On the undrained unloading behaviour of gassy soils

Soils that contain large amounts of dissolved gas within the pore fluid are called gassy soils. Gassy soils are common in marine environments and it is important to further our understanding of the unloading behaviour of gassy soils because of their potential to initiate and propagate submarine slope failures. This thesis focuses on the pore pressure responses and volumetric strains of loose gassy sands under different undrained unloading stress paths in laboratory specimens. Special attention was given to the constant deviatoric stress (q-constant) undrained unloading stress path as it simulates the stress condition imposed by tidal drawdown; one of the potential triggers of landslides in gaseous marine sediments. Gas exsolution was observed when the pore pressure was reduced below the liquid gas saturation pressure. Upon further decreases in total stress, the resulting pore pressure change was much less than the total stress change; hence effective stress decreased rapidly, and at some point the samples tested under q-constant stress path collapsed. This thesis has experimentally and theoretically shown that gas in free and/or dissolved form is detrimental in undrained unloading stress paths.