Effect of gas bubbles on pore pressure response in peat beneath a railway embankment

This paper presents the pore pressure behaviour observed within peat beneath a newly constructed railway embankment. Piezometers installed at different depths beneath the structure showed episodic increases in pore pressure above hydrostatic pressure followed by a rapid pore pressure decrease. It was postulated that this fluctuation in pore pressure was due to the movement and expulsion of gases within the peat. Investigations were conducted in the laboratory to replicate this pore pressure response. The peat specimens were subjected to isotropic consolidation, within a triaxial cell, at constant confining pressure while measuring the change in the specimen's pore pressure and volume. The results of this investigation indicate that the gas bubbles move through the channeled network of voids towards the drainage boundary where they restrict the flow of water, which results in the increase of pore pressure towards an upper limit or escape pressure, ultimately causing an expulsion of gas bubbles. This expulsion of bubbles results in a rapid drop in pore pressure and volume change within the specimen. The implications of the volume change associated with this pore pressure behaviour are discussed.