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This paper is an attempt to apply the Palmer–Rice fracture mechanics approach to the shear band propagation in sands and normally consolidated clays. This approach, proposed 30 years ago for overconsolidated clays, had a tremendous advantage of treating a shear band evolution as a true physical process and not just as a sufficient mathematical condition for its existence. Extension of this approach to a wider variety of soils requires for non-elastic soil properties (e.g. isotropic hardening plasticity, strain softening, lack of tensile strength, dilatancy, active and passive failure modes, etc.) to be taken into account. This paper demonstrates how the energy balance and process zone approaches can be applied to the simple problem of the shallow shear band propagation in an infinite slope built of such a soil. The energy balance approach appears to be the most conservative one. It allows for catastrophic and progressive types of soil failure to be properly identified, and dramatically effects the results of the slope stability analysis.

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). L'analyse des tunnels a de plus en plus souvent recours à la méthode des éléments finis, mais toute prévision dépend (entre autres) de la modélisation du comportement des sols avant la rupture. L'article compare les mouvements de sol de tunnels simples et doubles creusés dans de l'argile rigide, sous l'effet de déformations planes, prédits dans les modèles suivants (a) isotrope, linéaire, élastique, parfaitement plastique (b) anisotrope, linéaire, élastique, parfaitement plastique (c) isotrope, non linéaire, élastique, parfaitement plastique, la rigidité au cisaillement variant en fonction des contraintes déviatrices et de la contrainte intergranulaire moyenne, et le module de compression variant en fonction de la contrainte hydrostatique et de la contrainte intergranulaire moyenne (d) anisotrope, non linéaire, élastique, parfaitement plastique, en utilisant le modèle cidessus (c) (e) isotrope, non linéaire, élastique, parfaitement plastique, la rigidité au cisaillement et la rigidité à la compression variant en fonction des contraintes déviatrices, de la contrainte intergranulaire moyenne et des contraintes subies récemment. Les analyses modélisent la géométrie des tunnels doubles du prolongement de la ligne de métro Jubilee Line sous St James's Park, et présentent des données relevées sur le terrain à titre de comparaison (Standing et al., 1996). En examinant le tassement de surface prédit, l'étude montre l'importance de la modélisation de l'élasticité non linéaire, et l'effet d'un module de cisaillement tendre indépendant. L'article fait ressortir les différences de déplacements souterrains des modèles isotropes et anisotropes. La modélisation subséquente de l'excavation d'un tunnel adjacent met en évidence des aspects plus détaillés de tous les modèles. L'article conclut (a) que les paramètres anisotropes propres à l'argile londonnienne n'améliorent pas les pré- visions des mouvements de sol sous l'effet de déformations planes dans la modélisation des déformations non linéaires avant la rupture; (b) qu'un module de cisaillement anisotrope tendre mène à de meilleures prévisions dans le cas de nouveaux chantiers, mais pas celui de tunnels doubles; et (c) que l'historique des contraintes récentes compte peu dans l'analyse de ce problème (tunnels doubles de St James's Park).

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Catastrophic landslides pose significant threats to life and property, and in the case of submarine landslides damage to offshore infrastructure. Although widely discussed, the triggering mechanisms and propagation criteria for catastrophic failure remain open issues. This study investigates a particular case of shear band initiation history: creation of a fully softened initial failure zone in a thin lens of a weaker material causing catastrophic failure of an infinite planar slope in sensitive clay under undrained conditions. The corresponding shear band propagation criteria were derived analytically using a process zone approach and validated numerically using a static large-deformation finite-element method. New analytical solutions taking account of elastic deformations within the shear band and in the entire sliding layer are established for both a linear strength degradation curve and an exponential strength degradation curve. Advantages of formulating propagation criteria in terms of the critical length of the fully softened initial failure zone (excluding process zones) are demonstrated, with the more realistic exponential degradation case producing a more stringent criterion than the linear degradation case.

A simple approach to slope stability analysis of naturally occurring, mild nonlinear slopes is proposed through extension of shear band propagation (SBP) theory. An initial weak zone appears in the steepest part of the slope where the combined action of gravity and seismic loads overcomes the degraded peak shear resistance of the soil. If the length of this steepest part is larger than the critical length, the shear band will propagate into the quasi-stable parts of the slope, where the gravitational and seismically induced shear stresses are smaller than the peak but larger than the residual shear strength of the soil. Growth of a shear band is strongly dependent on the shape of the slope, seismic parameters and the strength of soil and less dependent on the slope inclination and the sensitivity of clay. For the slope surface with faster changing inclination, the criterion is more sensitive to the changes of the parameters. Accounting for the actual nonlinear slope geometry eliminates the main challenge of the SBP approach-determination of the length of the initial weak zone, because the slope geometry can be readily obtained from submarine site investigations. It also helps to identify conditions for the early arrest of the shear band, before failure in the sliding layer or a change in loading or excess pore water pressures occurs. The difference in the size of a landslide predicted by limiting equilibrium and SBP approaches can reach orders of magnitude, potentially providing an explanation for the immense dimensions of many observed submarine landslides that may be caused by local factors acting over a limited portion of the slope.

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