ABSTRACT:

This paper presents the preliminary validation of a new stress calculation model for the triaxial CSIR cell that allows to include the heterogeneous non linear anisotropic stress-strain relationships found in most rocks. This interpretation methodology is an extension of a stress.calculation model developed for the doorstopper measuring technique (Corth?sy et al., 1993a). It is based on the modelling of rock behaviour using first and second order volumetric and shear stress-strain relationships. The hypotheses required for the passage from a 2D to a 3D model are presented. The validation of the methodology was done in the laboratory on hollow cylinders recovered from stress measurements performed at the URL near Pinawa, Manitoba, using the AECL modified CSIR triaxial cells. The stresses'calculated with the proposed methodology are compared to those calculated with more conventional stress calculation models:

INTRODUCTION

Overcoring stress measurement techniques are based on the recovery principle where elastic strains are recovered as the stresses are relieved by detaching a certain volume of rock from the rock mass. For the doorstopper technique (Leeman, 1965) the strains are measured at the flattened end of a borehole which is prolonged in order to induce the stress relief. When using CSIR triaxial cells, the recovered strains are measured at the wall of a pilot hole and the stress relief is accomplished by prolonging a borehole that overcores this pilot hole. In order to calculate the stresses from the recovered strains, deformational parameters are required as input in the stress-strain relationships. In the early days of overcoring techniques, these parameters were obtained by performing uniaxial compression tests on rock samples that were located in the proximity of the measurement area (Obert et al., 1962). It has long been realized that more representative parameters should be made available and one way of obtaining them is to reload the instrumented stress relieved core itself. For this purpose, devices have been developed to biaxially load a section of the stress relief core (e.g., Fitzpatrick, 1962). Although such a procedure solves the rock sample representativity problem, conventional stress calculation models consider the rock to have a homogeneous, isotropic and linear elastic beharour. In practice, the recovered cores can show non linear stress-strain relationships and anisotropic behaviour 0Vlartin and Christiansson, 1991) due to the existence of microcracks showing a preferential orientation (Walsh, 1965; Ribacchi, 1988). When such behaviour is observed, the conventional stress calculation models will lead to erroneous results. In this paper, a preliminary validation of a new stress calculation model for the triaxial CSIR cell that allows to include the heterogeneous non linear anisotropic stress-strain relationships found in most rocks is presented. It is an extension of a stress calculation model developed for the doorstopper measuring technique (Corth6sy et al., 1993a) which is based on the modelling of rock behaviour using first and second order volumetric and shear stressstrain relationships. The paper first describes the existing interpretation models that take into account anisotropy and/or non linear elastic behaviour for the interpretation of triaxial cell stress measurement results. A description of the stress calculation model developed for the doorstopper is then presented with emphasis on its extension to the CSIR triaxial cell, giving all the hypotheses required for the passage from a 2D to a 3D model.

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