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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Several different types of mineral processing wastes, particularly mill tailings and coarse coal refuse, have been successfully used to construct highway embankments. To a lesser extent, waste rock has also been sporadically used as a fill material in highway construction.
Waste rock derived from igneous or metamorphic rocks, as well as properly consolidated limestones, sandstones and dolomites, are generally suitable for use in embankment or fill construction applications, provided the rocks do not contain deleterious components and are not commingled with overburden. Prior to use, some consideration should be given to the leaching potential of waste rock from sulfide-based ore bodies (such as lead, zinc, or silver) or waste rock subjected to heap leaching.
Mill tailings have been used previously in embankment and fill applications by some state and local highway agencies. Generally, the coarser, sand-size fractions of mill tailings can also be used as a construction aggregate, provided there are no harmful or reactive chemical components concentrated from the host rock. Despite the fine size of most mill tailings, these materials can be blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range, or can often be classified prior to initial disposal in order to recover the coarser fraction for possible use. However, the metal leaching potential of these materials can be a cause for environmental concern and should be thoroughly investigated prior to embankment use.
Coarse coal refuse can be used in embankment applications. Proper compaction of coarse coal refuse to its maximum dry density is necessary to achieve stability and to minimize the potential for spontaneous combustion. Burnt coal refuse (red dog) is also a suitable embankment or fill material. Fine coal refuse slurry has little or no load carrying capability and is, therefore, unsuitable for use as a construction material.
Potential problems with spontaneous combustion associated with the carbonaceous content of coal refuse, its pyritic or sulfur composition, and acidic nature are causes for environmental concern.
Mineral processing wastes have been used in a number of states during the past 30 to 40 years for embankment applications where such materials have been available, acceptable, and economical.
Although uses of mining and mineral processing wastes for highway embankment construction have not generally been well documented over the years, it is known that these materials have been used for such purposes in at least 14 different states. There are also at least two other examples (feldspar tailings in North Carolina and coal refuse in Ohio) where these materials have been used on a local basis as fill and approved for highway construction. Table 9-6 is a summary of the known uses of mining and mineral processing wastes in embankments in these 16 different states.(1)
Waste rock derived from all sources may be used for embankment applications provided it satisfies applicable specification requirements for rock base. Waste rock should not contain deleterious components and must not be commingled with unsuitable materials. Currently, New York is the only state that is reportedly using waste rock as highway material. It is being used as stone fill for embankments and as rip rap for bank and channel protection. Performance has been acceptable in each application.(2) Other states that have made some use of waste rock for embankment construction in the past include Arizona, Colorado, Michigan, and Washington.(1)
The coarser, sand-size fractions of most mill tailings ordinarily make acceptable embankment construction materials, provided there are no harmful or reactive chemical components contained in the tailings. Despite the fine size of most of tailing materials, they can be readily classified or blended with coarser materials, such as natural gravel, to bring the overall fines content to a more acceptable range.
In the last 15 to 25 years, some very large highway embankments have been constructed using mill tailings. Copper tailings have been used in Utah and Michigan, lead-zinc tailings in Idaho, feldspar tailings in North Carolina, and gold tailings in California and Colorado. Although some embankment applications may not have been well documented, their performance has been generally described as good to very good for this type of application.(1)
In 1994, it was reported that at least 11 local or state agencies in Alaska, California, Colorado, Idaho, Michigan, Minnesota, Missouri, North Carolina, South Dakota, Utah, and
Table 9-6. List of mining waste embankment or structural backfill projects constructed in the United States.
Washington have been involved, at one time or another, in mill tailings use in embankment applications.(2)
Pennsylvania has had successful experiences with the use of coarse coal mine refuse in at least four embankment construction projects. Both anthracite and bituminous coal refuse have been used.(3) Other states with some experience in coal refuse embankment construction include Illinois, Indiana, Maryland, and Ohio.(2) Also, some local usage of coarse coal refuse as a fill material has been previously indicated in Colorado and Kentucky.(1) The Federal Highway Administration has prepared a manual recommending the appropriate methods for using coal refuse to build highway embankments.(4)
Great Britain has been a forerunner in the utilization of coal mine refuse in highway construction. The Ministry of Transport in Great Britain permits the use of coal refuse (well-burnt nonplastic shale) as an alternative material source for the construction of embankments.(5)
MATERIAL PROCESSING REQUIREMENTS
Crushing and sizing is the only processing required to make use of oversize waste rock in embankments. Waste rock should be free of overburden and vegetation before crushing..
For certain mill tailings sources, some processing (such as dewatering, reclaiming, and selective size classification) may be necessary, although this is not common practice and can be costly. Tailings reclaimed from ponds will normally require a reasonable period of time to dewater, depending on climatic conditions.
Some fine tailings can be size classified to recover a coarser fraction (between the 4.75 mm (No. 4) and 0.075 mm (No. 200) sieves) for use as an embankment construction material.
Separation or Cleaning
Various mineral processing techniques are used to separate the coal from the unwanted foreign matter in coal preparation plants. The equipment most frequently used in these plants to classify the refuse is based on the difference in specific gravity between the coal and the host rock.
Additional separation or cleaning may be required in the field in order to remove and recover the combustible portion of coarse coal refuse for use as fuel, prior to placing the remaining refuse material in an embankment. This is particularly the case for older refuse banks.
Some of the properties of waste rock that are of particular interest when waste rock is used in embankment or fill applications include gradation, specific gravity, and shear strength.
Gradation: Waste rock is generally coarse, crushed, or blocky material covering a range of sizes, from very large boulders to fine sand-size particles and dust. Waste rock can be processed and blended with other aggregates to generate a product suitable for use in embankment construction.
Specific Gravity: The average specific gravity of waste rock is about 2.65, with a range from 2.4 to 3.6 depending on the nature of the mineral constituents. Specific gravity may be used to determine other important properties such as void ratio or porosity.(6)
Shear Strength: Typical values of the angle of internal friction of most waste rock sources exceed 35 degrees and contribute to relatively high bearing capacity and stability.
Some of the properties of mill tailings that are of particular interest when mill tailings are used in embankment or fill applications include gradation, particle shape and texture, moisture-density characteristics, and unit weight. The chemical composition of the tailings should also be known prior to its use. It is difficult to definitively characterize representative samples of mill tailings materials because of the number of sources and variations in the degree of processing that can be encountered.
Gradation: Typically, mill tailings range from sand to silt-clay in particle size, with 40 to 90 percent passing a 0.075 mm (No. 200) sieve. They are usually disposed of in slurry form by pumping into large retention areas or settlement ponds.(7) The coarser, sand-size fractions, if any, of mill tailings are more highly recommended for embankment construction. Mill tailings can be classified prior to disposal or blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range, preferably less than 35 percent passing a .0075 mm (No. 200) sieve.
Shape/Texture: Mill tailings consist of hard, angular, siliceous particles.
Moisture-Density Characteristics: With the possible exception of iron ore or taconite tailings, most mill tailings have an optimum moisture content in the range of 10 to 18 percent. The maximum dry density of most tailings is in the range of 1600 to 2025 kg/m3 (100 to 125 lb/ft3).(8)
Unit Weight: Iron ore and taconite tailings typically have high unit weight values, up to as high as 2700 kg/m3 (170 lb/ft3). The unit of weight of most other tailings sources is expected to range from 1500 to 2000 kg/m3 (90 to 135 lb/ft3), which is comparable to that of most natural aggregates.(7)
Some of the properties of coarse coal refuse that are of particular interest when coarse coal refuse is used in embankment or fill applications include gradation, particle shape/ texture, moisture-density characteristics, strength, permeability, durability, resistance to wetting/drying, and frost susceptibility.
Gradation: Coarse coal refuse, which is greater than 4.75 mm (No. 4 sieve), is a well-graded material (can vary in size from 100 mm (4 in) to 2 mm (No. 10 sieve)) consisting mainly of slate or shale with some sandstone or clay. Most coarse refuse contains particles that may break down under compaction equipment, resulting in a finer gradation following placement.
Shape/Texture: Coal refuse is composed mainly of flat slate or shale particles with some coal, sandstone, and clay intermixed. Such particles may weather or break down easily.
Moisture-Density Characteristics: Based on available data, the optimum moisture content of coarse coal refuse is likely to range from 6 to 15 percent and its maximum dry density may vary from 1300 to 2000 kg/m3 (80 to 120 lb/ft3).(3,5)
Shear Strength: The shear strength of coarse coal refuse can be highly variable. The angle of internal friction values for coarse coal refuse have been reported to be between 18 and 42 degrees.(9) The shear strength of coal refuse is usually lower than that obtained for other granular materials with similar properties, but can be increased by proper compaction.(5,9,10) Previous experience with coal refuse usage as a construction material has demonstrated that the shear strength of the refuse is acceptable if proper compaction is achieved during construction.(9,11)
Permeability: The permeability of coarse coal refuse can also be highly variable and should be determined for each particular refuse source. It is related to the composition of the refuse, its degradation during compaction, and the degree of compaction.(9,12) The permeability of coarse coal refuse is less than that of other granular materials with a similar grain size distribution. Conventional formulas for estimating the permeability of coal refuse on the basis of size distribution and uniformity are not applicable for this material.(13)
It is preferable to attain lower permeability to reduce the air circulation, and void ratio and to eliminate spontaneous combustion, oxidation of pyrites, and acidic leachate.(14) The permeability can decrease rapidly when the percentage of particles minus 4.75 mm (No. 4 sieve) increases. However, at some point, as more minus 4.75 mm is added, the coarse particles may be displaced or pushed apart, creating higher permeability.(15)
Durability: If durability is a concern in the top layers of an embankment, fly ash can be added to the refuse to neutralize acidity of the refuse, increase its moisture-holding capacity and pore space volume, and reduce its erodability.(4) Lime and/or cement used as a binding agent with the fly ash produces a pozzolanic reaction, providing added strength and durability to the coarse coal refuse.(9)
Resistance to Wetting/Drying: Coal refuse begins weathering immediately after it has been placed in an embankment. Increases in the soluble sulfur content can induce oxidation of the pyrite. However, once the material is sealed within the embankment, oxidation is limited and weathering is greatly reduced. Water penetration is virtually eliminated, along with the degradation resulting from intermittent wetting and drying.
Frost Susceptibility: The top layers of coal refuse (especially burnt refuse) may be susceptible to damage from frost. Frost damage can be reduced or eliminated by the addition and mixing of cement into the top 1 meter (3 ft) of refuse.(16)
Waste Rock and Mill Tailings
The design requirements for waste rock or mill tailings in embankment construction are the same as for conventional aggregates or soils. These materials must meet appropriate sizing requirements and satisfy standard Proctor moisture-density criteria according to AASHTO T99.(17)
Structural design procedures to be employed for embankment or fill construction containing waste rock or mill tailings are essentially the same as design procedures that are used for conventional embankment materials. An analysis of the slope stability and consolidation characteristics of the embankment must be completed prior to construction. Some tailings sources may have an excessive amount of fines (greater than 35 percent passing the 0.075 mm (No. 200) sieve) which could necessitate prior classification or separation and use of only the coarse fraction of the tailings in an embankment or fill.
The design requirements for coarse coal refuse in embankment or fill construction are essentially the same as for conventional aggregates or soils. However, tests for standard Proctor moisture-density and spontaneous combustion potential should be carried out for all coal refuse that is considered for use in highway construction. Leaching and swelling indexes, porosity, freeze-thaw tests and wet-dry swelling tests are also required. Water-soluble sulfate testing methods/ specifications for determining the amount of sulfate found in coarse coal refuse and measures used to overcome such sulfate content are available from the British National Coal Board.
Design procedures for embankments or fill containing coal refuse are the same as design procedures for conventional embankment materials. Slope stability and settlement analyses should be conducted to ensure that the coal refuse embankment is stable at the design slope and will not settle excessively. The potential for weathering and frost heave must also be considered.
Coal refuse for use in embankments should be tested in accordance with AASHTO test methods T234(19) and T236(20) to determine the shear strength characteristics of the material tested. AASHTO T216(21) and T193(22) are also used to determine the consolidation characteristics of the refuse and evaluate its subgrade bearing capacity.(4)
Material Handling and Storage
Waste Rock and Mill Tailings
The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste rock and mill tailings.
Prior to using the refuse to construct embankments or fills, the bank should be cleaned or processed to recover the residual coal or combustible matter. This ordinarily involves a screening of the refuse, which also removes oversize and deleterious materials.
Placing and Compacting
The same methods and equipment used to place and compact conventional rock as embankment base or foundation material can be used for the placement of mine waste rock. Compaction operations and methods must be visually inspected on a continuous basis to ensure that the specified degree of compaction can be achieved, or that there is no movement under the action of compaction equipment. The construction of embankment bases or foundations containing rock or oversize materials usually requires a method specification, which describes how to place and compact such materials, but does not include test methods or acceptance criteria.
No modifications to normal construction equipment or procedures are needed for placing and compaction of mill tailings, except that mill tailings may need to be dried to near optimum moisture content prior to placement and compaction.
No modifications to normal construction equipment or procedures are needed, except that material breakdown under compaction equipment requires more repetitive testing in the field. The key to the success of placing coarse coal refuse is in proper compaction. Proper compaction of coal refuse reduces air voids to less than 10 percent, and can reduce the permeability to less than 10-5 to 10-6 cm/sec, which is very low. Well-compacted material is sufficiently dense for embankment construction with minimal potential for ignition because of spontaneous combustion.(5)
Waste Rock and Mill Tailings
The same test procedures used for conventional aggregate are appropriate for waste rock and mill tailings, although waste rock may have particles too large for certain in-place density tests. The same field test procedures used for conventional aggregate are recommended for embankment and fill applications when using waste rock or mill tailings. Standard laboratory and field test methods for compacted density are given by AASHTO T191,(23) T205,(24) T238,(25) and T239.(26)
Strict compaction control testing is necessary when building an embankment with coarse coal refuse. One of the best methods for controlling the compaction of coarse coal refuse embankments is to first place a test strip to determine the most appropriate compaction equipment and number of passes to ensure adequate compaction. The test strip will also assist in identifying the degree of particle breakdown and its effect on moisture-density characteristics for different types of compaction machinery.
The quality control test procedures described above for waste rock and mill tailings are also applicable to coarse coal refuse, except that some refuse particles may be too large for certain in-place density tests.
A determination of the sulfate levels that may be leached from coarse coal refuse is required in order to design for the protection of any adjacent concrete structures. The pH value of the refuse in water should also be determined for proper selection of type of underdrain or other drain pipes.
Waste Rock and Mill Tailings
General specifications and design methods should be developed for waste rock and/or mill tailings use in embankment or fill applications by those agencies where such materials are logistically available in large quantities and are suitable for embankment or fill use.
There is also a need to determine whether specific sources of such materials are environmentally suitable for embankment construction, particularly some sources of mill tailings. Engineering data are needed on the design properties and performance of waste rock and/or mill tailings that have been successfully used in highway embankment or fill applications.
There is a need to further evaluate environmental concerns regarding the potential for acidic leachate from coarse coal refuse used in embankments. The production of such leachate is caused by the oxidation of pyrite and marcasite with presence of high sulfur content. If acidic leachate were to be produced over time, it would contaminate ground water, adversely impact the ecosystem, and cause deterioration or corrosion of underdrains or other drain pipes.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling