非饱和土层中污染物运移过程的数值分析

针对填埋场下部为压实粘土衬里、非饱和土层和含水层的三层结构体系,建立了描述填埋场污染物运移的一维控制方程,在控制方程中考虑了压实粘土衬里的平衡非线性吸附方程与非饱和土层的线性吸附方程。采用数值法求解控制方程,通过变动参数探讨了污染物在三层结构体系中运移过程的各种影响因素。     

我国四类衬垫系统防污性能的比较分析

对我国填埋场采用的四类衬垫进行了防污性能的比较分析。评价参数包括渗漏率、污染物击穿时间及衬垫系统底部浓度值。除了2m压实黏土衬垫(CCL)外,其余3种均为包含土工膜(GM)的复合衬垫。分析模型采用了污染物通过有缺陷膜复合衬垫的一维运移解析解。以镉离子(Cd2+)为渗滤液中重金属离子的代表;以苯为其中挥发性有机污染物的代表。研究表明土工复合膨润土垫(GCL)复合衬垫的渗漏率最小,2m黏土最大,两者的差别可在3～5个数量级。GCL复合衬垫对重金属离子具有较好的防污性能,尤其是在高水头及复合衬垫接触较差的情形。厚度较大的2m黏土对挥发性有机污染物的防污性能较好,其击穿时间要比GCL复合衬垫大2～3个数量级。随着水头的增大,CCL复合衬垫的防污性能逐渐地优于2m黏土。在10m水头作用下,CCL复合衬垫底部的100年浓度可比2m黏土小近一个量级。单层膜衬垫的防污性能较差,不适合作为填埋场的衬垫系统。控制填埋场复合衬垫的施工质量和渗滤液水头尤为重要。     

Evaluation of interface shear strength of composite liner system and stability analysis for a landfill lining system in Thailand

The paper presents the case history of laboratory evaluation of the interface shear strength properties of various interfaces encountered in a modern day landfill with emphasis on proper simulation of field conditions and subsequent use of these results in the stability analyses of liner system. Over 70 large direct shear tests were systematically conducted to evaluate the interface shear strength properties of composite liner system using project specific materials under site specific conditions, being used at non-hazardous and hazardous landfills project situated in Sa Kaeo Province, Thailand. The critical interfaces were located between the geotextiles and the smooth geomembrane (GM), the smooth GM and the geosynthetic clay liner (GCL), and the smooth GM liner and the compacted clay liner (CCL) with the interface friction angles ranging from 6.5° to 10.5° for dry conditions and 6.5° to 9.5° in wet conditions. The residual shear stress for these interfaces was attained at a displacement less than 4 mm. Three methods, namely, limit equilibrium method (LEM), limit method (LM), and the simple composite column (SCC) approach were used to evaluate the tensile loads induced in the geosynthetic components. The SCC approach proposed by Liu, C.N. [2001. Tension of geosynthetics material regarding soils on landfill liner slopes. Proceedings, National Science Council ROC(A), 25(4), 211–218] that takes into account the force equilibrium as well as displacement compatibility yielded satisfactory results. The factor of safety for geosynthetic components in the liner was found to be greater than 3.0 for both types of landfill.     

污水厂污泥制备垃圾填埋场防渗衬层材料研究

以城市污水厂的污泥复合高岭土用作垃圾填埋场防渗衬层材料，研究了千污泥的掺入量及其颗粒大小、复合土密度、渗滤液性质对污泥复合土渗透性能的影响。结果表明，复合土中污泥的最佳质量分数为40％；提高复合土的干密度可显著降低其渗透系数，当干密度为1.08g／cm^3时复合土的渗透系数达10^-7cm／s数量级，符合垃圾填埋场防渗衬层的要求；渗滤液性质对渗透性能的影响较大，当其电解质浓度较大时复合土的渗透性能随之提高。此外，考察了污泥复合土对重金属的吸附效果，结果表明该复合土可有效吸附垃圾渗滤液中的Cu^2＋、Zn^2＋等重金属。     

Landfill Lining for Leachate Containment

There is an increasing need for engineered liners to prevent the escape of leachate and to control the migration of landfill gas. This has been influenced by new European and UK legislation and codes of practice.
General principles of site design are explained, followed by a review of the advantages and disadvantages of different liner systems, including mineral liners (natural clay, mudrocks and soil/bentonite admixtures), geomembranes and composite liners.
The need for conservative factors of safety in site design and high standards of quality control, involving full-time supervision and independent certification of the installation works, are considered essential to ensure the effectiveness of the barrier.
There is no one ideal liner material. Composite mineral/geomembrane liners provide the highest degree of security against leakage and, as such, should be considered as 'the norm'rather than for use only in particularly high-risk situations.     

混合粉质粘土和疏浚土填埋场防渗垫层的环境土工特性研究

充分利用疏浚土有机质含量高、吸附能力强和低透水性等特点,将其作为填埋场粘土衬垫的添加材料。等温吸附试验和扩散试验的结果表明:粉质粘土掺入疏浚土后,对金属离子的吸附能力得到增强,尤其对高价Cu2+的吸附能力显著提高,同时可以显著地降低Cu2+的扩散系数。研究表明,有机质对Cu2+的强吸附能力是Cu2+扩散系数降低的主要原因。粉质粘土掺入疏浚土后极大地提高了粘土衬垫的环境土工特性,非常适合作为填埋场粘土衬垫。     

Organic acid transport through a partially saturated liner system beneath a landfill

A one-dimensional model was developed to investigate the transport of organic acids (commonly found in landfill leachate) through a partially saturated composite liner system beneath a landfill. Specific attention was paid to the influence of water content distribution on aqueous-phase diffusion process. Composite liner system was investigated, which was consisted of a geomembrane and a compacted clay liner underlain by three kinds of attenuation layer: sand layer, sandy clay loam layer, and clay layer. Volumetric water content profile in soil layers was obtained by Van Genuchten model, and the Millington and Quirk model was employed to describe the non-linear relationship between volumetric water content and diffusion coefficient. Three cases were analyzed and compared, i.e., totally saturated condition, unsaturated condition without considering unsaturated diffusion model, and unsaturated condition considering unsaturated diffusion model. The numerical results show that the unsaturated sand attenuation layer could serve as excellent diffusion barrier to organic contaminant due to its low water retention capacity. When the dependence of diffusion coefficient on volumetric water content is sufficiently considered, the contaminant flux decreases significantly in all the three kinds of attenuation layer. Unsaturated diffusion model capturing the relationship between water content and diffusion coefficient enables a more reasonable prediction of contaminant transport and distribution in soils.     

粘土环境岩土工程特性对填埋场衬垫防渗标准的影响

采用溶质运移的一维迁移模型,研究了粘土环境岩土工程特性对填埋场衬垫防渗标准的影响,对目前采用的填埋场粘土类衬垫防渗设计标准及其有效性进行了多参数组合的计算分析;此外,还比较了粘土衬垫与复合衬垫对污染物离子的防渗阻隔性能,对土工膜在衬垫中的作用进行分析;研究表明,应针对不同地区的情况(干旱少雨或多雨等)考虑衬垫的形式。与其他因素相比,粘土的渗透系数对衬垫层渗漏影响最大,以水力渗透系数作为填埋场粘土衬垫的设计标准是可行的,但离子扩散系数的影响也不可忽略;而增强粘土的活性,提高粘土吸附离子的性能,则可延长离子通过衬垫的击穿时间;与单纯的粘土衬垫相比,复合衬垫对离子具有更好的阻隔性能,故在南方多雨地区宜采用复合衬垫;而用污染物的渗漏总量作为填埋场衬垫的设计标准比击穿时间更合理。     

膨润土防水毯在渗滤液环境下防渗性能的测试应用研究

膨润土防水毯作为一种优异的防渗材料,国内暂无其在渗滤液环境下的渗透性能报道。本文拟配置性能组分稳定的代表性合成渗滤液作为实际渗滤液的替代试验介质,研究合成渗沥液对膨润土原料膨胀性能和滤失性能的影响,测试在合成渗滤液环境中,不同压力和温度条件下膨润土防水毯的渗透系数,以此为垃圾填埋工程和其他固废填埋工程使用膨润土防水毯作为防渗衬垫提供指导。     

Redox Effects on Heavy Metal Attenuation in Landfill Clay Liner

Landfill leachate is characterized by high organic compounds that can be used by microorganisms as nutrients and induce a series of redox reactions. Thus, redox potential as well as pH is considered to have an effect on the behavior of contaminants in leachate from landfill sites. Modified batch tests, cultivating the native microorganisms in soil specimens, were conducted to evaluate the bacteria-induced redox and pH effects on the natural attenuation mechanisms of heavy metal in the bottom clay liner of landfills. The marine clay sampled from Osaka Bay, Japan was used as a natural clay liner material due to the consideration that some of the solid waste landfill sites in Japan are located in coastal areas. Iron and zinc were selected as target pollutants.Test results show that both pH and redox potential indicated a combined effect on the solubility of zinc and iron. Under the denitrification and Fe(III) reduction conditions, zinc was soluble and its solubility was only controlled by pH. When pH increased higher than 7.2, zinc precipitated as hydroxides and adsorbed on soil particle surfaces. Under the sulfate reduction condition, the formation of zinc sulfides became another attenuation mechanism. Iron was insoluble under the aerobic and denitrification conditions in natural pH conditions. Elevated levels of soluble iron were observed in moderately reduced and highly reduced conditions. A combination of nearly neutral pH and extremely low redox potential condition in landfill site tends to promote the insolubilization of zinc but the solubilization of iron.     

The Stability of the Oxford Clay as a Mineral Liner for Landfill

The paper discusses the mineralogical and chemical stability of the Oxford Clay as a landfill liner for the containment of domestic waste. The results from a series of batch equilibrium experiments with the mudrock and a synthetic leachate are compared with samples of a liner cored from a 15-year-old site in the Formation. The effects of leachate on the Clay include mineral dissolution, exchange of cations, particle-size reduction and collapse of illite-smectite. In situ mixed-assemblage mineral liners, such as the Oxford Clay, are capable of attenuating leachate components and buffering acid leachates whilst the predominant clay minerals, i.e. illite and kaolinite, remain stable. Alterations to the mineralogy and chemistry of the samples resulting from both short-term and long-term exposure to leachate are discussed with reference to the implications to landfill practice.     

Migration behavior of landfill leachate contaminants through alternative composite liners

Four identical pilot-scale landfill reactors with different alternative composite liners were simultaneously operated for a period of about 540 days to investigate and to simulate the migration behaviors of phenolic compounds (phenol, 2-CP, 2-MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP, 2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) and heavy metals (Pb, Cu, Zn, Cr, Cd, Ni) from landfill leachate to the groundwater. Alternative landfill liners of four reactors consist of R1: Compacted clay liner (10 cm + 10 cm, k = 10⁻⁸ m/sn), R2: Geomembrane (2 mm HDPE) + compacted clay liner (10 cm + 10 cm, k = 10⁻⁸ m/sn), R3: Geomembrane (2 mm HDPE) + compacted clay liner (10 cm, k = 10⁻⁸ m/sn) + bentonite liner (2 cm) + compacted clay liner (10 cm, k = 10⁻⁸ m/sn), and R4: Geomembrane (2 mm HDPE) + compacted clay liner (10 cm, k = 10⁻⁸ m/sn) + zeolite liner (2 cm) + compacted clay liner (10 cm, k = 10⁻⁸ m/sn). Wastes representing Istanbul municipal solid wastes were disposed in the reactors. To represent bioreactor landfills, reactors were operated by leachate recirculation. To monitor and control anaerobic degradation in the reactors, variations of conventional parameters (pH, alkalinity, chloride, conductivity, COD, TOC, TKN, ammonia and alcaly metals) were also investigated in landfill leachate samples. The results of this study showed that about 35-50% of migration of organic contaminants (phenolic compounds) and 55-100% of migration of inorganic contaminants (heavy metals) to the model groundwater could be effectively reduced with the use of bentonite and zeolite materials in landfill liner systems. Although leachate contaminants can reach to the groundwater in trace concentrations, findings of this study concluded that the release of these compounds from landfill leachate to the groundwater may potentially be of an important environmental concern based on the experimental findings.     

Shear strength of geosynthetic composite systems for design of landfill liner and cover slopes

Torsional ring shear tests were performed on composite specimens that simulate the field alignment of municipal solid waste (MSW) landfill liner and cover system components. Simultaneous shearing was provided to each test specimen without forcing failure to occur through a pre-determined plane. Composite liner specimens consisted of a textured geomembrane (GM) underlain by a needle-punched geosynthetic clay liner (GCL) which in turn underlain by a compacted silty clay. Hydrated specimens were sheared at eleven different normal stress levels. Test results revealed that shear strength of the composite liner system can be controlled by different failure modes depending on the magnitude of normal stress and the comparative values of the GCL interface and internal shear strength. Failure following these modes may result in a bilinear or trilinear peak strength envelope and a corresponding stepped residual strength envelope. Composite cover specimens that comprised textured GM placed on unreinforced smooth GM-backed GCL resting on compacted sand were sheared at five different GCL hydration conditions and a normal stress that is usually imposed on MSW landfill cover geosynthetic components. Test results showed that increasing the GCL hydration moves the shearing plane from the GCL smooth GM backing/sand interface to that of the textured GM/hydrated bentonite. Effects of these interactive shear strength behaviors of composite liner and cover system components on the possibility of developing progressive failure in landfill slopes were discussed. Recommendations for designing landfill geosynthetic-lined slopes were subsequently given. Three-dimensional stability analysis of well-documented case history of failed composite system slope was presented to support the introduced results and recommendations.     

Development of a numerical model for performance-based design of geosynthetic liner systems

A numerical model for performance-based design of the geosynthetic elements of waste containment systems has been developed. The model offers a rational alternative to the current state of practice for design of geosynthetic containment system elements in which neither the strains nor the forces in liner system elements are explicitly calculated. To explicitly assess the ability of the geosynthetic elements of a containment system to maintain their integrity under both static and seismic loads, a large strain finite difference model of waste-liner system interaction was developed. Modular features within the model allow the user to select the appropriate features required for any particular problem. A beam element with zero moment of inertia and with interface elements on both sides is employed in the model to represent a geosynthetic element in the liner system. This enables explicit calculation of the axial forces and strains within the liner system element. Non-linear constitutive models were developed to represent the stress-strain behavior of geomembrane and geosynthetic clay liner beam elements and the load-displacement behavior of the beam interfaces. The use of the various features on the model is illustrated using available experimental data, including shaking table test data on rigid and compliant blocks sliding on geomembranes. Analysis of geomembranes subject to waste settlement and subject to seismic loading demonstrate applications of the model and provide insight into the behavior of geosynthetic liner system elements subject to tensile loads.     

Assessment of consolidation-induced VOC transport for a GML/GCL/CCL composite liner system

In municipal solid waste landfills, a triple-layer composite liner consisting of a geomembrane liner (GML), a geosynthetic clay liner (GCL) and a compacted clay liner (CCL) is commonly used at the landfill bottom to isolate the leachates from surrounding environment. This paper presents a numerical investigation of the effect of liner consolidation on the transport of a volatile organic compound (VOC), trichloroethylene (TCE), through the GML/GCL/CCL composite liner system. The numerical simulations were performed using the model CST3, which is a piecewise linear numerical model for coupled consolidation and solute transport in multi-layered soil media and has been extensively validated using analytical solutions, numerical solutions and experimental results. The performed numerical simulations considered coupled consolidation and contaminant transport with representative geometry, material properties, and applied stress conditions for a GML/GCL/CCL liner system. The simulation results indicate that, depending on conditions, consolidation of the GCL and CCL can have significant impact on the transport results of TCE (i.e., TCE mass flux, cumulative TCE mass outflow, and distribution of TCE concentration within the GCL and CCL), both during the consolidation process and long after the completion of consolidation. The traditional approach for the assessment of liner performance neglects consolidation of the GCL and CCL and fails to consider the consolidation-induced transient advection and concurrent changes in material properties and, therefore, can lead to significantly different results. These differences for with and without the consolidation effects can range over several orders of magnitude. The process of consolidation-induced contaminant transport is complex and involves many variables, and therefore case-specific analysis is necessary to assess the significance of liner consolidation on VOC transport through a GML/GCL/CCL composite liner system.     

Landfill side slope lining system performance: A comparison of field measurements and numerical modelling analyses

Low permeability engineered landfill barriers often consist of a combination of geosynthetics and mineral layers. Even though numerical modelling software is applied during the landfill design process, a lack of data about mechanical performance of landfill barriers is available to validate and calibrate those models. Instrumentation has been installed on a landfill site to monitor multilayer landfill lining system physical performance. The lining system comprises of a compacted clay layer overlaid by high density polyethylene geomembrane, geotextile and sand. Data recorded on the site includes: geosynthetic displacements (extensometers), strains (fibre optics, Demec strain gauges, extensometers) and stresses imposed on the liner (pressure cells). In addition, temperature readings were collected by a logger installed at the surface of the geomembrane, at the clay surface using pressure cell thermistors and air temperature using a thermometer. This paper presents readings collected throughout a period of three years and compares this measured performance with the corresponding numerical modelling of the lining system for stages during construction. Numerical modelling predictions of lining system behaviour during construction are comparable with the measurements when the geosynthetics are covered soon after placement, however, where the geosynthetics are left exposed to the sun for an extended period of time, in situ behaviour of the geosynthetics cannot be replicated by the numerical analysis. This study highlights the significant influence of the effect of temperature on geosynthetics displacements. A simple thermal analysis of the exposed geosynthetics is used to support the explanation for observed behaviour.     

Dynamic shear behavior of GMB/CCL interface under cyclic loading

The dynamic shear behavior of composite liner interface is of great importance for landfill seismic analysis. In this study, an experimental investigation of the shear behavior of the interface between smooth high density polyethylene (HDPE) geomembrane (GMB) and compacted clay liner (CCL) is presented. A series of displacement-controlled cyclic shear tests were conducted to investigate the effects of displacement amplitudes, normal stress levels and number of cycles on the GMB/CCL interface shear behavior. Cyclic loading with higher displacement amplitude will produce greater vertical contraction and lower interface initial shear stiffness. Also, significant shear strength degradation was observed within the first 5 shearing cycles, then followed by slight interface reinforcement in subsequent cycles. The dynamic shear modulus of GMB/CCL interface is dependent on both normal stress levels and displacement amplitudes, while the damping ratio is only affected by displacement amplitudes. Finally, a method considering the GMB/CCL composite liner as an equivalent soil layer was proposed, which is useful for landfill seismic analysis.     

Prefabricated bentonite clay liners

The development and use of prefabricated bentonite clay liners is a relatively recent development, even for a technology as young as geosynthetics. There are four currently available commercial products, each of which consists of essentially dry bentonite clay agglomerates (with or without a dry, but water soluble, adhesive) placed on a geotextile or geomembrane carrier layer. Often a covering geotextile is used above the clay, and for two of the products, the entire ‘sandwich’ is needled throughout.

For landfill liner systems, the major use of these materials appears to be as the lower component of a composite primary liner. This use is particularly interesting in the light of response action plans (RAPs) which are being required by many regulating agencies. For landfill closure systems, these materials can readily form the lower component of a composite cap system. Of interest here is their possible capacity to follow subsidence of the landfill and their ability to deform in an out-of-plane manner.

With the newness and potential of prefabricated bentonite clay liners, however, comes a number of questions. This paper attempts to present these questions and the currently available answers regarding the various products. In concludes by giving a number of suggestions and cautions for field placement based on the authors' experiences to date.     

垃圾坝和界面强度对填埋场沿底部衬垫系统滑动的影响

为研究垃圾坝和界面强度对填埋场沿底部衬垫系统滑动的影响,将填埋场分为主动楔体、被动楔体和垃圾坝3个部分,对其进行极限平衡分析,建立平衡方程,求解填埋场的安全系数。考虑条间力的变化,可以计算填埋场的最大安全系数FSmax和最小安全系数FSmin,并且采用平均安全系数FSave来代替真实安全系数FStrue。填埋场的安全系数随着垃圾坝高度和底部摩擦角的增加逐渐增大,如果不考虑垃圾坝的作用,填埋场的稳定计算偏于保守。具有最小安全系数的潜在滑动面并不能仅仅通过比较衬垫中不同界面的黏聚力和内摩擦角来确定,随着衬垫界面摩擦角和黏结力的增加,FSave都在逐渐增大,衬垫界面黏结力对填埋场的整体稳定性影响较大,尤其是对于摩擦角较小、黏结力较大的情况。