Constitutive modelling of fine‐grained gassy soil: A composite approach

Abstract

SummaryFine‐grained marine sediments containing large undissolved gas bubbles are widely distributed around the world. Presence of the bubbles could degrade the undrained shear strength (su) of the soil, when the gas pressure ug is relatively high as compared with the effective stress in the saturated soil matrix. Meanwhile, the addition of bubbles may also increase su when the difference between ug and pore water pressure uw becomes smaller than the water entry value, causing partial water drainage from the saturated matrix into the bubbles (bubble flooding) during globally undrained shearing. A new constitutive model for describing the two competing effects on the stress‐strain relationship of fine‐grained gassy soil is proposed within the framework of critical state soil mechanics. The gassy soil is considered as a three‐phase composite material with compressible cavities, which allows water entry from the saturated matrix. Bubble flooding is modelled by introducing an additional positive volumetric strain increment of the saturated clay matrix, which is dependent on the difference between pore gas and pore water pressure based on experimental observations. A modified hardening law based on that of the modified Cam clay model is employed, which in conjunction with the expression for bubble flooding, can describe both the detrimental and beneficial effects of gas bubbles on soil strength and plastic hardening in shear. Only two extra parameters in addition to those in the modified Cam clay model are used. It is shown that the key features of the stress‐strain relationship of three fine‐grained gassy soils can be reproduced satisfactorily.