Effect of Gas on Pore Pressures in Wet Landfills

The waste in a landfill may become saturated due to many reasons, including leachate recirculation or extreme precipitation. As high saturation levels in waste are achieved, the permeability of the waste to landfill gas decreases. This may result in pore pressures that are greater than what would be predicted by fluid statics. A theoretical model for estimating the excess pore pressure at the bottom of saturated waste is derived. A finite difference procedure is then presented as an approximate solution to the model. It was found that below the level of saturation, the steady-state excess pore pressure distribution increases linearly similar to a hydrostatic distribution. Combining its effect with the static water pressure, the excess pore pressure may be accounted for by using an equivalent unit weight of fluid that is artificially higher than water. A parametric study of the input parameters showed that the equivalent unit weight of the pore fluid was highly dependent on the hydraulic conductivity of the waste, particularly if the hydraulic conductivity of the waste is lower than about 2*10^-3m/s.