Constitutive modelling of gassy clay

Fine grained sediments with undissolved gas bubbles are widely distributed in the seabed around the world. The gas bubbles are much larger than the clay particles and fit in the saturated clay matrix rather than the pore water. Generally, these bubbles tend to degrade the soil stiffness and strength. But when the difference between the gas and pore water pressure is sufficiently small, pore water in the saturated clay matrix can drain into the cavities, making the void ratio of the saturated matrix smaller, which makes the undrained shear strength of the gassy clay sample higher than that of a saturated one. Such soil response cannot be described based on the assumption that gassy clay is a soil with compressible pore fluid. A new constitutive model for describing the stress-strain relation for gassy clay is proposed. An important feature of the model is that the gassy clay is considered as a composite material with compressible cavities which could be flooded by pore water. Effect of gas cavities on plastic hardening on the saturated matrix is accounted for. The model has been used to predict the response of three gassy clays and good agreement between the test data and model simulations is observed. Potential improvement of the model is discussed.