Finite Element Modelling of Shear Band Formation in Soils Applying Multilaminate Framework and Homogenisation Technique

Currently, an increasing number of tunnels with low overburden is constructed according to the principles of NATM (New Austrian Tunnelling Method), even under very difficult geotechnical conditions. If these tunnels are analysed with respect to possible failure mechanisms or the deformation behaviour near failure, the formation of so-called shear bands in the ground has to be considered. Shear bands, also referred to as strain locali-sation, are zones in the ground where shear strains concentrate when the bearing capacity is approached. Therefore, for numerical analyses of tunnel excavation with low overburden an efficient numerical model for simulating the deformation behaviour of the ground involving shear band formation is highly welcome. It could improve the prediction capabilities of numerical methods significantly and contribute, thus, to a safer and more economic design of tunnels with low overburden. In this research project a Multilaminate model was applied within the framework of the finite element method. The constitutive relations for describing the soil behaviour are formulated on a certain number of so-called contact planes which are located within each integration point of the finite element mesh. The stress state is transformed onto each plane and evaluated on the plane using a yield criterion and a plastic potential function. The resulting plastic strains on the planes are integrated to obtain the global material behaviour. Depending on the stress state, plastic strains can develop on certain planes while other planes remain inactive. For numerical simulation of shear band formation this can be utilised in combination with a constitutive law employing deviatoric hardening plasticity and strain softening. Another advantage of the Multilaminate model if compared to most constitutive models formulated in terms of stress invariants is the potential to respond to rotations of the principal stress axes. The version of the Multilaminate model developed in the course of this research project has been applied to numerical simulations of NATM tunnel excavations with low overburden. Both, construction sequences with full face tunnel excavation and staged excavation sequences (top heading, bench, invert) have been analysed. Possible collapse mechanisms during excavation of the bench were investigated as well as the influence of soil parameters, the initial stress state and the overburden on the deformation behaviour of the tunnel and the failure mechanism. It can be concluded that the Multilaminate model developed within this research project is able to capture the formation of shear bands in the ground. Thus, possible failure mechanisms of tunnels with low overburden due to shear band formation can be analysed efficiently with the numerical tools developed.