sustainability

Review State-Of-The-Art Review of Geosynthetic Liners

De-Jun Kong 1,2, Huai-Na Wu 1,2,*, Jin-Chun Chai 3 and Arul Arulrajah 4 1 State Key Laboratory of Ocean Engineering and Department of Civil Engineering, School of Naval Architecture, Ocean, and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] 2 Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE), Shanghai Jiao Tong University, Shanghai 200240, China 3 Department of Civil Engineering, School of Naval Architecture, Ocean, and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] 4 Department of Civil and Construction Engineering, Swinburne University of Technology, Hawthorn, VIC 3122, Australia; [email protected] * Correspondence: [email protected]; Tel.: +86-21-3420-4301; Fax: +86-21-6419-1030

Received: 13 October 2017; Accepted: 11 November 2017; Published: 16 November 2017

Abstract: An important component of modern landfills is the liner system for the prevention of contamination of surrounding ground. Among landfill liner systems, geosynthetic clay liner (GCL) has gained widespread popularity across the world because of its lower as as its ability to self-heal local damage, which is almost unavoidable in the field. Over the past few decades, numerous studies have been conducted to examine the performance of GCLs, particularly in regard to hydraulic conductivity, chemical compatibility, water-swelling, self-healing capacity, diffusion characteristics, gas migration, and mechanical behavior. In this paper, a brief introduction on modern GCL products is firstly given. Subsequently, the main findings of previous publications on the critical properties influencing the long-term performance of GCLs are summarized in a comprehensive manner. Finally, further research perspectives on polymer-treated GCLs are presented. This paper provides general insights that help readers gain a state-of-the-art overview of GCLs and trends for future development.

Keywords: geosynthetic clay liner; engineering characteristics; ; polymer-treated GCL

1. Introduction Solid waste, which includes waste generated from mining, industrial and agricultural operations, urban construction [1–3], and community activities, commonly contains a variety of contaminants that may pose significant hazards to public health and the environment. In order to minimize the environmental impact, solid wastes generally require disposal in either a physical (e.g., crush, compaction, combustion) or a biogeochemical manner (e.g., composting, incinerating). No matter which treatment method is adopted, disposal by land-filling is often the eventual fate for most solid waste via cutoff from surrounding groundwater system [4–10]. In developed countries, the development of land-filling can be divided into three stages [11]: (i) the direct dumping of waste before the 1940s; (ii) sanitary landfills from the 1940s to 1970s, in which the waste was covered by dirt with no barrier liner; (iii) closed landfills after the 1970s, which had barrier liner systems to limit contaminant migration into the surrounding environment. The barrier system is a critical component of modern closed landfills. In the early 1980s, the barrier system of a landfill mainly comprised of compacted clay liner (CCL). This was progressively substituted by (GM) by the end of 1980s. Since the 1990s, the composite liner system GCL (geosynthetic clay liner) has gained widespread popularity. The geosynthetic clay liner is composed of a layer of bentonite supported or encased by or . Compared to the

Sustainability 2017, 9, 2110; doi:10.3390/su9112110 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 2110 2 of 18 to the compacted clay liners and geomembranes, GCLs have a variety of advantages, such as low compacted clay liners and geomembranes, GCLs have a variety of advantages, such as low hydraulic hydraulic conductivity, high mechanical behavior, and simple and rapid installation in the field. conductivity, high mechanical behavior, and simple and rapid installation in the field. Moreover, Moreover, the high-swelling capability of bentonite in GCL enables local damage caused by field the high-swelling capability of bentonite in GCL enables local damage caused by field installation installation to self-heal in some circumstances. Owing to these advantages, GCLs are also adopted in to self-heal in some circumstances. Owing to these advantages, GCLs are also adopted in many many other geotechnical applications, such as dams, artificial lakes, sewage-treatment ponds, storage other geotechnical applications, such as dams, artificial lakes, sewage-treatment ponds, storage tanks, tanks, , contaminated sites, etc. [12–21]. landfills, contaminated sites, etc. [12–21]. Over the past three decades, extensive studies have been conducted on the physical and Over the past three decades, extensive studies have been conducted on the physical and mechanical mechanical properties of GCL and/or bentonite-based barriers in geoenvironmental applications [22– properties28]. Generally, of GCL the and/or properties bentonite-based of GCLs have barriers been ininvestigated geoenvironmental according applications to the following[22–28] .aspects: Generally, (i) hydraulicthe properties conductivity of GCLs and have chemical been investigated compatibility; according (ii) self-healing to the following capacity; (iii) aspects: diffusion; (i) hydraulic (iv) gas migration;conductivity and and (v) chemical mechanical compatibility; behaviours, (ii) e.g., self-healing creep behavior capacity; [29]. (iii) These diffusion; properties (iv) gas are migration; closely relatedand (v) to mechanical bentonite’s behaviours, form and mineralogy, e.g., creep behaviorwhich have [29]. also These received properties much areattention closely in related the past to decade.bentonite’s Moreover, form and due mineralogy, to the increased which applicatio have alson receivedof GCL muchmaterials attention in mining in the and past industrial decade. processesMoreover, (e.g., due heap to the leach increased pads etc.), application many studies of GCL have materials been conducted in mining to and investigate industrial the processes effect of (e.g.,exposure heap to leach extreme pads environments etc.), many studies and conditions have been (i conducted.e., excessive to cold, investigate excessive the heat, effect hyper-salinity, of exposure to stronglyextreme environmentsacidic, strongly and alkaline, conditions etc.). (i.e., To excessivebetter deal cold, with excessive these effects, heat, hyper-salinity,the modification strongly of bentonite acidic, alsostrongly became alkaline, a research etc.). hotspot. To better One deal of withthe typical these innovations effects, the modification is polymer-treated of bentonite also in became GCLs [30–32].a research The hotspot. objectives One of of this the paper typical are: innovations (i) to provide is polymer-treated a comprehensive bentonites summary in GCLson the [30 main–32]. Thefindings objectives on the of above-mentioned this paper are: (i) five to provideaspects aof comprehensive GCLs over the summary past few ondecades; the main (ii) findingsto provide on perspectivesthe above-mentioned for further five studie aspectss, e.g., of GCLspolymer-treated over the past GCLs. few decades; (ii) to provide perspectives for further studies, e.g., polymer-treated GCLs. 2. Configuration of Geosynthetic Clay Liners 2. Configuration of Geosynthetic Clay Liners Figure 1 shows a typical liner system in a municipal waste with the configuration of the Figure1 shows a typical liner system in a municipal waste landfill with the configuration covering and bottom liners, which are generally composed of geomembrane, and other of the covering and bottom liners, which are generally composed of geomembrane, geotextile geomaterials. GCL is one of the most important impermeable materials in the liners. GCL comprises and other geomaterials. GCL is one of the most important impermeable materials in the liners. a layer of bentonite supported by a geomembrane or sandwiched between two geotextiles. The GCL comprises a layer of bentonite supported by a geomembrane or sandwiched between two thickness of a GCL is typically 5 mm to 12 mm. According to the type of adopted, GCL geotextiles. The thickness of a GCL is typically 5 mm to 12 mm. According to the type of geosynthetics can be divided into the following two categories: (i) GT-GCL (geotextile-encased GCL), which is adopted, GCL can be divided into the following two categories: (i) GT-GCL (geotextile-encased composed of two geotextiles and a thin bentonite layer sandwiched between them, mechanically held GCL), which is composed of two geotextiles and a thin bentonite layer sandwiched between together by method of needle punching, stitching or chemical adhesives; (ii) GM-GCL them, mechanically held together by method of needle punching, stitching or chemical adhesives; (geomembrane-supported GCL), which comprises of a layer of geomembrane and a layer of bentonite (ii) GM-GCL (geomembrane-supported GCL), which comprises of a layer of geomembrane and a layer pasted to it using non-polluting adhesive [33]. The photos for various types of GCL are given in of bentonite pasted to it using non-polluting adhesive [33]. The photos for various types of GCL are Figure 2. given in Figure2.

Figure 1. Cont. SustainabilitySustainability 20172017,, 99,, 2110 2110 33 of of 18 18 Sustainability 2017, 9, 2110 3 of 18 Sustainability 2017, 9, 2110 3 of 18

Waste Vegetation Waste Vegetation Waste Vegetation Filter (e.g., ) Geotextile Filter (e.g., sand) Geotextile Drain (e.g., stone) Filter (e.g., sand) DrainGeotextile (e.g., stone) Geotextile Drain (e.g., stone) Drain (e.g., stone) Drain (e.g., stone) GeomembraneGeotextile Geotextile Geomembrane Drain (e.g., stone) Geotextile GeomembraneGeotextile GCLGeomembrane GCL GeotextileGeomembrane GCL GeomembraneGCL GCL GCL Exhaust (e.g., stone) Exhaust (e.g., stone) Sub- Exhaust (e.g., stone) Sub-grade Sub-grade

Waste Compacted clay Waste Compacted clay (b) Waste (c) Compacted clay (b) (c) (b) (c) Figure 1. Structure of a typical landfill: (a) schematic of a typical landfill (modified from Du et al., FigureFigureFigure 1. 1. Structure1. Structure Structure of of aoftypical a a typicaltypical landfill: landfill:landfill: (a) schematic ((a)) schematicschematic of atypical of of a a typical typical landfill landfill (modifiedlandfill (modified (modified from Du from etfrom al., Du 2009)Du et al.,et [28 al.,]; 2009) [28]; (b) structure of covering liners; (c) structure of bottom liners. (b)2009)2009) structure [28]; [28]; (b of(b) )structure covering structure liners;of of covering covering (c) structure liners; ((c ofc)) structurestructure bottom liners. of of bottom bottom liners. liners.

FigureFigureFigure 2. 2. Basic2.Basic Basic types types types of of ofgeosynthetic geosynthetic geosynthetic clay clayclay liners linersliners (GCLs): (GCLs):(GCLs): ((a (aa)) ) geotextile-encasedgeotextile-encased geotextile-encased (GT-GCL);(GT-GCL); (GT-GCL); and and and (b () (b b) ) Figure 2. Basic types of geosynthetic clay liners (GCLs): (a) geotextile-encased (GT-GCL); and (b) geomembrane-supportedgeomembrane-supportedgeomembrane-supported (GM-GCL). (GM-GCL). (GM-GCL). geomembrane-supported (GM-GCL). FigureFigure 3 schematically 3 schematically illustrates illustrates the the configurat configurationsions ofof thethe GCLs, includingincluding three three types types of of GT- GT- GCLFigureFigure (needle-punched,3 3schematically schematically stitched-bonded, illustratesillustrates the andconfigurat configurations adhesive-bonded)ions of of the the GCLs,and GCLs, one including including type of three GM-GCL. three types types Theof GT- of GCL (needle-punched, stitched-bonded, and adhesive-bonded) and one type of GM-GCL. The GT-GCLGCLneedle-punching (needle-punched, (needle-punched, method stitched-bonded, of stitched-bonded, GT-GCL involves and andpierciadhe adhesive-bonded)sive-bonded)ng fibers from andupper andone geotextile onetype typeof to GM-GCL. the of GM-GCL.bottom The needle-punching method of GT-GCL involves piercing fibers from upper geotextile to the bottom Theneedle-punchinggeotextile. needle-punching These methodfibers method extend of ofGT-GCL through GT-GCL involvesthe involves bentonit pierci piercinge andng rely fibers fibers on naturalfrom from upper upper entanglement geotextile or being to thethe fused bottombottom geotextile. These fibers extend through the bentonite and rely on natural entanglement or being fused geotextile.geotextile.(called “thermal These These fibersfibers lock”) extend extend to the through through bottom the thegeotextile bentonite bentonit ine andorder and rely rely to onbond on naturalnatural the GCL entanglemententanglement together. The or orbeing beingstitching fused fused (called “thermal lock”) to the bottom geotextile in order to bond the GCL together. The stitching (called(calledmethod “thermal“thermal of GT-GCL lock”)lock”) is to toto sew thethe the bottombottom geotextiles geotextilegeotextile and bentonite inin orderorder together to to bond bond by the the using GCL GCL stitching-bonded together. together. TheThe stitchingyarns.stitching method of GT-GCL is to sew the geotextiles and bentonite together by using stitching-bonded yarns. methodmethodThe pasting ofof GT-GCLGT-GCL method is is to toof sew sewGT-GCL thethe geotextilesgeotextiles is to incorporate andand bentonitebentonite non-polluting togethertogether adhesive byby usingusing into stitching-bondedstitching-bonded bentonite and then yarns.yarns. The pasting method of GT-GCL is to incorporate non-polluting adhesive into bentonite and then TheThepaste pastingpasting on two methodmethod layers ofofof GT-GCLgeotextiles.GT-GCL isis This to to incorporate incorporatemethod creates non-pollutingnon-polluting a relatively adhesiveloweradhesive bonding intointo bentonite strengthbentonite than andand the thenthen paste on two layers of geotextiles. This method creates a relatively lower bonding strength than the pastepasteneedle-punching onon two two layers layers or of of stitching geotextiles. geotextiles. methods. ThisThis methodFormethod GM-GCL, createscreates the aa bentonite relativelyrelatively mixed lowerlower with bondingbonding adhesive strengthstrength is pasted thanthan to thethe needle-punching or stitching methods. For GM-GCL, the bentonite mixed with adhesive is pasted to needle-punchingneedle-punchingone layer of geomembrane. or or stitching stitching In methods. methods. order to ForprovideFor GM-GCL,GM-GCL, primar theythe protectionbentonite bentonite duringmixed mixed instawith withllation,adhesive adhesive a thinis is layerpasted pasted of to to one layer of geomembrane. In order to provide primary protection during installation, a thin layer of oneonegeotextile layer layer of of geomembrane.isgeomembrane. also adhered on InIn to orderorder the surface toto provideprovide of bentonite. primaryprimary protectionprotection during during installation, installation, a a thin thinlayer layerof of geotextile is also adhered on to the surface of bentonite. geotextilegeotextile is is also also adhered adhered on on to to the the surface surface of of bentonite. bentonite.

Figure 3. Cont. Sustainability 2017, 9, 2110 4 of 18 Sustainability 2017, 9, 2110 4 of 18 Sustainability 2017, 9, 2110 4 of 18

Figure 3. Configurations of GCLs: (a) GT-GCL (needle-punched); (b) GT-GCL (stitched-bonded); (c) FigureGT-GCLFigure 3. 3. (adhesive-bonded);ConfigurationsConfigurations of of GCLs: ( GCLs:d) GM-GCL (a) ( aGT-GCL) GT-GCL (adhesive-bonded); (needle-punched); (needle-punched); (modified (b) (GT-GCLb) from GT-GCL Koerner,(stitched-bonded); (stitched-bonded); 2005) [34]. (c) GT-GCL(c) GT-GCL (adhesive-bonded); (adhesive-bonded); (d) (GM-GCLd) GM-GCL (adhesive-bonded); (adhesive-bonded); (modified (modified from from Koerner, Koerner, 2005) 2005) [34]. [34 ]. The bentonite in GCL has extremely low hydraulic conductivity and high swelling capacity. BentoniteTheThe bentonite bentonitehas a high in increep GCL GCL behavior has has extremely extremely to keep low lowthe hydraulichydraulic hydraulic conductiviconductivity conductivityty of and andGCL high high low swelling [35–42]. swelling Forcapacity. capacity. a GT- BentoniteGCL,Bentonite the hasrole has a of high a the high creep bentonite creep behavior behavior is to to reduce keep to the keep the hydraulic hydraulic the hydraulic conductivi conductivity conductivityty of ofGCL GCL low of and GCL[35–42]. to lowrepair For [35 a local –GT-42]. GCL,damage.For a GT-GCLthe Forrole a , ofGM-GCL, the the role bentonite of since the bentonite isthe to geomembrane reduce is to reducethe hydraulic has the a hydraulic very conductivity low conductivityhydraulic of GCL conductivity, of and GCL to and repair the to repair mainlocal damage.functionlocal damage. ofFor the a For GM-GCL,bentonite a GM-GCL, issince to sincerepair the thegeomembrane local geomembrane damage. has The has a bentonitesvery a very low low hydraulic commonly hydraulic conductivity, conductivity,used in GCLs the the are: main main (i) functionsodiumfunction bentonite, of of the the bentonite bentonite and (ii) is calcium is to to repair repair bentonite. local local damage. damage. Calcium The Thebentonite bentonites bentonites has commonly a higher commonly hydraulic used used in GCLsconductivity in GCLs are: are: (i) sodiumthan(i) sodium sodium bentonite, bentonite, bentonite. and and (ii) In (ii) calcium engineering calcium bentonite. bentonite. practice, Calcium Calcium calcium bentonite bentonite bentonite has has ais ahigher usually higher hydraulic hydraulic treated withconductivity conductivity sodium thancompoundsthan sodium sodium such bentonite. bentonite. as sodium In In engineering engineeringcarbonite prior practice, practice, to use calcium calcium in order bentonite bentonite to reduce is isits usually usually hydraulic treated treated conductivity with with sodium [43– compounds45].compounds such asas sodiumsodium carbonitecarbonite prior prior to to use use in in order order to to reduce reduce its its hydraulic hydraulic conductivity conductivity [43 [43––45]. 45]. 3. Properties Properties of of GCLs GCLs 3. Properties of GCLs 3.1. Hydraulic Hydraulic Conductivity Conductivity and Chemical Compatibility 3.1. HydraulicWorking Conductivityas a hydraulic and barrier, Chemical hydraulic Compatibility conductivityconductivi ty is the most critical parameter to evaluate thethe effectiveness Working as a ofof hydraulic thethe GCLs.GCLs. barrier, Generally,Generally, hydraulic the the barrier barrier conductivi effect effect of tyof GCL isGCL the is ismost achieved achieved critical by by parameter the the bentonite bentonite to evaluate within within it, theit,which which effectiveness has has extremely extremely of the low GCLs.low hydraulic hydraulic Generally, conductivity conductivity the barrier similar similar effect to thatof to GCL that used usedis in achieved the in lubricantthe lubricant by the in bentonite pipe in pipe jacking jackingwithin [46]. it,[46].The which onlyThe hasonly exception extremely exception is GM-GCL low is GM-GCL hydraulic in which in conductivity which geomembrane geomembrane similar acts to as thatacts the usedas water the in water the barrier. lubricant barrier. in pipe jacking [46]. TheBentonite only exception is basically is GM-GCL composed in whichof of smectite, geomembrane wh whoseose crystal crystalacts as structurethe structure water cobarrier. consistsnsists of of a a layer of aluminium-oxygenBentonite is basically (or magnesium-oxygen) composed of smectite, octahedron,octahedron, whose (Al(Al 22crystal(OH)(OH)66, Mgstructure3(OH)(OH)6 ),),co sandwiched sandwichednsists of a betweenlayer between of 4− twotwo layers of silicon-oxygensilicon-oxygen tetrahedraltetrahedral ((Si((Si4OO10)4-).). Each Each clay clay sheet sheet is is composed composed of of several several or or dozens dozens aluminium-oxygen (or magnesium-oxygen)4 octahedron,10 (Al2(OH)6, Mg3(OH)6), sandwiched between of these structurestructure cells,cells, and and layers layers of of structure structure cells cells are are connected connected to eachto each other other via Vanvia Van derWaals der Waals force. two layers of silicon-oxygen tetrahedral ((Si4O10)4-). Each clay sheet is composed of several or dozens offorce.These these These connections structure connections cells, are comparatively and are layers comparatively of weakstructure soweak that cells so the are that water connected the molecules water to molecules each can other intervene can via intervene Van easily, der causing easily,Waals force.causingswelling These swelling of theconnections smectite of the onsmectiteare the comparatively macro on the level. macro weak Figure level. so4 givesthat Figure the an water illustration4 give moleculess an ofillustration the can crystal intervene of structure the crystaleasily, of causingstructuresmectite swelling andof smectite the crystalline-swellingof theand smectitethe crystalline-swelling on mechanism.the macro level.mechanism. Figure 4 gives an illustration of the crystal structure of smectite and the crystalline-swelling mechanism.

Figure 4. Smectite:Smectite: ( (aa)) crystal crystal structure structure of of smectite smectite [47]; [47]; ( (bb)) crystalline-swelling crystalline-swelling mechanism mechanism (modified (modified from Ruedrich, et al., 2011) [48]. Figurefrom Ruedrich, 4. Smectite: et al., (a) 2011)crystal [48]. structure of smectite [47]; (b) crystalline-swelling mechanism (modified from Ruedrich, et al., 2011) [48]. Sustainability 2017,, 9,, 21102110 5 of 18

The smectite has high specific surface area, e.g., about 800 m2 for 1 g of smectite, and a vast 2 numberThe of smectite negative has charge highs specificon it. To surface achieve area, charge e.g., balance, about 800 the msurfacefor 1 will g of adsorb smectite, the and cations a vast in numberthe pore ofwater, negative forming charges a “diffuse on it. Todouble achieve layer” charge (DDL) balance, [49–51]. the Figure surface 5 gives will adsorban illustration the cations of the in thetypical pore diffuse water, double forming layer. a “diffuse The thickness double of layer” DDL (DDL) (H) can [49 be– 51calculated]. Figure 5by gives the equation an illustration proposed of the by typicalBolt (1956) diffuse [52]: double layer. The thickness of DDL (H) can be calculated by the equation proposed by Bolt (1956) [52]: rDkT H = DkT (1) H =πεν22 (1) 8 n 8πnε2ν2 where, D = dielectric constant of solution; k = Boltzmann constant (k = 1.38 × 10−23 J/K); T = absolute where, D = dielectric constant of solution; k = Boltzmann constant (k = 1.38 × 10−23 J/K); T = absolute temperature; n = molar concentration; ε = elementary charge (ε = 4.8 × 10−20 esu); and ν = ionic valence. temperature; n = molar concentration; ε = elementary charge (ε = 4.8 × 10−20 esu); and ν = ionic Moreover, electrostatic forces can attract numerous ions into the space between the clay particles, valence. Moreover, electrostatic forces can attract numerous ions into the space between the clay which causes a large difference of ionic concentration between the clay mineral surface and the pore particles, which causes a large difference of ionic concentration between the clay mineral surface and solution. Driven by concentration gradient forces, water with lower electrolyte concentration is taken the pore solution. Driven by concentration gradient forces, water with lower electrolyte concentration up by osmosis between the clay particles thus pushing them apart, which makes the smectite further is taken up by osmosis between the clay particles thus pushing them apart, which makes the smectite swell. further swell.

Figure 5. Osmotic swelling of clay minerals (mod (modifiedified from Ruedrich et al., 2011) [[48].48].

After water-swelling,water-swelling, anan immobile immobile water water phase phase will will be be formed formed by by the the bound bound water water molecules molecules and anand impermeable an impermeable material material will be will formed be byformed hydrated by hydrated bentonite, bentonite, leading to theleading decrease to the of hydraulically decrease of activehydraulically pores, andactive manifested pores, and as manifested anti-seepage as anti-s of bentoniteeepage of [30 bentonite,43,45,53 ].[30,43,45,53]. Generally, Generally, the hydraulic the conductivityhydraulic conductivity of bentonite of bentonite to water to is withinwater is the within range the of range 2 × 10of− 212 × 10m/s−12 m/s to 2 to× 210 × −1010−10m/s, m/s, which decreases with the increase in confiningconfining pressurepressure [[15,54].15,54]. In some conditions, if the permeant solution containscontains high concentrations of cations, especially bivalent cations andand trivalenttrivalent cationscations (e.g.,(e.g., CaCa2+2+, MgMg2+2+,, Al Al3+3+),), ion ion exchange exchange will will occur occur and reach equilibrium, and the thickness of DDL will significantlysignificantly decrease. This will cause an increase of the hydraulic conductivity of GCL, which can be one order of magnitude or moremore for some permeating conditions [55[55–58].–58]. The The triggering triggering mechanism mechanism of ion of exchange ion exchange on increase on ofincrease the hydraulic of the conductivity hydraulic wasconductivity presented was by Egloffstein presented(2001) by Eglo [59ffstein]. Figure (2001)6 gives [59]. an exampleFigure 6 of gi ionves exchange an example between of ion monovalent exchange sodiumbetweenions monovalent and bivalent sodium calcium ions and ions. bivalent The ion calcium exchange ions. increases The ion the exchange inner-crystalline increases attraction,the inner- thuscrystalline reducing attraction, the space thus between reducing the the silicate space layers betw (layerseen the of silicate structure layers cells) (lay anders theof structure mass of bound cells) water.and the With mass this of bound change, water. the micro With structurethis change, of bentonite the micro changes structure from of bentonit smaller,e finelychanges distributed from smaller, clay mineralfinely distributed flakes to larger clay claymineral mineral flakes crystals, to larger leading clay to mineral an increase crystals, in hydraulic leading conductivity. to an increase in hydraulic conductivity. Sustainability 2017, 9, 2110 6 of 18 Sustainability 2017, 9, 2110 6 of 18

FigureFigure 6.6.Ion Ion exchange exchange of monovalentof monovalent sodium sodium ions againstions against bivalent bivalent calcium calcium ions in bentonite ions in (modifiedbentonite from(modified Egloffstein, from Egloffstein, 2001) [59]. 2001) [59].

GCLsGCLs areare exposedexposed toto leachatesleachates otherother thanthan waterwater underunder mostmost circumstances.circumstances. AssessmentAssessment ofof thethe effecteffect ofof chemicalchemical solutionssolutions onon thethe hydraulichydraulic behaviourbehaviour ofof GCLsGCLs isis ofof greatgreat importance.importance. TheThe hydraulic conductivityconductivity toto thethe permeantpermeant liquidliquid ofof GCLGCL needsneeds toto bebe assessedassessed onon aa case-by-case basis.basis. The common methodmethod adoptedadopted isis thethe “compatibility“compatibility test”,test”, inin whichwhich thethe samplesample isis permeated with a leachate similar toto thatthat anticipated. anticipated. Factors Factors influencing influencing the hydraulicthe hydraulic conductivity conductivity of GCLs of haveGCLs been have studied been throughstudied extensivethrough extensive compatibility compatibility tests [25, 53tests,55 ,[25,53,55,59–65].59–65]. Generally, Generally, the hydraulic the hydraulic conductivity conductivity can be affected can be byaffected both internalby both andinternal external and causes.external The causes. internal The factors internal involve factors the involve smectite the content, smectite aggregate content, size,aggregate exchangeable size, exchangeable metals and metals and ofvoid bentonite ratio of in bentonite GCL, and in the GCL, external and the factors external include factors the concentrationinclude the concentration of cations (monovalent of cations (monovalent and divalent) an ind divalent) the permeant in the liquid, permeant pre-hydration liquid, pre-hydration of the GCL, etc.of the In GCL, the lastetc. 10In the years, last increasing10 years, increasing concern hasconcern been has expressed been expressed about the about compatibility the compatibility of GCLs of inGCLs extreme in extreme environments environments and conditions, and conditions, such as such hyper-salinity, as hyper-salinity, strongly strongly acidic, strongly acidic, alkaline,strongly excessivealkaline, excessive cold and cold excessive and excessive heat environments, heat environments, which can which greatly can deteriorategreatly deteriorate their anti-seepage their anti- performanceseepage performance [14,61,66 –[14,61,66–68].68].

3.2.3.2. Self-Healing CapacityCapacity DuringDuring fieldfield installation, the the GCL GCL system system may may suffer suffer from from damage damage for for the the following following reasons: reasons: (i) (i)piercing piercing by by sharp sharp objects, objects, e.g., e.g., construction construction machineries machineries [15], [15], angular angular stones; stones; (ii) poor connectionsconnections betweenbetween panelspanels of of GCLs GCLs [69 ,[69,70];70]; (iii) (iii) migration migration of bentonite of bentonite in GCLs in dueGCLs to dailydue temperatureto daily temperature variation, formingvariation, spots/zones forming spots/zones with litter orwith no litter bentonite or no [ 29bentonite,56,71,72 [29,56,71,72];]; (iv) wet and (iv) dry wet cycles, and which dry cycles, leads which to the desiccationleads to the of desiccation clay and associated of clay and cracking associated [73]. Becausecracking of [73]. its highBecause expansion of its high capacity expansion when hydrated, capacity thewhen bentonite hydrated, in GCLsthe bentonite can be squeezed in GCLs intocan thebe damagesqueezed holes/slits into the damage under the holes/slits confining under pressure. the Suchconfining self-healing pressure. capacity Such self-healing can guarantee capacity that can the gu GCLarantee functions that the properly GCL functions in some properly circumstances; in some forcircumstances; example, holing for example, up to 30 mmholing in diameterup to 30 mm [74 ,in75 diameter]. Figure7 [74,75]. is a photograph Figure 7 is showing a photograph the self-healed showing resultthe self-healed of a GCL result pierced of aby GCL a bolt. pierced The by self-healing a bolt. The self-healing capacity of capacity a GCL liner of a GCL is commonly liner is commonly assessed usingassessed percent using swellpercent tests swell or tests direct or measurementsdirect measurements of hydraulic of hydraulic conductivity conductivity [76,77 [76,77].]. To assessTo assess in ain quantitative a quantitative way, way, Chai Chai et al. et (2016) al. (2016) [26] proposed[26] proposed a concept, a concept, the “self-healing the “self-healing ratio” (theratio” area (the healed area byhealed bentonite by bentonite divided bydivided the total by damaged the total area),damaged and anarea), explicit and empirical an explicit equation empirical for predicting equation thefor self-healingpredicting the ratio. self-healing ratio. Sustainability 2017, 9, 2110 7 of 18 Sustainability 2017, 9, 2110 7 of 18

Figure 7. Self-healing of GCL pierced by a bolt (modified from Koerner, 1990) [78]. Figure 7. Self-healing of GCL pierced by a bolt (modified from Koerner, 1990) [78].

Extensive laboratory tests have been conducted to investigate the factors that influence the self- healingExtensive capacity laboratory of GCLs. In tests general, have all been of the conducted factors affecting to investigate the swelling the factors capacity that of bentonite influence will the influenceself-healing the capacity self-healing of GCLs. ability In general,of GCLs all[70]. of theThe factors swelling affecting amount the of swelling bentonite capacity can be of influenced bentonite willby the influence following the self-healingfactors: (i) internal ability offactors, GCLs [e.g.,70]. Thebentonite swelling mass, amount binding of bentonite method; canand be (ii) influenced external factors,by the following e.g., permeant factors: solution (i) internal type, overburden factors, e.g., pressure, bentonite hydraulic mass, binding head. method;The most andvital (ii) element external of bentonitefactors, e.g., in GCLs permeant is smectite, solution which type, is overburden responsible pressure, for the swell hydraulic potential. head. It Thehas been most proven vital element that a higherof bentonite amount in GCLsof bentonite is smectite, in the which liner, or is responsiblea higher content for the of swellsmectite potential. in bent Itonite, has beencan contribute proven that to a better higher self-healing amount of bentonitecapacity [79,80]. in the liner,Furthermore, or a higher the content method of of smectite bindingin the bentonite, GCL components can contribute has a significantto a better self-healinginfluence to capacity self-healing [79,80 ability,]. Furthermore, e.g., needle-punched the method of GCLs binding have the better GCL componentsself-healing capacityhas a significant than stitch-b influenceonded to GCLs self-healing [75,81]. ability, e.g., needle-punched GCLs have better self-healing capacityThe thanhydrated stitch-bonded bentonite GCLsparticles [75 ,in81 ].a defect area can be carried away under huge seepage force [10,19,21,82,83],The hydrated resulting bentonite in the particles destruction in a of defect the self-healing area can be capacity carried [84]. away Overburden under huge stress seepage may affectforce [10the,19 self-healing,21,82,83], resulting capability in in the two destruction opposite of ways: the self-healing (i) squeezing capacity the bentonite [84]. Overburden into damaged stress holes/slits,may affect thethus self-healing increasing capability the capability in two oppositeof self-healing; ways: (i)(ii) squeezing constraining the bentonite the expansion into damaged of the bentonite,holes/slits, causing thus increasing reduction the of capability the self-healing of self-healing; capability. (ii) constraining The latter occurs the expansion when the of the overburden bentonite, stresscausing of reductionthe bentonite of theis greater self-healing than the capability. swelling Thepressure latter [70]. occurs The when type of the permeant overburden solution stress also of has the abentonite significant is greaterimpact, than and the all swellingof the factors pressure affecting [70]. The the typeDDL of around permeant the solutionbentonite also particles has a significant will also contributeimpact, and to all a change of the factors of self-healing affecting capability the DDL around[59,70,85]. the bentonite particles will also contribute to a change of self-healing capability [59,70,85]. 3.3. Diffusion 3.3. Diffusion Diffusion refers to the migration of ions or molecules as a result of their random thermal motion Diffusion refers to the migration of ions or molecules as a result of their random thermal motion from a higher concentration region to lower concentration region without the transportation of from a higher concentration region to lower concentration region without the transportation of seepage seepage water. For some typical situations, the advective flows are extremely slow and molecular water. For some typical situations, the advective flows are extremely slow and molecular diffusion diffusion may be the main mechanism of transporting through the GCL system [86]. The diffusion may be the main mechanism of transporting through the GCL system [86]. The diffusion process process of molecules and ions through GCLs can be described by the following equations proposed of molecules and ions through GCLs can be described by the following equations proposed by Fick by Fick (1855) [87]: (1855) [87]: dC JDJ ==−−D (2)(2) dxdx ∂C ∂  ∂C  = D (3) ∂∂∂∂CCt ∂x ∂x = D (3) ∂∂tx ∂ x Sustainability 2017, 9, 2110 8 of 18 where, J = diffusion flux; C = volume concentration of diffusive substance; dC/ dx , ∂∂Cx/ = concentration gradient; dC/ dt = concentration variation rate with time; and D = diffusion coefficient. The diffusion coefficient is a critical parameter for evaluating the diffusion capacity of molecules or ions through GCLs. Several apparatus and test methods for the evaluation of the GCL diffusion coefficient have been proposed [88–90]. The test involves placing a contaminant source solution on one side of a GCL specimen and an uncontaminated receptor solution on the other side (see Figure 8). Factors influencing the diffusion coefficient of GCLs have been widely investigated. Lake and Rowe (2000) [86] indicates that the diffusion coefficient has a significant linear correlation with the final bulk GCL void ratio (defined as the void volume divided by the solid volume in GCLs at the Sustainabilityend of testing).2017, 9 ,The 2110 diffusion coefficients for different chemical molecules or ions through GCLs8 of can 18 vary a lot. Lake and Rowe (2004) [23] investigated the diffusion of acetone through a GCL under where,normalJ conditions.= diffusion The flux; resultsC = indicated volume concentrationthe diffusion rate of diffusiveof mass through substance; the dCGCL/dx for, ∂ Cdifferent/∂x = concentrationmolecules, from gradient; high todC low,/dt = are: concentration dichloromethane, variation dichloromethane, rate with time; and benzene,D = diffusion trichloroethylene, coefficient. toluene.The diffusionRowe, et coefficiental. (2005) [89] is a further critical investig parameterated for the evaluating diffusion theof benzene diffusion and capacity concluded of molecules that the ororder, ions from through high GCLs. to low, Several is: toluene, apparatus ethylbenzene, and test methodsm & p-xylene for the and evaluation o-xylenen. of the GCL diffusion coefficientThe diffusion have been coefficient proposed can [88 –be90 ].also The significantly test involves influenced placing a by contaminant the concentration source solutionlevel of onthe onesolute, side which of a GCL is able specimen to modify and the an uncontaminatedmicrostructure of receptor the sensitive solution minerals on the other[86]. However, side (see Figure Lange8 ).et Factorsal. (2009) influencing [91] conducted the diffusion laboratory coefficient tests ofon GCLs metal have diffusion been widely through investigated. GCLs from Lake four and solutions Rowe (2000)collected [86 ]from indicates landfill that wastes, the diffusion and found coefficient no significant has a significantvariation of linear the metal correlation diffusion with coefficients. the final −10 bulkFor instance, GCL void the ratio diffusion (defined coefficient as the void of Cu, volume Ni, Fedivided and Cd byis within the solid the volumerange of in 0.67 GCLs × 10 at theto 0.98 end × −10 2 −10 −10 of10 testing). m /s, while The diffusion0.80 × 10 coefficients to 1.6 × 10 for for different As, Al, Mg chemical and Sr. molecules They also orpointed ions throughout that some GCLs other can + varycations a lot. (e.g., Lake Na and) can Rowe be sensitive (2004) to [23 the] investigated solution pH. the diffusion of acetone through a GCL under normalThe conditions. diffusion also The depends results indicated on temperature, the diffusion with lower rate of value mass at through a relatively the lower GCL fortemperature different molecules,[89]. What fromis noteworthy high to low, is that, are: by dichloromethane, contrast with the dichloromethane, influence on hydrauli benzene,c conductivity trichloroethylene, of GCLs, toluene.the type Rowe, of bentonite et al. (2005) contained [89] furtherin GCLs investigated (sodium or thecalcium) diffusion and ofthe benzene ion exchange and concluded in bentonite that (e.g., the order,from sodium from high to tocalcium) low, is: both toluene, have ethylbenzene, no obvious influence m & p-xylene on the anddiffusion o-xylenen. coefficient of GCLs [90,92].

Figure 8. Illustration of diffusion-testing instrument. Figure 8. Illustration of diffusion-testing instrument.

The diffusion coefficient can be also significantly influenced by the concentration level of the solute, which is able to modify the microstructure of the sensitive minerals [86]. However, Lange et al. (2009) [91] conducted laboratory tests on metal diffusion through GCLs from four solutions collected from landfill wastes, and found no significant variation of the metal diffusion coefficients. For instance, the diffusion coefficient of Cu, Ni, Fe and Cd is within the range of 0.67 × 10−10 to 0.98 × 10−10 m2/s, while 0.80 × 10−10 to 1.6 × 10−10 for As, Al, Mg and Sr. They also pointed out that some other cations (e.g., Na+) can be sensitive to the solution pH. The diffusion also depends on temperature, with lower value at a relatively lower temperature [89]. What is noteworthy is that, by contrast with the influence on hydraulic conductivity of GCLs, the type of bentonite contained in GCLs (sodium or calcium) and the ion exchange in bentonite (e.g., from sodium to calcium) both have no obvious influence on the diffusion coefficient of GCLs [90,92]. Sustainability 2017, 9, 2110 9 of 18 Sustainability 2017, 9, 2110 9 of 18

3.4.3.4. Gas Migration ExceptExcept for the the hydraulic hydraulic barrier, barrier, the the objective objective of of GCLs GCLs in incover cover systems systems should should also also include include the thecontrol control of leakage of leakage of oftoxic toxic gas gas from from waste waste and and the the migration migration of of oxygen oxygen into into waste. waste. As As with thethe movementmovement of liquidliquid through GCL, gasgas migrationmigration occursoccurs by twotwo means:means: permeationpermeation and diffusion. TheThe drivingdriving force force for for gas gas permeation permeation is the is the pressure pressure differentials differentials across across the two the sides two of sides theliner of the system, liner whichsystem, can which result can from result natural from fluctuations natural fluctuatio in atmosphericns in atmospheric pressure, or pressure, changes ofor thechanges temperature of the ortemperature water table or [water54,93, 94table]. Gas [54,93,94]. diffusion Gas occurs diffusion in response occurs in toresponse a concentration to a concentration gradient gradient of the gas. of Thethe gas. molecules The molecules move from move a high from concentration a high concentration area to a area low concentrationto a low concentration area. area. ToTo preventprevent gasgas migration,migration, thethe porespores inin bentonitebentonite need to be saturated during their lifetime. However,However, thisthis is difficultis difficult to achieve to achieve because because experience experience has indicated has indicated that the occurrence that the ofoccurrence unsaturated of flowunsaturated is unavoidable flow is inunavoidable liner systems in [liner94]. Vangpaisalsystems [94]. and Vangpaisal Bouazza (2004) and Bouazza [94] conducted (2004) [94] a group conducted of gas migrationa group of tests gas onmigration different, tests partially on different, hydrated partia GCLs,lly hydrated and the results GCLs, indicated and the results that gas indicated permeability that wasgas permeability closely related was to closely the variation related of to moisture the variatio andn volumetricof moisture water and volumetric content. As water the gravimetriccontent. As moisturethe gravimetric and volumetric moisture water and contentvolumetric increases, water thecontent gas migration increases, rate the of gas GCLs migration decreases rate accordingly of GCLs (seedecreases Figure accordingly9)[ 95]. Didier (see etFigure al. (2000) 9) [95]. [96 Didier] indicated et al. that(2000) the [96] gas indicated permeability that the was gas linearly permeability related towas the linearly volumetric related water to the content volumetric on a linear-logwater content scale. on Thea linear-log relationship scale. between The relationship the gas diffusion between coefficientthe gas diffusion and the coefficient effective diffusionand the effective has been diff investigatedusion has been by many investigated researchers by many [97–99 researchers], and the results[97–99], are and plotted the results in Figure are 10plotted. It is evidentin Figure that 10. the It is diffusion evident coefficientthat the diffusion of gas decreased coefficient with of thegas increasedecreased of with saturation the increase degree. of Environmental saturation degree factors. Environmental such as straining factors and such wet-dry as straining cycles can and induce wet- straindry cycles cracks can and induce desiccation strain cracks inand bentonite desiccation within cracks GCLs, in bentonite which provide within a preferentialGCLs, which flow provide path, thusa preferential accelerating flow gas path, permeation thus accelerating through thegas GCLpermeation [27,94]. through the GCL [27,94]. The structure of GCL can also influenceinfluence its gas permeability; for example, stich-bonded GCLGCL hashas aa higherhigher gas gas permeability permeability than than needle-punched needle-punched GCL. GC GCLsL. GCLs with awith poor a distribution poor distribution of needle-punched of needle- fiberspunched will fibers have will gas permeabilityhave gas permeability up to three up ordersto three of orders magnitude of magnitude higher than higher that than with that uniform with distributionuniform distribution [100,101]. [100,101]. Besides, Besides, in actual in applications, actual applications, it is not it uncommonis not uncommon for some for spacesome space to exist to atexist the at interface the interface of the of GM/GCL the GM/GCL composite composite cover, cove whichr, which will inevitablywill inevitably provide provide a flow a flow path path for gasfor migrationgas migration [22,33 [22,33].].

10-12

10-13

-14

) 10 2

10-15

10-16

10-17

10-18 Cured under 20 kPa Intrinsic Permeabiliry (m 10-19 Cured under 0 kPa (data from Bouazza and Vangpaisal, 2003) [95] 10-20 0 1020304050607080 Volumetric Moisture Content (%)

Figure 9. Gas permeability of nitrogen against volumetricvolumetric moisturemoisture content.content. Sustainability 2017, 9, 2110 10 of 18 Sustainability 2017, 9, 2110 10 of 18

10-5

10-6 /s) 2 10-7

10-8

10-9

10-10 Aubertin, et al. (2000) [97] Rouf, et al. (2016)[98], 2 kPa

-11 Rouf, et al. (2016)[98], 20 kPa Gas diffusion coefficent (m 10 Bouazza and Rahman (2007)[99], GCL1&2 Bouazza and Rahman (2007)[99], GCL3 10-12 0 20406080100 Bentonite degree of saturation S (%) r

FigureFigure 10. 10.Diffusion Diffusion coefficient coefficient of oxygenof oxygen against against bentonite bentonite degree degree of saturation.of saturation.

3.5.3.5. Mechanical Mechanical Behaviour Behaviour StabilityStability is anis an importation importation consideration consideration for for GCLs GCLs which which are are applied applied in coverin cover and and slope-lining slope-lining systemssystems of wasteof waste landfills. landfills. GCLs GCLs are are commonly commonly expected expected to sufferto suffer from from transient transient shear shear stress stress during during construction,construction, as wellas well as permanentas permanent shear shear stress stress in some in some cases. cases. During During the design the design stage, stage, two aspectstwo aspects of shearof strength should should be taken be intotaken account into account for stability for stability analysis: analysis: (i) the internal (i) the internal shear strength shear strength of GCL; of (ii)GCL; the interface (ii) the interface shear strength shear strength between between GCL and GCL adjacent and materials.adjacent materials.

3.5.1.3.5.1. Internal Internal Shear Shear Strength Strength TheThe internal internal shear shear strength strength and variablesand variables governing governing the strength the strength for different for different types of types GCLs of have GCLs beenhave investigated been investigated by many by researchers many researchers using direct using shear direct tests shear [37 ,tests102– 108[37,102–108].]. These indicate These indicate that all ofthat theall GCLs of the exhibit GCLs strain-softeningexhibit strain-softening characteristics characteristics under directunder shear. direct Forshear. unreinforced For unreinforced GCLs thatGCLs are that bondedare bonded together together with a chemical with a binder chemical (e.g., binder adhesive-bonded, (e.g., adhesive-bonded, GM-GCL), since GM-GCL), no reinforcement since no existsreinforcement in the bentonite exists the in the products bentonite provide the products a relatively provide low resistance a relatively to shearlow resistance force. For to reinforced shear force. GCLsFor (e.g.,reinforced stitch-bonded, GCLs (e.g., and stitch-bonded, needle-punched and GT-GCL),needle-punched the piercing GT-GCL), fibers the and piercing the stitch fibers will and act the asstitch reinforcement will act as materials reinforcement leading materials to a significant leading to increase a significant in shear increase strength in shear [103 ,strength104]. Typically, [103,104]. owingTypically, to the frictionalowing to connectionthe frictional of connection the reinforcing of the fibers, reinforcing needle-punched fibers, needle-punched GCL has a higher GCL peak has a shearhigher strength peak thanshear other strength types than of GCL other [105 types,106 of]. ForGCL each [105,106]. type of For GCL, each failure type occursof GCL, at failure the interface occurs at ofthe geotextile interface and ofbentonite. geotextile Theand stitch-bondedbentonite. The GCLstitch-bonded fails when GCL the stitchingfails when lines the ripstitching through lines the rip geotextile,throughwhile the geotextile, the needle-punched while the needle-punched GCL fails when reinforcingGCL fails when fibers reinforcing pull out of thefibers geotextile pull out [105 of ].the In termsgeotextile of stability, [105]. In reinforced terms of GCLsstability, are reinforced suitable for GCLs high-stress are suitable application, for high-stress while unreinforced application, GCLs while areunreinforced suitable for low-stress GCLs are applications.suitable for low-stress applications. ExceptExcept for for some some typestypes ofof GCLs,GCLs, the the hydratio hydrationn condition, condition, the the normal normal stress stress and andthe shear- the shear-displacementdisplacement rate ratewill willalso alsocontribute contribute to internal to internal shear shear strength strength variability variability [103]. [Hydration103]. Hydration will lead willto lead an increase to an increase in the inwater the watercontent content of the ofbentonite the bentonite as well as as well a significant as a significant decrease decrease of the ofinternal the internalshear shearstrength. strength. Hydration Hydration fluid, fluid, hydration hydration time, time, and andnormal normal stress stress during during hydration hydration are are the the key keyfactors factors influencing influencing the the hydrated hydrated shear strengthstrength [[104].104]. ForFor needle-punched needle-punched GCLs, GCLs, the the peak peak shear shear strengthstrength increases increases with with normal normal stress, stress, while while for for stitch-bonded stitch-bonded GCLs, GCLs, it seemsit seems to beto be independent independent of of normalnormal stress. stress. Under Under a low a low normal normal stress, stress, the the peak peak shear shear strength strength of reinforcedof reinforced GCLs GCLs was was observed observed to increaseto increase with with the the shearing shearing rate rate [103 [103,105];,105]; however, however, under under a high a high normal normal stress, stress, the the trend trend will will be be reversed [106]. This is because negative pore water pressures are generated during shearing under low normal stress, while positive pore water pressures are generated under high normal stress. Sustainability 2017, 9, 2110 11 of 18 reversed [106]. This is because negative pore water pressures are generated during shearing under low normal stress, while positive pore water pressures are generated under high normal stress.

3.5.2. Interface Shear Strength GCLs are usually laid under the geomembranes (GM) to form a composite liner system in landfills. As a critical factor for stability, the shear strength of the interface between GCL and GM has been widely studied [28,94,109–113]. Compared with GM sheared against normal geotextiles, what makes the GCL-GM interface specific is the presence of extruded bentonite, which can significantly reduce interface [23,24,110]. Laboratory tests have indicated that the GM-GCL interface has smaller peak shear strength and greater large-displacement strengths than GCL internal shear specimens [112]. The failure modes of a GM-GCL system can transform from interface failure to GCL internal failure as normal stress increases, according to many laboratory tests [102,112,114]. The measured value of the interface shear strength depends on several factors, such as product type, normal stress, hydration fluid, hydration condition, and shear conditions. The product type is determined by the type of geotextiles (woven or non-woven) and the roughness of the geomembranes’ surface. For example, a woven geotextiles–geomembrane interface has lower strength than non-woven geotextile interfaces and smooth GM interfaces, because the bentonite is more likely to be in the former [54,110]. The extrusion of bentonite is also relevant to the normal stress applied during shear. A higher normal stress will lead to a greater amount of extrusion and a smaller interface shear strength [110,115]. If the products are pre-hydrated, the bentonite will be extruded in the interface more easily than those without predisposal of hydration, and thus will have a lower interface strength [115]. The loading sequence and loading rate also play an important role; for example, GCL/GM products subjected to swelling before loading will have obvious bentonite extrusion, while those subjected to a reverse sequence will exhibit no extrusion in the interface. The amount of extruded bentonite increases as the loading rate increases [116].

4. Recent Advancement of GCL and Perspectives

4.1. Polymer-Treated GCL Exposure to inorganic solutions can cause significant degradation of the hydraulic properties of bentonite clay in GCLs. In order to enhance the hydraulic performance and chemical resistance of GCLs, numerous bentonite modification technologies, e.g., adding glycerol carbonate or propylene carbonate, have been developed [117–119]. Recent progress has been made by adding a polyelectrolyte polymer into the bentonite clay. A polymer is a large molecule, or macromolecule, composed of many repeated sub-units. The application of polymers (macromolecule resin, polymeric fiber) in environmental geotechnics has become more popular in some areas of the world, such as Europe, America and Asia, e.g., Japan. Due to the low hydraulic conductivity and high swelling potential, a macromolecule resin in GCL as a substituted material of bentonite has been explored in Japan. The main chemical composition of the water-swelling polymer is acrylic acid, and its water-absorbing capacity can be from several to several dozen times the value of the bentonite, e.g., 1 g of polymer material can absorb a few hundred grams or even several kilograms of water. The mechanism of water swelling of the polymer is illustrated in Figure 11. However, the water-swelling capacity of acrylic materials can be impaired when in contact with sea water or other solutions with a high concentration of cations. Since there are pros and cons to both bentonite and polymer, the new concept of combining bentonite and polymer to form a high-performance anti-seepage liner in GCL has been developed in recent years. The use of polymer for enhancing the mechanical and hydraulic behaviours of clay materials has been well studied [120–123]. However, research on a polymer-treated GCL is still at a preliminary stage, and references are relatively limited. Elhajji et al. (2001) [124] shows that polymers can improve the hydraulic performance of GCL when permeated with low-concentrated Sustainability 2017, 9, 2110 12 of 18 contaminant. Tests conducted by Ashmawy (2002) [125] indicate that polymer treatment will be more beneficial if the clay is first saturated with water and not directly with the leachate. When low hydraulic conductivity is required in the short term, pre-hydration is more advantageous than polymer treatment.Sustainability 2017 Razakamanantsoa, 9, 2110 et al. (2012) [126] conducted laboratory test on the different12 types of 18 of polymer-treated GCL, and found different polymer had different effects. For example, cationic polymerpolymer cancan improveimprove thethe swellingswelling capacitycapacity ofof thethe bentonite,bentonite, whilewhile anionicanionic polymerpolymer enhancesenhances thethe hydraulichydraulic behaviorbehavior ofof thethe product.product. ComparedCompared withwith thethe untreateduntreated variety,variety, bothboth cationiccationic andand anionicanionic polymerspolymers improveimprove thethe propertiesproperties ofof thethe bentonite.bentonite. ScaliaScalia etet al.al. (2014)(2014) [[127]127] conductedconducted laboratorylaboratory teststests thatthat enabledenabled thethe bentonite–polymerbentonite–polymer compositecomposite (BPC)(BPC) toto comecome inin contactcontact withwith aggressiveaggressive inorganicinorganic leachate.leachate. TheThe resultsresults indicatedindicated thatthat BPCBPC hadhad aa betterbetter long-termlong-term anti-seepageanti-seepage performanceperformance thanthan thethe naturalnatural bentonite,bentonite, with with low low hydraulic hydraulic conductivity conductivity and and high high swelling swelling capacity. capacity. In In particular, particular, Yang Yang et al.et (2017)al. (2017) [30,45 [30,45]] proposed prop aosed SHMP-amended a SHMP-amended calcium calcium bentonite bentonite which has which been found has been to be promisingfound to inbe contaminant-containmentpromising in contaminant-containment applications due applications to its superior due resistance to its tosuperior DDL compression resistance caused to DDL by heavycompression metal and caused salt exposure.by heavy Allmetal of thisand research salt exposu showsre. thatAll of the this present research polymer-treated shows that the technology present haspolymer-treated great potential technology in future GCLhas great applications. potential in future GCL applications.

FigureFigure 11.11. MechanismMechanism ofof waterwater swellingswelling ofof polymerpolymer (modified(modified fromfrom Fujida,Fujida, 2011)2011) [[128].128].

4.2.4.2. PerspectivesPerspectives forfor FurtherFurther StudyStudy PolymerPolymer chemistrychemistry hashas experiencedexperienced greatgreat developmentdevelopment inin recentrecent years,years, andand varietiesvarieties ofof newnew polymerpolymer materialsmaterials havehave beenbeen developed.developed. BentoniteBentonite isis aa naturalnatural productproduct whosewhose componentscomponents cancan varyvary fromfrom regionregion to region. The The diversity diversity of of both both polymers polymers and and bentonites bentonites provides provides great great possibilities possibilities for forthe thefuture future exploration exploration of a of polymer–bentonite a polymer–bentonite composite composite that that provides provides a high-performance a high-performance GCL. GCL. For Forfurther further study, study, the the microstructure microstructure of ofthe the polyme polymer-treatedr-treated bentonite bentonite needs needs to to be investigatedinvestigated inin comprehensivecomprehensive andand systematicsystematic ways.ways. TheThe microstructuremicrostructure ofof thethe polymer–bentonite compositecomposite createdcreated byby usingusing thethe technologytechnology of of free-radical free-radical polymerization, polymerization, with with its its relationship relationship to to the the macro macro behaviors behaviors of PBCof PBC (e.g., (e.g., hydraulic hydraulic conductivity, conductivity, water-swelling water-swelling capacity, capacity, chemical chemical compatibility, compatibility, self-healing) self-healing) should beshould further be investigated.further investigated.

5.5. ConclusionsConclusions ThisThis paperpaper providesprovides aa systematicsystematic summarysummary ofof researchresearch onon geosyntheticgeosynthetic clayclay linersliners overover thethe pastpast decades,decades, andand thethe followingfollowing conclusionsconclusions cancan bebe drawn:drawn: (a)(a) Research on geosynthetic clay liners has experiencedexperienced rapid growth over the past few decades. Numerous laboratory investigationsinvestigations and fieldfield teststests havehave beenbeen undertakenundertaken toto examineexamine thethe performance of geosynthetic clay liners, such as their hydraulic conductivity and chemical compatibility, water-swelling and self-healing capacity, diffusion, gas migration, and shear strength. These properties are found to be affected by a variety of factors, e.g., structural types, permeant solution, hydration condition, confining pressure, environmental factors (temperature, hydraulic head, etc.), which can vary from case to case. It is of great importance to assess the properties of GCL on a site-specific basis. Sustainability 2017, 9, 2110 13 of 18

performance of geosynthetic clay liners, such as their hydraulic conductivity and chemical compatibility, water-swelling and self-healing capacity, diffusion, gas migration, and shear strength. These properties are found to be affected by a variety of factors, e.g., structural types, permeant solution, hydration condition, confining pressure, environmental factors (temperature, hydraulic head, etc.), which can vary from case to case. It is of great importance to assess the properties of GCL on a site-specific basis. (b) With the wide use of GCLs in landfills and other geotechnical applications, more problems concerned with complicated environments, such as moisture-cycles, freeze-thaw cycles, thermal cycles, long-term exposure to solar radiation, GCL-lined slopes, can be encountered. Such particular environments may significantly influence the long-term hydraulic performance and durability of GCLs. Although primary studies on the impact of these factors can be found in some publications, the mechanism by which complicated conditions influence the physical and chemical properties of GCL components needs to be further investigated. (c) Polymer-treated technology has shown great potential for future GCL applications. The expansion capacity of bentonite in GCLs decreases when permeating aggressive leakages with high cation concentration. The addition of superabsorbent polymers, which have much higher resistance to aggressive leakages, can make up for the deficiency of bentonite and greatly improve hydraulic performance and self-healing capacity. Research on polymer-treated GCLs is still in its preliminary stages. Further study needs to be conducted on the microstructure of polymer–bentonite composite created by using the technology of free radical polymerization as well as on the macro behaviors of polymer–bentonite composites (e.g., hydraulic conductivity, water-swelling capacity, chemical compatibility, diffusion, gas migration, and shear strength).

Acknowledgments: This research work was funded by the National Nature Science of China (NSFC) (Grant No. 51578333). This financial support is gratefully acknowledged. Author Contributions: This paper represents a result of the collaborative teamwork. Huai-Na Wu and De-Jun Kong collected the data sources, De-Jun Kong wrote the manuscript, Jin-Chun Chai and Arul Arulrajah provided constructive suggestions for the manuscript. The four authors contributed equally to this work. Conflicts of Interest: The authors declare no conflict of interest.

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