Stability Of Municipal Solid Waste Landfill Within High Water Level

Municipal Solid Waste(MSW)in developing countries usually has a high kitchen waste content.Large leachate production and high leachate mounds are common in MSW landfills in China.At local area of landfill,the gas liquid mixture have high pore pressure,which threatening the stability of landfill.Based on the centrifuge equipment at China Institute of Water Resources and Hydropower Research(IWHR),a series of centrifuge model tests have been carried out.Combing with field monitoring results and numerical analysis method,the main research contents and research achievements include:(1)In this study,model MSW have been used which mainly consists of fine sand,peat,and kaolin clay,six centrifugal model tests have been conducted,in which the container could be rotated during the centrifuge flight.The effect of configuration of landfill,fill age of MSW and leachate water on the stability of landfill has been studied.The tests recreate the instability process of landfill.(2)There is a unified point view in obtaining failure strain values of MSW,because that the MSW will not destroyed even the deformation is large.Considering the influence of deformation on landfill structures,15%-20% was suggested as failure strain of MSW.Failure strain values should be appropriately adjusted accord to the different configuration of landfill and the water level.(3)Based on the centrifuge shaking table,three model tests have been carried out.Based on the acceleration response data,the soil dynamic parameters of MSW have been calculated.Comparing with actual MSW from the shear beam equation.It can be conclude that the model MSW have a similar dynamic properties with real MSW.In the second test,an unlined landfill has been constructed.The coupled horizontal and vertical seismic loading was applied on the model slope to investigate its stability under dynamic loading.The post-test observations indicate that the MSW landfill has a strong potential to against seismic loading,but cracks and large settlement may appear during the shaking event.In the third test,the coupled horizontal and vertical seismic loading was applied on the saturate model waste to investigate its possibility of liquefaction under dynamic loading.(4)In June 2008,the Shenzhen landfill slope failed.This case is used to study the deformation characteristics and failure mode of a slope induced by high water level.To calculate the relative interface shear displacements in the geosynthetic multilayer liner system,a series of numerical direct shear tests were carried out.Based on the numerical results,the composite lining system simplified and the centrifuge modeling technique was used to quantitatively evaluate the effect of water level on the landfill instability.(5)In order to obtain the contribution of gas generation on excess pore pressure quantitatively within high saturation municipal solid waste,a one dimensional coupling kinetics model was introduced based on the fluid dynamics theory of porous media,Darcy's Law,Fick's Law and the gas state equation.The coupling equations were implemented into Comsol Multiphysics and solved by the finite element method.Based on field monitoring data of Shenzhen landfill site,the process of gas accumulation had been simulated.The results show that gas will accumulated and produced excess pore pressure with gas generation.Finally,the sensitivity analysis of model parameters showed the gas generation rate,the intrinsic permeability coefficient of liquid and the diffusion coefficient of gas will all play a key role in simulating properly.(6)The method of gas genetated in soil has been investigated.A new method that the calcium carbonate particles embedded in the soil,the citric acid solution inject into the soil has been used in this study.A series of centrifugal model tests have been carried out to investigate slope instability induced by high gas-liquid pressure.The results showed that gas generation below the underground water level has great influence on the slope failure mode.Positive gas pressure will reduce the material strength,leading to instability at the toe of the model,and caused the slope failured steply.(7)To describe the characteristics of large deformation and flowing of landfill instability,the constitutive equations of Bingham fluid which based on Mohr-coulomb failure criterion have been impleted into the smooth particle hydrodynamics(SPH)program.Based on the cetrifugal model test results,the failure process was simulated by SPH method.The simulated results such as external contour of waste body at different time were used to compare with the prototype test results.The results show that SPH reflects the basic flow process of instability of MSW landfill.