A Double Compressibility Model for Gassy Soil

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Gassy Soil can be described as a soft marine soil containing methane gas bubbles in an otherwise saturated soil matrix.

Oedometer testing demonstrates two distinct stages of vertical deformation.  Undrained deformation takes place due to the transfer of the increase in total stress to the gas bubble pressure, so causing gas compressibility and subsequent undrained volume change. During this undrained stage, the porewater pressure increases by the same increment as the total stress increase, resulting in no increase in effective stress, and no volume change of the saturated soil matrix.

After the undrained deformation, the drained stage begins, where the increased pore water pressure dissipates to the drained boundary, and the effective stress within the saturated soil matrix increases by the total stress increment, and time-dependent consolidation occurs.

Laboratory tests demonstrate that the saturated soil matrix behaves similarly to a fully saturated soil, insomuch that the void ratio of the saturated component of the soil is dependent on the difference between the average vertical total stress over the sample, and the pore water pressure (operative stress).  However, the compression of the gas bubbles are governed by the increase in the gas pressure, which is strongly related to the mean total stress.

Based on these observations, the volume change of a gassy soil is then governed by two independent stress changes (i) the change in total stress, and (ii) the change in operative stress. This results in a gassy soil having two independent compressibilities, hence the term, Double Compressibility Model.

21st International Conference on Soil Mechanics and Geotechnical Engineering (Vienna)

TC106