The overcoring method for measuring in situ stresses is frequently used in rock engineering projects. Most models for its interpretation assume that the rock is homogeneous, linear elastic and isotropic. The elastic constants are determined by various methods, namely by biaxial tests on the recovered core.
A three-dimensional finite element model was developed, which allows to compute the stress state from the strain measurements obtained during overcoring, assuming a transversely-isotropic behaviour of the rock mass. Different testing methodologies for elastic constants determination of anisotropic rocks are introduced and discussed. Several numerical tests were performed to analyse the influence of the weakness planes that sometimes occur in the rock core owing to the tensile strains that are generated during the biaxial test. To avoid the tensile strains in the biaxial test, a new triaxial testing procedure of the overcored rock sample with the cell was developed. The effect of the degree of rock anisotropy on in situ stress determination and the error involved in neglecting rock anisotropy are analysed. Differences on the principal stresses and principal directions calculated considering the usual isotropic model and the developed numerical model for a real case of anisotropic rock are presented.
The knowledge of the in situ stress state is indispensable for design and construction of underground excavations in rock masses. However, determination of the state of stress in a rock formation is still a challenging problem in rock engineering.
Stress measurements by overcoring methods are widely used for determination of the state of stress in rock masses. In this indirect technique, borehole deformations induced by stress relief are measured, and they are then converted in stresses using the elastic constants of the medium.
The strain tensor tube (STT) is a 2 mm thick hollow cylinder with a 35 mm external diameter, with 10 electrical resistance strain gauges embedded midway from the inside to the outside surface, developed at LNEC for this purpose. Stress measurements using this technique consist of cementing into a 37 mm diameter borehole the strain cell and releasing the stresses by overcoring with a larger diameter, usually around 120 mm. The state of stress in the rock mass is computed from the strains obtained in the 10 gauges in the overcoring final stage and the elastic constants are determined from a biaxial test on the recovered large diameter core, with the cell. Comparing with other tests reviewed by Amadei (1996), the biaxial test presents the advantage of being carried out on the overcored rock. However, it has been verified by the authors that the axial tensile strains that result from the loading condition, often significantly affect the values calculated for the elastic constants.
The model currently used for interpretation of the overcoring test assumes linear elastic and isotropic behaviour of the rock, both for calculating the state of stress from the strain measurements and for calculating the elastic constants from the biaxial tests.
