title: Constitutive modelling of fine-grained soils containing gas bubbles
Creators: name: Cai, Hongjian
Publisher: name: University of Glasgow
Year: 2023

description: Fine-grained marine sediments containing free gas bubbles can be frequently encountered in the seabed worldwide, which can cause large-scale submarine landslides and offshore foundation failures. This soil has a unique composite structure with gas bubbles fitting within the saturated soil matrix. Therefore, its mechanical behaviour cannot be described using conventional unsaturated soil mechanics. The gas cavities have a detrimental effect on the soil stiffness and strength when they are filled with undissolved gas because gas has low bulk modulus and shear stiffness. The cavities can be filled with gas and pore water due to ‘bubble flooding’. Bubble flooding has a beneficial effect on the soil stiffness and undrained shear strength because it makes the saturated soil matrix partially drained under a globally undrained condition. The critical state constitutive modelling approach for fine-grained soils containing gas bubbles (FGS) is presented, which accounts for the composite structure of the soil and bubble flooding. The new lower and upper bounds for the undrained shear strength of FGS are derived firstly by considering the effect of total stress path and plastic hardening of the saturated soil matrix. For the upper bound, it is assumed that there is only bubble flooding, and the shear strength of an unsaturated soil sample is the same as that of the saturated soil matrix. Bubble flooding makes the saturated soil matrix partially drained and increases the undrained shear strength. The amount of bubble flooding is calculated using the Modified Cam-Clay model and Boyle’s law for ideal gas. The lower bound is derived based on the assumption that the entire soil fails without bubble flooding and the gas cavity size evolves due to plastic hardening of the saturated soil matrix. Compared to Wheeler’s upper and lower bounds which do not consider plastic hardening of the saturated soil matrix, the new theoretical results give a better prediction of the undrained shear strength of FGS, especially for the upper bound. Implications for constitutive modelling of FGS is discussed based on the new research outcomes. A constitutive model for normally consolidated FGS is then proposed based on the new bounds. The cavities are assumed to have a detrimental effect on the plastic hardening of the saturated soil matrix because they damage the soil structure. The variable found in the new upper and lower bounds is introduced to capture this detrimental effect of gas bubbles. Some of the bubbles can be flooded by pore water from the saturated soil matrix, increasing the soil stiffness and strength. The new model uses stress quantities which can be readily measured, and only one parameter is introduced (as compared to the MCC model) to describe the effect of gas bubbles on the mechanical behaviour of FGS, making it easy to calibrate and use. The soil response in triaxial compression and isotropic compression is considered in the model. However, there are limitations for the conventional elastoplastic constitutive model to describe the mechanical behaviour of overconsolidated FGS. A constitutive model for overconsolidated FGS is derived based on the structure of that of the normally consolidated FGS. The bounding surface and the dilatancy relation are considered to describe the response of the overconsolidated FGS matrix. The model has been validated by the results of a series of tests. Finally, the comparisons of predictions from three models with test data are shown to indicate a progressive relationship among the models proposed in the thesis.