Analysis Of Instability Failure Of Landfill Along Geosynthetic Interface And Tension Of Geomembranes Placed On Landfill Slope

Composite liners containing soil and geosynthetic materials provide effective isolation of waste and leachate from the surrounding environment.However,the shear resistance at soil-geosynthetic and geosynthetic-geosynthetic interfaces is often low and most geosynthetic interfaces are susceptible to strain softening,which often led to slippage and landfill slope failures.Investigationshave shown that the possibility of sliding within a composite liner is often the most critical mechanism in all possible landfill instability modes because of the low shear strength of the interfaces between the various components.For the rational evaluation and design of modern municipal solid waste landfills,this thesis focuses on the failure analysis of landfill along the liner interface,and the analysis of tension of geomembranes placed on landfill slopes.Strain softening of the liner interface is studied,and the new displacement～Softening constitutive model is developed to describe the sliding of geosynthetic interfaces.This new constitutive model uses the parameters of friction angle and cohesion intercept which would reduce with the increment of plastic shear displacement to describe the shear strength of geosynthetic interface.Then,the constitutive model of the geosynthetic interface is introduced to finite differential program and the failure of landfill along the single liner interface is analyzed.The effects of variations in waste material properties and geometric parameters of the landfill on the mobilized shear stresses and shear displacements on the interface are investigated.Results show that the slope gradients(including waste slope and the landfill side slope gradients)have the most important effects on the stability of single lining interface.Once either of the waste slope gradient or the side slopeo gradient exceeds its critical value,obvious local slippage will manifest itself at certain location of interface.However,in this case,global slippage along the interface has not yet occurred,which show that the factor of safety relates to complete failure of the landfill cannot be used to assess local failure mechanism.Mobilized shear stresses of the interface generated from the numerical analysis are used in a limit equilibrium stability assessment.These give factors of safety for global stability comparable to those obtained using peak on the base and residual on the side slope.It can be concluded that the approach using residual on the side slope and peak on the base is valid for assessment of global stability of the waste body.On the other hand,because the factor of safety for global stability cannot be used to assess local stability of the lining system,the partialfactor is introduced to evaluate local stability mechanisms.Research shows that when local failure occurs before global instability, calculated partial factor is much lower than limit equilibrium factor of Safety for global failure.Stability of lined waste impoundment with double interfaces is also investigated in this paper using finite difference method.The effects of variations in waste material properties and geometric parameters of the landfill on the mobilized Shear Stresses and shear displacements on double interfaces are studied.It can be seen that large slippage often appears along the interface with weaker strength,and softening of this interface would not result in softening of the other interface with higher strength.Because of softening of the weaker interface,the shear displacements between two interfaces have become obvious difference,and the values of shear stresses between them are no longer the same.This differential value of shear stresses would be undertaken by geosynthetic materials(e.g.geomembrane)between two interfaces and may cause tensile failure of geomembrane.Significant geomembrane tension is necessary to be studied so as to ensure the safety use.In this paper,the shear stress versus displacement relationship for interface between geomembrane and clay is divided into three states,including elastic one, softening One and residual strength one.Three-states elastic-plastic model is presented to simulate the shear deformation property for the interface,and the governing differential equations of three states are got.Because the demarcation points among three states are unknown,the iterative computation method should be used to compute the tension of geomembrane.Finally,the important factors,including landfill height, slope gradient and geomembrane parameters,which influence the tension of geomembrane,are studied.