A Theoretical Study On One-dimensional Degradation-consolidation Of Saturated Municipal Solid Waste

The landfill may be saturated for high-kitchen-waste-content municipal solid waste (MSW). For MSW experiencing solid mass loss, the consolidation process with external loading is complex. This paper paper focused on one-dimensional (1-D) degradation-consolidation properties of saturated MSW. The main research works and conclusions are followed:(1)The universal model for the 1-D degradation-consolidation was established. Unlike traditional soil, the volume caused by hydrolysis of degradable solid need external pore water filling, which leads to decrease of excess pore water pressure. The volume for intra-particle water release is filled with equal volume of released pore water, which causes no change in excess pore water pressure. Degradation-dependent compressibility contributes to increasing excess pore water pressure.(2)A simplified 1-D degradation-consolidation model considering hydrolysis of degradable solid, formation of pore water due to intra-particle water release and degradation-dependent compressibility was derived based on the existing researching achievements of degradability, compressibility and permeability of MSW. The top and bottom boundary conditions of landfill layer were assumed to be free-draining and impervious, respectively. Analytical solution to a simplified 1-D degradation-consolidation model was derived. Case studies were carried out for the fresh MSW in Chinese landfill. The result shows that excess pore water pressure being larger than the initial value is found in deeper waste during the early stage of consolidation under a constant surcharge load. This is mainly due to skeleton loose, increasing compressibility associated with solid mass loss. Excess pore water pressure is found to be negative in the whole waste during the later stage of consolidation. The main reason for this is that degradable solid mass loss causes increasing void ratio, while degradation-dependent compressibility is relatively smaller. Sensitivity analysis shows that an increase of secondary compression rate causes increasing excess pore- water pressure during the early stage of consolidation, and the dissipation of it will be delayed with increasing preconsolidation pressure.(3)A simplified 1-D degradation-consolidation model considering changing coefficient of consolidation was derived. Case studies were carried out based on COMSOL Multiphysics. Coefficient of consolidation increases with depth during the early stage of consolidation. Coefficient of consolidation is bigger than the initial value in the upper layer. However, it is smaller than the initial value in the lower layer. Excess pore water pressure is bigger than the value in the corresponding case whose coefficient of consolidation was assumed to be a constant.this is mainly due to the low coefficient of consolidation.(4) Hydrolysis rate of degradable solid, release rate of intra-particle water and secondary compression rate were derived based on the measured mass of degradable solid and intra-particle water and settlement. The relationship of three rate were studied. A simplified 1-D degradation-consolidation model considering the effect of hydrolysis of degradable solid and intra-particle water release on secondary compression was derived. Case studies were carried out. Sensitivity analysis showers that an increase of hydrolysis rate of degradable solid causes decreasing excess pore-water pressure when release rate of intra-particle water are the same.