In situ stress measurements in anisotropic rock

Abstract

The purpose of this paper is to discuss how to account for rock anisotropy when measuring in situ stresses by overcoring with instrumented devices such as the CSIRO hollow inclusion cell or the U.S. Bureau of Mines (USBM) gauge. The rock is described as an isotropic, a transversely isotropic or an orthotropic material. The computation of in situ rock stresses with the two devices mentioned above depends on the anisotropic rock character, the degree and type of rock anisotropy, the orientation of the rock anisotropy with respect to the hole in which the devices are located, and, for the CSIRO cell only, dimensionless parameters describing the relative deformability of the rock with respect to the deformability of the cell material. Using these parameters, the paper addresses the following problems: the effect of the degree and type of rock anisotropy on in situ stress determination; the error involved in neglecting rock anisotropy by assuming isotropy; the error involved in neglecting the geometry and properties of the CSIRO cell by assuming that its strain gauges are positioned directly on the walls of pilot holes, and how this error is affected by the rock anisotropy.