Development of Downward Compact Conical-ended Borehole Overcoring Technique for In Situ Rock Stress Measurement in Deep Borehole

Accurate determination of rock stresses at great depth is one of the most important subjects that should be resolved in the fields such as deep geological disposal of nuclear fuel waste and underground sequestration of carbon dioxide. To determine complete three-dimensional in situ stress at great depth with a high accuracy, the Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique, which is one of stress relief methods, was developed. DCCBO technique can be applied to a water-filled vertical/sub-vertical borehole. The main features of this technique are as follows: 1) This technique is applicable to a water-filled borehole by using an appropriate adhesive.2) A diving-bell-type bonding device keeps a strain cell dry until the moment of installation.3) Just before the strain cell reaches the bottom of the borehole, high-pressure nitrogen gas cleans the borehole bottom, where cuttings may exist.4) The process described above proceeds within a PQ bit which overcores the strain cell.5) The data during the overcoring is recorded by a multi-channel data logger which is installed at the top of the strain cell.6) A system consisting of a small video camera and compass for single shots measure the direction of the strain cell.In laboratory test, a measuring system consisting of a bonding device, a cleaning system and a multi-channel data logger were verified to work well in a water-filled borehole with cuttings on the bottom. Reasonable results were able to be obtained by measurement sensitivity test using a specimen of rock block. Thus, we confirmed the usability of DCCBO technique.