Geosynthetic Opportunities Associated With Coal Mining Spoils and Coal Combustion Residuals

by Bob & George Koerner Geosynthetic Institute

1. Overview of Energy Sources

2. Coal Spoil Tips

3. Coal Combustion Residuals 3.1 - Dry Disposal

3.2 - Wet Disposal

4. Related Geosynthetic Solutions

5. Summary and Conclusions

1.0 Overview of Energy Sources

Fuel type 2006 World

consumption

in PWh

2006 US

consumption

in PWh

Oil 50 13

Gas 32 6

Coal 37 7

Hydroelectric 9 1

Nuclear 8 3

Geothermal,

wind, solar,

wood, waste

1 1

TOTAL 137 31

Oil 36%

Gas 23%

Coal 27%

Hydro 7%

Nuke 6%

Other 1%

Oil 42%

Gas 19%

Coal 23%

Nuclear 10%

Hydro 3%

Renewables 3%

USA World

Energy Sources in the World and USA in 2006 (100 quadrillion BTUs = 29 PWh) Wikipedia

Coal Related Comments

• coal has fueled the industrial revolution

• it is embedded in our culture; “Coal is King!”

• global warming and pollution are challenges

• but business, employment, politics, interest groups, unions, etc., make it contentious

• whatever the pros and cons; both deep mining and strip mining will likely continue

2. Coal Spoil Tips

• 250 years of coal spoil is enormous

• precipitation creates acid rock drainage polluting streams and rivers

• erosion is commonplace and ongoing

• spontaneous combustion has occurred

• the tip itself can be unstable (later)

• vegetation does not take root since the mass traps solar heat and kills it

Spoil tips on the site Écopôle 11/19 in Loos-en-Gohelle (right). The town of Liévin is on the left

Abandoned spoil tip in northeastern Pennsylvania (Wikipedia)

Notable Coal Spoil Tip Failures

• Aberfan, Wales, in 1966 (144 people killed; 116 children in school)

• Coedely, Wales in 1968

– pile started in 1930

– moved 120 m by 1955

– another 20 m by 1966

– slide photo is at right

– let’s evaluate it… (Ghosh and Ferguson, 1991)

Coedely-Critical Cross Section

• major concern is stability of underlying glacial till • flows and springs were located over time

Spoil

Sand Stone Glacial Till

The Critical Parameters

• critical parameters used in the design are shown in the table below

• the maximum recorded water level was used • friction angle of the spoil was adjusted to obtain

factor of safety of 1.0, i.e., incipient failure • triggering mechanism was assumed to be the

elevated water level within the spoil

Material Unit Weight

(kN/m3) Friction angle

(deg.) Cohesion

(kPa)

Coal spoil 19 22 0

Glacial till 20 30 10

Analysis Using ReSSA (3.0) Code • evaluated both rotational and translational modes • failure surfaces include the triggering mechanism:

FS = 1.01

Spoil

Sand Stone Glacial Till

(a) Rotational failure with Trigger

Spoil

Sand Stone

Glacial Till

FS = 1.00

(b) Translational failure with Trigger

Re-Analysis • failure surfaces without triggering mechanism:

i.e., removal of the elevated internal water

Spoil

Sand Stone

FS = 1.03

Glacial Till

(c) Rotational failure without Trigger

Spoil

Sand Stone

FS = 1.06

Glacial Till

FS = 1.06

(d) Translational failure without Trigger

Analysis Summary

• the site is susceptible to both modes of failure • elevated water only slightly influenced the FS-values • the decrease was sufficient to cause creep failure • thus, the stability of the site was marginal, but the

elevated water caused the eventual failure

Failure Mode

FS with Trigger

FS without Trigger

Differences

Rotational 1.01 1.03 +2%

Translational 1.00 1.06 +6%

Geosynthetic Opportunities With Coal Spoil Tips

• MSE berms needed for containment (GG’s)

• geocomposites needed for drainage removal

• PVDs and high strength GTs could be used for soft foundation soil stabilization

• consider a massive GS-related final cover

• more details later…

3. Coal Combustion Residuals

• fly ash (which is 60% of all CCR’s)

• flue gas desulfurization (FGD) materials

• bottom ash

• boiler slag Comment: covers are critical

Data on the Generation of CCRs in 2008 (Tons)

Commodity Annual Quantity Generated

Annual Quantity Landfilled

Total Quantity Stockpiled as of 2006

Coal fly ash 72.4 M 42.31 M 100-500 M

Bottom ash 18.4 M 10.36 M Undetermined

Boiler slag 2.0 M 0.34 M Undetermined

Source: American Coal Ash Assoc. 2008*

*EPA estimates 240 M tons at 1000 sites scattered in 47 states.

U.S. EPA documented groundwater contamination sites from Coal Combustion Residuals (CCRs) Disposal

(ref. GSE/GMA, 2012)

3.1 Coal Combustion Residuals - Dry Disposal • represents ~ 65% of total CCRs

• traditionally placed directly on ground surface

• rarely includes a liner or cover of any kind

• also includes metals in varying amounts including lead, arsenic, selenium, cadmium and chromium

Component Bituminous Subbituminous Lignite

Silicon Dioxide 200,000-600,000 400,000-600,000 150,000-450,00

Aluminum Oxide 50,000-350,000 200,000-300,000 100,00-250,000

Iron Oxide 100,000-400,000 40,000-100,000 40,000-150,000

Calcium Oxide 10,000-120,00 50,000-300,000 150,000-400,000

Magnesium Oxide 0-50,000 10,000-60,000 30,00-100,000

Sulfur Trioxide 0-40,000 0-20,000 0-100,000

Sodium Oxide 0-40,000 0-20,000 0-60,000

Potassium Oxide 0-30,000 0-40,000 0-40,000

Loss on Ignition 0-150,000 0-30,000 0-50,000

Source: Coal Fly Ash Material Description - Turner-Fairbank Highway Research Center (data in ppm)

Oak Creek Wisconsin CCR Ash Failure

• power plant ash placed in ravine in 1950’s

• runoff water existed in three locations (?)

• seepage and groundwater recharge possible

• storm sewer and outlet piping more recently (??)

• FGD basin added on top very recently (???)

• failed abruptly on Oct. 31, 2011 (ca. 22,000 yd3)

• being investigated by WI-DNR & WE Energies

Plan view of site showing failure area and FGD basin (Lake Michigan is 50-100 ft. to the south)

Site failure photographs (compl. WI-DNR)

• placed directly on ground surface

• rarely are liner systems used at present

• federal regulations have passed U.S. House and are in the Senate presently but on hold until November (or longer)

(compl. A. Filshill) (compl. D. DiGuilio)

Disposal of Most Dry CCRs

Geosynthetic Opportunities With Dry CCR’s

• MSE berms needed for containment(GG’s)

• PVDs and high strength GTs and GGs for soft foundation soil stabilization

• lined base and sideslope systems are needed; single lined or double lined, with or without leachate collection systems (most GS’s)

• final cover systems (including most GS’s)

• more details later…

3.2 Coal Combustion Residuals - Wet Disposal

• represents ~ 35% of total CCRs

• ash is slurried and piped to disposal area

• traditionally placed directly on ground surface behind soil containment berms

• several recent massive berm failures

– Spain - mine sludge

– Hungary - aluminum dross sludge

– USA - slurried CCR

Failure in Spain (2010) (Wikipedia)

Failure in Hungary (2008) (Wikipedia)

The Kingston, TN (TVA) Failure 12/29/08 • Approximately 4.1 million m3 of slurried ash containment was spilled

into the nearby river and area beyond

Aerial Photo of the site after the failure (Walton and Butler, 2009)

Background of TVA Failure

• The site was first operated in 1954 and the “outer dike” was completed in 1958

• Vertical and lateral expansions were initiated in 1995

• “New dikes” were constructed over the slurried ash with an average slope of 4H-to-1V (14.0°)

Graph of the site (Walton and Butler, 2009)

Cell 1 Phase 1 Emergency Cell

Cell 2

Outer Dike

New Dikes

Phase 2 Expansion

Various Failure Stages Stage 1

Stage 2

(Walton and Butler, 2009)

(Walton and Butler, 2009)

Stage 3

(Walton and Butler, 2009)

(Walton and Butler, 2009)

Stage A - Initial failure of Cell 2

The Failure in Progressive Stages

(Walton and Butler, 2009)

Stage B - Failure progressing north

(Walton and Butler, 2009)

Stage C - Failed mass stressing of Dike C

(Walton and Butler, 2009)

Stage D - Cumulative mass moving into river

(Walton and Butler, 2009)

(compl. D. DiGuilio)

Stage E - Aerial views of failed site

Critical Factors

• Thin layer of “slime” was found between the ash fill and soft clayey foundation soil

– About 150mm (6 inches) thick

– Low shear strength, high water content (40 to 140%)

• Filling rate of Cell 2 was increased in 2008

– Over 2 m/year (highest increasing rate over the years)

– Low ash shear strength; design assumed strength would increase with depth; however, consolidation of the ash did not occur. In our analysis the low undrained shear strength was assumed to be the trigger.

Critical Parameters

• shear strength of ash layers and cohesion of the slime were systematically varied so as to obtain factor of safety of 1.0

• critical parameters:

• triggering mechanism was assumed to be the low undrained strength behavior of the previously placed ash due to recent high filling rate of the new slurried ash

Material Unit weight

(kN/m3)

Friction

angle (deg.)

Cohesion

(kPa)

Ash Layers 16.8 0 4.8 to 14.4

Slime 14.1 0 42

Analysis (using low undrained strength)

Dike

Dike

Coal Residuals

Dike Dike

Dike Dike

FS = 1.00

(a) Rotational failure with trigger

Dike Dike

Dike Dike Dike Dike

Coal Residuals

FS = 1.02

(b) Translational failure with trigger

Slime Clay

Coal Residuals

Slime Clay

Coal Residuals

Re-Analysis (with higher drained strength)

Dike

Dike

Dike Dike

Dike Dike

FS = 1.38

(c) Rotational failure without trigger

Dike

Dike

Dike

Dike Dike

Dike

FS = 1.34

(d) Translational failure without trigger

Slime Clay

Coal Residuals

Slime Clay

Coal Residuals

Analysis Summary

Failure Mode FS with Trigger FS without Trigger Differences

Rotational 1.00 1.38 +38%

Translational 1.02 1.34 +31%

• As seen in the table below the site is susceptible to both types of failure

• Low undrained condition of previously placed ash was critical for the slope stability of the newly placed slurried ash

• If old ash was drained, FS-values would have been reasonably adequate

Status of Containment Facilities Containing Slurried CCR’s

(Rated by U.S. COE for U. S. EPA, 2009)

High Hazard 25%

Significant Hazard

35%

Low Hazard 36%

Less than Low

Hazard 4%

Not Yet Rated

200 Units 429 Units

?

?

? ?

?

?

? ?

Geosynthetic Opportunities With Wet CCR’s

• engineered berms needed for containment (GG’s)

• PVDs and high strength GSs for soft ground stabilization

• engineered single or double liner systems are being recommended (using most GS’s)

• possible floating cover over slurried ash (GTs)

• more details next…

4. Related Geosynthetic Opportunities

4.1 Foundation soil stabilization with PVDs 4.2 Deep foundations and HS-GTs 4.3 Soil/gravel columns using GTs and GGs 4.4 Engineered MSE berms using GGs and GTs 4.5 Single composite liner systems 4.6 Double composite liner systems 4.7 Cover systems on spoils and dry CCRs 4.8 Cover systems on wet (slurried) CCRs 4.9 GT tubes for wet (slurried) CCRs

4.1 Foundation Soil Stabilization with PVDs

(Comp. Nylex Co.)

Progression of Activities Stabilizing Soft Dredged Soil at Maryland Port Authority

4.2 Deep Foundations and HS-GTs

(Original Concept by B. Broms, 1977)

The Load Transfer Platform (LTP) (ref. Alexiew & Gartung, Berlin Railway, 1999)

SRR Values using Seven Existing Methods (ref. Filtz and Smith, 2006)

Method

Stress Reduction Ratio (SRR)

d/s = 0.25 d/s = 0.50

H/s = 1.5 H/s = 4 H/s = 1.5 H/s = 4

British BS8006 (1995) 0.92 0.34 0.09 0.02

Adapted Terzaghi, KT = 1 0.60 0.32 0.34 0.13

Adapted Terzaghi, KT = 0.5 0.77 0.52 0.54 0.26

Kempfert et al. (2004) 0.55 0.46 0.23 0.15

Hewlett and Randolph (1988) 0.52 0.48 0.30 0.13

Adapted Guido (1987) 0.12 0.04 0.08 0.03

Carlsson (1987) 0.47 0.18 0.31 0.12

where: SRR = stress reduction ratio = v(on GS)/ z (soil above) = v(on GS) /(H+q)

d = pile width s = center-to-center pile spacing H = embankment height

Comment

• limit equilibrium methods vary greatly and are technically questionable

• current direction in design is using FEMs

• several papers are now available

• highest stresses are at the pile edges

• see following…

Finite Element Calculated Tensile Forces Showing Extremely High Values at Pile Edges, after Liu, Ng and Fei, JGGE, December 2007

High Edge Stresses are Obvious Even to the Contractor

VOID

PilePile

VOID

PilePile

PilePile

PilePile

PilePile

PilePile

Log Spiral Curvature of Pile Cap Edges

Inverted Form for Concrete Placement with Central Void for Pile

GSI Suggested Method for Pile Caps

Void

For

Pile

Soft Soil

4.3 Sand/Gravel Columns with Soft Edges using geogrids or geotextiles

Geogrid Encased Stone Columns (Comp. NAUE, Inc.)

Geotextile Encased Sand Columns (Comp. Huesker, Inc.)

Test field: Installation of geogrid column (compl. NAUE)

Geosynthetic reinforced soil columns

Installation of geotextile encased sand columns

Placement of high strength geotextile over encased sand columns

(Comp. Huesker Co.)

Mountain View Landfill Access Road Embankment

(GSI Photos)

4.4 Engineered MSE Berms with GS Reinforcement

Engineered berm (wall) at landfill, (comp. Tensar Corp.)

(comp. Waste Management Inc.)

4.5 Single Composite CCR Liner Systems

Single Composite Liner for Dry CCRs (Comp. D. DiGuilio)

Geotextile Cushion Ready for Dry CCRs (Comp. D. DiGuilio)

The Concept of a Composite Liner

(a) geomembrane over compacted clay liner (b) geomembrane over geosynthetic clay liner

Which clay component do you use ???

Can a Liner System Withstand Foundation Settlement?

• total settlement is easily accommodated

• anchor trenches can be designed to allow GM and/or GCLs to slide out of them

• differential settlement has to be evaluated

• response varies greatly for different materials

Differential Settlement Calculations

Geomembrane tensile stress:

Geomembrane tensile strain:

tL16

P4L2

22

2

Lfor100

L

L4L

4Lsin

4

4L

%

2

Lfor100

L

L4

4L

4L

4Ltan

%

22

122

22

22

1

Example:

0.5 m

2.0 m

with L = 2.0 m and = 0.5 m, = 15.9% is needed…

Since 0.5 < 1.0, use

100L

L4

4L

4L

4Ltan

%

22

22

1

How do CCL's Behave Undergoing Differential Settlement?

Type or Source of Soil

w1 (%)

P.I.2 (%)

t3

(%)

Clayey Soil Illite Kaolinite Anon. Dam Rector Creek Dam Woodcrest Dam Wheel Oil Dam Willard Embankment

19.9 31.4 37.6 16.3 19.8 10.2 11.2 16.4

7 34 38 8 16 n/p n/p 11

0.80 0.84 0.16 0.14 0.10 0.18 0.07 0.20

Data on Tensile Strain at Failure for Compacted Clay, LaGatta (1992)

1. Water Content 2. Plasticity Index 3. Tensile Strain at Failure Ave = 0.31%!

How do GCL's Behave Undergoing Differential Settlement?

To a Breakthrough in Permeability (via LaGatta & Boardman)

t (%) = 10 to 15 %

To Break in 3-D Tension Test (via Koerner, et al.)

f (%) = 15 to 26 %

How do GM's Behave Undergoing Differential Settlement?

(via GRI GM4 Test Method: Koerner, et al., ASTM STP 1081)

fPP-R = 12% HDPE = 25% PVC = 75% LLDPE = 75% fPP = 100%

Resulting f

0 20 40 60 80 100

0

10000

20000

30000

40000

Strain (%)

Stre

ss (

kPa)

HDPE

fPP-R

PVC

LLDPE

fPP

But is HDPE Limited to 20%?

• multi-axial 3D tension test is appropriate

• GRI-GM4 started at 1.0 psi/min. in 1990

• unfortunately ASTM D5716 did likewise

• following ongoing project varies pressure rate

Test Pressure Rate Strength (psi) Elongation (%)

1 2 3 4 5 6 7 8

1.0 psi/min 0.5 psi/min 0.1 psi/min 1.0 psi/hr. 1.0 psi/12 hr. 1.0 psi/day 1.0 psi/week 1.0 psi/mo.

2260 2500 3050 2700 2780 2950 3330

working

13.5 37.5 57.1 62.1 63.8 69.9 81.4

working

Current Project Evaluating 1.0 mm (40 mils) Smooth HDPE Sheet

Conclusion: HDPE elongation at break increases greatly as pressure rate decreases

Conclusions

• CCL's should not be the selected barrier material for deforming subgrade soils.

• GM's and GCL's are better both technically and based on benefit/cost.

• The preferred barrier is a GM by itself or a GM/GCL composite… HDPE is a “player”

• GSI has a published paper on closures.

4.6 Double Composite Liner Systems

GT

GG

GN

GCL

GM

CCL

Gravel w/ perforated pipe

Optional GCL on

Subgrade or GT

Secondary GM

GN/GT Leak Detection

Primary GM over GCL

Berm Separating Cells with Completed Cells in Background

GT Cushion over Primary GM

Gravel Leachate Collection Layer over GT Cushion

Downgradient Sump

Double Lined Cell Layer-by-Layer (note; single lined system is similar but without secondary liner or leak detection layer)

4.7 Cover Systems on Spoils or Dry CCRs

Savannah River Cover over Low Level Rad-Waste

GC GM GCL

GT

GG

Waste cover soil

4.8 Cover Systems for Wet (Slurried) CCRs

(Comp. J. Guglielmetti)

Prefabrication of High Strength GT for Covering of Sludge

• GT covering required two days • backhoe and dozer pulled while laborers guided

the movement • leading edge sagged into sludge with several seam failures • they were repaired by hand sewing

Sequence of Soil Cover the GT

GT Being Pulled Over Sludge with Soil Covering Being Staged

4.9 Geotextile Tubes for Wet CCRs

(a) Dewatering at (or near) power plant and moving dried material to a landfill

(b) Pumped sludge to a tube within a landfill where the effluent becomes leachate and is treated as such

Dewatering Plus Decontamination (hypothetical example follows)

• add activated carbon for removal of organic pollutants (drinking water method)

• charcoal is also a strong sorbent for organic pollutants which can be added

• phosphoric rock reacts with heavy metals to form insoluble phosphate salts

Example 1: Contaminated slurry: 15% solids/85% water; density = 1.2 kg/L; 100 ppm (100 mg/kg) of following pollutants. Charcoal added at a rate of 5 gm/L of slurry. What is the theoretical % reduction of each pollutant?

Solution: Units are aqueous solution concentrations in “mg/L”

Type of Pollutant Before (mg/L)

After (mg/L)

Reduction (%)

Polynuclear Aromatic Hydrocarbons

Naphthalene Fluorene Phenanthrene Anthracene Pyrene Benzo(a)pyrene

4.53 0.69 0.28

0.030 0.077

0.0033

0.43 0.053 0.021

0.0022 0.0057

0.00024

90.6 92.3 92.5 92.6 92.6 92.6

Chlorinated Chemicals

1,2-dichlorobenzene 2-chlorobiphenyl 2,5-dichlorobiphenyl 2,2’,5,5’-chlorobiphenyl

4.32 0.31

0.063 0.0069

0.40 0.023

0.0047 0.00051

90.7 92.5 92.6 92.6

5. Summary and Conclusions

• massive amounts of CCR’s and spoils exist

• represents a significant ongoing environmental challenge; both groundwater and airborne

• Failures in Spain, Hungary and Tennessee heightened the situation for CCR slurries

• U. S. EPA regulations are a work-in-progress

• State EPAs are presently actively involved

• geosynthetics opportunities are enormous

Conclusions

• unlike shale gas opportunities (which are driven by benefit/cost ratios); CCR and spoil containment is largely regulatory driven

• federal EPA should be the focus of containment regulations

• GMA activities are commendable • fall-back is to the individual state regulations

which appears to be the present status • whatever the situation, geosynthetics are

fundamental to any solution going forward

Thanks for Listening

Questions???