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

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 ${\mathrm{N}}_2$ 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 ($u_{\mathrm{w}0}=0–600\mathrm{kPa}$) 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 $u_{\mathrm{w}0}$ 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 $u_{\mathrm{w}0}$, respectively. This implies that water depth would significantly influence the cyclic deformation and stiffness degradation of offshore foundations in fine-grained gassy sediments.