Final Covers for Mine Tailings William H Albright, PhD Desert - - PowerPoint PPT Presentation

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Final Covers for Mine Tailings

William H Albright, PhD

Desert Research Institute Desert Research Institute Reno NV USA (775) 771-1296 bill@dri.edu bill@dri.edu

SLIDE 2

Functions of Covers

• Physical containment of waste
• Control percolation into waste
• Control gas movement

Ingress (O2) Egress (Rn, CH4, CO2) C t l t i t i

• Control vector intrusion
• Persist for design life of containment facility
• Persist for design life of containment facility

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SLIDE 3

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 conductivity is the primary impediment to leakage and gas flow. clay covers composite covers GCL covers 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 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

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Conventional Resistive Covers with a Soil Barrier with a Soil Barrier

Simple Soil Cover Geosynthetic Clay Liner (GCL) Cover Compacted Clay Cover (GCL) Cover

Soil Soil Soil Waste Compacted l Waste Clay GCL Waste

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Conventional Resistive Covers with a Composite Barrier with a Composite Barrier

GCL-Geomembrane Composite Clay-Geomembrane Composite Composite Composite

Soil Soil Compacted Clay Soil Geomembrane (GM) Clay Waste Waste GCL

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Water Balance Covers

M lithi C ill Monolithic Barrier Capillary Barrier

Fine Textured Fine Textured Soil Textured Soil Soil C W Coarse Soil Waste Waste

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Cover Percolation Gas Cost Cover Type Percolation Rate Gas Flux Cost ($/ac) Simple Soil Highest Highest 25 000 Simple Soil Highest Highest 25,000 Clay Modest Modest 75,000 GCL Modest Modest 75 000 GCL Modest Modest 75,000 Composite Very low Very Low 125,000 Very low ET Monolithic Very low - low Modest 50,000 Capillary Very low Capillary Barrier Very low - low Modest 50,000

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SLIDE 8

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

• select layering to meet a quantitative performance criterion
• analyze to ensure alternative cover meets performance

criterion criterion

SLIDE 9

Issues with Prescriptive Regulation

• 1. With conventional designs typically no performance criteria
• 2. Alternative designs typically required to show equivalent
• 2. Alternative designs typically required to show equivalent

performance (see 1) 3 Equivalency demonstration is difficult

• 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

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An Alternative Regulatory Philosophy

• Focus on primary goal (ex. protection of ground water)
• Prescriptive design process
• Type of waste?
• Waste packaging?
• Climate?

C a e

• Depth to groundwater?
• Attenuation capacity of unsaturated zone?

Distance to nearest receptor (ex pumping well)?

• 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 R i i t it i

• Require appropriate monitoring

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USEPA’s Alternative C

• Twenty

Twenty-

• four test covers at

four test covers at

Cover Assessment Program (ACAP)

eleven sites in seven eleven sites in seven states. states.

• Ten conventional covers

Ten conventional covers (seven composite and (seven composite and three clay) three clay)

• Fourteen alternative

Fourteen alternative covers (eight monolithic covers (eight monolithic ( g ( g barriers and six capillary barriers and six capillary barriers) barriers)

• Eight sites with side

Eight sites with side-

• by

by-

• side comparison of

side comparison of conventional and conventional and

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alternative covers alternative covers

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ACAP Drainage Lysimeters

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Full-scale construction methods

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Hundreds of samples and instruments

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Lysimeters are the only method for direct t f d i measurement of drainage

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Conventional Covers Evaluated by ACAP

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SLIDE 17

Compacted Clay Covers

Objectives:

(1) Construct a soil barrier (compacted clay) with low saturated hydraulic saturated hydraulic conductivity. (2) Protect the clay barrier from

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( ) y damage that may increase hydraulic conductivity

SLIDE 18

Types of Damage

• Frost

D i ti

• Desiccation
• Differential settlement (normally a

Differential settlement (normally a problem with municipal solid waste, but not mining wastes coal ash etc ) not mining wastes, coal ash, etc.)

• Erosion

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SLIDE 19

Sensitivity to Frost y Damage

Freezing of compacted clay barriers Freezing of compacted clay barriers causes: f ti f i l ki

• formation of ice lenses: cracking
• formation of desiccation cracks as

water moves to freezing front

• cracking that causes increases in

cracking that causes increases in hydraulic conductivity

Protect clay barrier with insulation (synthetic or burial).

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Sensitivity of Compacted Clay to Desiccation Damage Desiccation Damage

Drying of compacted clay barriers causes desiccation cracks to form 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

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Conventional Clay Cover Performance

8 2 2 5

• Soil dried for first time

Soil dried for first time d i 6 d i 6 k d ht k d ht

5 5 1 5

during 6 during 6-week drought week drought

• Change in response of

Change in response of percolation to precipitation percolation to precipitation

3 7 5 Precipitation Soil water storage

No rain

p p p p p p events events

– Quantity Quantity “Stair step” response “Stair step” response

3 7 5 Percolation

mm water mm water – Stair step response Stair step response

• No evidence that defects in

No evidence that defects in clay barrier healed when soil clay barrier healed when soil

5 7 /1 /0 9 /1 /0 1 1 /2 /0 1 /3 /0 1

3/4/02 3/4/02

m

water increased water increased

Data from ACAP field site in Albany GA

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Field Hydraulic Conductivity Measurements on Clay Barrier 4 Years After Construction

Hydraulic

Clay Barrier 4 Years After Construction

Test Hydraulic Conductivity (cm/s) Kfinal/Kas-built 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 - 2 3.2x10-5 800 TSB - 3 3.1x10-3 77,500

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Typical Composite Cover

Surface Layer

150-1000 mm thick (6 – 40 inches) Drainage Layer

Compacted

450 900 mm thick Drainage Layer Geomembrane

• Geomembrane added directly

Compacted Clay

450-900 mm thick (18 – 36 inches)

• Geomembrane added directly
• n top of clay barrier or GCL
• Drainage layer frequently

Waste

• Drainage layer frequently

added on top of geomembrane to enhance stability by limiting

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pore water pressures.

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1.5 mm LLDPE Textured Geomembrane Geomembrane

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Geocomposite Drain Drain

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For covers, chemical compatibility normally is not a concern when selecting geomembrane polymer. Key issues are:

• constructibility
• durability

cost

• 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. with higher strength than parent material. LLDPE and HDPE geomembranes are most commonly used for covers used for covers

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SLIDE 27

Drainage Layers

Functions:

• Reduce Head on Barrier Layer
• Reduce Pore Pressure Build Up

Reduce Pore Pressure Build Up Materials:

• Coarse-Grained Soil (clean sand, crushed rock)
• Geocomposite Drain

Design Approach: S l t d i th t id t bl h d

• Select drain that provides acceptable head
• Adequate hydraulic conductivity
• HELP, conservative (over-predicts lateral drainage)
• Giroud & Houlihan's Method

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SLIDE 28

Conventional Composite Cover Performance Performance

1800 120

Precipitation Precipitation Lateral flow Lateral flow

• Percolation

Percolation correlated correlated with with Heavy precipitation Heavy precipitation

1700 80

Surface flow Surface flow

– Heavy precipitation Heavy precipitation events events – Surface flow Surface flow

1600 mm water) 40

P l i P l i

– Lateral flow on Lateral flow on geomembrane geomembrane

1500 5/1/03 5/16/03 5/31/03 (m

Percolation Percolation

Data from ACAP field site in Cedar Rapids IA

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Damage to Geomembrane Affects Performance

900 150

• No

No cushion between the cushion between the geomembrane and the geomembrane and the soil punctures likely in soil punctures likely in

800 100

Surface flow

soil, punctures likely in soil, punctures likely in geomembrane geomembrane

• Relatively high rate of

Relatively high rate of

Precipitation Percolation

percolation percolation

• Illustrates importance of

Illustrates importance of f l b f l b

700 50

Lateral flow

careful geomembrane careful geomembrane installation installation

600 8/22/02 10/11/02 11/30/02 1/19/03 3/10/03

Data from ACAP field site in Marina CA

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Summary: Conventional Designs

• Composite designs

– Restrict percolation to low (<~5 mm/yr) levels at all locations Restrict percolation to low ( 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 D lik l t i t – Damage likely to persist – Probably not suitable for near-surface applications that require low-permeability barrier p y

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UW Desiccation Study:

10-5

Effect on Hydraulic Conductivity of GCLs

Divalent for monovalent

(cm/s) 10-6 10 DI Water DI Water + CaCl2 Tap Water + CaCl2 C Cl

cation exchange results in inability to close

Conductivity 10 8 10-7 CaCl2

to close desiccation cracks, resulting in large increase

Hydraulic C 10-9 10-8

in large increase in K.

Number of Wetting Cycles 1 2 3 4 5 6 7 8 9 10-10 g y

*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.

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UW Desiccation Study: Effect on Swelling of GCLs*

Divalent for monovalent monovalent exchange results in loss f lli

30 40 DI Tap m)

• f swelling

capability … and

20 p CaCl2 DI-CaCl2 Tap-CaCl Swell (mm

potentially healing capability

10 1 2 3 4 5 6 7 8 Tap-CaCl2

capability

1 2 3 4 5 6 7 8 Number of Wetting Cycles

*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.

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UW Study: GCLs Exhumed from In-Service Caps

10-3

All specimens underwent Ca/Mg for Na exchange

10

• 4

10 3 Site N Site S Site D Sit O s)

for Na exchange Only those with w

10

• 5

Site O uctivity (cm/s

> 120% maintained low K

10

• 7

10

• 6

draulic Condu

Need to protect GCL from drying and/or cation

10

• 8

Hyd

and/or cation exchange.

10-9 50 100 150 200 250 In Situ Water Content (%) 33

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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 not experience desiccation C lik l i d f i

• This case study has relatively
• Cover likely retained function

due to intact GM and GCL

small distortion

• Differential settlement an

issue with waste containers

• Need more research

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Precipitation Precipitation Surface Surface R ff R ff Lateral Lateral Fl Fl ET ET Percolation Percolation

ACAP Data for Conventional Covers

Cover Type Cover Type Site Site p Average Average (mm/yr) (mm/yr) Runoff Runoff Average Average (mm) (mm) Flow Flow Average Average (mm) (mm) Average Average (mm) (mm) Average Average (mm/yr) (mm/yr) Altamont Altamont 379 379 59.0 59.0 4.0 4.0 1.5 1.5 0.1* 0.1* Composite Composite Apple Valley Apple Valley 169 169 6.8 6.8 0.0 0.0 0.0 0.0 Trace Trace Boardman Boardman 177 177 0.0 0.0 0.2 0.2 0.0 0.0 0.0* 0.0* Marina Marina 433 433 98.7 98.7 47.4 47.4 23.1 23.1 28.3 28.3 Polson Polson 350.0 350.0 17.7 17.7 40.5 40.5 0.4 0.4 Trace Trace Cedar Rapids Cedar Rapids 981 981 54.1 54.1 96.2 96.2 12.2 12.2 2.8* 2.8* Omaha Omaha 731 731 86 8 86 8 43 3 43 3 6 0 6 0 0 7* 0 7* Omaha Omaha 731 731 86.8 86.8 43.3 43.3 6.0 6.0 0.7 0.7 Soil Soil Barrier Barrier Apple Valley Apple Valley 169 169 3.4 3.4 0.0 0.0 0.0 0.0 7.4 7.4 (4.1%) (4.1%) Albany Albany 1263 1263 359.4 359.4 NA NA 195.2 195.2 195.2 195.2 (17 1%) (17 1%) Barrier Barrier (17.1%) (17.1%) Cedar Rapids Cedar Rapids 981 981 79.6 79.6 29.5 29.5 51.6 51.6 51.6 51.6 (6.0%) (6.0%)

*Composite percolation data are scaled from field measurements to account for x10 increase in

= semi = semi-

• arid/sub

arid/sub-

• humid/arid.

humid/arid. = humid. = humid. 35

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

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Summary: Field Performance of Conventional Covers Field Performance of Conventional Covers

• Percolation rates for composites are very low:

Percolation rates for composites are very low: < 1 mm/yr in semi-arid and arid climates < 5 mm/yr in humid climates y

• Percolation rates for soil covers much higher than expected:
• 195 mm/yr at Albany GA

195 mm/yr at Albany, GA

• appears dominated by preferential flow

Surface runoff is a small fraction of the water balance (<10%)

• Surface runoff is a small fraction of the water balance (<10%)
• Lateral drainage is a small fraction of the water balance (< 5%)

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SLIDE 37

Water Balance Covers Evaluated by ACAP

Helena, MT Polson, MT Boardman, OR Altamont, CA Monticello, UT Marina, CA Albany, GA Marion, IA Omaha, NE Sacramento, CA Apple Valley, CA 300 0 mm 600 900 1200 1500 1800 2100 2400 2700

Soil-Gravel Admixture Gravel Topsoil Compost / Soil Mix

2700 3000

Storage Layer Compacted Vegetative Cover Clean Sand Silty Sand

Target Target percolation percolation

Interim Cover Vegetation (Grass) Vegetation (Grasses, forbs, and shrubs) Vegetation (Hybrid-Poplar Trees with a grass understory)

percolation percolation rates ~ 3 rates ~ 3 mm/yr or less. mm/yr or less.

SLIDE 38

ACAP Site Characteristics

Site Location Elev. (m) Annual Precip. (mm) Annual Snowfall (mm) Annual P/PET Climate Monthly Avg. Air Temp. A l V ll CA 898 119 38 0 06 id 1 37 Apple Valley, CA 898 119 38 0.06 arid

• 1, 37

Boardman, OR 95 225 185 0.23 semi-arid

• 2, 32

Helena MT 15 312 1288 0 44 semi-arid

• 11 28

Helena, MT 15 312 1288 0.44 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

S t CA 320 434 0 33 i id 3 34 Sacramento, CA 320 434 0.33 semi-arid 3, 34 Underwood, ND 622 442 813 0.47 semi-arid

• 19, 28

Marina, CA 31 466 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 1 03 humid 8 23 Cedar Rapids, IA 290 915 724 1.03 humid

• 8, 23

Albany, GA 60 1263 3 1.10 humid 8, 33

SLIDE 39

Marina, CA

• Costal semi

Costal semi-

• arid

arid climate climate

• Precipitation = 466

Precipitation = 466 mm/yr mm/yr

• P/PET = 0.46

P/PET = 0.46 Capillary barrier Capillary barrier

• Capillary barrier

Capillary barrier (theory), but (theory), but effectively a effectively a monolithic barrier monolithic barrier monolithic barrier monolithic barrier

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2000 500 m)

Water Balance of Capillary Barrier: Marina, CA

2000 400 500 ration (mm Cumulat Missing Data Storage Capacity = 300 mm 1500 300 400 apotranspir tive Percol and Surfa Soil Water Storage Data 1000 200 300

• n and Eva

lation, Soil ace Runoff 500 200 Precipitatio Water Sto f (mm) Precipitation Evapotranspiration 100 umulative P

• rage,

Percolation No Surface Runoff

Percolation occurs every year when storage capacity is exceeded. Percolation occurs every year when storage capacity is exceeded.

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 Cu

SLIDE 41

Polson, MT

Cool and Seasonal Semi- Humid Climate Humid Climate Capillary Barrier Precipitation ~ 380 mm/yr) 380 mm/yr) P/PET = 0.58

SLIDE 42

1600 80

Capillary Barrier: Polson, MT

piration, Cumulativ Precipitation (b) Alternative

1200 60

Evapotransp rage (mm) ve Percolat

800 40

cipitation, E l Water Sto tion and Sur Evapotranspiration

400 20

mulative Prec and Soil rface Runof Soil Water Storage Surface Runoff Cum ff (mm) Percolation

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. 0.8 mm percolation over 5 yr! Less than composite.

SLIDE 43

ACAP Data for Water Balance Covers

Maximum Average Site Maximum Average Precip. (mm) Perc. (mm) Year Precip. (mm) Perc. (mm) Albany, GA 1380.2 218.3 4 1202.3 109.2 y 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) 0 0 0 0 Boardman, OR (Thin) 210.8 0.0 3 181.4 0.0 Boardman, OR (Thick) 0.0 0.0 Cedar Rapids, IA 898.4 366.1 4 930.0 207.3 Helena MT 351 5 0 1 5 272 4 0 0 Helena, MT 351.5 0.1 5 272.4 0.0 Marina, CA 406.9 82.4 4 462.8 63.3 Monticello, UT 662.9 3.4 5 387.0 0.7 O h NE (Thi ) 101 0 56 1 Omaha, NE (Thin) 612.4 101.0 1 732.5 56.1 Omaha, NE (Thick) 57.9 27.0 Polson, MT 308.1 0.4 349.1 0.2 S CA (Thi ) 361 2 108 4 54 8 Sacramento, CA (Thin) 361.2 108.4

• 422.0

54.8 Sacramento, CA (Thick) 455.7 8.5 3 2.7 Underwood, ND 585.2 9.4 1 384.1 7.1

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2000 500 m)

Water Balance Covers: How They Function

1500 2000 400 500 spiration (mm Cumulative a Total storage capacity = 300 mm Required storage capacity 1000 1500 300 d Evapotrans e Percolation and Surface R 1000 200 cipitation and n, Soil Water Runoff (mm) Soil Water Storage Precipitation 500 100 mulative Prec Storage, Percolation Storage 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 Cum

Key to design is Key to design is availabl available storage capacity storage capacity (soil properties (soil properties Key to design is Key to design is availabl available e storage capacity storage capacity (soil properties (soil properties and cover thickness) must equal or exceed and cover thickness) must equal or exceed required required storage storage (climate characteristics) (climate characteristics)

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A Two-Step Method for Design of Water Balance Covers Design of Water Balance Covers

• 1. Preliminary design: estimate

required thickness by matching required and available storage 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

SLIDE 46

Regulatory Requirements Regulatory Requirements A Regulatory Framework Site and Engineer Site and Engineer Regulatory Requirements Regulatory Requirements

• The goal: Protect human health and

The goal: Protect human health and environment! environment!

Responsibilities Responsibilities

• Conduct prescribed site and waste

Conduct prescribed site and waste

• Factors to consider

Factors to consider

• Waste characteristics

Waste characteristics

• Hazardous life of waste

Hazardous life of waste W t k i W t k i analysis analysis

• Define required closure performance

Define required closure performance

• Percolation

Percolation

• Waste packaging

Waste packaging

• Depth to ground water

Depth to ground water

• Attenuation capacity of geo

Attenuation capacity of geo strata strata

• Gas release

Gas release

• Erosion

Erosion

• Containment life

Containment life S l t i t l t S l t i t l t

• Distance to nearest receptor or

Distance to nearest receptor or sensitive environment sensitive environment

• Climate

Climate

• Stakeholder views public

Stakeholder views public

• Select appropriate closure concept

Select appropriate closure concept (composite, water balance, ?) (composite, water balance, ?)

• Result is site

Result is site-

• specific, performance

specific, performance-

• b

d d i b d d i

• Stakeholder views, public

Stakeholder views, public acceptance acceptance

• Containment Philosophy

Containment Philosophy

• Minimize release

Minimize release based design based design

• Monitor and maintain

Monitor and maintain

• Engineer and regulator must develop

Engineer and regulator must develop long long term relationship based on past term relationship based on past

• Controlled release

Controlled release

• Modify list to be site

Modify list to be site-

• specific

specific long long-term relationship based on past term relationship based on past performance, trust, and a shared performance, trust, and a shared dedication to the ‘real’ goals. dedication to the ‘real’ goals.

SLIDE 47

Albright, W.H., Benson, C.H., and Waugh, W.J., 2010. Water Balance Covers for Waste Containment: Principles and Practice ASCE Press Reston VA

Publications

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 Albright, W., Benson, C., Gee, G., Abichou, T., Tyler, S., Rock. S., 2006. Field Performance

• f 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

• f a Compacted Clay Landfill Final Cover at a Humid Site J of Geotech and Geoenv
• f 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

• n the Hydraulic Properties of Water Balance Cover Soils J of Geotech and Geoenv
• n 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 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/