The stress-strain behaviour of overconsolidated soil is non-linear and the stiffiiess depends on the current state and stress history. In addition to the overall stress history which is best described by the overconsolidation ratio, the recent stress history- either a sudden change in the direction of the stress path or a period of time at a constant stress state-has a major effect on subsequent stiffiiess. The effects of a change in the direction of the loading path on the stiffness of overconsolidated soil were examined in laboratory stress path tests on reconstituted samples of London Clay. Further tests were carried out on reconstituted samples of other soils with a range of plasticities. The test results showed that the behaviour of over-consolidated soil is largely inelastic and highly nonlinear, and that the recent history has a major effect on the subsequent stiffiiess, particularly for relatively small changes of stress or strain. For reconstituted samples of London Clay the stiffiiess near the start of a loading path was found to vary by up to an order of magnitude following different rotations of the stress path. For other soils with lower plasticities the variations were smaller but still significant. Le comportement contrainte-déformation d'un sol surconsolidé est non-linéaire et al rigidité dépend de l'etat existant et de l'historique des contraintes. En addition à l'historique général des contraintes, qui est le mieux representé le rapport de surconsolidation, l'historique recent des contraintessait un changement abrupt dans la direction du chemin des contraintes sait une période de temps dans un état de contrainte constante-a un eftet primordial sur la rigidité ultérieure. Les effets d'un changement dans la direction du chemin de chargement sur la rigidité du sol surconsolidé ont été examinés au cours des essais concernant les chemins des contraintes effectués en le laboratoire sur des échantillons reconstitute d'argile de Londres. D'autres essais ont été effectués sur des échantillons d'autres sols avec diverses de plasticites. Les résultats des essais montrèrent que le comportement du sol surconsolidé est en grande partie non-élastique et très non-linéaire et que l'historique récent a un effet primordial sur la rigidité ultérieure, particulièrement pour des changements relativement petits de la tension ou de la contrainte. On trouvé que pour des èchantillons reconstitués d'argile de Londres la rigidité dans le voisinage d'un chemin de chargement variait jusqu'à un ordre de grandeur siuvant les differentes rotations du chemin des contraintes. Pour d'autres sols ayant des plasticités inférieures les variations étaient moins grandes mais encore significatives.
The stress—strain response of overconsolidated clay depends both on its current state and on the loading history followed to reach that state, in particular the relative directions of the current and previous loading paths. A constitu-tive soil model is developed which predicts this behaviour by allowing elasto-plastic deformations controlled by two nested kinematic hardening surfaces inside a conventional Modified Cam-clay state boundary surface. This relatively straightforward model requires only eight para-meters, each with a rational basis, and which can be determined from a small number of well-controlled stress path tests. Predictions of soil behaviour using this model are compared with data from triaxial stress path tests. The close agreement confirms that the essential features of soil behaviour are predicted by the model. The wider implications of the use of the model in geotechnical analysis are illustrated by compar-ing predictions made using the model (in conjunction with finite element analysis) with data from a specially commissioned series of centrifuge tests of a circular foundation loaded on overconsolidated clay. The stress history of the soil was carefully controlled in the experiments and was replicated in the course of the analyses. The computations reproduced the main characteristics of the observed ground movement, in particular the surface profile. In contrast, conventional constitutive models of soil behaviour show very poor predictions. This demonstrates the importance of using a model that simulates the behaviour of soil over a wide range of strain increments and with changes in load path direction. La réaction tension/déformation de ĺargile préconsolidée dépend à la fois de ĺétat de ĺargile et des charges antérieures, notamment des directions relatives de la charge présente et des charges précédentes. ĺexposé décrit un mod&gave;le de sol constitutif qui prédit ce compor-tement en permettant des déformations élastoplastiques engendrées par deux surfaces ciné matiques de durcissement emboîtées dans une surface limite classique ´argile du Cambrien modifiée. Ce modèle relativement simple ńexige que huit paramètres, chacun avec une base rationnelle, et pouvant être déterminés à pardr ´un petit nombre ´essais bien contrôlés des parcours de tension. Les prédictions du comportement du sol faites à ĺaide de ce modèle sont comparées aux données fournies par des essais triaxiaux des parcours de tension. La bonne corrélation entre les deux ensembles de valeurs confirme que le modèle pent prédire les caractéristiques essentielles du comportement du sol. ĺexposé illustre les autres applications possibles de ce modèle à ĺanalyse géotechnique en comparant les prédictions faites à ĺaide du modèle (en conjunction avec une analyse des é1éments finis) aux données fournies par une série spéciale ´essais centrifuges ´une fonda-tion circulaire chargée sur de ĺargile prĺcon-solidée. On a soigneusement documenté les tensions successives dans les essais pour pouvoir les reproduire dans les analyses. Les calculs ont reproduit les principales caractéristiques du mouvement de sol observé, notamment le profil de surface. Par contraste, les modéles constitu-tifs classiques du comportement du sol donnent de très mauvaises prédictions. Cela montre qúil est important ´utiliser un modèle qui simule le comportement du sol sur une vaste gamme de tensions et avec des changements de direction des charges.
A simple constitutive model for structured clays is presented. The model is based on an existing constitutive model for reconstituted clays. It uses a fixed relationship between change in sensitivity and plastic strain to represent destructuration so that all predicted effects of structure are included in the value of sensitivity. Only three added parameters are required, which can each be derived from a single isotropic compression test. All other parameters are the same as those used in the base model, which are derived from test data on reconstituted specimens. A value of ultimate sensitivity greater than unity allows the model to simulate clays with stable and/or metastable structure. Results from a parametric study show that the model predicts responses during compression and shearing typical of natural clays. The model was applied to test data from a natural soft clay using parameters derived where possible from isotropic compression tests. Results from the analyses prove that the simplified formulation of the model is sufficient to significantly improve predictions.
Citation: Bilotta, E. and Stallebrass, S. E. (2009). Prediction of stresses and strains around model tunnels with adjacent embedded walls in overconsolidated clay. Computers and Geotechnics, 36(6), pp. 1049-1057. doi: 10.1016/j.compgeo.2009 This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent
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