Assessment of HDPE geomembrane performance in a municipal waste landfill double liner system after eight years of service

In the late 1970s and early 1980s, environmental regulations were upgraded in a general national movement to effect secure management of our municipal and residual solid wastes. The new regulations required varying combinations of natural and/or synthetic barrier and drainage layers to prevent the unrestricted release of contaminants.

The acceptable barrier materials included synthetic flexible membrane liners (FMLs) of various types. One of those most commonly used has been high-density polyethylene (HDPE) geomembrane. HDPE has been selected because of its good chemical resistance characteristics, among others. Background compatibility testing has shown the HDPE geomembrane to be extremely resistant to the leachates that are generated by municipal and residual solid waste landfills. The background testing for design has generally been based on relatively short-term tests that are conducted under extreme conditions to ‘forecast’ service life.

Recently, a municipal solid waste landfill double liner system that was constructed in 1988 was exhumed. The HDPE geomembranes of this liner system had been exposed to varying degrees of leachate since 1989. Samples of the HDPE were extracted from the in-place liner system and were laboratory-tested for physical, mechanical and endurance properties. The selected suite of tests duplicated the test protocol conducted in 1988 as part of the liner system construction quality assurance (CQA) program.

The results of this testing show that the HDPE properties are still within the range of data generated by the original testing in 1988. No degradation in properties was indicated by this testing program. The HDPE had been exposed to the leachate, methane, and static and dynamic stresses for approximately 8 years. The results of this test program support the design selection of HDPE as the synthetic barrier component of this landfill liner system.     

Numerical study of strain development in high-density polyethylene geomembrane liner system in landfills using a new constitutive model for municipal solid waste

Tensile strain development in high-density polyethylene (HDPE) geomembrane (GMB) liner systems in landfills was numerically investigated. A new constitutive model for municipal solid waste (MSW) that incorporates both mechanical creep and biodegradation was employed in the analyses. The MSW constitutive model is a Cam-Clay type of plasticity model and was implemented in the finite difference computer program FLAC?. The influence of the friction angle of the liner system interfaces, the biodegradation of MSW, and the MSW filling rate on tensile strains were investigated. Several design alternatives to reduce the maximum tensile strain under both short- and long-term waste settlement were evaluated. Results of the analyses indicate that landfill geometry, interface friction angles, and short- and long-term waste settlement are key factors in the development of tensile strains. The results show that long-term waste settlement can induce additional tensile strains after waste placement is complete. Using a HDPE GMB with a friction angle on its upper interface that is lower than the friction angle on the underlying interface, increasing the number of benches, and reducing the slope inclination are shown to mitigate the maximum tensile strain caused by waste placement and waste settlement.     

Tension in geomembranes on landfill slopes under static and earthquake loading—Centrifuge study

Geomembranes are some of the most commonly used geosynthetics in landfill liner systems. Geomembranes may experience harsh environmental conditions such as extreme temperatures or earthquake loading. Earthquake loading can be an extreme loading case for landfills located in seismic regions. This study, based on dynamic centrifuge testing, investigates the effects of simulated earthquake loading on the tension experienced by the geomembrane on a landfill slope. The landfill modelled in the dynamic centrifuge tests was a 7 m high municipal solid waste (MSW) landfill with a single geomembrane-clay liner system (45° side slope and 10 m slope length). Results show that moderate earthquake loading (base acceleration between 0.1 and 0.3 g) can result in a permanent increase in geomembrane tension of 5–25%.     

Contaminant mitigating performance of Chinese standard municipal solid waste landfill liner systems

In 2004, Chinese Government prescribed standard municipal solid waste (MSW) landfill bottom liners. However, very limited research has been conducted to evaluate the performance of the standard MSW landfill bottom liners prescribed by the Chinese Government. In this paper, the performance of the two types of Chinese MSW landfill bottom liner systems was evaluated based on: (1) the maximum leachate head; (2) leakage rate; (3) peak concentration of the target contaminant in an aquifer which was underlain the assumed landfill, and (4) total mass per unit area of the target contaminant discharged into the aquifer. The performance of the German standard MSW landfill bottom liner system was evaluated and compared with that of Chinese ones. It is found that the calculated maximum leachate head for the Chinese landfill liner systems was much higher than that for the German one. The calculated leakage rate, peak concentration and the maximum total mass per unit area in the aquifer of the target contaminant show that the performance of the Chinese standard landfill liner Type 2 is practically the same as that of the German standard landfill liner with holed geomembrane wrinkles, while the Chinese standard liner Type 1 is less effective, with regarding the mitigation of the impact of landfills to the groundwater quality. It is concluded that the overall performance of the Chinese standard landfill liner systems is less strict than that of the German standard landfill liner system.     

Interface friction: An owner''s perspective

The introduction of geosynthetics to landfill applications has generally resulted in significant environmental protection improvements. Most significant applications have been the use of geomembranes in landfill liners and final covers. Other geosynthetics that are commonly utilized, often in conjunction with and adjacent to geomembranes, include geotextiles and geonets. However, when geosynthetics are used, especially when they are placed adjacent to each other in a liner or final cover configuration, the frictional characteristics of the resulting interfaces can become a very important factor affecting the design, construction and operation of a landfill. The impact of the frictional characteristics of such interfaces and the related potential stability issues on the design, construction and operation of landfills, from a landfill owner's perspective, is the subject of this paper.     

Magnitude and significance of tensile strains in geomembrane landfill liners

The implications of the tensile stress/strain developed in high density polyethylene (HDPE) geomembranes (GMB) is explored in the context of a reduction in stress crack resistance due to ageing in contact with leachate in a municipal solid waste (MSW) landfill. The experimental evidence of GMB cracking and ultimately failure when subject to excessive tensile strains is discussed to highlight the need to limit the maximum tensile strain sustained by an HDPE GMB to an acceptable level if good long-term performance is to be ensured. The effect of both local GMB indentations induced by gravel in an overlying drainage layer or an underlying clay liner on tensile strain is reviewed. In addition, the tensile strains caused by down-drag in the GMB on side slopes with settlement of the waste is examined. The key research related to tensile strains developed in GMBs from these sources is reviewed and new data presented. It is shown that an appropriate protection layer over the GMB can limit local GMB tensile strains to less than 3% and that the selection of a suitable slope inclination and stiffness of a geotextile reinforcement layer can limit the GMB strains due to down-drag to less than 2% and geotextile strains to less than 4% after long-term waste settlement.     

Subsidence performance of landfills

This paper presents field measurements of landfill subsidence for five municipal solid waste landfills. Data from the.five sites are within the range of landfill subsidence reported in other publications.

At one site with variable subsidence of up to 10 percent of the landfill height in 3 years after capping, portions of an in-situ HDPE cap geomembrane were uncovered. The geomembrane showed no visual signs of subsidence-induced distress.     

The effects of reduction of the deposited waste on short‐term landfill leachate composition of a landfill: a case study in Norway

Municipal solid waste landfill leachate was sampled over from a landfill receiving varying amounts of municipal solid waste. The investigation aimed to provide information on expected leachate changes in the short term, either after closure of an active landfill, or after a strong decline in the amount of waste deposited at smaller landfills. It was found that during a two year period following a sudden decline in the amount of waste deposited, the levels of various chemical and physical parameters all dropped sharply. The reasons for the decline in discharge levels are thought to be aerobic decomposition taking place in the municipal solid waste just after landfilling, and thus the decline in the impact of this process when there were less fresh waste masses available, and formation of preferential flow paths for the leachate as the municipal solid waste stabilized in the landfill.     

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.     

垃圾填埋场边坡上土工膜的受力分析

在垃圾填埋场中,土工膜可以隔离垃圾体与周围环境,避免地下水受到污染。在土工膜上填埋垃圾和土层,沿其长度方向将有剪应力作用,使得土工膜发生变形,内部产生拉力,为保证土工膜的安全使用,需要对土工膜的拉力进行分析。将土工膜与黏土界面的剪应力–位移关系曲线分为弹性、软化和残余强度3阶段,采用三阶段弹塑性模型来描述土工膜与黏土界面的剪切变形特性,推导出了界面处于弹性、软化和残余强度3阶段时土工膜位移–拉力的微分控制方程,由于3个阶段分界点的位置是未知的,利用迭代法求解土工膜的拉力,分析了填埋高度和坡度的变化对土工膜拉力的影响。     

Quality control of flexible membrane liners for waste disposal facilities

Flexible membrane liner systems have become required by law in many parts of the world for hazardous waste landfills and surface impoundments. Although a number of liner types can be considered in waste containment, there is a rapidly increasing trend toward the use of polymeric liners of the high-density polyethylene variety.

With such emphasis and confidence given to flexible membrane liners, how can we be sure that the liner will provide maximum security for containment of hazardous wastes? Comprehensive quality control of the membranes provides the answer. A comprehensive quality control program is outlined in this paper in the light of recent developments in membrane liner selection and regulatory actions.     

Performance issues of barrier systems for landfills: A review

The objective of the paper is to give an update in key topics related to performance issues of barrier systems for landfills. The objective of using barrier systems is to minimize the impact of contaminants on the surrounding environment. To achieve this goal puncture protection of the geomembrane must be ensured. An update is first given with respect to this matter. The question of the stability on slope of geosynthetic barrier systems is then discussed and an insight is given in modeling and laboratory measurement of parameters required to perform reliable modeling, especially as regards the case of piggy-back landfills. Geotechnical centrifuge modelling tests are very important for simulation of landfill stability induced by the failure of geosynthetic interfaces and validation of complicated numerical models, especially for the high food waste content landfills. The seismic design or assessment of landfill stability with respect to geosyntheics needs to be investigated. Finally, the question of transfers through bottom barrier systems is addressed, giving an update especially in the analytical solutions developed in the past 10 years in China in this matter. The breakthrough time based design method for landfill liner system was then summarized. The behaviour of double-liner system and its simplified performance-based design method should be further investigated in the high food waste landfills with high leachate level.     

三峡库区污水处理厂污泥处置现状研究

目前,重庆三峡库区共建有污水处理厂18座,日产污泥132．95t／d。污泥处理主要采用浓缩＋机械脱水的工艺,然后送至垃圾填埋场填埋。运行过程中,由于脱水污泥含水率较高（80％左右）,且未经稳定化处理,已经极大的影响了填埋场的正常运行。针对这种情况,本文提出：在有垃圾填埋场的情况下,通过机械脱水、石灰稳定化等处理后进行填埋,不具备卫生填埋条件的实行堆肥后资源化利用。     

复杂荷载作用下填埋场HDPE土工膜受拉计算

采用FLAC程序对典型荷载作用下城市垃圾填埋场HDPE土工膜的受力状况进行了计算分析。3种荷载条件包括:36 m高垃圾堆体自重荷载(分层填埋);下卧软弱黏土层不均匀沉降引起的荷载;地震引起的动力荷载。计算结果显示:①土工膜内的拉应力随着垃圾土分层填埋、基础不均匀沉降、地震荷载的作用而积累;②基础不均匀沉陷是影响衬垫层土工膜局部拉应力的主要因素;③中等强度水平的地震动输入(例如,峰值加速度为0.25g)可使覆盖层土工膜端部锚固位置拉应力超过极限拉应力。     

Controlling strain in geosynthetic liner systems used in vertically expanded landfills

According to relevant new regulations in China,a composite liner system involving geosynthetic materials must be installed at the bottom of an expanded landfill.The deformation and integrity of the composite liner under a variety of factors are important issue to be considered in the design of a landfill expansion.In this paper,we investigate the strain distribution in geosynthetic materials within the composite liner system of expanded landfills,including strains in geosynthetic materials resulting from overall settlement and lateral movement of landfills,localized subsidence in landfills,and differential settlement around gas venting wells.The allowable strains of geosynthetic materials are discussed based on the results of tensile tests,and the corresponding design criteria for composite liner systems are proposed.Meanwhile,practical measures allowing strain control in geosynthetic materials used in landfill engineering are proposed.     

考虑垃圾体成层性的渗滤液回灌运移规律

渗滤液回灌不仅能处理渗滤液、节约费用和保护环境,而且加速了填埋场稳定化,增加了产气率。以上海某填埋场四期工程为计算模型,本文考虑垃圾土沉降导致的成层性,运用SEEP/W有限元软件模拟渗滤液喷洒回灌。垃圾土在自重作用下会产生较大的沉降变形,导致垃圾土的孔隙率随深度变化,残余含水量、饱和含水量及饱和渗透系数也随之变化。在数值模拟中,以VGM(van-Genuchten-Mualem)函数描述垃圾土渗透系数与基质吸力关系,以流量边界模拟填埋场顶部、底部的渗滤液回灌和导排,分析了回灌过程中渗滤液的饱和-非饱和渗流情况,进而探讨了垃圾土成层性、回灌强度和排水流量对渗滤液运移规律的影响。     

Planung einer thermischen Abfallverwertungsanlage in Großbritannien,ERF Newhaven

Im Jahre 2000 trat die EU‐Richtlinie [1] für Abfalldeponien in Kraft, mit der eine erhebliche Reduzierung der Mengen an Hausmüll verbunden ist, die deponiert werden darf. Da in Großbritannien ein beträchtlicher Anteil des Hausmülls auf Deponien entsorgt wird, muss das Müllmanagement nun komplett umgestellt werden, um die Anforderungen dieser Richtlinie zu erfüllen. Zum einen ist die stetig ansteigende Abfallmenge auf ein Minimum zu begrenzen und der anfallende Hausmüll so weit wie möglich zu recyceln und zu kompostieren, zum anderen sind Alternativen zur Mülldeponierung zu suchen. Eine entscheidende Rolle wird dabei sicher die thermische Abfallbehandlung spielen, die in anderen EU‐Ländern schon einen weitaus größeren Stellenwert einnimmt. Neben der eigentlichen Entsorgung entsteht bei dem kontrollierten Verbrennungsvorgang unter hohen Temperaturen als Nebenprodukt Wärme. Diese kann zur Energieerzeugung genutzt werden und macht die Anlagen somit in zweierlei Hinsicht attraktiv. In der Region East Sussex County haben die Mülldeponien, in denen der überwiegende Teil des Hausmülls der Region entsorgt wird, ihre Lagerungskapazität erreicht. Um das Abfallproblem zu lösen und die Auflagen der EU zu erfüllen, vereinbarten die Grafschaft East Sussex County Council und die Städte von Brighton & Hove City Council mit dem weltweit führenden Umweltdienstleister Veolia Environment Services die Übernahme des Abfallmanagements und als Bestandteil des Vertrages den Bau und den Betrieb einer hochmodernen thermischen Abfallverwertungsanlage in Newhaven, East Sussex. Design of a energy recovery waste facility in Great Britain, ERF Newhaven. The Landfill Directive, more formally, Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste [1], is a European Union directive issued by the European Union to be implemented by its member states. The Directive's overall aim is “to prevent or reduce as far as possible the negative effects of landfills on the environment. In particular, the objectives are the minimization of the pollution of surface water, groundwater, soil and air, and on the global environment, including the greenhouse effect, as well as any resulting risk to human health, from the landfilling of waste, during the whole life‐cycle of the landfill”. This legislation also has important implications for waste disposal. Due to the wide availability of landfills, Great Britain, has in the past, disposed a substantial part of its municipal waste in landfills. The UK government has now imposed waste legislation designed to reduce the release of greenhouse gases produced by landfills. It is the UK government's position that thermal waste treatment will play an increasingly large role in the treatment of municipal waste and supply energy in the UK. This position is consistent with other EU members. The landfills have all reached their capacity in East Sussex County. In order to manage the waste disposal problem and simultaneously respect the new rigorous legislation, East Sussex County Council, the cities of Brighton & Hove city council and the worldwide environmental services provider Veolia Environment Services have contracted to provide a waste management service and as part of the new waste infrastructure build, commission and operate a state‐of‐the‐art energy recovery facility in Newhaven, East Sussex.     

垃圾填埋场渗沥液水位壅高及工程控制

 我国垃圾填埋场渗沥液水位普遍壅高,严重影响安全运行。测试和总结垃圾的持水特性、饱和渗透系数以及渗沥液导排层渗透和淤堵特性。通过数值分析,揭示垃圾初始含水率、持水特性以及垃圾和导排层渗透系数对填埋场水位的影响规律。结果表明,垃圾饱和渗透系数随深度和龄期减小、导排层淤堵、垃圾初始含水率高导致水位明显壅高;而垃圾饱和渗透系数较小时,堆体内易形成局部滞水。结合实际工程,验证数值模拟结果,提出并实施水位壅高控制的工程措施,效果十分显著。     

Geoenvironmental approaches in an old municipal waste landfill reclamation process: Expectations vs reality

Nowadays, environmental geotechnics and engineering are facing several challenges to be addressed during landfill reclamation works. The purpose of the technical paper is to present examples of reclamation works designed to reduce the soil–water environment impact of an old municipal solid waste landfill and to ensure geotechnical safety. The authors tried to prove that even though the initially designed reclamation works could be difficult to implement and execute, they could bring the desired effect in real conditions. Several technical solutions to re-engineer shape the landfill body and reinforce the slope stability are discussed in the article. Commonly known pollution-reducing methods were also presented and discussed. For many years of operation, a landfill study area was located in difficult hydrogeological conditions, with no systems in place to prevent contamination. Using monitoring and numerical modelling, it has been shown that reclamation works increase the quality of groundwater and improve the conditions for landfill slope stability over time. The study also showed that both vertical barriers and leachate drainage systems can improve the condition of plant communities in landfill surroundings. Accordingly, reclamation works do not fundamentally change the function of a landfill but do limit its negative impacts on the environment, such as reducing the spread of pollutants into soil and water, protecting the slopes from erosion, reducing dust, allowing establishing of new vegetation cover, and improving the visual quality of the landscape. The research proved that expected reclamation results could only be reached if complex approaches and constant monitoring are provided when executing the reclamation works.