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The influence of pre-failure soil stiffness on the numerical analysis of tunnelling

Abstract:

Models the geometry of the twin Jubilee Line Extension tunnels beneath St. James's Park. By considering the predicted surface settlement, the study shows the importance of modelling nonlinear elasticity, and the effect of introducing a soft independent shear modulus. The differences in subsurface displacements for isotropic and anisotropic models are highlighted. It is concluded that anisotropic parameters appropriate to London Clay do not enhance the plane strain predictions of ground movement as long as nonlinear pre-failure deformation is being modelled; that a soft anisotropic shear modulus improves greenfield predictions but not twin tunnel predictions; and that accounting for load reversal effects does influence an analysis of this problem. (Abstract quotes from original text) The use of the finite-element method to analyse tunnels is becoming more widespread, but any prediction is dependent (among other things) on the model adopted for the pre-failure soil behaviour. This paper compares and contrasts plane strain predictions of ground movement for both single- and twin-tunnel excavations in stiff clay modelled as (a) isotropic linear elastic perfectly plastic (b) anisotropic linear elastic perfectly plastic (c) isotropic non-linear elastic perfectly plastic with shear stiffness dependent on deviatoric strain and mean effective stress, and bulk modulus dependent on volumetric strain and mean effective stress (d) anisotropic non-linear elastic perfectly plastic employing the model in (c) above (e) isotropic non-linear elastic perfectly plastic with shear and bulk stiffness dependent on deviatoric strain level, mean effective stress, and loading reversals. The analyses model the geometry of the twin Jubilee Line Extension Project tunnels beneath St James's Park (London, UK), and field data are presented for comparison (Standing et al., 1996). By considering the predicted surface settlement, the study shows the importance of modelling non-linear elasticity, and the effect of introducing a soft independent shear modulus. The differences in subsurface displacements for isotropic and anisotropic models are highlighted. The subsequent modelling of an adjacent tunnel excavation exposes more detailed features of all the models. It is concluded (a) that anisotropic parameters appropriate to London Clay do not enhance the plane strain predictions of ground movement as long as non-linear pre-failure deformation behaviour is being modelled; (b) that a soft anisotropic shear modulus significantly improves greenfield predictions but not twin-tunnel predictions; (c) and that accounting for load reversal effects does influence an analysis of this problem (St James's Park twin tunnels).

Authors:

Addenbrooke, T. and Potts, D.M. and Puzrin, Alexander

Index Terms:

geomechanics; GeomechanicsGroup; Addenbrooke, T.; Potts, D.M.; Puzrin, Alexander

Further Information:

Date published: 1997