Designer s Forum GCL design series Par 1: GCL performance as a fluid barrier By Richard B. Erickson, Richard Thiel, P.E., and Gregory N. Richardson, P.E., Ph.D. Over he pas 18 monhs, GSE in conjuncion wih indusry design and academic professionals have developed he indusry s firs comprehensive GCL design guidance documen, he GSE GundSeal Design Manual (Thiel e al. 21). Alhough he manufacurer of one paricular ype of GCL produc sponsored he manual, i feaures design mehodologies and procedures for evaluaing all ypes of GCLs in a wide range of composie liner applicaions. I presens sae-of-he-pracice design principles relaed o hydraulic performance evaluaion, slope sabiliy analyses, consrucion and durabiliy issues in uilizing GCLs in boom liner sysems, caps, ponds, and secondary conainmen lining applicaions. An overview of he fundamenal design issues presened in he manual is summarized in his hree-par GFR series, wih implicaions for making a design uilizing GCLs simpler, quicker and more effecive. Par 1 presens an overview of he GCL design issues and GCL insallaion opions, and discusses design principles relaed o hydraulic performance and leakage. Par 2 focuses on GCL slope sabiliy, followed by Par 3 which expands on GCL insallaion and durabiliy. Types of GCL composie lining sysems Convenional composie-liner applicaions in wase-conainmen indusries require a geomembrane ha overlays a low-permeabiliy compaced clay liner. Alernaely, GCLs are now commonly used as an alernaive o replace all or par of he lining sysem. Fabric-suppored GCLs have a hin layer of benonie (ypically a sodium-based monmorillonie clay) carried beween various combinaions of woven and nonwoven needlepunched geoexiles. Producs are available in a non-reinforced configuraion (benonie glued beween he fabrics), or in Figure 1: Composie liner consising of a primary geomembrane and a reinforced or unreinforced fabricencased GCL. Unreinforced benonie or needlepunch reinforced benonie Figure 2: Overlapped GM-GCL wih shingle seams. backing (smooh or exured).4 mm hru 2. mm Benonie coaing B Fabric-encased GCLs geoexile/benonie/geoexile 12 in. (3 mm)* *Overlap lengh dependen on subgrade condiion and anicipaed selemen a reinforced configuraion (ouer geoexiles are siched or needlepunched ogeher). To creae a composie liner sysem, hese ypes of GCLs are overlain by a coninuous geomembrane, as depiced in Figure 1. The geomembrane-suppored GCL (GM- GCL) is comprised of a hin layer of benonie mixed wih a waer-based adhesive ha aaches i o a polyehylene geomembrane. This produc has been used as a oneproduc composie liner in boom liner and cap applicaions, effecively replacing boh he geomembrane and compaced clay componens of radiional prescripive composie liners. There are wo general design configuraions for he GM-GCL produc: Single composie mode In his insallaion, he benonie side of he maerial is generally insalled face down and he geomembrane side face up, o form a one-produc composie (geomembraneclay) liner. Normally, he overlaps are no mechanically joined, bu are overlapped for self-sealing, as shown in Figure 2. I is also possible o weld he geomembrane componens ogeher uilizing convenional geomembrane welding echniques, including eiher dual-rack ho-wedge welding or exrusion welding procedures (Figure 3). Encapsulaed mode In his insallaion, a supplemenal geomembrane is insalled agains he benonie side of he maerial, as shown in Figure 4. In his applicaion, he GM-GCL produc is usually insalled wih he benonie side face up and he geomembrane side facing agains he subgrade, wih a supplemenal geomembrane insalled over he benonie surface. This configuraion, however, can also be reversed so ha he GM- GCL is deployed on op of a previously deployed geomembrane, wih he benonie side face down. Noe ha his configuraion can also be used wih fabric-encased GCLs by deploying he GCL beween wo geomembranes. The encapsulaed design mode offers he following disinc perfor-
mance advanages: improved fluid conainmen; improved benonie durabiliy during consrucion by prevening pre-hydraion of he benonie; and improved slope sabiliy. Performance as a fluid barrier The hydraulic performance of GCLs can be evaluaed using sandard acceped leakage models. Leakage hrough a composie liner may be caused by: defecs in he primary geomembrane resuling primarily from insallaion damage; coinciden defecs in he upper and lower geomembranes in encapsulaed GCL insallaions (geomembrane-claygeomembrane); and seepage a overlapped seams when he geomembrane seams are no welded for he GM-GCL produc. The design equaions and general approach for evaluaing composie liner leakage, and environmenal proecion comparisons beween he GM-GCL and an equivalen geomembrane-compaced clay composie liner are summarized below. Leakage modes Defecs in composie-liner geomembranes Empirical modeling and field observaions (Giroud and Badu-Tweneboah 1992; Giroud 1997) have resuled in he Giroud equaion for esimaing leakage hrough a hole in he geomembrane porion of a composie liner. The empirical equaion akes he form of Equa- ion 1. For hydraulic head 3 m, he empirical equaion akes he form of Equaion (2) (Thiel e al. 21): Equaion (1) Q GM = C [1 +.1(h w / ).95 ] a.1 h w.9 k s.74 [For h w < 3 m, and defec diameer a 5x1-4 m 2 (25 mm dia.)] Equaion (2) Q GM = C [1 +.1(h w / ).95 ] a.1 h w.9375 k s.74 Figure 3: Exrusion-welded GM-GCL seams. backing (smooh or exured) 3 mil (.75 mm) hru 8 mil (2. mm) 6 in. (15 mm) 12 in. (3 mm)* Figure 4: Encapsulaed GM-GCL. [For h w 3 m, and defec diameer a 5x1-4 m 2 (25 mm dia.)] where Q GM = rae of leakage hrough a defec (m 3 /s), C = a consan relaed o he qualiy of he inimae conac beween he geomembrane and is underlying clay liner, h w = head of liquid on op of he geomembrane (m), = hickness of he soil componen of he composie liner (m), a = area of defec in geomem- Approx. 3 in. (75 mm) Benonie free geomembrane edge for welding backing (smooh or exured) 3 mil (.75 mm) hru 8 mil (2. mm) 6 in. (15 mm) 12 in. (3 mm)* Benonie coaing Exrusion weld (grind geomembrane prior o welding *Overlap lengh dependen on subgrade condiion and anicipaed selemen Overlying geomembrane Benonie coaing *Overlap lengh dependen on subgrade condiion and anicipaed selemen Figure 5: Drawing illusraing he facors aken ino accoun by he Giroud equaion for esimaing leakage hrough a hole in he geomembrane par of a composie liner. Conac C-Value h W Q GM Defec wih area a Clay wih k s m/s
brane (m 2 ), and k s = hydraulic conduciviy of he underlying clay liner (m/s) (see Figure 5). The basis for Equaion (1) is referenced in he U.S. EPA Technical Manual (1993), and is incorporaed ino he laes versions of he HELP compuer model (U.S. EPA 1994) used for predicing landfill leachae generaion and leakage. Defecs in encapsulaed benonie sysem For an encapsulaed design (Figure 4), he size of he defec in he lower geomembrane would conrol leakage, and leakage would occur when an even caused coinciden defecs in he upper and lower geomembranes. In his case, Darcy s law conrols he advecive flow rae hrough a defec of a given size. The leakage equaion would ake he following form: Equaion (3) Q enc = k s i a = k s [(h w + ) / ] a where Q enc = leakage (m 3 /s), k s = hydraulic conduciviy of he benonie, i = hydraulic gradien [(liquid head h w + ) /, where = benonie (m/s) hickness], and a = area of coinciden defecs hrough an encapsulaed liner sysem (m 2 ) (see Figure 6). Figure 6: Drawing illusraing he facors aken ino accoun by he equaion, derived from Darcy s law, for esimaing leakage hrough a hole in an encapsulaed benonie sysem. Figure 7: Drawing illusraing he facors aken ino accoun by he equaion, derived from Darcy s law, for esimaing seepage a overlapped (unwelded) GM-GCL seams. h w Q olap Q enc B Coinciden defecs wih area a Benonie wih k s m/s h w s Benonie wih k s m/s Seepage a overlapped (unwelded) GM-GCL seams In he case of overlapped GM-GCL seams (Figure 2), liquid will seep direcly ino and possibly hrough he overlaps. Therefore, he seepage rae hrough overlapped GM-GCL seams mus be quanified in a leakage evaluaion. Due o he weigh of is benonie coaing, an insalled GM-GCL lays fla on he subgrade. This virually eliminaes wrinkles and resuls in excellen conac beween overlapped panels a heir seam areas. For a ypical overlap disance of 3 mm, i would ake more han 5 years before seepage would begin hrough he GM-GCL overlap wih a fluid buildup of up o 3 mm. Seadysae leakage would mos likely ake several more years o develop as documened by Dr. David Daniel in Thiel el al. (21). This seam performance is based on daa provided by he large-scale ank ess repored by Esornell and Daniel (1992) and he Cincinnai U.S. EPA GCL es plo P exhumed afer 4.5 years of performance. Leakage per uni lengh due o seepage along a sauraed GM-GCL overlap would be calculaed in accordance wih Darcy s law as follows: Equaion (4) Q olap = k s (h w /B) where Q olap = flow rae per uni lengh (m 3 /s m), k s = hydraulic conduciviy of he benonie (m/s), h w = hydraulic head on op of he liner (m), B = widh of overlap (m), and = hickness of he benonie (m) (see Figure 7). To deermine leakage due o seepage a overlap seams, he oal linear lengh of seam for a given projec mus be calculaed. The general lengh of overlap seams (S) in an insallaion area (A) is: Equaion (5) S = A (1/L + 1/W) where L = average lengh of panels less overlap (ypically 51.2 m), W = average widh of panels less overlap (ypically 5. m). Applying Equaion (5) o a ypical GM- GCL insallaion hus resuls in approximaely 22 m of overlap seam per hecare of lined area. The acual lengh of overlap seam would increase slighly if he complexiy of he insallaion increased due o srucures, for example, or irregulariies. Toal leakage a overlapped seams is subsequenly deermined by muliplying Q olap by he lengh of seam S for a given lined area A. Facors affecing leakage Inimae conac C-Value The Giroud equaion conains he facor C which accouns for he degree of inimae conac beween he geomembrane and adjacen clay. In Thiel e al. (21), Dr. J.P. Giroud evaluaes he conac C-facor beween he benonie componen of a GM- GCL and an adjacen geomembrane by analyzing he approaches developed and expounded by Rowe (1998),
Designer s Forum Foose e al. (21), and Harpur e al. (1993). Using he resuls published in hose references, Giroud recommends a conservaive value of C =.1 for conac beween he benonie componen of he GM-GCL and is geomembrane. The second auhor uses a value of C =.5 for fabric-encased GCLs (NWNP side agains a geomembrane) o represen excellen conac condiions. Hydraulic conduciviy The hydraulic conduciviy k s of sodium benonie in GCLs is affeced by he level of normal sress applied o he GCL, and chemical aleraions caused by differen permeaing liquids ha may increase he hydraulic conduciviy of sodium benonie. Guidance o selecing he appropriae hydraulic conduciviy value(s) for a projecspecific GCL applicaion and liquid is presened in Chaper 2 of Thiel e al. (21) as compiled by Dr. David Daniel. Projec-specific design assumpions Liquid head buildup, h w The buildup may vary from less han 25 mm for cap applicaions, up o 3 mm for regulaed allowable buildup above boom liners, and elevaed liquid head for secondary conainmen leakage evens and impoundmen applicaions. Defec area, a, and frequency of defecs per uni area Indusry average sandards for esimaing defecs in an insalled geomembrane assume ha approximaely wo o en 1 mm 2 holes per ha exis afer a geomembrane is deployed and covered wih soil. The number and size of hese defecs can be reduced hrough more horough CQA procedures, such as he use of an elecric defec-deecion survey afer he overlying soil has been placed. The qualiy of insallaion and he assumed size and frequency of geomembrane defecs should be evaluaed on a projec-specific basis. Clay liner hickness, The hickness of compaced clay liners is generally given by prescripive requiremens. The hickness of he GCL benonie layer is based on he mass loading of benonie (sandard 37 g/m 2 a % moisure) a he design normal load. The hickness of he hydraed benonie componen of he GM-GCL as a funcion of effecive compressive sress ranges from 8.5 mm o 3 mm for a normal load range from 1 kpa o 1 kpa, respecively (Thiel e al. 21). Leakage rae comparisons In evaluaing hydraulic performance, each liner sysem is analyzed by uilizing he projec-specific design crieria oulined above and applying he applicable leakage equaions. The oal poenial leakage for he composie liner sysem is calculaed by combining he leakage hrough he assumed frequency of geomembrane defecs wih he leakage a he overlapped seams, if he geomembrane seams are no welded. The mehodology for deriving a design leakage rae for composie liners is presened in Thiel e al. (21) wih examples demonsraing various design assumpions and performance crieria in each design chaper for boom liners, caps, ponds, and secondary conainmen applicaions. Figure 8 (p.2) presens an example of leakage rae comparison beween he various GM-GCL seam configuraions (overlapped seams, welded seams, and encapsulaed benonie alernaives) and a prescripive U.S. EPA Subile D geomembrane-compaced clay composie liner for a ypical landfill boom liner applicaion (Erickson and Thiel 22). For he calculaions in his comparison, he design assumpions were: liquid head h w = 3 mm; benonie hickness ben = 5 mm; assumed area of defecs a =.1 m 2 ; defecs per hecare n = 1; and overlap disance B = 3 mm. Design assumpions for he prescripive compaced clay liner included hickness ccl = 6 mm and hydraulic conduciviy k ccl = 1 x 1-9 m/s. As shown in Figure 8, he simple-overlap design wih he one-produc composie liner will environmenally ou-perform a prescripive Subile D liner (geomembrane over 6 mm compaced clay layer) even when is benonie s hydraulic conduciviy is increased o k ben = 1 x 1-9 m/s. The environmenal performance of he encapsulaed GM-GCL design is excepional, wih esimaed leakage raes beween 1 and 1, imes lower han he prescripive geomembrane-compaced clay liner (showing as nearly zero leakage on he graphical scale in Figure 8), depending on he hydraulic conduciviy of he benonie. The hydraulic analysis and design mehodology presened above can be adaped o all design applicaions in order o evaluae he environmenal performance and equivalency of a GM-GCL composie liner o convenional geomembrane-compaced clay composie liner sysems for a projec-specific se of design parameers. Summary Wheher a GCL is used in a boom liner or cover, as a single-composie liner or wih a separae geomembrane, hree fundamenal design issues should be considered for GCL applicaions, including: performance as a fluid barrier; slope sabiliy; and insallaion and durabiliy. Par 1 of his series on GCL design guidance focused on he general approach and design equaions required for evaluaing GCL hydraulic performance based on sandard acceped leakage models. The hree poenial modes of leakage hrough a geomembrane-clay composie liner and leakage analyses were presened, including (1) leakage hrough geomembrane defecs, (2) geomembrane defecs in encapsulaed benonie (geomembrane-benoniegeomembrane) liners, and (3) seepage a overlapped (unwelded) GM-GCL seams. These leakage mechanisms, combined wih projec-specific design facors (including inimae conac, hydraulic conduciviy, liquid head, frequency and size of geomembrane defecs, and clay liner hickness) provide he basis for evaluaing global projec leakage. Finally, a mehodology for comparing poenial lining sysem leakage raes hrough a geomembrane-compaced clay composie liner vs. a GM-GCL composie liner was presened. This leakage analysis allows he design praciioner o effecively evaluae environmenal performance and equivalency of a GM-GCL compared o convenional compaced clay-geomembrane composie liners for a given se of design parameers. The subsequen pars of his GCL design guidance series will focus on GCL slope sabiliy (Par 2) and insallaion and durabiliy (Par 3). References Erickson, R.B., and Thiel, R. 22. Design and applicaion of he geomembrane suppored GCL in one-produc and encapsulaed composie liner sysems. Geosynheic Clay Barriers In-
Figure 8: Comparison of hydraulic performance of example composie boom liner sysems wih (a) GM-GCL alernaive liners and (b) U.S. EPA Subile D composie boom liners. Esimae Leakage vs. Hydraulic Conduciviy 3 a Expeced leakage rae for GM-compaced clay liner Leakage Rae (liers per hecare per day) Overlapped GM-GCL GM-GCL welded seams Encapsulaed GM-GCL U.S.EPA Subile D a b d 1.E-8 1.E-9 1.E-1 1.E-11 c Benonie Hydraulic Conduciviy (m/s) 2 5 2 1 5 1 b c d head h w =3mm GM defecs n =1/ha k ccl =1 x 1-9 m/s Expeced leakage rae for encapsulaed GM-GCL Typical k-value for benonie exposed o ash leachae Range of k-values for benonie under 2 kpa load ernaional Symposium. Nuremberg, Germany, April. Esornell, P., and D.E. Daniel. 1992. Hydraulic conduciviy of hree geosynheic clay liners. ASCE Journal of Geoechnical Engineering, vol. 118, no. 1: 1592 166. Foose, G.J., C.H. Benson, and B.E. Tuncer. 21. Predicing leakage hrough composie landfill liners. Journal of Geoechnical and Geoenvironmenal Engineering, June: 51-52. Giroud, J.P., and K. Badu-Tweneboah. 1992. Rae of leakage hrough a composie liner due o geomembrane defecs. Geoexiles and s, vol. 11:1 28. Giroud, J.P. 1997. Equaions for calculaing he rae of liquid migraion hrough composie liners due o geomembrane defecs. Geosynheics Inernaional, vol. 4, nos. 3-4: 335-348. Harpur, W.A., R.F. Wilson-Fahmy, and R.M. Koerner. 1993. Evaluaion of he conac beween geosynheic clay liners and geomembranes in erms of ransmissiviy. Proceedings of he 7h Annual GRI Seminar. Drexel Universiy, Philadelphia, PA, Unied Saes. 138 149. Koerner, R.M., D.A. Carson, D.E. Daniel, and R. Bonapare. 1996. Curren saus of he Cincinnai GCL es plos. Proceedings of he 1h GRI Seminar. Drexel Universiy. Philadelphia, PA, Unied Saes. Rowe, R.K. 1998. Geosynheics and he minimizaion of conaminan migraion hrough barrier sysems beneah solid wase. Proceedings of he Sixh Inernaional Conference on Geosynheics. Indusrial Fabrics Associaion Inernaional, Roseville, MN, Unied Saes. 27 12. Thiel, R., D. Daniel, R. Erickson, E. Kavazanjian, and J.P. Giroud. 21. The GSE GundSeal GCL Design Manual. GSE Lining Technology Inc., Houson, TX, Unied Saes. U.S. EPA. 1993. Technical Manual for Solid Wase Disposal Faciliy Crieria. Office of Solid Wase and Emergency Response, U.S. EPA. Repor U.S. EPA53-R-93-17, November. U.S. EPA. 1994. The Hydrologic Evaluaion of Landfill Performance (HELP) Model, User s Guide and Engineering Documenaion for Version 3. Office of Research and Developmen, Unied Saes Environmenal Proecion Agency, Repor U.S. EPA/6/R- 94/168b.