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Technical Advisory

Use of Coal Ash in Embankments and Bases

T 5080.9

May 16, 1988

  1. PURPOSE. To set forth guidance and recommendations relating to the use of coal ash in bases and embankments. This Technical Advisory covers the history of coal ash use in these applications along with discussions on environmental, design, and construction considerations.
    1. On April 2, 1987, the Surface Transportation and Uniform Relocation Assistance Act (STURAA) became law. The law provided for an additional 5 percent Federal-aid match for projects that included a significant amount of coalash. The law did not provide for any additional obligational authority or apportionment.
    2. On August 28, 1987, the FHWA issued a Notice which included the implementation instructions for the coal ash provisions of the STURAA. This Notice was published in the Federal Register, September 8, 1987. The Notice defines a significant amount as a total of l,000 tons of coal ash for all applications in a project. The Notice allows all uses except for those applications which include bottom ash.
    3. The two Highway applications which have the most potential for large use of coal ash are in embankment and bases.
    1. History of Use. Fly ash is the most frequently used coal ash product for embankment construction. At least eight States have used fly ash in embankments, and it also has been used as embankment material in many private projects. If properly designed and constructed, the material can function successfully as embankment material. It should be noted that limited research has been done on the environmental effects of using fly ash in embankments.
      • (1) It should also be noted that pond ash is sometimes referred to as fly ash. Pond ash is sluiced fly ash which often contains various quantities of bottom ash. Due to the possibility of the bottom ash deteriorating under loading, care needs to be taken when using pond ash to ensure that it does not contain a significant amount of bottom ash. In addition, pond ashes may not be uniform due to the way the material is placed in the pond. Since uniformity may be a problem, particular attention needs to be taken in picking the target density for the material.
      • (2) Since there has been limited use of this material in embankments, this use should be treated as an experimental feature and subject to the reporting requirements of Federal-Aid Highway Program Manual (FHPM) 6-4-2-4.
    2. Environmental Concerns. The environmental concerns center around possible leaching of heavy metals from embankments constructed from the material. Also, ionic solutions containing calcium, sulfate, magnesium, sodium, potassium, and silicates may also leach out.
      • (1) Only a small percentage of coals produce an ash which yields leachate that is hazardous. Congress has asked the EPA to perform a study to determine if coal ash products are hazardous. Congress has also indicated that coal ash will not be considered as a hazardous waste until the EPA's study is completed. If leaching of heavy metals isidentified as a possible problem, hydrated lime may be used to elevate the pH of the system to prevent the leaching of heavy metals.
      • (2) Leachate containing calcium, sodium, potassium, magnesium sulfate and silicate ions may adversely affect the quality of ground water but are not in themselves hazardous.
      • (3) A chemical analysis should be performed on the coal ash prior to allowing its use in an embankment to determine if any compounds in the coal ash will cause environmental problems. A leaching test should be performed to determine if there are compounds which will leach out of the embankment.
      • (4) Coal ash is treated differently in the various States. In some States, no special requirements are placed on the construction of embankments with coal ash products. In other States, fly ash is considered to be a solid waste and is subject to appropriate requirements for disposal. Those disposal procedures may require a permit to build an embankment, i.e., the embankment is treated as a disposal site. In general, permitting requirements will typically include a chemical analysis of the coal ash, geological and hydrological survey of the site and a detailed design plan. The permit will typically require ground water monitoring to determine if compounds are leaching from the site.
      • (5) Ground water monitoring plans include the utilization of water samples from monitoring wells for testing. Up gradient and down gradient wells are necessary to determine if leaching is occurring. Sampling must be done prior to construction of the embankment to obtain background levels and must continue on a routine basis for an extended period of time. The testing of the water on a routine basis will be limited to only a few compounds varying from site-to-site depending on the nature of the compounds in the embankment. The values from these tests are compared to the test values taken before the embankment is constructedto determine if any leaching is occurring. If leaching is occurring more detailed testing and appropriate corrective action will be required.
    3. Design Consideration
      • (1) Embankments constructed with boiler slag will not require additional special considerations over those listed above. It is not likely that boiler slag would be used for embankment material due to the lack of a sufficient quantity of material.
      • (2) Fly ash is a silt size nonplastic material. In general embankments designed with fly ash should be handled with the same care as silts.
        • (a) Since fly ash will lose shear strength when saturated, several features should be incorporated into the project.
          • 1 The fly ash should be placed on an aggregate drainage blanket to prevent water from rising into the embankment by way of capillary action. The fly ash should also be separated from the drainage blanket by an appropriate filter fabric.
          • 2 The side slopes of the embankments need to be covered with approximately 5 feet of another soil to prevent erosion.
        • (b) Since fine grain materials are susceptible to frost heave due to freezing, untreated fly ash should not be placed in an embankment above the frost line. Fly ash treated with lime or cement can be placed above the frost line.
    4. Construction Considerations. Construction lifts should be limited to 6 to 8 inches loose. Fly ash should be placed at a moisture content below optimum to avoid stability problems.
      • (1) Some publications suggest using 90 percent of American Association of State Highway and Transportation Officials (AASHTO) T-99 for a target density when fly ash is used as embankment material. However, conventional criteria for fine grain materials indicate that 90 percent to 95 percent of T-180 should be used. One reason for using T-180 for compaction criteria is to limit the optimum moisture content of the material.
      • (2) There is some disagreement relative to the appropriate equipment to be used to achieve proper compaction. Some publications indicate that pneumatic-tired and vibratory rollers have worked best in achieving density. In these publications, it has been reported that sheepsfoot rollers will "fluff up" the material. However, pneumatic-tired and sheepsfoot rollers are typically specified for fine grain materials in order to avoid the build up of pore water pressures which would hinder compaction efforts. Because of the disagreement in this area, a test strip should be used to determine the most effective method of obtaining compaction.
      • (3) Fly ash is difficult to dry. For this reason it is important to maintain the proper cross-slope on the embankment to prevent any ponding which would result in localized loss of stability during construction. It is also important to place the side slope protection as embankment construction proceeds to avoid the erosion that could occur in a rain storm.
    5. Items to be Included in an Experimental Work Plan. The following items should be included in an experimental work plan for a project using coal ash as embankment material: chemical analysis of the coal ash, laboratory leeching studies, periodic ground water monitoring, test strips to determine the best method for compaction, strength testing, modified proctor tests, and field moisture-density determinations.
  4. BASES
    1. History of Use. Lime fly ash aggregate (LFA) bases have been used by at least a dozen States. The States ofIllinois, Pennsylvania, and Ohio have used LFA bases the most. Pennsylvania and Ohio have not used LFA bases recently due to a lack of successful bidders on the materials. Proposed projects located in States that have had insufficient experience to develop and verify mix design and pavement design criteria should be considered experimental and subject to the reporting requirements of FHPM 6-4-2-4.
    2. Pavement Design Considerations. Since LFA bases were not included in the AASHTO test road, structural coefficients were not established. However, as reference the structural coefficients established for cement treated bases range from 0.15 to 0.23. States which have used LFA bases have used various structural coefficients ranging from 0.28 to 0.35 with the trend toward using the lower structural coefficients. States that have not constructed or evaluated LFA bases should use a conservative number until a history is established with the material.
    3. Mix Design Considerations. The typical proportions for LFA mixes consist of the following: 10 to 15 percent fly ash, 3 to 4 percent lime, and 81 to 87 percent aggregate. Typically the lime to fly ash ratio is 1 to 3.
      • (1) Fly ash used in stabilization is typically specified by American Society for Testing and Material (ASTM) C-593. However, some States specify fly ash by the more restrictive ASTM C-618 or AASHTO M-295 which is used to specify fly ash for use in portland cement concrete.
      • (2) Evaluation of the mix consists of determining the optimum moisture content, maximum density, compressive strength of laboratory specimens, and freeze thaw durability of the mixture. The procedures as defined in ASTM C-593 include the use of a modification of AASHTO T-180 for compaction of the specimens, i.e., 10-pound hammer with an 18-inch drop and 3 lifts, curing at 100 degrees F for 7 days and the use of a vacuum saturation procedure to determine freeze-thaw durability. The ASTMC-593 has a minimum compressive strength of 400 psi for both the vacuum saturated and unsaturated specimens.
      • (3) The States that have used LFA material have not universally accepted the procedures in ASTM C-593 and have made various revisions. The following are the significant variations that have been made.
        • (a) Some States have used AASHTO T-99 proctor for specimen compaction, i.e., 5.5-pound hammer, 12-inch drop, and 3 lifts.
        • (b) In some States, curing of the specimens was changed to 72 degrees F for 14 days. The 100-degree cure may induce chemical changes that would not occur in the field.
        • (c) Some States also use the maximum density of the mixes as a design parameter. In this case a State would test mixes with several different proportions and pick the mixture with the highest density meeting the minimum compressive strengths for both saturated and unsaturated conditions.
      • (4) Some LFA mixes can reach 1,500 to 2,000 psi at 7 days at 100-degree F curing. It is likely that mixes in this range will be more susceptible to cracking. Research has not yet established the maximum strength of these mixtures to minimize cracking; however, it is suspected that this point is in the range of 800 to 1,000 psi. The proportions of the mixes exceeding 1,000 psi should be adjusted accordingly.
    4. Construction Considerations. LFA base material can be placed in a number of methods. It can be end dumped and spread with a motor grader, placed with a spreader box, or placed with an asphalt or concrete paver. In all but low-volume road situations an asphalt or concrete paver should be used. A paver has several advantages: the paver can place the material at a more uniform cross-section and the material can be placed faster which canbe critical with some mixes. An asphalt paver has one additional advantage. The material behind the paver will be compacted to a large degree due to the vibrating screed or tamper bars on the paver.
      • (1) There are differences in the handling characteristics of various LFA mixes. Mixes which contain Type C fly ashes in general will set up faster than mixes which contain Type F fly ashes. In some cases, compaction problems may result from the material setting up prior to final compaction. In a few cases the mixture may actually set up in the trucks. In these situations, it will be necessary to use a retarder to increase the set time. On the other end of the scale, mixes containing Type F fly ash typically will allow sufficient time for proper compaction.
      • (2) The compaction specifications used for LFA mixes range from 95 to 100 percent of AASHTO T-99 and 95 percent of AASTHO T-180. It is critical to keep the moisture content on the dry side of optimum in order to maintain stability under construction equipment.
      • (3) The mat must be sealed to ensure that the pozzolanic reaction will continue. The typical seal consists of an emulsion placed at a rate of 0.1 to 0.2 gallons per square yard within 24 hours of placement of the mat. The mat must be kept moist until the seal is placed.
      • (4) Another concern with LFA bases is the temperature at which the pozzolanic reaction occurs. The reaction which develops the strength of the mixture will not take place below approximately 40 degrees. If the mixture does not obtain sufficient strength prior to freezing and thawing, the pavement could be weakened. Most States have established cutoff dates for the placement of LFA bases. The dates are based on the number of degree days that will occur prior to freezing. A method for determining the cutoff date appears in National Cooperative Highway Research Program (NCHRP) Synthesis 37. Some States also allow the use of cement or additional lime and fly ash to gain earlier strength. When this is done, care needs to be taken to avoid excessive ultimate strengths which can lead to excessive cracking.
      • (5) The mixture is controlled by running gradations on the aggregate at the plant and running titrations to determine the lime content of the mixture. Plant calibrations need to be made and checked periodically to ensure a uniform mixture.
    5. Performance Concerns
      • (1) A cracking problem has been reported with the use of LFA in some States. Some of the cracking may be due to the high strength that can be achieved with the material. A research project has been initiated by one State to determine if sawing and sealing can be used to effectively control the cracking problem.
      • (2) A deterioration problem at cracks has also been reported by one State. The deterioration consisted of a swelling of the material at the joints which caused a "tenting" effect. The deterioration has been attributed to salt brine attack. Sealing cracks and joints appears to prevent the problem.
    6. Use of other Reactants with Fly Ash. Due to the shortage and cost of lime in some areas, other reactants have been used to replace the lime in LFA bases. These reactants include Portland cement, kiln dust, and hydrated byproduct lime. Fly ash bases constructed with these materials have not been widely used or evaluated so not much is known about their properties.
    7. Items to be Included in an Experimental Work Plan. The following are items that need to be examined in an experimental project containing LFA bases:
      • (1) On the first couple of projects, several variations in mix design procedures should be used to help determine the optimum mix design criteria. Thevariations that should be looked at include: the 7-day 100-degree cure, the 14-day 72-degree cure, specimen compaction by proctor and modified proctor, and evaluation of various proportions by density of the resultant mixes.
      • (2) During production, specimens should be made for compressive strength determinations at 7, 14, 28, and 56 days.
      • (3) Gradations and titrations for lime contents should be run for production control of the mix. Moisture and density determinations should be performed to control the placement operations.
      • (4) Over a period of 5 years, an evaluation of the pavement should be made which will include: cores for strength determination, flexible strength determination by the use of a dynaflect or falling weight deflectometer, traffic counts, and crack surveys. This information will help to determine the pavement performance which in turn will help to define the mix design and pavement design criteria.
  5. REFERENCES. Additional information on the use of coal ash in these applications can be found in the following publications:

    "Fly Ash a Highway Construction Material," Federal Highway Administration, Implementation Package 76-l6, May 1976.

    "Soil Stabilization in Pavement Structures," Federal Highway Administration, Implementation Package 80-2, 1980.

    "Fly Ash Facts for Highway Engineers," Federal Highway Administration, Report-DP-59-8, July 1986.

Ronald E. Heinz
Associate Administrator for
Engineering and Program

Updated: 04/07/2011

United States Department of Transportation - Federal Highway Administration