Failure mechanisms of geosynthetic clay liner and textured geomembrane composite systems

The objective of this study was to evaluate shear behavior and failure mechanisms of composite systems comprised of a geosynthetic clay liner (GCL) and textured geomembrane (GMX). Internal and interface direct shear tests were performed at normal stresses ranging from 100 kPa to 2000 kPa on eight different GCL/GMX composite systems. These composite systems were selected to assess the effects of (i) GCL peel strength, (ii) geotextile type, (iii) geotextile mass per area, and (iv) GMX spike density. Three failure modes were observed for the composite systems: complete interface failure, partial interface/internal failure, and complete internal failure. Increasing normal stress transitioned the failure mode from complete interface to partial interface/internal to complete internal failure. The peak critical shear strength of GCL/GMX composite systems increased with an increase in GMX spike density. However, the effect of geotextile type and mass per area more profoundly influenced peak critical shear strength at normal stress > 500 kPa, whereby an increase in geotextile mass per area enhanced interlocking between a non-woven geotextile and GMX. Peel strength of a GCL only influenced the GCL/GMX critical shear strength when the failure mode was complete internal failure.     

挥发性气体通过填埋场复合覆盖层的一维扩散解析解

为了评价填埋场覆盖层对挥发性气体的防渗性能,建立了挥发性气体在复合覆盖层中的扩散运移模型并采用分离变量法获得了模型解析解。解析解的计算结果与数值解结果吻合得较好。分析结果表明,对于土工膜/土工复合膨润土垫（GM/GCL）与土工膜/压实黏土（GM/CCL）组成的覆盖层,覆盖层顶部气体扩散达到稳态的时间比较接近,约为1.6 a;对于压实黏土（CCL）系统达到稳定状态的时间为0.5 a;稳态时通量依次为6.0×10⁵,1.0×10⁶和7.4×10⁵ mg/ha/a。在这3个覆盖层系统中,GM/CCL对气体扩散的控制性能最差。对于GM/GCL,GCL含水饱和度从0.85增加到1时,覆盖层顶部通量减小了82.5%。饱和时GM/GCL系统顶部气体达到稳态的时间是非饱和情况下的约100倍。对于GM/CCL,CCL含水饱和度从0.1增加到0.85时,覆盖层顶部通量减小了近1个数量级。含水饱和度的变化可导致覆盖层顶部挥发性气体稳态时通量发生数量级的变化。     

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.     

Geotechnical design considerations for geomembrane lined slopes: Slope stability

The design of geomembrane lined containment systems is a multi-faceted problem requiring the combined talents of many diverse disciplines. One of these disciplines is that of geotechnical engineering, within which falls the topic of soil slope stability. Since the integrity of the liner is dependent on this support system its proper design is of obvious importance. This paper addresses various approaches of soil slope stability and also includes the stability of cover soils which are often placed above the geomembrane for its protection. Related topics of anchor trench design at the top of the slope and of post-construction stability problems will be treated in subsequent papers.     

Change pattern of geomembrane surface roughness for geotextile/textured geomembrane interfaces

Applying textured geomembrane improves the frictional performance of geotextile and geomembrane interfaces. However, very limited research has been conducted to analyze the variation in textured geomembrane roughness during geotextile/geomembrane interface shear processes. In this study, a geomembrane surface roughness measurement method for measuring asperity height data with fixed intervals was presented. Normalized profile length and fractal dimension were used to quantitatively describe the geomembrane surface deformation during the geotextile/textured geomembrane interface shear process. It was found that applying normal stress led to a reduction of the roughness parameters. After the mobilization of the peak shear stress during the shear process, the chosen roughness parameters decreased with the shear displacement. And, increasing the normal stress made the shear-induced reduction of roughness parameters more obvious. The hyperbolic model can be used to describe the quantitative relationship between the geomembrane roughness parameters and the shear displacement. This study can help explain the displacement-softening post-peak behavior of the geotextile/textured geomembrane interfaces.     

Relationship between the shrinkage crack characteristics and the water content gradient of compacted clay liner in a landfill final cover

Shrinkage cracking is the primary reason for the anti-seepage failure of compacted clay liner (CCL) in a landfill final cover. With a focus on the surface crack characteristics and the water content distribution of three CCLs with different liquid limits and their mineral compositions, experiments were conducted to investigate the cracking mechanism of a CCL during the drying process. The results showed that the total crack ratio (TCR), the sum of the surface shrinkage crack ratio (SCR) and the surface boundary shrinkage ratio (BSR), is a function of the surface water content of a CCL. The change in the TCR with surface water content is consistent with the soil shrinkage characteristic curve (SSCC). The surface SCR is a function of the surface water content gradient of a CCL. The variations in the SCR with the water content gradient can be divided into the following three stages: the crack open stage, the crack linear expansion stage and the crack linear close stage. The effect of sample size, surface boundary shrinkage and shrinkage cracking are the main deformations of CCL specimens with low and high liquid limits, respectively, during the drying process. An increase in the amount of clay minerals in CCLs enhances the soil shrinkage capacity, leading to an increase in the SCR under the same water content gradient. A unified linear relationship exists between $\mathit{SCR}/K_{\text{j}}$ (where K_j is the slope of the SSCC) and the water content gradient in the crack linear expansion stage and the crack linear close stage for different CCL types.     

垃圾填埋场覆盖层现场试验及特性评价(英文)

填埋场覆盖系统由不同的土层组成,它们具有不同的性质和功能。覆盖层直接与大气和植被接触,因此土的含水率随季节和天气条件不断变化。冬天,土的饱和度增加。夏天,土的含水率减少,同时土中吸力增加。如果土中吸力达到某个极限值,将出现干缩裂缝。这时,粘土阻隔层的密封功能将受到损害。为了研究粘土覆盖系统的长期性状,我们进行了两个大规模现场试验。本文介绍第一批现场试验结果,包括水流量、土中吸力和温度的变化规律。重点介绍保护层(营养层)厚度对覆盖系统水平衡的影响。     

Performance-based indicators for controlling geosynthetic clay liners in landfill applications

The results of a project aimed at identifying performance-based indicators that can be used by landfill operators to check the suitability of GCLs for bottom barrier applications are presented. The general methodology consisted of performing detailed characterization of the prevalent GCLs used in France for landfill barrier applications, before and after prolonged contact with several fluids during oedo-permeameter tests. Results of mineralogical analysis illustrate the variety of composition of the tested bentonites, which in addition to smectite clay contain a large number of accessory minerals. For one of the GCLs tested, the proportion of smectite was lower than 30 wt%, which highlights the limitations of the generic designation “bentonite” when referring to GCLs destined to landfill applications. Results also underline the correlation between cation exchange capacity (CEC) and smectite content, the correlation between free swell volume and proportion of exchangeable sodium and the influence of the bentonite's calcium carbonate fraction on hydraulic conductivity. Transmission electron microscopy (TEM) photographs illustrate the effect of cation exchange on clay microstructure, with the formation of clay particles which lead to increased hydraulic conductivity. The exchange is also documented by exchangeable cation analyses. Results of isotopic analyses indicate that information provided by suppliers with respect to the “natural” versus “activated” nature of the bentonite, may sometimes be arbitrary and related to factors that are very difficult to check in practice, even by the suppliers themselves. This further underlines the need for performance-based indicators, rather than generic designations, to provide objective information regarding GCL suitability for landfill applications. Several performance-based indicators are selected in order to provide practical tools for checking the suitability of sodium-bentonite GCLs in bottom barrier applications and limit values are proposed.     

Design concerns and performance of geomembrane anchor trenches

The main purpose of an anchor trench is to secure geosynthetics at the top of slopes. As such, the tendency for most designers is to firmly secure the geosynthetics so that movement or pullout does not occur. However, the basic design consideration should be extended to allow pullout of the geosynthetics from the anchor trench rather than have the geomembrane tear if excess stresses are developed. The emphasis should be placed not on the construction condition, where potential problems may be easily remedied, but on the long-term performance where liner problems may have a greater impact. A realistic design procedure is therefore needed to determine the resistance provided by the anchor trench to check the potential stresses that can be resisted while confirming that the allowable tensile strengths of the geosynthetics are not exceeded.

Following a review of anchor trench designs and methodologies, case studies will be presented to show anchor trenches that have been demonstrated to work. Perhaps more importantly, cases where problems have occurred as a result of poorly designed or constructed anchor trenches will also he presented.     

A new constitutive model for textured geomembrane/geotextile interfaces

The paper presents a study of the frictional behaviour of geosynthetics used for municipal solid waste landfills. Direct shear tests of several geomembrane/geotextile interfaces were performed to investigate the shear behaviour. Furthermore, analytical and numerical models were developed to describe the observed behaviour, especially to simulate progressive geomembrane/geotextile interface failure and the factors controlling its significance.     

红黏土覆盖层水力参数演变规律与防渗设计讨论

以红黏土为对象,通过室内单元体并结合填埋场现场原位试验,从建设施工和建成后长期服役两个不同时间尺度对土质覆盖层水力参数开展了5a的跟踪监测;分析对比了从实验室到现场、从建设施工到建成长期服役水力参数的劣化衰减规律.结果 表明:①5a长期服役中,无植被红黏土覆盖层入渗系数从10-7 cm/s增大到10-3 cm/s,增大...     

有明粘土作为垃圾填埋场隔离层的环境岩土工程评价(英文)

通过室内实验并应用现有的计算软件,反算出在文中所述的试验条件下,两种重金属离子(Cd2+,Pb2+)在有明粘土中的有效扩散系数和分配系数。在此基础之上,对假定的垃圾填埋场于其下卧含水层的水质影响进行了计算分析和评估。     

围堰防渗墙与复合土工膜联接型式离心模型试验研究

防渗墙上接复合土工膜是围堰常用的防渗体系,联接部位的有效性是决定防渗成败的关键.以某围堰工程为原型开展离心模型试验,通过监测复合土工膜在堰体与防渗墙变形过程中的应变,揭示复合土工膜的受力性状和联接部位的破坏机理.结果表明一定上覆荷载作用下,复合土工膜与堰体之间的摩阻力超过其所承受的拉伸力,导致预留伸缩节不能发挥作用;复合土工膜下部风化砂的固结变形使之与刚性防渗墙之间产生较大的差异沉降,以及防渗墙与上游堰体的水平脱离,此两种变形不协调均可导致与防渗墙联接的复合土工膜因受拉而破坏.基于此,提出了防渗墙与复合土工膜联接型式的改进方法,使联接部位的复合土工膜不承受拉应力,通过离心模型试验验证其合理性和可行性,为指导围堰防渗体系的设计和施工提供了可靠的试验依据.     

Coupling effect of landfill leachate and temperature on the microstructure of stabilized clay

This paper studies the microstructure of stabilized clay polluted by landfill leachate at different temperatures. For this purpose, dynamic corrosion-stabilized clay was used to prepare mercury intrusion porosimetry and scanning electron microscopy samples by lyophilization. The results showed that a rise in temperature affects the pore structure of corrosion-stabilized clay. Macropores are easily produced when the temperature ranges from 40 to 60 °C, while cryptopores and ultramicropores appear in significant numbers if the temperature reaches 80 °C. The corresponding micrographs show a dispersed structure at temperatures of 40 to 60 °C and a clearly flower-like structure at 80 °C. Landfill leachate has obvious effects on the microstructure of stabilized clay. After corrosion processes, pore size is reduced while average pore radius is increased. Macropores increase and span a wider range. The peak of the pore size distribution curve shifts from the middle to both ends; porosity initially decreases and then increases. From the chemical point of view, this corrosion mechanism is mainly due to the growth of new material such as calcium chloro-aluminates, ettringite or dihydrate gypsum that were generated by the reaction between landfill leachate and stabilized clay.

     

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.     

聚氨酯防水涂料/改性蒙脱土纳米复合材料初探

对双组分聚氨酯防水涂料／改性蒙脱土纳米复合材料进行了研究，结果显示。该复合材料比聚氨酯防水涂料具有更好的力学性能和耐水性。     

Numerical analysis of moisture and gas transport in earthen final covers considering effects of vapor and temperature gradient

Biochemical decomposition of high kitchen wastes in landfills in China results in (i) a high content of vapor in landfill gas (LFG) and (ii) a noticeable temperature gradient across the earthen final cover (EFC). Yet, it is unclear how vapor and temperature gradients affect moisture and gas transport through EFC and microbial aerobic methane oxidation (MAMO). A theoretical model of moisture-gas-heat reactive transport considering MAMO and effects of vapor flow and temperature gradients was developed, and a series of parametric studies were included. The model was verified by published test results. The parametric simulation results show that vapor from landfill wastes can replenish water to EFCs in dry seasons, which results in (i) the relief of water shortage of vegetation and MAMO and (ii) LFG emission reduction. Effects of vapor inflow become more significant under larger temperature gradient. In cold seasons, temperature gradients can promote MAMO to reduce methane emissions, and enhance evaporation to reduce percolation. When evaporation rate at the surface exceeds 30 times vapor influx, vapor inflow has negligible effects on moisture and gas transport in EFCs. Neglecting effects of vapor and temperature gradient can lead to misjudgments on anti-seepage performance and methane emission reduction performance of EFCs.     

饱和黏性土中散体桩复合地基极限承载力系数研究

埋置深度对复合地基承载力的影响规律一直缺乏深入研究,工程中采取最低程度考虑基础埋深的保守方法导致复合地基承载力被不同程度的低估。采用有限差分法建立饱和黏土中散体桩复合地基数值模型,对刚性基础下散体桩变形模式及机理进行分析,在此基础上,进行考虑桩、土为无重介质的散体桩复合地基极限承载力系数研究。结果表明散体桩复合地基破坏模式受置换率、桩长和埋深耦合作用,可能发生浅基础型、复合型或实体基础类型的破坏模式。深度修正系数随着桩长的增加而增大,随埋深的增加而减小;当桩长和埋深一定时,存在一个最优置换率使深度修正系数取得极大值。而现行规范中深度修正系数取1.0,低估了基础埋深的作用。最后基于M o h r–C o u l o m b破坏准则推导群桩复合地基的等效强度,并利用极限分析法推导了浅基础破坏模式下复合地基的极限承载力系数解答,结果与有限差分法的结果吻合较好。     

垃圾填埋场防渗层的设计与施工

介绍了常见的用于垃圾填埋场的防渗材料，以工程实例探讨了垃圾填埋场防渗层的结构设计及其优化方案，并对HDPE膜的焊接和粘土层的施工作了介绍。     

软黏土中劲性复合桩水平承载特性p-y曲线研究

劲性复合桩(SC桩)是一种将高强度混凝土桩与水泥土桩相结合的新型桩基。为研究软黏土中SC桩水平承载力理论计算方法,将水泥土视为硬黏土,基于现有软黏土和硬黏土中桩基的p-y曲线形式,考虑水平荷载作用下桩周水泥土和软黏土的土抗力分担比例,推导了p-y曲线中两个重要参数p_u和y₅₀的修正因子,进而建立了软黏土中SC桩水平承载特性p-y曲线计算方法。通过与3个现场试验的实测结果的对比分析,验证所建立的p-y曲线法的准确性与可靠性,继而开展SC桩水平受荷性能影响因素分析。结果表明:所建立的理论计算方法可以有效预测SC桩的水平承载特性,且当桩身变形较大时应考虑混凝土芯桩的非线性影响。水泥土桩桩径(D)对SC桩水平承载性能影响显著,当水泥土桩与混凝土芯桩的桩径比(D/d)从1.0增至3.0时,120 kN水平荷载下的桩头位移从25.8 mm减至5.1 mm,且桩身最大弯矩值减小51.0%;桩身水平承载性能受水泥土桩桩长(L)的影响较大,但当长径比(L/d)超过10后,桩身内力位移趋于稳定值;适当地增加水泥土桩强度与混凝土芯桩弹性模量也可提高SC桩的水平承载性能。