DESIGNING WITH GEOSYNTHETIC CLAY LINERS (GCLs) Presentation Topics • About Bentonite • GCL Products and Properties • Design Considerations – Hydraulic Performance – Chemical Compatibility – GCL Shear Strength and Slope Stability Chris Athanassopoulos, P.E. CETCO Construction Engineering Group [email protected] Why is bentonite such a good lining material? What is Bentonite? • Bentonite is an ore whose main ingredient is the clay mineral montmorillonite • It was formed hundreds of millions of years ago through the aqueous deposition of volcanic ash • • • • Myth #1: Myth: “Powdered bentonite is superior to granular bentonite” Truth: There is no difference in hydraulic performance between powdered and granular. The individual bentonite platelets (approximately 1 nm thick and 0.2 to 2 microns in length) are the same in both materials. It swells when hydrated! Bentonite swells up to 15 times its original volume if unconfined. Bentonite expands and conforms to irregular surfaces, penetrations, and infiltrates cracks and voids. Swell pressure makes it an active sealing agent. DRY WETTED Powdered Bentonite GCL Properties • A manufactured hydraulic barrier consisting of clay bonded to a layer or layers of geosynthetics (ASTM D4439) • Very low permeability (< 5 x 10-9 cm/sec) • Self-healing if punctured • Self-seaming at overlaps • Replaces compacted clay liners (CCLs) Bentomat DN (Bentoliner BL200 and BL300) – World’s most popular GCL, established 1990. – Ideal for most sloping applications – Certified permeability of 5 x 10-9 cm/sec. – Used in landfill liners, closures, tank farms, wetlands, heap leach pads Key Design Elements • Hydraulic Performance—will the GCL provide the required level of containment? • Chemical Compatibility—what about compatibility with contaminated liquids? • Shear Strength and Slope Stability—will the liner system remain stable on sloping surfaces, berms, ramps, and roadways? – Reinforced GCL with two high interface friction nonwoven geotextiles – Needlepunched to same specifications as Bentomat ST – High tensile strength and elongation Hydraulic Performance • Governed by Darcy’s Law: Q Bentomat ST (Bentoliner BL080 and BL100) k uiu A where: Q = flow through liner k = permeability i = hydraulic gradient = (head + thickness) / thickness A = lined area • Note: Flux = q = Q / A = k i The “Permeability Test” • It’s really a flux test! • k is calculated by solving Darcy’s Law • ASTM D 5084 is for determining k value of soil • ASTM D 5887 is for determining flux (and now k) of GCLs. What Darcy Didn’t Tell You: Composite Liner Performance GCL Permeability varies with Effective Stress! • A composite liner will leak only where there is a hole in the geomembrane (diffusion is ignored). • Flow from the hole depends on: K (c m /s e c ) 1.00E-07 1.00E-08 10x – – – – Size of hole Hydraulic head on liner system Permeability of materials above and below the geomembrane Contact between membrane and underlying soil Landfill Liners Liners/Slopes Landfill Bottom Caps, Mine Closures, PondsLeach Pads Heap 1.00E-09 1.00E-10 1 10 100 1,000 10,000 Effective Stress (kPa) Petrov, R.J. et al (1997) “Selected Factors Influencing GCL Hydraulic Conductivity”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE Giroud Leakage Equation Q Cqo 1 0.1 h t s 0.95 a 0 .1 h 0.9 k 0.74 where: Q = flow through hole in geomembrane Cqo = quality of intimate contact between membrane/soil h = liquid head on liner system (< 3 m) ts = thickness of soil beneath geomembrane a = area of hole in geomembrane (circular, max diam 25 mm) Ks = permeability of soil under geomembrane (constant) Giroud Equation Results • Shows that GCL-based composite liner outperforms CCL-based composite liner by a factor of 30 • These calculations include many assumptions that require designer’s best judgment—actual results will vary Reference: Giroud, J.P., (1997) "Equations for Calculating the Rate of Liquid Migration Through Composite Liners Due to Geomembrane Defects,“ Geosynthetics International, Volume 4, Nos. 3-4, pp. 335-348. Liner Leakage Rates • There is ACTUAL leakage data available! • Major study conducted for USEPA (Bonaparte, Daniel, and Koerner - 1999) • Encompassed 91 Landfills and 287 cells • Cells monitored for 10 years • Double-lined • Leakage through primary liner (either GM/CCL or GM/GCL) was evaluated by measuring flow into the underlying leachate detection system Field Performance Comparison Composite Liner Leakage Rate Performance (Bonaparte, et al. 2002) 400 Sand as leak detection layer 300 Leakage, lphd GM 200 GM/CCL 100 GM/GCL 0 1 2 Life Cycle Stage 3 As a result of this data, USEPA recognizes GCLs as equivalent or superior to CCLs as the clay component of MSW landfill liners. GCL Chemical Compatibility is a function of: • • • • Chemical Compatibility How will the GCL perform when exposed to leachate and other chemicals? Diffuse Double Layer Theory Type of chemicals and concentration Prehydration Exposure to wet/dry cycles Confining pressure Daniel, D. E. and Stark, T. D., GCLs for Containment of Wastes, Chemicals and Liquids, Short Course, University of Illinois, 2000. Contaminants of Concern • Main contaminants of concern: – Cations (Na+, Ca+2, Mg+2) – Miscible (water-soluble) Organic Solvents at high (> 50%) concentrations Testing for Compatibility • ASTM D6141, Standard Guide for Screening the Clay Portion of a GCL for Chemical Compatibility to Liquids. • Uses free swell and fluid loss tests • Results using test liquid are compared to results using clean water Fluid Loss Test Swell Index Air Pressure FS 24 mL/2g in clean water Filter Paper Slurry FL < 18 mL in clean water Alternate Test Method Concentration Effects on Permeability for a more conclusive assessment • D6766, Standard Test Method for Evaluation of Hydraulic Properties of GCLs Permeated with Potentially Incompatible Fluids. Mg = 240 ppm – Modified version of D5887 and D5084. – Termination criteria: OUTFLOW Na = 2,300 ppm Ca = 400 ppm Jo et al. (2001) • E.C.INFLOW§E.C.OUTFLOW INFLOW Landfill Bottom Liner Research (TR-254) Jo, H.Y. et al., Hydraulic Conductivity and Swelling of Nonprehydrated GCLs Permeated with Single-Species Salt Solutions, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, July 2001. Kolstad (2004) modified • Kolstad (2004) tested long-term compatibility (hydraulic conductivity) of numerous GCLs in contact with various salt solutions. • Found that a GCL’s long-term hydraulic conductivity depends on 2 properties: – Ionic Strength, P – Ratio of Monovalent to Divalent Cations, RMD RMD >Na @ >K @ >Ca @ >Mg @ 2 2 Most leachates causes very little increase in k Prehydration Benefits Confining Pressure Mine Waste with EC = 4,000 Pmho/cm Prehydration improves k Hydrating the GCL with fresh water prior to contact with contaminants can have a significant positive affect on permeability Shackelford, C.D. et al., Evaluating the Hydraulic Conductivity of GCLs Permeated with Non-standard Liquids, Geotextiles and Geomembranes, Elsevier Ltd., Vol. 18, 2000. Confining stress improves k Daniel (2000) Wet-Dry Cycling Hydraulic Conductivity (cm/s) 10 10 10 10 10 10 Compatibility Recommendations -5 DI DI-CaCl 2 -6 • Once hydrated, bentonite is compatible with most waste leachates Tap-CaCl 2 CaCl2 -7 – Compatible with most inorganics except high Ca and Mg (salts). • Prehydration with clean water and increased confining pressure improve compatibility • Desiccation should be avoided • Project-specific conditions will influence GCL response to contaminants • GCL screening and permeability testing should be performed in cases of uncertainty -8 -9 -10 0 1 2 3 4 5 6 7 8 9 Number of Wetting Cycles GCL Shear Strength and Slope Stability GCL Shear Strength • GCLs need cover for physical protection, overlap performance, and lowest permeability • Soil or waste cover on sloping surfaces creates shear stresses on liner system • Shear strength of liner system must exceed shear stresses in order to be stable Bentonite Shear Strength Bentonite Shear Strength High Normal Loads Low Normal Loads 3000 Peak Unreinforced Bentonite @ Low Normal Loads 400 Daniel 1992, slow shear Large displacement Fox et al, 1998 peak 2500 Hyperbolic fit for peak Fox et al, 1998, post peak Hyperbolic fit for post peak 300 Shear Stress (psf) Daniel & Shan 1991, slow shear post peak Gilbert et al, 1996 post peak Shear stress (psf) Leisher, 1992 peak Peak Hyperbolic Curve 10º Post peak Hyperbolic Curve 200 2000 Mesri & Olson 1500 1000 2.4º 100 500 7.0º 18º 0 0 200 400 600 800 1000 1200 1400 1600 - 5,000 10,000 Shear Strength of Unreinforced Hydrated Bentonite • Approximately 10o at low normal loads (less than 3,000 psf) • Decreases to 4o at high normal loads • Not acceptable for most sites with appreciable slopes 15,000 20,000 25,000 30,000 Normal Stress (psf) Normal Stress (psf) What do engineers want? • We don’t want to rely on unreinforced hydrated bentonite. • We want the GCL internal shear strength to be greater than interface strength H 1 4 Subgrade L MSW (JI) 2 Liner I( ) Needlepunch Reinforcement 3 to 5x improvement in internal shear strength 1 Benefits of High Needlepunch Density • Higher internal shear strength – Higher factor of safety – Allows steeper slopes or higher shear loads – Not the “weakest link” in a multi-layer system • More consistent mass/area – Fewer thick and thin spots – Uniform hydraulic performance – Minimal edge loss during handling/installation • Better resistance to unconfined hydration – Resists swelling pressure of bentonite GCL Peak Strength Comparison Myth #2: Myth: “Thermal Locking is necessary to achieve high internal shear strength” Truth: There are other GCL manufacturing methods which can yield even higher shear strength. Needled fibers heat-bonded to geotextile Bentomat with needled fibers Slope Stability Issues Bentomat (Not thermal locked) 33.5 degrees Bentomat BentofixGCL Thermal Locked 28.9 degrees Veneer Stability (McCartney, 2002) Veneer Stability Issues Resisting Forces Driving Forces - Interface friction - Tension - Toe buttressing - Weight of cover - Pore pressure If Downslope driving forces > Resisting forces, slide could occur Tension Weight of cover Toe buttressing Interface friction Global Stability Why do veneer slides occur? 1. Pore pressures (no. 1 cause) 2. Inadequate shear strength for the slope conditions: -Long-term static conditions -Dynamic conditions (during construction and earthquakes) Consult the literature for methods of stability assessment Global Stability Issues Global Stability Analysis for bottom liner systems • Depends on geometry of fill • Evaluated by limit equilibrium methods or finite element analysis • Too complex to address here! • Best practice is to ensure that a large-displacement FS > 1 is maintained for all interfaces Stability of the entire mass of waste or earth must be assessed Internal failure mode 4 H Interface failure mode MSW (JI ) 1 Subgrade L 1 2 Liner ( I ) Myth #3: Torn Edge of Geomembrane Myth: “Tensile strength and elongation are important GCL performance properties” Truth: In waste sliding failures, GCLs and geomembranes are “tissue paper” and should not be relied upon for tensile reinforcement. NonNon-woven geotextile Exposed clay liner Myth #4: Peel and shear strength 300 Myth: “Peel strength is not an important GCL property” 250 P eak S hear Strength (k P a) Truth: Peel strength is a measure of internal reinforcement, and as such, an indicator of GCL internal shear strength. Since it is difficult to control, some manufacturers do not certify peel strength. Torn edge of geotextile 200 150 100 50 0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Peel Strength (N/cm) 14.0 16.0 18.0 20.0 Slope Stability General Rules • Consult the Data! CETCO has a vast database of internal and interface shear test results • In general, the GCL interface with a geomembrane is the most critical • Internal shear strength usually exceeds peak design requirements Slope Stability Recommendations Slope Stability General Rules (cont’d) • All interfaces have unique characteristics and the designer must perform testing to ensure that sloped systems will be stable • Conduct material-specific tests across full range of normal loads • Work with a lab well-experienced in shear testing For more information • Web site: www.cetco.com/LTE • Attempt to position critical plane above primary geomembrane • Good practice to check FS > 1 everywhere using Residual strength (seismic events, construction activities, waste placement/settlement can cause liner deformation) • Use Residual strengths of interfaces that have the lowest peak strengths over each normal load range • Think about unexpected pore pressure scenarios – Register to access free technical references, specifications, and installation guide • E-mail: [email protected] [email protected] [email protected]