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  Bulletin of the PLAXISUsers Association (NL)PLAXIS bulletinP.O. Box 33022601 DH Delft,The NetherlandsE-mail:bulletin@plaxis.nl IN THIS ISSUE: Editorial   Column Vermeer   Engineering Practice   Plaxis Practice   Plaxis symposium   New developments   VACANCIES   Users forum   Courses   Agenda   P L X S Nº 7-JANUARY1999 Editorial Some people fear the next millennium. Weon the other hand look forward to anexhilarating 1999 and thriving futurebeyond the year 2000. The first International Plaxis Symposium has a similar title: Beyond 2000 in Computational Geotechnics . In this bulletin you will find the program and registration form. Thesymposium is preceded by the International course for Experienced Plaxis Users . Both events have been scheduled successively to give your trip extra merit.Other events are listed in the calendar onthe back of this bulletin, or on our web-siteat http://www.plaxis.nl. I n 1998 about 250 people signed up for oneof the courses, workshops or users meetingthat were organised in Amsterdam (2), Boston,Delft, Karlsruhe, Paris, Singapore, Stuttgart andTrondheim. Most events are in English, except for the courses in Stuttgart and Paris and theusers meeting in Trondheim which were in thenational languages. Participants and lecturers in French course (Paris). In September another memorable event tookplace. On this day, the Dutch Public Works andthe Indonesia Public Works (PU) signed aMemorandum of Understanding (MOU). ThisMOU describes a close cooperation on thesubject of Soft Soil Engineering. Directly aftersigning the MOU Mr. Hendro Moeljono (PUDirector General) accepted the first of ten Plaxislicenses for the Indonesian Public Works.Besides the delivery of software, an extensivetraining program is planned in the frameworkof the cooperation. Last October Plaxis BV celebrated their fifthyear of existence. Since it's establishment thecompany has grown substantially and is nowlooking for further expansion (see alsovacancies).In November Version 7.1 was released on CDand sent to all registered Version 7.0 users. Thepast year has been very successful as a resultof the release of the new Windows Version.This can also be seen from the users groupwhich has increased to well over 1000 people.For the next year and beyond 2000 newdevelopments are scheduled as described inan outline by Ronald Brinkgreve. Other plans for the near future include movingto a new office. Around summer time PlaxisBV will move to the city of Delft, the Dutchknowledge centre of civil engineering andnumerical analysis. Of course we will keep youinformed on further details. We hope you willenjoy reading this extra thick issue of the Plaxisbulletin and look forward to see you duringthe Plaxis Symposium or one of the otherevents.Editorial staff:Nisa Nurmohamed, chief editorEric Sluimer, chairman Plaxis Users AssociationPeter Brand, Plaxis bvScientific committee:Prof. Pieter Vermeer, Stuttgart UniversityDr. Ronald Brinkgreve, Plaxis bv 1  P L X S C olumn Vermeer O n re-reading my previous column on creepin oedometer tests, I am struck by the factthat geotechnical engineers have so manydifferent soil parameters. It would seem thatthere are two entirely different reasons for this.The first is that we are dealing with a complexcohesive, frictional material and that the formulation of its mechanical behaviour simplyrequires several different (model) parameters.The second reason is that different countrieshave different geotechnical traditions, includingdifferent soil parameters. Especially in the fieldof one-dimensional compression, we seedifferent traditions and different definitions ofsoil compressibility parameters. In the followingI will review some of them.For primary oedometer consolidation, the USAprovides us with the well-know compressionindex C  C  , as used in a 10-log compression law.From critical-state soil mechanics, as developedin the UK, we have an e -ln compression law,and consequently a slightly different indexcalled  . For the Soft Soil Creep model, thecompression law involves strain rather thanvoid ratio, and so we introduced the modifiedcompression index  *. The conversion rulesmay be simple, but there is still a considerablerisk of confusion. The essentials are: C  C  =compression index  = C  C  / 2.3  *=   / (1+ e ) E ref  =  p ref   /  * The logarithmic law for oedometriccompression can be differentiated and oneobtains for the tangent stiffness E oed =  /  *.Hence stiffness moduli are proportional to theapplied vertical stress. In geotechnical literaturedifferent symbols are used for E oed , e.g. M  or E S as used in German literature.In Plaxis 7, both E ref and  * are used as modelparameters for oedometer compressibility. TheSoft Soil Creep model uses * and the HardeningSoil model uses E ref  . For the sake of clarity, itwould have been much nicer to use the sameinput parameters for both models, but oldtraditions die hard. Definition of E ref  in oedometer test results. No doubt, the use of compression indices,such as C  C  ,  or  * relates to the use of alogarithmic compression law. It dates back topioneering work of Terzaghi 1925, and it is thusdeeply rooted in soil mechanics. Ten year later,however, the general applicability was alreadyrefuted by Ohde. The latter was the first topropose a more general power law of the form E oed =   m . During the second World WarOhde's German writings were more or less lost,but the power law was re- discovered byJanbu. The beauty of the power law is that itis accurate for all different types of soil. Testdata on sands, for example, tend to fit wellwhen using m   0.5, whereas soft soils requirethe exponent m   1. For m = 1, it can also beshown that the power law reduces to thelogarithmic law.Of course, the power law is much more generalthan the logarithmic law. Using m = 1, and E ref  /  *, it reduces completely to the logar-ithmic law. This does not mean that the entireHardening Soil model will become identical tothe Soft Soil (creep) model. In fact, agreementwill only be obtained for oedometer-typeloading paths. To properly compare and assessvarious soil models, it is necessary to considermodel responses in drained and undrainedtriaxial testing as well. And this could well bean interesting topic for another bulletin. P.A. Vermeer, Stuttgart University 2 oedoedoedoedoed  P L X S Engineering Practice On October 15 the Dutch Plaxis usersassociation held it’s annual workshop. The mainpurpose of these meetings is to upgrade users'modelling abilities and knowledge. This yearthe workshop’s theme was: Soil tests,parameters and constitutive models andattracted over 40 enthusiastic users. Questionsrelated to the theme were:- Which in-situ and laboratory tests arecurrently available ?- Which material parameters can be deter-mined from the available tests ?- Which constitutive models are most appro-priate ?In practice these questions will not appear inthe above sequence. Usually we start off withthe nature of the geotechnical problem:loading, retaining structure, excavation, etc.The key to a successful analysis is not a matterof finding the right buttons, but to my opinionto model the situation correctly.Let me explain. Modelling a situation meansrecognition of the essential geotechnicalphenomena and including these in a numericalanalysis. This requires theoretical insight fromthe user:- How does (real) soil behave under differenttypes of loads ? - What types of stress-paths are significant inmy project ?- Which constitutive model is capable ofcapturing the soil behaviour for the dominantstress-paths in my project ?Only after being able to answer the abovequestions we can make an appropriate decisionas to which material parameters are neededand wich soil tests should be performed.Therefor, the order of themes of the workshopshould have been: Constitutive models,parameters and soil tests .Unfortunately the above ideal situation is notcommon practice. For standard engineeringprojects, site investigation is usually limited.However, when dealing with projects thatinclude a considerable risk for failure ordamage to the surrounding area, engineersneed to take their responsibility. Thisresponsibility includes , the creation of extrabudget for thorough soil investigation andanalysis !The workshop proved that continuousattention is needed on the subject: models,parameters and soil tests , not only for theuser, but also for the Plaxis developers. Whatis the point of investing in more advancedconstitutive law’s if the user cannot benefit from these developments. From thisperspective the organizers of the ExperiencedUsers Course (15-17 March 1999) will focus theattention on practical usage of advancedmodels. To this end I would recommendpeople to participate in this course ! Eric Sluimer, chairman PLAXIS UsersAssociation (NL) PLAXIS Practice THE VAASA TRIAL EMBANKMENTGeometry, instrumentation andconstruction: In [1] a trail embankment in Vaasa (Finland) waspresented. The main purpose of the con-struction was the testing and development ofthe calculation methods for the planning ofroad embankments. The trial embankment hasa rectangular shape (fig. 1). The height of theembankment is 2 metres and the gradient ofslopes is 1:1.5. The width of the embankmentis 15 m. and the length 25 m. The instru-mentation of the trial embankment is alsopresented in fig. 1. 3  P L X S Figure 1 Geometry and instrumentation of Vasaa trial embankment. Settlements at the base level of theembankment are measured with settlements plates, and deeper in the ground withmagnetic extensometer. The pore pressuresare measured using open and closedpiezometer tips installed at different depths.The construction of the embankment took 3days. Furthermore, extensive laboratory soiltests using the triaxial apparatus and theoedometer have been done in order todetermine the soil parameters. Figure 2 FEM mesh. Ground conditions: The subsoil consists of more than 40 m. thicklayer of soft silty clay with high organic contentand sulphur. The clay layer is homogeneousand most of the laboratory investigations areconcentrated only to 15 m. depth. The groundwater level is 0.5 m below the srcinal surface.The parameters used for the calculations areobtained from the results of oedometer tests. They are presented in table 1 and table 2. Calculations: For the calculations with the FEM code Plaxisthe Soft Soil Creep model has been used.The Vaasa embankment and the subsoil havebeen schematised using the mesh presentedin figure 2. The mesh consists of 6-nodetriangular elements. In [1] three node elementswere used.The calculations have been done in plane strainconditions. The staged construction option inthe Plaxis code has been used. At first thestress distribution due to the body weight ofthe sub-soil is calculated (drained). In the following calculation step the load due to theweight of the embankment has been appliedin an undrained state, followed byconsolidation and creep. In the Soft Soil Creep model computations theinput parameters listed in table 1 have been 4
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