膨润土防水毯渗透性能实验研究

采用特定pH值的多级浓度Pb~(2+)溶液,对膨润土进行了自由膨胀量测试,同时以Pb~(2+)溶液作为水化液,对膨润土防水毯(GCL)的渗透系数进行了测试,指出GCL铺设时应注意强酸性和高离子浓度工程环境对GCL抗渗性能的影响。     

赤泥渗滤液对改性GCL防渗性能的影响

旨在评价商用土工合成材料黏土衬垫(GCL)用于阻隔赤泥渗滤液的有效性。赤泥渗滤液作用下GCL的防渗特性是评价其防污性能的关键因素。以商用改性GCL中膨润土为研究对象,通过自由膨胀试验,研究了4种赤泥渗滤液中膨润土的自由膨胀指数。以商用改性GCL为研究对象,通过改进滤失试验,研究了4种赤泥渗滤液作为渗透液作用下GCL渗透系数的变化规律,评价了预水化作用对渗透系数的影响。研究还采用清洁自来水作为GCL的渗透液作为对照。结果表明,随着离子强度的增加,改性GCL中的膨润土自由膨胀指数随之减小。预水化处理改性GCL的渗透系数相较于未预水化处理试样降低了5倍左右。在实际工程应用中,建议采用自来水预水化处理GCL,以此充分发挥其防渗性能。随着改进滤失试验中施加气压的增大,不同赤泥渗滤液作用下的改性GCL渗透系数均下降。随着离子强度和一价二价离子摩尔数比的增加,改性GCL的渗透系数随之增大。随着膨润土自由膨胀指数的增加,改性GCL的渗透系数随之减小。与自来水渗透情况相比,赤泥渗滤液渗透作用下,改性GCL渗透系数增大4.35~12.0倍。     

GCL对铜离子吸附和隔离性能试验研究

采用蒸馏水和0.01 mol/L铜离子溶液分别水化钠基膨润土防水毯(以下简称GCL)进行渗透试验,并用0.01 mol/L铜离子溶液代替蒸馏水测试GCL渗透系数和渗滤液中铜离子浓度;分析了不同水化处理对GCL吸附铜离子和防渗性能的影响。结果表明:不同水化方式预水化的GCL对铜离子都具有优异过滤效果,两种水化方法对铜离子过滤能力都达到99%以上;相比于0.01 mol/L铜离子溶液直接水化,蒸馏水预水化后GCL吸附性能较好,但防渗性能随着时间的增加而变差。     

GCL膨润土衬垫膨胀量对渗透性能的影响

GCL膨润土衬垫有很好的吸水膨胀性能,作为防渗材料被广泛应用在环境工程和水利工程。在上覆荷载作用下,GCL膨润土衬垫的膨胀量会受到影响。采用膨胀试验和渗透试验研究上覆荷载与膨胀量之间的关系,以及膨胀量对渗透系数的影响。结果表明,随着上覆荷载的增加,GCL膨润土衬垫膨胀量明显减小,荷载超过1000 k Pa后几乎不会发生膨胀现象。GCL膨润土衬垫膨胀量大,防渗性能低,膨胀量达到12 mm时渗透系数增大1个数量级。根据拟合得到的膨胀量与上覆荷载、膨胀量与渗透系数的关系式,计算了不同堆载高度下的膨胀量和渗透系数,GCL膨润土衬垫上部铺设0.2 m的砂土,能大大提高安全性。     

盐溶液对膨润土-砂混合物膨胀性能的影响

利用自制固结仪,开展膨润土-砂混合物在蒸馏水及不同浓度的NaCl、KCl和CaCl₂溶液下的膨胀变形试验,研究盐溶液浓度和阳离子交换反应对混合物膨胀变形的影响。试验表明,试样在蒸馏水与不同浓度的NaCl溶液下可用同一条e_m-p_e曲线表示,表明NaCl主要通过渗透吸力的作用影响混合物的膨胀变形。K⁺、Ca²⁺主要通过置换吸附在钠基膨润土表面的Na⁺,使得膨胀性能减弱,因此NaCl溶液的抑制性小于KCl和CaCl₂。值得关注的是,膨润土-砂混合物在KCl与CaCl₂溶液下随浓度的变化产生了不同的膨胀变形规律。当浓度为0.1 mol/L时,由于同浓度下CaCl₂溶液中的总离子浓度大于KCl,从而通过渗透吸力的提高增大了有效应力,此时CaCl₂的抑制效果强于KCl;而浓度在0.5 mol/L时,CaCl₂的抑制效果弱于KCl,因为此时溶液中离子浓度较高,一方面K⁺可以充分置换吸附在膨润土表面的Na⁺;另一方面K⁺更容易嵌入相邻晶层间硅氧四面体中氧原子形成的孔穴里,使得相邻晶层连接起来,导致水分子不易进入层间,无法正常吸水,从而明显地抑制了膨胀变形。     

竖向应力作用下GCL的膨胀特性和渗透性能

近年来,土工织物膨润土垫(GCL)被越来越多地应用到各种防渗工程之中,它的防渗有效性也成为了设计人员和研究人员所关注的焦点。GCL的防渗有效性包括渗透性能、吸附能力和内部剪切强度三个方面。通过水化膨胀试验和渗透试验研究了GCL在竖向应力作用下的膨胀特性和渗透性能,并分析了正应力和加压水化顺序的影响。试验结果表明:(1)随着竖向应力的增大,GCL的膨胀量不断减小,而GCL的渗透系数则出现先减小后略有增大的规律;(2)水化加压顺序对GCL的膨胀量和渗透系数均有影响;(3)在实际工程应用中,GCL铺设完成后在堆载之前最好完全水化,这样能够大大提高GCL的防渗有效性。     

钠钾盐梯度循环作用下高压实膨润土膨胀力衰变特性

高放废物深地质处置库近场环境中,高压实膨润土将长期遭受含盐地下水的循环化学作用,导致其膨胀性能不断衰变。针对干密度为1.7 g/cm3的高压实高庙子(GMZ)膨润土,分别开展了0.5 mol/L和1.0 mol/L两种盐梯度、Na Cl-水-KCl和Na Cl-水不同循环路径下的恒体积膨胀力试验,探讨盐梯度循环化学作用下其膨胀力的衰变特性。结果表明:盐梯度循环作用下,膨润土膨胀力的发展与离子种类、浓度和循环次数等因素有关。盐化阶段膨胀力不断降低,淡化阶段膨胀力有所提高;低盐度梯度循环下各阶段的稳定膨胀力均高于高盐度梯度循环时的膨胀力。随着循环次数的增加,稳定膨胀力逐渐下降,入渗溶液浓度越高,降幅越大,且衰减幅度随着循环次数的增加而减小。KCl溶液的入渗会引起膨润土发生矿物相变,膨胀力显著降低;当KCl溶液浓度达到一定值时,蒙脱石的充分溶解导致膨润土丧失膨胀能力。     

Ca~(2+)浓度对膨润土掺砂混合土渗透性能的影响

垃圾填埋场底部铺设含黏土或膨润土掺砂混合土的防渗系统,防止渗滤液流入地下污染周边环境。采用模型试验研究Ca~(2+)离子浓度和基础局部沉降对膨润土掺砂混合土渗透特性的影响。试验结果表明Ca~(2+)浓度对膨润土掺砂混合土的渗透性能有较大影响,渗透液中Ca~(2+)浓度超过一定值后混合土渗漏;Ca~(2+)离子浓度与发生渗透时间之间有很大关系,浓度越高发生渗透的时间越短;同样沉降量条件下,含Ca~(2+)离子溶液渗透的混合土均发生渗漏,而用蒸馏水作渗透液的没有出现渗漏;钠基膨润土遇Ca~(2+)离子溶液后,当Ca~(2+)离子浓度大于两倍Na+就会发生离子交换,钠基膨润土转化为钙基膨润土,混合土的防渗性能下降。因此开展Ca~(2+)离子浓度对膨润土掺砂混合土渗透特性的研究具有较好的理论和实际意义。     

溶液作用下黏土的界限含水率及渗透试验

黏性土的界限含水率是用于土的分类和评价的重要指标,并可能与渗透系数等参数具有一定的相关性,但溶液作用下界限含水率的变化规律没有一致的结论。选取填埋场渗滤液中的几种代表性溶液,对0~15%含量的膨润土改性黏土的界限含水率及渗透性进行了试验。研究发现,CaCl₂溶液和乙酸溶液具有絮凝作用,使高液限黏土变为高液限粉土;膨润土改性黏土的液限和塑性指数与膨润土掺入量正相关;改性黏土的液限随CaCl₂溶液浓度的增大而减小;渗透系数与液限之间没有显著相关性,溶液作用下土的渗透性能还需要深入研究。     

重金属Cu~(2+)污染土渗透特性试验及微观结构分析

研究重金属Cu2+离子的侵入对土体渗透特性的影响。以人工配置的CuCl2溶液作为渗透液,采用三联式柔性壁渗透仪在室内开展两种工况下污染土渗透试验,分别考虑围压和渗透污染液浓度对土体渗透性的影响。试验结果表明,配制的CuCl2溶液浓度保持恒定时,渗透液无论采用纯净水还是CuCl2溶液,土体渗透系数均随着施加围压的增大而减小,但是相同围压作用下CuCl2溶液作为渗透液,测得土体的渗透系数小于纯净水作用的渗透系数,并且随着围压的增大,渗透系数的减小量呈现出逐渐缩小的趋势,直至围压超过500kPa后流体特性对土体渗透性的影响不大。施加围压相同而渗透液浓度不同的情况下,土体渗透系数随着CuCl2溶液浓度的增加先急剧减小随后则逐渐增大。微观结构分析结果表明,CuCl2溶液作为渗透液时土体渗透系数与纯净水渗透存在差异的主要原因是缘于重金属Cu2+改变了黏土的内部结构,影响了黏土的孔隙大小,从而造成了宏观渗透性的差异。     

Hydraulic performance and chemical compatibility of a powdered Na-bentonite geosynthetic clay liner permeated with mine drainage

The hydraulic and chemical compatibility of a geosynthetic clay liner (GCL), containing powdered Na-bentonite, was evaluated against artificial acid rock drainage (ARD) in terms of the swell index, hydraulic conductivity and heavy metal retention. Six artificial ARDs with an approximate pH of 3 and different metal concentrations (electrical conductivity, EC, ranging between 75 and 1000 mS/m; ionic strength ranging between 8 and 400 mM) were used in the experiments. The results of free swelling tests showed that high metal concentrations (EC higher than 70 mS/m) negatively impact the swell volume by lowering it. The hydraulic conductivity of the GCL permeated with distilled water was 1.2 × 10^?11 m/s, falling in the range of 7.9 × 10^?12 to 1.1 × 10^?10 m/s when prehydrated with distilled water and permeated with ARDs. The ion exchange and metal precipitation appeared to be the main mechanisms controlling the metal attenuation on the bentonite. The ion exchange mechanism starts with the release of Na from the bentonite and the sorption of the bi- and tri-metals present in the ARDs onto the bentonite. After the depletion of Na, the ion exchange reaction proceeds with the desorption of Ca and Mg from the bentonite and the sorption of cations present in the ARDs onto the bentonite layers. The depletion of Na from the bentonite and the subsequent release of Ca and Mg correlate to the sudden drop in pH and a gradual increase or equilibration of the hydraulic conductivity. It is possible to say that, after this point, hydraulic and chemical equilibrium is reached. From the overall results, the tested GCL showed acceptably low hydraulic conductivity and the potential to attenuate heavy metals present in ARDs.     

Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade

The effects of the silt aggregation, compaction density, and water content of the subgrade on the hydration of five different geosynthetic clay liner (GCL) products is reported based on a series of laboratory column experiments conducted over a six-year period. GCLs meeting typical specifications in terms of minimum hydraulic conductivity and swell index are hydrated to equilibrium from the same subgrade soil with sufficient cations to cause cation exchange during hydration. It is then shown that the GCL bentonite granularity and GCL structure can have a significant (~four orders of magnitude) effect on hydraulic conductivity under the same test conditions (from 8 × 10⁻¹² m/s for one GCL to 6 × 10⁻⁸ m/s for another GCL product). The effect of subgrade water content on the hydraulic performance of GCLs are not self-evident and quite dependent on the bentonite granularity, GCL structure, and permeant. Varying the subgrade water content from 5 to 16% and allowing the GCL to hydrate to equilibrium before permeation led to up to 5-fold difference in hydraulic conductivity when permeated with tap water and up to 60-fold difference when the same product is permeated with synthetic municipal solid waste leachate. When permeated with synthetic leachate, increasing stress from 70 kPa to 150 kPa led to a slight (average 37%; maximum 2.7-fold) decrease in hydraulic conductivity due to a decrease in bulk void ratio. It is shown that hydraulic conductivity is lower for GCLs with a scrim-reinforced geotextile, and/or with finer bentonite. It is shown that selecting a GCL based on the initial hydraulic conductivity and swell index in a manufacturers product sheet provides no assurance of good performance in field applications and it is recommended that designers pay more attention to selection of a GCL and preparation of the subgrade for important projects.     

Hydraulic Conductivity of Nonprehydrated Geosynthetic Clay Liners Permeated with Inorganic Solutions and Waste Leachates

To investigate systematically the effects of electrolytic solutions on the barrier performance of geosynthetic clay liners (GCLs), a long-term hydraulic conductivity test for 3 years at longest was conducted on a nonprehydrated GCL permeated with inorganic chemical solutions. The hydraulic conductivity test for waste leachates was also conducted. The results of the test show that the hydraulic conductivity of GCLs significantly correlates with the swelling capacity of bentonite contained in GCLs. GCLs have excellent barrier performance of k<1.0×10^-8 cm/s when the free swell is larger than 15 mL/2 g-solid regardless of the type and concentration of the permeant solution. In addition, when the results of the hydraulic conductivity test with chemical inorganic solutions were compared to those with waste leachates, the hydraulic conductivity of GCL permeated with chemical solution was almost the same within the electric conductivity of 0-25 S/m as that permeated with waste leachate having similar electric conductivity. The hydraulic conductivity of GCLs to be used in landfill bottom liners can be estimated by the hydraulic conductivity values obtained from the experiment using chemical solutions having the similar electric conductivity values, if the chemical solution had the electric conductivity within=25 S/m.     

Investigating factors influencing polymer elution and the mechanism controlling the chemical compatibility of GCLs containing linear polymers

A study was conducted to investigate (1) physicochemical factors that influence polymer elution from GCLs containing a blend of bentonite and linear (water-soluble) polymer (LPB GCLs) and (2) the mechanism that controls the chemical compatibility of LPB GCLs when polymer elutes. A series of hydraulic conductivity (k), free swell and viscosity tests were performed on a commercial LPB GCL using DI water, varying concentrations of NaCl and CaCl?. Comparable tests were also performed on a conventional bentonite (CB) GCL containing the same untreated bentonite and the same physical properties as the LPB GCL. The LPB GCL showed improved swelling and hydraulic performance compared to the CB GCL when permeated with salt solutions. Total organic carbon analysis of the effluents showed that polymer eluted from the LPB GCL regardless of the permeant solution. However, the rate at which polymer eluted increased as the concentration and valence of the dominant cation increased. The rate at which polymer eluted also increased with hydraulic gradient. The mass of polymer retained inside the GCL matrix did not correlate with the k of the LPB GCL. Free swell tests coupled with chemical analysis suggest that, the improved chemical compatibility of the LPB GCL was due to the ability of the polymer to scavenge cations from the solution which allows the bentonite to undergo adequate swelling during the initial hydration period. Analogous to water-prehydrated CB GCLs, the dispersed structure of the bentonite fabric and increased adsorbed water molecules attained during initial swelling controls the k of the LPB GCL when polymer elutes.     

Hydrodynamic assessment of bentonite granule size and granule swelling on hydraulic conductivity of geosynthetic clay liners

Flow in an idealized geosynthetic clay liner (GCL) containing bentonite comprised of equisized and equispaced square granules was simulated using a hydrodynamic model to quantitatively evaluate the premise that the hydraulic conductivity of GCLs diminishes as the bentonite granules hydrate and swell into adjacent intergranular pores, creating smaller and tortuous intergranular flow paths. Predictions with the model indicate that hydraulic conductivity decreases as granules swell and intergranular pores become smaller, and that greater granule swelling during hydration is required to achieve low hydraulic conductivity when the bentonite is comprised of larger granules, or the bentonite density is lower (lower bentonite mass per unit area). Predictions made with the model indicate that intergranular pores become extremely small (<1 μm) as the hydraulic conductivity approaches 10⁻¹¹ m/s. These outcomes are consistent with experimental data showing that GCLs are more permeable when hydrated and permeated with solutions that suppress swelling of the bentonite granules, and that the hydraulic conductivity of GCLs with bentonite having smaller intergranular pores (e.g., GCLs with smaller bentonite granules, more broadly graded particles, or higher bentonite density) is less sensitive to solutions that suppress swelling.     

重金属污染土物理性质强度定量关系试验研究

通过室内试验研究土体孔隙液中重金属离子影响土体物理力学性状的机理,采用不同浓度的重金属Cu2+、Zn2+、Pb2+掺入高岭土、伊利石类黏土和钠基膨润土,研究土体界限含液率、不排水抗剪强度随重金属离子浓度的变化规律。试验结果表明:以高岭石、伊利石为主要黏土矿物的低活性土的液限、塑性指数随离子浓度的增加而增大,钠基膨润土则表现出相反的变化规律;低活性黏土的不排水抗剪强度随离子浓度增加而增大,高活性黏土强度则随之减小;基于已有的无污染土体物理力学性状定量联系,分析重金属污染土的不排水抗剪强度与液性指数的定量关系,发现重金属离子引起的不排水抗剪强度的变化可以归结于重金属离子引起的液塑限变化,表明重金属Cu2+、Zn2+、Pb2+污染过程基本没有产生化学反应,其物理力学定量关系与已有的无污染土经验关系式一致。     

Numerical investigation on hydraulic and gas flow response of MSW landfill cover system comprising a geosynthetic clay liner under arid climatic conditions

Municipal solid waste (MSW) landfill cover systems are designed to minimize the infiltration of rainwater into waste and to mitigate the biogas emissions to the atmosphere. In the present study, the efficacy of such a landfill cover system consisting of a Geosynthetic clay liner (GCL)in mitigating the hydraulic flow and gas emissions under arid climatic conditions was investigated critically. For this purpose, the water retention curve (WRC), hydraulic conductivity function, and gas flow characteristics of the chosen GCL were studied through experimental methods in the laboratory. The unsaturated transient state seepage analysis utilizing coupled hydraulic and gas flow mechanisms was performed in the study to assess the performance of the cover system. The results obtained from the experiments were used as input parameters. The effect of drying/desiccation and the self-healing nature of GCL due to climatic changes were also analyzed by exposing the GCL directly to the climatic boundary for one year. It was observed that the GCL present in the chosen cover system effectively functions as a hydraulic barrier in arid climatic conditions. However, during the summer and winter seasons, an increase in gas flow from 0.02 g/h/m² to 24.7 g/h/m² was observed, probably due to the drying anddesiccation of GCL. Interestingly, due to the self-healing nature of the GCL, gas flow through the cover system was substantially reduced to 0.02 g/h/m² during the rainy season. The effect of drying was more pronounced when the GCL was exposed to the climatic condition, leading to an early gas breakthrough and an increase in gas flow from 0.02 g/h/m² to 957 g/h/m². The percolation through the cover system remains considerably low throughout the year, mostly due to the unsaturation and low hydraulic conductivity in GCL. A cumulative percolation close to 0.1 m³ was observed at the end of one year in arid climatic conditions.