【作者】 高登；

【导师】 陈云敏； 朱斌；

【作者基本信息】 浙江大学 ， 岩土工程， 2009， 博士

【摘要】 基于原址扩建是解决城市垃圾填埋场容量不足的最经济和有效手段之一,目前我国许多城市垃圾填埋场正在进行扩建。由于老填埋场采用了垂直防渗系统或其它原因,扩建时在新老填埋场之间需设置一层包含多种土工合成材料的中间衬垫系统作为扩建堆体的防渗屏障。然而由于垃圾的高压缩性、高度不均匀性及衬垫材料之间较低的界面强度,中间衬垫系统的以下变形和稳定问题成为其主要工程问题:老填埋场的整体沉降和侧向变形对中间衬垫系统产生附加应变、中间衬垫系统下卧垃圾体的局部沉陷破坏中间衬垫系统防渗材料、刚性构筑物周边的不均匀沉降拉裂土工膜、扩建垃圾堆体沿中间衬垫系统薄弱界面失稳破坏。能有效反映垃圾应变硬化特性的应力应变模型及中间衬垫系统的材料和界面特性是研究上述问题的基础。本文通过模型试验、解析方法和数值模拟等手段,围绕上述工程问题开展研究,所做工作和主要结论如下:(1)基于国内外垃圾三轴压缩试验结果,提出了垃圾的复合指数应力-应变模型,能较好地反映垃圾在小应变情况下的非线性变形特性及其在应变较大情况下(20%以内)的明显应变硬化特征,并建立了国内外垃圾的复合指数应力-应变模型参数数据库和建议值;同时系统总结了中间衬垫系统的材料特性和界面特性,并建议中间衬垫系统中土工合成材料的容许应变值取为8%。(2)基于所提出的垃圾复合指数应力-应变模型,针对山谷型扩建填埋场整体沉降和侧向变形对中间衬垫系统应变的影响开展数值模拟,考虑了中间衬垫系统刚度及界面特性、填埋场宽度、填埋场前坡坡度、填埋场背坡坡度、填埋场扩建高度及垃圾体模量参数对中间衬垫系统应变分布的影响。分析结果表明:老填埋场整体变形情况下的中间衬垫系统应变主要由老填埋场的侧向变形控制,最大拉应变发生在老填埋场背坡锚固沟处;老填埋场的整体沉降和侧向变形通常不会导致中间衬垫系统产生拉伸破坏,但当其背坡坡度较陡或扩建堆高较大时需值得关注。(3)分别针对砂土材料、压实粘土缓冲层和应变硬化材料开展了局部沉陷1g模型试验,考虑了堆体高度、加筋体刚度及缓冲层对土拱效应及加筋体变形的影响。试验结果表明:沉陷区加筋体的上覆土压力与加筋体挠曲变形大小密切相关,当扩建堆高较大时加筋体挠度不足以使土拱效应充分发挥,已有土拱理论可能低估沉陷区加筋体的实际上覆土压力;中间衬垫系统不宜仅采用压实粘土层缓冲层作为抗局部沉陷措施,但一定厚度的缓冲层结合土工格栅加筋可有效控制中间衬垫系统中防渗材料的变形;下卧土体局部沉陷条件下应变硬化材料的土拱规律及变形特性与砂土材料有明显差异,应变硬化材料自身具有良好的抵抗局部沉陷能力,附加加筋措施可进一步减小其变形量。(4)分别基于所提出的复合指数应力-应变模型及常规摩尔-库仑模型,首先针对所开展的局部沉陷1g模型试验开展了数值模拟,验证了数值模拟不同材料土拱效应及土工加筋抗局部沉陷问题的可靠性。进一步把研究对象拓展到扩建垃圾填埋场,提出了中间衬垫系统的抗局部沉陷加筋设计新方法并给出了相关设计曲线。该设计方法基于应变硬化垃圾材料的土拱规律,综合考虑了扩建堆体高度、垃圾体模量、加筋体刚度、中间衬垫系统坡度、缓冲层厚度等因素的影响,设计过程简单,具有良好的工程适用性。(5)首先基于传统薄膜理论建立了下卧土体沉降作用下圆形构筑物周边土工膜受力变形的分析模型,从理论上揭示了下卧土体沉降作用下圆形构筑物周边土工膜的褶皱产生机理。进一步通过引入“可变泊松比”概念并基于褶皱薄膜理论建立了褶皱区与张拉区土工膜受力变形的分析模型。与无限长构筑物相比,圆形构筑物周边土工膜在下卧土体沉降作用下将产生更大的变形和张力。减小下卧土体沉降、采用柔性土工膜及增大界面强度和圆形构筑物半径等措施可一定程度上降低褶皱的程度,而减小下卧土体沉降则是降低土工膜的内力和变形最直接有效工程措施。(6)针对含垃圾坝的山谷型新建或扩建填埋场沿衬垫界面的稳定性,提出了三楔体极限平衡分析方法,并给出了安全系数及屈服加速度系数的近似解。采用Newmark法对不同场地、不同高度垃圾填埋场沿衬垫界面的地震永久位移进行了分析。系统获得了填埋场衬垫界面强度、填埋场各坡坡度、填埋高度和宽度、垃圾坝高度和地震系数等参数对扩建堆体稳定性的影响规律。建议我国填埋场衬垫系统地震容许永久位移控制在15 cm内。(7)结合苏州七子山大型垃圾填埋场扩建工程,对中间衬垫系统的受力变形及扩建堆体沿其界面的静动力稳定性进行了综合评价。发现老填埋场整体不均匀沉降和侧向变形引起的中间衬垫系统的最大拉应变为2.8%～4.15%,建议采用LLDPE土工膜和GCL作为防渗材料。针对局部沉陷问题,建议水平区中间衬垫系统选用一层抗拉强度为90 kN/m(ε=8%)的双向HDPE土工格栅进行加筋并设置压实粘土垫层(厚0.6 m),斜坡区中间衬垫系统则选用一层抗拉强度为130kN/m(ε=8%)的双向HDPE土工格栅进行加筋并设置压实筛选垃圾缓冲层(厚1m)。土工膜与竖向导气井连接部位需采取措施进行处理,以控制土工膜的褶皱变形和最大应变。同时建议在老场垃圾坝上部设置反压平台,以有效控制扩建垃圾堆体沿中间衬垫界面的稳定性。更多还原

【Abstract】 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.更多还原