Rainfall-induced liquid-gas interactions and pore gas pressure modeling in municipal solid waste

Rainfall critically influences the moisture dynamics, gas generation, and pore gas pressure within landfilled waste. Previous studies, however, have predominantly examined the liquid or gas phase in isolation at single degradation times, leaving a gap in understanding the rainfall-induced liquid-gas interactions throughout the waste's degradation timeline. This study investigated these interactions through rainfall simulation experiments on waste columns at four key degradation times, including the onset of degradation, peak gas production rate, and 50% and 95% reductions from the peak rate. The results indicated that rainwater infiltration proceeded through three distinct phases, rainfall-controlled, unsaturated, and saturated infiltration. The infiltration capacity was highly sensitive to the degradation time. During rainfall, both volumetric water content and pore gas pressure increased rapidly to their peaks before stabilizing. The variation in the magnitude of these responses across degradation times suggested a coupled yet competing liquid-gas interaction. Furthermore, a predictive model for pore gas pressure was developed by incorporating rainfall-induced moisture dynamics, and an analytical solution was derived under simplified conditions. The model predictions agreed well with the experimental observations. These findings provide novel insights into liquid-gas interactions, aiding prediction and management of landfill behavior during rainfall events.