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This paper describes the results of numerical analysis of the effects of installing a driven pile. The geometry of the problem has been simplified by the assumption of plane strain conditions in addition to axial symmetry. Pile installation has been modelled as the undrained expansion of a cylindrical cavity. The excess pore pressures generated in this process have subsequently been assumed to dissipate by means of outward radial flow of pore water. The consolidation of the soil has been studied using a work-hardening elasto–plastic soil model which has the unique feature of allowing the strength of the soil to change as the water content changes. Thus it is possible to calculate the new intrinsic soil strength at any stage during consolidation. In particular the long-term shaft capacity of a driven pile may be estimated from the final effective stress state and intrinsic strength of the soil adjacent to the pile. A parametric study has been made of the effect of the past consolidation history of the soil on the stress changes due to installation of the pile. The results indicate that for anyinitial value of overconsolidation ratio, the final stress state adjacent to the pile is similar to that in a normally one-dimensionally consolidated soil except that the radial stress is the major principal stress. A method is described whereby the model of pile installation and subsequent consolidation may be extended to clays which are sensitive. The method is used to predict changes in the strength and water content of soil adjacent to a driven pile which compare well with measurementsfrom two field tests on driven piles. It is also shown that the rate of increase of bearing capacity of a driven pile may be estimated with reasonable accuracy from the rate of increase in shear strength of the soil predicted from the analysis. Cet article fait part des résultats d'analyses numériques des effets occasionnés par la mise en place d'un pieu battu. L'hypothése de conditions de déformation plane en plus dúne symétrie axiale a été adoptée pour simplifier la géométrie du problème. Le battage du pieua été représenté par l'expansion non drainée d'une cavité cylindrique. On a supposé par la suite que les surpressions interstitielles engendrées au cours de ce processus se dissipaient en raison de l'écoulement radial vers l'extérieur de l'eau interstitielle. L'étude de la consolidation du sol a fait appel à un modèle de sol élasto-plastique rigide, qui a pour propriété unique de permettre des modifications de la résistance du sol en fonction des variations dela teneur en eau. Grâce à cela, il est possible de calculer la nouvelle résistance intrinsèque du sol à une étape quelconque de la consolidation. Notamment, il est possible d'estimer le frottement latiére à long terme d'un pieu battu à partir de l'état de contrainte efficace finale et de la résistance intrinséque du sol adjacent au pieu. L'effet de l'historique de consolidation antérieure du sol sur les variations de contraintes dues a l'installation du pieu a fait l'objet d'une étude paramétrique. Les résultats montrent que pour toute valeur initiale du degré de surconsolidation, l'état de contrainte final au voisinage du pieu est identique à celui d'un sol à consolidation unidimensionnelle normale sauf que lacontrainte radiale constitue la contrainte principale majeure. L'article decrit une méthode suivant laquelle le modèle d'installation de pieux et de consolidation subséquente peut être appliqué aux argiles qui sont sensibles au remaniement. La méthode sert à prévoir les variations de la résistance et de la teneur en eau du sol adjacent à un pieu battu, lesquelles sont tres voisines des mesures provenant de deux essais in situ de pieux battus. Il appalaît également que le taux d'accroissement de la capacité portante d'un pieu battu peut etre estimé dans des limites de préision raisonnables à partir du taux d'accroissement de la résistance à la rupture au cisaillement du sol prévu par l'analyse.

>The response of skirted offshore foundations to combined vertical (V), moment (M) and horizontal (H) loading has been studied using two-dimensional finite-element analysis and upper-bound plasticity analysis assuming the soil to be undrained. New information has been gained about the shape of the yield locus and the soil deformation mechanisms occurring at yield from the finite-element analysis. The shape of the yield locus was found to be similar to that predicted by previous workers in V—M and V— H space but differed significantly in M—H space. This behaviour is explained using upper-bound plasticity mechanisms suggested by the soil deformation mechanisms calculated in the finite element analysis. This procedure is then used to give a good approximation to the shape of the yield locus and thus may form the basis for future design methods. Additionally, a simplifying transformation is suggested for the yield locus in H—M space based on plasticity analysis, which allows use of simple mathematical expressions to form a design envelope. Nous 6tudions la réponse de fondations offshore à jupe à une combinaison de charge verticale (V), instantanée (M) et horizontale (H) en utilisant des analyses d'é1éments finis en deux dimensions et une analyse de plasticité de limite supérieure en supposant un sol non drainé. Nous drons de Panalyse d'é1éments finis de nouveaux renseignements sur la forme du lieu d'é1asticité limite et sur les mécanismes de déformation du sol qui se produisent à la limite é1astique. La forme du lieu se révèle comme similaire à celle présagée par les travaux précédents dans les espaces Vmdash;M et V—H mais considérablement différente dans Pespace M—H. Nous expliquons cc comportement en utilisant les mécanismes de plasticité de limite supérieure qui sont suggérés par les mécanismes de déformation du sol calcu1és dans 1'analyse d'é1éments finis. Cette procédure est alors utilisée pour donner une bonne approximation de la forme du lieu qui pourra ainsi former la base des futures méthodes de design. De plus, nous suggérons une transformation simplificatrice pour le lieu d'é1asticité limite dans Pespace H—M sur la base de 1'analyse de plasticité qui permet Putilisation d'expressions mathématiques simples pour former une enveloppe de design.

SUMMARYThe problem of penetration resistance involves a continuously moving zone of plastic distortion in the soil medium. This has been explored for cone penetration and pile installation, where additional volume is intruded into the soil, using the strain path method with the flow field derived from classical fluid mechanics. This paper focuses on a new generation of penetrometers, which have a much greater projected area than the cone shaft, and introduces a version of the strain path method based on classical upper bound solutions for the penetrometers. The new approach is used to explore the effects of high strain rates, and gradual strength degradation, on the penetration resistance of cylindrical and spherical penetrometers.

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The capacity of surface foundations on clay under pure vertical (V), horizontal (H) or moment (M) loading may be expressed in non-dimensional form through the use of appropriate bearing capacity factors, with values that will be affected by the shape of the foundation and also any variation of undrained shear strength with depth. A common assumption has then been that the shape of the complete failure envelope in three-dimensional loading space (V, M, H) will be similar regardless of foundation shape and soil non-homogeneity, once scaled to the appropriate apex points. The appropriateness of this assumption has been explored by means of two- and three-dimensional finite element analyses of strip and circular footings, for a simple Tresca soil model where the shear strength varies linearly with depth. With a view to applications involving partially embedded foundations, such as offshore skirted foundations, full suction and ‘bonding’ with the underlying soil has been assumed. The paper documents the normalised capacities under uniaxial (V, M or H) loading, and compares the shapes of the failure envelopes in the three planes H = 0, M = 0 and V = 0 for a practical range of strength gradients. The broad conclusion is that a single shape does indeed hold in the M = 0 plane, for both strip and circular foundations, but that for the H = 0 and V = 0 planes the overall size of the normalised failure envelope reduces as the degree of strength non-homogeneity increases. Hence the assumption of a failure envelope shape derived for homogeneous strength conditions would be unconservative.

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