Development of Overcoring Stress Measurement Method Based on Borehole Deformation Taking into Account the Anisotropy and the Error by Assuming Isotropy

摘要

Overcoring rock stress measurement method has been used in most of projects of constructing large rock caverns for hydro-power plants and oil storage in Japan. This is because the principle of the method is clear and reliable measurement devices have been developed. Overcoring stress measurement method is applicable to both isotropic and anisotropic rocks, but only methods assuming isotropy have been actually available in engineering. Rock is sometimes very anisotropic and the importance of considering anisotropy in rock stress measurement has been pointed out by several authors. One of the major reasons for the fact that there has been no practical method taking into account the anisotropy is that it has been very difficult to determine anisotropy of the rock at the location of the overcoring. In this paper, adopting a new method for determining orthotropic anisotropy by pressurizing a hollow cylinder, a practically applicable rock stress measurement method is developed. Then an example of applying the method to a field data is shown. Further, the error in the resultant stress state when isotropy is assumed for anisotropic rock is discussed through many simulations.