IN-SITU IMPOUNDMENT Tim Silar, P.G. Principal Geoscientist CLOSURE AND John Magee, P.E. GROUNDWATER Principal Engineer CORRECTIVE ACTION World of Coal Ash 2019 TECHNOLOGY St. Louis, Missouri
PRESENTATION OBJECTIVE To present a permanent, technically sound, minimally disruptive and cost effective impoundment closure and groundwater corrective action option including: • Common impoundment challenges/concerns • Impoundment closure remedial technology • Proof of technology • Cost evaluation • Stakeholder acceptance and risk reduction • Applicability 2 FIRST UP CONSUL TANTS
CCPs below the water table Ponded water/increased hydraulic Common CCP head Impoundment Structural stability Challenges/ Remedial Construction safety Concerns Extensive dewatering and treatment Historic infrastructure and disposal practices
CCP Impoundment Profile
Technology – Hydraulic Containment via In Situ Solidification(ISS) ISS: In‐place mechanical mixing of media with dry reagent or an injected engineered grout mixture Result: Monolithic structure with increased strength/stability and decreased permeability Typical Reagents: Portland Cement, Slag, Bentonite Typical Performance Goals • Decreased Permeability (e.g. ≤ 1x10 ‐6 to 1x10 ‐8 (cm/sec)) • Increased compressive strength (e.g. ≥ 50 psi) • Reduction of leaching to meet regulatory requirements Construction Techniques • Conventional full depth ISS – proven technique • Discrete zone ISS – proven technique via CERCLA Site Pilot Test
ISS Drilling Methods Full Depth Construction Vertical Barrier and Geotechnical Stability Discrete Layer Construction Horizontal Hydraulic Barrier
Investigation and Constructability Evaluation Major Activities ‐ Review records and develop CSM ‐ Conduct Constructability evaluation ‐ Perform ISS treatability study to determine performance criteria/construction parameters =>Evaluate dewatering ‐ Conduct on‐site investigation => Evaluate Grading Plan =>Complete Geotechnical Eval =>Perform GW Assessment to establish baseline conditions & interface
Impoundment Site Preparation ASH IMPOUNMENT SATURATED/ ASH IMPOUNMENT FILL CAN BE POSSIBLE STANDING WATER CONCERN: Impoundment may require UNEVEN & UNSTABLE dewatering over time and hydraulic CONCERN: Grading & Stabilization may head can exacerbate leaching be required to construct liner. =>Begin dewatering to facilitate =>Regrade/stabilize ash to establish construction and overall effectiveness of remedy. construction platform
Impoundment Site Construction Begin ISS construction of linear system and perimeter columns. ISS full depth along perimeter berms to construct perimeter containment and provide stability.
Impoundment Site Construction Begin ISS construction of linear system and perimeter columns. ISS full depth along perimeter berms to construct perimeter containment and provide stability.
Plan View: Both Full and Discrete ISS provides complete coverage 40 ACRE POND: ~18,000 COLUMNS ~1,000 FULL ISS ~17,000 Discrete ISS
Final Cover Construction Complete ISS construction of liner system and final cover system Import fill or ISS swell to Low permeability layer (e.g. establish subgrade for final geomembrane, compacted clay, cover. Closure Turf TM ) Final Cover protective layer (e.g. soil, Closure Turf TM )
Proof of Technology Applications of ISS on CCP Previously need for active remediation was o limited Implemented for geotechnical/stability o applications Discrete ISS: Successfully Pilot Tested at CERCLA Site Baseline Laboratory Treatability Testing on CCP Material Properties – physical and chemical o Reagent and proportion selection (e.g. Portland o Cement, Slag, Bentonite) Performance testing (hydraulic conductivity, o compressive strength, monolithic leaching) Impoundment water reuse evaluation o Full depth ISS mixing recently proven at CCP impoundment
CCP ISS Treatability Testing 10% Portland 7.5% GGBFS 6% GGBFS 8% Portland 18% GGBFS Grout Amendments and Cement 2.5% Portland 8% Portland 2% Portland Cement 6% Portland Cement Proportions 0.5% Bentonite Cement Cement Cement 0.5% Bentonite 2% Bentonite Curing Days Prior to Testing 14 14 14 28 14 28 14 28 14 28 Moisture Content (%) (pretreatment) 31 31 37 37 37 37 37 37 23.5 23.5 Density (lbs/ft 3 ) (pretreatment) 102 102 97.63 97.63 97.63 97.63 97.63 97.63 97.63 97.63 Unconfined Compressive Strengh (psi) 221 184 80 101 38 76 70 86 -- 1039 Hydraulic Conductivity 4.1 x 10 -7 1 x 10 -6 8.1 x 10 -6 9.1 x 10 -6 6.4 x 10 -6 2.3 x 10 -8 (cm/sec) -- -- -- -- GGBFS = Ground Granulated Blast Furnace Slag
CCP ISS Treatability Testing 10% Portland 7.5% GGBFS 6% GGBFS 8% Portland 18% GGBFS Grout Amendments and Cement 2.5% Portland 8% Portland 2% Portland Cement 6% Portland Cement Proportions 0.5% Bentonite Cement Cement Cement 0.5% Bentonite 2% Bentonite Curing Days Prior to Testing 14 14 14 28 14 28 14 28 14 28 Moisture Content (%) (pretreatment) 31 31 37 37 37 37 37 37 23.5 23.5 Density (lbs/ft 3 ) (pretreatment) 102 102 97.63 97.63 97.63 97.63 97.63 97.63 97.63 97.63 Unconfined Compressive Strengh (psi) 221 184 80 101 38 76 70 86 -- 1039 Hydraulic Conductivity 4.1 x 10 -7 1 x 10 -6 8.1 x 10 -6 9.1 x 10 -6 6.4 x 10 -6 2.3 x 10 -8 (cm/sec) -- -- -- -- GGBFS = Ground Granulated Blast Furnace Slag
Compare in situ closure to excavation & off-site disposal
Cost Evaluation: Assumptions Volume Assumptions: Excavation Assumptions: In situ Closure Assumptions: • Excavation volume is 1.29 Assume the • ISS volume is 366,000 million cubic yards impoundment volume cubic yards including 5’ • Excavated ash will be is approximately 1.29 thick discrete bottom liner disposed within 20 miles million cubic yards (40 and 10’ wide fully of the impoundment penetrating perimeter acres by 20’ thick) • One cubic yard of ash walls to 25’ below grade weighs 2,700 lbs • Dewatered groundwater • Excavation will require will be used in ISS batch dewatering and water plant treatment • Impoundment will be • Impoundment will be capped with swell , partially backfilled and geotextile, and soil cover vegetated Cost evaluated on recent experience and discussions with contractors
Cost Categories Excavation & Off‐ Excavation & Off‐ General General Site Disposal vs Site Disposal vs Conditions Conditions lSS Technology lSS Technology Dewatering Dewatering Site Restoration Site Restoration
Cost Evaluation: Details Excavation and Off-Site Disposal In situ Closure (ISS) Estimated Item Units Unit Price $ Amount $ Unit Price $ Amount $ Quantity $ 2,750,000 $ 2,750,000 General Conditions Mobilization & Temporary $ 700,000 $ 700,000 $ 700,000 $ 700,000 1 Lump Sum Facilities Setup Site Preparation, Site Survey, $ 1,500,000 $ 1,500,000 $ 1,500,000 $ 1,500,000 1 Lump Sum Remove Ponded Water Clearing, Grubbing and Erosion $ 300,000 $ 300,000 $ 300,000 $ 300,000 Controls Installation, 1 Lump Sum Maintenance & Monitoring Demobilization & Record $ 250,000 $ 250,000 $ 250,000 $ 250,000 1 Lump Sum Documents
Cost Evaluation: Details Excavation and Off-Site Disposal In situ Closure (ISS) Estimated Item Units Unit Price $ Amount $ Unit Price $ Amount $ Quantity Excavation / Disposal vs. $ 69,373,351 $ 31,977,640 ISS Excavate Ash, Dewater, and 1,290,667 Cubic Yards $ 20 $ 25,813,340 $ - $ - Load Transport and Dispose of Ash 1,742,400 Tons $ 25 $ 43,560,011 $ - $ - Mobilize ISS Treatment 1 Lump Sum $ - $ - $ 500,000 $ 500,000 Equipment and Materials In situ Solidification/Stabilization 366,000 Cubic Yards $ - $ - $ 80 $ 29,280,000 (ISS) ISS Swell Management (15%) 54,900 Cubic Yards $ - $ - $ 4 $ 197,640 Geotextile Cap 40 Acres $ - $ - $ 50,000 $ 2,000,000
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