SYNOPSIS:

A method to evaluate the deformability of rock masses is presented. This method considers the geometrical and mechanical properties of discontinuities, and the mechanical properties of intact rock. The constitutive equations to represent normal joint and shear stiffnesses varying against stress-deformation are introduced. The applicability of this method is clarified by means of two-dimension loaded large-scale rock mass model s in the laboratory and by numerical analysis.

RESUME:

Ce texte presente une methode pour evaluer la deformabilite d'un massif rocheux. Cette methode considère comme proprietes geometriques et mecaniques des discontinuites et proprietes mecaniques de la roche intacte. Pour exprimer les diaclases normales et les raideurs de cisaillement, nous avons introduit des equations constitutives variant selon les contraintes normales et les deformations. L'applicabilite de cette methode est verifiee par les essais de compression biaxiale au laboratoire utilisant un modèle de massif rocheux de grande dimension et par l'analyse numerique.

ZUSAMMENFASSUNG:

Eingefuehrt wird eine Methode zur Auswertung der Deformierbarkeit von Fels, die die geometrischen und mechanischen Eigenschaften von Diskontinuitaten und die mechanischen Eigenschaften des intakten Felsens beruecksichtigt. Weiterhin werden die konstitutiven Gleichungen eingefuehrt, die die Normal- und Schersteifigkeiten von Klueften im Verhaltnis zur Spannungs- Verformung ausdruecken. Die Anwendungsmöglichkeiten dieser Methode werden mit Hilfe von zweidimensional belasteten Felsgroβmodellen im Labor und in numerischen Analysen erlautert.

1. INTRODUCTION

It is well known that the deformation behaviour of rock masses is greatly influenced by the existence of joints. Because of this reason, many studies have been performed about the effects of joints on rock mass. However, almost of them are concerned with the strength of rock masses, and there are few studies to evaluate the deformability of rock mass quantitatively. In this paper" we take a view points that the deformability of rock mass is the function of fundamental elements such as mechanical properties of intact rock and joints, and geometrical properties of joints. And we present a method to obtain the relation between these elements and deformation behaviour of rock mass by experiments and numerical analyses.

2. MECHANICAL PROPERTIES OF SINGLE JOINT

To obtain joint stiffness, the methods proposed by Goodman (et al.,1968) are adopted basically.

2.1 Compressibility of single joint

Compressibility of single joint is obtained under uniaxial compression and calculated by the difference of deformation between intact and jointed rocks.

2.2 Shear stiffness of single joint

Shear deformation of joint was measured by direct shear test equipment as shown in Fig.2. Many measuring points were pasted on the surface of side wall. Shear and dilation deformation were measured with contact gage. In Fig.6, plotted points for average joint length A, represent measured Ksi and Kn, and solid line shows the result calculated by eq.(5) with material constants given in Tab. II. The reasonable agreements are achieved in Fig.6, and this means that eqs. (1) and (2) are effective for the expression of normal and shear stiffnesses.

3. DEFORMATION EQUATION OF JOINTED ROCK MASS

In this paper, it is assumed that deformation of rock mass 0, equates to the summation of the deformations in both intact rock and joints, δi and δj respectively. Elastic constants of intact rock are given by Young's modulus Ec and poisson's ratio νc. Joint normal and shear stiffnesses for joint set 1 are Kn1 and KS1, Kn2 and KS2 for joint set 2. Shear strength of joint is represented by cohesion c, and friction angle ø of joint surface.

4. EXPERIMENTAL RESULTS AND ITS CONSIDERATION ON BIAXIAL COMPRESSION TEST RESULTS OF JOINTED ROCK MASS MODEL

Biaxial compression test was performed on the rock mass model shown in Fig.8. Total size of rock mass model is 50cm width, 50cm length and 30cm height, and that of element blocks which consists of the model, is 12.5 12.5 15.0 cm. Joints contained in rock mass, are continuous as shown in Fig.8. Sets of orientation in perpendicularly crossing joints are (0°,90°),(15°,75°), (30°,60°) and (45°,45°). Loading platens are rigid compared with used rocks. And the friction between platen and rock mass model is reduced sufficiently by lubricated teflon sheets. Confining pressure σ2 is held constant during loading process and the value of σ2 used are 0.196, 0.49, 0.98 and 1.96 MPa (2, 5, 10 and 20 kgf/cm2). A typical example of stress-strain curves of rock mass model, is shown in Fig.9. Tangent Young's modulus Et is calculated by the nearly straight part of stress-strain curves. Fig.10 shows the change of Et with θ in polar system. From this figure, it is clarified that the anisotropic characteristics of Et are reduced by increase of confining pressure σ2, and minimum value of Et appears at θ=30° or 45°, and maximum value of Et at θ=0° and 90° in Fig.10. The experimental results are analyzed by FEM with use of the developed technique described in Chs. 2 and 3.

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