Effect of Cylindrical THF Hydrate Veins on the Undrained Behavior of Fine-Grained Soils

Gas hydrate filled fractures and veins readily occur in fine-grained deep water marine sediments that increase sediment strength and restrict sediment consolidation. Subsequent hydrate destabilization can dramatically reduce sediment strength, which may lead to slope failures. To investigate the undrained behavior a series of consolidated undrained (CU) triaxial tests were carried out on fine-grained soils containing cylindrical tetrahydrofuran (THF) hydrate veins of varying diameter to mimic naturally occurring hydrate-bearing clays. Axial compressions tests on stand-alone hydrate veins showed brittle failure with axial stresses reasonably independent of vein diameter and confining stress and thought related to development of bending stresses and tensile cracking. Reduced axial strain rates led to ductile behavior, potentially suppressing tensile crack development, resulting in slightly higher failure stresses. CU shear tests on hydrate-bearing specimens showed increasing strength and stiffness with increasing vein diameter and confining stress, although the impact of confining stresses reduced for the largest diameter veins. Using a Mohr-Coulomb failure criteria, increased strength was associated with increase in cohesion and reduction in friction angle. The enhanced strength would reduce consolidation processes that would lead to sediment instabilities if the hydrate was destabilized. However, for larger diameter veins, increasing lateral stresses from the soil reduced buckling stresses with significant plastic deformation of the THF veins being observed at the end of tests, suggesting a strain rate dependent behavior. Further research is required to fully understand this behavior and its impact on sediment consolidation to fully consider the relationship between hydrate dissociation and sediment instability.