Final Covers for Mine Tailings
William H Albright, PhD
Desert Research Institute Reno NV USA
(775) 771-1296 [email protected] Functions of Covers
• Physical containment of waste
• Control percolation into waste
• Control gas movement
Ingress (O2)
Egress (Rn, CH4, CO2)
• CtlControl vec tititor intrusion
• Persist for design life of containment facility
2 Categories of Engineered Covers
Conventional covers – cover designs where a barrier layer (clay, geomembrane, etc.) having low saturated hydraulic conductivity is the primary impediment to leakage and gas flow.
clay covers, composite covers, GCL covers
Water balance covers – cover designs where leakage is controlled by balancing the water storage capacity of unsaturated finer-textured soils and the ability of plants and the atmosphere to extract water stored in the soil. Also known as water balance covers, evapotranspiration (ET) covers, store-and-release covers.
monolithic covers, capillary barrier covers 3 Conventional Resistive Covers with a Soil Barrier Simple Compacted Geosynthetic Soil Cover Clay Cover Clay Liner (GCL) Cover
Soil Soil Soil
Waste Waste Compacted Clay
GCL
Waste 4 Conventional Resistive Covers with a Composite Barrier
Clay-Geomembrane GCL-Geomembrane Composite Composite
Soil
Soil Geomembrane (GM) Compacted Clay Waste
Waste GCL 5 Water Balance Covers Monolithi c Cap illary Barrier Barrier
Fine Fine Textured Textured Soil Soil
Coarse Soil
Waste Waste
6 Cover Percolation Gas Cost Type Rate Flux ($/ac) Simple Soil Highest Highest 25, 000 Clay Modest Modest 75,000 GCL Modest Modest 75, 000 Composite Very low Very Low 125,000 Very low - ET Monolithic Modest 50,000 low Capillary Very low - Modest 50,000 Barrier low
7 Design Philosophy
Conventional Designs - regulatory engineering, not site specific - methods & materials requirements - no quantitative performance criterion
Alternative (Performance-based) Design - determine performance criterion (e.g., percolation ≤ prescriptive cover) - select layering to meet a quantitative performance criterion - analyze to ensure alternative cover meets performance criterion Issues with Prescriptive Regulation
1. With conventional designs typically no performance criteria 2. Alternative designs typically required to show equivalent performance (see 1) 3. Equivalency demonstration is difficult 4. Primary goal (protect HH&E) often neglected 5. Cost (to society) can be higher than necessary 6. An example of the rule of unintended, undesirable consequences 7. Common with indirect regulation An Alternative Regulatory Philosophy
- Focus on primary goal (ex. protection of ground water) - Prescriptive design process - Type of waste? - Waste packaging? - CCaelimate? - Depth to groundwater? - Attenuation capacity of unsaturated zone? - Distance to nearest receptor (ex. pumping well)? - Any sensitive environments or species? - Each site will have a different list - Require design engineer to demonstrate compliance with primary goal - RiRequire appropr ititiiate monitoring USEPA’s Alternative • TwentyTwenty--fourfour test covers at Cover Assessment eleven sites in seven Program (ACAP) states. • Ten conventional covers (seven composite and three clay)
• Fourteen alternative covers ((geight monolithic barriers and six capillary barriers)
• Eight sites with sideside--byby-- side comparison of conventional and alternative covers 11 ACAP Drainage Lysimeters Full-scale construction methods Hundreds of samples and instruments Lysimeters are the only method for direct measurementfdit of drainage Conventional Covers Evaluated by ACAP
16 Compacted Clay Covers
Objectives:
(1) Construct a soil barrier (compacted clay) with low saturated hydraulic conductivity.
(()2) Protect the clay barrier from damage that may increase hydraulic conductivity 17 Types of Damage
-Frost - DiDesicca tion - Differential settlement (normally a problem with municipal solid waste, but not mining wastes , coal ash, etc. ) - Erosion
18 Sensitivity to Frost Damage
Freezing of compacted clay barriers causes:
- ftifilformation of ice lenses: crac king
- formation of desiccation cracks as water moves to freezing front
- cracking that causes increases in hydraulic conductivity
Protect clay barrier with insulation (synthetic or burial). 19 Sensitivity of Compacted Clay to Desiccation Damage Drying of compacted clay barriers causes desiccation cracks to form, increasing the hydraulic conductivity.
Large-scale cracks may form, as in this clay barrier in southern Georgia four years after construction.
Dye tracer test in soil barrier cover showing preferential flow path
20 Conventional Clay Cover Performance
800 225 • Soil dried for first time diduring 6 --weeweekdk drought ht • Change in response of ppppercolation to precipitation 550 150 events Precipitation Soil water – Quantity No rain storage – “Stair step ” response 300 75 • No evidence that defects in Percolation
clay barrier healed when soil m waterm water mmmm water increased 50 0 7/1/00 9/1/00 11/2/00 1/3/01 3/4/02
Data from ACAP field site in Albany GA Field Hydraulic Conductivity Measurements on Clay Barrier 4 Years After Construction
Hydraulic Conductivity Test Kfinal/Kas-built (cm/s) As-Built 4.0x10-8 1.0 SDRI 2.0x10-4 5000 TSB - 1 5.2x10-5 1300 TSB - 2 3. 2x10-5 800 TSB - 3 3.1x10-3 77,500
22 Typical Composite Cover
Surface 150-1000 mm thick Layer (6 – 40 inches) Drainage Layer Geomembrane
Compacted 450-900 mm thick • Geomembrane added directly Clay (18 – 36 inches) on top of clay barrier or GCL
• Drainage layer frequently added on top of geomembrane Waste to enhance stability by limiting pore water pressures. 23 1.5 mm LLDPE Textured Geomembrane
24 Geocomposite Drain
25 For covers, chemical compatibility normally is not a concern when selecting geomembrane polymer. Key issues are:
- constructibility - durability - cost - availability with texturing
All of the cited geomembranes can be welded in the field using wedge or extrusion techniques to obtain welds with higher strength than parent material.
LLDPE and HDPE geomembranes are most commonly used for covers
26 Drainage Layers Functions: -Reduce Head on Barrier Layer -Reduce Pore Pressure Build Up
Materials: - Coarse-Grained Soil (clean sand, crushed rock) - Geocomposite Drain
Design Approach: -SlSelec tdit drain thtthat prov ides accep tblhtable hea d -Adequate hydraulic conductivity -HELP, conservative (over-predicts lateral drainage) 27 -Giroud & Houlihan's Method Conventional Composite Cover Performance
• Percolation correlated 1800 120 with Precipitation Lateral flow – Heavy precipitation events 1700 80 – Surface flow Surface flow
– Lateral flow on 1600 40
geomembrane m water) mm ( PliPercolation 1500 0 5/1/03 5/16/03 5/31/03
Data from ACAP field site in Cedar Rapids IA Damage to Geomembrane Affects Performance
900 150
••NoNo cushion between the geomembrane and the Surface flow soil, punctures likely in 800 100 geomembrane Percolation Precipitation • Relatively high rate of percolation 700 50
• Illustrates importance of Lateral flow careflful geomemb rane 600 0 installation 8/22/02 10/11/02 11/30/02 1/19/03 3/10/03
Data from ACAP field site in Marina CA Summary: Conventional Designs
• Composite designs – Restrict percolation to low (5 (<~5 mm/yr) levels at all locations – Percolation typically coincides with flow on membrane – Require careful construction practice and QA • Clay barrier designs – Performance quickly (<2 yrs) degrades – Percolation probably due to preferential flow through macro- features related to desiccation, freeze/thaw, roots – Damage likel y t o persi st – Probably not suitable for near-surface applications that require low-ppyermeability barrier UW Desiccation Study: Effect on Hydraulic Conductivity of GCLs
10-5 DI Water
DI Water + CaCl2 Divalent for 10-6 Tap Water + CaCl 2 monovalent (cm/s) CClCaCl 2 cation exchange 10-7 results in inability onductivity onductivity -8 CC 10 to close desiccation
10-9 cracks, resulting Hydraulic in large increase 10-10 in K. 0123456789 Number of Wettinggy Cycles
31 *Lin, L. and Benson, C. (2000), Effect of Wet-Dry Cycling on Swelling and Hydraulic Conductivity of Geosynthetic Clay Liners, J. of Geotech. and Geoenvironmental Eng., 126(1), 40-49. UW Desiccation Study: Effect on Swelling of GCLs* Divalent for monovalent 40 exchange DI 30 results in loss
) Tap mm CaCl of swelli ng 2 20 DI-CaCl capability … 2 Swell (m Swell and Tap-CaCl 10 2 potentially healing 0 0 1 2 3 4 5 6 7 8 capability Number of Wetting Cycles
32 *Lin, L. and Benson, C. (2000), Effect of Wet-Dry Cycling on Swelling and Hydraulic Conductivity of Geosynthetic Clay Liners, J. of Geotech. and Geoenvironmental Eng., 126(1), 40-49. UW Study: GCLs Exhumed from In-Service Caps
10-3 All specimens Site N -4 Site S underwent Ca/Mg 10
) Site D for Na exchange ss Site O 10-5
Only those with w ctivity (cm/ uu > 120% 10-6 maintained low K 10-7 raulic Cond dd
Need to protect Hy -8 GCL from drying 10 and/or cation 10-9 exchange. 0 50 100 150 200 250 In Situ Water Content (%)
33 Differential Settlement
• Distortion – ~300 mm V – ~450 mm H • No damage to GM • Large increase in K to soil barrier •GCL – Extensive cation exchange – Retained very low hydraulic conductivity – Humid climate and overlying GM – hydrated quickly, did • This case study has relatively not experience desiccation small distortion • Cover like ly reta ine d funct ion due to intact GM and GCL • Differential settlement an issue with waste containers • Need more research ACAP Data for Conventional Covers
Surface Lateral Precipitation ET Percolation RRffunoff Flow Cover Type Site Average Average Average Average Average (mm/yr) (mm) (mm/yr) (mm) (mm)
Altamont 379 59.0 4.0 1.5 0.1*
Apple Valley 169 6.8 0.0 0.0 Trace
Boardman 177 0.0 0.2 0.0 0.0*
Composite Marina 433 98.7 47.4 23.1 28.3
Polson 350.0 17.7 40.5 0.4 Trace
Cedar Rapids 981 54.1 96.2 12.2 2.8*
Omaha 731 86. 8 43. 3 606.0 070.7*
7.4 Apple Valley 169 3.4 0.0 0.0 (4.1%) Soil 195.2 Albany 1263 359.4 NA 195.2 Barrier (17.1%) 51.6 Cedar Rapids 981 79.6 29.5 51.6 (6.0%)
*Composite percolation data are scaled from field measurements to account for x10 increase in geomembrane flaws. Marina data not scaled due to geomembrane damage during construction 35 = semisemi--arid/subarid/sub--humid/arid.humid/arid. = humid. Summary: Field Performance of Conventional Covers
- Percolation rates for composites are very low: < 1 mm/yr in semi-arid and arid climates < 5 mm/yry in humid climates
- Percolation rates for soil covers much higher than expected: - 195 mm/ yr at Albany , GA - appears dominated by preferential flow
- Surface runoff is a small fraction of the water balance (<10%)
- Lateral drainage is a small fraction of the water balance (< 5%)
36 Water Balance Covers Evaluated by ACAP Helena, Polson, Boardman, Altamont, Apple Monticello, Marina, Albany, Marion, Omaha, Sacramento, MT MT OR CA Valley, CA UT CA GA IA NE CA
0 mm
300
600
900
1200
1500
1800
2100
Compost / Soil Mix Soil-Gravel Admixture 2400 Topsoil Gravel 2700 Storage Layer Clean Sand 3000 Compacted Vegetative Cover Silty Sand Target
Interim Cover Vegetation (Hybrid-Poplar Trees with percolation a grass understory) Vegetation (Grass) rates ~ 3 Vegetation (Grasses, forbs, and shrubs) mm/yr or less. ACAP Site Characteristics
Annual Annual Elev. Annual Monthly Avg. Site Location Precip. Snowfall Climate (m) P/PET Air Temp. (mm) (mm) AlVllCAApple Valley, CA 898 119 38 0060.06 arid -1371, 37
Boardman, OR 95 225 185 0.23 semi-arid -2, 32
Helena, MT 15 312 1288 0440.44 semi-arid -11, 28 Altamont, CA 227 358 2 0.31 semi-arid 2, 32 Monticello, UT 1204 385 1498 0.34 semi-arid -9, 29 StCASacramento, CA 320 434 0 0330.33 semi-arid 3343, 34 Underwood, ND 622 442 813 0.47 semi-arid -19, 28
Marina, CA 31 466 0 0.46 semi-arid 6, 22
Polson, MT 892 380 648 0.58 sub-humid -7 ,28 Omaha, NB 378 760 711 0.64 sub-humid -6, 25 Cedar Rapids, IA 290 915 724 1031.03 humid -8238, 23 Albany, GA 60 1263 3 1.10 humid 8, 33 Marina, CA
- Costal semisemi--aridarid climate
- Precipitation = 466 mm/yr
- P/PET = 0.46
- Capillary barrier (theory), but effectively a monolithic barrier Water Balance of Capillary Barrier: Marina, CA
) 2000 500 mm Cumula
Storage Capacity = 300 mm Missing ation (m t rr Data 400 Perco ive and Surf 1500 Soil Water Storage potranspi l a ation, Soil aa 300 ce Runof
1000 n and n and Ev f (mm) Water St Water oo 200 Evapotranspiration Precipitation recipitati
500 o PP 100 rage, Percolation
mulative mulative No Surface Runoff uu
C 0 0 1/31/00 7/20/00 1/7/01 6/27/01 12/15/01 6/4/02 11/22/02 5/12/03 10/31/03 Percolation occurs every year when storage capacity is exceeded. Polson, MT
Cool and Seasonal Semi- Humid Climate Capillary Barrier Precipitation ~ 380 mm/yr) P/PET = 0.58 Capillary Barrier: Polson, MT
1600 80 Cumulati (b) Alternative Precipitation iration, v pp 1200 60 e Percola t vapotrans ion and Suion rage (mm) rage EE
800 Evapotranspiration 40 ipitation, r Water Sto Water ll cc face Runo Soil Surface Runoff Water Storage
and Soi and 400 20 f ulative Pre f (mm) mm Cu Percolation 0 0 11/1/99 10/28/00 10/25/01 10/22/02 10/19/03 10/15/04
0.8 mm percolation over 5 yr! Less than composite. ACAP Data for Water Balance Covers
Maximum Average Site Precip. Perc. Precip. Perc. Year (mm) (mm) (mm) (mm) Albany, GA 1380.2 218.3 4 1202.3 109.2 Altamont, CA 498.6 139.3 4 379.7 44.8 Apple Valley, CA 272.0 1.8 3 167.4 0.5 Boardman, OR (Thin) 000.0 000.0 210.8 3 181.4 Boardman, OR (Thick) 0.0 0.0 Cedar Rapids, IA 898.4 366.1 4 930.0 207.3 Helena, MT 351. 5 010.1 5 272. 4 000.0 Marina, CA 406.9 82.4 4 462.8 63.3 Monticello, UT 662.9 3.4 5 387.0 0.7 OhNEOmaha, NE (Thin ) 101. 0 56. 1 612.4 1 732.5 Omaha, NE (Thick) 57.9 27.0 Polson, MT 308.1 0.4 349.1 0.2 SCA(Thi)Sacramento, CA (Thin) 361. 2 108. 4 - 54. 8 422.0 Sacramento, CA (Thick) 455.7 8.5 3 2.7 Underwood, ND 585.2 9.4 1 384.1 7.1 Water Balance Covers: How They Function
) 22000000 500 mm Cumulativ Total storage capacity = 300 mm Required storage capacity 400 a piration (m piration e nd Surface nd Surface ss
1500 Percolatio
300 Evapotran n R , Soil Water dd 1000 unoff (mm)
200 Soil Water ipitation an Precipitation cc Storage Storage, 500 100
ulative Pre ulative Percolation mm
Cu 0 0 7/1/00 10/27/00 2/22/01 6/20/01 10/16/01 2/11/02 6/9/02 10/5/02 1/31/03
Key to design is available storage capacity (soil properties and cover thickness) must equal or exceed required storage (climate characteristics) A Two-Step Method for Design of Water Balance Covers
1. Preliminary design: estimate required thickness by matching required and available storage using ACAP approach based on a robust, nation-wide field data set
2. Refine the design with numerical simulations to evaluate: • Important design parameters • “what if?” assessments A Regulatory Framework Regulatory Requirements Site and Engineer Responsibilities • The goal: Protect human health and environment! • Conduct prescribed site and waste • Factors to consider analysis • Waste characteristics • Define required closure performance • Hazardous life of waste • Percolation • WtWaste pack kiaging • Gas release • Depth to ground water ••ErosionErosion • Attenuation capacity of geo • Containment life strata • Distance to nearest receptor or • SlSelec t appropr itiate c losure concep t sensitive environment (composite, water balance, ?) • Climate • Result is sitesite--specific,specific, performanceperformance-- • Stakeholder views , public bddibased design acceptance • Monitor and maintain • Containment Philosophy • Engineer and regulator must develop • Minimize release • Controlled release long-term relationship based on past • Modify list to be sitesite--specificspecific performance, trust, and a shared dedication to the ‘real’ goals. Publications Albright, W.H., Benson, C.H., and Waugh, W.J., 2010. Water Balance Covers for Waste Containment: Principles and Practice . ASCE Press , Reston VA.
Apiwantragoon, P., Benson, C.H., Albright W.H., 2014. Field Hydrology of Water Balance Covers for Waste Containment. J. of Geotech. and Geoenv. Engr.
Albright W.H., Benson, C.H., Apiwantragoon, P., 2013. Field Hydrology of Landfill Final Covers With Composite Barrier Layers. J. of Geotech. and Geoenv. Engr. 139:1, 1–12
Albright, W., Benson, C., Gee, G., Abichou, T., Tyler, S., Rock. S., 2006. Field Performance of Three Compacted Clay Landfill Covers, Vadose Zone J., 5:1157-1171.
Albright, W., Benson, C., Gee, G., Abichou, T., Tyler, S., Rock. S., 2006. Field Performance of a Compacted Clay Landfill Final Cover at a Humid Site. J. of Geotech . and Geoenv. Engr., 132:11, p. 1393-1403.
Benson, C., Sawangsuriya, A., Trzebiatowski, B., and Albright, W., 2007. Pedogenic Effects on the Hydraulic Properties of Water Balance Cover Soils. J. of Geotech . and Geoenv. Engr, 133:4, p. 349-359.
Benson, C., Albright, W., Fratta, D., Tinjum, J., Kucukkirca, E., Lee, S., Scalia, J., Schlicht, P., Wang, X . 2011 . Engineered Covers for Waste Containment: Changes in Engineering Properties & Implications for Long-Term Performance Assessment, NUREG/CR-7028, Office of Research, U.S. Nuclear Regulatory Commission, Washington. http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr7028/