Stress-dilatancy behaviour of bubbled fine-grained sediments

Although the strength and stiffness of fine-grained gassy soil have been relatively well characterised, its dilatancy remains poorly understood. This study aimed to understand and formulate the stress-dilatancy behaviour of fine-grained gassy soil through a series of triaxial tests (19 in total) conducted on fine-grained gassy specimens where different initial gas volume fractions (ψ0) and initial pore water pressures (uw0) were considered. Compared to the stress-dilatancy relation of saturated fine-grained soil, the addition of gas bubbles was found to result in more contractive or dilative soil behaviour, depending on the combination of ψ0 and uw0. These distinct features cannot be captured by the existing stress-dilatancy functions for fine-grained saturated soils, which solely depend on the stress ratio. Therefore, a new stress-dilatancy function is proposed within the framework of critical state soil mechanics to quantify the distinct modification effects of bubbles (which either suppress or enhance dilatancy) on the dilatancy of gassy soils with different ψ0 and uw0 values in a unified manner. This new function has been validated and can be readily implemented in critical state based constitutive models for predicting the shear behaviour of fine-grained gassy soil and therefore, the response of various offshore structures (i.e., foundations and pipelines) founded on a gas-bearing seabed. •Stress-dilatancy of fine-grained sediment containing large bio-gas bubbles are investigated by 19 triaxial tests.•The bubble can make the soil either more contractive or dilative, depending on the initial pore water pressure.•A new dilatancy function considering initial pore water pressure and gas content is proposed.•The new dilatancy function can be readily implemented into critical state based constitutive models.