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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Coal fly ash has been successfully used as a structural fill or embankment material for highway construction projects in a number of different locations throughout the United States. Compared with conventional soils used to build embankments, fly ash is somewhat of a unique engineering material. When dry, fly ash is cohesionless and considered by many as a dusty nuisance. When saturated, fly ash becomes an unmanageable mess. But, as with most fine-grained soils, fly ash can be easily handled and compacted at more intermediate moisture contents, and does exhibit some cohesion.
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 potential handling problems and inability to achieve the required degree of compaction. Fly ash use as a structural fill or embankment material was pioneered during the 1950’s in Great Britain, where it is still bid as an alternate borrow material on roadway fill projects in areas where it is available.
It has been reported that since 1970 at least 14 states have used fly ash to construct or repair embankments.(1) Fly ash has also been used as a structural backfill material behind retaining walls and bridge abutments. States that have thus far used fly ash as an embankment or structural backfill material include Arizona, Delaware, Illinois, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Ohio, Oklahoma, Pennsylvania, Virginia, West Virginia, and Wisconsin. At least nine states have a specification for the use of fly ash as an embankment material.(1)
Table 5-4 provides a list of the number, estimated size, location, and year of construction of the known fly ash embankment or structural backfill projects that have been built in each of these states. This list, which may not be complete, encompasses at least 21 highway construction projects,(2) involving an estimated 2 million or more cubic yards of fly ash, in some cases mixed with bottom ash.
Although very few of these projects have been monitored for long-term performance, none of the states in which these projects have been constructed have indicated any dissatisfaction or concern with the embankments or backfills constructed using fly ash in their states. Two of the ash embankment projects listed in Table 5-4 (one in Delaware and one in Pennsylvania) were monitored for construction and postconstruction performance over a 3-year time period.(3,4) The monitoring consisted of sampling and analyzing ash physical and engineering characteristics, evaluating ash placement and compaction behavior, collecting and analyzing samples from groundwater monitoring wells, and periodically taking settlement readings from selected locations within each embankment. Neither of these two projects showed any signs of undue settlement or adverse environmental impacts over the 3-year monitoring period.
Table 5-4. List of fly ash embankment or structural backfill projectsconstructed in the United States.
When used in structural fills or embankments, fly ash offers several advantages over natural soil or rock. Its relatively low unit weight makes it well suited for placement over soft or low bearing strength soils, and its high shear strength, compared with its unit weight, results in good bearing support and minimal settlement. The ease with which fly ash can be placed and compacted, especially when placed at the proper moisture content, can reduce construction time and equipment costs. In areas where fly ash is readily available in bulk quantities, costs for the purchase, permitting, and operation of a borrow pit can be reduced or eliminated.(5)
There are some disadvantages of using fly ash in structural fills or embankments. Unless delivered to the project site within the proper moisture range, dust control measures may be needed. Also, since fly ash is a predominantly silt-size material, it is subject to erosion and, as a result, erosion control procedures may be needed.
MATERIAL PROCESSING REQUIREMENTS
The only property or characteristic of fly ash that may need to be adjusted or modified before delivery of the ash to the job site is its moisture content. The optimum moisture content of the fly ash to be used must be determined well in advance of starting the project. To achieve the desired degree of compaction in the field, the fly ash being supplied to the job must be delivered to the site as close to its optimum moisture content as possible.
Although specification requirements may vary somewhat from job to job, fly ash for embankment construction should usually be delivered to the job site within 3 to 4 percent of its optimum moisture content, preferably on the dry side of optimum.(3) 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 moisture 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, 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.
Some of the engineering properties of fly ash that are of particular interest when fly ash is used as an embankment or fill material are its moisture-density relationship, particle size distribution, shear strength, consolidation characteristics, bearing strength, and permeability. Moisture-Density Relationship: Fly ash has a relatively low compacted density, thereby reducing the applied loading and resultant settlement to the supporting subgrade. Conditioned fly ash tailgated over the slope of an embankment can have a loose dry density as low as 650 to 810 kg/m3 (40 to 50 lb/ft3). However, when it has been well compacted at an optimum moisture content (usually between 20 and 35 percent), the dry unit weight of fly ash may be greater than 1380 kg/m3 (85 lb/ft3), possibly even as high as 1620 kg/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, its particle size distribution falls essentially within the normally recognized limits for frost-susceptible soils.(6) The potential for frost susceptibility of fly ash may account for the reluctance on the part of some geotechnical engineers to use fly ash as a fill material. The fine particle sizing of fly ash, together with the relative uniformity of the gradation in the coarse silt range, makes it imperative that the ash be handled with sufficient water to prevent dusting. Since fine-grained soils can be fairly easily eroded, enough moisture must also be present to support compaction equipment and to permit the material to be well densified, in order to prevent or minimize erodibility.
Shear Strength: Shear strength tests conducted on freshly compacted fly ash samples show that fly ash derives most of its shear strength from internal friction, although some apparent cohesion has been observed in certain bituminous (pozzolanic) fly ashes.(7) The shear strength of fly ash is affected by the density and moisture content of the test sample, with maximum shear strength exhibited at the optimum moisture content.(8) Bituminous fly ash has been determined to have 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 value of 34°, with a fairly wide range.(9)
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 in silty clay soil because the fly ash has a higher void ratio and greater permeability than the soil. For fly ashes with age-hardening properties, including most "high lime" fly ashes from lignite or subbituminous coals, the age-hardening can reduce the time rate of consolidation, as well as the magnitude of the compressibility.
Bearing Strength: California bearing ratio (CBR) values for "low lime" fly ash from the burning of anthracite or bituminous coals have been found to range from 6.8 to 13.5 percent in the soaked condition (an optional procedure in the test method)(10) to 10.8 to 15.4 percent in the unsoaked condition.(11) For naturally occurring soils, CBR values normally range from 3 to 15 percent for fine-grained materials (silts and clays), from 10 to 40 percent for sand and sandy soils, and from 20 to 80 percent for gravels and gravelly soils.(12)
Permeability: The permeability of well-compacted fly ash has been found to range from 10-4 to 10-6 cm/s, which is roughly equivalent to the normal range of permeability of a silty sand to silty clay soil.(12) The permeability of a material is affected by its density or degree of compaction, its grain size distribution, and its 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 permeability values and minimizing seepage of water through a fly ash embankment.
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.
Fly ash, because of its predominance of silt-size particles, tends to wick water into itself, making it possible that the lower extremities of a fly ash embankment could become saturated, resulting in a loss of shear strength. It is, therefore, important that the base of a fly ash embankment not be exposed to free moisture, wetlands, or the presence of a high water table condition. Adequate provisions should be made to handle maximum flows anticipated from surface waterways, swales, or seepage from springs or high water table conditions.
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-drained granular material at the base of the embankment. An ASTM recommended practice for the use of fly ash in structural fills recommends placement of a drainage layer at a height that is at least 5 feet above the historical high water table.(13)
To determine a safe and appropriate design slope ratio (the ratio of vertical to horizontal distance), an analysis of the slope stability 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. For long-term stability of fly ash embankments, a factor of safety (ratio of the resisting forces to the driving forces along a potential failure surface) of 1.5 is recommended using the Swedish circle method of slope stability analysis.(14) Unless the fly ash is self-hardening, the cohesion (c) value should be zero for these calculations.
Erosion Control Analysis
The slope ratio described above is also a factor in the potential for erodibility of compacted fly ash slopes. These slopes must be protected as soon as possible after attaining final grade because they are subject to severe erosion by runoff, or even high winds, if left unprotected. 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. It is also possible to overfill the slopes and trim the excess fly ash back to the appropriate slope once the final layer is completed. 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 a determination of the California Bearing Ratio (CBR) for a flexible asphalt pavement system or by a determination of the 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.(15)
Although no frost susceptibility criteria have been established in the United States, the British Road Research Laboratory has developed a test method to evaluate frost susceptibility.(16) The test method involves subjecting a compacted 150 mm (6 in) high specimen to freezing temperatures that simulate actual field conditions. The test is run over a 250-hour time period, after which the total amount of frost heave of the test specimen is measured. Frost-susceptible materials heave 18 mm (0.7 in) or more after testing.
Some of the top portion of a fly ash embankment to frost heaving can be substantially increased by the addition of moderate amounts of cement or lime. Objections to the use of compacted fly ash within the frost depth can be overcome by substituting a soil that is not susceptible to frost for fly ash within the frost zone.
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 usually be compacted throughout much of the winter, although it is recommended that fly ash not be spread and compacted when the ambient air temperature is below -4°C (25°F).(14)
Protection of Underground Pipes and Adjacent Concrete
Chemical and/or electrical resistivity tests 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 its 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 pipes 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 possibly even using a Type V sulfate-resistant cement in the adjacent concrete.
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.
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
The minimum amount of 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.
The suitability of any proposed construction equipment should be verified by using it on a test strip prior to its use in actual construction. The test strip may also be used to evaluate the specified compaction procedure, as well as any proposed modifications to the procedure. If fly ash from a power plant's landfill or lagoon contains any lumps when spread for compaction, it may be necessary to break down the lumps using a disk harrow or a rotary tiller as a supplemental piece of equipment.(14)
Fly ash should be placed in uniform lifts no thicker than 0.3 m (12 in) when loose. Experience has shown that steel-wheel vibratory compactors and/or pneumatic tired rollers have provided the best performance. 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.(13)
In general, six passes of the roller are usually needed to meet specified 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.(17)
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 before the actual construction. If a vibratory compactor is to be used, the test strip can be used 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)
During periods of moderate rainfall, construction may proceed by reducing the amount of water added at the power plant or jobsite to compensate for precipitation. Dry fly ash can also be mixed into excessively wet fly ash to reduce the moisture content to an acceptable level.
Because fly ash obtained directly from silos or hoppers dissipates heat slowly, fly ash may be placed during cold weather. If frost does penetrate a few inches 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.(13) Construction should be suspended during severe weather conditions, such as heavy rainfall, snowstorms, or prolonged and/or excessively cold temperatures.
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.(13)
If allowed to dry out, fly ash surfaces can be susceptible to dusting. 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 (for "low lime") or in pneumatic tankers (for "high lime"), moisture conditioning fly ash at the power plant (especially "low lime"), wetting or covering exposed fly ash surfaces, and sealing the top surface of compacted fly ash by the compactor at the conclusion of each day's placement.(13)
Fly ash surfaces must be graded or sloped at the end of each working 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 must be protected as soon as possible after being finish graded because, if left unprotected, they can be severely eroded. 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)
Since coal fly ash consists of predominantly silt-sized particles, there is sometimes a concern about the possible frost susceptibility of fly ash as an embankment or structural backfill material, especially in northern climates. Normally, earthen materials that are primarily in the silt grain size range are frost susceptible. However, some fly ashes are frost susceptible while others are not. More testing of fly ashes needs to be done to determine why this is so and to develop a more accurate predictor for frost susceptibility.
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 the self-cementing characteristics of the latter type, which hardens 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.