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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Waste rock, mill tailings, and coarse coal refuse can be used as a granular base in pavement construction applications. Burnt coal refuse (or red dog) from banks or piles that have caught on fire has also been used as a granular base material.
Waste rock derived from igneous or metamorphic rocks, as well as properly consolidated limestones, sandstones, and dolomites are generally suitable for granular base and subbase applications, provided the waste rocks do not contain deleterious components and are not commingled with overburden.
Coarser-sized mill tailings can be used in granular base and subbase applications. Generally, the coarser, sand-size fractions of mill tailings can also be considered for use as construction aggregates, 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.
Coarse coal refuse can be used as aggregate in granular base applications. Burnt coal refuse (red dog) is also a suitable granular base material. Proper compaction of coarse coal refuse to its maximum dry density is necessary to achieve stability within a pavement structure. Fine coal refuse slurry has little or no load carrying capability and is, therefore, unsuitable for use as a construction material.
The carbonaceous content of coal refuse, its potential for spontaneous combustion, as well as its pyritic or sulfur composition and acidic nature are causes for environmental concern.
The use of mineral processing waste as a granular base material is not very common, since many mineral processing wastes are not close to urban areas where construction materials are needed. There is little current research or actual reported use of such wastes in granular base construction.
A review of the published and unpublished reports reveals that at least 13 states have made use of some source of waste rock in their state highway construction programs, sometimes dating back as many as 50 years. However, it does not appear that any state highway agencies or universities have been involved in research for waste rock use as aggregate in granular base or subbase applications.(1)
At least five states, including Alabama, Alaska, Arizona, Arkansas, and Virginia, have reported using mill tailings as aggregate in granular base course applications, although the Arkansas experience was not considered successful, due to poor performance or economics.(1) Other states that have made some use of mill tailings for granular base or subbase construction in the past include California, Colorado, Idaho, Illinois, Michigan, Minnesota, and Tennessee.(2)
Coal refuse has been successfully used in cement stabilized base applications in Europe. The success of this material for use in this application is reportedly dependent on proper compaction. There has been occasional use of coal refuse in Alabama, Kentucky, Virginia, and West Virginia as an alternative material for bases and subbases.(1,2,3)
The Pennsylvania Department of Transportation has rejected anthracite refuse usage as aggregate for base and subbase courses because of high percent losses in the sodium sulfate (soundness test).(4,5) West Virginia is evaluating the use of coal refuse as subbase material.
The Ministry of Transport in the United Kingdom permits the use of incinerated coal refuse (well- burnt, nonplastic shale) as a granular subbase material in Ministry controlled road work.(6)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Where the waste rock consists of hard, stable chunks of rock with no overburden or vegetation, granular aggregate material can be produced by crushing. Crushing and screening can be accomplished using conventional aggregate processing equipment.
Processing of certain tailings sources (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 tailings materials may contain sufficient coarse sizes (greater than 2.0 mm (No. 10 sieve) or 4.75 mm (No.4 sieve)) that could be classified and separated from the finer fraction for possible use as a granular base material.
Separation or Cleaning
The basic coal refuse processing techniques used in coal preparation plants are separation of the coal from the unwanted foreign matter (pyrite and marcasite). 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.
For older refuse banks, additional separation or cleaning may be required in the field to remove and recover the combustible portion of coarse coal refuse for use as fuel, prior to using the remaining refuse material as a granular base or subbase material. Such cleaning also serves to prevent potential spontaneous combustion of the refuse.
Some of the properties of waste rock that are of particular interest when waste rock is used in granular base 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 and rocks to sand-size particles and dust. Waste rock can be crushed and screened for use or blended with other aggregates to generate a product suitable for granular base or subbase aggregate.
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.(7)
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 granular base applications include gradation, particle shape and texture, unit weight, and moisture-density characteristics. It is difficult to definitively characterize representative samples of mill tailings materials because of the number of sources and the 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 normally disposed of in slurry form by pumping into large retention areas/settlement ponds.(8) Despite the fine size of most mill tailings, these materials can be classified prior to disposal or blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range for use as a construction aggregate.
Shape and Texture: Mill tailings consist of hard, angular, siliceous particles.
Unit Weight: Iron and taconite tailings typically have high unit weight values up to as high as 2700 kg/m3 (170 lb/ft3). The unit weight of most other tailings sources is expected to range from 1500 kg/m3 (90 lb/ft3) to 2200 kg/m3 (135 lb/ft3), which is comparable to that of most natural aggregates, which are approximately 2,000 kg/m3 (125 lb/ft3) to 2300 kg/m3 (140 lb/ft3).(8)
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 sources is in the range of 1600 kg/m3 (100 lb/ft3) to 2025 kg/m3(125 lb/ft3).(9)
Some of the properties of coal refuse that are of particular interest when coal refuse is used in granular base applications include gradation, particle shape/ texture, moisture-density characteristics, shear strength, permeability, and frost susceptibility.
Gradation: Coarse refuse, which can contain particles that are greater than a 4.75 mm (No. 4) sieve, is generally a well-graded material for particles up to 100 mm (4 in) in size. These particles consist 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 kg/m3 (80 lb/ft3) to 2000 kg/m3 (120 lb/ft3).(6)
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.(10) The shear strength of coal refuse is usually lower than that obtained for granular materials with similar properties, but can be increased by proper compaction.(6,10,11) Previous experience with coal refuse usage as a construction material has demonstrated that the shear strength of the refuse is acceptable if proper compaction measures are achieved during construction.(10,12)
Permeability: The permeability of coarse coal refuse can be highly variable and should be determined for each particular source and application. It is related to the composition of the refuse, its degradation during compaction, and the degree of compaction.(10,13) The permeability of coarse coal refuse is lower than that of other granular materials with a similar grain size distribution. Conventional formulas relating permeability to particle size distribution and uniformity are not applicable for estimating the permeability of coarse coal refuse.(14)
Low permeability values are desirable in order to reduce air circulation and to reduce the potential for spontaneous combustion, oxidation of pyrites, and acidic leachate. Fly ash may be added to the refuse for this purpose. The average permeability for coal refuse-fly ash mixtures is significantly lower (10-6 to 10-7 cm/sec) than that for the coal refuse alone (10-3 x 10-5 cm/sec) because fly ash fills the voids of the coal refuse.(14,15)
Frost Susceptibility: Coal refuse is susceptible to frost heave, especially burnt coal refuse. Frost damage can reportedly be reduced or eliminated by the addition of cement to the refuse.(16)
There are no standard specifications for the use of crushed waste rock in granular base applications. Most sources of waste rock are of a quality that is comparable to conventional aggregates used as granular base materials, so specifications applicable to such aggregates can probably be used, provided sufficient compaction is achieved.
There are no standard specifications for mill tailings as an aggregate in granular base. The tailings must meet sizing requirements and satisfy standard Proctor moisture-density criteria.(17) Durability testing may also be required. Most tailings sources may have an excess amount of material finer than the 4.75 mm (No. 4) or 2.00 mm (No. 10) sieve. This will either limit their use in granular base course applications or necessitate separation and use of the coarser fraction of the tailings.
Mill tailings to be used in granular base should also be tested in accordance with AASHTO test methods T234(18) and T236(19) to determine the shear strength characteristics and to define the angle of internal friction and cohesion of the material tested. The California bearing ratio (CBR) test (AASHTO T193)(20) can also be used to evaluate the subgrade bearing capacity.(22)
Tests for combustion potential and standard Proctor moisture-density criteria should be carried out for all coal refuse that is considered for use in granular base construction. Leaching and swelling indexes, porosity, freeze-thaw tests, and wet-dry swelling tests are also recommended. Water soluble sulfate testing methods and specifications for determining the amount of sulfate found in coal refuse and measures used to overcome such sulfate content are available from the British National Coal Board.(21) The introduction of fly ash to coal refuse may help to neutralize the acidity of the refuse, increase its moisture-holding capacity, increase its pore space volume, and reduce its erodability.(22) Lime and/or cement added as a binding agent with the fly ash produces a pozzolanic reaction, which can provide added strength and durability to the coal refuse/fly ash mixture.(22)
Coal refuse to be used in granular base should also be tested in accordance with AASHTO test methods T234(23) and T236(24) to determine the shear strength characteristics and to define the angle of internal friction and cohesion of the material tested. The CBR test (AASHTO T193)(25) can also be used to evaluate the subgrade bearing capacity.(22)
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 or mill tailings.
Prior to using the refuse to construct a granular base, 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 aggregate can be used for the placement of waste rock. As with any other oversize rock placement, compaction operations must be 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.
No modifications to normal construction equipment or procedures are needed, except that tailings may need to be dried to near optimum moisture content prior to placement and compaction.
No significant modifications to normal construction procedures are needed, except that possible material breakdown under compaction equipment requires more repetitive testing in the field. 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. Under such conditions, the material is sufficiently dense for base course construction and the potential for ignition is substantially reduced.(6)
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 that are too large for certain in-place density tests. The same field test procedure used for conventional aggregate are recommended for granular base applications when using waste rock or mill tailings. Standard laboratory and field test methods for compacted density are given by AASHTO T191,(26 T205,(27) T238, (28)and T239.(29))
Strict compaction control testing must also be performed when using coal refuse as a base. A determination of the sulfate levels leached from the coarse refuse materials 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.
Relatively little is known about how variations in mineral processing operations can alter the quality of mineral processing wastes.(30)
Waste Rock and Mill Tailings
There is also a need to determine whether specific sources of such materials are environmentally suitable for use in granular base construction. In addition, there is a need to develop engineering data on the design properties and performance of potential waste rock and/or mill tailings used in granular base applications.
There is a need to further evaluate the environmental concerns regarding the potential for acidic leachate from coarse coal refuse used in granular base applications. The production of such leachate is caused by the oxidation of pyrite and marcasite with the presence of high sulfur content. If acidic leachate were to be produced over time, it could contaminate groundwater, adversely impact the ecosystem, and cause deterioration or corrosion of underdrains or other drain pipes.
More information may be needed to completely mitigate concerns associated with the spontaneous combustion potential of coarse coal refuse.