energy is lost in disrupting these bonds. Maximum density achieved therefore decreases
as the hydration reaction progresses after blending of the soil, fly ash and water.
Self-cementing fly ash hydrates more rapidly than Portland cement; therefore, a 2-
hour delay in compaction can result in a decrease in maximum density of up to 1.6 kN/m
(10 pcf) or more. Usually a 2-hour delay time can be achieved even with rudimentary
equipment. When pulvamixers are used with experienced personnel a 1-hour compaction
time can be readily achieved.
The allowable range in moisture content must be specified and be monitored
during construction to ensure that moisture contents of the stabilized section are near the
optimum for maximum strength. If the actual compaction in the field will be completed
within the specified 2-hour delay period, actual strengths achieved in the field would be
between the laboratory test results with 0 and 2 hour compaction delay.
No standard methods have been adopted for the design of materials stabilized
with fly ash. (ASTM C-593 and ASTM D-1633). Depending upon the application either
standard or modified Proctor compactive energy may be used. For most county road
application, standard Proctor compaction should be adequate.
For cohesive soils, the moisture content should be up to 10 percent below
optimum moisture content for maximum density. Test specimens should be cured for 7
days at 38C (100F) in accordance with C-593 after which compressive strength should be
determined. The optimum moisture content for maximum strength has been shown to be
consistent for cure periods of 7, 28, and 56 days. Therefore, optimum moisture content
can be determined using 7-day strengths.
The reduction of PI for clay soils will be less for fly ash compared to lime.
220.127.116.11.3. Construction Procedures and Concerns
The laboratory mix design is usually conducted to establish the optimum ash and
moisture contents. Maximum dry density and strength gain for design and construction
testing are determined. A general construction specification is presented in Reference 38.
The following goals must be achieved to result in a good project:
• Uniform distribution of the fly ash
• Proper pulverization and thorough mixing of the fly ash with the material to
• Control of moisture content for maximum density and strength
• Final compaction within the prescribed time frame (usually 2 hours)
Typical design specifications call for fly ash contents of 1 to 2 percent greater than
optimum contents determined in the laboratory. Pneumatic tankers or bottom dump
trailers are used to transport fly ash to the project. Careful blading of the fly ash over the
exposed grade from uniform windrows deposited by the transports is the best way to
obtain uniformity of application. The quantity of ash can be calculated knowing the
depth, width, length and design percent of fly ash. Uniform distribution can be
accomplished using metered gates on the transport of direct metering of the ash into the
mixing drum of a mobile mixer.
Construction discs can effectively blend the ash with cohesive soils. The depth the
disc is cutting must be closely monitored. Where higher degrees of stabilization are
required the use of a self-propelled mixer (pulvamixer) is required to ensure adequate
pulverization and uniform distribution of moisture and fly ash. One or two passes of a
mixer can be used to obtain good mixing.
Control of moisture content is both critical and difficult. Strengths of the
stabilized materials decrease significantly as the moisture increases above the optimum
moisture for maximum strength. Strength also decreases on the dry side of optimum
moisture and increased compactive effort is required.
Maintaining moisture contents within a range of 0 to 4 percent above optimum
moisture content for maximum compressive strength is typically recommended and is
readily achieved with proper equipment.
Significant quantities of water may be required to bring the moisture to the design
level. The following aspects of moisture control must be considered.
If water is added after the fly ash is blended the final strength of the stabilized
material will be reduced due to hydration of the ash before compaction is completed.
• Adding sufficient water to the pulverized material prior to distribution of the
ash may make the untreated material unstable, hampering distribution and
operation of construction equipment.
• Applying water directly onto the fly ash distributed on the surface in not
advisable since this increases the rate of hydration.
• Water can be added after the fly ash has been incorporated; however,
additional passes with the mixing equipment will be required to achieve
• Introducing water directly into the drum of a rotary mixer is the most effective
procedure in controlling moisture content so it falls within the desired range
and providing the most uniform mixing without additional delay in
Moisture contents can be monitored using a nuclear density gauge. The nuclear
gauge may not give an accurate moisture measurement; however, it can give a good
indication of uniformity.
Compaction of the mixture must be accomplished as soon as possible following
the final pass of the mixing equipment. Using paving train type operations initial
compaction can easily be achieved within 15 minutes of the final pass of the mixing
Initial compaction is most often accomplished using a vibratory padfoot or a self
propelled padfoot roller operated immediately behind the mixing equipment. The
padfoot provides good compaction from the bottom of the stabilized layer and imparts
a kneading action which can give some additional mixing.
After initial compaction the materials should be shaped to final grade by blading
and final compaction done using a self-propelled, pneumatic-tired roller. Shaping
should not be delayed.
The surface of the stabilized lift should be maintained in a moist condition to
help hydration of the fly ash. Curing can be accomplished through periodic
application of water on the surface until the nest lift or a wearing surface is
constructed over the stabilized material.
Stabilization with fly ash can be performed satisfactorily down to temperatures
of 10C (50F). Construction can be accomplished at cooler temperatures with
modified procedures. At cooler temperatures two passes of a pulvamixer may be
required to reduce the maximum size of the material to less than 25 mm (1 in.).
Cooler temperatures may be beneficial apparently because the cooler temperature
retards hydration. However, cooler temperatures also result in decreased density for
the same compactive effort. With additional compactive effort, and in-place densities
are adequate, the strength of the compacted section can be near design strength when
constructed below 4.5C (40F).
Cooler temperatures have greater impact on soil pulverization and compaction
than on ash hydration. Soil temperatures below 10C (50F) help retard ash hydration
which increases long-term strength of the stabilized material. Multiple passes of the
pulvamixer may be required to achieve pulverization and mixing with the ash.
Additional compactive effort may also be required to obtain specified density.
Effective stabilization of clay soils as long as soil temperature is above 0C and
construction procedures are modified to attain proper mixing and compaction of the
stabilized materials (47).
18.104.22.168.4. Concerns when Using Fly Ash for Soil Stabilization
There are two common high-sulfate content ashes: fluidized bed combustion
(FBC) and flue gas desulfurization (FGD) ash. These materials can exhibit self-
cementing properties similar to subbituminous coal ashes. These materials may
cause serious expansion characteristics when hydrated. Therefore, the following
should be considered when evaluating the sulfate content of an ash.
• Ash with SO
contents of 5 to 10 percent should be considered potentially
expansive until laboratory testing indicates otherwise
• Ash with SO
contents greater than 10 percent should not be used for
• Soluble sulfates in the soil or groundwater can influence swell potential and
be considered in addition to the amount of sulfate in the ash
The relative damage/deterioration of a high-sulfate ash-stabilized material can be
categorized based on combined clay and colloid content as follows:
Clay and Colloids Content
Greater than 30%
The availability of free moisture in the stabilized material is critical to long term
performance. With saturated or near-saturated conditions, sulfate, silica and alumina ions
within the fluid are mobile and free to react (47).
The primary environmental concern when using self-cementing ashes is the
migration of metals. Data from four roadbases and one embankment suggested that
very localized migration of ash derived metals had occurred into the underlying soils.
Depth of migration was less than 0.7 m (2 ft) below the stabilized section on two
Most applications of fly ash stabilized soils or bases would be designed such that
the material would be above the water table and water flow through the material
would be minimal. This is necessary to maintain the structural integrity of the
stabilized and layers of the pavement section. If there is a groundwater associated
problem, the stabilized section is encapsulated in a geofabric.
To evaluate the potential of leaching particular materials the specific metals in a
given ash should be determined. The source of coal for a given generating plant the
coal source is usually the same because the burning system is setup for that coal
An EPRI Demonstration Project was conducted in Kansas to assess the migration
of metals from the stabilized section in to the underlying subgrade. Of the 23 metals
evaluated only one was present in a higher concentration in the fly ash than in the soil
below the section to be fly ash-stabilized. Barium was the only metal that was
present in significantly higher concentrations than in the soil.
The Toxicity Characteristic Leaching Procedure (TCLP) has been used by a
number of agencies to what and how much of various metals are leached from various
situations and environments. Studies at specific locations showed that the metals
leached from the ash were a small percentage of the total metals present in the
existing soils. Overall, it was found that the hydration and solidification of the ash in
addition to the natural soil attenuation characteristics caused a reduction in leachable
Fugitive Dust can be a problem just as for any other construction process.
Maximum dust is generated at the time the ash is discharged from the tankers or end
dump trailers onto the pavement subgrade. Construction activity will generally
minimize fugitive dust. When a rotary mixer is used, water is added in the mixer,
which minimizes fugitive dust. This is the procedure that also is most effective in
constructing a good stabilized soil subgrade (47).
22.214.171.124.5. Summary of Fly Ash Soil Stabilization Procedures
- Damp or dry
- Little or no wind
- Temperature above 40F (4.5C)
- Temperature below 40F (4.5C)
- Fly ash is delivered to The the project either in tarped trucks or tanker trucks with
pressurized pumping systems
Measurement of Quantities
- Fly ash either metered from the truck and trucks counted.
- Moisture added to grade as needed.
- Disking may be sued to decrease moisture content.
Method(s) of Mixing
a. Trucks dump fly ash in uniform windrow (if no wind);
b. Spread laterally across the embankment with a bulldozer
c. Mix with a recycler (BOMAG) traveling at 20-30 ft/min or disked to design or lift
d. If water needed, the truck is pulled through the grade with a bulldozer
e. Shape the grade with a bulldozer
a. Initial compaction - pad roller or sheepsfoot roller
b. Final compaction – steel wheeled roller to provide smooth surface and help shed
c. Compaction control – Mn/DOT Specification 2105 allows for specified density
based on a Procter with the given percent fly ash or quality compaction with
d. Compaction must be accomplished within two (2) hours because working of
the mixture after that may break up the products of hydration which
stabilize the soil.
Curing of a Soil-Fly Ash Mixture
When self cementing fly ash is mixed with water, hydration of the material
creates the gel which binds (stabilizes) the soil and results in the stronger more
uniform lower permeability material. The hydration requires water. Therefore, the
surface of the grade should be kept damp.
About 0.4 to 0.6 km (¾ to 1 mi) of stabilized grade can be constructed in one
1. Wind: watch out for windy conditions if fly ash laid out on the grade.
2. Mixing: mix in fly ash as soon as possible.
3. Protection: Workers should wear protective equipment to avoid burning skin,
eyes, nose and mouth
Life: With proper mix design and construction it is expected the grade would last
at least 50 years.
Contacts: Jeff Blue, Waseca County, John Seikmeir, Mn/DOT
4.5.5. Subgrade Soil Enhancement using Geosynthetics
Geosynthetics are a class of textile materials that are extruded petroleum polymer-based
thin pliable sheets of varying permeability. There are many different varieties, such
as geotextiles, geogrids, geonets, geocells, and geomembranes. One difference is the
size of the aperture, with geogrids having the largest aperture. Most varieties of
geosynthetics used for pavement applications in Minnesota are of Mn/DOT Type V
and VI (Spec 3733.1) classification.
Table 4.8 Mn/DOT Geosynthetic Classifications (Mn/DOT Spec 3733.1)
For use in wrapping subsurface drain pipe or for other specified
For use in wrapping joints of concrete pipe culvert and as a cover
over drain field aggregate.
For use under Classes I and II random riprap, gabions, and revet
For use under class III and IV random riprap, hand-placed riprap, and
For use in separating materials (stabilization).
For use in earth reinforcement and Class V random riprap.
Geosynthetics are used in many areas of ground construction. Common highway
applications include separation, reinforcement, drainage and filtration. The usefulness and
effectiveness are directly dependent on the application, the type of geosynthetic, and the
design in which the geosynthetic is incorporated.
Interpretation of the benefits associated with geosynthetics can be difficult. Some of the
most common benefits are cost savings, longer life, and improved performance. Obtaining
quantifiable improvement using geosynthetics requires careful design along with correct and
careful installation procedures.
Proper design procedure requires more information than what is presented in this report.
The purpose of this overview is to provide an introduction to geosynthetic applications and
construction procedures. This information can be used to facilitate the decision whether
geosynthetics are appropriate for specific pavement design applications.
Types of Geosynthetics
Geotextiles are permeable textile-like materials most commonly composed of a
polymer like polypropylene and polyester (48). The two most common geotextile varieties are
woven and non-woven. The woven varieties are made from both monofilament and
multifilament fibers. The non-woven (multifilament) varieties are bonded together after
extrusion by one of three processes: melt-bonding, needle-punching, or resin-bonding. The
spectrum of geotextile variations is vast, providing flexibility for design.
Geotextiles are used in three major categories of pavement system improvement:
The most common pavement application for geotextiles is separation of dissimilar materials
(49). Separation between an underlying fine-grained soil and an aggregate base or granular
subbase to prevent contamination of the base material has been used for many applications.
Separation is mostly needed for grades that will be saturated or close to saturation.
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