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GEOMEMBRANE LINER DURABILITY: CONTRIBUTING FACTORS AND THE STATUS QUO

Ian D. Peg gs

I-CORP INTERNATIONAL, Inc .

AbstractReg ulators and eng inee rs have suffic ient c onfidenc e in the durab ility and long-term integ rity of

ge om em brane lining system s to req uire their use as ba rriers be twe en po tential c onta minants

and g roundwa ter. Yet expe rienc e with suc h lining system s covers only ab out 30 yea rs.

How ever, in that period ad eq uate p erforma nce ha s be en de monstrated . But how long will

such ge om em brane ma terials last be fore ultima te d eg rada tion or failure?

In the c ase o f municipa l solid wa ste land fills c hem ica l dissolution a nd de grad at ion o f the

typical high density polyethylene (HDPE) geomembrane is considered to be a non-issue.

Ultima te durab ility will be a func tion o f the stress c rac king resista nc e of the spec ific HDPE resin

used , the e ffec tiveness of its antioxida tion a dd itives, the stresses gene ra ted in the

geo me mb rane during insta lla tion and land fill op erat ion, and the stress relaxation ra te. The

po tential influence s of e ac h of the se p henomena ind ividua lly, and synergistica lly, on the lifetime

of g eom emb rane s are c onside red .

Introduction

It is interesting to note tha t env ironme nta lists freq uent ly c laim tha t the p lastic b ags tha t float

a round in the oc ea ns are a pe ril to wildlife for ever, yet they also c laim that spe c ially formulate d

and designed p lastic b ased land fill lining systems are bo und to fa il in a relatively short time!

In our tec hnica l world the lifetimes of HDPE ge om em branes in land fill lining systems ha ve

been va riously estima ted to b e b etw een 200 and 750 yea rs. At the o ther end o f the sc ale

installed HDPE lining systems in other applications, typically exposed pond liners or cast-in

c onc rete liners, have not lasted 6 mo nths without failing. Failing is p rac tica lly defined a s

de veloping a leak.

Of the ma ny HDPE ge om em brane liners that have failed in the pa st 20 years, all have failed

in a ve ry limited numb er of ways, but no ne ha ve just wo rn-out o r ge nerally de grad ed to

nothing, nor is it expec ted that the y will. How eve r, our prac tica l expe rienc e w ith HDPE

ge om em branes is limited t o a bo ut 25 yea rs. Polyvinyl Chloride (PVC) has be en eva luated afte r

30 yea rs, and polyprop ylene (PP) is quite young at a bout 10 yea rs. North America n municipa l

solid wa ste (MSW) lea c hate is typically quite benign, as show n b y the mode l for a standa rd

testing lea cha te in Append ix A, to the e xtent tha t in the USA c hemica l resista nc e tests of HDPE

are now ra rely req uired . Ma ny EPA 9090 Co mp atibility Test for Wastes and Memb rane Liners

tests have b een p erformed with MSW lea cha tes and none ha ve b een shown to da ma ge the

ge ome mb rane the d eg rad ative effect o f MSW lea c hate on HDPE ca n p rac tic ally be ignored.

HDPE liners in land fills and other app lic a tions fail or a re ma de to fail as follow s:

Inad eq uate w elding a nd a ttac hment to structures Imposed stresses during construction Mec hanica l da ma ge d uring c onstruction Stress c rac king a t stress points Service stresses tha t sep arate weld s


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Excep t for po or we ld ing a nd d am age induc ed during installation HDPE ge om em brane s have

ge nerally only failed by stress c racking (a fund amenta l performa nce c harac teristic o f HDPE) , or

as a com bination o f oxida tion fo llow ed by stress c rac king (SC). Stress c rac king is essent ially a

brittle c racking phe nom enon tha t occ urs at a c onstant stress lower than the short term yield

strength or b rea k strength of the material. It is a c onseq uence of the sem i-c rysta lline

mic rostructure tha t gives the HDPE its go od c hem ica l resistanc e a nd h igh streng th. PVC liners

have c racked from loss of p lasticizer at e levated tem pe ratures and unde r ultraviolet rad iation(UV) exposure, and PP has also e xperienc ed c rac king at elevate d tem pe ratures but without UV

exp osure. However, PVC, PP, and LLDPE, a re not susc ep tib le to SC in the a s-ma nufa c tured

c ond ition as is HDPE. Break time s as a func tion o f co nsta nt stress for five a s-ma nufa c tured HDPE

geo me mb ranes are shown in Figure 1. At the h ighe r stresses c lose to the yield stress, b reak

oc c urs in a d uc tile ma nner. At lowe r stresses, below the knee in the curve, b rea k oc curs in a

b rittle ma nner the d uc tile slope c anno t be extrap olated t o g ive a lifetime a t a lower stress.

Figure 1 Stress rupture c urves for five HDPE geo me mb rane s (Hsua n e t a l. 1992)

It is freq uently sta ted by som e in the g eo me mb rane a nd gas p ipe industries (Peg gs (2003)),

Thom as (2002) Brow n (1993)) that the o nly mea ningful pa rame ter that req uires spec ifica tion for

HDPE is its stress c rac king resista nc e (SCR). This is the on ly param ete r that reflec ts the widerange of me c hanica l durab ilities of ge om em branes ma de from the d ifferent HDPE resins. All

other index properties (tensile, punc ture, and tear) a re e ssent ially identica l in a ll HDPE

geo me mb ranes. Fortuna tely, as a result of the failures tha t have oc c urred , resins,

ge om em brane s, and we lding eq uipment/ proc ed ures used in land fill lining system s have

signific antly imp rove d. LLDPE and PP do suffer from SC, but o nly whe n the ir antioxidants a re

de pleted and they oxidize.


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Suc h fa ilures have b ee n mo re evident in expo sed lining system s in ponds, lag oo ns, and

c onc rete basins whe re restra ined c ontrac tion stresses are c yclic as temp eratures cha nge ,

where the g eom emb rane is not c onfined be twee n two layers, where leakag e is more evident,

and whe re the da ma ge c an be seen. There have e ffectively been no known in-service failure

events that ha ve o cc urred in solid wa ste fa c ilities in North America that c anno t be a sc ribed to

external influenc es. Howe ver, HDPE and PP have c rac ked o n wrinkles und er a hyd rosta tic head

and there ha s rec ently be en c racking in reinforced PP (RPP) on the und erside o f floa ting c ove rsat the bo ttom o f drainag e troughs.

Doub le lining system s in US land fills tha t a llow c ont inuous mo nitoring of lea kag e flow ra tes

through primary liners into the leakag e c ollec tion, draina ge , and rem ova l systems (LCDRS) have

show n no sp ikes rela ted to p unc tures or liner deg radation in service caused by events solely

within or very c lose to the lining system. However, HDPE liner lifet imes c onside rab ly in exc ess of

those experienc ed to d at e (ma ximum ab out 25 yea rs) are desired and o bt aining such lifetimes is

the subjec t of this pa pe r.

Discussion

Ge om em brane liners a re ide a lly de signed to b e insta lled without stress. They a re simply

intend ed to a c t as a b a rrier. Clea rly, a zero stress insta lla tion is p rac tica lly imp ossible to

achieve wrinkles a re unavo ida b le. But without mec ha nica l tensile stress a liner cannot be

ma de to b rea k and lea k, ma king such an objec tive, or the mea ns to tolerate it, desirea ble.

Howe ver, while ge neral chem ic al deg rad ation due to leac hate d oes not oc c ur, the presenc e

of chemica ls suc h as c hlorina ted solven ts, ac ids, and d ete rge nts in c ontac t with a stressed

HDPE ge om em brane may result in environm enta l stress c rac king (ESC) where the c hem ica l

acc elerate s the funda mental stress c racking p henomenon. ESC is taken ad vantage of in

lab orato ry tests that are p erformed in a surfac e ac tive d eterge nt a t elevated temp erature

(500C) to a c c elerate failures so they oc c ur in a reasona ble time, but w ithout c hang ing the

frac ture m ec hanism a nd morphology.

Impermeability

It must be rec og nized tha t nothing is absolutely impe rme ab le. Ap pa rent leakag e m ay also

oc c ur throug h d iffusion of va po r (solvent a nd wa ter) throug h a morpho us reg ions of the HDPEge om em brane, which then rec ond enses on the op posite side. Sang am a nd Row e (2002), Park

et a l. (1995), and others have show n the d iffusion rates of va rious orga nic liquids and solutions

through ge om em branes, but wh ile the se liquids a re a bsorbe d b y the HDPE, which c auses it to

soften they do no t ca use a c ontinuing a nd p erma nent deg rad ation. When the liquid

environm ent is rem oved the solvent vap ors volatilize out o f the geo memb rane which rec ove rs

its original p roperties. In ge nera l the softe ning of the ge om em brane while in service will be

be neficial in a llow ing the liner to b ette r c onform to subg rad e p rofiles and differential settlement

without signific ant stress, thereby red uc ing the possibility of stress c racking . In fac t the

d iffusing/ absorbed organics ac t a s a plasticizer for PVC w hich, when it d ries out m ay then c rac k.

Thus it rem ains flexible in service , but be c om es brittle when exposed and te sted . Howe ver, a

similar phe nom enon ma y have o c c urred in one c ase in which HDPE wa s exposed to c reosote in

a c hem ica l resista nc e te st - a 70% red uc tion in SCR wa s ob served when the HDPE wa s rem ove dfrom the c reo sote a nd a ll orga nics ha d desorbe d. It is not know n if this is a sta nda rd

oc c urrenc e a fter expo sure to organic liquids. In ano ther ca se, in the p resenc e o f sulphuric

ac id, wrinkles in 3 mm and 5 mm thick HDPE liners c aused by naphtha lene, kerosene , and

aroma tic hydroca rbo n a bsorption d id suffer stress c rac king as a result of oxida tion c aused by

the ac id a t temp eratures of ab out 700C.

Giroud and Bona parte (1989) have show n (Tab le 1) that wa ter vap or diffusion through 1 mm

HDPE ge om em brane w ith a hea d o f 300 mm (the ma ximum a llow ed in MSW land fills), is


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Oxidation

The m ajor env ironm enta l agenc y of co nc ern in US land fills is oxida tion. The Geo synthe tic

Resea rc h Institute ha s performed a number of the rma l ag ing stud ies (Hsua n a nd Koe rner (1998),

Hsuan and G uan (1998)) to deve lop an estima ted liner lifetime . Samp les of different HDPE

ge om em branes we re p laced in ovens or simulated landfill environm ents at te mp eratures

be twee n 55 and 1150C for up to 2 yea rs. Typica l me c hanical prop erties we re period ica lly

measured . The de p letion of antioxida nts wa s de termined by me asuring stand ard oxidat ioninduc tion t imes (OIT) at 2000C and high p ressure oxida tive induc tion t imes (HP-OIT) a t 1300C.

Hsuan and Koerner (1998) hea ted five d ifferent geo me mb rane samp les in forced -air ove ns. The

as-rec eived OIT and HP-OIT va lues va ried wid ely, ag a in demo nstrating t he d ifferenc es in

durab ility betw een geom em branes from d ifferent ma nufac turers. Reta ined O IT values a fter

ag ing a t 650C are show n in Figure 2, and c hanges in OIT and mec hanic al p rop erties at 950C a re

show n in Figure 3. The latte r show s tha t mec hanica l p rop erties a re not chang ing as AO

de pletion occ urs. Hsuan and Gua n (1998) sta te that mec hanica l prop erties do no t cha nge

until a ll AO ha s bee n c onsume d. An Arrhen ius p lot for OIT test da ta is show n in Figure 4 from

which the ac tiva tion energy for dep letion of AO indica tes that a ll AO is dep leted a t a service

tem pe rature of 200 C in three o f the ge om em branes a fter ap proxima tely 60, 80, and 100 yea rs.

Note, aga in, that the d ifferent HDPE ge om em branes be have quite d ifferently. At the p oint of

c om plete AO d ep letion the mec hanica l prop erties start to deg rad e. Thus there is ano ther

pe riod afte r AO d ep letion d uring which t he p olymer itself deg rad es (oxid izes) and during w hic h

the m ea sured p rop erty deg rad es to a defined c ritica l level, often c onsidered to be 50% of its

orig inal va lue. The time a t which this oc curs is termed the ha lf-life.

Figure 2 Ag ing at 65C


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Figure 3 Ag ing at 95C

Figure 4 Arrhenius p lot fo r OIT test da ta


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Figure 5 Equipm ent fo r ag ing in simulate d land fill env ironm ent

Hsuan and Koe rner (1998) rep ort da ta on one HDPE ge om em brane exposed in a simulated

land fill environm ent a s show n in Figure 5. The geo memb rane samp le was confined b etw een

two sand layers at a p ressure o f 260 kPa. A 300 mm hea d of wa ter wa s ma intained a bo ve the

sam ple and the com plete a ssem bly wa s hea ted . Similar c hang es in prop erties we re ob served

as show n in Figure 6. In this c ase the estimated time to AO d ep letion wa s ca lculated to be

betw ee n 200 and 215 yea rs. It is longer tha n in the laboratory tests bec ause of the mo re limited

acc ess of fresh oxygen to the surfac e of the g eomemb rane in the c onfined environm ent.

Ag ing p roc ed ures a re still unde rwa y to assess the p ost-AO-dep letion d eg rada tion of thema terials p rop erties.

Figure 6 Cha nge s in prope rties of g eomemb rane in simulate d land fill environm ent


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Hsuan and Koe rners (1998) hypo thesis is tha t deg radation of the ge om em brane is a three

sta ge process, as show n in Figure 7: 1) dep letion of AO; 2) ind uc tion time to o nset o f po lymer

deg rada tion, and; 3) deg rad at ion of the po lymer and loss of mec hanica l prop erties. This will

likely occ ur when samp les a re exposed without stress in an ove n and und er a uniform

c om pressive stress in a simulated field environme nt. How eve r, when the geo me mb rane is

und er tensile stress or ha s shear stresses imp osed on / in surfac e la yers a t the same t ime as

oxidat ion is oc c urring, the kinetics of de grad at ion will not b e a s simp le to mod el. Oxidat ion andSC w ill interac t synergistica lly. Hessel (1990) ind ica tes tha t when a n HDPE spec imen is the rmally

aged und er stress it fa ils c om pletely when the AO is c onsume d, as show n in Figure 8. At the

highe r stresses c lose to the yield stress the ma terial fails in a duc tile m od e b efo re oxida tion

oc c urs. At the intermed iat e stresses a p rem ature (comp ared to d uc tile reg ion extrapo lation)

brittle SC b rea k oc c urs befo re o xida tion oc c urs. But a t the lowe r stresses when the AO is fully

c onsumed a nd o xida tion oc c urs be fore the e xtrap olate d SC c urve, break is even more

prema ture. Therefore, there is a c onstant c om pe tition be twe en the rate o f de pletion of AO

and pa rallel or subseq uent oxidat ion and the initiation o f stress c rac king as to w hich initiates

failure first. In prac tice, oxidation within a c ontinuously prop ag ating a nd op ening c rac k tip will

further acc elerate the c rac k growth rate .

Figure 7 Stag es of ag ing a nd d eg rad ation

Figure 8 Stress rupture curves as a func tion o f tem pera ture


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In fac t, a c onventional measurem ent o f OIT will not ind ic ate the t rue situation since oxida tion

will first oc c ur on the surfac e where c rac ks will be initiate d a s soo n as there is an adeq ua tely

thick surfac e laye r for the loc a l stress to initia te a c rack. When a c onvent iona l OIT test is

performed , using the full thic kness of the geo me mb rane a s the spec imen, the m ea sured O IT will

not b e a n indica tor of the c ond ition of the surfac e of the m ate rial. For instanc e if 10% of the

thickness is fully dep leted of AO t he me asured OIT will indica te a 90% retent ion o f OIT, the same

as if the c om plete thickness of the ge om em brane we re o xidized only 10% not som ething tha two uld norma lly ca use conc ern. But, a c om plete ly oxidized surfac e layer and the c rac ks that

wo uld b e initiated in it would be of conc ern. Thus there will be a c ontinuing synergism b etw een

the kinetic s of o xida tion through the thickness of the ge om em brane and the kinetic s of stress

cracking.

Stress Crac king Resista nc e

The significanc e o f the rate s of initiat ion of stress c racks on the surfac e o f a geo me mb rane

follow ed by c rac k prop ag ation into the bo dy of the g eom emb rane wa s further shown b y

Ca dw allad er (2001). He found tha t coe xtruded textured m ate rial ma de w ith a surfac e laye r of

low stress c rac king resistanc e (app a rently rec ycled po lymer) would c ause the acc elerate d

c rac king o f core ma teria l with othe rwise high SCR. Thus a c ore ma terial that ha d a single p oint

not ched c onstant te nsile loa d stress c rac king resista nc e o f ove r 1000 hr fa iled in 324 hr in an

unnotc hed tests whe n c oextruded with a textured surfac e laye r ma de with inferior qua lity resin.

The c racks we re ea sily initiate d in the textured surfac e layer but d id not slow d ow n when the y

me t the c ore layer. Thus it is ea sier for a c rac k to p rop agate into a core layer than it is for a

c rac k to initiate and propa ga te within that ma terial alone. In general it wa s c onc lude d tha t

rando m surfac e te xtures ma y red uc e the SCR of the basic smo oth ge om em brane . This will not

oc c ur in struc tured -surfac e ge om em brane s with their designe d rep rod uc ible p rofiles on shee t o f

a uniform thickness.

In p rac tice a confined HDPE ge om em brane will only fail in the long term e ither by stress

c rac king at p oints of consta nt stress stone protrusions, stresses ac ross sea ms, c reased wrinkles,

textured surfac es. Stressed area s have a lso bee n see n at tem porary divid ing be rms where the

vertica l p ressure of wa ste ha s caused the b erm to sprea d la tera lly on the c ont inuous liner there

ma y be wrinkling on o ne side o f the b erm a nd signific ant tension on the othe r side

The a utho r c learly has c onc erns about doub le textured liners on side slopes where the re is a

higher shea r resistance on the top surfac e tha n on the b ottom surfac e of the g eo memb rane

with the result that the geo mem brane b ec ome s a loa d-bea ring m emb er of the system d ue to

the ind uc ed shea r stress. This is a ma jor d isc onnec t since on o ne ha nd the liner is designed t o

be w ithout stress but on the o ther hand it is textured to hold soil on slopes. And , as indica ted

abo ve, the p resenc e o f the surfac e te xture w ill, at the sam e time, c ause a red uc tion in the SCR

of the ge om em brane itself to different d eg rees in the d ifferent typ es of textured and struc tured

p rofiles. When a slide oc c urs on a slope and the g eo me mb rane tears, it is a lwa ys assume d tha t

the g eo me mb rane tea rs as a result of the soil moveme nt. It is eq ua lly possible tha t the

ge om em brane m ay experienc e stress c rac king, as a c onseq uenc e o f the induc ed shea r stress,

tha t initiates c ritic al move ment of the soil. All such geom em brane tea rs should b e exam ined forreg ions of stress c rac king within mo re e xtensive ove rload tea rs with the ir duc tile e longations.

Shea r stresses induc ed in textured surfac es will be o f muc h mo re signific anc e in the fo rthc om ing

b iorea c to rs with the ir highe r tem peratu res and mo re extensive sett leme nt a long side slopes.

The use o f smooth t op surfac es on ge om em brane s will have signific ant po sitive impa c t on the

servic e life of a ge om em brane cove ring soils wo uld bette r be provided with veneer stability by

geo grids or high streng th g eo textiles, or by using a n HDPE geo me mb rane with m uc h highe r SCR.


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The kinetics of stress c rac k initiation a nd p rop ag ation increase at elevat ed tem pe rat ures as

show n sc hem atica lly in Figure 8. Howe ver, stress relaxation a lso inc rea ses as tem perature

increa ses resulting in a p erma nent rac e b etw ee n stress c rac king and stress relaxation a s to

wh ich will p reva il. If the induc ed stresses can be sufficiently red uc ed befo re c racking is initiate d

c rac king will not oc c ur. Also to be fa c tored into this a rgum ent is oxida tion of the

ge om em brane, for a ll HDPE ge om em brane s have req uired antioxida nt ad d itives that p rotec t

them ag ainst oxidation a t the eleva ted tem peratures during a nd a fter extrusion, during w elding,during we ld rep a irs, and during servic e. Onc e the ad ditives are all c onsumed in p roviding

protec tion, on ly very sma ll tensile stresses will be sufficient to c ause frac ture.

The influences of the d ifferent textures and p erformanc e c ha rac teristics on the durab ilities of

HDPE ge om em brane a re reflec ted in rec ent w ork pe rformed by Peg gs et a l. (2003b) to evaluate

the m aximum allowa ble stra in in HDPE and other ge om em brane ma terials used as a sep aration

barrier betw ee n old wa ste and new waste in an MSW land fill vertica l expansion. This wo rk wa s

done in response to the reg ulato rs req uiring no mo re than 1% strain in the sep a ra tion

ge ome mb rane indep ende nt of the po lymer used . At another projec t the engineer wa s

req uiring a n HDPE ge om em brane in a lining system to experienc e no mo re than 0.25% strain at

any loc a tion. This is p rac tica lly impossible to ac hieve. These spec ific a tions a re c learly a

misund erstand ing o f the G erma n BAM req uirem ents (See ger and Mller, 1996) for a m aximum

globa l strain of 3% and ma ximum loca l strain (at ind ividua l stone p rot rusions, for exam ple) of

0.25%. More rea listica lly, the follow ing maximum strains a re be ing rec om me nded :

HDPE smooth SCR1500 hr 8% HDPE rand om texturing 4% HDPE struc tured profile 6% LLDPE density 0.935 g/ c m3 10% LLDPE rand om texture 8% LLDPE struc tured p rofile 10% PP unreinfo rce d 15%

The mea surem ent o f strain is used as an ind irec t m ea sure o f the stress tha t exists in ageo me mb rane tha t might result in stress c rac king. While this is c lea rly imp orta nt for HDPE, it is

not a s signific ant for othe r ma terials tha t a re no t susc ep tible to SC unless oxidized . The o b jec tive

is to limit stress to a subc ritic a l value w here stress c rac king w ill not b e a p rac tica l prob lem.

How eve r in a c onfined situat ion the stress will be app lied very slowly to the ge om em brane as the

adjacent soils move, and the g eo memb rane will be a ble to relax resulting quite rap idly in

ge om em brane stresses that are ma ybe 50 % of the va lue imp lied by the de formation.

Stress Relaxation

While the bene fits of stress relaxation a re ap parent it is not a top ic tha t ha s been tho roug hly

stud ied fo r geo me mb ranes. Soo ng e t al. (1994) investiga ted stress relaxation in a 1.5 mm thick

HDPE geom em brane w ith initia l stresses of 40, 50, and 60% of yie ld stress (at te st temp era ture)

and initial strains of 1, 3, and 5%, at te mp eratures betw ee n 10 and 70o

C. These we re qua si-biaxial tensile tests using 100 mm wide by 50 mm ga ge length wide width tensile spe c imens.

Initia l load ing wa s done quite qu ickly to m inimize stress relaxation on loa d ing. As show n in

Figure 9 what ever the sta rting c ond itions, there w as a trend to a very narrow range of final, but

still significant stresses, a fte r about 100 days. The relaxa tion m od ulus c urves (stress/ strain a s a

function o f time ) for a g iven sta rting cond ition c ould b e supe rimposed into a ma ster curve for a

g iven relaxation te mp erature, as show n in Figure 10 for an initia l 3% strain and a tem peratu re o f

10oC.


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Figure 9 Ge om em brane stress relaxation da ta


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Figure 10 Ma ster stress re laxa tion c urve fo r 3% strain a t 10oC

In this case 50% of the app lied stress is rem ove d by relaxation a fter 50 minutes with fina l

eq uilibrium b eing a c hieved a t ab out 30% of app lied stress a fter 11.4 yea rs. At highe r

temp era tures the stress wou ld relax more quic kly. The equilib rium residua l stress is betwee n 2500

and 4000 kPa, b etw een about 13 and 21% of the roo m te mp erature yield stress. Note t hat the

stra in w as ap plied far more quic kly tha n w ill oc cur during subg rad e settleme nt, so in the land fill

significant stress relaxation w ill occ ur during defo rma tion. Soo ng e t a l. (1994) sta ted :

Trial tests we re p erformed initially to dete rmine the suitab le loa d ing rate . The results

suggested a rate o f 12.7 mm / min as being approp riate.. At slow er rates a ve ry significant

am ount of stress relaxation occ urred during the loa d ingprocess.

Also, note that Soo ng et a l. (1994) conc luded :

other HDPE ge om em branes will und oub ted ly respond d ifferently than the HDPE

studied.

Thus a ll HDPE geo me mb ranes are no t the same , just a s the ir SCR performa nc es are no t the

same.

These stress relaxation rates c om pare w ell with those g ene ra ted by Soo ng and Koe rner (1997)

for stress relaxation in waves in HDPE geo me mb ranes under a uniform vertic a l load ing. Afte r

1000 hr a t temp eratures of 23, 42, and 55oC the y found stresses relaxed b etw ee n 60 and 78%

leaving residua l stresses of b etw ee n 1% and 22% of the yield stress. Rec ollec t tha t SC o c c urs

below a bout 40% of the yield stress, in the range o f these residua l stresses. However, the se tests

we re d one unde r sem i-c onfined cond itions (wa ves ra ised off a flat supp ort surfac e) w hile the

Soo ng e t al. (1994) tests we re done under unco nfined c ond itions. Under sem i-con fined

c ond itions the residua l stresses were low er tha n fo r unconfined spec imens, po ssibly a result of the


13/24


stress relaxation o c curring d uring load ing. Under fully confined c ond itions the residua l stresses

wo uld p rob ab ly be even lower.

Fracture Mechanics

The HDPE na tural ga s d istribution p ipe resea rch supported by the Ga s Resea rc h Institute (now

part of the Ame ric an Ga s Assoc iation) sinc e the late 1970s has involved the d eve lopm ent o f

frac ture m ec hanic s me thod ology to forec ast lifetimes of high and me dium d ensity PE pipe andjoints fo r syste m operating c ond itions typ ica lly a well estab lishe d interna l pressure and

tem pe rat ure. Slow c rac k grow th tests on lab oratory spe c ime ns at e levated te mp eratures are

used to d eve lop emp irica l relations for the initiation a nd rate of c rack grow th as a func tion of a

measure of the c rac k driving force and temp erature. Kanninen et a l. (1993) found tha t biaxial

stress and tem peratu re shifting ra ther tha n convent iona l uniaxial time tem perature shifting

(sup erpo sitioning ) wa s mo re ap prop riate fo r gas p ipe m aterials. This is bec ause the sem i-

c rysta lline mic rostructu re c auses a c hange in strength of HDPE as tem peratu res cha nge a nd this

c hange a lso c ont ribute s to c hanges as a func tion of time. The shift func tions for pipe HDPEs are

very simple:

a T= exp[ -0.109(Ts-Tt)] for horizon ta l (time ) shifting

b T = exp [0.016(Ts-Tt)] for vertic al (depe ndent variab le) shifting

where Ts is an a rb itra ry (servic e) temp eratu re (0C) relat ive to a referenc e (test) tem pe rature TR.

Howe ver, note tha t while these shift func tions a re the same for all MD/HDPEs tested the

refe renc e b eha viors of the va rious PEs we re d ifferent. As show n in Figure 11 ra te curves and

duc tile/ brittle transitions c an be rep rod uc ibly shifted to any te mp erature w ithin the va riab ility of

da ta g enerated at that temp erature

Figure 11 Use o f bi-direc tiona l shifting to consolida te SC d a ta on HDPE ga s p ipe


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The sma ll amount of testing pe rformed on g eomemb rane s implied the same beha viour as for

p ipes and w ith very simila r simp le shifting func tions. The m ec hanica l durab ility of HDPE

ge ome mb rane s wo uld b e a func tion of the resin used to ma nufacture the ge omem brane, the

ge om etry of the liner feature being e valuate d (plain geom em brane , extrusion sea ms, fusion

sea ms, textures/ struc tures), the stress d istribut ions, and the temp era ture. All the se pa ram eters

synergistica lly influenc e the stress intensity fac to r responsible for c rac k initiation and

prop aga tion. They wo uld be very d iffic ult to pred ic t for ge om em branes, althoug hsimp lifica tions could be ma de to a ssure lifetime s in exce ss of 100 yea rs rela tively qu ickly (Pop ela r

et a l. (1998).

The p ote ntial lifetime o f an HDPE geo me mb rane a s a result of crack initia tion and

prop aga tion unde r a given set o f environm enta l param ete rs has be en initiated but is far from

being finished . Along with the rate of AO d ep letion at service te mp eratures, this is the te sting

that w ill provide the da ta ne cessa ry to p red ic t the d urability of any g iven HDPE ge om em brane .

A sta rt on a pp lying the lessons learned from stud ies on HDPE ga s p ipe has bee n ma de b y

Kanninen, et al. (1992, 1993) who investigated the fracture mechanics of HDPE geomembranes

and the p ossibility of p erforming a cc elerat ed tests at eleva ted tem pe ra tures then shifting rat e

c urves to low er servic e temperatu res. Two heuristic ca lcula tions we re ma de of the lifetime s of

a sea med ge ome mb rane with stress c rac ks in the c enter of the weld and in the g eom emb rane

at the e dg e o f the sea m (Figure 12) simp ly as a result of a lowe r servic e te mp erature co mp ared

to the insta lla tion temp era ture i.e. as a result of con trac tion stresses. As shown in Figure 13, a

stress c rac k ap proxima tely 0.2 mm d eep w ould p rop aga te through a liner at a tem pe rat ure 30C

lower than the installation temp erature in ap proxima tely 1.5 yr. At a tem pe rature differenc e of

120C fina l failure would oc c ur in 0.3 yr. And a t a 120C d ifferenc e a stress c rack 0.08 mm d ee p

wo uld have a fa ilure time o f ab out 0.4 yr, while a 0.3 mm d eep c rac k would ha ve a failure time

of a bout 0.2 yr. While these scop ing ca lculations ge nerate ve ry short crac k pene trat ion times it

should b e no ted that ba seline m ea surem ents we re m ad e o n a ma terial w ith low stress c rac king

resista nc e. The c a lcula tions a lso a ssume a c onstant load (no stress relaxation) and no c on fining

pressures. Neve rthe less, these ca lcula tions do show the ab ility of frac ture me c hanics,

ac c elera ted testing, and shifting of d at a to pred ic t the failure times of spec ific HDPE

ge om em brane s with given flaws in spe c ific environm ents. Then, armed with a d efinition of

c ritic al flaw sizes, CQA mo nitors will be c om e mo re effec tive and e quipm ent c an b e deve lopedto q uant ify observed d efec ts and to m ark them a s c ritica l or sub-c ritica l. The latter need not b e

rep a ired . This is far be tte r tha n the present b lanket spec ific a tions which typ ica lly req uire no

surfac e d efec t to exce ed 10% of the thickness of the geomemb rane while not ha ving a n

instrument to ea sily m ea sure it.

Figure 12 Model used in lifetime ca lcula tions


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Figure 13 Brea k time s a t notc hes due to con trac tion stress

Spe c ific at ions

At p resent p rote c tion ag ainst SC is typica lly c onsidered to be provided if the ge om em brane has

a b reak time e xceed ing 200 hr in the ASTM D5397 notc hed c onsta nt tensile loa d te st as

p romulga ted in the G eo synthet ic Resea rch Institute GRI.GM13 sta nd ard Test Properties, Testing

Freq uenc y and Rec om me nded Warrant for High Density Polyethylene (HDPE) Smo oth a ndTextured Ge om em branes Rev ision 2, 1999. Som e m aterials have b rea k time s of 250 hr, others

have passed 10,000 hr w ithout b rea king . Thus, all HDPEs a re not ident ica l som e a re fa r

superior to others in their resistanc e to SC. These are the one s tha t shou ld b e used for ma ximum

durab ility. Spec ifying HDPE fo r a c ritica l ge om em brane is akin to spec ifying Stee l for b ridg e

c onstruc tion without identifying type s and grad es.

In the same GRI.GM13 sta nda rd adeq ua te o xida tion resistanc e is assume d if a

de c om po sition time e xcee d ing 100 min is ob ta ined in the ASTM D3895 oxida tion induc tion time

(OIT) test. But this test pe rformed a t 2000C d oe s not nec essarily reflec t the o xida tion resistanc e

at low er servic e tem pe ratures, since d ifferent AO pa c kage s have d ifferent c om po nents that

protec t the g eomemb rane ove r d ifferent te mp erature rang es, as show n in Figure 14. For

examp le, pho spha tes only protec t a bo ve 1500C w hile hindered amine light sta b ilizers (HALS)

only protec t b elow 1500C. Thus, a p assing OIT a t 2000C do es not nec essa rily gua rante eac ce ptab le b ehaviour at 800C, and v ice versa . How eve r, in most insta nc es GRI has show n a

relationship b etween o xidation rate s at the two tem pe ratures, but Peg gs (2003) repo rts two

instanc es whe re ad eq uate SCR and OIT values did not result in ad eq uate long te rm

performanc e. In the first case a n SCR of 240 hr and a n OIT of 101 min did not p reve nt c rac king

of an HDPE ge om em brane on exposed land fill line r slopes a fter 8 yea rs. Crac ks oc c urred on the

longitudinal folds of the round -die manufa c tured g eo memb rane , in and a long sea ms, and in

the c ove ring pa tc h at burn-through protrusions. The material ha d lost all of its AO a dd itives and

had me asured OITs of zero a nd 3 min.


16/24


Figure 14 Effec tive tem perature ranges of no a dd itive c om pone nts (Fay & King (1994))

In the sec ond case a ne w HDPE ge om em brane that had a n SCR of 540 hr and an O IT of 240

hr , far exceeding the GRI.GM13 specifications, just met the specification for thermal aging (at

850C) b ut m iserab ly fa iled the UV resista nc e test w ith a reta ined OIT of 35% c om pared to the

spe c ified >50% reta ined OIT. Thus there is muc h that we do no t yet know abo ut the oxidat ion

rat e of HDPE ge om em branes at different tem pe ratu res.

Peg gs et al. (2002) are atte mp ting to de velop a single ma terial d urab ility fac tor (MDF) tha t

c om bines the SCR with an o xidat ion fa c tor de termined at 850C. A Fourier Transfo rm Infrared

Spe c troscop y spe c imen (simulat ing a thin surfac e layer on a bulk samp le) is hea ted in a n oven

for a g iven time the n the chang e in ca rbo nyl group c ontent, rep resenta tive of oxida tion, is

mo nitored. It wa s found nec essary to hea t the spec imen in an oxygen ric h air strea m a t 900C

for at least 24 hr in orde r to sta rt see ing signific ant cha nges in the c arbonyl group pea k. More

rec ent testing by Thom as (2003) sugge sts tha t te sting a t 850C in a high-p ressure oxyge n

at mosphere ma y be nec essa ry to g ene rat e suffic ient oxida tion in a thin spe c imen in area sona b ly short time 20 to 50 hr. Suc h an MDF will not q uantify the t ime a t wh ich lea ks w ill

oc c ur in a given lining system but it will fac ilitate a qua lita tive ranking o f the d urab ilities of HDPE

ge om em brane s ma de from different resins and with different AO packages. Then whe n

expe riments and c a lculations are ma de t o determine the lifetime of one o r two produc t, othe rs

ca n be sca led ac co rdingly.

How eve r, to further comp lica te ma tters, the exac t com binations of c ircumstances tha t

gene ra te stress c rac king a re a lso no t we ll estab lished . In a pulp mill b lac k liquor pond

(effec tively a co nfined situation) at an inco ming liquo r tem pe ra ture of a bo ut 700C

environm enta l stress c rac king (due to dete rge nt in the liquo r) oc c urred at the top s of w rinkles in

an indisc riminate fashion sma ll wrinkles on the floo r were c rac ked but large kinked wrinkles a t

the toes of slopes we re not . Inte rme d iate w rinkles on the slopes a lso c racked ind isc riminate ly.

Therefo re, it is impossible to p red ict the c om b inations of p arame ters tha t will ge nerate

env ironm enta l stress c rac king .

Wrinkles have also caused problems in HDPE liners in concrete basins in mining facilities where

c ast-in liner has be en used on wa lls and a round the p eriphery of the floo r, and loose liner has

be en used o n the floor. Ab sorption of orga nic co mp one nts of heap leac h process solutions

and swe lling of surfac e laye rs has c aused large w rinkles to b uild up a ga inst the p eriphera l we ld

between loose a nd anchored liner with the result tha t eve ry millimet er of we ld expe rienc es a


17/24


significant peel stress. The we akest seg me nts of the we lds have sep ara ted . Tha t this happens

ma y not b e to o surprising whe n liner sea m spe c ific at ions often allow o ne o f the five pee l and

shea r spec imens to fa il a 20% failure rate . When suc h a sea m ha s sep ara ted the long te rm

durab ility of the liner is com promised even mo re b ec ause it is very difficult to m ake an effec tive

rep air we ld on liner c onta ining ab sorbed o rga nics. Such rep air/ pe el/ rep air/ pe el be havior

c ontinues. Ultima tely the ac id co mp one nt of the eleva ted tem pe rat ure solution might oxidize

the liner with resultant stress c rac king on the to ps of w rinkles and a long the e dge s of w elds.

A survey of ma ny co llea gues revea ls that none are aw are of a ny lea kage that ha s

deve loped in a land fill bottom liner after a fa c ility has be en p lac ed in servic e tha t is not d ue to

external influenc es. Howe ver, a land fill in Minnesota ha d a b ulldozer nick nea r a sump during

c onstruc tion that wa s rep a ired . The system , with wa ste on the floo r, op erated without any leak

indica tion for ab out three mo nths. Lea kage the n sta rted a t a rate o f abo ut 5000 lpd,

eq uivalent to a hole of ab out 6 mm d iam eter unde r a 300 mm hydraulic head (Giroud and

Bona parte (1989)). Elec tric a l leak surveys on top o f 9 m of waste a nd d ie testing suggested tha t

the lea k wa s not at the same loc at ion as the sump a nd the rep a ired p atc h. Unfortunat ely, the

suspe c ted lea k was not excava ted to c onfirm its existenc e a nd to de termine its c ause. There

have be en a numb er of instances whe re leakage rate s have sudde nly inc rea sed afte r som e

time, b ut these have gene rally be en found due to a n increa se in the prima ry leac hate level

ab ove original d efec ts in the liner that previously were not leaking.

Surveys by Bona pa rte a nd Gross (1990) and others since then have show ed tha t leakag e

ra tes through the p rima ry liners of p onds and land fills va ry significantly from effec tively zero to

quite signific ant va lues (3300 lphd ). A typ ica l Ac tion Lea kag e Rate in US land fills a t whic h lea ks

must be found a nd repa ired is 200 lphd . This is not d ifficult to ac hieve . How eve r, when a

req uirem ent for 70 lphd wa s not m et a t one ha zardo us wa ste p rojec t atte mp ts to m ake repa irs

only resulted in a highe r leakag e rate . At a doub le lined c onc rete b asin projec t a few drips

from the leakag e detec tion system w as not c onsidered sat isfac tory pe rforma nc e b y the owne r

who insisted that rep airs be ma de ; the d rips increased to a stea dy flow which could no t be

stopped . Surveys performed by Koe rner et a l. (2000) have show n (Figure 15) leakag e rates a t

d ifferent stages of a land fill lifetime in d ifferent types of lining systems to ta per off d uring c losure

to b e ve ry low less tha n 1 lphd Koe rner (2003 persona l comm unica tion) is no t awa re o f anyHDPE land fill liner that has deve lope d a hole in servic e from anything othe r than a n e xternal

influence, suc h as a b ulldozer.


18/24


Figure 15 Primary liner leakag e ra tes a t d ifferent stages of land fill lifetime .

Sta ge1: Initial life Sta ge 2: Ac tive life Sta ge 3: Post-c losure

Nosko e t a l. (1996, 2000), and Rollin (1999) have c learly show n the loca tions, freq uenc y, and

c auses of lea ks ma de in liners during the ir insta lla tion, co vering, and ea rly sta ge s of operat ion.

Their da ta a re summ arized in Tab le 3 and Figure 16. In cove red liners mo st dama ge (ove r 70%)

is c aused during p lac em ent o f the co ver soil, and o nly 24% of leaks oc c ur in sea ms. Howe ver, in

exp osed liners a lmost 80% of leaks a re on seams.


19/24


Tab le 3: Sta tistic s of Liner Damage

WHEN/ WHERE AMOUNT DETAILS AMOUNT

Line r insta llation 24% Extrusion

Melting

Stone Punc ture

Cuts

61%

18%

17%

4%Co vering 73% Stone Punc tures

Hea vy Equipm ent

Grad e Stakes

68%

16%

16%

Post-Co nstruc tion 2% Heavy Equipme nt

Construction

Weather, etc.

67%

31%

2%

Flat Floor 78% Sto ne s

Hea vy Equipm ent

81%

13%

Co rner, Edge 9% Stones

Hea vy Equipm ent

Welds

59%

19%

18%

Und er Pipes 4% Sto nesWelds

Hea vy Equipm ent

Worker

Cuts

30%27%

14%

15%

14%

Pipe Penet rations 2% Welds

Worker

Cuts

91%

8%

1%

Roa d , Sto rage, etc . 7% Hea vy Equipm ent

Stones

Worker

Welds

43%

21%

19%

17%

Figure 16 Freq uency of lea ks in p rima ry geom em brane liners


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Summary

In summ ary, an HDPE geo me mb rane used in a land fill is mo st unlikely to fail due to c onventiona l

c hem ica l de grad ation as a result of be ing in co ntac t with MSW leac hate s. HDPE

ge ome mb rane s have ad eq uate c hemica l resistanc e to endure a nd retain their integrity we ll

be yond othe r fac tors that w ill cause a liner to fa il. How eve r, if the lea chat es conta in unusua lly

high c onc entrations of oxid izing ac ids, chlorina ted solvents, or dete rge nts tha t rema in c onstantly

on the liner for c onsiderab le time s, env ironm enta l stress c racking may oc c ur. By fa r thepred om inant mod e of failure is due to ma n-induced da ma ge during c onstruc tion, suc h as stone

punc tures, bulldozer da ma ge , and d ep th stake punc turing. When this type of da ma ge is

p rec luded or sta b ilized , premature fa ilures will then only oc c ur by simp le stress c rac king , or

oxida tion fo llow ed by stress c rac king a t reg ions of ind uc ed stress suc h a s c rea ses and wrinkles,

stone p rotrusions, sea ms, textured surfac es, et c . The susc ep tibility of the liner to these kinds of

stresses will be a func tion of the SCR of the spec ific resin used , and suc h resista nc es presently

vary by a fac tor of abo ut 500.

The d urab ility of an HDPE geo me mb rane is a func tion of the following ;

The SCR and OIT of the resin used and of the g eo me mb rane itself The know led ge of the d esign eng ineer in selec ting and spe c ifying t he m ost approp ria te

HDPE, and designing the liner for minimum stress on slop es, a t sumps, at penet rations,

and in anc hor trenc hes.

The know led ge of the engineer in de signing the system to ac com mo da te the interimstresses betw een insta lla tion a nd design o pe rating c ond itions

The knowledg e o f the eng ineer in spe c ifying ad eq uate punc ture protection for thegeomembrane.

The a bility of the m anufac turer to produc e a c onsistent homoge neous ma terial with aminimum num be r of internal and surfac e flaws and with effec tive antioxidat ion a dditives

The smo othness, uniformity, and density of t he sub grade The q ua lity of the insta lla tion lac k of wrinkles, intima te c ontac t w ith subgrade, sea ms,

penetrations, minimum extrusion welding, minimum shear stress on slopes.

Qua lity of CQA Plac em ent of cove r layers Op eration of eq uipm ent on c over layers Plac em ent of first laye r of w aste

If all of these item s are op timized it is expec ted tha t an HDPE ge om em brane in an MSW

land fill shou ld last for about 400 yr. Exposed liners a re another mat ter altog ether, c lea rly

dep end ing on the e xpo sure c ond itions and req uiring a b ette r unde rstanding o f oxidat ion rates.

Perhap s 75 yea rs is an ap p rop riate p lac e to sta rt. But lifetimes excee d ing 100 yea rs ma y not be

nec essary since, b y then, there w ill be be tter things to d o w ith p resent wa ste and it w ill be be ing

mined w ith unusab le co mp onents dispo sed of in bette r wa ys ma ybe even ato mized to

not hing. Future generat ions will not want to ma intain sea led c ells of their forebea rs waste, in

the sam e w ay tha t there a re p resently very few infrastruc tures that we are using tha t a re in their

1875 1900 as-insta lled cond ition. We delude ourselves if we think we have the ultima te solutionto wa ste conta inment and d isposa l. Neve rthe less we shou ld still ta rge t the maximum

ge om em brane a nd liner durab ility within existing tec hnology.

Onc e a liner has suc c essfully be en insta lled and t here is no lea kag e the only inte rna l

influenc es that c an c ause ad ditiona l lea ks a re such things as:

Wrinkles and prot rusions causing SC befo re stress relaxation c an o cc ur


21/24


Wrinkles at sea ms c ausing slow p ee l sep a ration a nd prop ag a tion of c ritic al, but no t thenpenetrating, flaws in seams

Crazes induc ed by pee l sep ara tion initiating stress c racking a t the stressed sea m p rior tostress relaxa tion oc curring

Shea r and pe el stresses at ove rhea ted and ove r-ground sea ms adjacent t o w rinklesinitiating stress c rac king . Elevated g eomemb rane tem pe ratures c ausing oxida tion and a c c elerating stress

c rac king if stress relaxation is not a c c elerate d in p rop ortion.

Stress c rac king on slopes with rand om ly textured liner on the t op surfaces p rior to wastestabilization.

All of the se p ossibilities c an be m inimized by the use o f an HDPE ge om em brane w ith high

stress c rac king resistanc e and g oo d o xida tion resistanc e. In prac tice , if sett lement stab ilizes and

if no fa ilures ha ve a lrea dy oc curred it is unlikely that any subseq uent fa ilures will oc c ur.

Conclusion

In prac tice , while one c an m ake a ny numb er of ag ing a nd d eg rad ation ca lculations of

lifetimes, half lives of spec ific index me chanic al a nd physica l prope rties, and ac tivation

ene rgies, the p rac tica l performa nce o f a lining system is p resently co ntrolled by huma n

ac tivities. Onc e the liner is insta lled a nd w orking without leakag e the development of further

lea kag e is a func tion o f its stress c rac king resista nc e, its oxida tion resista nc e, the stresses

gene ra ted , and the stress relaxation ra te . The synergism betwe en these performa nc e

c harac teristics is extrem ely difficult to pred ic t. The m ost me aningful tec hnique wo uld be to use

a frac ture mec hanics app roa c h. Howe ver, as is ev iden t, this still req uires a signific ant am ount of

resea rc h effort. In assessing the d eve lopme nt of flaws the most imp ortan t thing to note is tha t

a ll HDPEs are no t the same - their me chanica l durab ilities c an va ry by a fac to r of 500.

Spec ifying HDPE for a c ritic a l lining system is som ew ha t a kin to spec ifying Stee l fo r a b ridg ewithout identifying types or grad es. Should the go lf c ourse de corative pond be lined w ith the

same liner as a hazardous wa ste liquid p ond a s is p resently done ?

In the m ea ntime, the b est solution is to selec t a g eo me mb rane w ith the highest stress

c rac king resistanc e a nd the b est p erformance in the GRI.GM13 thermal a ging test, and to install

it ca refully. Expo sed liners w ill a lso req uire the UV resista nc e test. With a h igh SCR HDPE liner

the em pha sis will even m ore b e on the c are o f design eng inee rs, installation c ontrac tors,

gene ra l contrac to rs, CQA firms, and ow ners to e nsure tha t the liner has no ho les in it when it is

placed in servic e.

References

1. Hsua n, Y.G., Koerner, R.M., and Lord, Jr., A.E. (1992), The Notc hed Consta nt TensileLoa d (NCTL) Test to Eva luate Stress Crac king Resista nc e , Proc ee d ings of the 6th GRI

Seminar, Folsom, PA, pp 244-256.

2. Peg gs, I.D., (2003), Forensic A na lysis of the Performanc e G eo me mb rane and GC LLining Syste ms, IFAI, Roseville , MN, Tab 7.

3. Thomas, R., (2002), Priva te c om munica tion .4. Brown, N., and Lu, X., (1993), Co ntrolling the Qua lity o f PE Ga s Piping Systems by

Co ntrolling the Qua lity o f the Resin, Proc eed ings Thirtee nth Internat iona l Plastic Fuel

Ga s Pipe Symp osium, Am erican G as Assoc iation, p p 327-338.


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5. Sang am, H.P., Row e, R.K., Permea tion o f Orga nic Polluta nts Throug h a 14 yea r oldField-Exhumed HDPE Ge om em brane , Ge osynthe tics Sta te o f the A rt Rec ent

Developme nts Vol. 2, A.A. Balkem a Pub lishers, Lisse, The Netherland s, pp 531-534.

6. Park, J.K., Sakti, J.P., Hoopes, J.A., (1995), Effectiveness of Geomembranes as Barriersor Orga nic C om pounds , Proc ee d ings of G eo synthe tics 95, IFAI, Roseville, MN, pp

879-892.

7. Giroud , J.P., and Bonaparte , R., (1989), Lea kag e Through Liners Co nstructe d w ithGe ome mb ranes Part 1. Ge om em brane Liners , Geo textiles and Ge om em brane s,

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APPENDIX A

Mod el for Stand ard M SW Lea c hate forChemic a l Resista nc e Testing

(TRI/Environm enta l)

Documents

Id Pig Suk Paper