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。     

A prefabricated bentonite clay liner

This paper presents manufacturing information and engineering data on a prefabricated bentonite clay liner named Claymax^®. Details of the standard product that is currently available are given, along with possible new variations that are available on a site specific basis.     

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。     

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.     

Preliminary results of composite liner field performance study

This paper presents preliminary results of a study of the field performance of composite liners. The purpose of the study is to evaluate the ability of composite liners to contain municipal solid waste (MSW) leachate. The paper presents data for double-lined MSW landfills having composite top liners consisting of a geomembrane (GMB) upper component and a compacted clay liner (CCL) lower component. Data on flow volumes and flow constituents for the leachate collection and removal system (LCRS) and the leakage detection system (LDS) components of the double liner system are analyzed to assess whether leakage has occurred through the composite top liner. Data for nine MSW landfill cells with monitoring periods of up to eight years are presented. Preliminary results indicate that the nine composite liners are performing well and are effective in containing MSW leachate.     

土工合成材料在城市固体垃圾填埋场中的应用

对一系列新型化工产品———土工合成材料在城市固体垃圾填埋场中的应用 ,特别对其中土工织物 (GT)、土工膜 (GM)、土工合成材料粘土垫 (GCL)、土工网 (GN)、土工复合材料 (GC)以及土工泡沫 (GF)等在填埋场衬垫、封盖系统中的应用作了全面的总结 ,详尽地分析了各种材料的主要用途     

Geotechnical characterization of geomaterial blends with zeolitic tuffs for use as landfill liners

Mixtures of volcanic and zeolitic tuffs from Jordan with other natural materials such as marl, sand, kaolinite and bentonite were prepared and tested for Proctor compaction in order to determine the best blend to achieve the highest dry density for use in a landfill liner. Compaction tests indicate that volcanic tuffs have higher dry densities than the zeolitic tuffs. The presence of zeolitic tuff in a mix decreases the dry density and increases the optimum moisture content. The study indicated that two mixtures of zeolitic tuff, marl, bentonite, kaolinite and sand gave the highest dry densities which, combined with the high cation exchange capacity of zeolitic tuff and bentonite, provide mixtures suitable for potential low cost landfill liners, if the shrinkage tests confirm that they are appropriate. The liquid limits measured using the cone penetration method of the volcanic tuffs were higher than those obtained using the Casagrande method except for the Rmah Tuff (RT) which showed the reverse. This needs further investigation.     

Assessment of shear strength for clay liners using a dynamic probe

Liner-cover layers are top layers normally placed when closing landfill sites. The top layers are commonly subjected to the direct influence of moisture, temperature changes, and erosion. This study proposed the use of a dynamic cone penetrometer as a quick tool to assess the shear strength and density of sand-clay cover liners. Poor areas indicating low shear strength and dry density need to be investigated and identified. Changes in moisture content and dry density affect the soil compressibility and shear strength parameters. A lightweight dynamic probe was found to give reliable shear strength measurements when assessing bentonite sand mixture materials. The impact of the moisture content on the profile of penetration was established and a bilinear relationship between the cone penetration and moisture content is shown. Laboratory fall cone tests were conducted to verify the trends of penetration and the influence of dry density. It was found that a linear function can be established within the wet of the optimum zone of the compaction curve. The tests conducted support the effectiveness of using dynamic cone penetrometers for measurements and evaluation of shear strength and dry density for sand clay cover liners. This will pave the way for using a quick tool for the assessment of compaction uniformity and shear strength of sand clay cover liners.     

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.     

Investigating the mechanism of downslope bentonite erosion in GCL liners using X-Ray CT

Exposed composite GMB-GCL liners are at risk of downslope bentonite erosion caused by the release of low ionic strength condensed water onto the top surface of the GCL following daily solar heating. This paper investigates the use of X-ray computed tomography (X-ray CT) to quantify the thinning of the bentonite layer and the application of X-ray diffraction techniques (XRD) to investigate the changes in clay chemistry (if any) of the bentonite from the virgin GCL to the eroded bentonite. The effect of specimen size and scanning orientation was investigated resulting in a revised testing procedure in which the CT scanning orientation was changed from horizontal to vertical to permit a longer test specimen which was also sealed at the bottom edge to minimise the edge boundary condition. The X-ray CT results provide highly visual evidence that a) bentonite thinning immediately under the upper cover geotextile is the initial location of erosion, and b) the bentonite core erodes at a significantly higher rate when not covered by a geotextile than when covered by a geotextile. These observations indicate that the upper geotextile of the GCL plays a significant role in controlling the rate of bentonite erosion. Finally, a comparison of the virgin and runoff bentonite properties was conducted to investigate potential changes in swell index, X-ray diffraction results, and concentration of Na and Ca cations. The runoff bentonite was observed to had a significantly higher swell index (40?ml/2?g) than the virgin bentonite (28?ml/2?g) and lower Na and Ca concentrations. This finding is consistent with the observation from XRD analyses of the runoff bentonite which illustrate that the clay fraction of the bentonite is preferentially eroded by the application of DI water.     

Factors affecting the down-slope erosion of bentonite in a GCL

Leaving a composite liner exposed for an extended period can sometimes lead to down-slope bentonite erosion from geosynthetic clay liners (GCLs). This laboratory study examines a number of factors that can affect the erosion of bentonite particles with an imposed flow of water for one particular geotextile-encased, needle-punched GCL. The factors examined include the effect of an initial wet/dry cycle, water chemistry, flow rate, slope, prior cation exchange, and the effect of no-drying phase in the test cycle. No erosion was observed unless the GCL had been hydrated and dried to create a wet/dry cycle. The most critical factor was found to be the water chemistry. No erosion was observed with tap water (39 ppm calcium) with up to 360 cycles and a flow of 3 L/hour. Tests simulating the evaporation and condensation of water below an exposed composite liner by imposing deionized water on the GCL surface developed erosion holes within 5–6 cycles.     

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.     

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.     

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。     

Irrigated composite liner designs for fast hydration and prevention of thermal desiccation of geosynthetics clay liners

In composite liners made of geomembrane (GMB)-geosynthetics clay liners (GCLs), maintaining bentonite in the GCL in a suitably hydrated state is critical for their performance. Hydration of GCL from subsoil, following industry best practice, is time consuming and conditional on suitable water chemistry in subsoil. In addition, under thermal gradients, dehydration occurs, with moisture migrating downwards to the subsoil, leading to the development of cracks in the bentonite and hence loss of performance.Two novel ideas are proposed in this paper, namely hydration of GCLs by artificial irrigation and hydraulic separation of the liner system from the underlying subsoil. Three new composite liner designs allowing for actively irrigating a geosynthetic clay liner (GCL) through a geocomposite layer were investigated. In two of the three designs, the hydraulic connection between the GCL and the subsoil was broken by placing an additional GMB between them. The new designs were tested in column experiments under 20 kPa overburden pressure and temperatures of up to 78 °C applied to the top of the liner. The performances of the new designs were compared to that of a standard GCL-GMB design where GCL was allowed to hydrate from a well-graded sandy subsoil. Three scenarios for the staging of hydration and thermal load application were investigated.Under active hydration of the composite liners, it took less than 14 days for the GCLs to reach a gravimetric water content ω of 110–130%, compared to 49 days taken to reach ω~95% under hydration from the subsoil. GCLs in the new designs in which the hydraulic connection with the subsoil was broken, remained well-hydrated (ω>100%) after 14 days of heating and no cracks appeared in the bentonite. On the other hand, the GCL in the conventional design experienced severe desiccation under the same conditions. The new designs hence offer a viable solution to the problem of slow hydration and/or thermal desiccation of GCLs.     

Design and placement considerations for clay and composite clay/geomembrane landfill final covers

The performance of clay and clay geosynthetic combinations for landfill barrier layers has not always proved predictable and the integration of geosynthetic materials into conventional landfill designs presents some interesting problems which were previously unforeseen.

This paper looks at identifying and mapping the approach to some specific considerations for clay and geomembrane/clay barriers. These considerations include: long-term performance of clay barriers, short-term and long-term stability of clay barriers, clay/geomembrane interface behavior, and the phenomena of moisture build-up beneath exposed membrane liners.     

水化针刺GCL+GM复合衬里的单剪破坏特征

针刺GCL和HDPE土工膜(GM)广泛应用于填埋场防渗衬里,GCL的内部剪切强度和GCL/GM界面剪切强度是填埋场复合衬里边坡滑移稳定性的控制因素。通过开展不限定剪切破坏面的水化针刺GCL+GM复合衬里大单剪试验,获得了剪切过程中GCL/GM界面位移和GCL内部位移发展规律,分析了GM的糙面分别与GCL的有纺面和无纺面接触时的峰值强度,揭示了GCL+GM复合衬里的整体剪切破坏特征。试验结果表明:大单剪试验能够正确和合理地模拟GCL与GM间的相互作用,GCL+GM复合衬里中的极限破坏面不仅会随着法向应力的增加而发生转移,甚至出现GCL内部和GCL/GM界面同时成为剪切破坏面的临界状态。     

Membrane behavior of bentonite-amended compacted clay

The performance of landfill liners can be enhanced if they exhibit a semipermeable membrane behavior, which restricts the migration of contaminants. Consequently, enhancing the membrane property of clays used for liners is becoming increasingly important. As bentonite has already proven to be an excellent additive for improving the membrane behavior of clay, the hydraulic conductivities and membrane behaviors of a locally available clay, known as Fukakusa clay, amended with different amounts of dry bentonite (5%, 10%, 15%, and 20%) were herein evaluated. The chemico-osmotic efficiency coefficient, ω, was obtained under different concentration differences of KCl solution (0.5, 1, 5, 10, and 50 mM) for specimens in rigid-wall cells. The test results show that the ω of unamended clay is very low and can be ignored unless amended with bentonite, and that hydraulic conductivity k is suitably low (1.58×10⁻⁹ m/s). Additionally, ω decreased as the KCl concentration increased, which is consistent with the theory that increasing concentration causes progressively greater shrinkage of the diffuse-double layers of the clay particles. Furthermore, the mechanisms that influence the membrane behavior are discussed from the viewpoints of the diffuse-double layer and the interparticle pore size with the assistance of SEM.     

Addressing the special concerns of landfill closures: VLDPE and textured geomembranes

Landfill closures often require a somewhat different set of properties for synthetic liners than do landfill bottom liner installations. In particular, cap design usually presents the geotechnical engineer with greater concerns regarding long-term slope stability and accommodation of differential settlement. Friction between synthetic liners and materials contacting those liners, multiaxial elongation, and flexibility increase in importance. Since leachate does not contact the liners, chemical resistance becomes less important. Resistance to the components of landfill gas is, in most cases, all that is necessary.

As a result, synthetic liners with a textured surface to improve friction angles, and very low density polyethylene (VLDPE) geomembranes are becoming very attractive to geotechnical engineers. They provide considerable improvement in those areas which are important for cap installation. These materials, however, behave differently in standard index and performance testing of geomembranes, when compared with traditional polyethylene liners. These behaviors and differences are important for geotechnical engineers to understand. They are discussed in this paper.     

填埋场边坡土工衬垫结构的稳定性分析方法

本文针对现代卫生填埋场典型的防渗衬垫结构,分析了边坡衬垫破坏的原因和形式,结合计算分析了常用土工材料对边坡稳定的影响,并对不同条件下土工材料的选用以及工程设计中的一些注意事项提出了建议。