Geosynthetics for Waste Containment Overview and Latest Research Findings and Product Developments

Transcription

1 Geosynthetics for Waste Containment Overview and Latest Research Findings and Product Developments By: Randal Osicki, M.Sc., P.Eng. Senior Geotechnical Engineer Co-author: Brian Ayres, M.Sc., P.Eng. SustainTech 2018 March 22, Saskatoon, SK

2 A world leader Founded in 1911, SNC-Lavalin is one of the leading engineering and construction groups in the world and a major player in the ownership of infrastructure. From offices in over 50 countries, SNC-Lavalin's employees are proud to build what matters. Our teams provide EPC and EPCM services to clients in a variety of industry sectors, including oil and gas, mining and metallurgy, infrastructure and power. SNC-Lavalin can also combine these services with its financing and operations and maintenance capabilities to provide complete end-to-end project solutions. 2

3 Presentation Outline 1) Geosynthetics 101 2) Liner Systems 3) Design Considerations 4) Construction Considerations 5) Longevity of Geosynthetics (from (from MDH archives) (from 3

4 Geosynthetics 101

5 Types of Geosynthetics (from Koerner, 2012)

6 Geotextiles The Basics: A permeable geosynthetic comprised solely of textiles Commonly referred to as filter fabric Varying mass per unit area (or thickness) (from Typical Applications: Separation of dissimilar materials Reinforcement of weak soils / mine waste Filtration (cross-plane flow) Drainage (in-plane flow) Types of Geotextile: (from Nonwoven fabrics bonded w/ heat, needle-punching Best for separation, filter & drainage applications Woven fabrics weaved together Best for reinforcement (e.g. road construction) (from 6

7 Geotextiles (cont ) Typical Installation: Panels unrolled using labourers Adjacent panels overlapped ~45 to 90 cm (consider sewing seams for softer subgrades) Key-in trench for steeper slope applications (from Typical Costs: (FOB Saskatoon) Supply: $1.00 to $5.00 / m 2 Install: $1.00 to $1.50 / m 2 (from 7

8 Geomembranes (GMs) Definition: A very low permeability synthetic membrane liner or barrier used to control fluid and/or gas migration Also known as Flexible Membrane Liner (FML) Geomembrane permeability typ to m/s Most Widely Used GMs: High-density polyethylene (HDPE) Linear low-density polyethylene (LLDPE) Bituminous geomembrane (BGM) Key Attributes: Composition: PE resins, carbon black, antioxidants Thickness: Colour: Sheet Type: PE typ. 1 to 2 mm (40-mil to 80-mil) BGM typ. 4 to 5 mm Black, white, green Smooth, 1-side textured, 2-side textured (from (from 8

9 Geomembranes (cont ) Typical Installation: Panels unrolled using spreader bar and labourers Adjacent panels overlapped by ~15 cm and fused together w/ specialized welders (from Non-destructive and destructive testing of welded seams Key-in trench for upslope locations (from (from (from Typical Costs: (FOB Saskatoon) HDPE or LLDPE: $8 to $12 / m 2 supply & install BGM: $20 to $25 / m 2 supply & install 9

10 Geosynthetic Clay Liners (GCLs) The Basics: A factory-manufactured hydraulic barrier w/ bentonite layer supported by geotextiles or GMs, held together by needling, stitching, or chemical adhesives Permeability of Na-bentonite GCL typ to m/s GCL Types: (from Most common GCL Becoming more popular (from Benson, 2000) 10

11 GCLs (cont ) Typical Installation: Key-in trench for upslope locations Panels unrolled using spreader bar and labourers Adjacent panels overlapped 30 to 50 cm Seams sealed with bead of Na-bentonite (from Typical Costs: (FOB Saskatoon) Supply: $6 to $10 / m 2 (from Install: $1 to $2 / m 2 11

12 Liner Systems

13 Types of Liner Systems Single Clay Liner: Compacted clay liner (CCL) or GCL Best for water retention ponds or sewage lagoons Need to assess propensity for cation exchange Composite Liner System: The standard for waste barrier systems Underlying clay liner could be CCL or GCL Exploit desirable characteristics of both mediums Double Composite Liner w/ Leak Detection: Allows for monitoring leakage rates thru primary liner (c) Very costly, but potentially warranted when risks for environmental impacts are high (from Benson, 2000) 13

14 Defects in Geomembranes Typical Causes: Handling of rolls from factory to site Poor subgrade preparation Poor installation / welding of seams Adjacent soil particles too coarse Twisting of equipment on cover soil Action Leakage Rate (ALR): (from Rowe, 2016) Never assume zero leakage Alberta EPA (1996) suggests 2 holes/ha w/ hole dia. of 2 mm for strict CQA Giroud & Bonaparte (2001) suggest 2.5 to 5 holes/ha, dia. of 2 mm for strict CQA Benson (2000) assumes 12 holes / ha and hole dia. of 11 mm (A = 1 cm 2 ) 14

15 Wrinkles in Geomembranes Caused by: Poor construction (arrangement of panels) Thermal expansion (solar heating): Studied at Queen s University Experimental Liner Test Site (QUELTS) Not limited to geometric imperfections Thermal-induced wrinkles disappear Research Findings (Rowe): Typical L values for 60-mil HDPE: <20 m for T GM <20 C & cloudy day >500 m for T GM >30 C & sunny day Leakage rates much higher when cover soil placed over GM w/ wrinkles (due to loss of hydraulic impedance) Early morning in November, T a = 3 C Late morning in November, T a = 17 C (from Rowe et al, 2012) 15

16 Transmissivity (θ) at GM CL Interface Function of: Roughness / deformations in clay layer Stiffness (thickness) of GM product Saturated permeability of clay layer (from Rowe, 2012) Typical Values of θ: GM CCL w/ poor contact: 1 x 10-7 m 2 /s GM CCL w/ good contact: 1 x 10-8 m 2 /s GM GCL w/ 10 kpa stress: 4 x m 2 /s GM GCL w/ 50 kpa stress: 2 x m 2 /s Key Installation Points: (from MDH archives) CL should be as uniform as possible CL surface should be free of larger particles (10 mm maximum for PE products) Do NOT include a Geotextile between GM and subgrade to act as cushioning 16

17 Leakage Rates for Different Liner Systems (Source: Rowe, 2012) Key Points: Composite liners far better than single liners GCL better than CCL in a composite liner system Keep connected wrinkle length to a minimum prior to cover placement 17

18 Diffusion Through Liner Systems Diffusion often dominant transport mechanism for well-designed and constructed liners Considerable research has been conducted in the laboratory for various contaminants and geosynthetic liners (from Jones and Rowe, 2016) (from Rowe, 2012) Key Research Findings: PE GM is a great diffusive barrier for ions (e.g. Cl - ), some gases (e.g. CH 4 ), and non-vocs (e.g. PCBs) Co-extruded HDPE and underlying CCL needed to contain VOCs (e.g. Benzene) GCL requires underlying attenuation layer to be equivalent to CCL 18

19 Key Design and Construction Considerations

20 PE Geomembrane Design Considerations Key Design Considerations: 1) Required service life 2) Exposure time to atmospheric conditions 3) Chemistry of fluid or leachate 4) Temperature of fluid or solid waste 5) Maximum normal load / stress 6) Shear strength requirements Some Design Tips: Subgrade design / preparation is paramount!! Pond liners should have high initial stress crack resistance (SCR>1,500 hrs) to combat waves Use textured GMs for steeper slopes (for slope stability and safety reasons) LLDPE provides same puncture resistance as HDPE (see Rowe et al., 2013) (from Rowe et al., 2013) (from 60-mil HDPE 60-mil LLDPE 20

21 GCL Design Considerations Various Products Available: Bentonite... swell index (source), quantity (mass per unit area) Bentonite treatment polymer-enhanced? Needle-punching light vs. heavy Lower geotextile woven vs. scrim NW (from Option to add a LLDPE or HDPE coating (from Rentz et al., 2015) Some Design / Installation Tips: Coated GCL use to limit cation exchange effects or root penetration GCL requires hydration, but do not allow to saturate PRIOR to cover placement Must have timely covering of a GCL (by itself or in a comp. liner) to prevent desiccation and downslope bentonite erosion 21

22 Bituminous Geomembranes (BGMs) Manufacturers claim the following advantages of BMGs: Greater puncture resistance compared to HDPE Hence, larger particles in subgrade Simpler to fuse seams compared to HDPE Able to install down to -20 C ambient temps (from BGM installation at Diavik Mine (from Potential concerns with BGMs: Insufficient research into the ageing of BGMs compared to HDPE Potential softening of bitumen upon exposure to high temps (fluid or solar) Puncture resistance potentially not as great as first thought research on-going 22

23 Strain in HDPE/LLDPE Geomembranes Potential Causes of Excessive Strain: Localized depressions in the subgrade Welding panels during heat of day w/o allowing for thermal contraction (especially at changes in grade) Maximum Allowable Strains: (based on Peggs et al. (2005) study) (from HDPE, smooth: 6% HDPE, textured: 4% LLDPE (ρ >0.935 g/cm 3 ): 10% LLDPE (ρ <0.935 g/cm 3 ): 12% LLDPE, textured: 8% Reducing effects of Strain in the Field: Firm, uniform subgrade preparation Use an experienced contractor (from Do NOT cover a GM when it is under strain!! 23

24 Exposure Time for GMs in Composite Liners Issues: Black GM can heat to >40 C above ambient temp leads to: Wrinkling Evaporation of water from underlying CCL or GCL potential for desiccation cracking and thus higher leakage rates Desiccation Cracking of Compacted Till Layer (from MDH archives) Recommendations: Cover GM when there are relatively few wrinkles (<5% of area) Limit time between GM installation and cover placement (from MDH archives) Consider use of a White HDPE in cases where exposure time is higher 24

25 Longevity of Geosynthetics

26 Ageing of Geosynthetics Key Factors that Cause Ageing: Exposure to UV radiation High oxygen concentrations High temperatures Various hazardous chemicals (e.g. soaps) Stages of Failure: (from 1) Depletion of protective antioxidants 2) Induction to onset of polymer chemical degradation 3) Significant chemical degradation and ultimately failure (typically in form of stress cracking) Accelerated Ageing Experiments: Studied a variety of conditions (Rowe, Koerner) HDPE predicted to last >450 yrs for ideal conditions (from 26

27 Service Life of Geomembranes Definitions: Hsuan & Koerner (1998) the time up to where physical property of interest has decreased to 50% of its original value Rowe (2012) the time during which the GM will act as an effective hydraulic and diffusive barrier to contaminant migration Depends on relationship between resistance and demand Measures to Increase Service Life of Geomembranes: 1) Select appropriate GM for site conditions 2) Use a thicker HDPE or LLDPE GM 3) Limit atmospheric exposure time 4) Limit strain in the GM (short & long term) 5) Keep wrinkles to a minimum prior to covering (fromhttp:// 27

28 Concluding Remarks

29 Key Take-Away Messages Design: Define service life and design objectives Estimate all possible stresses in short, intermediate and long terms Design the subgrade gradation, w/c, strength Select geosynthetic materials to meet engineering requirements and remember: Not all geosynthetic products are the same!! What is cheap in the short term may be a very expensive solution in the intermediate to long term (Rowe, 2013) (from Construction: Ensure materials delivered to site conform with specs High level of CQA is essential!! Hire experienced / qualified contractors Limit exposure of GM liners prior to cover placement 29

30 References / Sources of Information

31 References / Geosynthetic Sources Benson, C.H Liners and covers for waste containment. In Proc. 4 th Int l Geotechnical Forum, Creation of a New Geo-Environmental, JGS, Kyoto, Japan, May 24-26, pp Giroud, J.P Equations for calculating the rate of liquid migration through composite liners due to geomembrane defects. Geosynthetics International, 4: Giroud, J.P. and Bonaparte, R Geosynthetics in liquid-containing structures. In Geotechnical and Geoenvironmental Engineering Handbook. Kluwer Publishing, pp Hosney, M.S. and Rowe, R.K Changes in geosynthetic clay liner (GCL) properties after 2 years in a cover over arsenic-rich tailings. Canadian Geotechnical Journal, 50: Jones, D.D. and Rowe, R.K BTEX migration through various geomembranes and vapor barriers. Journal of Geotechnical Engineering, DOI: /(ASCE)GT Koerner, R.M Designing with Geosynthetics. 6 th edition. Xlibris Corporation. Liu, Y., Bouazza, A., Gates, W.P., and Rowe, R.K Hydraulic performance of geosynthetic clay liners to sulfuric acid solutions. Geotextiles and Geomembranes, 43: Peggs, I.D., Schmucker, B., and Carey, P Assessment of maximum allowable strains in polyethylene and polypropylene geomembranes. In Geo-Frontiers ASCE, Reston, VA. Rentz, A.K., Take, W.A., Brachman, R.W.I., and Rowe, R.K Effect of geomembrane colour and cover soil on a solar-driven down-slope bentonite erosion from a GCL. Geosynthetics International, 31

32 References / Geosynthetic Sources (cont ) Rowe, R.K Short- and long-term leakage through composite liners. The 7 th Arthur Casagrande Lecture. Canadian Geotechnical Journal, 49: Rowe, R.K Performance of GCLs in liners for landfill and mining applications. Environmental Geotechnics, Rowe, R.K Design and construction of barrier systems to minimize environmental impacts due to municipal solid waste leachate and gas. Indian Geotechnical Journal, 42(4): Rowe, R.K Recent insights regarding the design and construction of modern MSW landfills. In EurAsia Waste Management Symposium, May 2-4, 2016, Istanbul, Turkey. Rowe, R.K. and Sangam, H.P Durability of HDPE geomembranes. Elsevier, Geotextiles and Geomembranes, 20 (2002): Rowe, R.K. and Yu, Y A practical technique for estimating service life of MSW leachate collection systems. Canadian Geotechnical Journal, 50: Rowe, R.K., Brachman, R.W.I., Irfan, H., Smith, M.E., Thiel, R Effect of underliner on geomembrane strains in heap leach applications. Geotextiles & Geomembranes, 40: Rowe, R.K., Chappel, M.J., Brachman, R.W.I., Take, W.A Field study of wrinkles in a geomembrane at a composite liner test site. Canadian Geotechnical Journal, 49: Rowe, R.K., Islam, M.Z., Hsuan Effects of thickness on the aging of HDPE geomembranes. Journal of Geotechnical and Geoenvironmental Engineering, 136:

33 Our values are the essence of our company s identity. They represent how we act, speak and behave together, and how we engage with our clients and stakeholders. We put safety at the heart of everything we do, to safeguard people, assets and the environment. We do the right thing, no matter what, and are accountable for our actions. We work together and embrace each other s unique contribution to deliver amazing results for all. We redefine engineering by thinking boldly, proudly and differently.