One-dimensional Compression Theory Based On Solid-liquid-gas Interactions Of Municipal Solid Waste

Recently municipal solid waste (MSW) aggrandized rapidly in China. Landfill has become the main means to deal with MSW and it will be existed as the final disposal method in the long run. Most of the geotechnical and environmental engineering problems including deformation, stability and seepage, are related to the mechanism of the solid-liquid-gas interactions in the landfill of MSW.A specially-designed equipment was developed to simulate the solid-liquid-gas interaction of MSW. Its functions include temperature controlling, leachate recirculating, loading and gas collecting. A sample of 5.0 kg refuse with its height of 320 mm, was filled in a 190 mm diameter cylinder. The sample was compressed under 150 kPa and decomposed under different conditions with various temperatures, gas pressure, and leachate recirculation .The test results showed that temperature was the dominant factor influencing the biodegradation rate, and lechate recirculation increased gas production rate. The refuse kept decomposing at 400 kPa (absolute gas pressure). The refuse sample was placed in a cylinder maintained 41 °C and operated with leachate recycled, and the biodegradation of which nearly completed after 6 months. The GC/MS analysis demonstrated that the gas generated from the refuse was mainly composed of CH₄, CO₂ and H₂O. The total strain of the sample was 0.44, among which the biodegradation compression strain was 0.24. The steady-state standard of biodegradation compression was developed, that the biodegradation compression could be thought to be completed when the last 3 days' production gas volume is little than 1 percent of the total production gas volume.A theoretical model considering solid-liquid-gas interaction was developed to compute the compression of landfill of MSW, assuming that the mechanical effect of suction in the MSW can be ignored because macro-pores are dominant in the MSW. According to the model, the biodegraded solid is the sink while the produced gas and liquid are the sources. The model incorporated Gibson and Lo's biodegradation and creep model, the Landgem's gas production equation, Darcy's law, the ideal gas law, the dynamics of fluids in porous media and the principle of effective stress. It isassumed the migration of landfill gas only occurs vertically, and hence the theoretical model can be simplified as a one-dimensional compression model. The excess pore pressure caused by the loading pressure was calculated by Hilf's analysis. A finite difference solution was obtained for the solid-liquid-gas interaction problem, which was a special case of solid-liquid-gas interaction problem. The results showed that the gas pressure increased along the depth. During the filling stage, the gas pressure kept increasing. During the closed stage, the gas pressure dispersed gradually. The landfill with high permeability has a high rate of the pore gas pressure dispersing. The decreasing of the settlement rate, caused by the increment of the gas pressure, will decrease the capacity for the landfill.Based the study on the mechanism of the MSW compression, a numerical model for MSW compression was developed. According to the study, the void of MSW includes the initial void and the biodegradation void. The initial void is the void of MSW after pre-compaction. The biodegradation void emerged from the biodegraded organic. The MSW compression consists of the primary compression and the biodegradation compression. The primary compression is caused by upload. The biodegradation compression is the compression on the biodegradation void and further compression on the initial void. The biodegradation compression is not only related to the stress but also the biodegradation condition, which contains the content of organic and the biodegrading rates. The MSW biodegradation leads to the decreasing of the modulus of compressibility.A calculated method for landfill settlement was given, which considered the multi-stage loading. Since the method has considered both the influences of the different biodegradation degree on primary compression and the different loading pressure value on biodegradation compression, it could be applied to calculate the landfill settlement under multi-stage filling.