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The influence of previous stress history and stress path direction on the surface settlement trough induced by tunnelling A. GRA MMATIKO POU LOU*, L. ZDRAV KOV IC † and D. M. P OTTS †The pre-failure behaviour of overconsolidated soils is well known to be non-linear and inelastic. Over the last two decades experimental work has shown that pre-failure behaviour is also dependent on the previous stress history of the soil. However, some research has also suggested that, if enough time is allowed for creep to take place, then there is no effect of recent stress history; moreover, what affects soil stiffness is the direction of the stress path relative to the gross yield surface. This paper investigates the effect of two scenarios on the surface settlement trough associated with tunnelling. The first assumes that the previous stress history of the soil has an effect on soil stiffness, whereas the second assumes that creep/ageing periods have erased any memory of the previous stress history, and it is only the stress path direction that influences soil stiffness. A three-surface kinematic hardening model, which can simulate both scenarios, is employed in finite element analyses of tunnel construction.

The paper investigates the effect of the deviatoric shape of the yield and plastic potential surfaces on the failure height of an embankment founded on a soft clay deposit. Different combinations of the shapes of the surfaces in the deviatoric plane are explored in a critical state model that predicts isotropic strength, and which is used in order to model the clay foundation. The shapes considered start from the widely used circular shape and move on to the Mohr-Coulomb hexagon and advanced shapes, such as the Lade shape. For all combinations the same undrained strength profile in triaxial compression is adopted for the clay foundation. Finite element analyses investigate the failure height of the embankment assuming plane-strain conditions. Moreover, two ways of modelling the fill material are adopted in the analyses, and the effect of each of these on the failure height of the embankment is explored.Cet article étudie l'effet de la forme déviatorique des surfaces de limite élastique et de potentiel plastique sur la hauteur à la rupture d'un remblai fondé sur un dépôt d'argile plastique. Différentes combinaisons de formes de surfaces dans le plan déviatorique sont explorées dans un modèle d'état critique prédisant la force isotrope, utilisé pour modéliser la fondation argileuse. Les formes considérées sont d'abord la forme circulaire, largement utilisée, puis l'hexagone de Mohr-Coulomb ainsi que d'autres formes complexes telles que la forme de Lade. Pour toutes les combinaisons, le même profil de résis-tance du sol non drainé en compression triaxiale est adopté pour la fondation argileuse. Des analyses par éléments finis permettent d'étudier la hauteur à la rupture du remblai dans des conditions de contrainte plane. Deux méthodes de modélisation sont adoptées pour les analyses, et l'effet de chacune d'entre elles sur la hauteur à la rupture du remblai est étudié.

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This paper compares non-linear and linear elasto-plastic models in the numerical analyses of a retaining wall in stiff clay. The paper aims to be applicable to routine design, where often a non-linear elasto-plastic model is not available and instead a linear elasto-plastic model is used. Current guidelines suggest deriving the appropriate linear elastic stiffness from the non-linear soil behaviour, on the basis of the expected straining in the ground. However, it is not always straightforward to choose a representative strain level. The paper presents profiles of linear elastic stiffness, back-calculated to match the maximum wall deflection of the non-linear analyses in the short term. On the basis of the non-linear soil stiffness–strain curve, the paper then derives a corresponding strain level for each linear elastic stiffness profile. Three different support systems are examined: a single-propped wall, a double-propped wall, and a top-down construction. A number of non-linear stiffness–strain curves are considered for the stiff clay and two K0 profiles are examined.

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