考虑破坏面转移和垃圾坝作用的边坡稳定分析

垃圾坝是山谷型填埋场和横向扩建填埋场中常采用的增稳措施;破坏面在衬里结构不同界面间发生转移也是被证实的规律,考虑破坏面转移和垃圾坝作用的垃圾体边坡稳定分析方法尚未见报道。通过将衬里结构中破坏面转移点作为分界点,将滑动垃圾体分成5个楔体,利用极限平衡条件建立了五楔体边坡稳定分析方法。研究结果表明,五楔体极限平衡分析方法能够分析考虑破坏面转移和垃圾坝影响的填埋体稳定性;考虑破坏面转移计算得到的安全系数低于不考虑考虑破坏面转移的计算结果,考虑破坏面转移的计算方法能够发现更危险的情况;填埋场安全系数随垃圾坝高度的增大而增大;垃圾坝的背坡有一最优坡度,垃圾坝的背坡小于这一坡度时,发生“坝背破坏”模式;垃圾坝的背坡大于这一坡度时,发生“坝底破坏”模式;最危险破坏面通过填埋场的背坡和底坡的衬里,再通过垃圾坝的坝背衬里界面或坝底。     

某填埋场垃圾堆体边坡失稳过程监测与反分析

某填埋场是国内首批在场底铺设复合衬垫系统的大型卫生填埋场,该场垃圾坝前堆体边坡于2008年6月连续强降雨期间发生失稳事件。介绍该堆体边坡失稳过程的现场监测结果,包括坡面水平位移、深层侧向位移和渗滤液水位。基于监测数据,开展堆体边坡稳定性反分析工作,探讨复合衬垫系统界面抗剪强度取值方法,提出抽排竖井迫降水位、铺膜防渗等应急抢险措施。现场监测和理论分析结果表明:堆体边坡中高渗滤液水位是导致其失稳的关键因素,堆体边坡水平位移速率和渗滤液水位高度呈明显正相关关系;该堆体边坡失稳模式是沿场底复合衬垫系统中软弱界面的深层滑移;斜坡场底上复合衬垫系统在滑移过程中发生位移-软化效应,其界面强度介于峰值强度和残余强度之间;抽排竖井迫降水位是最直接、有效的应急抢险措施。     

基于界面物态演变规律的衬垫界面动力模型

衬垫界面的动力剪切特性对填埋场动力稳定性有着重要影响,然而,已有界面/接触面动力本构理论无法完整揭示其动力剪切过程中的物态演变机理。将土工膜/GCL界面的动力剪切过程分为土质材料变形和界面滑动摩擦两个物态阶段,提出了触发界面物态演变的界面临界状态和临界应力,结合界面动力剪切基本规律,分别采用弹塑性模型与摩擦系数模型描述两个物态阶段,建立了衬垫界面动力模型,并与多次循环条件下的土工膜/GCL界面动力剪切试验进行对比,验证了该动力模型的准确性。研究表明,该模型能有效地模拟衬垫界面动力剪切变形的力学特性与物态演变规律,为填埋场动力稳定性分析提供了理论支持。     

关于具有多层复合衬里填埋场稳定安全的探讨

防止含有多层土工合成材料复合衬里的填埋场的平移破坏是填埋场设计中的最重要问题之一。发生在多层衬里界面上的平移破坏的破坏面可能会在背坡上通过衬里的一个界面,而在底部却通过衬里的另一个界面;具有最小安 全系数的衬里最危险界面会随着填埋高度和填埋体形状的变化从一个界面转移到另一个界面。在填埋高度较小时, 30 cm 的渗滤液水位对稳定分析计算的最危险界面和安全系数都会有较大的影响;但是随着填埋高度的增高, 30 cm 的渗滤液水位对最危险界面改变的和安全系数降低比例的影响变得越来越小。由于渗滤液水位的升高,不仅会导致安全系数的减小,而且还可能会导致多层衬里的最危险界面从一个界转移到另一个界面。提高设计水平,解决目前我国填埋场在长期运行中的渗滤液水位升高问题,保证填埋场的稳定性,是目前迫切需要重视和解决的问题。     

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.     

设垃圾坝卫生填埋场平移破坏的统一分析模型

 针对设垃圾坝卫生填埋场沿填埋场侧壁、底部及垃圾坝背部和沿填埋场侧壁、底部及垃圾坝底部这2种平移破坏模式,提出平移破坏的统一分析模型。该模型将滑移体分为主动、中间和被动3个楔体,通过对各个楔体的极限平衡分析,建立方程计算填埋场平移破坏安全系数。通过算例分析平均安全系数和真实安全系数的最大相对误差以及楔体间安全系数和整体安全系数的关系,并与未设垃圾坝和设垃圾坝,但只考虑单一破坏模式分析模型的计算结果进行比较,说明统一分析模型的正确性、可行性与兼容性。该模型既能考虑垃圾坝断面形状、垃圾坝背部与垃圾体作用力方向和复合衬垫系统界面的黏聚力等因素对平移破坏的影响,克服了有些模型对这些条件的限制;也能用于不设垃圾坝填埋场平移破坏分析。根据设垃圾坝填埋场平移破坏分析结果,还可对垃圾坝断面进行优化设计。     

Cyclic shear behavior of GMB/GCL composite liner

The composite liner system consisting of geomembrane (GMB) and geosynthetic clay liner (GCL) has been widely used in landfills. Although there have been a lot of studies on the monotonic shear behavior of GMB/GCL composite liner, the dynamic test data are still very limited and consequently, the dynamic shear mechanism is not clear. A series of displacement-controlled cyclic shear tests were conducted to study the shear behavior of GMB/GCL composite liner, including the shear stress versus horizontal displacement relationships, backbone curves, and shear strengths. Hysteretic loops in the shape of parallelogram were obtained and equivalent linear analyses revealed that the secant shear stiffness decreased and the damping ratio increased with the rise in loading cycles. According to the test results, it is generally acceptable to predict the dynamic peak strength of a GMB/GCL composite liner with its static strength envelope. Furthermore, the dynamic softening mechanism and rate-dependent shear stiffnesses were well described by the proposed equations, which also facilitate the accurate modeling of the cyclic shear behavior.     

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.     

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%.     

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。     

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.     

土工膜/土工织物界面大型斜坡模型试验研究

复合衬垫系统广泛应用于垃圾卫生填埋场,是防止渗沥液污染物渗漏扩散的重要屏障。在垃圾重力及沉降作用下易造成斜坡上复合衬垫系统拉伸破坏或沿其界面产生滑移而失稳。目前,由于缺乏对复合衬垫系统内部剪力传递机理的认识,仍难完全解决以上两大岩土工程问题。因此,设计并采用复合衬垫系统大型斜坡模型试验装置开展了其内部剪力传递机理的研究。该装置通过砂袋加载模拟填埋过程,采用手拉葫芦为核心的滑移控制系统再现了土工膜/土工织物界面的渐进累积破坏过程。试验结果表明：当外部剪力小于峰值强度时,界面不会进入残余状态,上覆的土工合成材料锚固端的拉力也非常小;但当外部剪力超过界面峰值强度时,界面就会逐渐进入残余状态,并最终达到残余强度。同时,薄弱界面上覆的土工合成材料锚固端的拉力也显著增加,严重时甚至被完全拉断。     

Oberflächenabdichtungen aus PE‐HD‐Dichtungsbahnen für Altdeponien

Surface lining with HDPE geomembranes of old municipal waste landfills. Until mid‐2005 approximately 250 old municipal waste landfills have to be closed, because they failed to meet the requirements according to the state of landfill engineering. After closure, a containment and remediation concept has to be put into action. At landfill sites with appropriate conditions (e.g. base liner and waste water collection system), one may try to stabilize the waste body by forcing up through water infiltration the anaerobic biochemical reaction within the waste body or by aerobic stabilization techniques. However, engineered capping systems are an indispensable tool in the containment strategy for waste disposal. Reliable, highly effective and long lasting liner systems can be installed even under the special conditions on the surface of old landfills. In particular, selected HDPE geomembranes are highly suited for these conditions. However, approval of geomembrane products for this application is essential, since geomembranes made of different HDPE resins as well as differently produced out of the same HDPE resin may extremely differ in their long‐term and installation properties.     

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.     

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.     

挥发性有机化合物在复合衬里中的一维扩散解

土工膜和粘土衬里组成的复合衬里已广泛用于填埋场的防渗。有机挥发性化合物在复合衬里中迁移时,其主要的机理是分子扩散作用。本文建立了有机挥发性化合物在复合衬里中的一维扩散模型,并得到了解析解。将该解析解和以往给出的数值解法作了比较,发现两者得到的结果较为接近,从而验证了本文解的可靠性。基于本文计算模型,分析了三种常用复合衬里对挥发性有机化合物的防渗性能。研究发现对于挥发性有机化合物甲苯,土工膜和GCL组成的复合衬里的浸出液总量要比土工膜和较厚粘土衬里组成的复合衬里大好几个量级。     

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.     

Dynamic shear behaviors of textured geomembrane/nonwoven geotextile interface under cyclic loading

Textured geomembrane (GMB) and nonwoven geotextile (GTX) are usually used together in liner systems of MSW landfills, but the low shear strength of GMB/GTX interface is extremely detrimental to the stability of landfills, especially under earthquake loading. To study the dynamic shear strength of the GMB/GTX interface, a series of displacement-controlled cyclic direct shear tests are conducted with a large-scale direct shear machine. Normal stress levels ranging from 100 to 1000 kPa and displacement amplitudes ranging from 5 to 25 mm are considered. To compare the failure mechanism, GMB and GTX specimens are tested in not only hydrated but also dry conditions. Different waveforms and excitation frequencies are also applied to analyze the effects of test conditions. It can be seen that the shear deformation develops totally along the GMB/GTX interface when specimens are fully hydrated, while the internal failure of GTX is induced in dry condition. Equivalent linear analyses reveal that the shear stiffness depends on normal stress and displacement amplitude, while the damping ratio is only affected by displacement amplitude. Variations of shear strength during the shear process indicate that the softening behavior of the GMB/GTX interface is closely related to cumulative displacement and normal stress level. Furthermore, based on test results, a positive correlation is summarized between the shear strength and displacement rate of the interface.     

下卧土体局部沉陷条件下复合衬垫系统的受力变形性能及设计

考虑上覆土体的土拱效应及衬垫的大变形,将下卧土体局部沉陷条件下的复合衬垫系统分为滑动区和沉陷区。建立了其受力变形分析模型,并以衬垫系统的最大拉应变作为控制标准,建立了工程上衬垫系统的抗沉陷设计方法。利用该分析模型研究了滑动区衬垫界面强度、衬垫上覆土体的厚度、重度、有效内摩擦角等参数和衬垫结构形式的影响,发现上覆土体厚度、重度及有效内摩擦角对衬垫受力变形影响很大,而衬垫的抗拉刚度直接决定了其拉应变大小。结合工程实例进行了衬垫系统的抗沉陷设计,并建议了衬垫的抗沉陷结构形式。     

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.