• Introduction
• Matrix New!
• Glossary New!
• Notice

• Environmental and Engineering Evaluation New!
• State Contacts New!
• Cost Issues
• Standards/Specifications New!

• Baghouse Fines
• Blast Furnace Slag
• Coal Bottom Ash/Boiler Slag Updated!
• Coal Fly Ash Updated!
• FGD Scrubber Material Updated!
• Foundry Sand Updated!
• Kiln Dusts
• Mineral Processing Wastes
• MSW Combustor Ash
• Nonferrous Slags
• Quarry Byproducts
• Reclaimed Asphalt Pavement Updated!
• Reclaimed Concrete Material Updated!
• Roofing Shingle Scrap Updated!
• Scrap Tires
• Sewage Sludge Ash
• Steel Slag
• Sulfate Wastes
• Waste Glass

• General Descriptions
• Asphalt Concrete Pavement
• Portland Cement Concrete Pavement
• Granular Base
• Embankment or Fill
• Stabilized Base
• Flowable Fill

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]

COAL BOTTOM ASH/
BOILER SLAGUser Guideline

Embankment/Structural Fill

INTRODUCTION

According to the American Coal Ash Association approximately 3.63 million metric tons (4.0 million tons) of bottom ash were utilized in structural fill and embankment applications in 2006.⁽¹⁾ Structural fill and embankment material is the largest use of bottom ash in the U.S. While specifications for bottom ash and boiler slag reuse depend on the application, there are material characteristics that must be met when using bottom ash or boiler slag in embankments or as a structural fill.

PERFORMANCE RECORD

Bottom ash and boiler slag have been successfully used as embankment or structural fill material both nationally and internationally. Bottom ash from WE Energies of Wisconsin was successfully used as a backfill material in Racine, Wisconsin⁽²⁾ as well as fill for a freeway spur in Milwaukee, Wisconsin and other projects.⁽³) Indiana DoT has used bottom ash in numerous transportation applications including as an embankment material.⁽⁴⁾ Approximately 1 million metric tons (1.1 million tons) of coal combustion products (mixtures of fly ash, bottom ash, boiler slag, and fluidized bed combustion ash) were used to raise embankments on an ash pond in Rihand, India.⁽⁵⁾ Coal combustion products including bottom ash have also been used in India for widening existing bridge approaches or building embankments for overpass bridges in Delhi.⁽⁶⁾ Bottom ash has been used as a light weight fill over soft soils in Sweden due to the low unit weight of bottom ash.⁽⁷⁾

ENGINEERING PROPERTIES

Mechanical characteristics of bottom ash that are important when bottom ash is used as an embankment or fill material are: moisture-density relationship (compaction curve), hydraulic conductivity, particle distribution, strength, stiffness, and compressibility.

Moisture-Density Relationship: Bottom ash and boiler slag are coarse-grained materials that are relatively insensitive to moisture content during compaction.⁽⁸⁾ Compacted dry unit weight of bottom ash is in the range of 15.7 to 17.6 kN/m³ (100 to 112 lb/ft³), but can be as low as 9.9 kN/m³ (63 lb/ft³).⁽⁹⁾ Dry bottom ash can sustain particle degradation during compaction.⁽⁹⁾ Crushing of the bottom ash particles during compaction contributes to an increase in the maximum dry unit weight. Compaction characteristics of mixtures of fly ash and bottom ash have more well-graded size distributions, which allows the fly and bottom ash particles to pack more closely, resulting in an increase in the maximum dry unit weight of the mixture.⁽¹⁰⁾

Hydraulic Conductivity: The hydraulic conductivity of well-compacted bottom ash ranges from 1 to 10^-3 cm/s, which is roughly equivalent to the hydraulic conductivity of sand.⁽⁷⁾ Bottom ash is free draining and not typically susceptible to either liquefaction or frost heave.⁽¹¹⁾

Particle Size Distribution: Bottom ash is a well-graded material with particles ranging in size from fine gravel to fine sand with low percentages of silt-clay sized particles. Bottom ash is predominantly sand-sized, usually with 50 to 90 percent passing the No. 4 sieve (4.75 mm) and 0 to 10 percent passing the No. 200 sieve (0.075 mm). Top particle size for bottom ash is typically between 19 mm (¾ in) to 38.1 mm (1^-½ in).

Shear Strength: Bottom ash, having a rough surface texture and angular particles, has a slightly higher friction angle than conventional granular soils. Direct shear tests conducted on bottom ash samples under loose and dense relative density conditions indicated that the angle of internal friction for most bottom ashes ranged from 35 to 50 degrees, depending on the extent of densification, with some dense bottom ash samples exhibiting friction angles as high as 55 degrees. The angle of internal friction for boiler slag was found to fall within the same range as that of most natural granular soils (between 36 and 46 degrees).⁽¹²⁾

Bearing Strength: California Bearing Ratio (CBR) tests on bottom ash indicate that soaking does not negatively affect CBR.⁽¹³⁾ Although the maximum CBR is obtained for bottom ash compacted slightly dry of optimum, where optimum is approximately a flushed conditioned, the general trend for CBR values is to increase with increasing moisture contents.⁽⁹⁾ The CBR values of bottom ash compacted a high moisture contents were found to be higher (CBR 40 to 70) than the CBR values of bottom ash compacted at low moisture contents (CBR 35 to 60) indicating that compacting at high moisture contents is advantageous.

Compressibility: Granular materials with angular particles are typically more compressible than those with well-rounded particles because the sharp edges of the angular particles tend to be break during compression as well as shear. Although some bottom ash particles are porous and weak, for low stress levels, the compressibility of bottom ash is comparable to that of sand placed at the same relative density.⁽¹⁴⁾ Coal ash consolidates rapidly, therefore compressibility typically is not a design concern.⁽¹¹⁾

Table 2 in the Material Description section for Coal Bottom Ash\Boiler Slag lists typical values for engineering parameters of interest when designing with bottom ash or boiler slag.

CONSTRUCTION PROCEDURES

Material Handling and Storage

Both bottom ash and boiler slag can be handled and stored using the same methods and equipment that are used for conventional aggregates.

Placing and Compacting

Prior to placement, a site should undergo preparations consistent with preparation requirements for soil fill materials. Construction equipment needed to properly place and compact bottom ash or boiler slag in an embankment or structural backfill includes a bulldozer for spreading the material, a compactor, either a vibrating or pneumatic tired roller, a water truck to provide water for compaction and to control dusting, and a motor grader where final grade control is critical.

Bottom ash and boiler slag is typically dumped and spread with a bulldozer or motor grader in lifts no thicker that 0.3 m (12 in.) when loose. Bottom ash and boiler slag should be compacted at, or slightly above, optimum moisture content as determined by standard Proctor compaction procedures.⁽¹⁾ Bottom ash loses stability at low moisture contents; therefore, high moisture contents should be maintained to allow construction equipment to operate. The addition of up to 30 percent fines in the form of fly ash may remedy the loss of stability upon drying.⁽¹²⁾ Compaction of bottom ash and boiler slag can be accomplished by static steel-wheeled rollers, pneumatic rollers, or vibratory compaction equipment. For each project, the type of compactor, the moisture content of the bottom ash or boiler slag at placement, the lift thickness, and the number of passes of the compaction equipment should be evaluated using a test strip.

Quality Control

Quality control programs for bottom ash or boiler slag embankments or structural backfills are similar to such programs for conventional earthwork projects. These programs typically include visual observations of lift thickness, number of compactor passes per lift, and behavior of fly ash under the weight of the compaction equipment, supplemented by laboratory and field testing to confirm that the compacted material has been constructed in accordance with design specifications.⁽¹¹⁾ More information on performance specifications and procedures and method specifications and procedures can be found in ASTM E2277.⁽¹¹⁾

ENVIRONMENTAL CONSIDERATIONS

As described in the Coal Bottom Ash\Boiler Slag Material Description, the use of bottom ash or boiler slag as an embankment of fill material is an unencapsulated use and therefore has the potential to leach trace elements. Use of bottom ash or boiler slag in embankments or as a fill requires good management and care to ensure that there is no negative impact on the environment. In particular, areas with sandy soils possessing high hydraulic conductivities and areas near shallow groundwater or drinking aquifers should be given careful consideration. An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe product that does not adversely affect the environment.⁽¹⁵⁾

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of bottom ash and boiler slag literature is available here.

1. American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. Aurora, CO: American Coal Ash Association; 2007. 2. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. We Energies; Milwaukee, WI: 2004.
2. Wisconsin Transportation Information Center. Using recovered materials in highway construction. University of Wisconsin-Madison Department of Engineering Professional Development; Madison, WI: 1999. Wisconsin Transportation Bulletin No. 20.
3. Siddiki NZ, Kim D, Salgado R. Use of recycled and waste materials in Indiana. Transportation Research Record 2004(1874):78-85.
4. Asokan P, Saxena M, Asolekar S. Coal combustion residues – environmental implications and recycling potentials. Resources, Conservation and Recycling 2005;43:239-62.
5. ASTM C331-05 standard specification for lightweight aggregates for concrete masonry units. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2005.
6. Rogbeck J, Knutz A. Coal bottom ash as light fill material in construction. Waste Management 1996;16(1-3):125-8.
7. Edil TB, Benson CH, Bin-Shafique MS, Tanyu BF, Kim W, Senol A. Field evaluation of construction alternatives for roadways over soft subgrade. Transportation Research Record 2002(1786):36-48.
8. Huang WH. The use of bottom ash in highway embankment and pavement construction. Purdue University; West Lafayette, IN:1990. p. 317 p.
9. Kim B, Prezzi M, Salgado R. Geotechnical properties of fly and bottom ash mixtures for use in highway embankments. Journal of Geotechnical and Geoenvironmental Engineering 2005;131(7):914-24.
10. ASTM E2277-03 standard guide for design and construction of coal ash structural fills. In: Annual book of ASTM standards. American Society for Testing and Materials; West Conshohocken, Pennsylvania: 2003.
11. Moulton LK. Bottom ash and boiler slag. In: Proceedings of the third international ash utilization symposium. U.S. Bureau of Mines; Washington, DC: 1973.
12. Lovell CW, Ke TC, Huang WH, Lovell JE. Bottom ash as highway material. In: 70th annual meeting of the transportation research board. Transportation Research Board; Washington, DC: 1991.
13. Seals RK, Moulton LK, Ruth BE. Bottom ash: An engineering material. Journal of the Soil Mechanics and Foundations Division 1972 April; SM 4:311-25.
14. Environmental Protection Agency (EPA), Federal Highway Administration (FHWA). Using coal ash in highway construction - A guide to benefits and impacts. 2005. Report nr EPA-530-K-002:ID: 151.

[ Material Description ] - [ Asphalt Concrete ] - [ Granular Base ] - [ Stabilized Base ] - [ Embankment/Structural Fill ]