Quantification of the Settlement of an Embankment Constructed on Peat due to the Expulsion of Gases

The mechanisms that result in the settlement of structures constructed on peat foundations have been the subject of numerous investigations. A recent study of the behavior of the peat subgrade showed that gas bubbles within the peat strongly influence the pore pressure. The thermal expansion of these gas bubbles results in an increase in pore pressure during the warmer months. The gas bubbles remain trapped until the pore pressure is sufficient to push them through the pore constrictions and toward the drainage boundary. The expulsion of the gas bubbles is followed by a sharp drop in pore pressure and a rapid change in volume. This paper presents the analysis of field data conducted to quantify the impact of this mechanism on the settlement of an embankment constructed on a peat subgrade. Measured pore pressures and deformations in the field are analyzed to show the acceleration of vertical deformation during the pore pressure drops. An approach developed from laboratory isotropic consolidation testing results is presented to correlate the magnitude of the drop in pore pressure to the corresponding volumetric strain. This correlation is then extrapolated to estimate the settlement of peat subgrade corresponding to the expulsion of gases. The results suggest that approximately 15% of the annual vertical settlement of the embankment occurs due to the cyclic dissipation of gas bubbles from the peat subgrade.