ABSTRACT:

A blind analysis for the trial excavation of the reduced-scale cavern preceding the construction of the Kazunnogawa power station is carried out employing MBC (Micromechanics-Base Continuum) model of jointed rock mass. The displacement in rock mass and the displacement between the cavern walls (convergence) predicted by the present method show accordance with measurement results.

RESUME:

Une analyse de prediction pour Ie modele d'excavation de la galerie precedent la construction de la centrale de Kazunogawa est realisee en employant MBC (Micromechanics-Base Continuum) pour modeliser les masses jointes de la roche. Les deplacements dans la masse de la roche et les deplacements entre les murs De la galerie pendant I'excavation predits par la methode decrite ci apres sont en accordance avec les resutats mesures sur place.

ZUSAMMENFASSUNG:

Eine Modellanalyse der Höhlenausgrabung, der Errichtung des Kazunogawa-Kraftwerkes vorangegangen, wurde mittels eines MBC (Micromechanic-Base Continuum) Models von gegliedertem Felsgestein durchgefuehrt. Die Verschiebungen innerhalb der Felsmasse und die Verschiebung der Höhlenwande untereinander wβhrend des Ausgrabungprozesses, vorhergesagt mit der vorliegenden Methode, stimmen gut mit den Meβergebnissen ueberein.

1
INTRODUCTION:

Opening and sliding of joints, which are initially closed by earth pressure, are considered to be the governing mechanism of behaviors of rock masses in underground excavation. In the present analysis, a micromechanics-based continuum model of jointed rock mass is employed. This MBC (micromechanicsbased continuum) model reflects effects of density, orientation and connectivity of joints as well as the property of joints itself. The constitutive equation is implemented into the FEM code for the analysis of problems with arbitrary geometry and boundary conditions. The distinctive features of the present method are the following: the information on dominant joint sets, such as strike and dip angles, and average distance, are employed as direct input data for the analysis; the opening and sliding displacement of joints are obtained as results of the numerical analysis. A trial excavation of a reduced-scale cavern was carried out in 1994 by Tokyo Electric Power Company prior to the construction of an underground power station for the Kazunogawa pumped storage project near Tokyo in Japan. The trial excavation is analyzed by the present method and distribution of opening and sliding displacement of joints as well as displacement and strain of rock mass are predicted. The predicted results are compared with measurements in the trial excavation.

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MICROMECHANICS-BASED CONTINUUM THEORY

Micromechanics-Based Continuum Theory is the continuum theory for the mechanical behavior of material which is governed by the existence and the growth of microstructures, such as, microcracks and inclusions. The behaviors of microstructures are modeled and the constitutive equation of the equivalent continuum is derived as the relationship between average stress and average strain over a representative volume element (R.V.E) which contains numbers of microstructures. This theory is suitable for the analysis of rock mass which contains a number of joints. We modeled the behavior of joints as follows. Joints have large-scale undulation which plays an important role. Sliding along one part of a joint leads to opening in the other part. The opening part of the joint surface is assumed to be a traction free surface. Columb's criterion is employed for sliding criterion of joints. It is assumed that the sliding criterion is maintained during sliding, and that there is no normal displacement across the sliding surface of the joint. Each joint is surrounded by rock mass and the amount of opening and sliding of joints depends on the constraint by the surrounding rock mass. The joint displacement is estimated by embedding the joint in the effective continuum.

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