0000013

3. FLEXIBLE MEMBRANE LINERS

IntroductionThis chapter discusses several material and designconsiderations for flexible membrane liners (FMLs).It highlights some of the problems encountered indesigning "bathtub** systems for hazardous wastelandfills and describes the impact of proposedregulations on material and design considerations,

Composite Liners: Clay versus SyntheticComponentsAfter a landfill site has been chosen and a basin hasbeen excavated, the basin is lined with one or morelayers of water-retaining material (liners) that forma "leachate bathtub.** The contained leachate ispumped out through a network of pipes and collectorlayers. Liners may be constructed of syntheticpolymer sheets or of clay. EPA's min imumtechnology guidance (discussed in Chapter One)relies on a composite liner that utilizes advantagesobtained from combining both liner systems.

Understanding the basic hydraulic mechanisms forsynthetic liners and clay liners is very important inappreciating the advantages of a composite liner.Clay liners are controlled by Party's law (Q = kiA)4 Darcy's law is discussed in more detail in ChapterTwo). In clay liners, the factors that most influenceliner performance are hydraulic Head and toil per me-ability. Clay liners have a higher hydraulicconductivity and thickness than do synthetic liners.Additionally, leachate leaking through a clay linerwill undergo chemical reactions that reduce theconcentration of contaminants in the leachate.

Leakage through a synthetic liner is controlled byPick's first law, which applies to the process of liquiddiffusion through the liner membrane. The diffusionprocess is similar to flow governed by Darcy's lawexcept it is driven by concentration gradients and notby hydraulic head. Diffusion rates in membranes arevery low in comparison to hydraulic flow rates evenin clays. In synthetic liners, therefore, the factor thatmost influences liner performance is penetration*.

Synthetic liners may have imperfect seams orpinholes, which can greatly increase the amount ofleachate that leaks out of the landfill.

Clay liners, synthetic liners, or combinations of bothare required in landfills. Figure 3-1 depicts thesynthetic/composite double liner system that appearsin EPA's minimum technology guidance. The systemhas two synthetic flexible membrane liners (FMLs):the primary FML, which lies between two leachatecollection and removal systems (LCRS), and thettcondary FML, which overlies a compacted clayliner to form a composite secondary liner. The ad-vantage of the composite liner design is that byputting a fine grain material beneath the membrane,the impact of given penetrations can be reduced bymany orders of magnitude (Figure 3-2). In the figure,QC is the inflow rate with gravel and Qc is the inflowrate with clay.

Figure 3-3 is a profile of a liner that appeared in anEPA design manual less than a year ago. Thissystem is already dated. Since this system wasdesigned, EPA has changed the minimum hydraulicconductivity in the secondary leachate collectionsystem from 1 x 10-* cm/sec to 1 cm/sec to improvedetection time. To meet this requirement, eithergravel or a net made of synthetic material must beused to build the secondary leachate collectionsystem; in the past, sand was used for this purpose.

Material ConsiderationsSynthetics are made up of polymers—natural orsynthetic compounds of high molecular weight.Different polymeric materials may be used in theconstruction of FMLs:

• Thermoplastics—polyvinyl chloride (PVC)• Crystalline thermoplastics—high density poly-

ethylene (HOPE), linear low density polyethylene(LLDPE)

*> O

33

2o - 0.033 u27

Pnmary FML-i - • "v,

SecooaryFML V

Q OranPip»» Q

(NotwScaiet

Figure 3-t. Synthetic'composfte double liner system.

..;:'"^ Soww• ~

Hy*au*c Conductvity^-*

PnmaryLCR

- Pnmary FML

Secondary LCR

Secondary FMi

Hydraukc Conductrvtiy

O11

Hyoraukc Conducuwtys 1 x ic- cnv«c

conductivity * now 1 cnvtec.

,-10*0,.

Figure 3-2. Advantage of composite ftner,

• Thermoplastic elastomers—chlorinated poly-ethylene (CPE), chiorylsulfonated polyethylene(CSPE)

• Elastomers—neoprene, e thy I en e propylene dienemonomer(EPDM) .' •

Typical compositions of polymeric geomembranesare depicted in Table 3-1. As the table shows, themembranes contain various admixtures such as oilsand fillers that are added to aid manufacturing of theFML but may affect future performance. In addition.many polymer FMLs will cure once installed, and the therefore to select polymers for FML constructionstrength and elongation characteristics of certain with care. Chemical compatibility, manufacturingFMKs will change with time. It is important considerations, stress-strain characteristics, sur-

F)Qjur«3-3. Profile ot MTG double Uner system.

28

vivability, and permeability are some of the keyissues that must be considered.

Chemical CompatibilityThe chemical compatibility of a FML with wa«tcleachate is an important material consideration.Chemical compatibility and EPA Method 9090 testsmust be performed on the synthetics that will beused to construct FMLs. (EPA Method 9090 tests arediscussed in more detail in Chapter Nine.) Unfor-tunately, there usually is a lag period between thetime these tests are performed and the actualconstruction of a facility. It is very rare that at thetime of the 9090 test, enough material is purchasedto construct the liner. This means that the materialused for testing is not typically from the sameproduction lot as the synthetics installed in the field.

The molecular structure of different polymers can beanalyzed through differential scanning calorimeteror thermogravimetric testing. This testing or"fingerprinting'* can ensure that the same materialused for the 9090 test was used in the field. Figure 3-4 was provided by a HOPE inanufacturer, and thefingerprints depicted are all from high densitypolyethylenes. Chemical compatibility of extrusionwelding rods with polyethylene sheets is also aconcern.

Manufacturing ConsiderationsPolyethylene sheets are produced in various ways:

• Extrusion—HOPE• Calendaring—PVC• Spraying—U re thane

In general, manufacturers are producing highqual i ty polyethylene sheets. However, the

Table* 3-t. Bwc CompotrOon ol Polynwtc G»om«mbran«

iSO'C. SOO ps»g

26

24

II 2°

Iu.

I

ifi-

a

4

A •

/ /•'

. X - . , , , _ _ , _ _ _6 10 20 30 40 50 60 70 SO 90 100 110 120

Figure 3-4. Companion of "fingerprints" of exothermic p«aftshapes.

compatibility of extrusion welding rods and highdensity polyethylene sheets can be a problem. Somemanufacturing processes can cause high densitypolyethylene to crease. When this material creases,stress fractures will result. If the material is takeninto the field to be placed, abrasion damage willoccur on the creases. Manufacturers have beenworking to resolve this problem and, for the mostpart, sheets of acceptable quality are now beingproduced.

Stress-Strain CharacteristicsTable 3-2 depicts typical mechanical properties ofHOPE, CPE, and PVC. Tensile strength is a

Component CrosstmkM

CornpoMon of Compound Type____< parti 0? *»yrt)_____

Thermoplastic SamcrystaftrwPorymer or aHoyOri or piasacuer

Carton Btac*tnorgancs

AnftdegradantsCrosshrtfcing sysmm:

Inonjar-c systemsystem

1005^05-40540

1-2

5-95-9

1005-55540540

1-2

1000-102-5

Souror Haxo. H. E. 1986. OuaMy Assurance of Geomemoranes Used as Unrigs tor Hazardous Waste Contanmem. In: GcoeitiicsGeomembranes. Vol. 3. No. 4 London. England.

29

fundamental design consideration. Figure 3-5 showsthe uniaxial stress-strain performance of HDPE,CPE, and PVC. As 600.800. 1,100, and 1,300 percentstrain is developed, the samples fail. When biaxialtension is applied to HDPE, the material fails atstrains less than 20 percent. In fact, HDPE can fail atstrains much less than other flexible membraneswhen subjected to biaxial tensions common in thefield.

Another stress-strain consideration is that highdensity polyethylene, a material used frequently athazardous waste facilities, has a high degree ofthermal coefficient of expansion - three to four timesthat of other flexible membranes. This means thatduring the course of a day (particularly in thesummer), 100-degrees-Fahrenheit (°F) variations inthe temperature of the sheeting are routinelymeasured. A 600-foot long panel, for example, maygrow 6 feet during a day.

SurvivabilityVarious tests may be used to determine thesurvivability of unexposed polymeric geomembranes(Table 3-3). Puncture tests frequently are used toestimate the survivability of FMLs in the field.During a puncture test, a 5/16 steel rod with roundededges is pushed down through the membrane. A veryflexible membrane that has a high strain capacityunder biaxial tension may allow that rod topenetrate almost to the bottom of the chamberrupture. Such a membrane has a very lowpenetration force but a very high penetration elonga-tion, and may have great survivability in the field.High density polyethylenes will give a very highpenetration force, but have very high brittle failure.Thus, puncture data may not properly predict Heldsurvivability,

PermeabilityPermeability of a FML is evaluated using the WaterVapor Transmission test (ASTM E96). A sample ofthe membrane is placed on top of a small aluminumcup containing a small amount of water. The cup isthen placed in a controlled humidi ty andtemperature chamber. The humidity in the chamberis typically 20 percent relative humidity, while thehumidity in the cup is 100 percent. Thus, aconcentration gradient is set up across themembrane. Moisture diffuses through the membraneand with time the liquid level in the cup is reduced.The rate at which moisture is moving through themembrane is measured. From that rate, thepermeability of the membrane is calculated with thesimple diffusion equation (Pick's first law). It isimportant to remember that even if a liner isinstalled correctly with no holes, penetrations.

punctures, or defects, liquid will still diffuse throughthe membrane.

Design ElementsA number of design elements must be considered inthe construction of flexible membrane liners: (1)m i n i m u m technology guidance, (2) stressconsiderations, (31 structural details, and (4) panelfabrication.

Minimum Technology GuidanceEPA has set minimum technology guidance for thedesign of landfill and surface impoundment liners toachieve de mini mis leakage. De minimis leakage is 1gallon per acre per day. Flexible membrane linersmust be a minimum of 30 mils thick, or 45 mils thickif exposed for more than 30 days. There may,however, be local variations in the requirement ofminimum thickness, and these variations can havean impact on costs. For example, membranes costapproximately $.01 per mil per square foot, so thatincreasing the required thickness of the FML from30 mils to 60 mils, will increase the price $.30 centsper square foot or $12,000 per acre.

StressStress considerations must be considered for sideslopes and the bottom of a landfill. For side slopes,self-weight (the weight of the membrane itself) andwaste settlement must be considered; for the bottomof the facility, localized settlement and normalcompression must be considered.

The primary FML must b« able to support its ownweight on the side slopes. In order to calculate self-weight, the FML specific gravity, friction angle,FML thickness, and FML yield stress must be known(Figure 3-6).

Waste settlement is another consideration. As wastesettles in the landfill, a downward force will act onthe primary FML. A low friction component betweenthe FML and underlying material prevents thatforce from being transferred to the underlyingmaterial, putting tension on the primary FML. A 12-inch direct shear test is used to measure the frictionangle between the FML and underlying material.

An example of the effects of waste settlement can beillustrated by a recent incident at a hazardous wastelandfill facility in California. At this facility, wastesettlement led to sliding of the waste, causing thestand pipes (used to monitor secondary leachatecollection sumps) to move 60 to 90 feet downs lope in1 day. Because there was a very low coefficient offriction between the primary liner and the geonet,the waste (which was deposited in a canyon) sliddown the canyon. There was also a failure zonebetween the secondary liner and the clay. A two-

30

TaWe 3-2. Typical Mechanical Properties

HOPE CPE PvcDensity, ynvcm3

Thermal coeffioeni of expansoiTen** strength, p*Puncture. Ibrtml

> 93512.5* 10*

480028

1 .3 - 1 374 i 10 *

1«00

1.2

1.24 • 1.3

3* 10-*22002.2

550.

4000

3000 -

2000

1000

To 3860 PSI at 1160%

300

Figure 3-5. FML «tre*«-ttra*n performance(uniaxial-Koemer. Richardson; biaaiai-Steffen).

dimensional slope stability analysis at the siteindicated a factor of safety greater than one. A three-dimensional slope stability analysis, however,showed the safety factor had dropped below one.Three-dimensional slope stability analyses shouldtherefore be considered with canyon and trenchlandfills.

Since more trenches are being used in double FMLlandfills, the impact of waste settlement along suchtrenches should be considered. Figure 3-7 is a simpleevaluation of the impact of waste settlement alongtrenches on the FML. Settlements along trencheswilt cause strain in the membrane, even if the trenchis a very minor ditch. Recalling that when biaxialtension is applied to high density polyethylene, thematerial fails at a 16 to 17 percent strain, it is

possible that the membrane will fail at a moderatesettlement ratio.

Another consideration is the normal load placed onthe membranes as waste is piled higher. Many of thenew materials on the market, particularly some ofthe linear low density polyethylene (LLDPE) liners,will take a tremendous amount of normal loadwithout failure. The high density polyetfaylenes, onthe other hand, have a tendency to high brittlefailure.

Structural DetailsDouble liner systems are more prone to defects in thestructural details (anchorage, access ramps,collection stand pipes, and penetrations) than singleliner systems.

31

Table 1-3. Ten Method* for Unexpoted Potymenc Geomembrane* —

Membrane Liner Without Fabric ReinforcementProperty

AnaMicat ProoernesVoiaMes

Eitractabies

Ash

Specific gravity

Thermal analysis:Differential scanning

catorimevy (DSC)Thermogravirnetry

(TGA)

Physical ProoertocsThickness - totalCoating over fabncTens*- — -)e*?te3

Tear resistance

Modulus of elasticityHartf-ttSS

Puncture restftance

Hydrostatic resistanceSeam strength:

tn shear

in peel

Piyaohesen

Environmental and ApjnoEffectsOzone crackingEnvironmental stress

cracktigLow temoerature testing

Thermoplastic

MTM-1*

MTM-2*

ASTM 0297. Section 34

ASTM 0792. Method A

NA

Yes

ASTM 0638NA

ASTM 0882.ASTM 0636

ASTM 01004(modrfieO)NA

ASTM 02240Duro A or 0FFMS 101B.Method 2065NA

ASTM 0882. Method A(modified)

ASTM 0413. MachMethod Type 1(modified)NA

ASTM 01 149NA

ASTM 01 790

Crossbnked

MTM-1*

MTM-2*

ASTM 0297. Section 34

ASTM 0297. Section 15

NA

Yes

ASTM 041 2

NA

ASTM 0412

ASTM 0624

NAASTM 022*0Duro A or 0FTMS 101 B.Method 2069NA

ASTM 0882. MetlCd A(modified)

ASTM 0413. MachMethod Type 1(modified)NA

ASTM D1149NA

ASTM 0746

Semoystattne

MTM-1*

MTM-2*

ASTM D297. Section 34

ASTM D792. Method A

Yes

Yes

ASTM 0638NA

ASTM 0638(matted)

ASTM 01004D*CASTM D882. Method AASTM 02240Duro A or 0FTMS 1016.Method 2065ASTM 0751. Method A

ASTM 0682. Method A(mod*ed)

ASTM 0413. MachMethod Type i(morjk&ed)NA

NA

ASTM 01693

ASTM D1790ASTM D746

Faorc Remforced

MTM-1*ton setvage andreinforced sheeting)MTM-2*(on servage andreinforced Sheeting)ASTM 0297. Section 34ion selvage)ASTM 0792. Method A(on setvage)

NA

Yes

ASTM 0751, Section 6

Optical methodASTM 0751. Method Aand B (ASTM 0638 onsefvage)ASTM 0751. Tonguemethod (modibed)NA

ASTM 02240 Ouro Aor 0 4 setvage only)FTMS 101 B.Methods 2031 and 2069ASTM 0751. Method A

ASTM 0751. Method A(modified)

ASTM O41 3. MachMethod Type 1(modified}ASTM 0413. MachMethod Type 1ASTM O7S1. Sector*39-42

ASTM Ol 149NA

ASTM 02136

Ten** prooene* atelevated temperature

Dimenscnai staWity ASTM 01204 ASTM 01204 ASTM 01204

) ASTM 0751 Method B(modrfwd)ASTM 01204

32

Table 3-3. Test Methods tor Unexposed Polymeric Geomwnoranet (contlmMd)

Membrane Uner Without Fabnc fletnter cementProosrty Tnermooiastc Crosshnked SemcrysuNmt Fabnc Re«ntorced

A*-ov*n ag*g ASTM 0573 (oxxHied) ASTM 0973 (moAted) ASTM 0573 (modified) ASTM DS73 imodfted)Water vapor transmission ASTM E96. Method BW ASTM E96. MBlfcod BW ASTM £96. Meffxx) BW ASTM E96, Mettod BWWater aosorptnn ASTM 0970 ASTM Q*7i ASTM DS70 ASTM 0570Immersionin standard ASTM 0471. 0543 ASTM 0471 ASTM 0543 ASTM 0471. 0543

Immersoi m wastetqurtS

Sod bundOuttoor exposure

TuDWst

ERA 9090

ASt M 03083ASTM 04364ft •

EPA 9090ASTM 030*3ASTM 04364b

EPA 9090

ASTM 03083ASTM 04364b

EPA 9090ASTM 03083ASTM 043646

•See reference (8)»See reference II2).NA • not spohcabie.Source: Haxo, 1967

Cell Component:

Required Material PropertiM Range Ttet Standard

Analy** Proceduret«it FML H«Vi

M*.. *»*

PM. ff, /»•

Design Ratio: References:

Example:

LJ= f o«*. »*J 4.« T* * iff tt

(fi

G'-

P t *

pc . «••/,» - lit

Example No.

Figure 3-6. Calculation ol seM-wotght

5. SECURING A COMPLETED LANDFILL

IntroductionThis chapter describes the elements in a closure orcap system of a completed landfill, including flexiblemembrane caps, surface water col lect ion andremoval systems, gas control layers, biotic barriers,and vegetative top covers. It also discussesinfiltration, erosion control, and long-term aestheticconcerns associated with securing a completedlandfill.

Figure 5-1 shows a typical landfill profile designed tomeet EPA's proposed minimum technology guidance(MTG) requirements. The upper sub profile comprisesthe cap. or -over, and includes the required 2-footvegetative tup cover, I-foot lateral drainage layer,and low permeability cap of barrier soil (clay), whichmust be more than 2 feet thick. This three-tiersystem also includes an optional flexible membranecap and an optional gas control layer. The guidanceoriginally required a 20-mil thick flexible membranecap, but EPA currently is proposing a 40 milminimum.

Flexible Membrane CapsFlexible membrane caps (FMCs) are placed over thelow permeable clay cap and beneath the surfacewater collection and removal (SWCR) system. FMCsfunction primarily in keeping surface water off thelandfill and increasing the efficiency of the drainagelayer. EPA leaves operators with the option ofchoosing the synthetic material for the FMC thatwill be most effective for site-specific conditions. Inselecting materials, operators should keep in mindseveral distinctions between flexible membraneliners (FMLs) and FMCs. Unlike a FML, a FMCusually is not exposed to leachate. so chemicalcompatibility is not an issue. Membrane caps alsohave low normal stresses acting on them incomparison with FMLs, which generally carry theweight of the landfill. An advantage FMCs have overliners is that they are much easier to repair, becausetheir proximity to the surface of the facility makesthem more accessible. FMCs wi l l , however, be

subject to greater strains than FMLs due tosettlement of the waste.

Surface Water Collection and Removal(SWCR) SystemsThe SWCR system is built on top of the flexiblemembrane cap. The purpose of the SWCR system isto prevent infiltration of surface water into thelandfill by containing and systematically removingany liquid that collects within it. Actual designlevels of surface water infiltration into the drainagelayer can be calculated using the water balanceequation or the Hydro logic Evaluation of LandfillPerformance (HELP) model. (A more detaileddiscussion of HELP is contained in Chapter Four.)Figure 5-2 shows the results of two verificationstudies of the HELP model published by EPA.

Errors in grading the perimeter of the cap oftenintegrates (or cross-connects) the SWCR system withthe secondary leak detection and removal system,resulting in a significant amount of waterinfiltrating the secondary detection system. Thissituation should be remedied as soon as possible if itoccurs. Infiltration of surface water is a particularconcern in nuclear and hazardous waste facilities,where gas vent stacks are found. A coi.tainmentsystem should be designed to prevent water fromentering the system through these vents.

In designing a SWCR system above a FMC, threeissues must be considered: (1) cover stability, (2)puncture resistance, and (3) the ability of the closuresystem to withstand considerable stresses due to theimpact of settlement. Figure 5-3 illustrates theeffects of these phenomena.

Cover StabilityThe stability of the FMC supporting the SWCRsystem can be affected by the materials used toconstruct the drainage layer and by the slope of thesite. In some new facilities, the drainage layer is ageonet placed on ton of the flexible membrane cap.

AncnorSecondary Anchor Trencn

Cap Ancfxx Trencrt

Figure 5-1 Typical geosynthetic cell profile.

with the coefficient of friction between those twoelements being as low as 8 to 10 degrees Such lowfriction could allow the cover to slide. One facility atthe Meadowlands in New Jersey is constructed on ahigh mound having side slopes steeper than 2:1. Inorder to ensure adhesion of the membrane to the sideslopes of the facility, a nonwoven geotextile wasbonded to both sides of the FMC. Figures 5-4 and 5-5give example problems that evaluate the slidingstability of a SWCR system in terms of shearcapacities and tensile stress.

Puncture ResistanceFlexible membrane caps must resist penetration byconstruction equipment, rocks, roots, and othernatural phenomena. Traff ic by operationalequipment can cause serious tearing. A geotextileplaced on top of or beneath a membrane increases itspuncture resistance by three or four times. Figure 5-6 shows the results of puncture tests on severalcommon geotextile/membrane combinat ions.Remember, however, that a geotextile placedbeneath the FMC and the clay layer will destroy thecomposite action between the two. This will lead toincreased infiltration through penetrations in theFMC.

Impact of SettlementThe impact of settlement is a major concern in thedesign of the SWCR system. A number of facilitieshave settled 6 feet in a single year, and 40 feet ormore over a period of years. The Meadowlands site inNew Jersey, for example, was built at a height of 95feet, settled to 40 feet, and then was rebuilt to 135feet. Uniform settlement can actually be beneficial

by compressing the length of the FMC and reducingtensile strains. However, if waste does not settleuniformly it can be caused by interior berms thatseparate waste cells. '

(n one current closure site in California, a wastetransfer facility with an 18-foot wall is being builtwithin a 30-foot trench on top of a 130-foot highlandfill. The waste transfer facility will settle fasterthan the adjacent area, causing tension at the edge ofthe trench. Electronic extensometers are proposed atthe tension points to check cracking strains in theclay cap and FMC.

Settlements can be estimated, although the marginfor error is large. Secure commercial hazardouswaste landfills have the smallest displacement, lessthan 1.5 percent. Displacements at new larger solidwaste landfills can be estimated at 15 percent, whileolder, unregulated facilities with mixed wastes havesettlements of up to 50 percent. Figure 5-7 gives anexample problem showing how to verify thedurability of a FMC under long-term settlementcompression.

Gas Control LayerGas collector systems are installed directly beneaththe low permeability clay cap in a hazardous wastelandfill. Landfills dedicated to receiving onlyhazardous wastes are relatively new and gas hasnever been detected in these systems. It may take 40years or more for gas to develop in a closed securehazardous waste landfill facility. Because the long-term effects of gas generation are not known, and

76

EDPM.75 mm PUC.75 mm

6 osy GeoteaMe ?

I Geornernorane Atone0 GeotexUe Bom S«oes

CPE.75 mm HOPE.75 mm S Geotertle Front9 Geoiejnie Bach

i2oty Geotwwe

EOPM.75 mm PUC.75 mmCPE.75 mm HOPE. 75 mm

'EOPM.75 mm PUC.75 mmCPE.75 mm HOPE.75 mm

•Geomembrane AloneOGaotwwe Both Sides

Front

ia osy GaotaxWe

EOPM.75 mm PUC.75 mmCPE.75 mm HOPE.75 mm

1 N * .225 to

a. Puncture Resistance

(Koemer. 1986)

' EDPM.75 mm PUC.75 mmCPE.75 mm MOPE 75 mm

1 J - 738 THb

b. Impact Resistance

Fiqurt 5-6. Puncture and impact resistance of common FML»

jvc -

1500

6 osy GeoiexUe

• Geomemtyane AloneEDPM.7S mm PUC.7S mm Q GeoiexUe Bom Soes

CPE.75 mm HDP6.73 mm z Geoteate Front8 Geofexnift Back

t2oiyGeotaxue

EDPM.75 mm PUC-75~mmCPE.75 mm HOPE.75 mm

'EDPM.75 mm PUC.75 mmCPC.75 mm HDPE.75 mm

•Geomembrane AloneQGeoiexMeBoth Sides• Geottxtte FrontSGeoiairMeBack

EDPM.75 mm PUC.75 mmCPE.75 mm HDPE.75 mm

1 N - .225 to.

a. Puncture Resistance

(Koemer, '986)

' EDPM.75 mm PUC.75 mmCPE.75 mm HOPE. 75 mm

1 J « 738tHb

b. impact Resistance

Figure S-«, Puncture and Impact resistance of common FMLa.

82

8. LINER COMPATIBILITY WITH WASTES

IntroductionThis chapter discusses chemical compatibility(resistance) of geosynthetic and natural linermaterials with wastes and teachates. Even in arelatively inert environment, certain materialsdeteriorate over time when exposed to chemicalscontained in bcth hazardous and nonhazardousleachate. It is important to anticipate the kind andquality of leachate a site will generate and selectliner materials accordingly. The chemical resistanceof any flexible membrane liner (FML) materials,geonets, geotextiles, and pipe should be evaluatedbefore installation.

Chemical compatibility testa using EPA Method9090 should always be performed for hazardouswaste sites, but some municipal waste sites alsocontain hazardous, nondegradable materials. EPAconducted a 5-year study of the impact of municipalrefuse on commercially available liner materials andfound no evidence of deterioration within thatperiod. However, in a current study of leachatequality in municipal landfills, the Agency hasdiscovered some organic chemical constituentsnormally found in hazardous waste landfill facilities.Apparently small quantities of household hazardouswaste enter municipal sites or are disposed of assmall quantity generator wastes. As a result of thesefindings, EPA is developing a position on the need forchemical compatibility tests for thousands ofmunicipal waste disposal sites.

In general, cover materials, including membranesand geosynthetics. do not need to be checked forchemical compatibility since these materials do not.encounter leachates. Research data indicate that thepredominant gases coming from municipal sites aremethane, hydrogen, and carbon dioxide, although afew others may be emitted from household hazardouswaste. These gases pass through cover materials bydiffusion and evidence to date indicates that theyhave caused no deterioration of membranes. Also,chemical compatibility of cover materials with gases

has not been a major problem at hazardous wastefacilities.

A primary objective of chemical compatibility testingis to ensure that liner materials will remain intactnot just during a landfill's operation but also throughthe post-closure period, and preferably longer It isdifficult, however, to predict f u tu r e chemicalimpacts. There is no guarantee that liner materialsselected for a site today will be the same as materialsmanufactured 20 years from now For example, thequality of basic resins has improved considerablyover the last few years.

The wastes themselves also change over time. Testsshould be performed to ensure that landfill leachatewill not permeate the liner layer. EPA recommends avariety of physical property degradation tests,including a fingerprint program of thermo-grtvimetric analysis, differential scanningcalorimetric tests, and infrared analysis.Fingerprinting involves analyzing the molecularstructure of the leachate components. Sometimes aparticularly aggressive leachate component can beidentified by evaluating the fingerprint analysistests after exposure of the membrane to the leachate.

Exposure ChamberThe first area of concern in chemical compatibilitytesting is the exposure chamber used to hold theleachate and membranes being tested. The exposurechamber tank can be made of stainless steel,polyethylene, glass, or a variety of other materials.Any geosynthetic liner material being consideredmust be tested for chemical compatibility with theleachate. Some leachates have caused rusting anddeterioration of stainless steel tanks in the past, andif polyethylene is being evaluated, the Uuk shouldbe of another type of material to prevent competitionbetween the tank material and the test specimen foraggressive agents in the leachate.

The conditions under which the material is testedare crucial. The top of the exposure chamber must be

109 Preceding page blank

scale*, and the tank should contain no free air spaceA stirring mechanism in the tank keeps the leachatemixture homogeneous and a heater block keeps it atan elevated temperature as required for the test.Stress conditions of the material in the Held alsoshould be simulated as closely as possible. Theoriginal EPA Method 9090 test included a rack tohold specimens under stress conditions but wasrevised when some materials shrank in the leachateDue to the hazardous nature of the material, testingshould be performed in a contained environment andsafety procedures should be rigorously followed.

In some cases a sump at the waste managementfacility can be used as an exposure chamber if it islarge enoilkh. The designer of a new landfill site candesigifa slWhtly larger sump especially for thispurpose However, since the temperature of a sumpis colder than room temperature (55*F instead of72T), the geosynthetics need to be exposed for alonger period of time. Instead of 120 days, the testmight take 6 months to a year or longer.

Representative LeachateIt is important that the sample being tested isrepresentative of the leachate in the landfill.Leachate sampled directly from a sump is usuallyrepresentative, but care .must be taken not to mix itduring removal. This will disturb the sample'shomogeneity and may result in componentsseparating out. Another problem is that municipalsolid waste landfill leachate will start to oxidize assoon as it leaves the sump and probably should besampled under an inert atmosphere.

A sampler should be familiar with the source of allthe leachate at a site before removing a sample. Ifradioactive materials are present, extra care must betaken.At some existing waste management facilities,operators have placed coupons of geosyntheticmaterials into sump areas to monitor leachateeffects. Information gathered from this monitoringprocedure provides an excellent data base. Regularrecording of data allows the operator to discovercompatibility problems as they develop, rather thanwaiting until a landfill liner fails. If the couponshows early signs of deterioration, the operator canrespond immediately to potential problems in thefacility.

When planning construction of a new site, anoperator first assesses the market to determine thequantity and quality of waste the landfill willreceive Representative leachate is then formulatedbased on the operator's assessment.

The Permit Applicant's Guidance Manual forTreatment, Storage, and Disposal (TSD) facilities

contains addi t ional i n f o r m a t i o n on leachaterepresentativeness (see Chapter 5. pp. 15-17;Chapter6, pp. 18-21; and Chapter 8, pp. 13-16).

Compatibility Testing of ComponentsGeosyntheticsEPA's Method 9090 can be used to evaluate allgeosynthetic materials used in liner and leachatecollection and removal systems currently beingdesigned. Method 9090 is used to predict the effectsof leachate under field conditions and has beenverified with limited field data. The test is performedby immersing a geosynthetic in a chemicalenvironment for 120 days at two differenttemperatures, room and elevated. Every 30 days,samples are removed and evaluated for changes inphysical properties. Tests performed on FMLs arelisted in Table 8-1. The results of any test should becross-referenced to a second, corollary test to avoiderrors due to the test itself or to the laboratorypersonnel.

S-1. Chemical Compttbttty Tests for FMU

• Hardness• ' MeRtodtx

• VotaMLoss• PatfAdneaon

• Spaofic Gravity

• Waiar AMXMon

*____ _ \• Dtfnenaona) StabMy \• Modufcis of BaaKrty• Bohdad S«am Srtngt*• Hydronafec Raaflttnoa• Carton Btadt Doparsnn• Tncknastt LanQffv Wrth• Tana«» at YwM and Braak• Enwonmenial Stew C-^ .• Etongakon at Yiaid and Bauk

Physical property tests on geotextiles and geonetsmost be designed to assess different uses, weights,and thicknesses of these materials, as well asconstruction methods used in the field. CPA has alimited data base on chemical compatibility withgeotextiles. Some tests for geonets and geotextilesrecommended by EPA are listed in Table 8-2. TheASTM D35 Committee should be consulted forinformation on the latest testing procedures.

no

S-2 Chemical Compatibrtrty Tests tor

• Puncture• Thickness

• Transnwsivity• Mass/Unit Area• Burst Strertgm• Abrasive Resistant• Percent Open ATM

• Grab Tensite/Etongason• Equn/atent Operung Soe• Hydrostaoc Bursbng Sb

• Teanng Strength (Trapezotial)• Compression B«Aavor/Crusrt Strength

Until recently, EPA recommended using I 1/2 timesthe expected overburden pressure for inplanetransmissivity tests. Laboratory research, however,has revelled that creep and intrusion cause a loss oftransmiumty. so the Agency has amended itsrecommendation to 2 to 3 times the overburdenpressure. EPA also recommends that the geotextileor geonet be aged in leachate, but that the actual testbe performed with water. Performing the test withthe leachate creates too great a risk of contaminationto test equipment and personnel. The transmissivitytest should be run for a minimum of 100 hours. Thetest apparatus should be designed to simulate thefield conditions of the actual cross section as closelyas possible.

PipesThe crushing strength of pipes also should be tested.There have been examples where pipes in landfillshave actually collapsed, and thus forced the site tostop opsrating. The ASTM D2412 is used to measurethe strength of pipe materials.

Natural Drainage MaterialsNatural drainage materials should be tested toensure that they will not dissolve in the leachate orform a precipitant that might clog the system. ASTMD2434 will evaluate the ability of the materials toretain permeability characteristics, while ASTMD1883 testa for bearing ratio, or the ability of thematerial to support the waste unit.

Blanket ApprovalsEPA does not grant "blanket approvals" for anylandfill construction materials. The quality of linermaterials varies considerably, depending on

quantities produced and on the manufacturer. Evenhigh density polyethylene (HOPE) does not receiveblanket approval The Agency, together with a groupof chemists, biologists, and researchers from theliner manufacturing industry, determined thatHOPE varies slightly in its composition amongmanufacturers. Because different calendaring aids,processing aids, or stabilizer packages can changethe overall characteristics of a product, eachmaterial should be individually tested.

Landfill designers should select materials on thebasis of site-specific conditions, as the composition ofspecific leacnates will vary from site to site. Adesigner working with the operator determines inadvance of construction what materials will be mosteffective. In recent years, EPA has restricted certainwastes, including liquids, from land disposal. Theseregulations have expanded the number of potentialcandidate materials, thus allowing more flexibilityto landfill designers.

Interpreting DataWhen liner material test data show the rate ofchange of the material to be nil over a period of time,then the membrane is probably not undergoing anychemical change. There have been instances,however, in which a material was tested for a yearwithout change and then suddenly collapsed. Forthis reason, the longer the testing process cancontinue, the more reliable the data will be. Whentest data reveal a continuous rate of change, then thematerial is reacting with the leachate in some way. Ifthe data show an initial continuous rate of changethat then tapers off, new leachate may need to beadded more often. In any case, the situation shouldbe studied in more detail.

A designer should consult with experts to interpretdata from chemical compatibility tests. To meet thisneed, EPA developed a software system calledFlexible Membrane Liner Advisory Expert System(FLEX) to assist in evaluating test data. FLEX is anexpert system that is based on data from manychemical compatibility tests and containsinterpretations from experts in the field.

Ill

9. LONG-TERM CONSIDERATIONS: PROBLEM AREAS AND UNKNOWNS

IntroductionThis chapter presents an overview of long-termconsiderations regarding the liner and collectionsystems of hazardous waste landfills, surfaceimpoundments, and waste piles. Included in thediscussion are flexible membrane liner and clay linerdurability, potential problems in leachate collectionand removal systems, and disturbance and aestheticconcerns in caps and closures.

In judging the impact of any facility, site-specificconditions such as geology and stratigraphy;s« is mi city; ground-water location and quality;population density; facility size; leachate quantityand quality; and nontechnical political, social, andeconomic factors must be taken into account Table9-1 summarizes areas of concern in various landfillmaterials and components.

One of the most important considerations inplanning a waste facility is estimating the length oftime the facility is expected to operate. Somerecommended time frames for different kinds offacilities are:

• Heap leach padse Waste piles• Surface impoundments• Solid waste landfills• Radioactive waste landfills

1-5 yean5-10 years5-25 yean30-100 yean100-1000+ yean

None of these time frames are set forth inregulations, however. The only time frame regulatedby EPA is the 30-year post-closure care period for allhazardous waste landfill facilities.

Rexible Membrane LinersThe major long-term consideration in anysynthetically lined facility is the durability offlexible membrane linen (FMLs). In the absence ofsunlight, oxygen, or stresses of any kind, a properlyformulated, compounded, and manufactured FML

will stay intact indefinitely. This is because stablepolymers in isolation have an equilibrium molecularstructure that prevents aging. The usual indicatorsof stress are changes in density, p, and glasstransition temperature, Tf. The glass transitiontemperature is the temperature below which theamorphous region of a crystalline polymer is rigidand brittle (glossy) and above which it is rubbery orfluidlike.

Polymers in the field, however, are subject to manyexternal influences and stresses. Experiments doneon FML materials attempt to simulate the long-termin situ aging processes. One approach is to place asample of material in a test chamber at roomtemperature (approximately 70*F), and anothersample in a chamber heated to 120° to 160*F. Theactivation energy for the two FMLs is evaluated.Then a model based on the Arrhenius equation isused to determine the length of time it would takethe sample kept at 70*F to degrade to the sameextent as the high temperature sample. Thisprocedure, however, assumes that temperatureincrease is physically equivalent to the time ofexposure in the field, an assumption with whichmany chemists disagree.

Therefore, in the absence of a direct measurementmethod for FML lifetime, it becomes necessary toevaluate all of the possible degradation mechanismsthat may contribute to aging.

Degradation ConcernsA number of mechanisms contribute to degradationin the field, many of which can be controlled withproper design and construction. A FML can beweakened by various individual physical,mechanical, and chemical phenomena or by thesynergistic effects of two or more of thesephenomena. Polymeric materials have an extremelyelongated molecular structure. Degradation cutsacross the length of this structure in a process knownas chain scission. The more chain breakages thatoccur, the more the polymer is degraded by loss of

113Preceding page blank

Tab** 9*1. Long-Term Concerns >n Landfill Mechanism* (Y • ye* N * no)

CJP • SWCR LCR Sec LTKX LDC«Mechanism FML

i Movement oi NSuDSO'l

2 SuDsKience of Ywaste

3 Aging Y

4 Degradation YS. Clogging N6 Disturbance Y

wnere:FML geomemorane hner

CuvN

Y

Y

N

N

Y

Nat SYNN N

Y Y

Y Y

N Y

Y Y

Y <

P-FML Nat. SYN FML Day N*t SYN

Y N N Y Y N N

N N N N N N N

Y Y Y Y N N Y

Y N Y Y N N Y

N Y Y N N Y Y

N N N N N N N

LCR le^cnate collection and removal systemSWCR surface water collection and removal systemNat. maoe 'rom natural soil materialsSyn. made from syntheoc pcitymenc matenali

strength and loss of elongation. Each of the processesinvolved in chain scission will be discussed in thefollowing sections.

Oxidation DegradationOxidation is a major source of polymer degradationleading tc a loss of mechanical properties andembrittlement. The steps in this process are asfollows:

• Heat liberates free radicals.• Oxygen uptake occurs.• Hydroperoxides accelerate uptake.• Hydrogen ions attach to tertiary carbons which

are most vulnerable.• Subsequent bond scission occurs.

At high temperatures (over 200*F) oxidation occursvery rapidly. Consequently, oxygen will createserious problems for FMLs built near furnaces,incinerators, or in extremely hot climates. Theimpact of oxidation is greatly reduced at umbienttemperatures.

One can minimize oxidative degradation of FMLs bydesigning facilities with PMLs buried to avoidcontact with oxygen and to dissipate heat generatedby direct rays of the sun. Oxygen degradation is avery serious problem with materials used for surfaceimpoundments, however, where the FML cannot beburied or covered.

Ultraviolet DegradationAll polymers degrade when exposed to ultravioletlight via the process of photooxidation. The part ofthe ultraviolet spectrum responsible for the bulk ofpolymer degradation is the wavelength UV-B (315-

380 nm). ASTM D4355 uses Xenon Arc apparatus forassessing the effects of this wavelength on polymerictest specimens. The Xenon Arc apparatus isessentially a wcatherometer capable of replicatingthe effects of sunlight under laboratory conditions.

Blocking or screening agents, such as carbon black,are commonly used to retard ultraviolet degradationFor this reason* FMLs are manufactured withapproximately 2 to 3 percent carbon black. Even thatsmall amount effectively retards degradation byultraviolet rays. Although the addition of carbonblack retards degradation, it does not stop itcompletely. Ultraviolet degradation, however, can beprevented by burying the material beneath 6 to 12inches of soil. FMLs should be buried within 6 to 8weeks of the time of construction, geonets within 3 to6 weeks, and geotextiles within 1 to 3 weeks, i.e., thehigher the surface area of the material, the morerapidly the geosynthetic must be covered.

In surface impoundments where PMLs cannot beburied, ultraviolet degradation also contributes tothe oxidation degradation. An attempt still should bemade to cover the FML, even though sloughing of thecover soils will occur unless the site has very gentleslopes. Various other covering strategies are beingevaluated, such as bonding geotextiles to FMLsurfaces.

High Energy RadiationRadiation is a serious problem, as evidenced by tneNuclear Regulatory Commission and U.S.Department of Energy's concerns with transuranicand high level nuclear wastes. High energy radiationbreaks the molecular chains of polymers and givesoff various products of disintegration.

As of 1992, low-level radioactive wastes, such asthose from hospitals and testing laboratories, must

114

V>«? inntami f l in landfil ls The effects of low levelriuliulum associated wi th these waste materials onpolymers s t i l l needs to he evaluated

Chemical Degradation: pH EffectsAll polymers swell to a certain extent when placed incontact with water tpl l = 7) because they acceptwater and/or wa te r vapor into their molecularstructure. Degrees of swell ing for some commonpolymers are listed below;

• Pulyvinyl chloride (PVC)• Polyamide(PA)• Polypropylene<PP)• Polyethylene IPE)• PolvestcrlPET)

10 percent4 to 4.5 percent3 percent0.5 to 2.0 percent0 4 to 0-8 percent

Polymer swelling, however, does not necessarilyprevent a material from functioning properly. TheU.S. Bureau of Reclamation has observed polyvinylchloride liners functioning adequately in watercanals for 20 to 25 years despite relatively largeincreases in the thickness of the material

In very acidic environments (pH < 3), somepolymers, such as polyamides, (i.e., Kevlar andnylon) begin to degrade. On the other end of thespectrum, certain polyesters degrade in extremelya l k a l i n e e n v i r o n m e n t s (pH > 12). Hightemperatures generally accelerate the chemicaldegradation process. Most landfill leachates are notacidic or basic enough to cause concern. However, incertain kinds of landfills, such as those used for ashdisposal, the pH of the leachate might be quitealkaline and needs to be taken into account whenchoosing liner materials.

Chemical Degradation: LeachateEPA's Method 9090 is a test procedure used toevaluate Icachate degradation of FML materials. Asdescribed in Chapter Eight, the FML must beimmersed in the site-specific chemical environmentfor at least 120 days at two different temperatures.Physical and mechanical properties of the testedmaterial are then compared to those of the originalmaterial every 30 days. Assessing subtle propertychanges can be difficult. Flexible Liner EvaluationExpert (FLEX), a software system designed to assistin the Resource Conservation Recovery Act tRCRA)permitting process, can help in evaluating EPAMethod 9090 test data.

Biological DegradationNeither laboratory nor Held tests have demonstratedsignificant evidence of biological degradation.Degradation by fungi or bacteria cannot take placeunless the microorganisms attach themselves to the

polymer and ii,id the end of a molecular chain, anex t remely u n h k e i v event. Chemical companies havebeen unable to manufac ture biological additivescapable of destroying h igh-molecu la r weightpolymers, l ike those used in KMLs and relatedgeosynthetic materials. Microbiologists have triecunsuccessfully to make usable biodegradable plastictrash bags for many years. The polymers in FMLshave 10,000 time* the molecular weight of thesematerials, thus are very unlikely to biodegrade frommicroorganisms.

There also is little evidence that insects or burrowinganimals destroy polymer liners or cover materials. Intests done with rats placed in lined boxes, none of theanimals were able to chew their way through theFMLs. Thus, degradation from a wide spectrum ofbiological sources seems highly unlikely.

Other Degradation ProcessesOther possible sources of degradation inc ludethermal processes, ozone, extraction via diffusion,and de lamina t ion . Kreeze-thaw cycling, or theprocess by which a material undergoes alternatingrapid extremes of temperature, has proven to havean insignificant effect on polymer strength or FMLseam strength. Polymeric materials experience somestress due to warming, thereby slightly decreasingtheir strength, but within the range of 0° to 1608F,there is no loss of integrity

Ozone is related to ultraviolet degradation in thephotooxidation process, and, therefore, creates amore serious problem for polymeric materials.However, ozone effects can be essentially eliminatedby covering the geosynthetic materials within thetime frames previously mentioned.

In the extraction via diffusion mechanism,plasticizers leach out of polymers leaving a tackysubstance on the surface of the material. The FMLbecomes less flexible, but not. apparently, weakernor less durable.

Delamination of scrim-reinforced material was aproblem until 15 years ago when manufacturersbegan using large polymer calendar presses. Thepresses thoroughly incorporate the scrimreinforcement, so that delamination rarely occurstoday.

Stress-induced MechanismsFreeze*thaw, abrasion, creep, and stress cracking areall stress mechanisms that can affect polymers. Thefirst two, freeze-thaw and abrasion, are not likely tobe problems if the material is buried sufficientlydeep. Soil burying will eliminate temperatureextremes. Abrasion is a consideration only in surface

115

impoundments in which water waves come into polyethy.direct contact with the FML. concern.

< II DPR) is the primary material of

CreepCreep refers to the deformation of the FML over aprolonged period of time under constant stress It canoccur at side slopes, at anchor trenches, sumps,protrusions, settlement locations, folds, and creases.

A typical creep test for a FML, or any othergeosynthetic material, involves suspending a loadfrom an 8-inch wide tensile test specimen. Initiallyan elastic elongation of the material occurs. Thematerial should quickly reach an equilibrium state,however, and experience no further elongation overtime. This is shown in the stabilized lower curve inFigure 9-1. The second and third curves in Figure 9-1show test specimens undergoing states of constantcreep elongation and creep failure, respectively

Creep Fa***

Figure 9-1. Typical rMutts of a sustained load (creep) teat

One can use the design-by-function concept tominimize creep by selecting materials in which theallowable stress compared with the actual stressgives a high factor of safety. For semicrystallineFMLs, such as polyethylenes, the actual stress mustbe significantly leu than the yield stress. For scrim-reinforced FMLs such as chlorosulfonatedpolyethylene (CSPE), or reinforced ethylenepropylene diene monomer (EPDM), the actual stressmust be significantly less than the breaking strengthof the scrim. Finally, for nonreinforced plastics suchas PVC or nonreinforced chlorinated polyethylene(CPE), the actual stress must be less than thtallowable stress at 20 percent elongation. In allcases, one should maintain a factor of safety of 3 orhigher on values for these materials.

Stress CrackingStress cracking is a potential problem withsemicrystalUne FMLs. The higher the crystallineportion of the molecular structure, the lower theportion of the amorphous phase and the greater thepossibility of stress cracking. High density

ASTM defines stress cracking as a brittle failure thatoccurs at a stress value less than a material's short-term strength. The test usually applied to FMLs isthe "Bent Strip Test," D1693. The bent strip test is aconstant strain test that depends on the type andthickness of the material being tested. In performingthe test, a specimen of the FML 0.5 inch wide by 1.5inches long is prepared by notching its surfaceapproximately 10 mils deep. Then the specimen isbent 180 degrees and placed within the flanges of asmall channel holder as shown in Figure 9-2.Approximately 10 replicate specimens can be placedin the holder simultaneously. The assembly is thenplaced in a glass tube containing a surface activewetting agent and kept in a constant temperaturebath at 122*F. The notch tips are observed forcracking and/or crazing. Most specifications call forstress-crack free result* for 500 or 1,000 hours.Commercial HOPE sheet usually performs very wellin this particular test.

There are two things, however, that the D1693 testdoes not allow: a constant stress testing of thematerial and an evaluation of the seams. The D1693process bends the test specimen initially, but thenallows it to relax into its original state. Furthermore,the notch cannot be made to span over separatesheets at seam locations.

ASTM D2552, 'Environmental Stress RuptureUnder Tensile Load," tests HOPE materials underconstant stress conditions. In this particular test,dogbone-shaped specimens under constant load areimmersed in a surface active agent at 122*F (seeFigure 9-3). The test specimens eventually enterelastic, plastic, or cracked states. Commerciallyavailable HOPE sheet material performs very wellin this test, resulting in negligible <» 1 percent)stress cracking. The test specimens are generallyelastic for stresses less than 50 percent yield andplastic for stress levels greater than 50 percent yield.

This apparatus can be readily modified to test »IDPEseams (see Figure 9-4). Long dogbone-shapedspecimens with a constant-width cross section aretaken from the seamed region. The same testprocedure is followed and the test results should bethe same. If cracking does occur, the cracks gothrough the fillet portion of the extrusion fillet-seamed samples or through the FML sheet materialon either side of the filbt For other types of seams,the cracking goes through the parent sheet on one orthe other side of the seamed region. Results on a widerange of field-collected HOPE seams have shown arelatively high incidence of cracking. Thisphenomenon is currently being evaluated withreplicate tests; carefully prepared seams; and

116

o*1o

)Iii :(A

am

Si VlT^Ji:T - \ « .^ - ——

var ia t ions of temperature , pressure, and otherg controls

\N

Front V« Eooe V*w

D«tat.« of IMI *o*Cim«n* from ASTM D2SS2modttUd IMI for HOPE

The test is sometimes crit ic ized as beingnonrepresentative of Held conditions since thewetting agent exposes all sides of the seamed regionto liquid, which does not happen in the field. Thespecimen also is too narrow to simulate real lifewide-width action. Also there is no cc ifinement onthe surfaces of the material. While these criticismsdo challenge the test as being nonrepresentative,there have been three field failures that mimickedthe laboratory cracking exactly. The first was in asurface impoundment lined with 80-mil HOPE *hatwas an extrusion fillet seam. The problem was netpicked up by construction quality assurance work,hut instead occurred after the facility was closed. Thesecond case involved extensive FML seam failures ata site in the Southwest. The seam used there was aflat extrudate. Cracking originated in several areasdue to high localized stresses and exposure to widetemperature fluctuations. The third failure occurredat a site in the Northeast that used 60 mil HOPE anda hot wedge seam. In this case a stress crack 1ft to 20inches long formed at the outer track of the wedge.

Of all the degradation processes reviewed, stresscracking in field seams is of the greatest currentconcern. It appears as though the Held problems onlyoccur in the exposed PML seam areas of surfaceimpoundments. Work is ongoing to investigate thephenomenon further and to understand themechanisms involved. An emphasis on carefullyconstructed and monitored field seams will certainlybe part of the final recommendations.

Clay LinersClay has a long history of use as a liner material.Data is available on the effects of leachate on clayliners over periods of 10 years or more, and theresults generally have been satisfactory. While claysdo not experience degradation or stress cracking,they can have problems with moisture ccntrnt andclods. High concentrations of organic solvents, andsevere volume changes and desiccation also cause^oncern at specific sites The rapid freezing andthawing of clay liner material* also affects theirintegrity, but freeze-thaw can usually be alleviatedwith proper design and construction considerations.For a more complete discussion of clay linerdurability, see Chapter Two.

Leachate Collection and RemovalSystemsThe leachate collection and removal system includesall materials involved in the primary leachatecollection system and the leak detection collectionsystem. For the proper functioning of these systems,all materials being used must be chemicallyresistant to the leachates that they are handling. Ofthe natural soil materials.-gravel and sand aregenerally quite resistant to leachates, with thepossible exception of freshly ground limestone. Withthis material, a solution containing calcium,magnesium, or other ion deposits can develop as theflow rate decreases in low gradient areas. A form ofweak bonding called "Umerock" has been known tooccur. Its occurrence would be disastrous in terms ofleachate collection and removal. Regardinggeotextiles and geonets, there are no established testprotocols for chemical resistivity evaluation like theEPA Method 9090 test for FMLs. Table 9-2 suggestssome tests that should be considered.

Tatta t-2. SuogarMd Ta« MutioOi for Aimnnq Cumleilf Ooosyntfietta* UMd in iMCHaia

(Y - yec N - no)G«on«t QcocompoM*

ThckncssMaWUnrtMtGrao»ns*WM* wdtt ttns*PunckirvfpntPutt** (C8R)TrapuoKW MarBust

YY

YYNYYY

YYNYNY

NN

YY

NNNYN

N

The system for collecting and removing the luachatemust function continuously over the anticipatedlifetime of the facility and its post-closure careperiod. During operation, leachate is removedregularly depending on the amount of liquid in theincoming waste and natural precipitation entering

118

the site l.cachate. however , cont inues to hegenerated long after landfill closure During the firstFew pest closure years the rate of leachate removal isalmost 100 percent cf that duri.ig constructionApproximately 2 to 5 years after closure, leachalegenerally levels off to a low-level constant cap leakrate or. in a very t ipht , nonleaking closure, falls tozero (see Figure 9-5)

Clogging is the primary cause of concern for the long-term performance of leachate collection and removalsystems Paniculate clogging can occur in a numberof locations First, the sand filler itself can clog thedrainage gravel Second, the solid material withinthe leachate can clog the drainage gravel or geonet.Third, and most likely, the solid suspended materialwithin the leachate can clog the sand filter orgpctextile f i l t e r The following breakdown ofpaniculate concentration in leachate at 18 landfillsshows the potential for paniculate clogging:

• Total solids• Total dissolved solids• Total suspended solids

Salts precipitating from high pH leachate, ironocher. sulfides, and carbonates can all contribute topaniculate clogging.

0 59.200 mg/L584-44,900 mg/L10 -700 mg/L

The potential for clogging of a filter or drainagesystem can be evaluated by modeling the system inthe laboratory. This modeling requires an exactreplica system of the proposed components, i.e.. coversoil, geotextile. geonet, etc. Flow rate plotted as a/unction of time will decrease in the beginning, buteventually should level off to a horizontal line at aconstant value It may take more than 1,000 hours/or this leveling to occur. Zero slope, at a constantvalue, indicates an equlibrium (or nonclogging)situation. A continuing negative slope is evidence ofclogging. As yet, there is no formula or criteria thatcan be substituted for this type of a long-termlaboratory How test.

Biological clogging can arise from many sourcesincluding slime and sheath formation, biomassformation, ochering. sulftde deposition, andcarbonate deposition. Ocher is the precipitate leftwhen biological activity moves from one zone toanother. It is an iron or sulfide deposit, and is mostlikely to occur in the smallest apertures of filtermaterials. Sand filters and geotextile filters are mostl ikely to clog, with gravel, geonets. andgeocumposites next in order from most to least likely.

The biological oxygen demand (BOD) value of theleachate is a good indicator of biological cloggingpotential; the higher the number, the more viablebacteria are present in the leachate. Bacterialclogging is more likely to be a problem in municipal

landfills than at hazardous waste facilities becausehazardous Icacnates probably would be fatal to mostbacteria. Currently, an KPA contractor is monitoringsix municipal landfills for evidence of aerobic andanaerobic clogging. The results should be availablein 1989.

The most effective method for relieving particulateand/or biological clogging is creating a high-pressurewater Hush to clean out the filter and/or the drain Incases of high biological growth, a hiocide may alsoneed to be introduced Alternatively, a biocide mightbe introduced into the materials dur ing the i rmanufacture. Geotextilps and geonets can include atime-release biocid* un their surface, or within theirstructure. WorL. by a number of geotextile andgeonet manufacturers is currently ongoing in thisarea. Measures to remedy clogging must beconsidered in the design stage.

The final factor to be considered in leachatecollection and removal systems is extrusion andintrusion of materials into the leak detection system.In a composite primary liner system, clay can readilyextrude through a geotextile into a geonet if thegeotextile has continuous open spaces, i.e., percentopen area (POA) > 1 percent. Therefore, relativelyheavy nonwoven geotextiles are recommended.Elasticity and creep can cause geotextiles to intrudeinto geonets from composite primary liners as well. AFML above or below a geonet can also intrude intcthe geonet due to elasticity and creep. Design-by-function and laboratory evaluations that simulatefield conditions should alert designers to thesepotential problems. For all geosynthetics. a highdesign ratio value or factor of safety for strengthshould be chosen.

Cap/Closure SystemsWater and wind erosion, lack of vegetation, excessivesunlight, and disturbance by soil-dwelling animals(or by people) all are potential problems for landfillclosure systems. The effects of rain, hail, snow,freeze-thaw, and wind are discussed in Chapter Five.Healthy vegetation growing over the cap minimizesthe erosion of soil on the slopes by these naturalelements.

The effects of animals and of sunlight (ultravioletrays and ozone) can be minimized by adequatelyburying the cap/closure facility. Soil depths overFMLs in covers range from 3 to 6 feet in thickness.Large rocks above the FML cover can also thwart theintrusion of animals into the area. Human intrusion,either accidental or intentional, can usually beprevented by posting signs and erecting fences.

The final long-term consideration related to cap andclosure systems is aesthetic. The New JerseyMeadowlands Commission is planning for the final

119

tooCofM I

Cao Leakage

To

Fifur* $•$. Apprommat* amount of lt*chat«

closure of 54 landfills in the northeastern part of thestate, all but 3 of which are already closed and readyto oe capped A graphic artist was ht.*ed to design anattractive landscape out of one facil i ty along aheavily traveled automobile and rail route. Thedesign included looping pipes for methane recovery,a solar lake, and an 8-foot concrete sphere allcontributing to a visually pleasing lunar theme.

The performance of a capped and closed wastefacility is critically important. If a breach shouldoccur many years after closure, there is a highlikelihood that maintenance forces would b*unavailable. In that event, surface water could enterthe facility with largely unknuwn consequences.Thus the design stage must be carefully thought outwith long-term considerations in mind.

120

tubular system can handle high levels of suspeif ^d solids withoutblocking or channeling. Systems are easily cleaned-in-place and energyconsumption is minimal.

three processing systemsTo accommodate liquids with different sizes of soluble or suspendedsolids and to meet desired end product characteristics, three types oftubular membrane systems are offered. One involves the use of multi-channeled hexagonal ceramic blocks within stainless steel housings formicrofiltration and some ultrafiltration duties. The others utilizeopen stainless steel tubes with inner membrane surfaces. Dependingupon the pore sizes of the membranes, these are used for eitherreverse osmosis or ultrafiltration.

ANSTROM UNfflPP(LOG SCALE)

RELATIVESIZE OF

COMMONMATERIALS

Not* 1 Micron - .OOOQ3M InchM1 Angstrom Unit = TO * mlcroni

reverse osmosisPolymer membranes are virtuallynon-porous but permit passage ofwater molecules during pre-concen-

±.\jm

t rat mn nnornt mnc

primarily in the degrReverse osmosis unitultrafiltration incorjmembrane inserts h;pore sizes. In either cinserts are fitted insisteel tubes mountedwithin a housing butat the ends to providtin series. The membibe interchanged as d>process modules useifor greater system aiobtain higher concen18 tubes per unit, thipreassembled for sin

stallation and may"ither batch or contii

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Under reverse osmgreater than the osmfeed liquid passing tlwater out of the feedsoluble and suspend(molecular structure,The permeate is virtiis retained for collectpressures of betweenthan heat to concent i

SIZE AND CHARACTERISTICS OF AIR-BORNE SOLIDS

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Equivokentsizes

, Electromagneticwaves

Technicsdefinitions

Common atmospheredisoersoids

Typical particlesand

. gas dispersoiQs

Methods forparticle -size

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settling*/for spheresi

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* Molecular diameters calculated from viscosity data at 0°C* Furnishes overage particle diameter but no size distribution.* Sue distribution may be obtained by special calibration.

Stokes - Cunnmgham factor included in values given lor air but not included for water

HG. 20-102 Characteristics of particles and particle dispersoids (Courtesy of the Stanford Research Institute; preparedby C, E Lapple.)

20-78