Document ID: EPA-HQ-RCRA-2008-0329-0139
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2009-01-02T05:00Z

HYPERLINK "http://www.tfhrc.gov/hnr20/recycle/waste/mwst3.htm" 
http://www.tfhrc.gov/hnr20/recycle/waste/mwst3.htm 

Turner-Fairbank Highway Research Center

	Granular Base

INTRODUCTION 

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 

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. 

Mill Tailings 

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. 

Coal Refuse 

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. 

PERFORMANCE RECORD 

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. 

Waste Rock 

A review of the published and unpublished reports reveals that at least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) 

Mill Tailings 

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 

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 

Waste Rock 

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. 

Mill Tailings 

Dewatering 

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. 

Screening 

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. 

Coal Refuse 

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. 

ENGINEERING PROPERTIES 

Waste Rock 

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. 

Mill Tailings 

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) 

Coal Refuse 

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) 

  

 

DESIGN CONSIDERATIONS 

Waste Rock 

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. 

Mill Tailings 

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) 

Coal Refuse 

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) 

CONSTRUCTION PROCEDURES 

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. 

Coal Refuse 

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 

Waste Rock 

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. 

Mill Tailings 

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. 

Coal Refuse 

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) 

Quality Control 

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)) 

Coal Refuse 

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. 

UNRESOLVED ISSUES 

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. 

Coal Refuse 

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. 

REFERENCES 

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