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COAL FLY ASHUser Guideline


Embankment or Fill

INTRODUCTION

According to the American Coal Ash Association, structural fill or embankment material is the second largest use of fly ash after concrete production.(1) Nearly all of the fly ash used for embankment construction is anthracite or bituminous coal fly ash. Lignite or subbituminous fly ashes, which are usually self-cementing, can harden prematurely when moisture is added, resulting in handling problems and an inability to achieve the required degree of compaction. Fly ash use as a structural fill or embankment material was pioneered during the 1950s in Great Britain as an alternate borrow material on roadway fill projects.

The Standard Guide for Design and Construction of Coal Ash Structural Fills, ASTM E2277-03, addresses fly ash in embankment or fill applications. The standard includes guidelines on site characterization considerations such as geologic and hydrologic investigations, laboratory test procedures, design considerations and methods, and construction considerations.(2)

PERFORMANCE RECORD

Fly ash has been used to construct or repair embankments since 1970.(3) When used in structural fills or embankments, fly ash offers several advantages over natural soil or rock. The relatively low unit weight of fly ash makes it well suited for placement over soft or low bearing strength soils. The high shear strength results in good bearing support and minimal settlement. The ease with which fly ash can be placed and compacted at the proper moisture contents, can reduce construction time and equipment costs. In areas where fly ash is readily available in bulk quantities, the cost of fly ash can be less expensive than borrow soil.

Two ash embankment projects (in Delaware and in Pennsylvania) were monitored for construction and post-construction performance over a 3-year time period. Monitoring consisted of sampling and analyzing physical and engineering characteristics of the ash, evaluating ash placement and compaction behavior, collecting and analyzing ground water samples monitoring wells, and settlement readings selected locations. No evidence of undue settlement or adverse environmental impacts were reported over the 3-year monitoring period.(4;5)

MATERIAL PROCESSING REQUIREMENTS

Moisture Control

Fly ash for embankment construction should be delivered to the job site within 3 to 4 percent of its optimum moisture content, preferably on the dry side of optimum.(4) A moisture content 1 or 2 percent above optimum can make fly as difficult to compact.(6) Dry fly ash from a silo must be water conditioned to the desired moisture content. Conditioned fly ash from a landfill should be excavated from the landfill, stockpiled, and additional water added, if needed, prior to delivery. Ponded fly ash must be removed from a lagoon, stockpiled until the moisture content has been sufficiently reduced for placement and then delivered to the job site.

Since most lignite or subbituminous fly ashes are self-cementing, the addition of moisture in amounts approaching the optimum moisture content may result in flash setting or sudden hardening of the ash. To prepare this type of fly ash for use as embankment material, the ash may need to be lightly conditioned with water (10 to 15 percent), stockpiled for several weeks, and passed through a crusher to remove agglomerations prior to its use as fill. Additional water, if needed, should be added only after the lignite or subbituminous fly ash has been placed and just prior to compaction.

ENGINEERING PROPERTIES

Engineering properties of fly ash that are of particular interest when fly ash is used as an embankment or fill material are moisture density relationship (compaction curve), particle size distribution, shear strength, consolidation characteristics, bearing strength, and hydraulic conductivity.

Moisture-Density Relationship: Fly ash has a relatively low compacted density, thereby reducing the applied loading and settlement of the supporting subgrade. Conditioned fly ash tailgated over the slope of an embankment can have a loose dry density as low as 6.3 to 7.9 kN/m3 (40 to 50 lb/ft3). However, when well compacted at optimum moisture content (usually between 20 and 35 percent), the dry unit weight of fly ash may be greater than 13.4 kN/m3 (85 lb/ft3), as high as 15.7 kN/m3 (100 lb/ft3).

Particle Size Distribution: Fly ash is predominantly a silt-sized nonplastic material. Between 60 and 90 percent of fly ash particles are finer than a 0.075 mm (No. 200) sieve. As such, fly ash can be considered to be frost-susceptible.(7) The potential reduction in strength from freeze-thaw cycles is a concern. Laboratory test results show that unconfined compressive strength of compacted fly ash was not affected by freeze-thaw cycles, because the degree of saturation was not 100 percent, which allowed for volumetric expansion of freezing water to occur without affecting the fly ash strength.(8) Therefore, fly ash fills should be free draining.

Shear Strength: The shear strength of freshly compacted fly ash samples is primarily from internal friction, although some apparent cohesion has been observed in bituminous (pozzolanic) fly ashes.(9) The shear strength of fly ash is affected by the density and moisture content with maximum shear strength occurring at optimum moisture content.(10) Bituminous fly ash has a friction angle that is usually in the range of 26° to 42°. A test program involving shear strength testing for 51 different ash samples resulted in a mean friction angle of 34°, and a standard deviation of 3.3°.(10) Therefore, a friction angle of 30° would be a reasonable estimate for design.

Consolidation Characteristics: An embankment or structural backfill should possess low compressibility to minimize roadway settlements or differential settlements between structures and adjacent approaches. Consolidation has been shown to occur more rapidly in compacted fly ash than fine-grained soil because fly ash has a higher void ratio and greater hydraulic conductivity than most fine-grained soils. For fly ashes with age-hardening properties, including most Class C fly ashes, the magnitude of the compressibility is reduced.

Bearing Strength: California bearing ratios (CBR) for Class F fly ash from the burning of anthracite or bituminous coals have been found to be similar to fine grained soil, 5 to 15 percent. For naturally occurring soils, CBR values normally range from 3 to 15 percent for fine-grained materials (silts and clays), from 5 to 40 percent for sand and sandy soils, and from 20 to 100 percent for gravels and gravelly soils.

Hydraulic conductivity: The hydraulic conductivity of well-compacted fly ash ranges from 10-4 to 10-6 cm/s, which is roughly equivalent to the hydraulic conductivity of a silty sand to silty clay soil. The hydraulic conductivity of fly ash is affected by the degree of compaction, grain size distribution, and internal pore structure. Since fly ash consists almost entirely of spherical shaped particles, the particles are able to be densely packed during compaction, resulting in comparatively low hydraulic conductivity that minimizes the seepage of water through a fly ash embankment.

DESIGN CONSIDERATIONS

Virtually any fly ash can be used as an embankment or structural backfill material, including ponded ash that has been reclaimed from an ash lagoon. The principal technical considerations related to the design of a fly ash embankment or structural backfill are essentially the same as the considerations for the design of an earthen embankment or backfill. There are certain special design considerations, however, that should be considered when fly ash is used in embankment or fill applications. If designed properly, fly ash has comparable strength and compressibility to most soil fill materials, while possessing lower dry unit weight.(11)

Site Drainage

Fly ash, because of its predominance of silt-size particles, tends to wick water, making it possible that the lower extremities of a fly ash embankment could become saturated. The base of a fly ash embankment should not be exposed to free moisture, wetlands, or the presence of a high water table condition. An effective way to prevent capillary rise or the effects of seepage in fly ash embankments and backfills is the placement of a drainage layer of well-draining granular material at the base of the embankment.(12)

Slope Stability

To determine a safe design slope, slope stability analysis of a design cross-section of the fly ash embankment must be performed. The basic principle of slope stability analysis is to compare the factors contributing to instability with those resisting failure. The principal resistance to failure is the shear strength of the embankment material. A long term and seismic slope stability analysis should be preformed. For long-term stability of fly ash embankments, a factor of safety of 1.5 against slope instability is recommended, while for seismic loadings a factor of safety of 1.2 is recommended.(12) Unless the fly ash is self-hardening, the cohesion (c) value should be zero for these calculations. Reference 13 provides state-of-the-art guidance on slope-stability analysis.

Erosion Control Analysis

Erodibility of compacted fly ash is affected by the slope angle. Slopes should be protected as soon as possible after attaining final grade to minimize erosion by runoff or even high winds. One way to prevent such erosion is to construct a fly ash embankment within dikes of granular soil, which serve to protect the slopes throughout construction. Another way is to cover the slopes with topsoil as the embankment is being constructed. Overfilling slopes and trimming excess fly ash back to the appropriate grade once the final height is achieved is another approach. Finally, short-term erosion control may be accomplished by stabilizing the surface fly ash on the slopes with a low percentage of Portland cement or lime,(14) or covering with a blanket of coarse bottom ash.

Soil Bearing Capacity

The ability of the top portion of a fly ash embankment to support a pavement structure can be predicted by the California Bearing Ratio (CBR) for a flexible asphalt pavement system or by a modulus of subgrade reaction (K-value) for a rigid or concrete pavement system. These bearing values can then be used to design pavement layer thicknesses in accordance with the AASHTO design guide for pavement structures.(15) Methods for determining the CBR can be found in ASTM D1883-05(16) and the modulus of subgrade reaction in D 1195 or D 1196, or bearing ratio by test methods D 1883 or D 4429, as appropriate.(12)

Climatic Conditions

During times of heavy or prolonged precipitation, the delivered moisture content of the fly ash may have to be reduced to compensate for the effects of the precipitation.

Fly ash, unlike most soils, can be compacted in the winter, although spreading and compacting fly ash when the ambient air temperature is below -4°C (25°F) is not recommended.(14) In addition, placing frozen fly ash is also not recommended. Because fly ash obtained directly from silos or hoppers dissipates heat slowly, fly ash may be placed during cold weather. If frost does penetrate into the top surface of the fly ash, the ash can be removed from the surface by a bulldozer, or recompacted after thawing and drying.(12) Construction should be suspended during severe weather conditions, such as heavy rainfall, snowstorms, or prolonged and/or excessively cold temperatures.

Strength reduction of fly ash during periods of cyclic freezing and thawing may occur.(8) The frost susceptibility of fly ash can be measured according to ASTM D5918.(17) This test method applies two freeze-thaw cycles to a compacted specimen 146 mm (5.75 in) in diameter and 150 mm (6 in) in height. The heave rate and California bearing ratio (CBR) after thaw give an indication of frost susceptibility. Frost susceptible materials have a heave rate greater than 4 mm/day and a CBR after thaw less than 10.(17) Frost heaving of the top portion of a fly ash embankment can be substantially decreased by the addition of moderate amounts of cement or lime.

Laboratory test results have shown that unconfined compressive strength of compacted fly ash was not affected by freeze-thaw cycles, because the degree of saturation was not 100 percent, which allowed for volumetric expansion of freezing water to occur without affecting the fly ash strength.(8) Therefore, fly ash fills should be free draining. When frost susceptibility remains a concern, substituting a soil that is not susceptible to frost for fly ash within the frost zone will elevate the potential problem.

Protection of Underground Pipes and Adjacent Concrete

Chemical and/or electrical resistivity tests (e.g. ASTM G187-05)(18) of some fly ashes have indicated that certain ash sources may be potentially corrosive to metal pipes placed within an embankment. Each source of fly ash should be individually evaluated for corrosivity potential. If protection of metal pipes is deemed necessary, the exterior of the pipes may be coated with tar or asphalt cement, the pipes may be wrapped with polyethylene sheeting, or the pipe can be backfilled with sand or an inert material.(14)

The sulfate content of fly ash, particularly self-cementing ash, has caused some concern about the possibility of sulfate attack on adjacent concrete foundations or walls. Precautions that can be taken against potential sulfate attack of concrete include painting concrete faces with tar or an asphalt cement, using a waterproof membrane (such as polyethylene sheeting or tar paper), or using Type V sulfate-resistant cement in the adjacent concrete.

CONSTRUCTION PROCEDURES

Material Handling and Storage

Bituminous (pozzolanic) fly ash is usually conditioned with water at the power plant and hauled in covered dump trucks with sealed tailgates. Subbituminous or lignite (self-cementing) fly ash may be partially conditioned at the plant and hauled in covered dump trucks to the project site, or hauled dry in pneumatic tank trucks from the plant to the project site, where it is placed in a silo and conditioned with water when ready for placement. Temporary stockpiling should be performed to reduce lagoon ash water prior to transportation to prevent road spillage during transportation.(19)

If a temporary stockpile of fly ash is built at the project site, the surface of the stockpile must be kept damp enough to prevent dusting. The stockpile should be placed in a well-drained area so the ash is not inundated with water following a rainfall.(14)

Placing and Compacting

Prior to fly ash placement, the site should undergo preparations consistent with preparation requirements for soil fill materials. The site must be cleared and grubbed and topsoil should be kept for final cover. Before and during construction, special attention should be given to site drainage and to preventing seeps, pools, or springs from contacting fly ash stockpiles.(19)

Construction equipment needed to properly place and compact fly ash 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 (if needed) and to control dusting, and a motor grader where final grade control is critical.

Fly ash is usually spread and leveled with a dozer, grader, or other equipment in lifts no thicker than 0.3 m (12 in) when loose. Using a disk harrow or a rotary tiller may be necessary if the fly ash contains lumps.(14) Fly ash lifts should be compacted as soon as the material has been spread and is at proper moisture content. Experience has shown that steel-wheel vibratory compactors and/or pneumatic tired rollers provide the best compaction. If a vibratory roller is used, the first pass should be made with the roller in the static mode (without any vibration), followed by two passes with the roller in the vibratory mode and traveling relatively fast. Additional passes should be in the vibratory mode at slow speed.(6;12)

In general, six passes of the roller are usually needed to meet compaction requirements. In most cases, 90 to 95 percent of a standard Proctor maximum dry density is the minimum specified density to be achieved. This is almost always achievable when the moisture content of the fly ash is within 2 or 3 percent of optimum, preferably on the dry side of optimum.(20)

For each project, the type of compactor, the moisture content of the fly ash at placement, the lift thickness, and the number of passes of the compaction equipment should be evaluated using a test strip. A vibratory compactor should use a test strip to evaluate the speed at which the compactor should be operated, the static weight, dynamic force and frequency of vibration of the compactor, and the number of passes required to achieve the specified density.(14)

Quality Control

Quality control programs for fly ash 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 fly ash has been constructed in accordance with design specifications.(12) More information on performance specifications and procedures and method specifications and procedures can be found in ASTM E2277.(12)

Dust Control

Dust control measures that are routinely used on earthwork projects are effective in minimizing airborne particulates at ash fill projects. Typical controls include hauling fly ash in covered dump trucks or pneumatic tankers, moisture conditioning fly ash at the power plant, wetting or covering exposed fly ash surfaces, and sealing the top surface of compacted fly ash with a smooth drum or rubber tire compactor at the conclusion of each day.(12)

Drainage/Erosion Protection

Fly ash surfaces must be graded or sloped at the end of each day to provide positive drainage and prevent the ponding of water or the formation of runoff channels that could erode slopes and produce sediment in nearby surface waters. Compacted fly ash slopes should be protected as soon as possible after finished grades to reduce erosion. Erosion control on side slopes is usually provided by placing from 150 mm (6 in) to 600 mm (2 ft) of soil cover on the slopes. An alternative approach is to build outside dikes of soil to contain the fly ash as the embankment is being constructed.(14)

ENVIRONMENTAL CONSIDERATIONS

As described in the Coal Fly Ash Material Description, the use of fly ash as an embankment of fill material is an unencapsulated use and therefore has the potential for contaminant leaching. Use of fly ash 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.(2)

UNRESOLVED ISSUES

Bituminous (pozzolanic) fly ash is more frequently used to construct embankments and structural backfills than subbituminous or lignite (self-cementing) fly ash. This is due in part to difficulties in placing and compacting self-cementing fly ash, which can harden almost immediately after the addition of water. Current practice is to lightly condition self-cementing fly ashes with water, allow them to stockpile for a period of time, then run the partially hardened fly ash through a primary crusher before taking it to the project site. There is a need to develop more well-defined handling and preconditioning procedures for using self-cementing fly ash as a fill material.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of coal fly ash 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 August 24, 2007.
  2. Environmental Protection Agency (EPA), Federal Highway Administration (FHWA). Using coal ash in highway construction - A guide to benefits and impacts. ; April 2005. Report nr EPA-530-K-002:ID: 151.
  3. Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products in highway construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board; Washington, DC: 1994.
  4. Collins RJ, Srivastava L. Use of ash in highway construction: Delaware demonstration project. Palo Alto, California: Electric Power Research Institute; 1989. Report nr GS-6540.
  5. Baykal G, Edinçliler A, and Saygili A. Ash utilization in highways: Pennsylvania demonstration project. Palo Alto, California: Electric Power Research Institute; 1989. Report nr GS-6431.
  6. Baykal G, Edinçliler A, Saygili A. Highway embankment construction using fly ash in cold regions. Resources, Conservation and Recycling 2004 10;42(3):209-22.
  7. Gray DH; Lin Y. Engineering properties of compacted fly ash. American society of civil engineers national water resources engineering meeting, Phoenix, Arizona, American Society of Civil Engineers; 1971.
  8. Cocka E. Frost susceptibility properties of Soma-B fly ash. Journal of Energy Engineering 1997;123(1):1-10.
  9. Di Gioia AM, Nuzzo WL. Fly ash as structural fill. Journal of the Power Division 1972 June;98(1):77-92.
  10. McLaren, RJ; DiGioia AM Jr. Typical engineering properties of fly ash. Geotechnical practice for waste disposal '87 University of Michigan, Ann Arbor, Michigan: 1987.
  11. 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.
  12. ASTM E2277-03 standard guide for design and construction of coal ash structural fills. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2003.
  13. Duncan JM, Wright SG. Soil strength and slope stability. John Wiley & Sons, Inc.; 2005.
  14. DiGioia AM, Jr., McLaren RJ, Taylor LR. Fly ash structural fill handbook. Palo Alto, California: Electric Power Research Institute; 1979. Report nr EA-1281.
  15. AASHTO. Guide for design of pavement structures. Washington, DC,: American Association of State Highway and Transportation Officials; 1993.
  16. ASTM D1883-05 standard test method for CBR (california bearing ratio) of laboratory-compacted soils. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 2005.
  17. ASTM D5918-06 standard test methods for frost heave and thaw weakening susceptibility of soils. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania; 2006.
  18. ASTM G187-05 standard test method for measurement of soil resistivity using the two-electrode soil box method. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2005.
  19. Federal Highway Administration (FHWA), American Coal Ash Association (ACAA). Fly ash facts for highway engineers. Federal Highway Administration (FHWA); 2003. Report nr FHWA-IF-03-019.
  20. Kinder, DL, Morrison RE. An engineering approach for using power plant ash in a structural fill. Fifth international ash utilization symposium, report no. METC/SP-79-10U.S. Department of Energy; 1979.

[ Material Description ] - [ Asphalt Concrete ] - [ Portland Cement Concrete ] - [ Embankment or Fill ] - [ Stabilized Base ] - [ Flowable Fill ]

Last Update 7/28/08