| Summary: | NGM 2016, 17th Nordic Geotechnical Meeting, Challenges in Nordic Geotechnic, Reykjavik, ISLANDE, 25-/05/2016 - 28/05/2016 Finite element simulations of the construction of shallow tunnels tend to provide poor predictions of the settlements induced at the surface. More precisely, the settlement trough width is generally overestimated by numerical simulations. Many parameters have an influence on the numerical results. In this paper, attention is focused on the constitutive law. Two classic methods of settlement predictions are introduced; a plane strain simulation with the load reduction method and a three dimensional model of conventional tunnelling with numerous excavation steps. After showing some particularity of stress state in soil in the case of a shallow tunnel, we develop a dedicated non-linear elastic law concentrating on the decrease of soil stiffness with increase of deviatoric stress. Indeed an increase of this stress invariant during excavation associated with a stability of the mean effective stress have been shown in the vicinity of the tunnel section especially for 3D modelling. The proposed formulation leads to an initial stiffness that does not depend on depth, which allows testing only the influence of the deviatoric stress variations, a feature which is generally renowned to have a significant influence on the settlement trough width. The influence of the differences between stress paths in the two dimensional and three dimensional simulations is precisely discussed as well as the determination of the stress relaxation factor in 2D modelling. Finally, it appears that, despite the differences in the redistribution of stresses, using a model in which stiffness decreases as the deviatoric stress increases does not make it possible to improve the prediction of settlements both in 2D or 3D. Les simulations par éléments finis du creusement des tunnels peu profonds tendent à donner de mauvaises prédictions des tassements en surface. La largeur de la cuvette de tassement est généralement ...
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