Nº 8 - JULY 1999

Editorial Editorial staff: Nisa Nurmohamed, chief editor We are very pleased to present this special Marco Hutteman, Plaxis Users Association (NL) edition of the PLAXIS bulletin. Besides the Peter Brand, Plaxis bv normal contents part of this bulletin is devoted to the PLAXIS symposium held last Scientific committee: March in . As requested by Prof. Pieter Vermeer, Stuttgart University participants the opening speech by Dr. Ronald Brinkgreve, Plaxis bv Margriet Janzs and a publication on creep by the Dutch Ministry of Public Works and Water management are published. In addition, a summary of the discussions is provided. Column Vermeer FIELD SITUATION WITH OCR ≈ 1 he first international PLAXIS symposium was Ta big success, nearly 100 participants from Very young soil deposits have experienced 20 different countries participated. The only vertical stresses due to self weight, Experienced Users Course directly after the without additional deformations due to conference was very well attended with 40 previous overburden or ageing, i.e. creep. Bulletin of the participants. Such soil may be truly normally PLAXIS Users Association (NL) consolidated with an over-consolidation Other courses included a PLAXIS short course factor of nearly one. PLAXIS bulletin P.O. Box 3302 in Egypt, and the course “Introduction to To my experience such situations are rare 2601 DH Delft The Computational Geotechnics” organised as a and even alluvial deposits are mostly E-mail: post-conference activity to the XIIth “European characterised by a pre-overburden pressure, [email protected] Conference on Soil Mechanics and Foundation either caused by real pre-loading, ageing Engineering” in Amsterdam. or structuring. This may be true for natural IN THIS ISSUE: soil deposits with a geological age, but it Editorial The course in Egypt, presented in both Arabic does not hold for man-made soil deposits. and English, showed that the PLAXIS program Extremely young soil layers stem for Column Vermeer is used more and more globally. This is again instance from human mining activities. Plaxis Practice shown by the overwhelming interest at the Mining remnants of all soils have been PLAXIS booth on the exhibitions at the created. They range from gravely slugs to New developments Amsterdam conference and the 3rd National sandy soils up to soft tailings, as created Recent activities Conference of the Geo-Institute in Illinois, after treatments of iron ore.

Plaxis Version 7.11 U.S.A. he first tailing problem I got involved in New Plaxis office All the above shows that the PLAXIS company Tconcerned a deposit with a thickness of Agenda 20 is on the move, which brings us to the last about 30 m over an area of some square topic. The PLAXIS company will move to a kilometres. On top of it there was nothing like SPECIAL: Plaxis symposium new office situated in Delft. More information a crust so that one could not walk on it. The The prehistroy of Plaxis on the office location is provided in this undrained shear strength simply vanished at Experiences with a viscoplastic model bulletin. the surface; it increased with depth according Summary of discussions

1 ≈ to rule Cc 2/4 , as may be expected from a model (SS-model), but in this case I had no soil with OCR ≈ 1.0. The clayey silt was classified preference. Indeed, our problem happens to by the parameters as listed in Table 1: be dominated by one-dimensional compression and both models will follow the Table 1 Soil parameters logarithmic compression law. Instead of *, as 2 Saturated weight ≈ 17 kN/m used as an input parameter for the SS-model, ∝ Porosity n ≈ 60 % the HS-model requires: Plasticity index I ≈ 40 % σ p ref ref Eoed = = 1000 kPa ≈ –9 λ* Vertical permeability kf 10 m/s ≈ Compression index CC 0,6

at least when choosing σ ref = 100 kPa. The These are averaged values as the deposit was HS-model also requires the input m = 1 as far from being homogeneous. Together with otherwise it does not yield the logarithmic other such “moorlands” the area had to be compression. The question rises whether or made stable and passable by adding a granular not we could have analysed that problem by fill on top. Calculations were not just needed means of the simple Mohr-Coulomb model for estimating the settlement, but also for (MC-model). Indeed, one might select a predicting the period of consolidation. In fact constant value of the stiffness modulus such the amount of water being squeezed out was that the final settlements comply to analyses important as the cleaning of this water would with more advanced models. The present entail substantial costs. To this end oedometer problem, however, is not only on the final tests were carried out and for the oedometer settlements, but also on the settlements module it was found that: during intermediate construction stages. The σ σ advanced models predict quite correctly E = d '= ' oed dε λ* relative large settlements for the first loading stage and relatively low settlements, for the with: C final stage, as indicated in Fig. 1. In order to λ* = c ≈ 0.1 (1+e)1n10 simulate such a phenomenon by using the MC- model, one would have to select appropriate This linear increase of the soil stiffness with stiffnesses for each stage of loading. No doubt stress, implies a nice logarithmic compression a cumbersome way of proceeding. law, as may be expected for soft soils. For many problems, especially excavation P.A. Vermeer, Stuttgart University problems, I would use the Hardening-Soil model (HS-model) rather then the Soft-Soil

PLAXIS Practice FINITE ELEMENT SIMULATION OF THE SOIL-STRUCTURE INTERACTION FOR A NAVIGABLE LOCK

Hanover-Anderten, Uelzen and Suelfeld locks are the corner points of the crest water race of the ‘Mittelland’ and ‘Elbeseiten’ channels in Germany. At present Sülfeld represents a needle eye for the ship passage between Hamburg, Figure 1 Comparison of the Time-Settlement Dortmund or Hanover and Berlin, which have curves between MC-model and advanced models. to be kept independent of the water level in

2 Elbe river [1]. The southern Suelfeld lock has to structure and the old existing constructions. be adapted to the rising traffic and to a new ship generation. For this purpose a modern and Lock structure efficient structure needs to be constructed on The length of the new lock is about 345 m, the old lock place, however on new location. including the inlet and outlet structures. The chamber walls are separated in 11 blocks (each block has a length of 15 m). Two or three blocks have a common foundation plate. For water saving purposes two stepped economizing basins will be constructed beside the lock, which will save about 50% of the water pumping demand. The foundation is located at + 46.4 m MSL, more as 20 m below ground surface in upper water area. The first economizing basin is founded at + 56.7 m MSL and the second at + 58.9 m MSL (Figure 1).

Soil profile Figure 1 The Suelfeld lock. 1 old lock; 2 new lock; retaining wall of the A characteristic soil profile of the site is northern economizing basin; northern economizing basin; 5 new economizing basin; 6 diaphragm wall; 7 anchors presented in Figure 1. The bedrock consists of Lias sandstone layers and marls of Malm age, The new structure consists of blocks, founded separated by a 40 - 50 m wide fault system, on massive concrete plates. Each plate which runs nearly perpendicular to the lock supports several blocks and is separated by axis. The location of the faults and the joints from the adjacent plates. The finite geometry of the rock surface was investigated element prognosis of the soil - structure using geophysical methods [2]. While the 2 behavior plays a decisive role in the design excavation pit is very large (about 27000 m ), process for two reasons: the soil conditions the impermeability of the diaphragm wall in are highly non-uniform and the evaluation of the disturbed rock area and the possibility of the settlements produced by the fluctuating water leakage through faults pose a major water level in the lock and in economizing problem for the construction. The influence basins controls the design of the joints. A series of a possible fault blocks movement on the of 2D and 3D simulations performed by BGS structure behavior was also simulated using Hanover and BAW is under way. The aim of this 3D nonlinear finite element analyses. paper is to present the results of some 2D The bedrock is overlain by overconsolidated finite element analyses performed using PLAXIS glacial deposits: clayey silt, sand and glacial till. Version 7.1 in order to evaluate the structure For the evaluation of the sand layer properties, loading and the interaction between the new correlations with the results of heavy dynamic penetration tests were used. The cohesive layers, silt and glacial till, were characterized using laboratory tests on undisturbed samples (12 oedometer and 18 CIU triaxial tests). Since the soil is inhomogeneous a two step calibration procedure was used: (1) statistically averaged parameters were obtained from the tests results and (2) the tests were back-analyzed and compared with the measured behavior of selected samples. The fitting was fairly good Figure 2 Calibration of lab tests on glacial till; a CIU triax; b oedometer for the significant strain domain (Figure 2).

3 This ‘common sense’ approach seems the lock, an active earth pressure is mobilized

reasonable, but some work needs still to be in the upper part of the backfill, a quasi-K0 done in order to reduce the uncertainties distribution in the middle and a silo effect related to the selection of calibration samples appears in the lower part. The distribution is and specially related to the safety reserve of controlled by the relative wall stiffness. When this procedure. The soil parameters presented the lock is filled with water, the walls deform in Table 1 are characteristic values, unaffected outwards and press the upper backfill area, by safety factors. inducing consequently an increase of earth pressure. This cyclic loading, which occurs at Results each ship passage, produces a supplementary The computation had simulated the whole compaction of the fill, which can be easily loading history: geological history, construction recognized in CPT profiles [3]. of the old lock, excavation, construction of the new lock and water loading. Only some Bending moments of the diaphragm selected results will be presented here. wall Figure 4 shows the computed bending moments of the diaphragm wall for two excavations phases: Phase 2, excavation up to 53.5 m MSL, upper anchor prestressed, lower anchor inactive and Phase 3, final excavation, both anchors active and prestressed. For 1 comparison the analytical calculated moment

Figure 3

Earth pressure distribution A main objective of the analysis was to obtain the distribution of the loads necessary for the design of structural elements. Figure 3 shows the earth pressure distribution on the new chamber wall. If no water pressure acts inside Figure 4 Bending moments of diaphragm wall Table 1 Soil parameters used in Plaxis simulations

ref ref ref v Soil Model _ c ψ ur EE50 E 50 Eoed v mRf [kN/m3][°] [kPa] [°] [MPa] [MPa] [MPa] [MPa] [MPa] Fill MC-model 20.0 30.0 0.1 0 10 - -- 0.30 - - Till HS-model 21.5 26.6 13.8 1 - 9.0 18 6.1 0.29 0.6 0.95 Sand MC-model 20.5 37.5 0.1 7.5 100 - -- 0.20 - - Silt HS-model 21.5 33.4 25 3 - 8.5 30 16 0.25 0.8 0.9

4 distribution is presented in the same figure. Conclusions The agreement between the results in the - PLAXIS is a very good tool to analyze complex upper part of the wall is reasonable good. The interaction problems. lower part of the analytical distribution is - The choice of soil parameters using statistical affected by the hypothesis used about restrain averaged values of lab tests and back conditions of the wall bottom. Indeed the computation of selected samples provides 2 analytical code (in standard manner ) searches a reasonable approach, but must be tested a restrain depth in order to obtain equilibrium, in order to evaluate the safety reserves. without taking into consideration the actual - If the boundary conditions are correctly non-classical wall-soil deformation. defined, analytical procedures furnish a good The analytical computed displacements estimation of the structural stresses, but represents only the structural displacements severely under-estimate the displacements. of the wall, neglecting the general - A good estimation of the soil-structure displacement field of the soil and subsequently interaction can be obtained using PLAXIS, but severely under-estimating the actual behavior. the choice of the constitutive law and the simulation of the complete loading history Displacement of existing structures play a decisive role. A critical point for the open pit design was 1 the behavior of existing adjacent economizing using the Blum theory, code AllPlus of basins belonging to the northern lock, which Nemetschek AG, Munich, Germany 2 must remain operational during the it is possible to use tricks in order to construction of the new lock. Special attention obtain the true restrain depth, but in this was paid to possible deformations of this case for comparison purposes a standard structure. In this case the soil parameters formulation was used were affected by safety factors. A large relative displacement between the retaining structure Literature: and the bottom of the economizing basin [1] Preser, F. & al. 1997. Ersatz der Sparschleuse was obtained (Figure 5). Consequently a Sülfeld-Süd. Binnenwasserstrassen; vol. 19, special constructive solution is to be used in Oct 1997 order to preserve the impermeability of the [2] Merkler, G.-P. & Hannich, D. 1997. structure. Schriftliche Darstellung der geophysikalischen Messergebnisse im Bereich der Schleuse Sülfeld. Univ. Karlsruhe, Rep. to NBA Hanover, Karlsruhe, Germany (unpublished) [3] Schwab, R. & Kayser, J. 1999. An A-type prediction for a deep excavation near an existing navigable lock. Proc. Int. Symp. Beyond 2000 in Computational Geotech., R. Bringreve ed., Amsterdam 18-20 March 1999, Balkema, Rotterdam

Frank Preser, Neubauamt fuer den Ausbau des Mittellandkanals (NBA), Hannover, Germany, e-mail: [email protected] Radu Schwab, Bundesanstalt fuer Wasserbau (BAW), Karlsruhe, Germany, e-mail: [email protected] Figure 5 Displacements of existing retaining wall

5 New Developments this way, the user is not bothered by features that have no relevance to the type of problem The PLAXIS Research & Development team he/she is dealing with. Nevertheless, the basic is working hard on the release of two concepts of the 3D modules, like the input additional modules for Version 7: A of soil layers, material modelling, mesh dynamics module and a 3D tunnel module. generation and staged construction are all Underneath a short description is given on similar to the 2D program. This makes the step these new features. into 3D modelling as simple as possible, without very strong restrictions on modelling Dynamics module features. The dynamics module enables 2D calculations The first 3D modulel that will be released is a in which inertia and damping are included in module for tunnel problems. With this 3D the system of equations: tunnel module it is possible to analyse bored . tunnels in soft and hard soils and as well as M ü + C + u + K u = F NATM tunnels in hard soils and soft rock. A preliminary release of this module is planned In this equation M is the mass matrix, C is in the second half of 1999. Other 3D module the damping matrix, K is the stiffness matrix for excavations and foundations are planned and F is the force vector. A single dot above in the near future. the displacement vector u indicates the time . derivative. Hence, u is the velocity vector and Ronald Brinkgreve, Plaxis BV ü is the acceleration vector. In a static calculation the velocity and acceleration components are absent. Dynamic forces usually lead to shear and compression waves in the soil. These Recent activities temporary varying stress increments may cause damage on surrounding structures. With COURSES the dynamic module one can apply different In addition to the traditional courses on kinds of dynamic loads (impacts, harmonic Computational Geotechnics, other courses loads or user-defined time-loading schemes) were organised in recent months. A ‘course and analyse the influence of the dynamic for experienced PLAXIS users’ was organised forces on surrounding structures. Applications in the same week as the PLAXIS Symposium may be found in the analysis of heavy traffic (15-17 March). In this course attention was loads (trucks and trains), pile driving, focused on the use of advanced soil models earthquakes, blasting, explosions, etc. A for practical applications. New case studies preliminary release of the dynamic module is were prepared in order to learn participants planned in the second half of 1999. how to deal with complex situations. The annual German course ‘Finite Elementen 3D tunnel module Anwendungen in der Grundbaupraxis’ was also When going from 2D to 3D modelling, organised in March. Over thirty participants from calculations generally become very difficult to Germany, Switzerland and Austria attended this handle. In order to keep the 3D modelling course in Ostfildern, near Stuttgart. simple, Plaxis has chosen for an approach In May, a course on Computational Geotechnics based on 2D cross sections, as explained in was organised in Cairo, Egypt. Also this course previous bulletins. A further simplification for was well attended by participants from Middle users can be achieved by considering specific East countries. types of applications and providing appropriate In addition to further developments of the modelling features in separate modules. In PLAXIS program, teaching and training is a

6 major activity of the PLAXIS group. Other Water Management (right) watches while Prof. courses that are scheduled for this year are in Verruijt opens the manual. preparation (see Agenda). An important issue for the Chinese Ministry of Water Resources is their concern for the stability and deformations of river embankments, for which they can now use PLAXIS.

PLAXIS Version 7.11 AVAILABLE ON THE INTERNET

Figure 1 Participants in the course for After the release of PLAXIS Version 7.10 in experienced PLAXIS users December 1998, some small improvements have been made. These have resulted in a free SINO-DUTCH SEMINAR ON WATER update version 7.11, which is currently available MANAGEMENT on the internet at www.plaxis.nl (‘user In the framework of a Chinese-Dutch services’). cooperation in the field of Water Management, a strong delegation, including members of the French and German documentation and Dutch Royal Family, visited Bejing, China on help files April 13 and 14. During one of the sessions As a service to our French and German users, Prof. Arnold Verruijt of Delft University (middle) the most important part of the full Plaxis handed over a PLAXIS Version to Dr. Dong manual, the Reference manual has been Zheren, Director General of the Department of translated (see picture). Additionally, the Help International Cooperation, Science and facilities in the program have been extended Technology of the Ministry of Water Resources to cover the English, French and German of The People’s Republic of China (left). The language. During the installation of Plaxis, a delegation leader Dr. G. Blom, Director General selection of these three languages can be of the Dutch Ministry of Public Works and made.

Figure 1 Dr. Dong Zheren (left), Prof. Verruijt (middle) and Dr. Blom (right) during the Sino-Dutch seminar. French and German Reference manuals.

7 Sheet-pile database institutes, which gives new opportunities for Another service to users is the availability of future developments. These and some other the material properties for the ProfilARBED advantages will enable improved service to the sheet-pile product range. We have asked users. several manufacturers of sheet-pile walls to provide data for their product range and While ‘old’ telephone numbers will remain expect to include the HOESCH product range active for a while, the new coordinates of in the next update, others may follow. The pre- PLAXIS BV are: defined properties can be loaded in the Global database. First select “Beams” and than use Correspondence: the “Open” button to select the arbed.DB file. PLAXIS BV P.O. Box 572 The updated programs are compressed in a NL-2600 AN DELFT 6.5 MB file. In order to download the file, the The Netherlands password PLX7UPD is required. The file is self- extracting, so that while running it will Visitors: automatically install the update programs in PLAXIS BV the Plaxis directory. BTC building Delftechpark 26 NL-2628 XH DELFT The Netherlands

PLAXIS office moves Communication: Tel: +31 15 2600 450 to Delft Fax: +31 15 2600 451 E-mail: [email protected] From 1993 on, when PLAXIS BV was founded, the office was located in Rhoon, The E-mail addresses will remain the same, Netherlands. On July 19th PLAXIS BV will move however, during the moving process, our their office to a new location in Delft, the Dutch Internet mail server must be disconnected for knowledge centre for civil engineering a short period. We ask for your understanding. research. The new location is the BTC building that accommodates a number of small to medium size high-tech companies (see picture). The main reason for moving to Delft is that it provides better contacts with Delft Figure 1 BTC Building in Delft, new location University of Technology and other research of PLAXIS BV office

8 PLAXIS BV searches for - High level of accuracy and efficiency - Good communication skills in English new staff members - Team spirit

PLAXIS BV is a growing high-tech company VACANCY 2: Programmer with experience in computational geomechanics, mainly with visual programming tools developing, marketing and supporting the PLAXIS finite element program for soil and Job description: rock analyses. Our future developments This job involves the creative development of a require new specialists to enter the new Windows user-interface for three- Research & Development team. At the dimensional finite element models. Most of the moment, PLAXIS is situated in Rhoon, near work is concentrated around 3D modelling and Rotterdam, but we will soon move to a visualisation, with convenience and new location in Delft, the Dutch knowledge userfriendliness as major keywords. Besides centre of civil engineering and numerical programming, the job involves contacts with analysis. Being market leader in Europe we users to discover the needs for further plan to become world leader in the developments. modelling of soil and rock problems. The company searches for two new talented Requirements: staff who are eager to help us reaching this - Completed professional education in a technical challenging goal. Working at PLAXIS BV direction; affinity with civil engineering means interesting teamworking on high- - Thorough experience with a visual tech software products in a stimulating programming tool (preferably Delphi) environment. - High level of accuracy and efficiency - Good communication skills in English VACANCY 1: Specialist on computational - Team spirit geomechanics APPLICATION: Job description: Inhabitants of the EC can apply freely for a job This job mainly involves the scientific in The Netherlands. Applicants outside the EC development of PLAXIS modules. The work is are subjected to formal governmental concentrated around numerical procedures for procedures. To apply for one of the above 2D and 3D finite element analyses, with positions at PLAXIS BV, please send your letter robustness and reliability as major keywords. of application including Resume (CV) to: Besides in-house research work the job involves cooperation with colaborating research centers PLAXIS BV and high-level user support on complex Attn: Dr. Ronald Brinkgreve engineering projects. P.O. Box 572 NL-2600 AN DELFT Requirements: The Netherlands - Research experience (PhD) in the field of Tel: +31 15 2600 458 computational geomechanics Fax: +31 15 2600 451 - Thorough programming experience in E-mail: [email protected] FORTRAN (90)

9 PLAXIS Symposium

The first PLAXIS Symposium entitled "Beyond 2000 in improvement of constitutive models, provided they are Computational Geotechnics - Ten Years of PLAXIS robust and easy to use, is necessary in order for PLAXIS to International" was held on 18-20 March, 1999, in keep contact with geotechnical research and to keep its high- Amsterdam, The Netherlands. Nearly hundred tech reputation. participants from 20 countries attended the Symposium In addition to the presentations and discussions, there was (see picture). time for social and cultural events, like the reception in the Amsterdam Historical Museum, the boat tour through the Amsterdam canals (see picture), the Symposium banquet and the excursion to the Zuiderkerk and the IJmuiden locks. Due to the success of this Symposium it was concluded that a PLAXIS Symposium could be organised on a more or less regular basis, say every five years.

Participants at symposium

he presentations were divided over six theme sessions: TGeneral geotechnical aspects, Ten years of PLAXIS international, Dams and embankments, Tunnels and deep excavations, Suburban and infrastructural works, Education and research. In addition, PLAXIS applications and related Boot tour through Amsterdam canals during Plaxis symposium research were presented as posters and discussed during the breaks. Symposium Proceedings Margriet Jansz presented her contribution, "The Prehistory All papers from the presentation and poster sessions are of Plaxis", a general story on the development of Geotechnical available in hard-bound proceedings and on CD-ROM (included Engineering in the Netherlands. This paper is enclosed in this in the book), which was distributed to the participants at the bulletin. In the special celebration session "Ten years of PLAXIS beginning of the Symposium. international", invited speakers gave their view on the past, Those who could not attend the present and future of PLAXIS. Dr. Harvey Burd focused on the Symposium and other interested early developments of PLAXIS at Delft University and the people can order the cooperation with the Dutch Ministry of Public Works and proceedings directly from Water Management, which have set a good basis for success. Balkema Publishers, P.O. Box Professor Steinar Nordal described the current status of 1675, 3000 BR Rotterdam, PLAXIS (Windows version) for practical design and education. The Netherlands, He expressed the wish that PLAXIS may contribute to a more fax: +31 10 413 5947 unified approach of geotechnical engineering over the world. http://www.balkema.nl Professor Vermeer emphasized on a further development of PLAXIS in the direction of '3D specials', in particular a tunnels special and a foundations special. Moreover, a continuous Symposium proceedings

10 THE PREHISTORY OF PLAXIS

A Story about Geotechnology in The Low Countries by Although many of the 'terpen' have disappeared, because Margriet Jansz, Technology Foundation STW: "It is a great farmers early this century found the soil to be very fertile pleasure to be here today to celebrate 10 years of Plaxis and used it to improve their fields, many can still be found International with you. The three remaining speakers of in the northern part of this country. Nowadays there are the afternoon will take us through the history of Plaxis, special programs to protect this evidence of early 'soil review its present - of which you all are part - and lead engineering'. us into its future - which we all hope to be glorious, of course. The next big geotechnological step in the Low Countries - That leaves prehistory to me. Against which background about 900 years ago - was the introduction of the dike to did it all happen? keep the water away from fields and living quarters. By building dikes to connect the 'terpen', the enclosed land o, please follow me back in time - let's have a look at the was also protected from high water ... and the '' was SDutch and their soil. As you undoubtedly know by now, invented. our soil is predominantly soft and soggy - if you will allow such a sloppy qualification from a relative outsider. Anyway, I am sure my forebears, who lived not too far north of here, would have agreed with my description. Building your home on such a soil can be a risky business, especially in a low-lying country where the water levels of the rivers and the sea are unpredictable. Accordingly, the early settlers looked for higher and dryer spots to live on. Unfortunately for them such spots were not always available where they wanted them to be, Fig. 2 Invention of the 'Polder' where the hunting and fishing were best for example. In the early days a polder was just a piece of land surrounded And here - we are talking about some 2500 years ago - I and thus protected by dikes. Later, windmills were used to proudly introduce the first Dutch geotechnician: instead of keep water levels low in the . This turned out to be looking for a high spot to build on, he decided to make one. very successful and the next logical step was to put dikes around a lake, remove the water and thus reclaim a piece of land. Windmills were used to pump the water out. By using windmills in series a considerable difference in level could be overcome.

A substantial part of the countryside around Amsterdam has been reclaimed in this way. The first of the large-scale reclaimed areas is the Zijpe. Around the middle of the 16th century several attempts to put dikes around the Zijpe (no. 1 in the map, which shows the situation around the year 1700) failed. The 4th try however, 1596-1597, was successful. In the Fig. 1 The first Dutch geotechnician following century many more polders were reclaimed (nos. 2 to 8). The details of this event are lost, alas, but the idea became popular. The practice of building farms, and - at a later stage It must have become clear to you now that geotechnology - whole villages on artificial hills - called 'terpen' or 'wierden' in this country was continually pushed forward by man's need - spread and was in use for many centuries. to keep the water in check - as far as that was possible. The

11 Middle Ages were particularly trying for the inhabitants of the low lands because of a rise in sea level. In a true meaning of the word this is 'reclaimed' land. These lakes were actually formed in historical times and, although they grew with each flooding, the memory of them as dry land was still alive.

Fig. 3: The Northern Netherlands around 1700.

The construction of another large-scale geotechnical work in this area was also Fig. 4: The Dam in Amsterdam (18th century). Still there today are the Palace and the Old Church. The prompted by the sea. In 1421 harbour, however, has been pushed out of the city center. a flood destroyed a considerable part of the In the following centuries, Amsterdam grew to be a dunes, the natural protection commercially very successful city and the harbour played an against the sea. In place of important role in its success. By the 17th century, however, the destroyed dunes a dike the harbour and the access to it silted up more and more. In system, the 'Hondsbossche Zeewering', was constructed. It 1772, finally, a committee (a good old Dutch institution) came still protects the land. up with a daring plan: to make a direct connection between But, as you can see, by the year 1700, most of the lakes in the IJ and the North Sea by digging through the dunes. This, the northern part are dry. Still open, though, are the IJmeer however, was considered too dangerous and as an alternative and the . the 'Noordhollands' canal to Den Helder was dug between 1819 and 1824. Unfortunately, the new canal did not solve The IJmeer, at that time, was of great importance for the city the problems and in 1862 it was decided to dig through the of Amsterdam, or more precisely, for its shipping activities. dunes after all. The history of this city, by the way, starts with geotechnical activity as well, as is preserved in its name: Amsterdam, The canal, the necessary sluices and locks and also the Aemstelledamme, a dam in the river Amstel, constructed conversion of much of the IJmeer into polders were finished towards the end of the 12th century. in 1876. For a long time those sluices have been the largest Much farmland in this area was lost in the 12th century floods in the world; they are now being renovated and those of you and the population turned to other means of support, such who join us for the excursion on saturday will visit them. as crafts, shipping and fishing. The construction of the dam, and later of sluices, turned the mouth of the Amstel into a But back to the 19th century, and to the other large body of natural harbour. This was situated at the Damrak, just across water still open, the Haarlemmermeer. This lake had a bad the street from here. Primarily, Amsterdam was a fishing port, reputation. In 1591 and again in 1611 a number of villages but gradually other trades developed, and these became the disappeared completely in its waters; many ships were source of the city's wealth. wrecked, especially in the north-eastern part. The lake was nicknamed the Waterwolf, its north-eastern corner the den An important milestone was the beer toll - the right to levy of the wolf, 'hol' in Dutch. taxes on all imported beer - granted to Amsterdam by the The first plan to reclaim this land and to tame the lake were Count of Holland in 1323, the most important privilege the published as early as 1641 by Jan Leeghwater, an engineer city ever managed to secure. By 1369 one third of the entire involved in the reclamation of several other areas. 166 Hamburg beer exports were shipped to Amsterdam. windmills were to be used to pump the water out of the lake.

12 But it was not considered feasible. There were many other eventually a special institute was created to combine plans, but no action was taken until after 1836, when the experimental and analytical methods and to integrate the 'Waterwolf' actually threatened the cities of Amsterdam and many basic principles with the treasure of practical knowledge. Leiden. The same change towards a more scientific approach took place at Delft University. An elevated canal was built to encircle the lake and the water was pumped into it. From the canal the water was then led 1918 was an important year also because of the decision to to the sea. Steam-driven pumping stations were used for the start reclaiming the largest body of water, the Zuiderzee. The major part of the job, an important innovation. One of those first dike, to the island of Wieringen, was begun two years pumping stations, the 'Cruquius' can still be visited today. It later. In spite of the economic crisis, the government decided took about 4 years to finish the job. to let the program continue with full force and in 1930 the first polder was dry. The enclosing dam, the 'Afsluitdijk', was There are many places in the Netherlands where the completed two years later: the 'Zuiderzee' became the waterlevel of a canal is higher than the neighbouring land. 'Ijsselmeer'. Practically the whole western half of the country lies below sea level. Water is continually pumped into canals like this and Three large polders have been reclaimed since then. One of on towards the sea in order to keep that land dry. the towns in the new land was called after the engineer who had presented the first plans for closing the Zuiderzee: Those of you who came to this meeting by plane have actually Lelystad, named for Cornelis Lely. already visited the reclaimed Haarlemmermeer. Holland's first and its largest airport today is situated in the north-east corner Another disaster, the great flood of 1953, prompted the of the 'Haarlemmermeerpolder' - Remember? Where most decision to shorten the coastline in the south-western corner ships were wrecked? And that is how it got its name 'Schiphol' of the country. 1800 people dead, 100.000 evacuated, 47.000 - the hole into which ships disappear. Fortunately, that usually damaged houses of which 9000 totally demolished and about does not apply to airplanes. 8% of the total country flooded by the sea provided a powerful incentive for our politicians to make available the Fig. 5: Failed railway necessary amount of money. embankment near Weesp, 1918 Since this is a quite recent project with many At the beginning of this geotechnically interesting century, in spite of all the aspects, I assume that most achievements geotechnology of you are familiar with it. in this country was still more of a craft than a science - primarily Fig. 6: Map of The Netherlands a matter of trial and error, even though many fundamental principles were already known. I hope that I have been able to show to you that there is some After all, Coulomb had published his essay on the behaviour truth to the saying that God made the world but the Dutch of soil as early as 1776. Unfortunately, it required a dramatic made Holland, and that the geotechnicians were key players. event to change the Dutch situation. In 1918 a railway This is the background, against which the development of embankment, weakened by heavy rains, failed. Plaxis started and it is with great pleasure that I leave it to the next speaker to talk about that." This disaster led to a call for more systematic study of soil mechanics. At first we had a committee, of course, but Margriet Jansz, Technology Foundation STW

13 EXPERIENCES WITH A VISCOPLASTIC CREEP MODEL FOR THE ENGINEERING PRACTICE.

For the reconstruction of an embankment near the town phenomena. At that moment the decision was made to stop of Alblasserdam traditional analysis using Bishop slip circle further reconstruction. analysis and FEM calculations with Plaxis has been done. The comparisons were done to explain the discrepancy 2 Finalizing the construction. between measured and calculated excess pore pressures. As stated before, the development of excess pore pressures FEM calculations were used to predict wether the design during the reconstruction of the embankment could not be will give a stable construction and to predict the correct explained by the loading condition and the available data of time scheme for finalizing the reconstruction. The the soil properties. discrepancy between measured and calculated excess So the question still remained why the proposed loading pore pressures were mainly due to the use of the scheme for the reconstruction could not be applied as had (regional) material properties within the Bishop analysis. been planned. At the mid of 1997 the height of the The permeability has a major impact on the creep embankment at Alblasserwaard Noord was 1.3 metre lower behaviour. Further research with respect to this aspect than the adjacent sections of the river embankment. To finalize will be necessary. the reconstruction of the embankment three clay layers of 0.4-0.5 m. should be placed on the current embankment (1997). RECONSTRUCTION OF THE ALBLASSERWAARD In order to prescribe a safe time scheme for finalizing the EMBANKMENT reconstruction and to be sure it would give a stable construction, it was decided the calculations will be done 1 Introduction using traditional (Bishop) slip surface analysis as well as finite In 1985 the reconstruction of the dike near the town element analysis with the Plaxis Soft Soil Creep model. In order Alblasserwaard was started. Because it was not possible to to find the appropriate material data, local soil investigation reconstruct the dike on the polder side, it was decided to had been carried out, thus extending the information from reconstruct the embankment at the riverside. The design the regional database with local material data). For the current of the reconstruction was done using Bishop slip surface situation as well as each loading stage (3) the stability of the analysis using data from a regional material database. At the embankment will be calculated; finally the long term stability riverside, minestone was placed on the bottom of the river will be determined. The results from the Bishop slip circle (NAP -5.429 m) until the top of the old embankment (NAP analysis are compared with the FEM calculations. - 0.729 m.) was reached. In 1990 clay was deposited on top of the minestone in 4 equal layers of about 0.5 m. During 3 Material data the reconstruction the development of settlements and For the calculations section 22 and two basic geometries of porewater pressures were monitored using settlement the embankment have been considered: the initial geometry plates and piezometers. In 1991 and 1993 additional layers before the minestone layers has been applied and the settled of clay were deposited. Due to the high water pressures in geometry based on the geotechnical profile which has been the sub-soil it was not possible to reach the desired level of determined by CPT tests recently done. See fig. 1. From NAP +4.7m within the scheduled period of time. In order several boreholes samples for lab testing has been taken. to speed up the consolidation process vertical drains over a length of 25 metre had been installed. At first the drains The strength parameters for the soft layers (Tiel clay, Holland had a positive effect on the consolidation process. But on peat and Gorkum clay light and Gorkum clay heavy) have been applying the next layers of clay, monitoring showed that determined by cell tests. The strength parameters for the there was a enormous build up of excess pore pressures. other layers has been determined using the regional database The increase of the excess pore pressures could not be of the Alblasserwaard. The parameters for the determination explained by the loading condition as shown by a of the creep and settlement behaviour have been found by consolidation analysis. It is assumed that the increase of the oedometer tests. For the material properties used in the FEM excess pore pressure could be explained by the creep calculation, see table 1.

14 The strength parameters conforming were chosen stress dependent using so called tau-sigma diagrams. For the current situation the degree of consolidation has been calculated. To determine the long term stability a consolidation analysis shows the hydrodynamic period after applying the last three load stages will be 60 years. Using Bishop slip analysis the stability factors shown in table 2 have been calculated. Fig. 1: Geotechnical profile of Alblasserwaard embankment The critical slip surface for the final stage (long term stability) is shown in fig 2.

Table 1. Material properties Table 2: Calculated safety factors 4 Bishop slip circle analysis The Bishop slip circle analysis has been done using the characteristic values for the strength parameters c’ and ’ from cell tests, which have been defined by the 5% fractile (5% lower limit of the student-t distribution). This way the results from the slip circle analysis gives the upper limit for te excess pore pressures and deformations.

The loading conditions considered for each analysis were:

1) current situation - recently measured pore water pressures; - using degree of consolidation; - mean river level (NAP +0.5 m); Fig 2: Critical slip circle, long term stability sf=1.227 2) stability during construction - pore water pressures monitored during construction; 5 Plaxis FEM calculations - using degree of consolidation; One of the reason for doing Plaxis FEM calculations is to find - mean high river water level (NAP +0.5 m); an explanation for the measured high excess pore pressures 3) end stability in the sub soil. So for the Plaxis FEM settlement calculations - fully drained condition; (incorporating creep and consolidation) the mean values for - trafic load; the strength parameters have been used (thus giving the - low level of the river water (NAP -0.5 m.) expected value for the excess pore pressures and deformations). Furthermore, due to the expected large

15 deformations, also triaxial strength parameters have been circle analysis described in paragraph 3.4. derived from cell test strength parameters for the Plaxis For the current situation, through trial and error, the calculations. In this case the ratio between the cell test values permeability of the soft soil layer are adjusted until the and the triaxial tests values for the strength parameters is calculated excess pore pressures are in good agreement with about 1.6. the measured ones, see fig. 3. This implies multiplication of the permeability with 6. The following analysis has been done: With the adjusted permeability and using the mean triaxial 1) Plaxis simulation with expected (mean) cell test values for strength parameters, Plaxis creep calculations have been the strength parameters c’ and ‘ using the soft soil model made and are compared with the Bishop slip circle analysis, (Cam Clay). This simulation shows that the development see table 3 and fig. 3. of excess pore pressures are overestimated with 1-2 m., with a maximum of 4 m. The current profile is not stable. The calculated settlements are smaller than the measured one; 2) Plaxis simulations with from cell tests derived (mean) triaxial strength parameters c’ and ‘ using the soft soil model. The calculated excess pore pressures are less overestimated as in simulation a); 3) Plaxis simulation with the Soft Soil Creep model with mean values for the triaxial strength parameters. The calculated excess pore pressures are in reasonable agreement with the measured values. The calculated settlements are smaller than the measured settlements. The current profile is not "stable". The creep model gives higher effective stresses (thus lower excess pore pressures) than the Cam clay model, as stress paths analysis shows. Fig. 3: Plaxis safety factors

In order to finalize the reconstruction 3 layers of clay should The values for triaxial strength parameters used in the Plaxis be placed on the current embankment. The Plaxis FEM "triaxial" calculations were 1.6 time higher than the cell test calculations have been extended in order to predict the values for the strength parameters. Therefor it can be current stability, the stability during construction and the long expected that the stability factor found with those calculations term stability. The results are compared with the Bishop slip will 1.68/1.6≈1. Plaxis calculations shows a safety factor of 1.1. It seems there is a discrepancies between the Bishop slip circle analysis (sf=1.415) and Plaxis calculations of the current situation using and c cell test cell test values. So for the long term situation additional Plaxis calculations have been made using fully drained conditions and using both the Soft Soil model and the Mohr Coulomb mode, see fig. 4a. Those calculations have been compared with additional Bishop (circular) slip surface analysis as well as a non circular slip circle analysis according Spencer, see tabel 4 and fig 4b.

Table 3: Plaxis vs Bishop safety factor

16 1.108. This results is in agreement with the Plaxis results in calculations 1) and 2) from table 4, see fig 5.

Table 4: Slip surface analysis vs Plaxis analysis Fig. 5: Plaxis, fully drained Soft soil model, for the long term stability _-c interpollated, safety factorª1.08

6 Conclusion The discrepancy between the measured excess pore pressures at Alblasserwaard Noord and the expected one from the Bishop analysis is mainly due to the difference in material parameters used in the Bishop calculations (regional material properties) and the material properties local present. The Fig 4a: Failure mechanism after -c reduction sf=1.08 Plaxis simulations show the permeability has a major impact in the behaviour of the embankment. Further research with respect to the influence of the permeability to the creep process is recommended. The difference between the safety factors found by the Bishop analysis and the safety factor found with Plaxis simulations can be explained by the difference in failure mechanism. The failure mechanism is non circular as the Plaxis simulations have shown. Choosing the circular slip circle results Fig 4b: Spencer, non circular slip surface in a overestimation of the safety factor. The long term stability factor (using the strength parameter from cell tests) is 1.08. For the non-circular slip analysis the same failure mechanism The design complies with the regulation in the Dutch code as has been found in the Plaxis calculations has been used. of practice. From results 3) and 4) in table 4 the conclusion can be made The Plaxis creep simulations, using the given material that the choosing the right failure mechanism reduces the properties, unexpectedly show lower excess pore pressures safety factor with 0.119. If the safety factor in the Bishop slip than the Plaxis simulations with the soft soil model (without surface analysis is also reduced with the same amount, a creep). corrected Bishop safety factor can be found of 1.227-0.119=

17 REFERENCES sophisticated model including probability would be convenient 1 Vermeer P. et al, 1991. PLAXIS, finite element code for soil in cases with a low safety factor. A full probabilistic analysis and rock plasticity, AA. Balkema would require extensive and reliable data. Two different 2 Brinkgreve et al, 1997, Backgrounds on numerical approaches were employed in the calculation of the safety modelling of geotechnical structures, part 2, Gouda, CUR factor, as proposed by Eurocode 7. One increasing the load publication 191. and another reducing the strength of the foundation. This 3 Den Haan E.J., Vertical compression of soils, thesis Delft second approach is similar to the PLAXIS -c reduction method. University of Technology, Delft University Press, ISBN 90- 407-1062-7. - Modelling of Soil/Slurry wall interaction of Diemerzeedijk, 4 Bjerrum L., 1967. Engineering geology of Norwegian Amsterdam normally-consolidated marine clays as related to the The bending moments in the bentonite wall were determined settlements of buildings. Geotechnique Vol 17, pg. 81-118. by imposing a curvature and applying a stress-strain 5 Vermeer P.A. et al. From the classical theory of secondary relationship for the bentonite. The stiffness of the wall was compression to modern creep analysis. Proc. Of the ninth not relevant to determine the horizontal displacements. For international conference on computer methods and the soil, a Mohr-Coulomb model has been used with a advances in geomecha-nics, Wuhan/China, Balkema R’dam, reduced shear modulus in order to obtain realistic vol. 4. deformation. A soft soil model would have been advisable 7 Van, M.A., Reconstruction Alblasserwaard North I, Report here. CO-356330/51, Delft Geotechnics. 8 Spierenburg, S.E.J., Case study Alblasserwaard North I, - Numerical study of a Double Cut and Cover Tunnel. Report 379210.20, Delft Geotechnics. A double cut and cover tunnel was modelled with PLAXIS, in order to determine the loads and spring constants, to be B.H.P.A.M. The; R.J. Termaat; P. Blomaart. Ministry of used for structural analysis. It can be concluded that, in fact, Transport, Public Works and Water Management, Delft, there is no need to use a structural model with springs, since The Netherlands all the structural information is provided by PLAXIS. In order to find the band of the possible moment distributions, several loading situations have to be considered. SUMMARY OF DISCUSSIONS - 3D modelling of tension piles. Presentations at the PLAXIS Symposium were organised A numerical calculation needs good data in order to provide in 6 theme sessions, with 4 to 5 presentations in each us with valuable information. The use of only CPT results is session. The program allowed for 5 minutes of discussion not enough to determine the soil parameters. Direct soil tests after each presentation and ten minutes of final are necessary. But even with little data, FEM calculations can discussion at the end of each session. Underneath an provide better results and give more insight in the problem overview is given of the main points raised during the than conventional calculations. The q-c method requires a discussions. For a description of the contents of the safety factor of 3, which could be lowered by using a more presentations, reference is made to the Symposium accurate model. proceedings (Beyond 2000 in Computational Geotechnics. R.B.J. Brinkgreve (editor), Balkema, 1999). Theme 2: Ten Years of PLAXIS international. - Past, present and future of PLAXIS. Theme 1: General - Finite element analysis for the safety evaluation of a dam Future PLAXIS developments will involve 3D modules for on a fractured rock foundation. specific applications. With respect to foundations, there are The safety factor presents a scale problem. A more discrepancies among the results obtained by classical solutions

18 for raft, pad, strip foundations, etc. A 3D FE model could situation. It is usefull to use different models and compare embrace all the classical models and close the gaps between the results. One should first focus on patterns of behaviour them. rather than on magnitudes. A combined foundation with footing and piles is a very complex problem; this will take some time to be implemented Theme 4: Tunnelling and deep excavations. in PLAXIS. - Modelling tunnel behaviour for soft soil conditions: Models for structural analysis and monitoring results. Theme 3: Dams and embankments. It is very difficult to model the pressures exerted on a tunnel - Finite Element Back analysis of loaded MSW Vertical cut. lining due to grouting. A 2D model is just a rough approximation. The analysis could be improved with a 3D model. A waste deposit presents a - 3D modelling of bored tunnels. scale problem, because there are elements of considerable A new kind of interface could improve the modelling of the size included. Also biological processes are taken place. cement-bentonite fill. Therefore these waste deposits are not conventional soils, and can not be modelled easily. - Monitoring and numerical analysis of tunnels in complex geological conditons. - Bank stability considering sand boil and excess pore water The hardening soil model can predict with good accuracy the pressures. displacements in unloading situations. This cannot be achieved The rapid lowering of the water level in a waterway does not with the Mohr-Coulomb model. change the internal pore water pressures immediately. There is a delay due to the fact that there is air included in the water. Theme 5: Suburban and infrastructural works. The pore water pressure remains constant and the effective - Finite element method and multi-anchored walls for stresses diminish. This is contrary to the common knowledge, excavations in urban areas. which considers the water incompressible and the load being An elastic model with a constant elasticity modulus equal in transmitted instantaneously to the water. loading and unloading cannot be accepted for the modelling This phenomenon has also been measured in the Scandinavian of soil behaviour. shoreline due to the changing tide. - Finite element analysis of basement construction at the - Back analyses of a staged embankment failure: The case British Museum. study Streefkerk. It is better and cheaper to use pressiometer test results than It is difficult to match the calculated pore pressures with the using results of triaxial tests, provided that the operators measured ones. An adjustment of permeabilities could make performing the pressiometer test are experienced. the results in better agreement. It may also be the case that the pressures from the piezometers differ from the actual Theme 6: Education and research. pressures. Summary of discussion: With a soft soil creep model it is more difficult to backcalculate - The possibility to develop a PLAXIS version for scientific the parameters of soil than with a Mohr-Coulomb model, purposes is being considered. because the parameters are interrelated. - To get a better understanding of the Soft Soil Creep model, very simple cases with drained soil should be calculated. Summary: - In the case of an excavation problem, good results that Calculation is not longer a problem. The problem is the match the field measurements quite well, were obtained determination of parameters. For some models, a change of with the hardening soil model. the parameters such that there is a direct link with - There is no need to create new models, but there is a need geotechnical engineering parameters can improve the for improving the existing models in some respect.

19 ACTIVITIES:

9-11 AUGUST, 1999 18 NOVEMBER, 1999 Short course on Computational Geotechnics 3rd Norwegian Users meeting (Norwegian), (English), Oslo, Norway Boulder, U.S.A. 17-19 NOVEMBER, 1999 1-3 SEPTEMBER, 1999 Short course on Computational Geotechnics NUMOG VII, Technical University Graz, Austria (French), web-site: ‘Pratique des éléments finis en http://www.cis.tu-graz.ac.at/geotechnical_gr Géotechnique’ oup/numog.html Paris, France E-mail: [email protected] 24-26 JANUARY, 2000 SEPTEMBER, 1999 Standard course on Computational Plaxis workshop (English), Singapore Geotechnics (English), Noordwijkerhout, the Netherlands 29, 30 SEPTEMBER, 1 OCTOBER, 1999 Short course on Computational Geotechnics 20-22 MARCH, 2000 (English), Short course on Computational Geotechnics Surabaya, Indonesia (German), ‘Finite Elemente Anwendungen in der 4-6 OCTOBER, 1999 Grundbaupraxis’, Short course on Computational Geotechnics Stuttgart, Germany (English), Bandung, Indonesia 27-29 MARCH, 2000 International course for experienced Plaxis OCTOBER, 1999 users (English), Workshop of Dutch Plaxis Users Association Noordwijkerhout, the Netherlands (Dutch), Delft, the Netherlands For more information on these activities 11-12 NOVEMBER, 1999 please contact: 8st European Users meeting (English), Karlsruhe, Germany Plaxis bv P.O. Box 572, 2600 AN Delft 15-17 NOVEMBER, 1999 the Netherlands Short course on Computational Geotechnics Tel: +31 15 26 00 450 (English), Fax: +31 15 26 00 451 Trondheim, Norway E-mail: [email protected]

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