Development of downward compact conical-ended borehole overcoring technique for in situ rock stress determination in deep borehole and its application examples

To determine the complete three-dimensional in situ stress in a deep borehole, the downward compact conical-ended borehole overcoring (DCCBO) technique, which is a type of stress-relief method, was developed. The DCCBO technique can be applied to a water-filled vertical/sub-vertical borehole. In a shallow in situ test at a gallery for an underground power station, a measuring system consisting of a bonding device, a system for cleaning the bottom of the borehole and a multi-channel data-logger were verified to work well in a water-filled borehole with cuttings at the bottom of the borehole. With regard to the direction of principal stress, reasonable results were obtained by an in situ test. Thus, we confirmed the practicality of this DCCBO technique.