Constitutive modeling of gassy sand behaviour

The behaviour of loose gassy sand was investigated with the use of a constitutive model. The constitutive model was modified from an existing model that focused on assessing the liquefaction of loose sands over a wide range of states and loading conditions. The modifications involved taking into account the compressibility and solubility of the pore gas and liquids. Hilf's equation, which calculates the pore-pressure change in a gassy or unsaturated soil subjected to an applied total stress, was incorporated into the model formulation. The initial degree of saturation is needed for model prediction, and the coefficient of volumetric solubility (Henry's constant) was introduced as a new model parameter. The modified model was used to predict the effect of gas on the undrained static behaviour of loose sand. The laboratory results of saturated specimens were modeled, and the predicted and observed behaviours were found to agree well. Results from gassy specimens were also predicted, and again the model predictions matched the test results. The model was used to confirm that gas has the effect of decreasing, but not eliminating, the susceptibility of loose sand to flow liquefaction. The major shortcoming of the modified model was its inability to predict the slight increase in effective normal stress that was observed in the initial stages of all the undrained triaxial tests. This shortcoming resulted in differences between the predicted and observed behaviour especially of strain-hardening specimens.