It is essential to consider the mechanical behavior of natural gas hydrate reservoir to have sustainable production of natural gas from the reservoir under the seafloor. Although the mechanical properties of marine sediments containing natural gas hydrates are essential to simulate the geo-mechanical response to gas production from the reservoir, they are not fully understood. In this study, effect of confining pressure on mechanical properties of artificial methane hydrate bearing sediment was experimentally examined and discussed. Drained triaxial compression tests were conducted for artificial sediment specimens containing Toyoura sand (average particle size D_(50) = 230 × 10~(-6) m and fine fraction content F_c = 0 %) and synthetic methane hydrate (hydrate-sand specimen), following the testing methods of Masui et al. (2005). During axial loading process in the tests, the specimen was compressed axially at 0.1 %/min of strain-rate under hydrate-stable condition, with 278 K of temperature and 8 MPa of pore water pressure. The cell pressure, or total confining pressure, was kept constant at 8.5 MPa, 9 MPa, 10 MPa or 11 MPa. Axial displacement and lateral displacement were measured with 25 mm and 5 mm linear variable differential transformers. Axial load was measured with a 50 kN load cell. The findings can be summarized as follows; (1) the hydrate-sand specimen becomes ductile by increase of confining pressure like many other geological materials; (2) the hydrate-sand specimen appears to be restrained from expanding diametrically by confining pressure; (3) strength and stiffness of the hydrate-sand specimen increases with confining pressure; (4) strength of No.8 silica sand containing methane hydrate (Masui et al. 2008) does not greatly differ from that of hydrate-sand specimen in this study; and (5) cohesion and internal friction angle of the hydrate-sand specimen, or the Mohr-Coulomb failure criterion, can be formulated as a function of methane hydrate saturation. These findings concerning the effect of confining pressure will be of considerable help not only in full understanding of the deformation mechanism of methane hydrate bearing sediments, but also in proposal of constitutive equation and numerical simulation in the future.
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