摘要
土工防渗系统是填埋场底部、边坡或封场覆盖中用于隔离渗滤液,填埋气体等物质的一种重要屏障,同时也是填埋场长期、有效运营的重要保证。分析防渗系统中土工合成材料的物理力学性质,研究土工防渗系统的变形破坏机理,探讨衬垫系统的安全稳定特性及其趋势预测已经成为国际环境岩土工程界的高度关注的问题。为此,本文采用试验和机理研究相结合的方法,依托相关学科研究成果,以填埋场现场地质资料为基础,以理论分析与数值分析计算为主导,探讨了土工防渗系统在填埋场运营过程中的变形破坏机理,构建了定量描述土工防渗系统变形破坏的力学模型,并运用有限元方法对建立的耦合模型进行数值求解,深入分析了土工防渗系统在不同环境载荷条件下的力学响应,为填埋场土工防渗系统的设计及安全性评价提供了可靠的理论依据及技术支持。取得的主要研究成果包括以下几个方面:
(1)分析了填埋场土工防渗技术特点,得出防渗系统的破坏主要是拉剪破坏,验证了开展填埋场土工防渗系统变形破坏机理及稳定性研究符合国际国内环保方针政策。
(2)分析了不同结构形式防渗系统的特征,得出复合衬垫系统优于双层衬垫系统,双层衬垫系统优于单层衬垫系统;给出了填埋场土工防渗系统变形破坏机理。包括:土工合成材料防渗机理,渗滤液在填埋场中运移机理,固体废弃物在填埋场中沉降变形机理,土工膜衬垫填埋场边坡稳定性机理,降雨入渗作用机理。
(3)通过土工膜窄长条拉伸试验,得出了两种纹理形态(顺纹、逆纹)的拉伸-位移曲线。HDPE土工膜拉力与拉伸变形量初始阶段呈现线性变化,随着拉力的逐渐增加转为塑性变化,HDPE土工膜无明显屈服极限。将顺纹拉伸与逆纹拉伸对比知,逆纹最大拉伸量为顺纹最大拉伸量3~7倍;逆纹与顺纹最大破坏拉力基本相当。通过室内模拟填埋场土工防渗结构压力试验,得出了防渗结构中应变-时间的关系曲线。对曲线分析发现,压力作用下土工膜的应变极为微小,土工膜应变与时间呈线性关系。
(4)给出了土工膜衬垫填埋场概念模型,建立了描述填埋场土工防渗衬垫系统变形破坏的流固耦合数学模型,并对比分析了土工膜厚度计算理论模型公式,提出了土工膜应考虑为索单元且设置接触单元的有限元法对模型进行离散求解。
(5)通过建立填埋场防渗系统数学模型,模拟了不同工况条件下防渗结构的力学响应,其中包括:降雨作用下填埋场防渗系统渗流场特征,常载荷作用下防渗系统变形规律,线性变化载荷作用下防渗系统变形规律,渗滤液水分抽排过程中防渗系统变形规律,土工膜衬垫填埋场边坡稳定性。
a)常荷载、线性变化荷载作用下填埋场场底土工防渗系统的变形研究结果表明:填埋场各结构层中垃圾堆体在荷载作用下的变形最为明显。纵向变形随着水平距离呈现非线性变化,随着深度的增加纵向位移逐渐减小;横向变形随着水平距离和深度的改变均呈现波动变化,填埋场坡顶,坡角处为应力薄弱处;
b)通过对降雨持续作用影响分析得出,降雨入渗使填埋场结构层渗流条件发生变化,填埋场各结构层含水量逐渐增加,导致材料的容重逐渐增大,坡角处孔隙水压骤增而出现峰值;
c)通过模拟填埋场渗滤液水分抽排过程中防渗系统的变形规律分析得出,随着水头的降低,流体压力逐渐降低,垃圾层,粘土层,土壤层的孔隙压力下降,固体骨架有效应力增大,从而造成填埋场防渗系统位移的增加;
d)通过对土工膜衬垫填埋场边坡稳定性的数值模拟及参数灵敏度分析得出,边坡角度和界面摩擦角对土工膜衬垫填埋场边坡稳定性影响较大;边坡角度增大,稳定性降低;界面摩擦角增大,稳定性提高;在考虑了边坡渗流作用和地震影响时,安全系数降低。
Anti-seepage system is used to isolate the leachate and landfill gas as an important barrier which is located at the bottom and slope in landfill. It is also an important guarantee of landfill for long-term and effective operation. Analyzing physical and mechanical properties of geosynthetics, studying the deformation and failure mechanism of anti-seepage system and exploring the stability of liner system in landfill has become a new topic in environment science field, which has attracted broad attention of scientific researchers. With the combination of experimental and theoretical method and relying on existing research achievements, deformation failure mechanism of anti-seepage system in landfill during its service has been discussed, and mechanical model of deformation failure has been built, and then a finite element method is used to solve the coupling model which has been established. Meanwhile, a thorough analysis of mechanical response of anti-seepage system under various environment loads is given. The results can offer technical support for the design, and it can also provide theoretical basis to the defense of stability of landfill. The main research results are as follows:
(1)The current applications, testing process and theoretical study of geosynthetics have been summarized on investigating a large number of landfill data.The anti-seepage system is mainly shear failure on analyzing its technical characteristics.It is in accordance with the international and domestic environmental policy in studying deformation mechanism and numerical simulation of anti-seepage system in landfill.
(2)Composite liner system is superior to double-liner system, and double-liner system is superior to single-layer liner system by analyzing the different structural forms of anti-seepage system. Meanwhile, the deformation and failure mechanism of anti-seepage system is given, including impervious mechanism of geosynthetics, leachate migration mechanism, and solid waste deformation mechanism, stability mechanism of slope padded by geomembrane and rainfall infiltration mechanism in landfill.
(3)The tensile-displacement curves were obtained (cis texture, reverse texture) through tensile test of geomembrane. There is a linear relationship between the tension and tensile deformation in initial stage, but then turned to plastic variation with the tension increased gradually. HDPE geomembrane has no obvious yield limit. The maximum extensional capacity of reverse texture is 3 to 7 times larger than the cis one through the comparison of cis texture with reverse one.Strain-time curve of anti-seepage structure was obtained through simulating the pressure test in laboratory. The curve showed that geomembrane strain is very minimal under pressure effect and there is a linear relationship between geomembrane strain and time.
(4) Conceptual model of landfill was given. Fluid-solid coupling mathematical model that describe the deformation failure of liner system (including saturated-unsaturated percolation model of porous media and stress field mathematical model) was established. The thickness formulas of geomembrane were given, and geomembrane was considered as cable element in finite element analyzes.
(5)A mathematics model of the seepage field for municipal landfill liner system was established and mechanical response of anti-seepage system in different working conditions has been simulated, such as seepage field features of liner system under rainfall condition, deformation laws under constant and linear variation loads, the deformation of anti-seepage system considering fluid-solid coupling and numerical analysis on stability of landfill slope padded by Geomembrane.
a) Considering constant and liner variation loads, the deformation laws of anti-seepage system at the bottom of landfill were obtained. Longitudinal deformation decrease with the depth increase and it is nonlinear variation along with the horizontal distance. Lateral deformation is fluctuation change with the horizontal distance and depth change.
b) Considering rain infiltration and selecting a typical section of landfill cover slope, numerical simulation showed that the storage capacity of each structural layer was gradually increased, which lead to the unit weight increased. Pore water pressure can be easily increased with increase of seepage flow at the foot of slope.
c) Deformation law was obtained by simulated water pumping in anti-seepage system. The results showed that the water head decreased, the pore pressure is progressively lowered, which makes virtual stress increase. Therefore, the displacement of anti-seepage system increases.
d) Some beneficial laws are obtained by analyzing the various parameters of the model of landfill slope padded by geomembrane. The slope angel and the interface friction angel influence greatly of the stability of the slope padded by geomembrane. The increasing of the slope angel, the length,the seepage forces and the seismic coefficient will lead to the safety factor of the landfill decrease, the increasing of the interface friction angel will lead to the safety factor increase, the variation of the friction angel of cover soil have little influence on the safety factor; the safety factor is decreased considering permeability factor and seismic action.
引文
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