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. Plain Language Summary: Gas hydrate, an ice‐like material, grows in marine sediments that which increases the sediments strength and prevents its consolidation. If the hydrate disappears the loss of strength may lead to slope failures. Laboratory tests were carried out to understand the behavior of similar soils containing cylindrical tetrahydrofuran (THF) hydrate veins of varying diameters to mimic natural hydrate‐bearing sediments. Testing of stand‐alone hydrate veins showed that they failed rapidly at similar vertical stresses regardless of vein diameter and confining stresses. This behavior was thought related to bending stresses causing cracking of the vein. Reducing how quickly the stress was applied slightly increased failure stresses. Testing of hydrate‐bearing soil specimens showed increasing strength and stiffness with increasing vein diameter and confining stresses, although the impact of confining stress reduced for the largest diameter veins. This suggests that natural hydrate‐bearing sediments would experience similar behavior and therefore might fail if the hydrate disappeared. However, for larger diameter veins, increasing confining stresses reduced buckling stresses on the vein so that THF veins bent rather than broke. Further studies are required to understand how this might change sediment consolidation which would affect sediment instability if the hydrate disappears. Key Points: Consolidated undrained (CU) tests were carried out to determine the undrained behavior of fine‐grained soils containing cylindrical tetrahydrofuran veinsCU tests showed increased strength and stiffness of the hydrate‐bearing soil with increasing vein diameter and confining stressesIncreasing lateral soil stresses suppress bending stresses in the vein leading to ductile behavior and significant plastic deformation [ABSTRACT FROM AUTHOR]