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Horisontella jorddeformationers påverkan på ett pålgrundlagt brofundament i lös lera: Baserat på finita elementanalyser i en tredimensionell geometri
2012 (Swedish)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

During excavation work and other soil unloading effects, the stress state in the soil will change. The change of stress state will lead to deformations in the remaining soil. The walls will move towards the centre in a circular excavation and the bottom will lift a bit. When excavation/unloading takes place around a foundation, it is more difficult to forecast the deformations. Most difficult is to foresee how much the moving soil will displace the foundation.More complex and advanced problems could be suitable to analyse in commercial FE-software with a two dimensional interface. The use of finite element simulation in two dimensions are limited, since it only can handle models that are either axisymmetric or in a plane-strain. During 2010, PLAXIS introduced a new FE-software for three dimensional analyses. This program, PLAXIS 3D, is based on pure three-dimensional modelling, which for instance can be used to model piled foundations.The soil mechanical engineers at Norconsult wanted to investigate the applicability of PLAXIS 3D on modelling foundations in soil. That is also the reason behind writing this master thesis work and its objective is to model the horizontal deformations in loose clay in a project where the excavation is finished and the deformations are measured. The project is placed in Lärjeholm north of Gothenburg and includes a long excavation under two newly built railway viaducts. Some of the foundations of the railway viaducts are placed in the slopes of the excavation. The main issue in the project has been to forecast how much the moving soil would displace the viaduct foundation. In the forecast that was done it was presumed that the foundation would not move more than the surrounding soil, which is a very conservative assumption.In the thesis it is investigated whether it, with help from the software PLAXIS 3D, is possible to model the soil deformations and the displacements of the foundations. The model is verified by comparing the model deformations with the measured deformations in the project. In the thesis, it is also evaluated when it may be preferable to use a 3D-model instead of a 2D-model.The result from the FE-analysis shows that the soil deformations in a slope are largely affected if there is a piled foundation placed in the slope. The horizontal deformations decrease in between the foundation and the top of the slope. Since not enough cases has been compared, it cannot be verified how the magnitude of the calculated deformations correlate with the measured ones. The modelled displacements in the foundations are a slightly too small compared to the measured ones in the project. However, the models give anyway a good view of the principle of soil deformations during unloading around foundations.Modelling in a three-dimensional interface is much more time consuming than to work in a two-dimensional interface, both when it comes to building the model as well as when analysing the model and computing time. The accuracy in a three-dimensional model compared to the two dimensional model is theoretically decreasing since the mesh has to be coarser. Despite all drawbacks, there are a lot of soil mechanical problems that have a three-dimensional nature and can’t be solved with a two-dimensional model. When deciding which kind of model that should be used, the two-dimensional model should always be the first option. If it’s not possible to solve the problem with a two-dimension model, a three-dimensional model should be used.

Place, publisher, year, edition, pages
2012. , 78 p.
Keyword [en]
Life Earth Science
Keyword [sv]
Bio- och geovetenskaper, Horisontella deformationer, brostöd, FEM, PLAXIS 3D, lös lera
URN: urn:nbn:se:ltu:diva-53380Local ID: a66a201e-212b-411b-a54a-5c23215f8fd6OAI: diva2:1026754
Subject / course
Student thesis, at least 30 credits
Educational program
Civil Engineering, master's level
Validerat; 20120229 (anonymous)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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