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Institute for Soil Mechanics and Foundation Engineering, Computational Geotechnics Group, Graz University of Technology Garber and Dalmatiner, Consulting Engineers

Abstract. A constitutive model formulated within the framework of the multilaminate concept for soils is presented. It is shown that anisotropic material behaviour is easily accounted for with these types of models. The potential of the formulation to predict plastic volumetric strains purely caused by rotation of principal stress axes is demonstrated. Furthermore the model is able to capture the formation of shear bands due to an enhanced strain softening formulation including both frictional and cohesive softening behaviour. Due to a simple regularisation technique mesh independent results are obtained with sufficient accuracy for practical purposes. The latter feature will be highlighted by solving the practical problem of a shallow tunnel excavation constructed using the principles of the NATM. It is shown that the development of plastic shear strains leading to a failure mechanism that involves shear banding is realistically predicted by the proposed formulation.

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Introduction

Constitutive modelling is still an issue in geotechnical engineering, in particular if solutions to complex practical problems have to be found under the financial and time restraints usually associated with this kind of work. Although a large number of sophisticated constitutive models has been proposed in the literature more advanced formulations are still not routinely employed for solving practical problems. The reason for this is either that these models are sometimes very difficult to understand and therefore not accepted in practice or that the computational effort is too high or both. However, as commercial software packages include more advanced models together with fast solution algorithms the advantages of powerful constitutive models over simple linear elastic - perfectly plastic models in predicting the deformation behaviour of soils and rocks under working load conditions and in defining possible failure mechanisms are increasingly appreciated in practice. By doing so the limitations of higher order models also become obvious because it is found that some deficiencies in modelling soil behaviour remain not only from a scientific but also from a practical point of view. Many important features of soil behaviour are still not included in most of the advanced models available in commercial software, possibly with a few exceptions. These include G. Beer (ed.), Numerical Simulation in Tunnelling © Springer-Verlag/Wien 2003

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H.F. Schweiger, H. Schuller anisotropic behaviour, small strain stiffness behaviour, the behaviour of partially saturated soils, the effects of rotation of principal stress axes, formation of shear bands including post peak behaviour.

There is a strong need therefore to formulate constitutive models for practical engineering which at one hand include all relevant features of soil behaviour and on the other hand are not too