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Effect of Anisotropy on Tensile Stresses at the Bottom of a Base Course in Flexible Pavements
3D Finite Element Analysis of a Deep Excavation in Monaco
Issue 29 / Spring 2011
Table of Contents
ColophonAny correspondence regarding the Plaxis Bulletin can be sent by e-mail to:
or by regular mail to:
Plaxis Bulletinc/o Annelies VogelezangP.O. Box 5722600 AN DelftThe Netherlands
The Plaxis Bulletin is a publication of Plaxis bv and is distributed worldwide among Plaxis subscribers
Editorial board:Wout BroereRonald BrinkgreveErwin BeerninkArny Lengkeek
Design: Jori van den Munckhof
For information about PLAXIS software contact your local agent or Plaxis main office:
Plaxis bvP.O. Box 5722600 AN DelftThe Netherlands
Tel: +31 (0)15 251 7720Fax: +31 (0)15 257 3107
The Plaxis Bulletin is the combined magazine of Plaxis bv and the Plaxis users association (NL). The bulletin focuses on the use of the finite element method in geotechnical engineering practise and includes articles on the practical application of the PLAXIS programs, case studies and backgrounds on the models implemented in PLAXIS.
The bulletin offers a platform where users of PLAXIS can share ideas and experiences with each other. The editors welcome submission of papers for the Plaxis Bulletin that fall in any of these categories.
The manuscript should preferably be submitted in an electronic format, formatted as plain text without formatting. It should include the title of the paper, the name(s) of the authors and contact information (preferably e-mail) for the corresponding author(s). The main body of the article should be divided into appropriate sections and, if necessary, subsections. If any references are used, they should be listed at the end of the article. The author should ensure that the article is written clearly for ease of reading.
In case figures are used in the text, it should be indicated where they should be placed approximately in the text. The figures themselves have to be supplied separately from the text in a vector based format (eps,ai). If photographs or scanned figures are used the author should ensure that they have a resolution of at least 300 dpi or a minimum of 3 mega pixels. The use of colour in figures and photographs is encouraged, as the Plaxis bulletin is printed in full-colour.
04 New Developments
05 PLAXIS Expert Services Update
06 Effect of Anisotropy on Ten sile Stresses at the Bottom of a Base Course in Flexible Pavements
10 3D Finite Elements Analysis of a Deep Excavation in Monaco
14 Recent Activities
www.plaxis.nl l Spring 2011 l Plaxis Bulletin 3
Since the release of the new PLAXIS 3D program last summer, more than 300 licenses have been sold. It seems that many users have been waiting for an easy-to-use true three-dimensional modelling environment for their complex geotechnical applications, and that PLAXIS 3D accommodates their requirements.
In this bulletin a first practical application using PLAXIS 3D is described by some early users. In addition to another interesting article by PLAXIS users, this bulletin also describes some recent and future activities. In fact, the list of activities is growing, in line with the steady growth of the Plaxis organization itself, which enables us to serve you even better. For more information see the end of this bulletin.
The first users contribution involves a study on the effect of anisotropy on tensile stresses in a granular base for flexible pavements using an axi-symmetric finite element model. Anisotropic stiffness in the granular base was modelled using the elastic part of the Jointed Rock model. Different anisotropic stiffness properties were used to investigate the influence on the tensile stresses. It was concluded that anisotropy can lead to a reduction in the tensile stress in the granular base layer. A recommendation was given to allow for a wider range of Poissons ratios to be selected for anisotropic materials.
The second users contribution, as mentioned before, involves a three-dimensional finite element model of a deep excavation with adjacent buildings in Monaco, with the purpose to analyse the deformations as a result of the excavation process. The excavation is retained by diaphragm walls which are supported by several rows of anchors in different directions. The article gives a
detailed description of the complex situation and shows some interesting model sections, including a global view of all the anchors. It was concluded that the new PLAXIS 3D program is quite capable and efficient to model this complex situation and was able to produce satisfactory results.
In addition to the contributions by PLAXIS users, the New Developments column describes a new dimension which the Plaxis research team is working on: the modelling of thermal flow and thermo-hydro-mechanical coupling. This enables temperature to be taken into account in the analysis of soil behaviour and soil-structure interaction. The bulletin also describes the positive experiences of a company with a dedicated in-house training course in the framework of PLAXIS Expert Services.
All together we trust to have composed another interesting bulletin for you. Do not hesitate to send us your comments or contact the corresponding author in case you like to discuss some items. We wish you an interesting reading experience and look forward to receiving your contributions for future issues of the Plaxis Bulletin.
4 Plaxis Bulletin l Spring 2011 l www.plaxis.nl
where temperature could play a role, and it would be convenient if you could take this into account within your PLAXIS analysis.
The development of THM coupling is still a research project. At the moment we are working on a 2D research version and later this year we will make a full 3D implementation. However, it will take some time before this feature will become available in future PLAXIS versions. Nevertheless, by moving into this new dimension we are confident to provide you with the right tools to solve your future geo-engineering challenges as well.
A further step is now taken by adding temperature as a degree-of-freedom in the finite element calculations and considering thermal flow in the soil as a result of temperature differences. The first aim is simply to calculate a change of the temperature distribution in the soil by thermal flow and diffusion, and to impose this on a mechanical model of the soil structure with the purpose to calculate thermal expansion or shrinkage (using the thermal expansion coefficients of the respective materials). In this case, thermal flow and deformations are semi-coupled. Another situation to be considered is the effect that temperature differences may cause convective groundwater flow to occur, whereas the groundwater itself will carry heat (advection) and will change the temperature distribution of the ground. Here, a full coupling between thermal flow and groundwater flow is required. The ultimate goal is to include a full coupling between thermal effects, hydraulic effects and mechanical effects, including phase transition of the pore medium (ice water vapour). This so-called thermo-hydro-mechanical (THM) coupling allows for very special situations to be analysed, such as the sustainability of nuclear waste deposits in underground repositories.
Considering more common civil engineering applications, there are several examples in which temperature and the coupling with the mechanical behaviour plays an important role, such as: Cyclic loading effects on soil-structure inter-
action as a result of day/night temperature changes.
Soil freezing as a mitigation method to stabilize tunnels and excavations.
Stability of slopes considering precipitation and evaporation using temperature-dependent water retention curves.
Geotechnical engineering in permafrost areas. Efficiency and sustainability of geothermal en-
ergy systems (borehole heat exchangers, heat/cold storage, energy piles)
Not all these examples are as complicated as the term THM coupling would suggest. Not in all cases a full coupling is necessary. We will implement simplified calculation options to avoid unnecessary complexity. If you think about it, you may find an example from your own experience
The full coupling between deformations and changes of pore pressures as a result of undrained loading and/or changes in
hydraulic conditions, has become available with the release of PLAXIS 2D 2010, last autumn. The new Advanced calculation
mode allows for complex flow-deformation analyses to be performed, taking into account unsaturated soil conditions.
Ronald Brinkgreve, Plaxis bv
Figure 1. Temperature distribution from THM Analysis
www.plaxis.nl l Spring 2011 l Plaxis Bulletin 5
TPS had several people requiring training on the modeling of deep excavations using PLAXIS 2D and more particularly in the case of propped cantilever retaining walls. The intention of TPS was to give a number of their geotechnical and civil engineering graduates a course with a balance between an overview of PLAXIS 2D and its modeling capabilities together with a worked example requiring practical hands-on computer