The 14-year Performance of a Compacted Clay Liner used as Part of a Composite Liner System for a Leachate Lagoon

The migration of contaminants through a 2.9 m thick compacted clay liner (CCL) for a landfill leachate lagoon is examined 14 years after construction. The clay liner formed the lower portion of the composite liner system but the geomembrane (GM) was found to have defects that had allowed leachate to migrate between the GM and CCL. Chloride, sodium, potassium, calcium and magnesium pore water profiles through the CCL are examined. It is shown that chloride migrated approximately 1.7 m into the CCL during the 14 years of the lagoon operation, sodium approximately 1.2 m, and potassium 0.7 m. Diffusion and sorption data from laboratory diffusion testing are utilized in combination with a finite layer contaminant transport model to predict field contaminant migration profiles through the composite liner system and to establish the time of ‘failure’ of the geomembrane at sometime between 0 and 6 years after installation. Relatively high sorptive uptake of potassium by the CCL soil is observed from batch testing and diffusion testing with field data suggesting an even larger amount of sorption. It is hypothesized that organic sludge matter at the base of the lagoon is responsible for potassium uptake from the leachate. This field case highlights the importance of the compacted clay liner as part of the composite liner system in acting as a diffusion barrier during the lifetime of the lagoon as well as using relatively non-conservative contaminants such as chloride and sodium to estimate geomembrane ‘failure’ times     

模拟渗滤液作用下压实黏土的力学性质

垃圾填埋场中渗滤液的作用可能对压实黏土衬垫及地基土的力学性质产生一定的影响,但目前研究成果存在矛盾之处。以CaCl2溶液和苯酚溶液作为渗滤液中无机和有机污染物的代表,对添加10%钠基膨润土的压实黏土进行力学性质的试验研究。结果表明,（1）两种模拟渗滤液的作用使土的压缩性增强,抗剪强度降低。（2）苯酚对压实黏土力学性质的影响更为显著,不排水强度约为自来水和0.05molL-1CaCl2溶液的55%。（3）土样初始孔隙比相同时,溶液作用下扩散双电层变薄造成孔隙变大,可能是产生这种现象的主要原因。（4）模拟渗滤液的作用对压实黏土衬垫及地基土具有不利的影响。     

Shear Strength Behavior of Two Landfill Clay Liners

Direct shear tests were conducted to obtain both the shear strength of compacted clay liners (CCLs) specimens and the interface shear strength between compacted clay liner and base soil. These experiments were conducted under the conditions of five different water contents. The experimental results show that shear strength of both CCLs and CCLs/base interface decreases with the increase in the water content of CCLs and base soil. In addition, the considerate concentration of NaCl in leachate has no deteriorating effect on the shear strength of liners. Triaxial shear tests were also conducted on clay liner specimens to obtain total and effective shear strength under a fast compression. The shear strength c‘=100 kPa for sand-bentonite, respectively. These results indicate that the compacted clay-bentonite shows normal consolidation, but that the compacted sand-bentonite exhibits over-consolidation.     

Parametric sensitivity analysis of coupled mechanical consolidation and contaminant transport through clay barriers

In this paper, an extensive parametric sensitivity analysis of coupled consolidation and solute transport in composite landfill liner systems has been undertaken. The analysis incorporates results of more than 3000 simulations for various combinations of barrier thickness, waste loading rate, initial void ratio, compression index, hydraulic conductivity and dispersion coefficient. However, it is noted that to limit the extent of the study a constant coefficient of consolidation is assumed in the analysis presented here, though this assumption is easily relaxed. Results of the parametric sensitivity analysis are succinctly presented using dimensionless plots, which allow the comparison of results for a large number of parameter values, and so the clear identification of the most important determinants on contaminant transport through the liner system. The dimensionless plots demonstrate a pessimum (for which the ‘breakthrough time’ is minimised). Numerical results reveal that in cases of extreme liner compressibility an order of magnitude reduction in contaminant transit time may arise due to coupling between solute transport and consolidation, while for barriers of low compressibility and porosity (such as well-engineered composite compacted clay landfill liners), it is found that the contaminant transit time may still be reduced by more than 30%. The numerical results suggest that the use of coupled consolidation–contaminant transport models are sometimes required for informed and conservative landfill liner design.     

An analytical solution for one-dimensional contaminant diffusion through multi-layered system and its applications

An analytical solution for one-dimensional contaminant diffusion through multi-layered media is derived regarding the change of the concentration of contaminants at the top boundary with time. The model accounts for the arbitrary initial conditions and the conditions of zero concentration and zero mass flux on the bottom boundary. The average degree of diffusion of the layered system is introduced on the basis of the solution. The results obtained by the presented analytical solutions agree well with those obtained by the numerical methods presented in the literature papers. The application of the analytical solution to the problem of landfill liner design is illustrated by considering a composite liner consisting of geomembrane and compacted clay liner. The results show that the 100-year mass flux of benzene at the bottom of the composite liner is 45 times higher than that of acetone for the same composite liner. The half-life of the contaminant has a great influence on the solute flux of benzene diffused into the underlying aquifer. Results also indicates that an additional 2.9–5.0 m of the conventional (untreated) compacted clay liner under the geomembrane is required to achieve the same level of protection as provided by 0.60 m of the Hexadecyltrimethylammonium (HDTMA)-treated compacted clay liners in conjunction with the geomembrane. Applications of the solution are also presented in the context of a contaminated two-layered media to demonstrate that different boundary and initial conditions can greatly affect the decontamination rate of the problem. The method is relatively simple to apply and can be used for performing equivalency analysis of landfill liners, preliminary design of groundwater remediation system, evaluating experimental results, and verifying more complex numerical models.     

基于同伦分析法的非饱和土层固结问题的级数解

由于非饱和土的渗透系数是基质吸力的函数,使得控制方程带有强非线性的特征,进而使得控制方程的解析求解变得十分困难。同伦分析法对级数基函数和辅助线性算子的选择具有更大的自由性、灵活性,且收敛性的控制和调节更加容易实现,求解强非线性微分方程时在选择线性算子以及辅助参数上具有明显的优势。因此,针对非饱和土固结方程的非线性特征,对于处于地表浅层的非饱和土层,假设孔隙气压力为大气压力,在Richard经验公式与非饱和土一维固结理论的基础上,推导了非饱和一维固结无量纲控制方程;应用同伦分析法,通过选取适当的初始猜测解与辅助参数,将该非线性方程转换为线性的微分方程组并求解得到固结问题的级数解。此外,以压实高岭土为研究对象,在收集相关试验参数基础之上,将由同伦分析法求得的固结问题的近似解析解与有限差分法数值结果相对比,分析结果验证了解析解的正确性。     

考虑Hansbo渗流的二维Biot固结有限元分析

为进一步深入研究弹性饱和黏性土地基的二维固结机制,引入Hansbo渗流方程描述固结过程中的非达西渗流,修正Biot二维固结方程。基于加权残数法,给出相应的有限元数值求解格式。通过和饱和黏性土一维非达西渗流固结理论有限体积法数值结果的对比,证明数值计算方法的有效性。在此基础上,探讨Hansbo渗流参数对二维地基固结进程的影响。计算结果表明,在固结初期,Hansbo渗流将增强Mandel-Cryer效应,增大孔压的峰值,并延长孔隙水压力达到峰值的时间;在固结中后期,整个土层存在孔隙水压力滞后现象。同时,Hansbo渗流将阻碍地基沉降的发展。而且,上述影响会随着Hansbo渗流参数的增大而更加明显。     

非等温分布条件下考虑半透水边界时饱和黏土的一维固结解析解

温度的变化会导致土体的物理力学性质改变,且在一些实际工程中,饱和黏土会处于非等温分布状态。为此,针对非等温分布条件下饱和黏土的一维固结问题,考虑了更具普遍性的半透水边界,通过某些假定推导了单级线性加荷形式下饱和黏土一维固结控制方程,并利用分离变量法求解得到了控制方程的解析解。通过将所提解析解分别与已有解析解和有限差分解展开对比分析,验证了所提解答的正确性。基于所提解析解,利用某一算例分析了温度梯度、半透水边界参数及加荷时间对固结性状的影响。结果表明：温度梯度 M 越大,土体的渗透性越大,土体的固结速率越快;半透水边界参数 R1和 R2越大,相同时间内土体的超孔隙水压力越小,土体的平均固结度越大;土体的平均固结度随加荷时间 tc 的增大而减小,这主要是由于加荷阶段所施加的外荷载小于最终荷载,但加荷时间tc的延长可一定程度减小土体中产生的最大超孔隙水压力。     

Diffusion of municipal waste contaminants in compacted lateritic soil treated with bagasse ash

Compacted clay can minimize infiltration of liquid into waste or control the release of contaminated liquids to the surrounding soils and groundwater. Compacted lateritic soil treated with up to 12 % bagasse ash and municipal solid waste (MSW) leachate sourced from a domestic waste land fill were used in diffusion test studies to access the diffusion characteristics of some inorganic species present in the municipal solid waste leachate. Diffusion set-up were prepared containing 0, 4, 8 and 12 % bagasse ash—soil mixes compacted at 2 % wet of optimum using the modified proctor effort. The set up was saturated with water for 30 days before the introduction of MSW leachate and initiation of diffusion test for another 90 days. After diffusion testing, water content within the soil column showed a decrease with depth. Diffusion test results generally showed that diffusion is an active means of transport of chemical species even at very low flow rates in the compacted soil-bagasse ash mixes, and the effective diffusion coefficient is affected by bagasse ash. The pore fluid concentration profile for the various chemical species tested showed that the compacted soil-bagasse ash mix has the capacity to attenuate Ca²⁺, Pb²⁺ and Cr³⁺ ions.     

Landfill Base Liners: Assessment of Material Equivalency and Impact to Groundwater

This paper gathered available flow and transport solutions and used them for two composite liners, consisting of geomembrane (GM) overlying either a compacted clay liner (CCL) or a geosynthetic clay liner (GCL). Its aim is to provide a guiding framework for the possible choices of (a) approaches to bottom liner design, (b) respective analytical solutions to flow and transport equations, as well as (c) parameters required for each type of solution. On the basis of the obtained results, the following recommendations are made. When the goal of analysis is to determine material equivalency, leachate flow rate is an adequate key parameter for GM-CCL composite liners. For GM-GCL composite liners, it is necessary to compute contaminant concentration or mass flux, considering (a) transport through defects for inorganic contaminants and (b) diffusion and the contribution of any available attenuation layer for organic contaminants. When the goal of analysis is to assess impact to groundwater, it is advised to calculate both discharge rate and contaminant mass flux regardless of liner type. The critical parameter for the transport calculations is the retardation factor of the contaminant, for the case of CCLs, while the results for GCLs are much less sensitive to this parameter.     

Relative performance of lime stabilized amended clay liners in different pore fluids

Successful design of soil liner and covers for landfills and waste impoundments involves selection of liner material and assessment of chemical compatibility of the liner material. In this work locally available materials with different mineralogy have been evaluated from the viewpoint of liner application. Many local soils may not meet the requirements of liner material; often it is necessary to amend these soils with commercially available processed bentonite. Clay liners may be attacked by chemical waste or leachate, concentrated organic chemicals can attack compacted clay effectively destroying the characteristics of liner material. Thus the main aim of this work has been to assess the suitability of different types of locally available materials for their potential use as liners for waste containment facilities. The materials studied are kaolinitic red earth, illite and fly ash. Based on literature survey and experimental work in this laboratory, 20% by weight of bentonite has been chosen for amending selected materials. Addition of 20% bentonite to selected liner material improved the hydraulic conductivity and adsorption capacity of the amended mixture but, reduced the volume stability. To improve the volume stability of the amended material and to have better resistance towards chemicals attack, stabilizing the amended mixture with 1% by weight of lime has been considered. The relative advantages and disadvantages of four materials namely; red earth with 20% bentonite, illite with 20% bentonite, fly ash with 20% bentonite and illite alone, stabilized with 1% by weight of lime were brought out. The chemical compatibility of the materials to electrolyte solution (0.5 N NaCl), alkaline solution (0.5 N NaOH), acid (0.5 N HCl) and organic fluid (CCl₄) has been studied. The relative efficiency of the selected materials with selected pore fluids was compared.     

软黏土层一维有限应变固结的超静孔压消散研究

根据土力学固结理论计算分析软黏土层固结过程的超静孔隙水压力值,确定软黏土体固结过程的强度增长,对排水固结法处理软土地基至关重要。软黏土层固结过程中土体变形较大时,有限应变固结理论和小应变固结理论计算分析软黏土固结所得结果差异较大。利用非线性有限元法及程序,通过对软黏土层固结工程算例的计算结果分析,研究了有限应变固结理论和小应变固结理论计算分析软黏土层一维固结超静孔压值消散的差异;探讨了软黏土体一维固结过程中,几何非线性、土体渗透性变化和压缩性变化对超静孔隙水压力消散的影响。研究结果表明,当土体的变形较大时,有限应变固结理论计算出的超静孔压要比小应变固结理论得到的值消散的更快。考虑土体固结过程中渗透性的变化时,超静孔压消散变慢;可用软黏土渗透性变化指数ck 反映渗透性变化对超静孔压消散的影响,渗透性变化指数ck值越小、超静孔压消散越慢。固结过程中软黏土压缩性的大小及变化也影响超静孔压的消散,可用软黏土的压缩指数cc反映固结过程中压缩性的大小及变化对超静孔压消散的影响,软黏土的压缩指数cc越小,固结过程软黏土层中的超静孔压消散越快。     

Leachate studies on fly ash-stabilised expansive clay liners

This paper presents leachate studies on fly ash-stabilised expansive clay liners. Fly ash in different contents (0%, 10%, 20% and 30%) by dry weight of the expansive clay was added to the clay, and the ash-clay blend was compacted as a liner overlying a compacted lateritic clay layer. Deionised water (DIW) and calcium chloride (CaCl₂) solutions of varying concentration (5 mM, 10 mM, 20 mM, 50 mM, 100 mM and 500 mM) were used as the permeating fluids in the leachate studies. Chemical analysis of the leachate was performed. For a given CaCl₂ concentration, the concentrations of both calcium ion and chloride ion in the leachate decreased up to a fly ash content of 20%, and thereafter they increased when the fly ash content was increased to 30%. Further, for a given fly ash content, concentrations of calcium ion and chloride ion increased with increasing CaCl₂ concentration.     

考虑非达西流的弱透水层固结计算

由于成岩作用的影响,深部弱透水层中的黏性土较为坚硬、致密,在这种黏土层中的孔隙渗流规律有偏离达西定律的现象,而在现有的地面沉降研究中,对深部弱透水层的固结变形计算始终沿用以达西定律为基础的固结方程进行,这显然是与实际情况不相符的。鉴于此,以孔隙渗流为非达西流为基础,对太沙基一维固结方程进行了重新推导,并采用半解析法对非达西流-太沙基一维固结方程进行了求解。分别采用达西流-太沙基固结方程和非达西流-太沙基固结方程对同一固结问题进行了计算,结果表明,考虑非达西流后土层达到某一固结度所需的固结时间较之不考虑非达西流时明显增加了,而且固结时间增长的趋势与实测的增长趋势较为一致。     

非饱和土拟二维平面应变固结问题的解析计算方法

基于Fredlund非饱和土一维固结理论,建立了二维平面应变条件下的固结方程组,并得到了单层非饱和土平面应变条件下的解析解。基于相关理论,假设体变系数和渗透系数都为常量,同时考虑到瞬时加载条件下,沿着土体深度方向上产生均匀或者线性分布的初始超孔隙压力,建立了二阶二元偏微分方程组。求解时,引入函数方法来降低方程的阶数,然后通过分离变量法获得方程的通解。在此基础上,结合一个针对单面排水条件下二维平面应变问题案例,通过与数值解对比,验证了所提方法的正确性。并采用所提方法计算获得了二维平面下超孔隙水压力、气压力沿垂直和水平方向消散的等时线,通过计算对比,分析了不同线性分布情况下,初始超孔隙压力对固结消散过程的影响。研究结果表明：初始超孔隙压力的不同分布对超孔隙气压力消散的影响几乎可以忽略,而对超孔隙水压力消散的影响更大。     

双层黏弹性地基一维固结分析

软土具有黏滞性,对其固结和变形会产生一定程度的影响。采用现有基于广义Voigt流变模型的单层黏弹性地基一维固结问题求解方法,获得各土层的孔压通解表达式。根据两层土体接触面处孔压和流量连续条件及边界条件,给出了系统的正交关系,进而确定通解中待定系数。广义Voigt模型反映了土体应力应变关系在不同时期的特征,因此该解有广泛的适用性。采用岩土工程中应用较广的Merchant流变模型对一工程算例进行了分析。分析结果表明,土体的黏滞性降低了土体的固结速度,且深度越深,影响幅度越大。     

饱和软粘土在冲击荷载下的动力特性研究

通过对淤泥质饱和软粘土施加冲击荷载进行室内动力固结试验,研究了软粘土在冲击荷载作用下的应力、孔隙水压力、轴向变形等动态响应特征,并通过三轴剪切试验结果,分析冲击荷载对淤泥质饱和软粘土的加固效应。试验结论对于动力固结理论的完善和现场施工具有指导意义。     

Large deformation elastic electro-osmosis consolidation of clays

This paper presents the theoretical background of an elastic electro-osmosis consolidation model for saturated soils experiencing large strains, which considers volumetric strains induced by changes in both the hydraulic and electric driven pore water flows. Three fully coupled governing equations, considering the soil mechanical behaviour, pore water transport and electrical field, and their numerical implementation within an updated Lagrangian finite element formulation, are presented. The proposed model is first verified against a classical one-dimensional analytical solution for electro-osmosis consolidation to demonstrate its accuracy and efficiency. Then, various numerical examples are investigated to study the deformation characteristics and time dependent evolution of excess pore pressure. Finally, the importance of considering large strains in a consistent and proper way is demonstrated, and differences compared to models based on small strain theory are highlighted.     

Numerical simulation of elasto-plastic electro-osmosis consolidation at large strain

In this paper, a numerical solution for the electro-osmosis consolidation of clay in multi-dimensional domains at large strains is presented, with the coupling of the soil mechanical behaviour, pore water transport and electrical fields being considered. In particular, the Modified Cam Clay model is employed to describe the elasto-plastic behaviour of clay, and some empirical expressions are used to consider the nonlinear variation of the hydraulic and electrical conductivities of the soil mass during the consolidation processes. The implementation of the theoretical model in a finite element code allows for analysis of the evolution of the transient response of the clay subjected to electro-osmosis treatment. The proposed model is verified via comparison with data from a large strain electro-osmosis laboratory test, to demonstrate its accuracy and effectiveness. Various numerical examples are also investigated to study the deformation characteristics and time-dependent evolution of the excess pore pressure. Finally, a well-documented field application of electro-osmosis is simulated to provide further verification. The results show that the numerical solution is effective in predicting the nonlinear behaviour of clay during electro-osmosis consolidation.     

Application of an optimum design technique for determining the coefficient of consolidation by using piezocone test data

For normally consolidated clay, several researchers have developed a number of theoretical time factors to determine the coefficient of consolidation from piezocone test results. However, depending on assumptions and analytical techniques, it could vary considerably, even for a specific degree of consolidation. In this paper a method is proposed to determine a consistent coefficient of consolidation by applying the concept of an optimum design technique over all ranges of the degree of consolidation. Initial excess pore pressure distribution is assumed to be capable of being obtained by the successive spherical cavity expansion theory. The dissipation of pore pressure is simulated by means of a two-dimensional linear-uncoupled axi-symmetric consolidation analysis. The minimization of differences between measured and predicted excess pore pressure was carried out by the BFGS unconstrained optimum design algorithm with a one-dimensional golden section search technique. By analyzing numerical examples and in-situ test results, it was found that the adopted optimum design technique gives consistent and convergent results.