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。     

水泥固化体淋滤液对GCL防渗性能的影响

水泥固化/稳定化是危险废弃物处理的经济、高效方法,然而,水泥固化体的淋滤液中含有大量Ca~(2+),其长期渗透有可能导致填埋场底部土工合成黏土衬垫(geosyntheticclayliner,GCL)的防渗性能下降,从而引发二次污染。使用柔性壁渗透仪,测定有效应力和水泥固化体淋滤液共同作用下GCL的渗透系数,探讨了淋滤液浓度以及不同有效应力对GCL渗透系数的影响。试验结果表明:当有效应力为24kPa时,水泥固化体淋滤液的持续渗透会使GCL的渗透系数增大179～721倍,淋滤液中Ca~(2+)浓度越高,GCL渗透系数增大的幅度越大。通过增加有效应力,可以降低固化体淋滤液对GCL防渗性能所造成的负面影响,当有效应力增大至438 kPa时,固化体淋滤液对GCL防渗性能所造成的负面影响全部被抵消。     

利用膨润土的膨胀和稠度特性对GCL渗透系数进行预测的试验研究

以土工合成粘土衬垫(Geosynthetic Clay Liner,GCL)在尾矿库防渗层中的应用为背景,研究不同浓度重金属离子(Cu和Zn)作用下,膨润土的自由膨胀量、液限及GCL渗透系数的变化规律,并分析它们之间的对应关系。试验结果显示,当重金属离子浓度在0.01mol/L到0.1mol/L之间递增时,膨润土的自由膨胀量和液限会随着重金属离子浓度的增大而大幅度减小,但当重金属离子浓度从0.1mol/L增加到0.5mol/L时,膨润土的自由膨胀量和液限则只有微小变化。在渗透试验中,当渗透溶液中重金属离子浓度小于0.01mol/L时,GCL的渗透系数能够保持稳定;但当重金属离子浓度大于0.02mol/L后,GCL的渗透系数会随着渗透溶液中重金属离子的浓度增加而不断升高。研究结果表明,当尾矿库渗滤液中重金属离子浓度大于0.02mol/L时,GCL的渗透系数与膨润土的自由膨胀量和液限之间具有良好的数学对应关系,可以利用自由膨胀量和液限对渗透系数进行预测。     

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。     

水化状态对含针刺GCL复合衬里抗剪强度的影响分析

针刺GCL和HDPE土工膜(GM)在防渗工程中应用广泛,含多层界面的复合衬里整体抗剪强度是边坡稳定性分析的关键。介绍了含针刺GCL复合衬里的大单剪试验方法,并且对比分析了针刺GCL初始状态分别为干燥和完全水化两种情况下的复合衬里抗剪强度。结果表明,复合衬里的剪切破坏不会发生在干燥针刺GCL内部界面,而GCL干燥状态下的复合衬里单剪强度未必高于GCL完全水化状态下的复合衬里单剪强度。结合含GCL复合衬里的剪切破坏机理,阐述了针刺GCL的水化状态对复合衬里抗剪强度的影响。含GCL复合衬里在不同水化状态下的界面滑移稳定性都应引起工程人员的重视。     

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。     

水化针刺GCL剪切破坏机理的试验研究

土工膨润土防水毯(GCL)的内部剪切强度是影响复合防渗衬里边坡稳定性的关键因素,加筋纤维的存在使针刺GCL的内部剪切强度明显高于非加筋GCL。含水化GCL的复合防渗衬里结构剪切试验会发生应力小峰值现象,但大都被研究人员忽视;加筋纤维对GCL峰值剪切强度的贡献处于定性阶段。通过开展GCL内夹钠基膨润土的饱和剪切试验和理论分析,证实应力小峰值现象为水化针刺GCL应力位移曲线的固有特征,并且小峰值应力代表了GCL内夹膨润土的抗剪强度贡献。以应力位移曲线上的应力小峰值现象为基础,将加筋纤维对GCL峰值剪切强度的贡献定量化,结合破坏机理分析了针刺GCL内部剪切破坏和应力位移发展过程。考虑针刺GCL剪切强度各部分贡献,提出了一种能反映针刺GCL破坏机理的峰值强度准则。     

冻融-干湿循环作用下土工合成黏土衬垫防渗性能变化规律

土工合成黏土衬垫(Geosynthetic Clay Liners,GCL)是废弃物处置场底部衬垫系统和上部覆盖系统建设中经常使用的一种材料。由于气候的周期性变化对GCL的防渗性能会造成一定的影响,分析了冻融循环和干湿循环作用下GCL渗透系数变化的内在机制及规律。膨润土黏土粒径及双电层变化是影响GCL渗透系数改变的主要原因,冻融循环对GCL防渗性能的影响较小,而干湿循环对GCL渗透系数的影响较大。     

饱和高庙子膨润土的渗透特性

在核废料深层地质处置工程中,膨润土的渗透特性是需要考虑的重要因素之一。对浸水饱和的高庙子钙基膨润土压实试样进行压缩试验,利用时间平方根法测定压缩试验中每级荷载下的固结系数,然后用太沙基一维固结理论计算土的饱和渗透系数。试验研究表明:在弹塑性阶段,计算所得饱和渗透系数随竖向应力的增大而减小,在双对数坐标下,计算渗透系数与竖向应力呈线性关系;孔隙比对渗透性起着主导作用,在半对数坐标下,计算渗透系数随孔隙比的减小而线性减小,由该线性关系得到的某一孔隙比下的计算渗透系数与公开发表的相同孔隙比的高庙子钙基膨润土的渗透系数接近。研究结果及相关文献证明应用固结理论方法间接推算膨润土饱和渗透系数的方法是可行的,该方法适用于低渗透性黏土的饱和渗透系数的量测。     

Hydraulic conductivity of bentonite-polymer composite geosynthetic clay liners permeated with bauxite liquor

The high ionic strength of the porewater in red mud (bauxite liquor from digestion) can suppress swelling of montmorillonite, resulting in geosynthetic clay liners (GCLs) that are too permeable to be effective as liners in red mud disposal facilities. Bentonite-polymer composite GCLs (BPC GCLs) have been developed as more resilient lining materials, and some BPC GCLs have been shown to have very low hydraulic conductivity to bauxite liquors that have extreme ionic strength and pH. In this study, a nationwide investigation was conducted in China to evaluate the characteristics of bauxite liquor in Chinese impoundments, and to evaluate the suitability of GCLs containing granular sodium bentonite or BPCs for containment. Hydraulic conductivity tests were conducted on six BPC GCLs with two characteristic Chinese bauxite liquors that are hyperalkaline (pH > 12) and had ionic strengths of 76.9 mM and 620.3 mM. The BPC GCLs had hydraulic conductivity ranging from 10^?8-10^?12 m/s, which is higher than the hydraulic conductivity of BPC GCLs to deionized water (10^?12-10^?13 m/s), but lower than the hydraulic conductivity of conventional GCLs with granular sodium bentonite GCLs to the same liquors (10^?7-10^?8 m/s). The hydraulic conductivity of the BPC GCLs depends on the chemical properties of the leachate, the polymer loading, and the type of polymer. Microstructural analysis by scanning electron microscopy (SEM) suggests that the hydraulic conductivity of BPC GCLs is controlled by pore-blocking by polymer hydrogel, which is affected by the bauxite liquor.     

Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkaline solution

GCLs containing powdered Na-bentonite treated with different dosages of a proprietary additive intended to reduce the impacts of chemical interactions were permeated with three solutions: a hyperalkaline solution (1 M NaOH and 1.3 mM CsCl) having similar pH to aluminum refining leachate, a 1.3 mM CsCl solution (no NaOH), and DI water. For a given permeant solution, the hydraulic conductivity of both GCLs was similar. Thus, the higher additive dosage had no measureable impact on hydraulic conductivity. Hydraulic conductivity of both GCLs decreased by a factor of approximately 1.5–1.8 during permeation with CsCl in response to osmotic swelling induced by the low ionic strength of the CsCl solution entering the pore space. In contrast, permeation with the NaOH–CsCl solution caused the hydraulic conductivity of both GCLs to increase modestly (<50 times the hydraulic conductivity to DI water), and then level out (or decrease slightly) as a result of reduced osmotic swelling in the interlayer combined with dissolution of the mineral. For the tests conducted with CsCl solution, nearly all of the Cs was adsorbed by the bentonite. In contrast, Cs broke through readily when the NaOH–CsCl solution was used as the permeant solution. Permeation with the NaOH–CsCl solution also increased the sodicity of the bentonite by replacing bound K, Ca, and Mg on the mineral surface.     

Laboratory investigation of GCL hydration from Lateritic subsoils

A laboratory investigation on the hydration behavior of GCLs from lateritic soils was conducted under isothermal and thermal conditions (tropical climate), varying subsoil moisture contents, GCLs bentonite particle size and mineralogy. GCL hydration levels from lateritic subsoils under isothermal conditions (55%) were similar to literature findings. A slight decrease in water content of some GCLs after long periods of contact with the lateritic soils indicates that equilibrium can demand long time in these soils. GCL with granular bentonites were less efficient to hydrate from lateritic subsoils. GCLs with activated-calcium bentonites maintained hydration levels in long-term. Nonwoven geotextile facing down favored capillary effects. Thermal cycles significantly influenced GCLs hydration from subsoils. Capillary connections developed during hydration under isothermal conditions due to suction gradient reductions. Post-hydration tests under isothermal conditions showed more alterations in GCLs swelling and cation exchange properties than thermal cycles test. An increase in the saturated hydraulic conductivity of GCLs was observed in both lateritic soils, mainly for isothermal condition, although continued attending hydraulic conductivity requirements for barrier applications.     

Effects of Water Content Distribution on Hydraulic Conductivity of Prehydrated GCLS against Calcium Chloride Solutions

When geosynthetic clay liners (GCLs) are applied as bottom liners at waste containment facilities, they are naturally prehydrated by absorbing moisture in the underlying base layers. In order to evaluate the effects of cations contained in waste leachates, this study investigated the effects of the water content distribution of the GCLs prehydrated with actual soils on their hydraulic conductivities against CaCl₂ solutions. The “prehydration tests”, which were conducted prior to the hydraulic conductivity tests, showed that the water content distribution of the prehydrated GCLs depends on the properties of the GCLs and the base layers. In particular, drastic differences between GCLs with powdered bentonite and GCLs with granular bentonite were observed in the prehydration water content and its distribution. Prehydrated GCLs with powdered bentonite had a higher water content and a more homogenous distribution than those with granular bentonite. The hydraulic conductivity tests showed that most of the prehydrated GCLs exhibit a low hydraulic conductivity of k?1.0×10^-8 cm/s against CaCl₂ solutions with 0.1-0.5 M. However, GCLs with granular bentonite may be difficult to homogeneously prehydrate and exhibit an unstable hydraulic conductivity, which varies from k=2.9×10^-9 cm/s to k=1.5×10^-6 cm/s. The homogeneity of the water content distribution has been considered an important factor to obtain a required barrier performance under prehydration conditions, which are naturally generated in actual sites.     

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.     

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.     

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.     

Effect of stress on water retention of needlepunched geosynthetic clay liners

Geosynthetic clay liners (GCLs) are placed at the bottom of waste disposal facilities where they hydrate from the subsoil and eventually from a hydraulic head on geomembranes (GMs) defects. Predicting hydration behavior of GCLs requires knowledge of the water-retention properties of the GCL along wetting paths. Given that GCLs could be subjected to different ranges of vertical stresses that are induced by the weight of the supported waste, the confining stress could affect water-retention properties of GCLs and should be investigated. To do so, a laboratory methodology to establish the water-retention curves (WRCs) of needlepunched GCLs under stress was undertaken. Various constant vertical stresses corresponding to different weights of the supported waste were applied to GCL specimens placed in controlled-suction oedometers. Suction values were selected so as to mimic a wetting path from the initial dry state to zero suction. Suction was controlled by using controlled suction techniques with controlled humidity imposed by a saturated saline solutions and using the osmotic technique with polyethylene glycol (PEG) solutions. Measurements were undertaken on oedometer systems as to apply confining stresses and have been complemented by standard saturated oedometer swelling tests. The data obtained confirm that increasing the stress on to the GCL results in less, albeit faster, water uptake, which could emphasize on recommendations about rapidly covering GCLs after they are placed at the bottom of a waste disposal facilities. Finally, the potential validity of the state-surface concept, which was developed in unsaturated soil mechanics, is discussed using van Guenuchten's and Fredlund and Xing's equations for water retention curves.     

Experimental study on the permeability and self-healing capacity of geosynthetic clay liners in heavy metal solutions

Geosynthetic clay liners (GCLs), which have a very low permeability to water and a considerably high self-healing capacity, are widely used in liner systems of landfills. In this study, a series of experimental tests were carried out under complex conditions on typical commercial GCLs from China. In particular, the effects of pH values and lead ions (Pb²⁺) were tested in addition to other factors. The swelling properties of natural bentonite encapsulated between geotextile components in the GCLs were tested first. The swelling capacity was reduced rapidly at pH values < 3 and concentrations of Pb²⁺ >40 mM. Permeability tests on GCLs with different concentrations of lead ions were then performed by using the self-developed multi-link flexible wall permeameter, and data showed that increases in lead ion concentrations greatly improved the permeability. Finally, self-healing capacity tests were conducted on needle-punched GCLs under different levels of damage. Results showed that the GCLs have a good self-healing capacity with small diameter damage holes (2 mm, close to three times the original aperture), but with a damage aperture larger than 15% of the sample area, the self-healing capacity could not prevent leakage; hence, in certain situations it will be necessary to repair the damage to meet the anti-seepage requirement.