The Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique is one of the overcoring methods. In this study, a measurement theory for application of the DCCBO technique to orthotropic rock is proposed. Numerical experiments with the use of a 3D-FEM analysis were conducted to confirm the applicability and efficiency of the proposed measurement theory. The results showed that the measurement includes a non-negligible error if we do not consider the anisotropy of rock when such anisotropy is strong. On the other hand, this measurement is both applicable and efficient if we consider the anisotropy of rock. Finally, laboratory experiments for four kinds of orthotropic rocks using a true-triaxial compressive apparatus were carried out to verify the efficiency of the proposed measurement theory. These experiments also verified that the proposed method is suitable for use in orthotropic rock.
The Downward Compact Conical-ended Borehole Overcoring technique (Sakaguchi et al., 2003(a), 2003(b), 2004, 2006) is a stress-measurement method based on the Compact Conical-ended Borehole Overcoring (CCBO) technique (Sakaguchi et al., 1994; Sugawara & Obara, 1999). This stress-measurement technique can be applied to a water-filled vertical borehole to measure in-situ stress at great depth. Stress-measurement methods based on the overcoring method assume that the rock mass is linearly elastic, isotropic, continuous, and homogeneous. However, a rock mass is actually anisotropic to some degree. Amadei (1996) noted that the anisotropy of rock must be considered in stress measurement when it reaches a certain level. In this study, we carried out theoretical, numerical and experimental studies to apply the DCCBO technique to an orthotropic rock. First, we proposed a theory of measurement to apply the DCCBO technique to orthotropic rock. Second, a numerical experiment was carried out to confirm the effectiveness of the proposed measurement theory.