In mining and civil engineering, it is essential to understand an in situ stress state when dealing with underground excavations. An in situ stress state can be complicated due to rock mass heterogeneities, geological structures, topography, tectonic movements, etc. Several in situ stress measurement techniques have been developed to provide more reliable data for the design of underground openings. The results obtained from field measurement, however, often only represent a local stress state in a rock mass where the measurement was taken. This study presents a back analysis based on three-dimensional finite element analysis in order to estimate regional stress fields and the elastic moduli of a rock mass simultaneously from in situ measurements. In order to improve the accuracy of the estimation of a three-dimensional stress field, heterogeneities in a rock mass consisting of geological layers with different elastic properties were considered. Back analysis was applied to determine the regional stresses for a broad field study that includes the Tono Mine, the Shobasama Site and the Mizunami Underground Research Laboratory (MIU) Construction Site.

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This paper presented the formulation of a divided linear strain softening model of soft rock using the unconfined compressive strength for incorporation into a numerical analysis code, and applied the model to sequential tunnel excavation analysis. The effectiveness of the model and problems in relation to numerical analysis were discussed. As problems in making analysis, the volumetric change (dilation) during strain softening causes turbulence in the distribution of displacements, and the variations of stress in elements due to stress release during strain softening greatly affect the results of analysis. It was therefore suggested that properly controlling stress variations would produce excellent results.

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The purpose of this study was to clarify the relationship between axial point load strength and uniaxial compressive strength in hydrothermally altered rocks, which are typical of the soft rocks found in northeastern Hokkaido, Japan. The numbers of specimens tested were 1,755 rock specimens for the axial point load strength test, whereas 329 rock specimens for the uniaxial compressive strength test. These came primarily from the earth’s surface in ancient hydrothermal fields. The rock specimens underwent axial point load strength and uniaxial compressive strength tests using a laboratory testing machine with specimens in forced-dry and forced-wet states.The correlation between the axial point load strength and the uniaxial compressive strength was linear. The forced-dry and forced-wet states can be combined to giving a relationship in which the relation of axial point load strength (I_s) and uniaxial compressive strength (q_u) was q_u = 12.9 I_s in the soft rocks with axial point load strengths below 1.5 MPa. This relationship might be applied to on-site tests of rocks with natural moisture content. The number of tested specimens satisfied accuracy requirements, based on the coefficient of variation. The axial point load strength was strongly correlated with the uniaxial compressive strength. Therefore, the relationship between axial point load strength and uniaxial compressive strength established in this study was highly precise. We could calculate the uniaxial compressive strength from axial point load strength tests in the soft rocks only when their axial point load strengths were below 1.5 MPa.

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This study aims to investigate the permeability and storativity characteristics of sedimentary rocks injected with supercritical CO2. Recent development of CO2 storage in sedimentary rocks requires knowledge of CO2 behavior in deep undergroundcharacterized as geological layers with low hydraulic gradient, high pressure and high temperature. Therefore, we developed a new flow pump permeability test, so that the test will be able to reproduce similar physical condition of deep underground whereCO2 behaves in supercritical state. The injection of supercritical CO2 was conducted on a cored Ainoura sandstone saturated withwater. A numerical simulation based on the theoretical analysis of flow pump permeability test incorporating Darcy's law for two phases flow was also undertaken in order to interpret the experimental results especially to examine the relative permeability and saturation of the saturated water and supercritical CO2 including the specific storage of the sandstone during supercritical CO2 injection. It is observed that, supercritical CO2 apparent to spend a very long time in flowing through the sandstone pores. The specific storage of the sandstone increases due to the displacement of the saturated water by incoming supercritical CO2. Very slow process of supercritical CO2 migration in the sandstone pores might be caused by the low hydraulic gradient employed in the experiment and the effect of capillary pressure functioning as such capillary trapper. These results indicate low permeable rocks could become an effective storage of supercritical CO₂. This study also shows that a new developed flow pump permeability test has been able to work with supercritical CO₂ injection to low permeable rocks.

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In this research, newly developed numerical approaches using the Distinct Element Method (DEM) were presented, and a series of DEM simulations were performed for better understanding the physical phenomena and mechanism for the following two fundamental issues in rock engineering field. The first issue is the Class II behavior of the brittle rocks under uniaxial compression. The radial strain control method for uniaxial compression tests was introduced in the DEM codes and the Class II behavior of rocks was simulated. The simulation results suggest that the DEM can reproduce the Class II behavior of the rock successfully and revealed that the loading condition of rocks (radial strain control) will play an important role for the Class II behavior. The second issue is the hydraulic fracturing behavior in rocks. A series of simulations for hydraulic fracturing in rock was performed by using the flow-coupled DEM code. Simulation results clearly show that the fluid infiltration behavior depends on the fluid viscosity. The fluid infiltrates into the fracture immediately, when a low viscosity fluid is used and the fluid infiltrates slowly into the cracks after the fracture generation and propagation, when a high viscosity fluid is used. Moreover, the tensile cracks are dominantly generated in the DEM simulations as expected in the conventional theory. However, the energy released from tensile cracks becomes smaller due to the fact that the tensile strength of rock is usually smaller than the compressive one. Such a small AE events is not distinguishable from noise and hard to recognize during laboratory experiments. Therefore, in AE measurements, shear type AE events with large energy are dominantly observed.

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Sedimentary soft rock is expected to be one of the potential host rock for the high level radioactive waste geological disposal. It is necessary to develop some evaluating methods for long-term stability of caverns in sedimentary soft rock which is sensitive for changes of environmental factors such as temperature and groundwater. Accordingly, in-situ heating test was conducted in an underground cavern at a depth of 50m for the purpose of improving thermo-hydro-mechanical coupled analysis code, and numerical results were compared with measured results. The comparison between numerical and measured results for the heater's temperature of 90 °C is reported in this paper. Deformation of soft rock during the heating test was able to be broadly reproduced by numerical model considering linear elasticity and thermal expansion.

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Natural gas hydrate, existing in marine sediments worldwide and in permafrost regions, is anticipated to be a promising energy resource. It is essential to consider the mechanical properties, including their time dependence, of a gas hydrate reservoir to simulate the geomechanical response to gas extraction from a reservoir. Recently it has been revealed that gas-hydrate-bearing sediments have rocklike mechanical characteristics due to the cementation effect of the hydrate between soil particles. To obtain information about the time dependence of gas-hydrate-bearing sediments, experimental methods for drained triaxial compression tests including a procedure for the preparation of artificial methane-hydrate-bearing sediment specimens have been established. Using these methods, constant-strain-rate tests and creep tests on artificial methane-hydrate-bearing sediment specimens have been conducted. In this report, the methods and results of the tests are presented, and the time-dependent behaviors of methane-hydrate-bearing sediment are discussed. On the basis of the results, the strain-rate dependence of the peak strength was examined, and it was found that the time dependence of the artificial methane-hydrate-bearing sediment is as strong as that of frozen sand and stronger than that of many other geological materials. It was also found that the creep deformation of methane-hydrate-bearing sediment is much larger than that of water-saturated sand without the hydrate. The experimental data presented in this report are expected to be used to obtain a full understanding of the deformation mechanism of methane-hydrate-bearing sediments and to formulate a constitutive equation for methane-hydrate-bearing sediments in future studies.

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