Pore-scale investigation of bubble evolution in shallow coastal gassy silt with micro-computed tomography

Shallow coastal gassy silts, characterized by isolated bubble trapping and heterogeneous bubble distribution within the sediment matrix, often serve as foundational substrates in nearshore and subsea engineering projects. Understanding the dynamic behavior of bubbles at the pore scale is crucial for evaluating the mechanical strength of gassy silts and their influence on engineering stability at a regional scale. In this study, we employed in-situ micro-computed tomography to investigate the effects of varying initial gas content and applied loading conditions on bubble evolution within gassy silts. Specimens were prepared using zeolite powder to simulate natural gas nucleation processes, with gas content levels of 2.0 % and 3.0 %, representing typical ranges of gas saturation observed in field conditions. These specimens were subjected to incrementally increasing vertical loads from 0 to 60 kPa, corresponding to pore scale stress conditions that simulate changes due to increasing water depths or geotechnical loads in coastal engineering applications. Our findings indicate that, with increasing stress, bubbles generally decrease in size, become more numerous, and shift toward more spherical shapes, aligning preferentially along directions parallel to stress transmission paths. The initial gas content plays a critical role in determining the processes of bubble evolution. Additionally, as loading intensifies, the total surface area of the bubbles decreases, suggesting a progression toward a more stable state for the gassy silt system overall. This work provides new pore-scale insights into bubble evolution in shallow coastal gassy silt.