Mizunami Underground Research Laboratory project. Rock mechanical investigations in situ stress measurements and laboratory tests using core on GL.-100m level of research gallery

In order to establish the scientific and technical basis of geological disposal of high level radioactive waste, Japan Atomic Energy Agency (JAEA) is advancing the geo-scientific research at the Mizunami Underground Research Laboratory (MIU). In this project, the surface-based investigation phase (Phase I) finished in fiscal year 2004. The Construction phase (Phase II) was started in fiscal year 2001 to overlap with the Phase I activities. In Phase II, construction of a conceptual model of the geological environment and the observation and study of excavation effects are two of the scientific and technical objectives. Accordingly, additional investigations in Phase II, the checking of the relationship between the accuracy of results and the amount of information required and evaluating the effectiveness of the stepwise investigations were needed. Therefore in situ stress measurements and laboratory tests on core from the GL.-100m level in the MIU research gallery were planed as additional rock mechanical investigations in Phase II, to develop further understanding of the rock mechanical environment. This report describes the results of these rock mechanical investigations. A brief summary is presented as follows. 1) The average physical and mechanical properties of the rock mass determined using core from the Toki Lignite-bearing Formation are: - apparent specific gravity (dry): 1.51; - unconfined compressive strength (U.C.S.): 10.7MPa; - tangent modulus at 50% U.C.S: 4.97GPa; - poisson's ratio: 0.42. These results are consistent with the results from the preliminary investigations in Phase I in the MIZ-1 borehole. 2) Estimation of in situ stress by measurements on core. Two different methods (AE and DRA) were applied to numerous samples from three boreholes. However, about half of the AE/DRA samples provided no information on the existing stress state in each sample because of the likely release of any pre-existing stresses. The maximum principal stress is oriented in the NW-SE direction. But the vertical principal stress is could not be determined. 3) Determining of in situ stress from stress relief. Two kinds of measurement tools were used. The CSIR triaxial strain cell was not suited to the investigation conditions (i.e., used in a horizontal borehole, in the Toki Lignite-bearing Formation etc.). Another tool, the conical-ended strain cell worked successfully. The maximum principal stress is oriented in the NW-SE direction. It is equal to the regional surface strain state. 4) Determining in situ stress by hydraulic fracturing. Fracturing induced by hydraulic pressure did not develop parallel to the axis of borehole. Therefore the usual stress analysis was not applicable. Fortunately, the pressure needed to re-open six non-parallel fractures could be determined and thus could be treated as six independent components of stress, were used for the successful determination of the initial stress state. This state gives the maximum principal stress oriented in the NW-SE direction. It is equal to the regional surface strain state. 5) Comparison with investigation results in MIZ-1 and at the GL.-100m level in the MIU. The average rock physical and mechanical properties at the GL.-100m level research gallery are consistent with the results from MIZ-1 testing. This means that the surface based investigations in MIZ-1 are valid at this level. Almost all in situ stress measurements indicate that the maximum principal stress is oriented in the NW-SE direction. The stress states at GL -100m depth match the observations in MIZ-1 at GL.-200m depth. But the observed stress level is not the expected value extrapolated from the MIZ-1 results. This suggests that the in situ stress has likely been decoupled at the rock boundary located at about the GL.-150m level between the Toki Lignite-bearing formation and the Toki granite. (author)