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
Functions of Covers • Physical containment of waste • Control percolation into waste • Control gas movement Ingress (O 2 ) Egress (Rn, CH 4 , CO 2 ) • C Control vector intrusion t l t i t i • • Persist for design life of containment facility 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 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 3
Conventional Resistive Covers with a Soil Barrier with a Soil Barrier Geosynthetic Simple Compacted Clay Liner Soil Cover Clay Cover (GCL) Cover (GCL) Cover Soil Soil Soil Waste Waste Compacted Clay l GCL Waste 4
Conventional Resistive Covers with a Composite Barrier with a Composite Barrier Clay-Geomembrane GCL-Geomembrane Composite Composite Composite Composite Soil Soil Soil Geomembrane (GM) Compacted Clay Clay Waste Waste GCL 5
Water Balance Covers Monolithic M lithi C Capillary ill Barrier Barrier Fine Fine Textured Textured Textured Soil Soil Soil C Coarse Soil W Waste Waste 6
Cover Cover Percolation Percolation Gas Gas Cost Cost Rate Type Flux ($/ac) Simple Soil Simple Soil Highest Highest Highest Highest 25 000 25,000 Clay Modest Modest 75,000 GCL GCL Modest Modest Modest Modest 75 000 75,000 Composite Very low Very Low 125,000 Very low Very low - ET Monolithic Modest 50,000 low Capillary Capillary Very low 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 select layering to meet a quantitative performance criterion - analyze to ensure alternative cover meets performance criterion criterion
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 3. Equivalency demonstration is difficult 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? - 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 Require appropriate monitoring i i t it i
USEPA’s Alternative • Twenty Twenty- -four test covers at four test covers at C Cover Assessment eleven sites in seven eleven sites in seven states. states. Program (ACAP) • 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 alternative covers alternative covers 11
ACAP Drainage Lysimeters
Full-scale construction methods
Hundreds of samples and instruments
Lysimeters are the only method for direct measurement of drainage t f d i
Conventional Covers Evaluated by ACAP 16
Compacted Clay Covers Objectives: (1) Construct a soil barrier (compacted clay) with low saturated hydraulic saturated hydraulic conductivity. (2) Protect the clay barrier from ( ) y damage that may increase hydraulic conductivity 17
Types of Damage - Frost - Desiccation D i ti - 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 18
Sensitivity to Frost y Damage Freezing of compacted clay barriers Freezing of compacted clay barriers causes: - formation of ice lenses: cracking f ti f i l ki - 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). 19
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 20
Conventional Clay Cover Performance 8 0 0 2 2 5 • Soil dried for first time Soil dried for first time d d during 6-week drought during 6 i i 6 6 week drought k d k d ht ht • Change in response of Change in response of 5 5 0 1 5 0 p percolation to precipitation percolation to precipitation p p p p p events events Precipitation Soil water No rain storage – Quantity Quantity 3 3 0 0 0 0 7 7 5 5 – “Stair step” response “Stair step” response Stair step response Stair step response mm water mm water • No evidence that defects in No evidence that defects in Percolation clay barrier healed when soil clay barrier healed when soil m water increased water increased 5 0 0 3/4/02 3/4/02 7 /1 /0 0 9 /1 /0 0 1 1 /2 /0 0 1 /3 /0 1 Data from ACAP field site in Albany GA
Field Hydraulic Conductivity Measurements on Clay Barrier 4 Years After Construction Clay Barrier 4 Years After Construction Hydraulic Hydraulic Conductivity Test K final /K as-built (cm/s) As-Built 4.0x10 -8 1.0 SDRI 2.0x10 -4 5000 TSB - 1 5.2x10 -5 1300 TSB TSB - 2 2 3 2x10 -5 3.2x10 -5 800 800 TSB - 3 3.1x10 -3 77,500 22
Typical Composite Cover Surface 150-1000 mm thick (6 – 40 inches) Layer Drainage Layer Drainage Layer Geomembrane • Geomembrane added directly • Geomembrane added directly Compacted Compacted 450 900 mm thick 450-900 mm thick Clay on top of clay barrier or GCL (18 – 36 inches) • Drainage layer frequently • Drainage layer frequently added on top of geomembrane to enhance stability by limiting Waste pore water pressures. 23
1.5 mm LLDPE Textured Geomembrane Geomembrane 24
Geocomposite 25 Drain Drain
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 26
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: -Select drain that provides acceptable head S l t d i th t id t bl h d -Adequate hydraulic conductivity -HELP, conservative (over-predicts lateral drainage) 27 -Giroud & Houlihan's Method
Conventional Composite Cover Performance Performance • Percolation Percolation correlated correlated 1800 120 with with Precipitation Precipitation Lateral flow Lateral flow – Heavy precipitation Heavy precipitation Heavy precipitation Heavy precipitation events events 1700 80 Surface flow Surface flow – Surface flow Surface flow mm water) – Lateral flow on Lateral flow on 1600 40 geomembrane geomembrane (m Percolation Percolation P P l l i i 1500 0 5/1/03 5/16/03 5/31/03 Data from ACAP field site in Cedar Rapids IA
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