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Sapporo, Japan

Aoyagi K.,Japan Atomic Energy Agency | Tsusaka K.,INPEX Corporation | Nohara S.,Japan Central Research Institute of Electric Power Industry | Kubota K.,Japan Central Research Institute of Electric Power Industry | And 3 more authors.
ISRM International Symposium - 8th Asian Rock Mechanics Symposium, ARMS 2014 | Year: 2014

In the construction of any deep underground facility such as repository for high-level radioactive waste (HLW) disposal, significant changes in hydrogeomechanical properties around excavated galleries can be expected. This zone is called an Excavation Damaged Zone (EDZ). For the safety of HLW disposal, it is necessary to investigate the extent and hydrogeomechanical characteristics of an EDZ. In this research, the authors conducted in situ surveys such as seismic refraction surveying, geological mapping of the galleries, borehole television surveying, and hydraulic testing in the Horonobe Underground Research Laboratory. From the results of these surveys, the authors concluded that the extent of an EDZ, the area with a high-density of fractures, resulting in high hydraulic conductivity, was estimated to be about 0.2 to 1.2 m into the rock wall of the gallery. The authors also compiled the information of the extent of an EDZ and the hydrogeomechanical properties inside and outside of an EDZ as a conceptual model. Since the conceptual models provide the basic idea for determining flow and solute transport in an EDZ, the result of this research provides a useful data for safety assessment of HLW disposal. © 2014 by Japanese Committee for Rock Mechanics.

Mitsuia Y.,Hokkaido University | Harab S.,Dia Consultants Co. | Akabanea K.,Hokkaido University | Kodamaa J.,Hokkaido University | And 3 more authors.
ISRM International Symposium - 8th Asian Rock Mechanics Symposium, ARMS 2014 | Year: 2014

Surfaces of rock slopes in a cold region often get frozen in winter. Therefore, understanding time-dependent behavior of frozen rock as well as that of unfrozen rock is significant for long-term stability assessment of the rock slopes. In this study, creep tests were carried out on Shikotsu welded tuff under dry and wet conditions at -20 °C (253 K) to clarify effect of water content on the time-dependent behavior of frozen rock. The results showed that deformation of frozen wet specimens was much more than that of frozen dry specimens. It was also found that axial strain rate-axial creep strain curves of the frozen wet specimens were different from those of the frozen dry specimens. Ratio of tertiary creep region of frozen wet specimens was relatively longer. These differences can be explained by existence of pore ice. Furthermore, creep life of un-failure specimens could be estimated with axial strain rate because creep life showed negative correlation with the axial strain rate in the early stage. Based on both observed creep life of rupture specimens and estimated creep life of un-failure specimens, the creep life of frozen wet specimens was expected to be shorter than that of frozen dry specimens at creep stress below about 50% of uniaxial compressive strength of frozen wet specimens. This result indicates that the effect of water content should be investigated for long-term stability assessment of rock slopes in a cold region. © 2014 by Japanese Committee for Rock Mechanics.

Nakata E.,Japan Central Research Institute of Electric Power Industry | Tanaka S.,Japan Central Research Institute of Electric Power Industry | Suenaga H.,Japan Central Research Institute of Electric Power Industry | Mahara Y.,Kyoto University | And 3 more authors.
Applied Geochemistry | Year: 2012

Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ 13C 1=-44.3‰ was obtained for gas in the container and δ 13C 1=-36.3‰ for seeping gas in a fault zone. However, typical CH 4 in this dissolved-in-water gas-field is mainly depleted in 13C, and δ 13C 1 values range from -66‰ to -68‰ owing to microbial degradation of organic material. 13C-rich CH 4 is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north-south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30m. Gas seepages were not observed below a depth of 40m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. 13C-rich CH 4 (adsorbed CH 4) was found to have desorbed from drilled mudstone core samples taken at depths of 1400-1900m in the main gas-production strata. Similarly, 13C-rich CH 4 was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200-1500m along a tectonic line. Methane generated by microbial degradation of organic material through CO 2 reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of 13C-rich CH 4, derived from thermogenic gas without oil components in strata. It is assumed that 12C-rich CH 4 is easily detached from core or pore water through gas production, whereas 13C-rich CH 4 is strongly adsorbed on the surfaces of particles. The 13C-rich CH 4 rises along the major tectonic line or up the 50m wide normal fault zone from relatively deep sources in the Kanto region. © 2012.

Kawaida M.,Central Nippon Expressway Company Ltd | Kosaka T.,Dia Consultants Co. | Yamada K.,Central Nippon Expressway Company Ltd | Nakakuma K.,Maruyama Industry Co.
Zairyo/Journal of the Society of Materials Science, Japan | Year: 2012

Maizuru-Wakasa expressway was constructed on the soft ground area consisting a thick layer of peat and soft clay. In trial embankment construction, a large lateral displacement and settlement due to embankment load were observed. Therefore, vacuum consolidation (air-water separation system type) was selected to apply to another trial area nearby the previous one to improve the ground for embankment construction. It was found that the vacuum pressure could maintain at high level even after a successive consolidation settlement up to about 10m, A significantly incremental soil strength after consolidation settlement were observed. The analysis of soil strength increment induced by vacuum consolidation compared to another case history was analyzed and described in this paper. © 2012 The Society of Materials Science, Japan.

Watada S.,University of Tokyo | Kusumoto S.,University of Tokyo | Kusumoto S.,Dia Consultants Co. | Satake K.,University of Tokyo
Journal of Geophysical Research: Solid Earth | Year: 2014

Systematic tsunami traveltime delays of up to 15min relative to the numerically simulated long waves from the 2010 Chilean and 2011 Tohoku-Oki earthquakes were widely observed at deep ocean tsunamimeters. Enigmatic small negative phases appearing before the main peak were commonly found only at the trans-oceanic locations. The frequency dependence of the measured tsunami phase velocities shows reverse dispersions at long periods, i.e., the tsunami speed becomes slower at periods beyond 1000s. This is consistent with the phase velocities of a tsunami mode coupled with a self-gravitating elastic Earth, suggesting that the effects of compression and dilatation of seawater, elastic tsunami loadings on a solid Earth, and the geopotential variations associated with the motion of mass during tsunami propagation are responsible for the traveltime delays and the initial negative phases. Simple 1-D tsunami propagation tests confirm that the reverse dispersion creates a small negative phase that precedes the main peak at large distances. A new method to simulate tsunami waveforms on real ocean bathymetry that takes into account seawater compressibility, the elasticity of the Earth, and geopotential perturbations has been developed by applying a phase correction to the simulated long waves. The simulated waveforms, in which phase corrections are applied for the dispersion effects, accurately reproduce the observed waveforms, including a small initial negative phase that appears at distant locations. The traveltime difference between the observed and simulated waveforms has been decreased to less than 5min and the waveform difference between them remarkably diminishes. ©2014. American Geophysical Union. All Rights Reserved.

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