1:45 PM - 3:15 PM
[HCG21-P03] Verification study on borehole investigation technologies: (4) Physical, mechanical, and thermal properties of rock mass
Keywords:Radioactive waste, Geological disposal, Boring survey, Hayama Group, Laboratory rock test
1. Introduction
In the preliminary investigation of geological disposal, it is necessary to obtain data on physical, hydraulic, mechanical and thermal properties, which will contribute to a design of the repository, such as a depth of the repository and the spacing for placement of the waste. These data are obtained mainly by borehole investigations, but data that cannot be obtained by borehole investigations are measured by laboratory tests using the boring cores. However, it is necessary to confirm whether the test methods are applicable to a design of the repository. In this study, laboratory tests were conducted using borehole cores from the NUMO-CRIEPI joint study "Verification study", and the effectiveness and issue of the methods and results were discussed.
2. Test methods
In this study, the physical property tests are density test, ultrasonic velocity test, cation exchange capacity test, effective porosity test. The hydraulic property tests are water permeability test, water content ratio test, gas permeability test, water retention test, and unsaturated permeability test. Mechanical property tests are uniaxial compression test, splitting tensile test, triaxial compression test, slaking test, and axial swelling strain and stress test. Thermal property tests are specific heat test and thermal conductivity test. In principle, these tests were conducted in accordance with the method of previous studies and the standards established by JIS, academic societies and other organizations.
3. Results
The specimens assumed in the standards are often a hard rock. In contrast, the specimens in this study are soft Hayama Group (muddy rock). Therefore, we confirmed the applicability to soft muddy rocks. As a result, many of the data similar to previous studies e.g., 1) with accuracy according to the standards. The Nationwide Map of "Scientific Features" for Geological Disposal published by METI, indicates "within distance of 20 km from the coast" as criteria to identify preferable features, and there is a potentiality of preliminary investigations in coastal areas where soft geological formations are expected. Thus, we think previous methods are applicable in the tests of soft rocks for the disposal.
However, many of the cores in the joint study obtained were in poor condition, for example, with many fractures. If the cores are in poor condition, it may not be possible to evaluate the effectiveness and quality of the tests. Therefore, for the mechanical and hydraulic property tests, which are particularly important in the evaluation of a design of the repository, we confirmed the quality of the tests using Opalinus clay (hard clay shale from the Jurassic Opalinus clay formation in northern Switzerland) with known properties.
The core diameter obtained in this joint study is 83 mm. On the other hand, the specimen dimensions of the standard are 50 mm in diameter which is widely used. In general, the borehole core is shaped to the diameter of the standard and tested. However, if the core is in poor condition and coring is difficult, it is reasonable to use the core for testing as is without coring. Therefore, we performed pre-tests of Opalinus clay with standard and non-standard dimensions to confirm the consistency of data and the applicability of method.
And, in the axial swelling strain and stress tests, specimens contained swelling clay sometimes required more than 14 days to end the test. In the tests where the values change gradually and converge, it is difficult to determine the end of the test, but this is not clearly indicated in the standard. Therefore, we considered how to determine the end of the swelling test. We applied and compared the hyperbolic approximation to the results until 7 days and 48 days. The results showed that applying the approximation to the results for the 7 days obtained values that were almost equivalent to the convergence values.
References
1) Kondo et al. (2014), Jour. Geol. Soc. Japan, 120, 12, 447-471.
In the preliminary investigation of geological disposal, it is necessary to obtain data on physical, hydraulic, mechanical and thermal properties, which will contribute to a design of the repository, such as a depth of the repository and the spacing for placement of the waste. These data are obtained mainly by borehole investigations, but data that cannot be obtained by borehole investigations are measured by laboratory tests using the boring cores. However, it is necessary to confirm whether the test methods are applicable to a design of the repository. In this study, laboratory tests were conducted using borehole cores from the NUMO-CRIEPI joint study "Verification study", and the effectiveness and issue of the methods and results were discussed.
2. Test methods
In this study, the physical property tests are density test, ultrasonic velocity test, cation exchange capacity test, effective porosity test. The hydraulic property tests are water permeability test, water content ratio test, gas permeability test, water retention test, and unsaturated permeability test. Mechanical property tests are uniaxial compression test, splitting tensile test, triaxial compression test, slaking test, and axial swelling strain and stress test. Thermal property tests are specific heat test and thermal conductivity test. In principle, these tests were conducted in accordance with the method of previous studies and the standards established by JIS, academic societies and other organizations.
3. Results
The specimens assumed in the standards are often a hard rock. In contrast, the specimens in this study are soft Hayama Group (muddy rock). Therefore, we confirmed the applicability to soft muddy rocks. As a result, many of the data similar to previous studies e.g., 1) with accuracy according to the standards. The Nationwide Map of "Scientific Features" for Geological Disposal published by METI, indicates "within distance of 20 km from the coast" as criteria to identify preferable features, and there is a potentiality of preliminary investigations in coastal areas where soft geological formations are expected. Thus, we think previous methods are applicable in the tests of soft rocks for the disposal.
However, many of the cores in the joint study obtained were in poor condition, for example, with many fractures. If the cores are in poor condition, it may not be possible to evaluate the effectiveness and quality of the tests. Therefore, for the mechanical and hydraulic property tests, which are particularly important in the evaluation of a design of the repository, we confirmed the quality of the tests using Opalinus clay (hard clay shale from the Jurassic Opalinus clay formation in northern Switzerland) with known properties.
The core diameter obtained in this joint study is 83 mm. On the other hand, the specimen dimensions of the standard are 50 mm in diameter which is widely used. In general, the borehole core is shaped to the diameter of the standard and tested. However, if the core is in poor condition and coring is difficult, it is reasonable to use the core for testing as is without coring. Therefore, we performed pre-tests of Opalinus clay with standard and non-standard dimensions to confirm the consistency of data and the applicability of method.
And, in the axial swelling strain and stress tests, specimens contained swelling clay sometimes required more than 14 days to end the test. In the tests where the values change gradually and converge, it is difficult to determine the end of the test, but this is not clearly indicated in the standard. Therefore, we considered how to determine the end of the swelling test. We applied and compared the hyperbolic approximation to the results until 7 days and 48 days. The results showed that applying the approximation to the results for the 7 days obtained values that were almost equivalent to the convergence values.
References
1) Kondo et al. (2014), Jour. Geol. Soc. Japan, 120, 12, 447-471.