Effect of tangent and secant elastic constants on CCBO stress evaluation in transversely isotropic rock

The Compact Conical Borehole Over-coring (CCBO) method is employed for the determination of the three-dimensional stress state in rock environments. The method utilises a strain gauge cell to quantify the deformation of over-cored rock, the deformation characteristics of which are evaluated in the laboratory. It is assumed that the in situ rock material is homogeneous, elastic and transversely isotropic and it is, therefore, imperative to ascertain the deformation parameters of the transversely isotropic material present in situ. The tangent and secant deformation parameters were taken from previous research and then employed throughout the stress determination process. This paper outlines the fundamental principles of the method and presents results from practicalmeasurements carried out at the Grimsel Underground Research Laboratory in Switzerland. The stress results vary depending on whether alternative deformation characteristics (i.e. tangent or secant) are taken into account during the evaluation process.