3D Elastoplastic Model for Fine-Grained Gassy Soil Considering the Gas-Dependent Yield Surface Shape and Stress-Dilatancy

Fine-grained sediments containing large discrete gas bubbles are widely distributed in the five continents throughout the world. The presence of gas bubbles could either degrade or enhance the hardening behavior and undrained shear strength (su) of the soil, depending on the initial pore water pressure (uw0) and initial gas volume fraction (psi 0). The existing constitutive models, however, can solely capture either detrimental or beneficial effect owing to the presence of gas. This study presents a new three-dimensional (3D) elastoplastic constitutive model that describes both the damaging and beneficial effects of gas bubbles on the stress-strain behavior of fine-grained gassy soil in a unified manner. This was achieved by incorporating (1) a versatile expression of yield function that simulates a wide range of yield curve shapes in a unified context, and (2) a dilatancy function capturing the distinct stress-dilatancy behavior of fine-grained gassy soil. Given the lack of direct experimental evidence on the shape of the yield curve of fine-grained gassy soil, new experiments were performed. This has led to the identification of three distinct shapes of yield curve-bullet, ellipse, and teardrop-as well as the formulation of the yield function considering the dependency of yield curve shapes on uw0 and psi 0. The new model was shown to reasonably capture both the damaging and beneficial effects of gas on the compression and shear behavior of three types of fine-grained gassy soils with a broad range of uw0 and psi 0 by using a unified set of parameters.