Development of geomembrane strains in waste containment facility liners with waste settlement

The development of tensile strains in geomembrane liners due to loading and waste settlement in waste containment facilities is examined using a numerical model. Two different constitutive models are used to simulate the waste: (a) a modified Cam-Clay model and (b) a Mohr-Coulomb model. The numerical analyses indicate the role of the slope inclination on the maximum geomembrane liner strains for both short-term loading (immediately post closure) and long-term waste settlement. A geosynthetic reinforcement layer over the geomembrane liner is shown to reduce the maximum geomembrane liner strains, but the strain level of the geosynthetic reinforcement itself may become an engineering concern on steeper slopes (i.e., greater than 3H:1V) for cases and conditions examined in this paper. The paper considers some factors (e.g., slope inclination, use of a high stiffness geosynthetic over the geomembrane liner) and notes others (e.g., the designer selection of interface characteristics below and above the geomembrane, use of a slip layer above the geomembrane) that warrant consideration and further investigation to ensure good long-term performance of geomembrane liners in waste containment facilities.     

Geosynthetic liner integrity and stability analysis for a waste containment facility with a preferential slip plane within the liner system

The paper examines the effects of settlement-induced downdrag on geosynthetic liner systems for a waste containment facility with steep side slopes for different design scenarios, and conducts the stability analysis of the waste mass during waste filling operations. Without the presence of a reinforcing layer above a geomembrane (GMB) liner, the liner experiences unacceptable tensile strains under both short- and long-term downdrag waste settlements. It is shown that an anchored high strength/stiffness geotextile (HS-GTX) reinforcement over the GMB can reduce the GMB tensile strains to less than 3%, but the HS-GTX itself may be overloaded. A geosynthetic slip layer over the full or partial HS-GTX reinforcement overlying the GMB can reduce the tensile strains of the GMB to less than 3% and of the HS-GTX to less than 5% by providing a preferential slip plane between the geosynthetic slip layer and the HS-GTX. A rupture of the geosynthetic slip layer is likely to occur resulting in the exposure of the HS-GTX to the waste, but the protection of the GMB by the HS-GTX is still expected. The results from the stability analysis show that, during waste filling operations under a given factor of safety, there is a critical relationship between the width of the top of the waste pile and the total waste thickness.     

基础局部沉降下土工格栅加筋衬垫系统变形特性的试验研究

基础局部沉降会引起垃圾填埋场衬垫系统中的土工膜产生较大的拉应变,有可能导致衬垫系统性能下降,因此正确评价衬垫系统的应变就显得非常重要。通过模拟试验,采用应变片和位移计对基础发生局部沉降后土工格栅加筋衬垫系统的变形进行试验研究。试验结果表明:环境温度对衬垫系统的变形影响较大;相同组成材料下土工格栅和土工膜叠放在一起比其他方案更能降低土工膜的应变;衬垫系统刚度对沉陷范围影响不大,但对最大应变值影响较大。所得结果对垃圾填埋场衬垫系统的设计具有一定的指导意义。     

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.     

Seismic performance and numerical simulation of earth-fill dam with geosynthetic clay liner in shaking table test

In this paper, shaking table tests were carried out on both a small-scale and a full-scale earth-fill dams with geosynthetic clay liners to examine their seismic performance. The behavior of these fully instrumented earth-fill dams when subjected to seismic loading was also simulated by numerical analysis. Firstly, in the small-scale shaking table test, no failure was observed along the geosynthetic clay liner when the earth-fill dam was subjected to seismic motion. Numerical analysis confirmed that the behavior of the model earth-fill dam was unaffected by the geosynthetic clay liner. Secondly, a comparative shaking table test was carried out on full-scale earth-fill dams, one with a sloping core zone and another with a geosynthetic clay liner. Both model dams showed similar acceleration response and deformation behavior. It should be mentioned that the acceleration response increased gradually toward the top of the dam, and the deformation, after shaking, was relatively large near the foot of the slope. These observations were successfully simulated by the numerical analysis.     

Shear strength behavior of geotextile/geomembrane interfaces

This paper aims to study the shear interaction mechanism of one of the critical geosynthetic interfaces,the geotextile/geomembrane,typically used for lined containment facilities such as landfills.A large direct shear machine is used to carry out 90 geosynthetic interface tests.The test results show a strain softening behavior with a very small dilatancy(0.5 mm) and nonlinear failure envelopes at a normal stress range of 25-450 kPa.The influences of the micro-level structure of these geosynthetics on the macro-level interface shear behavior are discussed in detail.This study has generated several practical recommendations to help professionals to choose what materials are more adequate.From the three geotextiles tested,the thermally bonded monofilament exhibits the best interface shear strength under high normal stress.For low normal stress,however,needle-punched monofilaments are recommended.For the regular textured geomembranes tested,the space between the asperities is an important factor.The closer these asperities are,the better the result achieves.For the irregular textured geomembranes tested,the nonwoven geotextiles made of monofilaments produce the largest interface shear strength.     

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.     

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.     

ABAQUS钢与混凝土复合材料模型的二次开发

为了找出结构的薄弱位置,往往需要对各种复杂的建筑结构进行整体结构层次上的弹塑性数值模拟。为提高建模以及数值计算效率,需要采用梁单元计算模型模拟钢筋、钢骨以及钢管混凝土梁柱等复杂的构件。在混凝土弹塑性损伤模型的基础上,基于ABAQUS平台开发了能够描述以上复杂梁柱构件力学行为的梁单元复合材料模型。模型的提出使整体结构的弹塑性数值计算可以采用较为简单的梁单元和板壳单元模拟,提高了超高层建筑结构整体弹塑性分析的效率。     

上海世博会中国馆结构弹塑性时程分析

上海世博会中国馆东方之冠建筑造型和结构体系独特,结构为复杂大跨大悬挑钢-混凝土混合结构,主体结构为四个混凝土筒体,上部楼屋面采用混凝土梁板体系、型钢梁-混凝土板梁板体系和钢桁架梁-混凝土板梁板体系。由于上部结构悬挑较大,结构1阶振型为扭转振型,不满足现行建筑抗震设计规范的要求。采用NosaCAD2005有限元程序建立整体结构分析模型,压弯构件采用纤维截面模型,墙体采用非线性平板壳单元,以反映构件非线性复杂受力情况。通过7度多遇和7度罕遇烈度下的弹塑性时程分析,研究了该结构的变形、内力、破坏情况的发展历程。计算结果表明,小震情况下,结构构件未出现损坏;大震情况下,结构最大层间位移角满足1/100的限值要求,筒体构件损坏顺序和分布较为合理,能在一定程度上耗散地震输入能量。出现塑性铰的杆件未超过极限承载能力,结构可以满足"小震不坏"、"大震不倒"的抗震设防要求。最后根据构件的受力或损坏情况,给出了设计改进建议。     

Seismic displacements of landfills and deformation of geosynthetics due to base sliding

Seismic design of waste landfills has been a subject of intense research over the past two decades, primarily due to the severe environmental impact of a potential failure. The majority of the related studies have been focused on the stability assessment of landfills utilizing permanent deformation methods. However, previous investigations have not fully addressed the impact of the composite liner system on the seismic performance of the geostructure, mainly expressed as potential sliding development, which is greatly affected by the geometry and the resulting initial static stress state of the landfill. Therefore, the aforementioned issues are investigated via a detailed parametric study, where the dynamic behaviour of the composite liner system is examined both analytically and numerically. The conducted coupled analyses indicated that the most significant parameters of the complex dynamic response of waste landfills can be reduced in two ratios that comprise functions of the main characteristics of the geostructure and of the excitation. Moreover, two distinct failure patterns have been identified with respect to the characteristics of the distribution of the permanent displacements along the interface and the axial deformation along the geosynthetic. The occurrence conditions of these failure patterns can be determined in terms of the two abovementioned ratios as verified by the analytical results of the critical acceleration of a simple SDOF system.     

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.     

考虑蠕变、地震效应的土工格栅砂性土加筋挡墙弹塑性有限元分析

土工合成材料加筋土挡土墙具有优良的抗震性能,但是由于加筋用的土工合成材料具有显著的蠕变及应力松弛特性,需要深入研究加筋土挡墙在经历蠕变后的地震动力行为及震后的进一步蠕变以理解其全面的静动力学性能。在已有研究的基础上,应用笔者提出的土工合成材料循环受载、蠕变和应力松弛统一本构模型模拟土工格栅的力学行为,考虑到土工格栅加筋土挡土墙的填土一般为砂性土,而砂性土一般蠕变变形较小,本文应用可以模拟砂土述砂性土非线性静动力性能的广义塑性模型模拟填土,未考虑其蠕变变形。结果表明,在正常加筋长度和密度情况下,加筋土挡墙在经受地震前的蠕变变形会趋于稳定,但筋材内力重分布明显;地震作用使得加筋土挡墙产生较大变形,加筋内力出现较大增长,但结构并未破坏;地震后加筋土挡墙蠕变变形继续发展,而土工格栅加筋会出现内力松弛现象。     

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。     

Lateral buckling of web-tapered I-beams: A new theory

This paper presents a new theory for the lateral buckling of web-tapered I-beams. Linear analysis is first conducted by taking account into the tapering effects of web-tapered I-beams, where the deformation compatibilities of the two flanges and web are considered in terms of the basic assumptions of thin-walled members. Subsequently, the total potential for the lateral buckling analysis of web-tapered I-beams is developed, based on the classical variational principle for buckling analysis. The lateral buckling loads of web-tapered cantilevers and simply supported beams of I-sections from the proposed theory are compared with those from the finite element (FE) analyses using two shell element models and two widely used beam element models. The two beam element models respectively represent the equivalent method using prismatic beam elements and the typical tapered beam theory in existing literature. These comparisons show that the results based on the total potential proposed in this paper are more accurate in predicting the lateral buckling loads of web-tapered I-beams than those in existing theories, indicating that the theory proposed in this paper is superior to existing theories. It is also found that the equivalent method using prismatic beam elements may yield unreliable buckling loads of tapered beams.     

垃圾填埋场多层复合衬垫的破坏面特征

多层土工合成材料复合衬垫的极限破坏面特性是垃圾填埋场稳定分析的重要问题,单一固定破坏面的观点已经被广泛接受。在多层土工合成材料复合衬垫的整体叠环式单剪试验后发现极限破坏界面并非单一固定,而是随着法向应力的变化发生由一个界面向另一界面转移,且在一定的法向应力范围内还可能同时出现两个具有相同剪切强度的极限破坏界面;多层土工合成材料复合衬垫中各层的剪切应力–位移曲线是硬化型的,衬垫系统的剪切强度总是低于极限破坏界面的剪切强度。试验结果表明,叠环式单剪仪能更正确和合理地模拟填埋场中的多层复合衬垫在加载过程中的实际剪切变形情况和复合衬垫中材料间的相互作用,从而能更好地揭示多层复合衬垫系统的整体剪切特性。     

Liner system design for heap leach pads

Heap leach pads are used in the mining industry as an integral component for metal extraction and processing. The design of modern heap leach pads utilize an engineered liner system that often is a blend of natural and geosynthetic materials in order to achieve a desired performance, such as ore heap stability, solution drainage, and efficient recovery. An important issue that is often overlooked in the design of heap leach pad liner systems is the compatibility between geosynthetic materials (e.g. geomembrane, geopipe, geotextile, etc), native materials, ore, and operational conditions/restrictions. This issue becomes more pronounced as heap leach pads are constructed in steep terrain, harsh climates (wet, cold, etc), and subjected to high ore loads (exceeding 4 MPa). This paper discusses design approaches for heap leach pad liner systems, with a focus on compatibility with geosynthetic materials.     

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.     

Numerical modeling of tight fitting flexible liner in damaged sewer under earth loads

Finite element analysis and closed form solutions are used to investigate and interpret tests conducted on a damaged rigid pipe fitted with an HDPE pipe liner. Details of the analysis are described, including the modeling of local interactions between the liner and the host pipe, different segments of the damaged host pipe across longitudinal fractures, and the host pipe and the surrounding ground. Laboratory tests on a lined sewer pipe under earth loads are studied to establish whether finite element and closed form solutions for the repaired pipe system are able to effectively represent the observed behaviour.

The deteriorated rigid pipe is fractured into four fragments, and these interact to impose pairs of closely spaced vertical line loads at the crown and invert of the liner. The finite element analysis demonstrates that significant local bending develops in the liner under these contact forces. Very little thrust develops in the liner, which is almost in pure bending provided the host pipe can still carry hoop thrust across fractures. The angular expansion of the fractures at crown and invert had a negligible effect on the local bending in the liner, allowing the use of theory for rings under parallel plate loading to provide simple calculations of liner response.

Nonlinear finite element analyses indicate that the interface between the liner and the host pipe is close to the full-slip (or smooth) condition. Analysis results for that condition are within 10% of the measured values. The finite element procedure also indicates that the host pipe and the liner within it deform under the effect of the full overburden pressure if the host pipe fractures after the liner is installed. Safe liner design could be achieved by ensuring that local bending associated with host pipe fracture and deformation under the full overburden pressures does not exceed tensile capacity of the polymer (in addition to liner design to resist buckling under external fluid pressure, in accordance with previous liner stability studies). Alternatively, measures might be used to avoid local bending in the liner by preventing deterioration of the host pipe and the surrounding soil during the design life of the repair.     

Novel experimental techniques to assess the time-dependent deformations of geosynthetics under soil confinement

A new experimental approach to assess the impact of soil confinement on the long-term behavior of geosynthetics is presented in this paper.The experimental technique described herein includes a novel laboratory apparatus and the use of different types of tests that allow generation of experimental data suitable for evaluation of the time-dependent behavior of geosynthetics under soil confinement.The soil-geosynthetic interaction equipment involves a rigid box capable of accommodating a cubic soil mass under plane strain conditions.A geosynthetic specimen placed horizontally at the mid-height of the soil mass is subjected to sustained vertical pressures that,in turn,induce reinforcement axial loads applied from the soil to the geosynthetic.Unlike previously reported studies on geosynthetic behavior under soil confinement,the equipment was found to be particularly versatile.With minor setup modifications,not only interaction tests but also in-isolation geosynthetic stress relaxation tests and soil-only tests under a constant strain rate can be conducted using the same device.Also,the time histories of the reinforcement loads and corresponding strains are generated throughout the test.Results from typical tests conducted using sand and a polypropylene woven geotextile are presented to illustrate the proposed experimental approach.The testing procedure was found to provide adequate measurements during tests,including good repeatability of test results.The soilegeosynthetic interaction tests were found to lead to increasing geotextile strains with time and decreasing reinforcement tension with time.The test results highlighted the importance of measuring not only the time history of displacements but also that of reinforcement loads during testing.The approach of using different types of tests to analyze the soilegeosynthetic interaction behavior is an innovation that provides relevant insight into the impact of soil confinement on the time-dependent deformations of geosynthetics.