Deformation And Stability Of Intermediate Liner For Landfill Expansion And Controlling Measures

Expansion over the old landfill site is one of the most economic and effective ways to overcome the shortage of landfill capacity,and the way is presently being undertaken in many cities of China.For some reasons such as the liner system of the old landfill is under-standard,an intermediate liner system involving kinds of geosynthetics should be installed between the old landfill and the expanded waste, which acts as a barrier system of the expanded waste.However,due to the high compressibility and non-homogeneity of the waste in old landfill and the weak strength of geosynthetic interfaces,the deformation and stability of the intermediate liner system has become its main engineering problems.This thesis addresses in following four main deformation and stability problems of the intermediate liner system,which includes additional liner strain caused by global settlements and lateral movements of the old landfill,.damage of liner components caused by local subsidence under the liner,crack of geomembrane caused by differential settlements around the circular structure and sliding of the expanded waste along the weak geosynthetic interfaces.Studies of above mentioned problems should be based on an effective stress-strain model which can behave the stain hardening characteristic of the waste as well as the material and interface properties of the intermediate liner system.To solve these fundamental and engineering problems,model tests,analytical methods and numerical modeling were carried out in this thesis.The main contents and conclusions are as follows:(1) A composite exponential stress-strain model for Municipal Solid Waste (MSW) was proposed based on the available triaxial compression testing results.It can not only behave the non-linear characteristic of the waste within small strains,but also reflect the strain-hardening characteristic of the waste within larger strains (maximum 20%).Further,its parameter database for the foreign and domestic waste was built respectively and parameters for engineering design were also suggested.The material and interface properties of the intermediate liner system were summarized systematically based on these available data.The allowable strain of geoysnthetics in the intermediate liner system was recommended as 8%.(2) Based on the proposed composite exponential stress-strain model,global settlements and lateral movements of a valley landfill expansion were numerical analyzed.Effects of the stiffness and the interface strength of the intermediate liner stiffness,the landfill width,the front-slope gradient,the back-slope gradient,the expansion height,the expansion mode and the waste modulus on the intermediate liner strains were studied.The calculated results show the intermediate liner strains are mainly controlled by the lateral movements of the old landfill subjected to the vertical stress by the weight of expanded waste.The maximum tensile strain of the intermediate liner appears near the anchor trench in the back-slope of the old landfill. In general,the global deformations of the old landfill will not cause damage of the intermediate liner,however,special attentions should be paid to if the back-slope of the old landfill is steep and the landfill expansion height is large.(3) 1g model tests of local subsidence were carried out for sand,compacted clay buffer layer,and the strain hardening soil.These tests were design to study influences of the filling height,the reinforcement stiffness and the buffer layer thickness on the soil arching and the reinforcement deformation.It was found that the vertical soil pressure acted on the deflected reinforcement is related to its deflection amplitude.If the reinforcement deflection is not enough to form a complete soil arching,the existing design methods will underestimate the actual vertical pressure acting on the deflected reinforcement.The compacted clay buffer layer will not be functional well if it is singly used as the anti-subsidence measure,however,the compacted clay buffer layer combined with the reinforcement will reduce the deformations of the sealing material and the reinforcement effectively.The arching laws and the deformation characteristics of the strain hardening soil are significantly different from that of sand. The strain hardening soil can resist the underlying subsidence in some degree by itself, and the additional geosynthetic reinforcement will further reduce its deformation.(4) Based on the proposed composite exponential stress-strain model and the conventional Mohr-Coulomb model,numerical modeling was carried out for the 1g model tests mentioned above,which proves the effectivity of numerical modeling the geosynthetic reinforcement resisting the local underlying subsidence for different soils. Then the numerical model was extended to the landfill expansion,and based on which a new design method and associated design curves of intermediate liner reinforcement were proposed.This design method is presented on the basis of the soil-arching laws of the strain hardening waste,and it has considered the effects of the landfill expansion height,the waste modulus,the reinforcement stiffness,the slope gradient of intermediate liner and the thickness of the buffer layer.Additionally with a simple using way,it is an attractive method for the engineering application.(5) A simplified model to analysis tensions and strains of the geomembrane around circular structures subjected to differential settlements was presented based on the conventional membrane theory.It is found that wrinkles will be induced in the geomembrane around the circular structure and its mechanism is investigated theoretically.Further,the analysis model is given by introducing the concept of "variable Possion's ratio" and based on the wrinkled membrane theory so that the tension and strain of the geomembrane considering induced wrinkles can be analyzed. The calculated results show that larger tensions and strains will be induced in the geomembrane around the circular structure subjected to a differential settlement than those for the infinite long structure.The behaved degree of wrinkles can be reduced by decreasing differential settlements,using flexible geomembrane,increasing interface shear strength and radius of the circular structure.The maximum tension and strain of the geomembrane can be controlled by decreasing differential settlements effectively.(6) A three-part wedge limit equilibrium method for stability analysis of the valley landfill along liners was presented.The analytical solutions of the factor of safety and the yield acceleration coefficient were obtained.Based on Newmark method,the permanent displacement of the expanded waste along the intermediate liner system was investigated.The influence of the interface shear strength,the angle of slopes,the height and the width of the landfill,the height of the retaining wall and the horizontal seismic coefficient on the expanded waste's stability along the liner were obtained systematically.The allowable permanent displacement of the landfill along liners in China is suggested to be 15 cm.(7) The deformation and stability of the intermediate liner system for expansion of Suzhou Qizhishan landfill were integrated evaluated.It is founded that the maximum tensile strain in the intermediate liner is between 2.8%and 4.15%due to the global deformation of the old landfill.Linear low density polyethylene(LLDPE) geomembrane and geosynthetic clay liner(GCL) are recommended as the sealing materials in the intermediate liner.To resist the local underlying subsidence,a layer of biaxial geogrid reinforcement(with tensile strength 90 kN/m at a strain of 8%) combined with a underlying compacted clay layer(with 0.6 m thickness) is recommended for the horizontal intermediate liner,but a layer of biaxial geogrid reinforcement(with tensile strength 130 kN/m at a strain of 8%) combined with a compacted waste buffer layer(with 1 m thickness) are recommended for the intermediate liner located on the slope.The connecting area between the geomembrane and the vertical gas venting well should be dealt with by some practical measures,so that the wrinkles and the maximum strain in the geomembrane can be controlled.It is suggested that a back-pressure waste platform be set above the old retaining wall to guarantee the stability of the expanded waste along the intermediate liner.