No abstract

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.

No abstract

A new escalator tunnel and associated passageways have been constructed at Waterloo station in London, passing only a few metres beneath two sensitive masonry structures. Compensation grouting was undertaken in the ground between the tunnels and the overlying structures to limit settlements to acceptably small levels. This comprised injections of grout during tunnelling in response to detailed observations of ground and structure movements. The observational method was used in the control of the compensation grouting operations. This was achieved by comparison of field observations with defined limits of acceptable behaviour, and by planned contingencies in the event that these limits were approached. The extensive instrumentation installed to monitor the movements of the ground and the structures during tunnelling is described. Details are given of the performance of the instrumentation, of the measurements made and of the interpretation of the observations required to evaluate the soil-structure interaction behaviour. The specified requirements for compensation grouting and a summary of the grouting operations are presented. Settlements of the structures due to tunnelling were restricted to less than 15 mm. The relatively novel technique of compensation grouting has proved very successful in limiting settlements. Un nouveau tunnel d'escalator et les couloirs associés ont été construits à la gare de Waterloo, à Londres. Ils passent quelques mètres seulement au dessous de deux structures de maçonnerie particulièrement sensibles. Un coulis compensateur a été injecté dans le terrain entre les tunnels et les structures sus-jacentes pour limiter le tassement à des valeurs faibles acceptables. Ceci consiste à injecter du coulis lors du percement du tunnel en fonction d'observations minutieuses de mise en mouvement du terrain ou des structures. La méthode d'observation a été utilisée pour contrôler la mise en place du coulis compensateur. Ceci a été réalisé en comparant les observations de terrain à des limites définies de comportement acceptable, et en parant à l'imprévu au cas où ces limites seraient approchées. On décrit l'instrumentation mise en place pour enregistrer à la fois les mouvements du terrain et ceux des structures. On détaille les performances de cette instrumentation, les mesures réalisées et l'interprétation des observations permettant d'évaluer l'interaction sol-structure. On présente également les conditions requises et le résumé des opérations de mise en place du coulis compensateur. Le tassement des structures dû au tunnel a été inférieur à 15 mm. La technique du coulis compensateur s'est donc révélée efficace pour limiter les tassements.

No abstract

This is the accepted version of the paper.This version of the publication may differ from the final published version. used to capture images of a deforming soil surface. The images were used to reconstruct the observed scene to a high density, accurate 3D point cloud. A new method has been developed to process the obtained 3D point clouds and images to determine 3D displacement vectors. The procedure is highly automatic which allows large data sets to be processed with minimal manual intervention. Two series of quantification experiments were carried out to assess the performance of the system which showed the overall accuracy to be within 0.05mm over a field of view of Permanent repository link500×250mm. An example application is presented to demonstrate the capabilities of the 3D imaging system.

A series of plane strain centrifuge model tests of single tunnels in moderately stiff clay has been conducted at City University, London. Ground movements were measured using conventional displacement transducers at the ground surface and by digital image-processing techniques below ground level. The relatively simple tests are shown to result in ground movements which are consistent with field measurements, but the tests produced considerably more high-quality data, which have been analysed to improve predictions of both surface and subsurface movements in the plane transverse to a tunnel in clay. The measured data are assessed against commonly used empirical predictive techniques, indicating some deviations from current practice. A procedure for predicting horizontal movements as a function of the vertical settlement profile is suggested.

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTGeotechnical centrifuge modelling provides a means by which geotechnical events and processes can be better understood. In particular, the technique has proved invaluable when investigating collapse mechanisms in small scale models that can be