ABSTRACT:

The Cosserat continuum theory has several advantages over the classical theory when the behavior of a discontinuous rock mass is to be studied by means of the equivalent continuum approach, especially if the bending stiffness of the rock layers and relative rotation of the blocks play an important role in the problem to be solved. In this paper an equivalent Cosserat continuum is used to study the elasto-plastic behavior of a shallow tunnel in a blocky rock mass. Several FEM analyses were carried out to investigate the influence of the block size - tunnel radius ratio on static behavior.

RÉSUMÉ:

Le modèle du continu de Cosserat presente plusieurs avantages par rapport à celui du continu classique lorsqu'il s'agit d'analyser le comportement d'un massif rocheux fracture avec la methode du continu equivalent, notamment dans toutes les situations geotechniques où la rigidite à la flexion des couches rocheuses et les rotations relatives des blocs jouent un rôle important. Dans ce papier, Ie continu equivalent de Cosserat est applique à l'analyse du comportement elasto-plastique d'une galerie superficielle creusee dans un massif rocheux ayant une structure en blocs reguliere. On a effectue plusiers FEM analyses en variant la taille des blocs pour montrer l'influence du rapport entre celle-ci et le rayon de la galerie.

ZUSAMMENFASSUNG:

Wenn das Verhalten einer diskontinuerlichen Felsmasse wird durch die Kosseratsche Theorie des aquivalenten Kontinuum studiert, hat das verschiedene Vorteile gegenueber dern klassischen Modell, vor allem wenn die Biegefestigkeit derFelsschichten und die relative Blockrotation eine bedeutende Rolle spielen. Das Kosseratsche Modell wurde gebraucht, das elastoplastische Verhalten eines in einem Blockfelsmasse angelegten Oberflachentunnels festzustellen, Der Einfluß des Blockengröße-Tunnelradius-Verhaltnisses wurde durch Untersuchungen an Blöcken von unterschiedlichen Größen bewertet.

INTRODUCTION

Rock masses with one or more sets of joints are traditionally modeled as an equivalent continuum or as a discontinuous medium. In the first case, the rock mass is treated as a homogeneous material with mechanical properties that try to capture the overall mechanical behavior of the rock mass; in the second case the rock mass is considered as a discontinuous system of interacting blocks and. hence each discontinuity needs to be modeled.

The equivalent continuum model based on the Cauchy theory is simple to implement in a FEM code (Gerrard 1982) but in many cases it cannot adequately account for the bending stiffness of blocks or layers of intact rock.

The discontinuum model can be implemented both in FEM codes, in which joint elements are introduced (Goodman et al, 1968). and in DEM codes (Cundall, & Hart 1992), but the discretization process is more cumbersome and the analysis much more time-consuming.

The typical advantages of the equivalent continuum approach can be maintained while also taking into account the actual blocky structure of the rock mass by using an equivalent continuum based on the micropolar continuum theory, which represents the simplest microstructured continuum (Muehlhaus 1993).

The micropolar continuum was developed by the Cosserat brothers (1909) and was further investigated and reproposed by several Authors in the 1960s (Grioli 1960; Mindlin 1964).

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