Application and Integrated Evaluation of the Compact Conical-Ended Borehole Overcoring Technique in a Fractured Rock

Several rock stress measurement techniques have been proposed by different research groups. Due to factors that cannot be visually confirmed, such as for instance rock type, geological structure and heterogeneity, the applicability and accuracy of the different measurement techniques can vary widely. Validation and engineering judgement with respect to the measured results is therefore necessary. This study focusses on the Compact Conical-ended Borehole Overcoring (CCBO) technique (Sakaguchi at al., 1994, 1995 and 1999), which is a stress relief method. With the CCBO technique, the three-dimensional stress field can be determined with a single measurement. Measurements can be carried out in a small zone and are therefore also applicable in fractured rock. In this study, in-situ stress measurement was conducted by the CCBO technique in a horizontal borehole drilled into a natural slope that is composed of fractured rock. In-situ stress measurements were performed at four measurement points in the borehole. To verify the applicability to the fractured rock of the CCBO technique, three-dimensional FEM numerical analysis of the topography based gravitational stress distribution was carried out separately in order to compare the results. Furthermore, a new evaluation method was developed to integrate the several measurements results in order to perform a more reliable assessment of the in-situ stress. Based on the results of the integrated evaluation method, in-situ stress values could be calculated that were highly consistent with the numerical analysis results.