Distinct Undrained Cyclic Behavior of Fine-Grained Gassy Soil at Various Initial Pore Water Pressures: Fid-Bau Portal

Distinct Undrained Cyclic Behavior of Fine-Grained Gassy Soil at Various Initial Pore Water Pressures

Hong, Y. / Chen, X. Y. / Wang, L. Z. / Yang, B. / Sanchez, M.

Undissolved gas bubbles, which are widely formed in marine sediments, could modify the behavior of the soil. To date, the cyclic behavior of fine-grained gassy soil has been rarely studied, limiting any reliable analysis of cyclically loaded offshore foundations in fine-grained gassy seabed. This study presents a series of undrained cyclic triaxial tests on normally consolidated (NC) fine-grained gassy soil. Each sample was charged with the same amount of $N_{2}$ gas before consolidation. Then, samples were consolidated to an identical mean operative stress (total stress minus pore water pressure) of 200 kPa, but at different initial pore water pressures ( $u_{w 0} = 0 - 600 kPa$ ) before cycling. Compared with a saturated sample, the inclusion of a small amount of gas bubbles can either significantly decrease the level of cyclic stiffness degradation when the initial pore water pressure $u_{w 0}$ is lower than a threshold value (150 kPa in this study), or vice versa. This behavior is associated with two competing mechanisms during the cyclic shearing: (1) collapse of gas-filled pores, which increases cyclic pore water pressure (CPWP), and therefore stiffness degradation; and (2) partial water drainage into gas cavities, which reduces CPWP and stiffness degradation. The former and the latter are likely to dominate at relatively high and low $u_{w 0}$ , respectively. This implies that water depth would significantly influence the cyclic deformation and stiffness degradation of offshore foundations in fine-grained gassy sediments.