As a result of recent investigations, carried out mainly by the MH21 research consortium of Japan, methane hydrate, referred to hereafter as ''MH'', is expected to become a possible future energy resource. In order to evaluate the stability of the seabed during MH production, previous research on the mechanical properties of MH-bearing sand have been conducted under triaxial stress conditions. However, these were very specific conditions whereas several stress conditions are assumed to exist in the Nankai Trough where MH production is carried out. Therefore it is essential to consider these stress conditions in order to understand the changes in stress and deformation behavior of the seabed. In this study, we describe the effects on MH-bearing sand under plane strain conditions with changing confining pressure. The apparatus used for plane strain tests has both a confining plate and an observation window. Local deformation of MH-bearing sand is shown using photographs of specimens taken during shear tests. A marked increase in the stress ratio of the host sand (Toyoura sand) and MH-bearing sand was more apparent in the plane strain tests than in the triaxial tests. The stress ratio decreased dramatically after the appearance of the peak stress ratio. Moreover, the peak stress ratio of MH-bearing sand was higher than that of the host sand as shown in previous research. After shear, in the case of the host sand, particle crushing increased as the effective confining pressure increased. Additionally, more particle crushing occurred inside the shear band than outside. In the case of the host sand, the failure strength for plane strain tests shifted from the SMP criterion to the Mohr-Coulomb criterion as the effective confining pressure increased, due to particle crushing. In the case of MH-bearing sand, the failure strength for every effective confining pressure lay between the SMP criterion and Mohr-Coulomb criterion. Expansive volumetric dilation increased within the shear band as the axial strain increased for each case, with more dilation occurring for MH-bearing sand compared with host sand.

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Methane hydrates are viewed as not only a new energy resource but also a trigger of marine geohazard such as large deformations and landslides of seabed ground. Many researcher have showed that methane hydrate containing sand specimens become stronger than that of without hydrates, however, few study have been performed to investigate the mechanical properties subjected to the hydrate dissociation. The main purpose of our study is to investigate behaviors of the deformation associated with dissociation of the hydrates. In this study, we prepared carbon dioxide hydrate containing specimens and conducted a series of dissociation tests using a heating method under undrained conditions. These experiments require the high pressure and the low temperature conditions to form the carbon dioxide, and we have developed a new triaxial apparatus. The dissociation results indicates that even a small percentage of hydrate saturation produced the large pore gas pressure and the sequential reduction of the effective confining pressure due to the hydrate dissociation, and extensive strain was observed.

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Recently there has been much research into Methane Hydrate (referred to as MH hereafter) as a developable material in the deep seabed in Nankai Trough. In this study, specimens were prepared to simulate the sediments in Nankai Trough, and a series of triaxial tests were performed on samples with varying density and fines content Fc of 0%, 8.9% and 25%. MH was produced with almost 30 and 50% MH saturation in the specimens. The apparatus can simulate the deep sea-bed environment. From the results of only the host sands, it was found that as the fines content increased, stiffness decreased and the volumetric strain increased. In the case of MH bearing sand however, an increase in fines content tended to lead to an increase in stiffness and maximum axial deviator stress, with positive dilative behavior occurring during shearing.

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In this study, to clarify the distribution of gas-hydrate and its formative environment of seabed methane, and to evaluate the potential as resource of gas-hydrate, the geologic surveys were conducted by NT13-20 and OS263 cruises covering a wide area of Abashiri and Shari Submarine Canyons in the Okhotsk Sea. The conducted investigations are the seismic geologic survey, the bathymetry and the gas-flare observation by an echo sounder, and core sampling of sea-bottom sediments. Moreover, to examine the soil properties of sea bottom sediments, some kinds of shipboard tests such as handy vane shear and cone penetration tests, and physical properties tests such as water content and grain size distribution tests, were performed for the samples.

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This paper presents numerical simulations on frost heave phenomenon observed at the earth structures and pavement structures using a coupled thermo-hydro-mechanical (THM) FE analysis, which can consider the change in deformation-strength characteristics and water retention-permeability characteristics due to freeze-thaw actions, and can reproduce frost-heave of soils under unsaturated conditions based on the results of frost heave tests. In order to evaluate the influences of freeze-thaw action on the interaction between frost-susceptible soil ground and structures quantitatively, a series of numerical simulations for cut slope and pavement structure subjected to freeze-thaw actions were performed. As the results, it was revealed that the coupled FE analysis employed in this study has a sufficient applicability to the precise prediction for the frost-heave behavior of soil ground, and that the frost-heave behavior has a considerable influence on the deformation and failure of earth structures in terms of the deformation and stress state of structures by examining the relation between frost heave and frost penetration depth.

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Frost susceptibility of soil is important parameter in construction of buildings in silt (clay) soil and cold regions. Therefore experimental investigation of frost susceptibility has high importance. Definition of frost characteristics soil predicts unsuspectible deformations of soil ground. Also extra measures could be predicted for lowering frost susceptibility. In-situ monitoring of frost depth is measure which also important for designing of soil ground. Frost depth monitoring system gives freezing depth results and moisture content for prediction of frost susceptibility of soil.

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