ABSTRACT:

The stress analysis of shallow tunnels in weak rocks, or "hard" soils, may present some difficulties caused by its three dimensional nature and by the particular mechanical behaviour of these materials, which show a reduction of stiffness and shear strength with increasing shear deformation. This problem is here approached, with particular reference to the stability of the tunnel face, from both numerical and experimental view points. The experimental part of this study consists of a series of tests on a 3D, reduced scale, 1g model that led to a quantitative evaluation of the displacements induced by the excavation. The problem was then analyzed through 2D and 3D finite element analyses, accounting also for "softening" material behaviour. The comparison between experimental and numerical results permits to draw some conclusion on the effectiveness of the adopted numerical models in the analysis of tunnels.

RESUME:

L'analyse de la stabilire des excavations souterraines à faible profondeur dans un milieu de roche meuble ou de sol "dur" peut rencontrer des difficultes, dues à la geometrie en trois dimensions du problème et au comportement mecanique de ces materiaux qui montrent une diminution de la raideur et de la resistance au cisaillement à l'augmenter des deformations de cisaillement. Le problème est etudie ici en se referent en particulier à la stabilite du front de taille, au point de vue experimental comme numerique. La partie experimentale traite une serie de tests sur modèle reduit, en trois dimensions, sous gravite normale, qui permettent une evaluation quantitative des deplacements causes par Ie creusement. En plus Ie problème a ete etudie par analyses aux elements finis sur modèles en deux et trois dimensions, en tenant compte du comportement radoucissant du materiel aussi. La comparaison entre les resultats experimentaux et numeriques permet de tirer quelques conclusions sur l'efficacite des modèles numeriques utilises dans I'analyse des tunnels.

ZUSAMMENFASSUNG:

Stabilitatsanalysen unterirdischer, untiefer öffnungen in weichem oder verwirtertem Gestein ergeben einige Schwierigkeiten, abhangig von der drei-dimensionalen Geometrie des Problems und des mechanischen Verhaltens dieser Materiale, welche im Algemeinen eine Reduzierung der Steifigkeit und Scherfestigkeit mit zunehmender Scherdeformation zeigen. Zur Untersuchung des Problems werden' I g reduzierte Skala Test' an einem drei-dimensionalem maßstabsgetreuem Modell durchgefuehrt, welche zu einer quantitativen Evaluierung der durch die Ausschachtung hervorgerufenen Verschiebungen fuehrt. Außerdem werden zwei- und dreidimensionale elastoplastische Analysen zum finite element abgeleitet die auch die Verweichung des Materials einbeziehen. Die experimentellen und numerischen Resultate werden verglichen um die Validitat der unterschiedlichen Annahmen, gemacht anhand des numerischen Modells, zu beurteilen.

1. INTRODUCTION

Shallow tunnels are frequently excavated nowadays due to the growing need of underground urban transportation and services. New technologies have been introduced in the last decade for excavation, and for supporting the tunnel face and walls. Since alternative methods are still proposed, it would be of interest to analyze them, in view also of possible improvements of their performance. In addition, a reliable evaluation of the stability of the tunnel face, and of the pressure necessary for stabilizing it, would also permit a prediction of the displacements caused by excavation and of the area interested by the surface settlements. In the literature the stability of the tunnel face has been approached from different view points. Both analytical [Davis et aI., 1980] and numerical [Lee and Rowe, 1990] models have been proposed, as well as physical models in full or reduced scale [Chambon and Corte, 1994; Hisatake et aI., 1995]. Our attention is focused here on numerical and small scale physical models.

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