ABSTRACT:

Redistribution of stresses in jointed rock mass occurs as a result of underground excavation or any other activity that changes the stresses in the region. The alteration in stress pattern leads to changes in porosity or void ratio of rock material as well in the aperture, i.e. closure or opening of the rock joints. The present study proposes a 3-D coupled stress-flow analysis relating the stress and flow by indirect method. Opening or closure of joints due to normal and shear stresses has been considered with due attention to the dilatant nature of rough discontinuities. As a result, the aperture of joint changes and, in turn, the permeability has too. Joint properties including the attitude of joint, stiffness, aperture, spacing and roughness have been taken into account in the derivation of the constitutive laws and the behavior of a tunnel excavated in jointed rock mass has been examined. More disturbances in the contours of hydraulic head and pore water pressure have been observed near the tunnel periphery. The change in permeability upon redistribution of stresses is a function of the opening/closure of the joints. It has also been shown that the orientation and aperture of the joints have a pronounced effect on the fluctuation of the phreatic surface.

1. INTRODUCTION

In geotechnical engineering problems, the effect of flow through fractured or porous media can be studied in terms of the effective stresses. Barton et al. [1] developed a constitutive model which provides shear stress " displacement " dilation-conductivity coupling. Based on the concept of joint roughness coefficient, JRC, a shear-dilation model was established. The normal closure of joint aperture was expressed based on a hyperbolic model for loading and unloading. Elsworth and Goodman [2] related the changes in hydraulic conductivity with induced deformation in fractured rock masses on the basis of their observations of sawtooth and sinusoidal fissure surfaces. Wei and Hudson [3] related hydraulic aperture with the closing and opening of a joint in a jointed rock mass using a hyperbolic function. The variation in permeability appears to be correlated to the variation in normal stress in a major zone of granite boreholes [4]. Ouyang and Elsworth [5] proposed a model to represent the mechanics of enhancement of hydraulic conductivity of fractured rock caused by deformation, through the use of a modulus reduction ratio, Rm. This ratio considers the expression of hydraulic conductivity to account for the influence of joint aperture, joint spacing, joint stiffness and modulus of intact rock on the change of conductivity. Barton et al. [6] showed that interaction between the state of stress and the fracture characteristics is determined by both the orientation of aperture and the hydraulic conductivity of the individual fractures and also by the magnitude and orientation of all the three-principal stresses. When faults are artificially stressed, increased permeability may cause a movement of fluid along the faults. Liao and Hencher [7] demonstrated, through the use of UDEC, that the stress change significantly controls fluid flow along the fractures. This behavior depends upon fracture intensity, orientation, distribution, and the connectivity.

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