7.13 of theLings, M.L. & C.W.W. Ng. & D.F.T. Nash, 1994,Plaxis Material Models Manual [1]). In order forThe lateral pressure of wet concrete inthe preconsolidation stress to follow thediaphragm wall panels cast under bentonite,effective stress level, time is needed. Hence,Proc. Instn. Civ. Engrs. Geotech. Engng, 163-when loading the soil very quickly, the OCR-172.value can (temporarily) become less than 1.0.De Wit, J.C.W.M., 1998, Design of undergroundOn the other hand, if the load remainsstations on the North/South line, Proceedingsconstant, the creep process continues in time,of the World Tunnel Congress, Sao Paolo.which results in a gradual increase of theDe Wit, J.C.W.M., J.C.S. Roelands & M. de Kant,preconsolidation stress and OCR (withoutFull scale test on environmental impact ofphysical loading). The latter process can bediaphragminconceived as ageing. The idea that ageingofwouldAmsterdam,walltrenchGeotechnicalexcavationAspectsleadtoanincreaseoftheUnderground Construction in Soft Ground, ISpreconsolidation stress was introduced by1999 Tokyo Japan, Balkema Publ.Bjerrum in his creep formulation [2]. This ideais also adopted in the SSC model. In the SSCmodel the increase of the creep strain in time12PLAXISPLAXIS(= creep strain rate) depends (in addition to thewest of The Netherlands) the initial OCR-value*) on the ratio of theis, per definition, equal to 1.0. On the othereffective stress and the preconsolidation stress,hand, even very soft soils often show ai.e. on the inverse of OCR. Hence, starting frompreconsolidation stress that is over 20 kPaan initial effective stress state, the initial creepbeyond the in-situ stress level. Particularly instrain rate depends on the initial OCR-value.the top layer (just below the ground surface)modified creep index,crust forming may occur, which is associatedThe latter is not very known among Plaxiswith relatively stiff and strong behaviour. Theusers. Since practical situations always involvereason for this can be drying of the soil,effective stresses from the very beginning, thevariations of the phreatic level, temporarycreep process (settlement) starts immediatelyloads, temperature changes, etc. Nevertheless,without additional loading, whereas thethese soil are still considered to be normallysettlement velocity depends on the OCR-consolidated, but the actual OCR-value is oftenvalue. For reasonable combinations of SSChigher than 1.0.parameters, the use of a default initial OCRvalue of 1.0 in the K0-procedure may lead toexcessive initial settlement velocities. Hence,layer under standard boundary conditions withthe initial OCR-value needs to be selected withproperties as listed in Table 1. The parameterscare. An initial OCR-value larger than 1.0 isFigure 1:Time-settlement curvesfor different initialvalues of OCRLet us consider, for example, a 10 m thick clayare arbitrary, but realistic for a normallygenerally recommended.consolidated clay. Initial stresses are generatedIt is generally said that for normallywith the K0-procedure, using different initialconsolidated soils (like the soft clays in thevalues of OCR (OCR0). On using different OCR0values, Plaxis proposes different K0-values, butall K0-values are reset to their original value ofK0nc=0.703. For all values of OCR 0 a drainedcalculation is performed for a total time of1000 days, just to let the soil creep under itsself weight without additional loading. Theresults of these calculations are presented inFig. 1.From Fig. 1 it can be seen that there is aremarkable difference in settlement, which isparticularly caused by the initial inclination ofthe time-settlement curve, i.e. the initial creeprate. For OCR0=1.0 the initial creep rate is quiteunrealistic. Considering rather soft normallyconsolidated soils, it is quite realistic to havea settlement of 0.05 m a year, decreasing downto 0.01 m a year when the soil hasnt beendisturbed for a some years. In the abovesituation this is well reflected by the choice ofTable 1. Properties of arbitrary clay layer,OCR0=1.4. At t=1000 days the inclination of allmodelled with the Soft Soil Creep modelwet17***0.020.100.005c0.151.0K0nc26.00.0curves is almost the same, except for0.70OCR0=2.0, which would suggest that the actualOCRs have increased to almost the same value.kN/m3Evaluation of the actual OCR-values at t=100013PLAXISPLAXISdays gives OCR 1.8 for all cases, except forUsers ForumOCR0=2.0. In the latter case OCR has hardlyincreased.Question:I have some difficulties in generating the initialFrom the above results it could be concludedstresses in my project. My project is a tunnelthat an initial OCR-value of 1.4, to be used in200.0 m underground and is not including thethe K0-procedure, would be a good choice. Thisground surface; otherwise the model will becould indeed be said for the above exampletoo big. Therefore, the initial stresses at theand perhaps for other cases, but it cannot betop of the model are not zero and shouldstated in general. It is a good habit for ainclude the overburden above the top of thepractical application to simulate a certain creepmodel. Could you please give me a hint solvingperiod in a drained calculation withoutthis problem?additional loading and to evaluate the initialOCR-value. Please note that when using initialAnswer:OCR-values larger than 1.0, Plaxis will proposeIf the model becomes too large you canK0>K0nc,whereas for normally consolidatedeventually omit the upper part, but you havesoils it is recommended to reset K0=K0nc. If theto compensate for the missing soil weightresulting settlement velocity is too high andotherwise non-realistic stresses will bethe other parameters of the SSC model havegenerated. To include an overburden pressurebeen properly determined, then the initial valueyou have to do the following: Create a thinof OCR should be increased (or the modifiedlayer with thickness h at the top of your modelcreep index* should be reevaluated).representing the omitted soil (see Figure 1).The weight of the soil in the thin layer is higherIn conclusion, the initial OCR-value in the Softthan the omitted soil and equal to (Soil Creep model needs to be selected with(real * hreal / hvirtual. For example in Figure 1,care. In principle, the initial value depends on(virtualthe ratio between the initial preconsolidationthis way you can generate realistic initialstress and the effective in-situ stress (which isstresses by means of the K0 procedure.virtual== 18.0 * 130.0 / 1.0 = 2340.0 kN/m3. Ingenerally slightly larger than 1.0), but it shouldbe realised that the initial OCR-value alsodetermines the initial settlement velocity. Afirst estimate could be OCR=1.4, but it isadvisable to simulate the creep process andadapt OCR if necessary.RONALD BRINKGREVE,DELFT UNIVERSITY OF TECHNOLOGY& PLAXIS BVReferences[1] Brinkgreve R.B.J., Vermeer P.A. (1998),PLAXIS Finite Element Code for Soil andRock Analysis, Version 7, Part: MaterialModels Manual. Balkema, RotterdamFigure 1: Replacement of 130.0 m of soil by1.0 m of soil with a higher density.[2] Bjerrum L. (1967), Engineering geology ofNorwegian normally consolidated marineQuestion:clays as related to settlements of buildings.Why are the stresses of non-porous materialsGotechnique 17 (2), pp. 81-118.not shown and how can I visualise them anyway?14PLAXISPLAXISAnswer:Non-porous material is general used forstructural purposes like concrete, and stressescan therefore become quite large. As a result,stresses in the soil cannot be viewed in detail.By excluding the stresses in structuralelements, the soil stresses remain clearly visible.The stresses in non-porous material arecalculated though and can be viewed in tablesand cross-sections.However, there is a trick to view the stressesin non-porous material:Calculate the problem and if all the calculationsare finished and you want to have a look at thestresses in output then save the calculationdata by pressing the save icon. Then go directlyto Input and open the concrete material dataset. Change <non-porous> to <drained> andexit the material data set. Save the Input dataand go directly to Output by pressing theOutput icon. In Output the stresses in the nonporous material set are now visible. Output istricked in believing that the non-porousmaterial set is drained and shows the stresses.15PLAXISPLAXISACTIVITIES03-06 JANUARY, 200127-31 AUGUST, 2001Short course on ComputationalXVth ISSMGE International ConferenceGeotechnics (English)on Soil Mechanics and GeotechnicalOne day Dynamics course includedEngineering (XV ICSMGE)Berkeley, California, U.S.A.Istanbul, Turkey22-24 JANUARY, 200101-03 SEPTEMBER, 2001Course on ComputationalPost-conference event (XV ICSMGE)Geotechnics (English)Short course on ComputationalNoordwijkerhout, The NetherlandsGeotechnics (English)Istanbul, Turkey19-21 MARCH, 2001Course on ComputationalWINTER, 2001Geotechnics (German)Short course on ComputationalFinite Elementen Anwendungen in derGeotechnics (French)GrundbaupraxisPratique des lments finis enStuttgart, GermanyGotechnique20-22 MARCH, 2001Paris, FranceShort course on ComputationalGeotechnics (English)20-23 JANUARY, 2002Course on ComputationalKuala Lumpur, MalaysiaGeotechnics (English)26-28 MARCH, 2001Noordwijkerhout, The NetherlandsInternational course for experiencedPlaxis users (English)24-27 MARCH, 2002Noordwijkerhout, The NetherlandsInternational course for experiencedPlaxis users (English)Noordwijkerhout, The Netherlands02-04 APRIL, 2001Course on ComputationalGeotechnics (English)Numerical methods in GeotechnicalEngineeringEynsham Hall, Oxfordshire, United KingdomFor more information on these23-25 APRIL, 2001activities please contact:Short course on ComputationalGeotechnics (Arabic/ English)Plaxis bvCairo, EgyptP.O. Box 5722600 AN06-09 AUGUST, 2001DELFTShort course on ComputationalThe NetherlandsGeotechnics (English)Tel: +31 15 26 00 450One day Dynamics course includedFax: +31 15 26 00 451Boulder, U.S.A.E-mail: info@plaxis.nl16...
View Full Document

Ask a homework question - tutors are online