ABSTRACT:

We present a new, iterative approach for determining the elastic parameters of transversely isotropic rocks, based on the mechanical testing and the numerical modeling of hollow cores. Cores recovered during overcoring were submitted to a given pressure on their external boundary while the strains were recorded within the inner hole by a CSIRO HI cell. The strains were also calculated numerically for different values of the elastic parameters of the rock, using a 3D finite element method. Charts were used for the graphical determination of the elastic parameters that allow the closest modeling of the measured strains. Elastic parameters of the Tournemire argillite rock estimated by this method display a much lower anisotropy ratio than the one obtained from classical laboratory tests. The possible origin of this difference is discussed.

1 INTRODUCTION

The determination of elastic parameters of transversely isotropic rocks is classically carried out by performing uniaxial or triaxial tests on small rock cylinders with different orientations of the plane of isotropy. A possible alternative is biaxial testing of large hollow rock cylinders. This technique has been applied for years to the measurement of elastic parameters needed for determination of in situ stress by overcoring. It consists in placing the hollow rock core retrieved after overcoring, still equipped with its strain measurement cell (e.g. a CSIROHI cell in this study), in a Hoek-Franklin pressure chamber and submitting it to a biaxial loading/ unloading cycle. The elastic parameters of the rock core are then derived from the recorded pressure strain curves (Fig. 1). Nevertheless, determining rock elastic parameters from a biaxial test is delicate due to the particular geometry of the rock sample and to the presence of a 1,5mm thick epoxy cement layer bonding the strain gauges to the rock, which affects the strain measurements.

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