The effect of gas on general stability in wet landfills

Abstract

Due to regulatory legislation enacted over the past few decades, the design of municipal sanitary landfills has evolved from simple open dumps to relatively sophisticated storage and treatment facilities. Bioreactor landfills, which were first introduced in the mid 1990s became a popular landfill management technique and. A bioreactor landfill uses leachate recirculation to enhance the degradation processes thus degrading the waste constituents much faster than would be experienced with arid landfills. However, the well-accepted set of design criteria and methods of analysis for stability, settlement, degradation, gas generation, and water infiltration that had been developed for the arid landfills were not necessarily appropriate for the bioreactor landfills. Whether due to leachate recirculation or extreme precipitation, the waste in a landfill may tend to become saturated. The high saturation levels then reduce the permeability of the waste to landfill gas thus leading to pore pressures that are greater than what would be predicted by fluid statics. In this case, the factor of safety for global stability within the landfill will be lowered, potentially creating failure. A theoretical model predicting the time and depth dependent development pore pressures due to the formation of landfill gas in a wet landfill is presented. The model is then quantitatively approximated with a finite difference scheme. It was found that below the level of saturation, the steady state pressure distribution appears to be hydrostatic except that the unit weight of the fluid is significantly heavier (14.0 kN/m³) compared to water (9.81 kN/m³). In the ten days preceding July 10, 2000, approximately 0.75 m (30 inches) of rain fell onto the Payatas Landfill near Manila, Philippines. Results of HELP modeling indicate that this precipitation caused a 10-m deep zone of saturated waste at the bottom of the waste mass. Through a back-analysis of the failure, a factor of safety of 1.0 for stability was calculated by using an increased unit weight of pore fluid of 20.9 kN/m³, which is similar to that predicted by the finite difference scheme of 14.0 kN/m³. Hence, this research shows that the build-up of landfill gas can play a significant role in the stability of wet landfills.