SOIL STABILIZATION AND DRYING
USING FLY ASH
by
H. A. Acosta, T. B. Edil, and C. H. Benson
Geo Engineering Report No. 03-03
Geo Engineering Program
University of Wisconsin-Madison
Madison, Wisconsin 53706 USA
January 5, 2003
i
EXECUTIVE SUMMARY
The objective of this study was to evaluate the effectiveness of stabilizing soft
subgrade soils with self-cementing fly ashes.
A laboratory testing program was
conducted using seven soils and four fly ashes. The soils varied in terms of source,
composition, and texture. The fly ashes differed in terms of geographical availability and
composition. Both Class C and ‘off-specification’ fly ashes (i.e., ashes that do not meet
the C or F classification) were used for stabilization. All of the soils were fine-grained
and represented poor subgrade materials.
Test specimens were prepared with each soil using a range of fly ash contents
(0, 10, 18 and 30%) at different soil water contents ranging from optimum water content
to 18% wet of optimum water content. Specimens prepared at optimum water content
were used as controls, whereas those compacted at higher water contents were
intended to simulate the soft and wet subgrade conditions often observed in the field.
Three tests were performed on each soil-fly ash mixture: California Bearing Ratio (CBR),
resilient modulus (Mr), and unconfined compression. Tests were also conducted on the
fly ash alone for comparison.
All of the soils had very low CBR (0-5) in their natural condition. A substantial
increase in the CBR was achieved when the soils were mixed with fly ash. Specimens
prepared with 18% fly ash at optimum water content showed the best improvement, with
CBRs ranging from 20 to 56. Specimens prepared with 18% fly ash and compacted at
7% wet of optimum water content showed significant improvement compared to the
untreated soils, with CBR ranging from 15 to 31. Specimens prepared with 18% fly ash
at very high water content (e.g., 18% wet of optimum) had lower CBRs (8 to 15), but
were appreciably stronger than untreated soils at the same water content.
Soil-fly ash mixtures prepared with 18% fly ash and compacted 7% wet of
optimum water content had similar or higher resilient modulus than untreated specimens
ii
compacted at optimum water content. The resilient modulus of specimens prepared at
very high water contents generally had lower resilient moduli compared those prepared
at optimum water content, but much higher resilient modulus than untreated soils having
high water contents. The resilient modulus also increased with increasing curing time,
with increases as large as 40% between 14 and 28 d of curing.
Unconfined compressive strength of the soil-fly ash mixtures increased with
increasing fly ash content. Soil-fly ash specimens prepared with 18% fly ash and
compacted 7% wet of optimum water content had unconfined compressive strengths
that were 4 times higher than those of untreated specimens compacted at the same
water content.
The effectiveness of fly ash stabilization generally depended on soil type.
Greater increases in CBR and resilient modulus were obtained for inorganic soils having
more fines and higher plasticity (i.e., poorer subgrade materials).
However, the
improvement in compressive strength was independent of soil type. Organic matter also
affected stabilization. All but one of the fly ashes was ineffective in stabilizing the soft
organic soil used in the testing program. The one fly ash that was effective had high
organic content, which suggests that high carbon fly ashes may prove useful for
stabilizing problematic organic soils and sludges
The CBR and resilient modulus data were used to illustrate how stabilizing soft
subgrades with fly ash can make flexible pavement designs more economical. Design
calculations showed that the base course thickness could be reduced by as much as
40% when a soft subgrade is stabilized with fly ash.
iii
ACKNOWLEDGEMENT
Financial support for this study was provided by the US Department of Energy
through the Midwestern Combustion Byproducts Recycling Consortium in Carbondale,
Illinois, the University of Wisconsin-Madison Consortium for Fly Ash Use in Geotechnical
Applications (funded by Mineral Solutions, Inc., Alliant Energy Corporation, and Excel
Energy Services, Inc.), and the Wisconsin Department of Transportation. The opinions
and conclusions described in the paper are solely those of the authors, and do not
necessarily reflect the opinions or policies of the sponsors.
iv
TABLE OF CONTENTS
ABSTRACT............................................................................................................ i
ACKNOWLEDGEMENT .......................................................................................iii
TABLE OF CONTENTS....................................................................................... iv
LIST OF FIGURES ..............................................................................................vii
LIST OF TABLES................................................................................................. xi
SECTION 1-INTRODUCTION .............................................................................. 1
SECTION 2-BACKGROUND ................................................................................ 3
2.1
2.2
2.3
2.4
2.5
NEED FOR SUBBGRADE STABILIZATION ........................................... 3
WHAT IS FLY ASH? ............................................................................... 3
TYPES OF COAL.................................................................................... 6
TYPES OF FLY ASH............................................................................... 7
FACTORS AFFECTING PHYSICAL AND CHEMICAL PROPERTIES
OF FLY ASHES....................................................................................... 7
2.6 STABILIZATION EFFECTS ON CLAYS................................................ 11
2.7 REVIEW OF RECENT LIME AND FLY ASH STABILIZATION RESEARCH
.............................................................................................................. 12
2.7.1 Lime Treatment ............................................................................ 12
2.7.2 Lime-Fly Ash Treatment ............................................................... 13
2.7.3 Fly Ash Treatment ........................................................................ 15
SECTION 3-MATERAILS AND METHODS ........................................................ 21
3.1 FLY AHSES........................................................................................... 21
3.1.1 Sources and Compositions........................................................... 21
3.1.2 Index Properties and Compaction Characteristics........................ 27
3.2 WISCONSIN SUBGRADE SOILS ......................................................... 29
3.2.1 Sources ........................................................................................ 29
3.2.2 Soil Index Properties .................................................................... 29
3.2.3 Compaction Characteristics and California Bearing Ratio ............ 30
3.3 TEST PROCEDURES ........................................................................... 35
3.3.1 CBR Test...................................................................................... 35
3.3.2 Resilient Modulus Test ................................................................. 35
3.3.3 Unconfined Compressive Strength ............................................... 42
SECTION 4-CALIFORNIA BEARING RATIO .................................................... 46
v
4.1 CBR OF FLY ASHES ............................................................................ 46
4.2 CBR OF UNTREATED SOILS .............................................................. 48
4.3 CBR OF SOIL-FLY ASH MIXTURES .................................................... 51
4.3.1 General Effects of Fly Ash Stabilization........................................ 52
4.3.2 Effect of Soil Water Content ......................................................... 54
4.3.3 Effect of Soil Type ........................................................................ 58
4.3.4 Effect of Organic Content ............................................................. 61
4.4 SYNTHESIS .......................................................................................... 66
SECTION 5-RESILIENT MODULUS ................................................................. 67
5.1 RESILIENT MODULUS OF FLY ASHES .............................................. 67
5.2 RESILIENT MODULUS OF UNTREATED SOILS................................. 69
5.3 RESILIENT MODULUS OF SOIL-FLY ASH MIXTURES ...................... 75
5.3.1 General Effects of Fly Ash Stabilization........................................ 75
5.3.2 Effect of Soil Water Content ......................................................... 80
5.3.3 Effect of Curing Time.................................................................... 80
5.3.4 Effect of Soil Type ........................................................................ 83
5.3.5 Effect of Organic Content ............................................................. 87
5.4 SYNTHESIS .......................................................................................... 99
SECTION 6-UNCONFINED COMPRESSIVE STREGTH................................... 91
6.1 UNCONFINED COMPRESSIVE STRENGTH OF FLY ASHES ............ 91
6.2 UNCONFINED COMPRESSIVE STRENGTH OF UNTREATED
SOILS.................................................................................................... 93
6.3 UNCONFINED COMPRESSIVE STRENGTH OF SOIL-FLY ASH
MIXTURES ............................................................................................ 95
6.3.1 General Effects of Fly Ash Stabilization........................................ 96
6.3.2 Effect of Soil Water Content ....................................................... 101
6.3.3 Effect of Curing Time.................................................................. 101
6.3.4 Effect of Soil Type ...................................................................... 103
6.3.5 Effect of Organic Content ........................................................... 103
6.4 SYNTHESIS ........................................................................................ 109
SECTION 7-PRACTICAL APPLICATION ......................................................... 111
7.1 FLEXIBLE PAVEMENT DESIGN.......................................................... 111
7.2 BASE THICKNESS DESIGN COMPARISON....................................... 112
SECTION 8-CONCLUSIONS ........................................................................... 117
8.1 CBR ...................................................................................................... 117
8.2 RESILIENT MODULUS......................................................................... 118
8.3 UNCONFINED COMPRESSIVE STRENGTH ...................................... 119
vi
8.4 PRACTICAL APPLICATIONS FOR FLEXIBLE PAVMENT DESIGN.... 119
..................................................................................................................
..................................................................................................................
SECTION 9-REFERENCES ............................................................................. 120
APPENDIX A .................................................................................................... 126
APPENDIX B .................................................................................................... 128
APPENDIX C .................................................................................................... 130
APPENDIX D .................................................................................................... 132
vii
LIST OF FIGURES
Fig. 2.1.
Poor subgrade soils in Wisconsin (WisDOT 1997).......................... 4
Fig. 2.2.
Comparison of a fly ash grain size distribution with those of
several soils..................................................................................... 5
Fig. 3.1
Particle size distribution curves for Dewey, Columbia, King, and
Edgewater fly ashes ...................................................................... 26
Fig. 3.2.
Compaction curves for Dewey, Columbia, King, and Edgewater
fly ashes ...................................................................................... 28
Fig. 3.3.
Location where soils were sampled............................................... 31
Fig. 3.4.
Particle size distributions for the Wisconsin soils ......................... 33
Fig. 3.5.
Compaction curves for Wisconsin soils ......................................... 34
Fig. 3.6.
California Bearing Ratio test being conducted on soil-fly ash
mixture........................................................................................... 36
Fig. 3.7.
Resilient modulus soil-fly ash specimen being compacted inside
a mold............................................................................................ 37
Fig. 3.8.
Specimen undergoing resilient modulus test ................................. 39
Fig. 3.9.
Resilient modulus of synthetic specimens A and B ....................... 41
Fig. 3.10.
Unconfined compression test being conducted on soil-fly ash
mixture........................................................................................... 43
Fig. 3.11.
Comparison between Proctor and Harvard compaction
methods......................................................................................... 44
Fig. 4.1.
Stress-penetration data from CBR tests for different fly ashes
prepared at 35% water-fly ash ratio for a curing period of 7
days............................................................................................... 47
Fig. 4.2.
The CBR of soil-fly ash specimens normalized to CBR of
untreated soil for soil-fly ash mixtures prepared at 7% wet of
optimum water content. (RSCT (CBR = 5), LRC (CBR = 2), and
BS (CBR = 3)) ............................................................................... 53
viii
Fig 4.3.
CBR gain as a function of fly ash content of red silty clay till,
lacustrine red clay, and brown silt prepared with Columbia,
Dewey, and King fly ashes compacted at 7% wet of optimum
water content................................................................................. 55
Fig. 4.4.
CBR of specimens prepared with (a) Dewey (b) King fly ashes
normalized to the CBR specimens prepared with Columbia
fly ash as a function of fly ash content (Specimen compacted
with 7% wet of optimum water content) ......................................... 56
Fig. 4.5.
Effect of water content on CBR of soils prepared with 10% and
18% (a) Columbia and (b) Dewey fly ash ...................................... 57
Fig. 4.6.
Effect of water content on CBR for red silty clay till (CL) and
brown silt (CH) at 10% Columbia and Dewey fly ashes................. 59
Fig. 4.7.
CBR as a function of soil water content (wSOIL - wOPT) and fly
ash content.................................................................................... 60
Fig. 4.8.
Effect of (a) liquid limit (LL) and (b) plasticity index (PI) on the
CBR ratio of soil-fly ash specimens prepared 7% wet of optimum
water content and 9-12% wet of optimum water content,
normalized to the CBR of the untreated soil specimen.................. 62
Fig. 4.9.
Effect of (a) group index (GI) and (b) liquidity index (LI) on the
CBR ratio of soil-fly ash specimens prepared 7% wet of optimum
water content and 9-12% wet of optimum water content,
normalized to the CBR of the untreated soil specimen.................. 63
Fig. 4.10.
CBR of organic Theresa Silt Loam with three fly ashes................. 65
Fig. 5.1.
Resilient modulus versus (a) deviator stress and (b) bulk stress
for fly ashes prepared at 35% water fly ash ratio for a curing
period of 14 days ........................................................................... 68
Fig. 5.2.
Resilient modulus of untreated soil specimens compacted
at optimum water content as a function of (a) deviator stress
and (b) bulk stress ......................................................................... 70
Fig. 5.3.
Comparison between resilient modulus estimated from the CBR
(7 days curing) and measured resilient moduli (14 days) at a
deviator stress of 21 kPa and confining pressure of 21 kPa.
ix
The conventional Modulus (EDYN) CBR relationship reported by
Heukelom and Foster (1960) is shown as the solid line. ............... 74
Fig. 5.4.
Resilient modulus of three soils prepared with Columbia, Dewey,
and King fly ashes compacted at 7% wet of optimum water
content and cured for 14 days ....................................................... 76
Fig. 5.5.
Moduli of the untreated soil compacted at W OPT to soil-fly ash
mixtures prepared with Columbia, Dewey, and King fly ashes
compacted at 7% wet of W OPT. Moduli of untreated soil
at 7% wet of WOPT were estimated based on CBR ........................ 78
Fig. 5.6.
Moduli of soil-fly ash specimens prepared with (a) Dewey and (b)
King fly ashes normalized to the moduli of the soil-fly ash
specimen prepared with Columbia fly ash. All specimens cured
for 14 days. (All resilient moduli are at deviator stress of 21
kPa). .............................................................................................. 79
Fig. 5.7.
Effect of water content on resilient modulus of several soils at
18% (a) Columbia and (b) Dewey fly ashes .................................. 81
Fig. 5.8.
Effect of water content on resilient modulus of several Plano
Silt Loam (CL) prepared with 12% Columbia fly ash and cured
for 7 days....................................................................................... 82
Fig. 5.9.
Resilient modulus tested at different curing time (i.e.7, 14, 28, and
56 days) for red silty clay till prepared with Columbia and
Dewey fly ashes at 18% fly ash content, normalized to the
resilient tested at 14 days curing time ........................................... 84
Fig. 5.10.
Effect of (a) liquid limit (LL) and (b) plasticity index (PI) in the
resilient modulus ratio of soil-fly ash mixtures prepared at water
contents at 7% wet of optimum and the very wet condition ........... 85
Fig. 5.11.
Effect of (a) group index (GI) and (b) liquidity index (LI) in
the resilient modulus of soil-fly ash mixtures prepared at 7%
wet of optimum water content and the very wet condition ............. 86
Fig. 5.12.
Resilient modulus of organic Theresa Silt Loam and in
combination with 30% Dewey fly ash at different water content
and cured for 7 days...................................................................... 88
Fig. 6.1.
Stress-strain data of unconfined compressive strength tests
performed on fly ash specimens prepared at 35% water-fly ash
x
ratio for a curing period of 14 days ................................................ 92
Fig. 6.2.
The unconfined compressive strength of soil-fly ash specimens
normalized to the unconfined compressive strength of the
untreated soil compacted at 7% wet of optimum water content..... 97
Fig. 6.3.
Strength gain as a function pf fly ash content for red silt clay
till, lacustrine red clay, and brown silt prepared with Columbia,
Dewey, and King fly ashes compacted at 7% wet of optimum
water content................................................................................. 99
Fig. 6.4.
Unconfined compressive strength prepared with (a) Dewey and
(b) King fly ashes normalized to the unconfined compressive
strength prepared with Columbia fly ash as a function of
fly ash content. (Specimens prepared at 7% wet of
optimum water content) ............................................................... 100
Fig. 6.5.
Effect of soil water content on the unconfined compressive
strength of soil-fly ash specimens prepared with 10 and 18% fly
ash content and cured for 14 days. ............................................. 102
Fig. 6.6.
Effect of fly ash content and curing time on the unconfined
compressive strength of soil-fly ash specimens prepared
with Columbia, Dewey, and King fly ashes at 18% fly
ash content.................................................................................. 104
Fig. 6.7.
Effect of (a) liquid limit (LL) and (b) plasticity index (PI)
on the unconfined compressive strength ratio of soil-fly
ash specimens ............................................................................ 105
Fig. 6.8.
Effect of (a) group index (GI) and (b) liquidity index (LI)
on the unconfined compressive strength ratio of soil-fly
ash specimens ............................................................................ 106
Fig. 6.9.
Effect of fly ash content on unconfined compressive strength
of organic Theresa Silt Loam prepared with Columbia and
Dewey fly ashes in very wet conditions and cured for 7 days...... 107
Fig. 7.1.
Diagram of a typical flexible pavement structure with its four
components................................................................................. 113
xi
LIST OF TABLES
Table 2.1
Chemical requirements for fly ash classification.............................. 8
Table 2.2
Typical chemical composition of fly ash........................................... 8
Table 3.1
Physical properties and chemical properties of fly ash .................. 22
Table 3.2
Factors affecting physical and chemical properties of fly ash........ 23
Table 3.3
Chemical composition of fly ashes ................................................ 25
Table 3.4
Index Properties of the soils .......................................................... 32
Table 3.5
AASHTO T 292-91 (1996) test sequence for cohesive soils ......... 40
Table 4.1
CBR of soil-fly ash mixtures compacted 2-hours after mixing
and cured for 7 days prior to CBR test .......................................... 49
Table 4.2
CBR of soil-fly ash mixtures compacted 2-hours after mixing
and cured for 7 days prior to CBR test .......................................... 50
Table 5.1
Resilient moduli (MPa) of soil-fly ash mixtures compacted 2
hours after mixing at a deviator stress of 21 kPa........................... 71
Table 5.2
Resilient modulus coefficients K1 and K2 for soil and soil-fly
ash mixtures .................................................................................. 72
Table 6.1
Unconfined compressive strength (kPa) of soil and soil-fly ash
mixtures......................................................................................... 94
Table 7.1
Recommended subbase layer coefficients (a3) of soil and
soil-fly ash mixtures from CBR and resilient modulus tests at
different water content ................................................................. 114
Table 7.2
Base course thicknesses calculated using CBR and resilient
modulus subbase layer coefficients (a3) ...................................... 116
1
SECTION 1
INTRODUCTION
Every year burning coal for production of energy in power plants in the
United States produces over 72 Mg of coal ash, including bottom ashes and fly
ashes (Collins and Ciesielski, 1992). Traditionally the fly ash has been disposed
in landfills at considerable cost. However environmental regulations in many
states now promote the reuse of fly ash and many other industrial by products, in
a variety of applications. One application of considerable interest is stabilization
of soft soils for roadway construction.
Soft subgrade soils are a common problem in Wisconsin. The typical
approach for remediating soft subgrade has consisted of removal of poor soil,
and replacement with large quantities of crushed rock, also known as “breaker
run.” The high cost for removal of poor soils and transportation of select
aggregates, along with increasing interest in re-used industrial by products, has
prompted investigations to find solutions that complement the needs of highway
construction with those of the environment. Use of fly ash for stabilization of soft
subgrade is one these solutions being evaluated.
The objective of this study was to determine how the use of different types
of fly ashes could improve the engineering properties of several soft Wisconsin
soils. To achieve this objective, a laboratory-testing program was conducted
where compacted soil-fly ash mixtures were prepared at several fly ash contents,
and then tested to determine their engineering properties relevant to highway
construction. The laboratory program included California Bearing Ratio (CBR),
2
resilient modulus, and unconfined compressive strength tests. Results of the
tests were used to formulate guidelines for engineers designing pavements with
fly ash stabilized soils in Wisconsin.
3
SECTION 2
BACKGROUND
2.1 NEED FOR SUBGRADE STABILIZATION
The distribution of poor subgrade soils in Wisconsin is shown in Fig. 2.1.
Nearly 60% of the surficial soils is classified as poor subgrade, with two-thirds of
which are soft silts in the southern and central regions, and one-third of which is
soft clay in the northern and eastern regions (Edil et al. 2002). Poor subgrade
soils tend to have low shear strength and are highly compressible. As a result,
they must be removed and replaced or stabilized before a highway can be
constructed. One stabilization method is to mix poor subgrade soil with fly ash to
improve the engineering properties. Reactions that occur in the soil-fly ash
mixture result in lower water contents, higher shear strength, and lower
compressibility.
2.2 WHAT IS FLY ASH?
Fly ash is a by-product produced from burning coal in electric power
plants. In 1993, approximately 43 million Mg of fly ash was produced in the
United States (Palmer et al. 1995). A comparison of fly ash particles sizes to
those of several types of soils is presented in Fig. 2.2. Fly ash is a fine residue
composed of unburned particles that solidify while suspended in exhaust gases.
Fly ash is carried off in stack gases from a boiler unit, and is collected by
mechanical methods or electrostatic precipitators. Because it is collected from
4
Poor Subgrade Soil
Good Subgrade Soil
Fig. 2.1.
Poor subgrade soils in Wisconsin (WisDOT 1997).
5
Fig. 2.2.
Comparison of fly ash particles to those of several soils (from
Meyers et al. 1976).
6
exhaust gases, fly ash is composed of fine spherical silt size particles in the
range of 0.074 to 0.005 mm (Ferguson 1993). Fly ash collected using mechanical
precipitators usually has coarser particles than fly ash collected using
electrostatic precipitators.
2.3 TYPES OF COAL
Electrical power generation produces fly ash from four major types of coal:
bituminous, anthracite, sub-bituminous, and lignite. Bituminous and anthracite
coals have low amounts of calcium oxide (CaO) (usually less than 10%). Subbituminous and lignite coals have higher amounts of calcium oxide (CaO), in the
range of 20% or more (Meyers et al. 1976, Ferguson 1993, ACAA 1995).
Bituminous and anthracite coals are usually found in the eastern United States.
Sub-bituminous and lignite coals are usually found in the western United States.
Fly ashes produced from bituminous and sub-bituminous coals can be
used in several civil engineering applications. One common application is an
admixture to Portland cement to increase workability, strength, and reduce heat
of hydration of concrete. Fly ash can be used in combination with lime, or by itself
for soil stabilization of road base and subbases to increase the bearing capacity
of soil. Fly ash is also combined with water, Portland cement, and sand to
produce flowable fills that flow like liquid and set up like a solid. Other fly ash
applications that have been reported include use in grouts, fast-track concrete
pavements, and as structural fills and backfills (ACAA 1995).
7
2.4 TYPES OF FLY ASH
ASTM C 618-99 (AASTO M 295) provides the classification requirements
for fly ash. The chemical requirements for classification and typical chemical
compositions are summarized in Tables 2.1 and 2.2. There are two types of
ashes: C and F. Class F fly ash is produced from bituminous coals and does not
have self-cementing properties due to its low calcium oxide (CaO) content (Table
2.2). Class F fly ashes are usually mixed with an activator such as lime or
Portland cement to generate self-cementing properties. Class C fly ash is
produced from sub-bituminous coals, and exhibits self-cementing properties.
When exposed to water, Class C ashes form cementitiuos products similar to
those produced during hydration of Portland cement. As a result, Class C fly ash
is convenient for soil stabilization (Ferguson 1993).
2.5
FACTORS AFFECTING PHYSICAL AND CHEMICAL PROPERTIES OF
FLY ASH
Variability of the chemical and physical properties of fly ash depends on
several factors such as coal type and source, type of boiler, conditions during
combustion, type of emission control devices, and storage and handling methods
(Toth et al. 1988). Changes in any of these factors affect the characteristics of
the fly ash, and its engineering properties.
Coal type is one of the factors having the greatest effect on the fly ash
characteristics. Coal source affects the calcium oxide (CaO) content, also
eastern ashes typically have considerably higher sulfur (S) contents compared to
8
Table 2.1. Chemical requirements for fly ash classification.
Class of Fly Ash
Chemical Requirements
F*
C*
Silicon Dioxide (SiO2) plus Aluminum Oxide
(Al2O3) plus Iron Oxide (Fe2O3), min (%)
70
50
Sulfur Trioxide (SO3), max (%)
5
5
Moisture Content, max (%)
3
3
Loss on Ignition, max (%)
6
6
*After ASTM Standard C 618-99
Table 2.2. Typical chemical composition of fly ash.
Class of Fly Ash
Compounds
F*
C*
SiO2
54.9
39.9
Al2O3
25.8
16.7
Fe2O3
6.9
5.8
CaO (Lime)
8.7
24.3
MgO
1.8
4.6
SO3
0.6
3.3
*After Ferguson et al. (1999)
9
western fly ashes, (Adriano and Weber 2001). In general as the amounts SiO2,
Al2O3 and free lime (CaO) increases, the pozzolanic activity of the fly ash
increases (Meyers et al. 1976).
The chemical composition of fly ash influences its color. The color of fly
ash ranges from light brown or cream to dark brown or gray. Fly ashes with
lighter color have higher calcium oxide content, whereas fly ashes with darker
colors have higher carbon content (Meyers et al. 1976).
The specific gravity of fly ashes varies depending on the coal source.
Specific gravities typically range from of 2.11 to 2.71 (Chu and Kao 1993).
The compaction characteristics of fly ash are similar to those of cohesive
soils. A typical compaction curve is obtained, but the curve is flatter relative to the
typical bell shape curve of most cohesive soils. Fly ashes with higher carbon
contents and less calcium oxide generally have a flatter compaction curve, lower
dry unit weight, and higher optimum water content. The maximum dry unit weight
for standard Proctor generally ranges from 8 to 17 kN/m3, and optimum water
contents range of 15 to 35%. Both of these properties vary with composition of
the fly ash (Meyers et al. 1976).
There are three main categories of boilers in which coal is combusted:
pulverized coal-fired units, stoker-fired units, and cyclone furnaces. Pulverized
coal-fired units are the most common (ACAA 1995). The combustion temperature
inside the boiler affects the degree to which many minerals elements in the coal
may volatilize (Adriano and Weber 2001). The mineralogy and crystallinity of the
fly ash is controlled by the boiler design and operation, since the boiler controls
10
the rate at which the fused matter is cooled. Boiler type accounts for the
differences in the ash chemistry; this difference can be observed in fly ashes
from different sources (Ferguson et al. 1999).
Collection method affects fineness of the ash. Fly ashes collected
mechanically are typically coarser, and have fineness in the order of 1700
cm2/gm. Fly ashes collected using electrostatic precipitators are finer, having
fines of approximately 6400 cm2/gm. The fineness and gradation of the fly ash
are influenced primarily by the degree of pulverization of the coal. Fineness is
important because it affect the pozzolanic activity, and the workability when the
fly ash is used in Portland cement. As the fineness of the fly ash increases, the
pozzolanic activity also increases (Meyers et al. 1976). A fineness specification
for concrete typically requires at least 66% of the ash to pass 325 sieve (ACAA
1995). Carbon content in the fly ash is also controlled by the fineness of the coal,
and the efficiency of the boiler unit. In general, old boilers are less efficient and
produce fly ashes with higher carbon contents (Meyers et al. 1976).
There are two primary techniques for handling fly ash, dry and wet
methods. Dry methods usually involve short-term storage of the fly ash. The fly
ash is stored in hoppers or silos, and is discharged through gates or doors into
trucks or rail cars for distribution. The wet method involves addition of water to
the fly ash to form slurry that is pumped to settling ponds or lagoons. Slurry
disposal affects the compaction curve significantly. The compaction curve
becomes flat; there is no variation in dry unit weight over a broad water content
(Meyers et al. 1976).
11
2.6 STABILIZATION EFFECTS ON CLAYS
Fly ash has been used extensively for stabilization of high plasticity clays
(Torrey 1978, Lamb 1985, Ferguson 1993, Turner 1997). Even though the
calcium oxide content of fly ash typically is not as high as in lime, fly ash often
stabilizes high plasticity clays as well as lime. One of the major benefits of using
self-cementing coal fly ash for soil stabilization in lieu of lime is the bond formed
between the soil grains and cementitiuos products in the fly ash when it is mixed
with water (Ferguson et al. 1999).
There are three primary mechanisms that contribute to stabilization when
soil is mixed with fly ash. The strength of the soil increases due to the
cementation resulting from hydration of tricalcium aluminate present in the fly
ash. Free lime (CaO) in the fly ash also reacts with the clay minerals, causing
compression of the absorbed layer and a corresponding reduction in plasticity.
Free lime not reacting with the clay minerals is available for additional
cementation through pozzolanic reaction with silica and alumina compounds. The
pozzolanic reaction is the primary factor responsible for the long-term stability of
soil fly ash mixtures (Turner 1997).
12
2.7 REVIEW OF RECENT LIME AND FLY ASH STABILIZATION RESEARCH
2.7.1 Lime Treatment
Prakash et al. (1989) evaluated the behavior of a montmorillonitic soil
treated with different percentages of lime (2 to 12%) and cured for different
periods (0 to 60 days). Three tests were conducted on the mixture: liquid limit
test, shrinkage limit test, and standard Proctor compaction test. The initial effect
of lime treatment was an appreciable decrease in the liquid limit with increasing
lime content. However, over time the liquid limit was increased with an increase
in lime content due water entrapment between large void spaces caused by
flocculation of the soil fabric. Lime treatment produced an immediate increase in
the shrinkage limit, and the effect on the shrinkage limit was a more pronounced
as time increased. For compaction, the maximum dry unit weight decreased with
lime content, and curing time. Optimum water content increased with lime
content, and decreased with curing time.
Robnett and Marshall (1976) studied the effects of lime treatment and
freeze-thaw on the resilient characteristics of several fine-grained subgrade soils.
Specimens were treated with 5% lime and compacted at dry and wet of optimum
water contents. The immediate response was a 27-34% increase in modulus
compared to untreated specimens. The resilient modulus of untreated specimens
ranged from 80 to 90 MPa at low deviator stress, whereas the resilient modulus
for uncured treated specimens ranged from 120 to 140 MPa.
Freeze-thaw was found to have a detrimental effect on the subgrade
moduli. The resilient modulus of untreated specimens ranged between 20 to 41
13
MPa after one freeze-thaw cycle. In contrast, uncured compacted soil-lime
specimens had moduli ranging from 96 to 140 MPa, even after 10 freeze-thaw
cycles. Specimens treated with 5% lime and cured for a period of 20 days had
even higher moduli (124 to 165 MPa) after 10 freeze-thaw cycles.
2.7.2 Lime-Fly Ash Treatment
Nalbantoglu and Tuncer (2001) and Nalbantoglu and Gucbilmez (2002)
evaluated swell potential, compressibility, hydraulic conductivity, and cation
exchange capacity (CEC) of an expansive clay from Cyprus that was chemically
treated with fly ash and lime. A Class C fly ash with a calcium oxide (CaO)
content of 15% was used.
Swell testing showed that the same treatment level could be achieved with
either lime or fly ash, at different treatment percentages. Swelling of the
expansive clay decreased with increasing amounts of lime or fly ash, and also
with increasing curing time. A decrease in compressibility of the compacted
treated soil was also observed, but the initial void ratio increased with increasing
lime and fly ash content. The pre-consolidation pressure was found to increase
with increasing lime and fly ash content. A combination of 3% lime and 15% fly
ash yielded approximately the same pre-consolidation pressure as was obtained
using only 7% lime.
The hydraulic conductivity of compacted treated soil increased with
increasing lime and fly ash content, and with increasing curing period. The CEC
14
was reduced as the lime and fly ash content increased. The largest reduction in
CEC was obtained using a combination of 15% fly ash and 3% lime.
Nicholson and Kashyap (1993) and Nicholson et al. (1994) evaluated how
fly ash and lime affect the engineering properties of tropical soils from Hawaii.
The fly ash did not meet the requirements for Class C or F fly ash, and contained
19% calcium oxide (CaO). Tests that were conducted included Atterberg limits,
compaction, California Bearing Ratio (CBR), free swell, and unconfined
compression.
Soils with a high liquid limit that were treated with 15% fly ash showed a
sharp decrease in liquid limit when fly ash was added. An additional small
decrease in liquid limit was observed when the fly ash content was increased to
25% fly ash. The reduction in plasticity index was as large as 50% for smectitic
clays. For the other clays the reduction in liquid limit and plasticity index was
more gradual with the addition of fly ash.
For all soils, compaction testing showed that the maximum dry unit weight
decreases and optimum water content increases with increasing fly ash content.
Swell potential was reduced for all soils when fly ash was added. The reduction
in swell potential was as large as 25% for the smectitic clays.
Little improvement in CBR was found when the soils were prepared with
up to 25% fly ash. However, for several soils prepared with 15% fly ash and 3%
lime, a CBR of 23 was obtained, which is appreciably larger than the CBR of 4
obtained for untreated soil.
15
Unconfined compression testing of the soil-fly ash mixtures showed that
the compressive strength depends on the fly ash content and the length of the
curing period. The stabilization effect also varied considerably with soil type. The
unconfined compressive strength ranged between 300 to 1993 kPa for soil fly
ash mixture with 25% fly ash at 28 days. Combinations of 15% fly ash and 3%
lime yield unconfined compressive strengths ranging between 470 to 1951 kPa at
28 days. Unconfined compression with 7% lime and no fly ash at 28 days yielded
compressive strengths between 1110 and 2306 kPa.
2.7.3 Fly Ash Treatment
Ferguson (1993) evaluated the effectiveness of self-cementing coal ashes
from Kansas City for several geotechnical applications such as water content
reduction by addition of solids, shrink-swell reduction, and stabilization for
improvement of engineering properties such as bearing capacity, and unconfined
compressive strength.
Using fly ash as a drying agent showed that water contents could be
reduced by 10 to 20% during construction and that compaction of soil fly ash
mixtures could be performed within an hour or less.
Laboratory tests were conducted on soils of three different textures, a
shale clay, a glacial clay, and a fat clay. Stabilization was performed with
different percentages of Class C fly ash that had a calcium oxide (CaO) content
between 28.0 to 33.0%. Evaluation of the shrink-swell potential showed that the
amount of fly ash required for stabilization depends on the properties of the fly
16
ash and the type of soil. Swell potentials between 0.8 and 2.4% were obtained
using 16% fly ash, as compare to untreated specimens, which had swell
potentials between 8.9-14.7%. CBR tests showed that using 16% fly ash could
increase the CBR from 2-5 to 19-34, respectively.
Compaction and unconfined compression strength tests showed that the
dry unit weight decreases, optimum water content increases, and the unconfined
compressive strength decreases when stabilized soils are compacted two hours
after mixing rather than immediately. The unconfined compressive strength for a
16% soil fly ash mixture after 7 days showed that the strength reached 780, 790,
and 820 kPa, as compared to the untreated specimens with unconfined
compression strengths of 260, 190, and 240 kPa, respectively.
Turner (1997) conducted a laboratory study to evaluate the effectiveness
of using low sulfur western coal fly ashes from Wyoming for stabilization of
subgrade soils. Tests were performed to evaluate improvements in unconfined
compressive strength, resilient modulus, and wet-dry and freeze-thaw durability.
Five low plasticity clayey soils were used, along with one off-specification fly ash,
four Class C, and two Class F fly ashes. Unconfined compressive strengths of
the compacted soil-fly ash mixtures ranged between 380 to 780 MPa when
mixtures were prepared with Class C fly ash and cured for a period of 7 to 28
days. Resilient modulus of the fly ash treated soils ranged between 834 to 6237
MPa, whereas the untreated soils failed during the pre-conditioning stage of the
test. Wet-dry and freeze-thaw durability tests showed that compacted soil fly ash
mixtures exhibited a cement loss of more than 14%.
17
Toth et al. (1988) evaluated Canadian Class F fly ash and bottom ash as
an alternative to natural materials for the construction of structural fills. Several
case studies of embankments projects constructed with fly ash and bottom ash
were monitored for several years to see how fly ash performs as a structural fill,
and also to evaluate how leachate from the ash affects groundwater quality.
Monitoring data has shown that the ash embankments behave similar to
embankments constructed with soils. The fly ash and bottom ash were
considered to be similar based on ease of compaction during construction, and
the amount of settlement of the fill and underlying soil.
One of the case studies involved a fly ash embankment 1.2 m high was
constructed using a Class F fly ash brought to the construction site with a
moisture content in the range of 16 to 18%. During compaction operations, the
dry unit weight and water content were monitored to evaluate the effect of
number of passes on the dry unit weight of the fly ash. Average dry unit weights
of 10 to 12 kN/m3 were achieved with 2 passes of the equipment over the fly ash.
A second case study described the use of Class F fly ash and bottom ash
of two bridge approaches that were 8 m high. Construction of the embankments
was performed using a lower layer of bottom ash, a fly ash core, and an upper
outside layer of bottom ash. Up to 2.45 mm of settlement was measured within
the 8 m high embankment. Settlements of an underlying clay layer were 295 mm
after 10 months monitoring.
A third case study described an open area (20 ha) where an embankment
of fly ash and bottom ash fill 12 m high was placed for agricultural purposes. The
18
landowner wanted to construct an industrial building partially located over the fly
ash fill and partially over the native soil. A geotechnical investigation showed that
N values ranging from 10 to 55 for the fly ash, and up to 75 for the bottom ash.
Consolidated undrained triaxial tests yielded effective friction angles of 35 to 36o.
To control settlements, the fly ash and bottom ash were excavated and
recompacted to 100% of standard Proctor maximum density. After construction,
settlements were monitored for two foundations constructed over the ashes,
settlements of 1.14 and 1.62 mm were recorded over a 10-month period.
An environmental case study is described were a Class F fly ash fill (17 m
high) constructed over an abandoned area. An extensive hydrogeology study
was performed at the site to evaluate the impact of the fly ash on the
groundwater quality. A silt layer with a thickness of 5 m underlay the fly ash. A 3
to 4 m thick layer of clayey silt was beneath the upper silt layer. Concentrations
of calcium, sulphate, potassium, and boron were reported. Leachate from the ash
had sulphate and calcium concentrations of 1230 and 485 mg/L, and potassium
and boron concentrations of 24 and 5.4 mg/L. The sulphate concentrations
exceeded the maximum concentration stipulated in the Guidelines for Canadian
Drinking Water Quality (GCDWQ). Concentrations of trace elements and metals
in the leachate complied with requirements in the GCDWQ.
Lee and Fishman (1992) evaluated two fine-grained industrial by products
for potential use in pavement construction. One by-product was a Class F fly ash,
and the other a fine-grained residue from processing aggregates. The Class F fly
ash was non-plastic and had a fines content of 76%. The residue was clayey,
19
and had a liquid limit of 31 and plasticity index of 14. Resilient modulus,
unconfined compression, and CBR tests were conducted on the materials alone,
and mixtures of the two materials.
Resilient modulus of the residue was found to range between 17 to 43
MPa for a deviator stress of 100 kPa, which corresponds to a soft to medium stiff
subgrade (Asphalt Institute 1982). Resilient modulus of the fly ash was
comparable to that of a granular material, with dependency on bulk stress.
Resilient modulus of the fly ash ranged from 11 to 21 MPa for bulk stresses
ranging from 21 to 138 kPa, which corresponds to very poor subgrade (Asphalt
Institute 1982). The resilient modulus of the compacted mixture of residue and fly
ash (10:3 by weight) ranged between 31 to 48 MPa for bulk stresses ranging
from 21 to 138 kPa, which corresponds to good subgrade. Unconfined
compression strength of the mixtures was approximately 200 kPa for specimens
cured for 7 days and compacted wet of optimum water content. CBRs of the
residue, fly ash, and the mixture (10:3) were reported as 2, 7, and 13 for
compaction at optimum water content.
Edil et al. (2002) conducted a field evaluation of several alternatives for
construction over soft subgrade soils. The field evaluation was performed along a
1.4 km segment of Wisconsin State Highway 60 and consisted of several test
sections. By products such as fly ash, bottom ash, foundry slag, and foundry
sand were used. A Class C fly ash was used for one test section. Unconfined
compression testing showed that 10% fly ash (on the basis of dry weight) was
sufficient to provide the strength necessary for construction on the subgrade.
20
Data were obtained before and after fly ash placement by testing undisturbed
samples in the laboratory and by using a soil stiffness gauge (SSG) and a
dynamic cone penetrometer (DCP) in the field.
Unconfined compressive strength, soil stiffness, and dynamic cone
penetration of the native soil before fly ash placement ranged between 100 to
150 kPa, 4 to 8 MN/m, and 30 to 90 mm/blow, respectively. After fly ash addition,
the unconfined compressive strength reached as high as 540 kPa, the stiffness
ranged from 10 to 18 MN/m, and the DPI was less variable and ranged between
10 and 20 mm/blow. A CBR of 32 was reported for the stabilized subgrade,
which is rated as “good” for subbase highway construction. CBR of the untreated
subgrade was 3, which is rated as “very poor” (Bowles 1992).
21
SECTION 3
MATERIALS AND METHODS
3.1 FLY ASHES
3.1.1 Sources and Composition
Four different fly ashes were used in this study: Columbia, Edgewater,
Dewey, and King. Columbia fly ash is from Unit-2 of the Columbia Power Plant in
Portage, Wisconsin. Edgewater fly ash is from Unit-5 of the Sheboygan Power
Plant in Sheboygan, Wisconsin. Dewey fly ash is from the Nelson Dewey Power
Plant in Cassville, Wisconsin. King fly ash is from the Allen S. King Plant in
Bayport, Minnesota. The Columbia, Edgewater, and Nelson Dewey Plants are
operated by Alliant Energy. The King plant is operated by Xcel Energy. These fly
ashes collectively provide a wide geographical area coverage as a source and
also represent a wide range characteristics of fly ashes available in Wisconsin.
Physical properties of the fly ashes are summarized in Table 3.1 along
with typical physical properties of Class C and F fly ash. Columbia, Edgewater,
and King fly ashes have a powdery texture. Dewey fly ash has a more granular
texture. Both Columbia and Edgewater are light brown in color, which indicates
these fly ashes have higher calcium oxide content (Meyers et al. 1976). The
Dewey and King fly ashes are dark gray and dark brown in color, which indicates
higher amounts of carbon (Meyers et al. 1976). Factors affecting physical and
chemical properties of fly ashes are shown in Table 3.2. Some of these factors
are fineness, coal type and source, collection method, storage method, and type
22
Table 3.1. Physical properties and chemical composition of fly ashes.
Cu
Percent
Fines
Moisture
Content
(%)
LOI
(%)
Lime
(CaO)
(%)
Other Oxides
(SiO2 + Al2O3
+ FeO3)
(%)
Sulfur
Trioxide
(SO3)
(%)
-
-
-
3
6
24.3*
50
5
-
-
-
-
3
6
8.7*
70
5
Columbia
C
2.70
9
95.3
0.09
0.7
23
55.5
3.7
Dewey
Off-Spec
2.53
103
39.6
0.23
16.2
9.8
38.7
11.8
King
Off-Spec
2.68
11
91.9
0.44
5.4
23.7
49.5
6.4
Edgewater
C
2.71
14
92.8
0.03
0.1
20.8
62.3
1.0
Fly Ash
Classification
(ASTM C 618)
Gs
Class C**
-
Class F**
Notes: Gs = Specific gravity, LOI = Loss on ignition, Cu = Coefficient of uniformity. Minimum and maximum
percentages for fly ash classification refer to Table. 2.1.
*After Ferguson et al. (1999),
**After ASTM Standard C 618-99.
23
Table 3.2. Factors affecting physical and chemical properties of fly ash.
Properties
Columbia
Dewey
King
Edgewater
Fineness (%)
14.4
12.7
10.4
24.8
Pozzolanic
Activity at 7
days (%)
95.8
82.7
77.7
71.7
SubBituminous
SubBituminous
SubBituminous
SubBituminous
Wyoming
PRB with
Colorado or
Petroleum
Coke
80% Montana
Coal with
Colorado or
Petroleum
Coke†
30% Montana
PRB
60% Wyoming
PRB
10% Petroleum
Coke
Wyoming
PRB with
Colorado or
Petroleum
Coke
Type
of
Coal
and
Source
Collection
Electrostatic
Electrostatic
Electrostatic
Electrostatic
Method
Storage
Dry Silo
Dry Silo
Dry Silo
Dry Silo
Type
Type of
Pulverized
Cyclone
Cyclone
Pulverized
Boiler
Notes: PRB = Powder River Basin coal, †Percentage remaining varies
throughout the year, information source: Randy Polleck (Alliant
Energy) and Michael Thomes (Xcel Energy).
24
of boilers used. Chemical composition of the fly ashes is summarized in Table
3.3, along with typical compositions of Class C and F fly ashes.
The Columbia and Edgewater fly ashes are classified as Class C fly ashes
following ASTM C 618. These fly ashes have high calcium oxide (CaO) content
(23.0 and 20.8%, respectively) and exhibit self-cementing characteristics. Dewey
fly ash classifies as off-specification fly ash, has a calcium oxide content of 9.8%,
and an organic content of 16.2%. Dewey fly ash is “off-specification“ because the
SiO2 + Al2O3 + Fe2O3 content is below 50%, the sulfur trioxide (SO3) content is
above 5%, and the loss on ignition exceeds 6%. King fly ash also classifies as
off-specification fly ash because the SiO2 + Al2O3 + Fe2O3 content is less than
50%, and the sulfur trioxide (SO3) content exceeds 5%. King fly ash has 24%
calcium oxide, and is close to a Class C fly ash. Dewey fly ash is closer to a
Class F fly ash.
The silicon dioxide (SiO2) contents for the Columbia, Dewey, and King fly
ashes are below the typical amounts for Class C fly ash. Only Edgewater fly ash
has a SiO2 content (39%) close to typical Class C fly ash. All of the fly ashes
have Al2O3 and Fe2O3 contents characteristic of Class C fly ashes, and the
magnesium oxide (MgO) contents for Columbia, Dewey, and Edgewater are
close to typical Class C fly ash. The MgO content of King fly ash is closer to that
of a typical Class F fly ash. The sulfur trioxide (SO3) content is higher for both
Dewey and King fly ashes (11.8% and 6.4%), compared to typical SO3 contents
for Class C and Class F ashes (3.3% and 0.6%).
25
Table 3.3 Chemical compositions of fly ashes.
Chemical
Compounds Columbia
Percent of Composition
Dewey
King
Edgewater
Typical*
Class C
Typical*
Class F
CaO (Lime)
23.1
9.8
23.7
20.8
24.3
8.7
SiO2
31.1
19.8
27.3
38.7
39.9
54.9
Al2O3
18.3
13.0
16.3
15.8
16.7
25.8
Fe2O3
6.1
6.0
5.9
7.8
5.8
6.9
MgO
3.7
3.1
1.8
3.4
4.6
1.8
SO3
3.7
11.8
6.4
1.0
3.3
0.6
*After Ferguson et al. (1999)
26
100
Percent Passing (%)
80
60
40
Columbia
Dewey
King
Edgewater
20
0
Fig. 3.1.
10
1
0.1
0.01
Particle Size (mm)
0.001
0.0001
Particle size distribution curves for Dewey, Columbia, King, and
Edgewater fly ashes.
27
3.1.2 Index Properties and Compaction Characteristics
Specific gravities and the percent fines for the fly ashes are summarized in
Table 3.1. The specific gravity of the Dewey fly ash is low relative to the other
three fly ashes, because it has higher carbon content. For the other fly ashes, the
specific gravity ranged from 2.68 to 2.71. The percent fines for Dewey fly ash is
also lower (39.4%); the other fly ashes have at least 90% fines.
Particle size distributions for the fly ashes are shown in Fig. 3.1. Columbia
fly ash is slightly finer than the other ashes. The King and Edgewater fly ashes
have similar particle size distributions. The Dewey fly ash is appreciably coarser
than the other fly ashes, and is also gap graded. The Columbia, Edgewater, and
King fly ashes have particles ranging from fine sand to silt and clay size, whereas
Dewey fly ash has particles as large as medium sand and as small as clay. The
unusual characteristics of the Dewey ash are also reflected in the coefficients of
uniformity (Cu), which are summarized in Table 3.1. The Cu for Columbia,
Edgewater, and King are similar, (9, 11, and 14), where as the Cu for the Dewey
fly ash is 103.
Compaction characteristics of the fly ashes using the standard Proctor
compaction procedure (ASTM D 698) are shown in Fig. 3.2. The compaction
curves are flat relative to the typical bell-shaped curves of fine-grained soils. The
variation in dry unit weight between the fly ashes is partly due to variations in
organic content. The Columbia and Edgewater fly ashes have the lowest organic
content, and the highest dry unit weight. Dewey fly ash has the highest organic
content, and the lowest dry unit weight. Furthermore, it has a coarser texture.
28
18
Columbia
Dewey
King
Edgewater
3
Dry Unit Weight (kN/m )
16
14
12
10
8
6
0
10
20
30
40
50
60
70
Water Content (%)
Fig. 3.2.
Compaction curves for Dewey, Columbia, King, and Edgewater fly
ashes (Compacted immediately after adding water).
29
King fly ash falls in the middle in terms of optimum water content, dry unit weight,
and organic content.
3.2 WISCONSIN SUBGRADE SOILS
3.2.1 Sources
Seven subgrade soils were considered for the testing program. The
locations where the soils were collected are shown in Fig. 3.3. These locations
were identified through discussions the Wisconsin Department of Transportation,
chief Geotechnical Engineer and are intended to represent the range of soft
subgrades typically encountered in Wisconsin. Samples of each soil were
collected along the highway shoulder, at a depth of 0.6 m to 0.9 m.
3.2.2 Soil Index Properties
Index and compaction properties and classifications of the soils are
summarized in Table 3.4. All of the soils are fine-grained and classify as clays
according to the Unified Soil Classification System. One of the soils (organic
Theresa silt loam) is a highly plastic organic clay (LOI = 10%). The Theresa silt
loam and the red silty clay till are low plasticity clays. The Brown Silt and
Lacustrine Red Clay are high plasticity clays. The inorganic clays have LOI less
than 4%.
Particle size distributions for the soils are presented in Fig. 3.4. Lacustrine
Red Clay is finer than the other soils. The Brown Silt, Red Silty Clay Till, and
30
Theresa Silt Loam have similar particle size distributions. All of the soils contain
at least 90% fines, all but the red silty clay till which has 70% fines.
Natural water contents of the soils are presented in Table 3.4. All natural
water contents are above optimum water content. On average the natural water
content is 7% wet of optimum water content, except one soil which has a water
content near its optimum water content. Thus, when the specimens were
prepared to simulate the natural wet condition observed in the field, they were
prepared approximately 7% wet of optimum water content.
3.2.3 Compaction Characteristics and California Bearing Ratio
Compaction curves corresponding to standard Proctor effort were
determined for each soil following the procedure in ASTM D 698. Typical bellshaped compaction curves were obtained (Fig. 3.5). The maximum dry unit
weights and the optimum water contents are summarized in Table 3.4. Organic
Theresa silt loam has the highest optimum water content (29%) and the lowest
dry unit weight (13.5 kN/m3), because of its high organic content. Red silty clay
till has the highest dry unit weight (18.4 kN/m3), and the lowest optimum water
content (13%), which reflects the larger fraction of coarse particles in the till.
California Bearing Ratio (CBR) tests were conducted on each soil following the
methods described in ASTM D 1883-87. The CBRs are summarized in Table 3.4.
The CBR of each soil was measured on a specimen prepared 7% wet of
optimum water content using standard Proctor effort. The CBRs range from 0 to
5, which implies the soils classify as very poor to fair subgrades (Bowles 1992).
31
Lacustrine Red Clay
Red Silty
Clay Till
Organic
Theresa
Silt Loam
and
Theresa
Silt Loam
Brown Silt
Plano Silt Loam
Fig. 3.3.
Locations where soils were sampled.
Joy Silt Loam
32
Table 3.4. Index properties of the soils.
Soil Name
Organic Theresa
Silt Loam
Theresa Silt Loam
Brown Silt
Lacustrine Red
Clay
Red Silty Clay Till
Joy Silt Loam
Sampling
Location
STH 28
Mayville, WI
LL
PI
Percent
Fines
Gs
LOI
(%)
Classification
USCS
AASHTO
CBR
wN
(%)
gd
(kN/m3)
wOPT
(%)
61
19
97
2.24
10
OH
A-7-5
0.3
35
13.5
29
45
19
99
2.58
2
CL
A-7-6
3
19
15.9
18
USH 151
Platteville,
WI
STH 13
Cloverland,
WI
STH 54
Luxemburg,
WI
60
35
97
2.58
4
CH
A-7-6
0.4
32
16.4
19
69
38
97
2.71
2
CH
A-7-6
2
35
15.7
24
47
22
71
2.69
2
CL
A-6
5
19
18.4
13
STH 60 Lodi,
WI
39
15
96
2.70
1
ML
A-6
3
25
16.5
19
Scenic Edge
44 20
96
2.71
2
CL
A-7-6
1
27
16.2
20
in Cross
Plains, WI
Notes: LL = Liquid limit, PI = Plasticity index, Percent Fines = percentage passing No. 200 sieve, Gs = Specific gravity,
LOI = Loss on ignition, CBR = California Bearing Ratio (performed approximately 7% wet of optimum water content), wN
= Natural water content, gd = Maximum dry unit weight, wOPT = Optimum water content.
Plano Silt Loam
33
Percent Passing (%)
100
80
60
Red Silty Clay Till
40
Lacustrine Red Clay
Organic Theresa Silt Loam
Theresa Silt Loam
20
Brown Silt
Joy Silt Loam
Plano Silt Loam
0
10
1
0.1
0.01
0.001
0.0001
Particle Size (mm)
Fig. 3.4.
Particle size distributions for the Wisconsin soils.
34
Red Silty Clay Till
Lacustrine Red Clay
Organic Theresa Silt loam
19
Theresa Silt Loam
Brown Silt
Joy Silt Loam
3
Dry Unit Weight (kN/m )
18
Plano Silt Loam
17
16
15
14
13
12
0
5
10
15
20
25
30
35
40
Water Content (%)
Fig. 3.5.
Compaction curves for the Wisconsin soils.
35
3.3 TEST PROCEDURES
3.3.1 CBR Test
Specimens for CBR testing were prepared in accordance with ASTM D
3668. A typical soil fly-ash specimen undergoing a CBR test is shown in Fig. 3.6.
Specimens were prepared 7% wet of optimum water content using standard
Proctor effort to simulate the wet and soft condition typically observed when the
soils were collected in the field. Additional specimens were prepared at optimum
water content, as well as 9 to 18% wet of optimum water content to simulate a
very wet and soft subgrade. For the soil fly-ash mixtures, specimens were left in
the mold, sealed using a plastic wrap, and left to cure for 7 days at 25oC and
100% relative humidity prior to testing.
3.3.2 Resilient Modulus Test
Fig. 3.7 shows a soil-fly ash specimen being compacted inside a mold.
Specimens for resilient modulus testing were prepared using the same
compactive effort as specimens prepared using the standard Proctor procedure.
The effort was matched by adjusting the numbers of blows per layer so that the
same energy per volume was delivered (600 kN/m3). The mold used to prepare
the resilient modulus specimens had a diameter of 102 mm, height of 203 mm,
and volume of 1.65 L. Specimens were compacted in the mold in 6 layers with 22
blows per layer using a standard Proctor hammer.
After compaction, the specimens were extruded. Soil specimens were
tested shortly after compaction. Specimens prepared with soil-fly ash mixtures
36
Fig. 3.6.
California Bearing Ratio test being conducted on soil-fly ash
mixture.
37
Fig. 3.7.
Resilient modulus soil-fly ash specimen being compacted inside a
mold.
38
were sealed with plastic wrap and cured at 25oC and 100% humidity.
Most specimens were cured for 14 days. However some specimens were cured
for as long as 56 days to understand how the resilient modulus changes over
time as the fly ash cures.
The procedure described in AASHTO T 292-91 (1996) was followed for
the resilient modulus test. A photograph of the resilient modulus cell in the
loading frame is shown in Fig. 3.8. The loading sequence for cohesive soils was
used, as is summarized in Table 3.5.
Before testing began, several tests were conducted using synthetic
specimens to assess the repeatability of the procedure. The synthetic specimens
(A and B) had different aspect ratios (1:1 and 2:1), and were made out of
different materials, but had similar density (1.18 Mg/m3 and 1.21 Mg/m3), as
shown in Fig. 3.9. The resilient moduli for Specimen B are in the range of 10
MPa. For Specimen A, the resilient moduli range from 140 MPa to 160 MPa, with
higher moduli obtained with increasing deviator stress. In general, similar moduli
were measured in each replicate test. Except for the one measurement of
Specimen B during the second replication, the resilient moduli differ by no more
than 2% at a given deviator stress.
39
Load Cell
for applied
deviator
stress (Dd)
Linear
Variable
Displacement
Transducers
Compacted
soil-fly ash
specimen
encased in
latex
membrane
Line for
confining
pressure (s3)
Fig. 3.8.
Specimen undergoing resilient modulus test.
40
Table 3.5. AASHTO T 292-91 (1996) test sequence for cohesive soils.
Phase
Sequence
Number
Deviator Stress
(kPa)
Number of Repetitions
Specimen
Conditioning
0
41
1000
1
21
50
2
34
50
3
48
50
4
69
50
5
103
50
Testing
Note: A confining pressure of 21 kPa and a seating load of 13.8 kPa were used.
41
Resilient Modulus (MPa)
200
150
A
100
A-First Test
A-Second Test
B-First Test
B-Second Test
B-Third Test
50
B
0
0
20
40
60
80
100
Deviator Stress (kPa)
Fig. 3.9.
Resilient modulus of synthetic Specimens A and B.
42
3.3.3 Unconfined Compressive Test
Unconfined compression tests were conducted following the procedure in
ASTM D 5102. A photograph of a soil-fly ash specimen being subjected to
unconfined compression is shown in Fig. 3.10. ASTM D 5102 recommends that
the strain rate should be between 0.5% and 2%/min. However, a strain rate of
0.21%/min was used because the soil-fly ash mixtures were expected to be
stiffer than typical cohesive soils. This reduction in strain rate is consistent with
Note 7 in ASTM D 5102, which suggests that stiffer specimens be tested at lower
strain rates. Unconfined compression tests were performed on the same
specimen used for the resilient modulus test. The specimen was removed from
the resilient modulus cell, and then subjected to unconfined compression in a
load frame.
Additional unconfined compressive tests were conducted on small
specimens prepared using Harvard compaction equipment following the
procedure in ASTM 4609. The Harvard mold is 33 mm in diameter and 72 mm in
height. To achieve approximately the same dry unit weight as in a standard
Proctor test, a trial-and-error procedure was followed to find the appropriate
number of layers and tamps per layer. Testing showed that 3 layers with 25
tamps/layer were adequate to achieve approximately the same dry unit weight as
obtained with the standard Proctor effort. The similarity of the compaction curves
obtained with the standard Proctor and Harvard compaction methods is shown in
Fig. 3.11 for organic Theresa silt loam, Theresa silt loam, and silt brown clay
soils. The differences in maximum dry unit weight and optimum water content are
43
Fig. 3.10.
Unconfined compression test being conducted on soil-fly ash
mixture.
44
Proctor-Organic Theresa Silt Loam
Harvard-Organic Theresa Silt Loam
Proctor-Theresa Silt Loam
Harvard-Theresa Silt Loam
17
Proctor- Brown Silt
16
3
Dry Unit Weight (kN/m )
Harvard-Brown Silt
15
14
13
12
10
15
20
25
30
35
40
Water Content (%)
Fig. 3.11.
Comparison
methods.
between
Proctor
and
Harvard
compaction
45
in the range of 0.9 kN/m3 and 3%.
After compaction the specimens were extruded, sealed in plastic, and
stored in 100% humidity for curing for 7 days. After the 7-day curing period, the
specimens were tested in unconfined compression at a strain rate of 0.15
mm/min.
46
SECTION 4
CALIFORNIA BEARING RATIO (CBR)
CBR is a measure of strength gain due to fly ash stabilization and is a
property that is readily recognized by pavement design engineers. To
characterize the CBR of fly ash stabilized soils, CBR tests were conducted on a
range of soils, fly ashes, and soil-fly ash mixtures. Specimens were prepared
over a range of water contents to determine how water content affects
stabilization.
4.1 CBR OF FLY ASHES
CBR tests were conducted on specimens of fly ash (i.e., not mixed with
soil) prepared at a water-to-fly ash ratio of 0.35 as recommended in ASTM D
5239. Load penetration curves the CBR tests on these specimens after 7 days of
curing are shown in Fig. 4.1. King fly ash has the highest CBR (300), followed by
Columbia (135), Edgewater (73), and Dewey (30). The King, Columbia, and
Edgewater fly ashes behave more like a lean concrete, whereas Dewey fly ash is
similar to a stiff soil.
The Columbia and Edgewater fly ashes are typical Class C fly ashes. Both
fly ashes have lower CBR compared to the off-specification King fly ash. One
possible explanation for their lower CBR is that Columbia and Edgewater have a
stronger hydration process, making the material stiffer and more brittle after 7
days curing time. When the CBR test is performed, the specimen is plunged with
47
35
Columbia
Dewey
King
Edgewater
Piston Stress (MPa)
30
25
20
15
10
5
0
0
1
2
3
Penetration (mm)
4
5
Fig. 4.1. Stress-penetration data from CBR tests for different fly ashes
prepared at 35% water-fly ash ratio for a curing period of 7 days.
48
a small piston (area = 1936 mm2). The CBR is calculated as the ratio of the
stress at a penetration of 2.5 mm (0.1 in.) divided by the stress at the same
penetration of a standard granular material. Penetration of the specimen over
such a small area may cause the specimen to lose strength before the piston
penetrates 2.5 mm, resulting in lower CBR for more brittle fly ashes.
4.2 CBR OF UNTREATED SOILS
CBRs for the seven soils compacted at optimum water content and at 7%
wet of optimum water content are shown in Table 4.1. Specimens compacted at
optimum water content have CBRs ranging from 2 to 26, with most less than 20.
The specimens compacted at 7% wet of optimum water content are soft, with
CBRs ranging from 0 to 5, with most less than 2. Thus, in their typical moist
condition, each of these soils would be considered a poor subgrade.
The highest CBR at optimum water content was achieved with the soil that
had the lowest amount of fines (71%), whereas the lowest CBR at optimum water
content was achieved with a soil that had a high fines content (99%), as well as
high organic content (10%). These CBRs are consistent with CBRs reported by
other investigators for soft subgrades (e.g., Ferguson 1993, Nicholson et al.
1997).
The CBRs in Tables 4.1 and 4.2 needs to be interpreted in the context of
the general relationship between CBR and quality in terms of a pavement
application. CBRs between 0-7 are typical of unstabilized subgrade materials.
CBRs between 0-3 are associated to very poor subgrades, whereas CBRs
49
Table 4.1. CBR of soil and soil-fly ash mixtures compacted 2-hours after mixing and cured for 7 days prior to
CBR test.
Soil Optimum Moisture Content (wOPT)
Soil
Name
Columbia
Soil
Alone
7% wet of wOPT
Dewey
Edgewater
Fly Ash Content (%)
Columbia
Soil
Alone
Dewey
King
Fly Ash Content (%)
0
6
10
12
14
16
18
20
10
14
18
10
18
0
10
18
10
18
10
18
RSCT
26
(-1.0)
-
33
(2.0)
-
-
-
35
(4.0)
-
24
(2.0)
-
20
(4.0)
-
-
5
(4.7)
11
(7.0)
30
(7.0)
17
(7.0)
23
(7.0)
14
(7.0)
26
(7.0)
LRC
17
(-0.9)
-
19
(2.0)
-
-
-
20
(4.0)
-
25
(2.0)
-
20
(4.0)
-
-
2
(7.0)
8
(7.0)
24
(7.0)
14
(7.0)
26
(7.0)
9
(7.0)
27
(7.0)
BS
17
(1.0)
-
-
-
-
-
-
-
20
(2.0)
20
(3.0)
25
(3.0)
-
-
3
(5.0)
12
(7.0)
15
(7.0)
10
(7.0)
31
(7.0)
9
(7.0)
20
(7.0)
OTSL
2
(0.5)
-
2
(2.0)
-
-
-
5
(4.0)
-
4
(2.0)
-
10
(4.0)
2
(2.0)
2
(4.0)
0.3
(6.9)
-
-
-
-
-
-
TSL
12
(-0.4)
15
(2.0)
25
(2.0)
-
23
(3.0)
-
30
(3.0)
-
-
-
-
-
-
3
(6.3)
-
-
-
-
-
-
JSL
5
(1.0)
-
32
(2.0)
-
36
(2.7)
-
38
(3.4)
-
-
-
-
-
-
3
(6.0)
-
-
-
-
-
-
PSL
5†
-
-
34
(2.4)
-
51
(3.2)
-
56
(4.0)
-
-
-
-
-
1
(7.0)
-
-
-
-
-
-
Notes: RSCT = Red Silty Clay Till (wOPT = 13%), LRC = Lacustrine Red Clay (wOPT = 24%), BS = Brown Silt (wOPT =
19%), OTSL = Organic Theresa Silt Loam (wOPT = 29%), TSL = Theresa Silt Loam (wOPT = 18%), JSL = Joy
Silt Loam (wOPT = 19%), PSL = Plano Silt Loam (wOPT = 20%). Number in parenthesis indicates the water
content of the soil relative (wSOIL - wOPT) to the soil optimum water content. †CBR for Plano Silt Loam was
assumed to be the same as Joy Silt Loam due to similarities in soil properties.
50
Table 4.2 CBR of soil and soil-fly ash mixtures compacted 2 hours after mixing and cured for 7 days prior to
CBR test.
Very Wet Condition (9-12% wet of wOPT)
Columbia
Soil
Name
6
10
9
(10.0)
7
(10.0)
9
(10.0)
14
Dewey
Fly Ash Content (%)
18
6
-
-
-
-
-
10
14
18
30
30
-
-
-
-
-
-
3
(9.0)
11
(10.0)
14
(10.0)
5
(10.0)
8
(11.0)
10
(12.0)
13
(12.0)
15
(12.0)
3
(9.0)
-
-
-
-
14
(9.0)
2
(9.0)
8
(12.0)
-
-
6
(10.0)
-
18
(12.0)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
(12.0)
8
(12.0)
11
(12.0)
30
Edgewater
RSCT
-
LRC
-
BS
-
OTSL
-
-
-
-
TSL
4
(9.0)
4
(10.0)
9
(11.0)
JSL
-
-
PSL
-
-
-
-
Notes: RSCT = Red Silty Clay Till (wOPT = 13%), LRC = Lacustrine Red Clay (wOPT = 24%), BS = Brown Silt (wOPT
= 19%), OTSL = Organic Theresa Silt Loam (wOPT = 29%), TSL = Theresa Silt Loam (wOPT = 18%), JSL
= Joy Silt Loam wOPT = 20%), PSL = Plano Silt Loam (wOPT = 19%). Number in parenthesis indicates the
water content of the soil relative (wSOIL - wOPT) to the soil optimum water content.
51
between 3-7 are generally associated with poor to fair subgrades. Materials with
CBRs between 7-20 are generally associated with subbase and are categorized
as “fair”. Thus, the untreated soils are generally considered as poor subgrades in
their wettest condition, and fair subgrades when compacted at optimum water
content. Base or subbase materials that are considered as “good” materials have
CBR between 20-50. Excellent materials have CBR > 50, and are usually used
as base coarse (Bowles 1992).
4.3 CBR OF SOIL-FLY ASH MIXTURES
Soil-fly ash mixtures were prepared with fly ash contents ranging from 0 to
30%, and soil water contents (i.e., weight of water divided by the weight of soil)
corresponding to optimum water content, 7% wet of optimum water content, and
a very wet condition with water contents between 9 and 12% wet of optimum
water content. These water contents correspond to the water content of the soil
before addition of fly ash (i.e., they represent the initial wetness of the soil). All
soil-fly ash specimens were cured for 7 days prior to CBR testing.
Specimens were compacted at optimum water content as a standard
reference condition. The condition 7% wet of optimum water content simulates
the typical moisture condition observed in Wisconsin subgrades. Specimens
prepared between 9 to 12% wet of optimum water content simulate a scenario
where a very wet condition exists. CBR of the soil-fly ash mixtures are
summarized in Tables 4.1 (optimum water content and 7% wet of optimum water
content) and Table 4.2 (9-12% wet of optimum water content).
52
Repeatability of the CBR test procedure was assess at the beginning of
the testing sequence. Several soil-fly ash specimens were tested to inferred the
general trends of soil-fly ash specimens with increasing water content and
increasing fly ash content. Based on these results the rest of the testing
sequence was performed.
4.3.1 General Effects of Fly Ash Stabilization
The general effect of fly ash stabilization on typical subgrade soils in
Wisconsin is illustrated in Fig. 4.2 and 4.3. These data are from CBR tests
conducted on soil-fly ash specimens compacted 7% wet of optimum water
content. Three soils (Red Silty Clay Till, Lacustrine Red Clay, and Brown Silt) are
shown in Figs. 4.2 and 4.3 that represent the range of soft subgrade soils in
Wisconsin. These soils also are geographically distributed throughout Wisconsin.
The specimens were compacted two hours after mixing with water to simulate
the construction delay that typically occurs in the field before subgrade
compaction due to construction operations.
The ratio of the CBR of the soil-fly ash specimens normalized to that of the
CBR of the untreated soil (referred to as the “CBR gain”) is shown in Fig. 4.2 as a
function of fly ash content. In general, the CBR gain increases with an increase in
fly ash content. The CBR gain also varies with the type of soil, and to some
extent the type of fly ash. The specimens prepared with 10% fly ash have similar
CBR gain, whereas the CBR gain of specimens prepared with 18% fly ash varies
more with soil type.
53
CBR
SOIL-FLY ASH
/CBR
SOIL
14
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
12
10
Columbia
2-Hour Delay
8
6
4
2
0
(a)
0
5
10
15
Percent Fly Ash (%)
20
CBR SOIL-FLY ASH/CBRSOIL
14
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
12
10
Dewey
2-Hour Delay
8
6
4
2
0
(b)
0
CBRSOIL-FLY ASH/CBRSOIL
14
5
10
15
Percent Fly Ash (%)
20
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
12
10
King
2-Hour Delay
8
6
4
2
0
(c)
0
5
10
15
Percent Fly Ash (%)
20
Fig. 4.2. The CBR of soil-fly ash specimens normalized to CBR of untreated
soil for soil-fly ash mixtures prepared at 7% wet of optimum water
content. (RSCT (CBR = 5), LRC (CBR = 2), and BS (CBR = 3)).
54
CBR gains for different soils stabilized with Columbia, Dewey, and King fly
ashes are shown in Fig. 4.3. The addition of 10% fly ash caused the CBR to
increase by a factor of 4, on average, whereas 18% fly ash caused the CBR to
increase by a factor of 8. Also, the largest CBR gains generally were obtained
with the highly plastic Lacustrine Red Clay, and the smallest with the more well
graded Red Silty Clay Till.
A comparison between the CBR of specimens prepared with the offspecification Dewey and King fly ashes, is shown in Fig. 4.4. The CBR have
been normalized by the CBR of identical specimens prepared with Columbia fly
ash, a typical Class C fly ash. The CBR of the soils stabilized with the offspecification fly ashes (Dewey and King) are similar or higher compared to the
CBR obtained with the typical Class C fly ash (Columbia). The soil-fly ash
specimens prepared with 10 and 18% Dewey or King fly ashes have CBR
between 0.8 and 2.0 times the CBR obtained using Columbia fly ash. Thus, the
off-specification ashes appear as effective at stabilizing soft subgrades as the
more valuable Class C ashes.
4.3.2 Effect of Soil Water Content
The effect of soil water content on the CBR of soil-fly ash mixtures is
shown in Fig. 4.5. Data from Six soils (Red Silty Clay Till, Lacustrine Red Clay,
Brown Silt, organic Theresa Silt Loam, Theresa Silt Loam, and Joy Silt Loam)
are included in Fig. 4.5. Each was mixed with 10% or 18% Columbia or Dewey
fly ash.
55
CBRSOIL-FLY ASH/CBRSOIL
16
14
12
10
(a) Red Silty Clay Till
Columbia
Dewey
King
8
6
4
2
0
10
18
Percent Fly Ash (%)
CBRSOIL-FLY ASH/CBRSOIL
16
14
12
10
(b) Lacustrine Red Clay
Columbia
Dewey
King
8
6
4
2
0
10
18
Percent Fly Ash (%)
16
12
10
CBR
SOIL-FLY ASH
/CBR
SOIL
14
(c) Brown Silt
Columbia
Dewey
King
8
6
4
2
0
10
18
Percent Fly Ash (%)
Fig. 4.3. CBR gain as a function of fly ash content for red silty clay till,
lacustrine red clay, and brown silt prepared with Columbia, Dewey,
and King fly ashes compacted 7% wet of optimum water content.
56
3.0
(a) Dewey
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
CBR
DEWEY
/CBR
COLUMBIA
2.5
2.0
1.5
1.0
0.5
0.0
10
18
Percent Fly Ash (%)
3.0
(b) King
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
CBRKING/CBRCOLUMBIA
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 4.4.
10
Percent Fly Ash (%)
18
CBR of specimens prepared with (a) Dewey and (b) King fly ashes
normalized to the CBR specimens prepared with Columbia fly ash as
a function of fly ash content. (Specimens compacted 7% wet of
optimum water content).
57
40
(a)
35
10% Columbia
18% Columbia
30
CBR
25
20
15
10
5
0
10
15
20
25
30
Soil Water Content (%)
35
40
40
35
(b)
10% Dewey
18% Dewey
30
CBR
25
20
15
10
5
0
10
Fig. 4.5.
15
20
25
30
Soil Water Content (%)
35
40
Effect of water content on CBR of soils prepared with 10% and 18%
(a) Columbia and (b) Dewey fly ash.
58
Scatter exists in the trends because the data are from different soils, and each fly
ash has a unique stabilization effect on each soil. However, CBR generally
decreases as the water content increases, regardless of the fly ash content. The
sensitivity to water content is greater for specimens prepared with Columbia fly
ash.
The influence of soil type on sensitivity of 3 CBR to water content is shown
in Fig. 4.6. Columbia and Dewey fly ashes were used to stabilize both soils. For
both soils CBR decreases with increasing water content. However, the red silty
clay till is more sensitive to water content than the brown silt. Thus, the
engineering properties of soil-fly ash mixtures are soil and fly ash specific.
The general effect of water content on CBR is shown in Fig. 4.7 in terms
of water content relative to optimum water content (wSOIL - wOPT). Significant
scatter exists because data for all soils and all fly ashes have been plotted on the
same graph. In general, soil water content decreases with increasing water
content, and increases with increasing fly ash content. The sensitivity to water
content is also similar, regardless of fly ash content.
4.3.3 Effect of Soil Type
Sensitivity of CBR to soil type was illustrated in Fig. 4.6. Two index
parameters that are used as indicators of soil type are the liquid limit (LL) and
plasticity index (PI). Both the LL and PI are indices of the quantity of clay mineral
in the soil and the activity of the clay mineral. Soils with a larger clay fraction or
more active clay minerals have larger LL and PI (Mitchell 1993).
59
40
(a)
10% Columbia
10% Dewey
35
30
Red Silty Clay Till
CBR
25
20
15
10
5
0
14
16
18
20
Soil Water Content (%)
22
24
40
(b)
10% Columbia
10% Dewey
35
30
Brown Silt
CBR
25
20
15
10
5
0
25
26
27
28
29
30
Soil Water Content (%)
Fig. 4.6.
Effect of water content on CBR for red silty clay till (CL) and brown silt
(CH) at 10% Columbia and Dewey fly ashes.
60
60
10% FA
18% FA
50
CBR
40
30
20
10
0
Fig. 4.7.
-5
0
5
wSOIL - wOPT
10
15
CBR as a function of soil water content (wSOIL - wOPT) and fly ash
content.
61
Two other index parameters that are used as indicators of soil type are
group index (GI) and the liquidity index (LI). GI is an index property used in
pavement engineering for rating the performance of subgrade soils. GI is an
index based on three major soils properties: percent fines, liquid limit, and
plasticity index. As the GI increases, the subgrade usually becomes poorer
(Yoder and Witczak 1975).
Liquidity index (LI = (w - PL)/(LL - PL)) is an index of soil water content
relative to a range of water contents in which a soil is plastic. This range varies
depending on the composition of the soil. A soil with, LI less than zero tends to
be brittle, whereas a soil with LI greater than one is soft and sensitive (Holtz and
Kovacs 1981).
Graphs were made to determine if sensitivity to soil type could be defined
using the CBR ratio, LL, PI, GI, and LI. The relationship between CBR ratio, and
liquid limit, or plasticity index, is shown in Fig. 4.8. Similar graphs for GI and LI
are shown in Fig. 4.9. The CBR ratio increases appreciably with each any of
these index properties. The sensitivity to each index property also increases as
the fly ash content increases.
4.3.4 Effect of Organic Matter
Only one of the soils that was used Theresa silt loam is considered an
organic soil. Addition of fly ash had a much smaller effect on the CBR of the
organic Theresa silt loam in comparison to the other soils, as shown in Fig. 4.11.
Adding Columbia and Edgewater fly ashes had little or no effect on CBR, even
62
14
(a)
12
10% FA
18% FA
CBR Ratio
10
8
6
4
2
0
40
45
50
55
60
Liquid Limit (LL)
65
70
14
(b)
12
10% FA
18% FA
CBR Ratio
10
8
6
4
2
0
15
20
25
30
35
40
Plasticity Index (PI)
Fig. 4.8.
Effect of (a) liquid limit (LL) and (b) plasticity index (PI) on the CBR
ratio of soil-fly ash specimens prepared 7% wet of optimum water
content and 9-12% wet of optimum water content, normalized to the
CBR of the untreated soil specimen.
63
14
(a)
12
10% FA
18% FA
CBR Ratio
10
8
6
4
2
0
10
15
20
25
30
35
Group Index (GI)
40
45
14
(b)
12
CBR Ratio
10
10% FA
18% FA
8
6
4
2
0
-0.3
Fig. 4.9.
-0.2
-0.1
0
0.1
Liquidity Index (LI)
0.2
0.3
Effect of (a) group index (GI) and (b) liquidity index (LI) on the CBR
ratio of soil-fly ash specimens prepared 7% wet of optimum water
content and 9-12% wet of optimum water content, normalized to the
CBR of the untreated soil specimen.
64
when the fly ash content was 30%. Increases in CBR were obtained when
Dewey fly ash was used. However, the CBR obtained with the Organic Theresa
silt loam are lower than the CBR achieved for the other soils using Dewey fly
ash. The increase in CBR obtained with Dewey fly ash may be due its high
organic content. Dewey fly ash has a LOI of 16.2%, whereas Columbia and
Edgewater fly ashes have LOI of 0.7 and 0.1%, respectively.
The ineffectiveness of the Columbia and Edgewater fly ashes is believed
to be caused by inhibition of hydration by the organic matter in the soil. Similar
results were reported by Tremblay et al. (2002), who evaluated the undrained
shear strength of two inorganic clays prepared with 13 different organic
compounds and mixed with 10% cement. Soil-cement mixtures were prepared
with 10% organic matter by weight (i.e., either solid or liquid) to evaluate how
different organic compounds affect the hydration process of cement. From the 13
different organic compounds tested only five (acetic acid, humic acid, tannic acid,
sucrose, and EDTA) showed to inhibit the cement hydration process, since the
undrained shear strength of these specimens (less than 15 KPa) were below the
undrained shear strength of the inorganic clays (above 800 kPa) tested with 10%
cement. Tremblay et al. (2002) conclude that some organic compounds inhibited
hydration, whereas others have no major effects.
65
15
Organic Theresa Silt Loam
2-Hour Delay
Columbia
Dewey
Edgewater
CBR
10
5
0
0
5
10
15
20
25
30
35
Percent Fly Ash (%)
Fig. 4.10. CBR of organic Theresa Silt Loam with three fly ashes.
66
4.4 SYNTHESIS
The effect of fly ash addition on CBR appears to be unique for each soil,
and fly ash although a general trend of increasing CBR with increasing fly ash
content or decreasing water content is apparent for all of the soils that were
tested. The CBR for soil-fly ash mixtures compacted 7% wet of optimum water
content with 10% fly ash content typically ranges from 8 to 17, with an average
CBR gain of 4. For 18% fly ash, the ranges between 15 to 31, with an average
CBR gain of 8.
Water content is a critical factor that affects CBR regardless of type of fly
ash type or fly ash content. As the soil water content increases, CBR decreases.
Soil-fly ash mixtures compacted under very wet conditions (9-12% wet of
optimum water content) typically have CBR ranging from 7 to 18, which is
considered as “fair” for subbase construction. Soil-fly ash mixtures compacted at
optimum water content typically have CBR between 15 to 56, which is
considered as “excellent” for subbase construction. For typical water contents
encountered in Wisconsin, the CBR typically ranges between 8 to 31, which is
considered to be between “fair to good” for subbase construction.
Stabilization of soft organic soils with typical fly ashes may not be practical
or effective due to inhibition of hydration by the organic matter. In such cases, a
fly ash with higher organic content should be considered as a potential
stabilization agent.
67
SECTION 5
RESILIENT MODULUS
Resilient modulus is a fundamental property that describes the
deformation behavior of materials subjected to repetitive loading similar to that
imposed by vehicle traffic. The resilient modulus is typically determined on
specimens of pavement materials compacted in the laboratory following a cyclic
loading protocol that varies the confining pressure and the repetitive deviator
stress.
To characterize the resilient modulus of soils and soil-fly ash specimens,
resilient modulus tests were conducted on a range of soils with different
properties mixed with three different fly ashes at three different water contents.
The curing period for resilient modulus specimens varied from 7 to 14 days.
5.1 RESILIENT MODULUS OF FLY ASHES
Results of resilient modulus tests on fly ash (i.e., not mixed with soil) after
14 days of curing are shown in Fig. 5.1, as a function of deviator stress and bulk
stress. The specimens were prepared at a water-to-fly ash ratio of 0.35, as
recommended in ASTM D 5239.
The typical Class C fly ashes (Columbia and Edgewater) have the highest
moduli (300-350 and 250 MPa), followed by the off-specification fly ashes (King
and Dewey) (200-250 and 50 MPa). The Columbia, King, and Dewey exhibited a
modest trend of increasing modulus with increasing bulk stress, or deviator
stress, whereas the resilient modulus of Edgewater fly ash is essentially
68
350
(a)
Resilient Modulus (MPa)
300
250
200
150
Columbia
Dewey
King
Edgewater
100
50
0
0
350
20
40
60
Deviator Stress (kPa)
80
100
(b)
Resilient Modulus (MPa)
300
250
200
Columbia
Dewey
King
Edgewater
150
100
50
0
Fig. 5.1.
80
90
100
110 120 130 140
Bulk Stress (kPa)
150
160
Resilient modulus versus (a) deviator stress and (b) bulk stress for fly
ashes prepared at 35% water-fly ash ratio for a curing period of 14
days.
69
independent of deviator stress or bulk stress, at least within the range of stresses
employed. From a practical perspective, all of the fly ashes have nearly a
constant resilient modulus. The higher moduli of the Columbia, Edgewater, and
King fly ashes (200 to 350 MPa) relative to the Dewey fly ash (50 MPa), indicates
that their hydration process is stronger than that of Dewey ash.
5.2 RESILIENT MODULUS OF UNTREATED SOILS
Resilient moduli of soil and soil-fly ash mixtures corresponding to a
deviator stress of 21 kPa are summarized in Table 5.1. This deviator stress (the
first deviator stress level applied) corresponds to the deviator stress expected in
situ for typical subgrades stabilized with fly ash. The general relationship
between resilient modulus (Mr) and deviator stress (sd) obtained from each test
was characterized with the power function corresponding to cohesive soils. The
parameters K1 and K2 for each test are summarized in Table 5.2.
Mr = K1(σ d )K 2
(5.1)
Resilient moduli of the untreated soils as a function of deviator stress and
bulk stress are shown in Fig. 5.2 for compaction at optimum water content. The
resilient modulus curves exhibit the typical behavior of cohesive soils (i.e., a
monotonic decrease in resilient modulus with increasing deviator stress).
Maximum axial strains of the soil specimens were in the range of 0 to 5%
(Appendix C).
Resilient modulus of the untreated soils could not be measured at 7% wet
of optimum water content because the specimens were too soft to withstand the
70
120
(a)
Red Silty Clay Till
Lacustrine Red Clay
Organic Theresa Silt Loam
Theresa Silt Loam
Brown Silt
Plano Silt Loam
Resilient Modulus (MPa)
100
80
60
40
20
0
0
20
40
60
Deviator Stress (kPa)
80
100
120
(b)
Red Silty Clay Till
Lacustrine Red Clay
Organic Theresa Silt Loam
Theresa Silt Loan
Brown Silt
Plano Silt Loam
Resilient Modulus (MPa)
100
80
60
40
20
0
Fig. 5.2.
60
80
100
120
140
Bulk Stress (kPa)
160
180
Resilient modulus of untreated soil specimens compacted at optimum
water content as a function of (a) deviator stress and (b) bulk stress.
71
Table 5.1. Resilient moduli (MPa) of soil and soil-fly ash mixtures compacted 2 hours after mixing at a deviator stress of
21 kPa.
wOPT
Soil
Name
RSCT
LRC
BS
Curing
Time
(Days)
7% wet of wOPT
Columbia
Soil
Alone
Fly Ash
Content
(%)
0
12
-
72.7
(-0.1)
-
-
43.3
(-1.0)
-
79.0
(-1.2)
-
-
Curing
Time
(Days)
Columbia
Very Wet Condition
Dewey
Soil
Alone
King
Curing
Time
(Days)
Fly Ash Content (%)
0†
10
12
18
10
18
10
18
14
15.0
(7.0)
12.1
(7.0)
-
65.0
(7.0)
21.1
(7.0)
76.3
(7.0)
22.0
(7.0)
90.9
(7.0)
14
6.0
(7.0)
28.3
(7.0)
-
50.6
(7.0)
47.7
(7.0)
74.2
(7.0)
31.3
(7.0)
106.3
(7.0)
9.0
(7.0)
49.4
(7.0)
-
71.4
(7.0)
60.0
(7.0)
82.8
(7.0)
18.7
(7.0)
68.3
(7.0)
14
Columbia
Dewey
Fly Ash Content (%)
10
18
10
18
30
7
14.2
(9.0)
20.7
(11.0)
15.2
(9.0)
41.8
(11.0)
-
7
10.1
(10.0)
11.7
(13.0)
15.2
(10.0)
25.4
(13.0)
-
14
22.9
(10.0)
41.6
(13.0)
-
-
-
-
-
-
-
-
25.6
(9.0)
OTSL
-
14.1
(-0.9)
-
-
0.9
(7.0)
-
-
-
-
-
-
-
7
F
(10.6)
F
(13.5)
F
(10.6)
F
(13.5)
TSL
-
13.3
(1.0)
-
-
9.0
(7.0)
-
-
-
-
-
-
-
-
-
-
-
-
-
JSL
-
-
-
-
9.0
(7.0)
-
-
-
-
-
-
-
-
-
-
-
-
-
7
3.0
(7.0)
-
144.9
(5.0)
-
-
-
-
-
-
-
-
-
-
-
PSL
7
34.0
(-3.0)
422.5
(-1.0)
226.1
(3.0)
7.8
(18.0)
Notes: RSCT = Red silty clay till (wOPT = 13%), LRC = Lacustrine Red Clay (wOPT = 24%), BS = Brown Silt (wOPT = 19%), OTSL = Organic Theresa Silt Loam (wOPT = 29%), TSL =
Theresa Silt Loam (wOPT = 18%), JSL = Joy Silt Loam (wOPT = 19%), PSL = Plano Silt Loam (wOPT = 20%). Number in parenthesis indicates the water content of the soil relative to the
soil optimum water content (wSOIL – wOPT). F = Failed during testing. †Estimated from CBR specimens prepared 7% wet of optimum water content using Equation 5.1.
72
Table 5.2. Resilient modulus model coefficients K1 and K2 for soil and soil-fly ash mixtures.
Soil
Name
Red Silty
Clay Till
Lacustrine
Red Clay
Organic
Theresa
Silt Loam
Theresa
Silt Loam
Brown Silt
Plano Silt
Loam
Fly Ash
Content
(%)
Curing
Time
(Days)
w
(%)
0
10
10
18
18
18
18
18
0
10
10
10
18
18
18
0
10
18
30
30
1
7
14
7
7
14
28
56
1
7
14
14
7
14
14
1
7
7
7
7
0
0
10
18
0
12
12
12
Columbia
K1
K2
-0.1
9.0
7.0
11.0
7.0
7.0
7.0
7.0
1.0
10.0
10.0
7.0
13.0
13.0
7.0
-0.9
10.0
13.5
-
185.0
6.6
4.2
12.4
161.7
68.8
77.5
73.3
124.0
9.0
33.7
40.1
7.7
89.8
90.5
12.5
F
F
-
-0.306
0.251
-0.349
0.166
-0.305
-0.019
0.063
0.098
-0.346
0.039
-0.127
-0.115
-0.138
-0.253
-0.191
0.040
F
F
-
1
-1.0
21.1
1
14
14
1
7
7
7
7.0
7.0
7.0
-3.0
-1.0
3.0
5.0
150.0
115.0
83.0
36.7
879.8
983.1
1550.2
w
(%)
Dewey
K1
K2
9.0
7.0
11.0
7.0
7.0
7.0
7.0
10.0
7.0
13.0
7.0
10.0
13.5
9.0
18.0
12.3
25.5
87.3
186.7
135.5
124.5
160.5
35.2
238.3
87.9
123.0
F
F
31.9
21.7
0.070
-0.062
-0.242
-0.299
-0.189
-0.129
-0.185
-0.276
-0.529
-0.408
-0.166
F
F
-0.072
0.423
-0.151
-
-
-0.210
-0.277
-0.050
-0.026
-0.241
-0.482
-0.779
7.0
7.0
-
334.5
125.0
-
w
(%)
King
K1
K2
7.0
7.0
7.0
7.0
-
22.0
167.0
62.2
144.6
-
0.001
-0.199
-0.225
-0.101
-
-
-
-
-
-0.565
-0.136
-
7.0
7.0
-
22.0
83.4
-
-0.054
-0.066
-
Notes: w (%) = Indicates the water content relative to the soil optimum water content, F = Failed during
testing.
73
applied stresses. Thus, for these specimens, the resilient moduli were estimated
from the CBR measurements described in section 4. The relationship between
resilient modulus at a deviator stress of 21 kPa and CBR was defined using the
CBR data in Tables 4.1 and 4.2 and the resilient modulus measurements in
Table 5.1, for both untreated clay soil at optimum water content and fly ash
stabilized clay compacted at 7% wet of optimum water content and wetter
conditions. This relationship is shown in Fig. 5.3 and can be defined as:
Mr = 3.0CBR
(5.2)
where Mr is in MPa. Fig. 5.3 shows with a dark line the conventional modulus
(EDYN) CBR relationship developed by Heukelom and Foster (1960). This
equation is based on low-strain dynamic modulus measurements by wave
propagation and therefore, it gives higher modulus compared to resilient
modulus, which is performed at higher strain (Das 1993). This relationship was
developed for several types of soils: course and fine sand and fine-grained soils
with an upper bound CBR value of 20.
The resilient moduli of the untreated soils agree well with resilient moduli
report in the literature. Lee et al. (1995) report the resilient moduli for a highly
plastic clay between 30 to 70 MPa at optimum water content, and Muhanna et al.
(1999) report resilient moduli of a low plasticity clay between 50 to 100 kPa at
optimum water content. Fredlund et al. (1975, 1977) report the resilient moduli
between 40 and 70 MPa for low plasticity Regina clay, and resilient moduli
between 6 and 60 MPa for a low plasticity glacial till, both at optimum water
content.
74
350
Heukelom and Foster (1960)
0% FA
10% FA
18% FA
Resilient Modulus (MPa)
300
250
200
EDYN = 10 CBR
Mr = 3 CBR (R2 = 0.6)
sd = 21 kPa
150
s = 21 kPa
3
100
50
0
0
5
10
15
20
25
30
35
CBR
Fig. 5.3.
Comparison between resilient modulus estimated from the CBR (7
days curing) and measured resilient moduli (14 days) at a deviator
stress of 21 kPa and a confining pressure of 21 kPa. The
conventional Modulus-CBR relationship reported by Heukelom and
Foster (1960) is shown as the solid line.
75
5.3 RESILIENT MODULUS OF SOIL-FLY ASH MIXTURES
Resilient moduli of soil-fly ash mixtures at a deviator stress of 21 kPa
prepared at different water contents are shown in Table 5.1. Soil-fly ash mixtures
were prepared with fly ash contents of 0, 10, and 30% and compacted at different
soil water contents: optimum water content, 7% wet of optimum water content,
and the very wet condition (water contents between 9% and 18% wet of optimum
water content). Optimum water content was used as a reference condition. The
condition 7% wet of optimum water content simulates the typical condition
observed in Wisconsin subgrades. Specimens prepared between 9 and 18% wet
of optimum water content simulate a scenario where very wet conditions exist.
The specimens were cured 7 or 14 days prior to testing.
Repeatability of the resilient modulus test procedure was assess by
testing synthetic specimens. The data for these specimens (e.g., Specimens A
and B) was presented earlier in Fig. 3.9.
5.3.1 General Effect of Fly Ash Stabilization
Resilient modulus specimens were prepared using three typical soils (Red
Silty Clay Till, Lacustrine Red Clay, and Brown Silt) found in Wisconsin. These
soils represent the range of soils encountered throughout the state. The soil-fly
ash specimens were compacted two hours after mixing with water, to simulate
the compaction delay usually found in constructions operations.
Typical resilient modulus curves for the stabilized soils are shown in Fig.
5.4. Resilient moduli of soil-fly ash specimens increases when the fly ash content
76
140
Resilient Modulus (MPa)
(a)
120 Columbia
2-Hour Delay
100
10%-Red Silty Clay Till
10%-Lacustrine Red Clay
10%-Brown Silt
18%-Red Silty Clay Till
18%-Lacustrine Red Clay
18%-Brown Silt
80
60
40
20
0
0
20
40
60
80
Deviator Stress (kPa)
100
Resilient Modulus (MPa)
140
(b)
120 Dewey
2-Hour Delay
100
80
60
40
20
Resilient Modulus (MPa)
0
0
20
40
60
80
Deviator Stress (kPa)
140 (c) King
120 2-Hour Delay
100
10%-Red Silty Clay Till
10%-Lacustrine Red Clay
10%-Brown Silt
18%-Red Silty Clay Till
18%-Lacustrine Red Clay
18%-Brown Silt
100
80
60
40
20
0
0
Fig. 5.4.
10%-Red Silty Clay Till
10%-Lacustrine Red Clay
10%-Brown Silt
18%-Red Silty Clay Till
18%-Lacustrine Red Clay
18%-Brown Silt
20
40
60
80
Deviator Stress (kPa)
100
Resilient modulus of three soils prepared with Columbia, Dewey,
and King fly ashes compacted 7% wet of optimum water content
and cured for 14 days.
77
increases. The increase in modulus varies with type of soil and type of fly ash.
For example resilient modulus exceeding 100 MPa were obtained when the
lacustrine red clay was mixed with 18% King ash, whereas, 18% Columbia ash
resulted in Moduli near 50 MPa. Similarly, the increase in modulus obtained with
Dewey ash was much larger for the red silty clay till than the brown silt.
A comparison between moduli of untreated specimens compacted at
optimum water content and soil-fly ash specimens compacted 7% wet of
optimum water content (i.e., 10 and 18% fly ash content) is shown in Fig. 5.5. All
of the moduli correspond to a deviator stress of 21 kPa. Moduli of soil-fly ash
mixtures prepared with 10% fly ash fall below the moduli of untreated soils
compacted at optimum water content. However, at 18% fly ash content, the
treated soils have similar or higher moduli compared moduli than the untreated
soil compacted at optimum. Addition of 18% fly ash produced modulus 0.8 to 2.5
times the moduli of the untreated specimen at optimum water content.
Resilient moduli of the soil-fly ash specimens prepared with the offspecification (Dewey and King) fly ashes, normalized to the resilient moduli
obtained with the typical Class C (Columbia) fly ash are shown in Fig. 5.6. The
resilient moduli obtained with both off-specification fly ashes range between 0.9
and 2.1 of the resilient moduli obtained with Columbia ash, except for the brown
silt mixed 10% King fly ash. Dewey fly ash provided a resilient modulus higher
than that obtained with a Class C (Columbia) fly ash in all cases.
78
Resilient Modulus (MPa)
140
Soil
(a) Red Silty Clay Till
Columbia
Dewey
King
120
100
80 wOPT
60
40
w
20
(OPT + 7%)
0
0
0
10
Percent Fly Ash (%)
18
Resilient Modulus (MPa)
140
Soil
Columbia
Dewey
King
120
100
(b) Lacustrine
Red Clay
80
60
40
w
OPT
20
0
w
0
(OPT + 7%)
0
10
Percent Fly Ash (%)
18
Resilient Modulus (MPa)
140
100
80
w
(c) Brown Silt
OPT
60
40
w
20
0
Fig. 5.5.
Soil
Columbia
Dewey
King
120
0
(OPT + 7%)
0
10
Percent Fly Ash (%)
18
Moduli of the untreated soil compacted at wOPT to soil-fly ash
mixtures prepared with Columbia, Dewey, and King fly ashes
compacted 7% wet of wOPT. Moduli of untreated soil at 7% wet of
wOPT were estimated based on CBR.
79
3.0
(a) Dewey
1.5
1.0
DEWEY
/Mr
2.0
Mr
COLUMBIA
2.5
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
0.5
0.0
10
18
Percent Fly Ash (%)
3.0
(b) King
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
MrKING/MrCOLUMBIA
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 5.6.
10
18
Percent Fly ash (%)
Moduli of soil-fly ash specimens prepared with (a) Dewey and (b)
King fly ashes normalized to the moduli of the soil-fly ash specimen
prepared with Columbia fly ash. All specimens cured for 14 days.
(All resilient moduli are at deviator stress of 21 kPa).
80
5.3.2 Effect of Soil Water Content
The effect of soil water content on the resilient modulus of soil-fly ash
specimens cured for 7 to 14 days is shown in Fig. 5.7 for the (red silty clay till,
lacustrine red clay, and brown silt). Each was mixed with 18% fly ash (Columbia
or Dewey). In general, as the soil water content increases, the resilient moduli
decrease, regardless of the curing time. The exception appears to be the soils
stabilized with Dewey fly ash that were cured for 14 days. However, the
maximum water content of these specimens was 31%. If a specimen had bee
prepared at a water content beyond 31%, a decrease in moduli probably would
have occurred.
The effect of soil water content on resilient modulus is soil specific as
shown for Plano Silt Loam stabilized with 12% Columbia fly ash in Fig. 5.8. The
resilient modulus of Plano silt loam decreases dramatically as the soil water
content increase from 19 to 24%. The data in Fig. 5.8 also illustrates how the
stabilization effect is soil specific. The lowest resilient modulus of the Plano silt
loam is 150 MPa, whereas the highest resilient modulus shown in Fig. 5.6 is 85
MPa.
5.3.3 Effect of Curing Time
Resilient modulus tests were conducted on soil-fly ash mixtures cured for
periods of 7, 14, 28, and 56 days to evaluate how curing time affects the resilient
modulus. The specimens were prepared with red silty clay till clay (7% wet of
optimum water content) mixed with 18% Columbia fly ash and 18% Dewey fly
81
100
Resilient Modulus (MPa)
(a)
18% Columbia-7 days
18% Columbia-14 days
80
60
40
20
0
15
20
25
30
35
Soil Water Content (%)
40
100
Resilient Modulus (MPa)
(b)
80
60
40
20
0
15
Fig. 5.7.
18% Dewey-7 days
18% Dewey-14 days
20
25
30
Soil Water Content (%)
35
40
Effect of water content on resilient modulus of several soils at 18%
(a) Columbia and (b) Dewey fly ashes.
82
450
12% Columbia-7 days
Resilient Modulus (MPa)
400
Plano Silt Loam
350
300
250
200
150
100
18
19
20
21
22
23
24
25
Soil Water Content (%)
Fig. 5.8.
Effect on water content on resilient modulus of Plano Silt Loam (CL)
prepared with 12% Columbia fly ash and cured for 7 days.
83
ash. These fly ashes were selected to evaluate the effect of curing time for Class
C and off-specification fly ashes.
The effect of curing time on the resilient modulus at a deviator stress of 21
kPa is shown in Fig. 5.9. The resilient modulus at each curing time has been
normalized by the resilient modulus measured at 14 days. Between 7 and 14
days the resilient modulus did not showed any significant increase (1.0 for
Columbia and 1.0 for Dewey). A larger increase (1.44 for Columbia and 1.10 for
Dewey) occurs between 14 and 28 days, with a greater increase occurring for the
Columbia fly ash. After 28 days, little additional increase in resilient moduli
occurs (1.52 for Columbia and 1.20 for Dewey).
5.3.4 Effect of Soil Type
As in section 4 the liquid limit (LL), plasticity index (PI), group index (GI),
and liquidity index (LI) were used as indicators of soil type. Relationships
between the resilient modulus and LL, PI, GI, and LI are shown in Figs. 5.10 and
5.11. The resilient modulus is reported as a resilient modulus ratio, which is the
resilient modulus data at a deviator stress of 21 kPa normalized by the resilient
modulus of the untreated soil estimated using Equation 5.2.
For all four parameters, the resilient modulus increases as the soil
parameters increases. Each of these parameters is indicative of the softness or
weakness of the soil. All other factors being equal, subgrade tends to be poorer
as the LL, PI, GI, or LI increases. Thus, fly ash tends to have a greater
stabilization effects as the subgrade becomes poorer.
84
1.6
Red Silty Clay Till
Mr / Mr (14-Days)
1.5
1.4
1.3
1.2
1.1
18% Columbia
18% Dewey
1.0
0.9
0
Fig. 5.9.
7
14
21
28
35
42
Curing Time (Days)
49
56
Resilient modulus tested at different curing time (i.e., 7, 14, 28, and
56 days) for red silty clay till prepared with Columbia and Dewey fly
ashes at 18% fly ash content, normalized to the resilient modulus
tested at 14 days curing time.
85
20
(a)
10% FA
18% FA
Mr Ratio
15
10
5
0
45
50
55
60
Liquid Limit (LL)
65
70
20
(b)
10% FA
18% FA
Mr Ratio
15
10
5
0
Fig. 5.10.
20
25
30
Plasticty Index (PI)
35
40
Effect of (a) liquid limit (LL) and (b) plasticity index (PI) in the
resilient modulus ratio of soil-fly ash mixtures prepared at water
contents 7% wet of optimum and the very wet condition.
86
20
(a)
10% FA
18% FA
Mr Ratio
15
10
5
0
20
25
30
35
Group Index (GI)
40
45
0
0.05
20
10% FA
18% FA
Mr Ratio
15
10
5
0
-0.25
Fig. 5.11.
-0.2
-0.15
-0.1
-0.05
Liquididty Index (LI)
Effect of (a) group index (GI) and (b) liquidity index (LI) in the
resilient modulus of soil-fly ash mixtures prepared 7% wet of
optimum water content and the very wet condition.
87
5.3.5 Effect of Organic Matter
Results of the CBR tests presented in section 4 indicated that fly ash had
little stabilization effect on the organic Theresa silt loam (LOI = 10%). To evaluate
if a similar response would be observed specimens of organic Theresa silt loam
were prepared at the very wet conditions with Columbia and Dewey fly ashes at
10% and 18% fly ash content. These specimens failed during the preconditioning stage of the resilient modulus test, indicating that they had very low
resilient. Consequently, a higher fly ash content (30%) was used to see if a
resilient modulus would be measured. The specimens were prepared at 9% and
18% wet of optimum water content. Dewey fly ash was used because it provided
a superior stabilization of the organic soil in the CBR tests. Resilient modulus of
the Theresa silt loam with 30% fly ash is shown in Fig. 5.12.
Nevertheless, even with 30% fly ash, the resilient moduli of the organic
Theresa silt loam is still below the moduli achieved for the inorganic soils (red
silty clay till, lacustrine red clay, and brown silt) with 18% fly ash. These soils with
18% fly ash had moduli that ranging from 50 to 110 MPa.
Addition of 30% Dewey fly ash caused an increase in resilient modulus for
both soil water content. At 18% water content, the resilient modulus stabilized
soil was as much as a factor of 2, smaller than the resilient modulus of the
organic Theresa silt loam compacted at optimum water content without fly ash.
However, at 9% water content, addition of 30% Dewey fly ash resulted in a
resilient modulus approximately 1.8 times that achieved by compacting the soil at
optimum water content.
88
50
Resili ent Modul us (MPa)
Organic Theresa Silt Loam 2-Hour Delay
Untreated Soil-OWC
Untreated Soil-From CBR
30% Dewey-9% wet of OWC
30% Dewey-18% wet of OWC
40
30
20
10
Estimated based on CBR
0
Fig. 5.12.
0
20
40
60
Deviator Stress (kPa)
80
100
Resilient moduli of organic Theresa Silt Loam and in combination
with 30% Dewey fly ash at different water content and cured for 7
days.
89
5.4 SYNTHESIS
A general trend of increasing moduli with increasing fly ash content is
evident for all of the soils that were tested. However, the effect of fly ash addition
on the resilient modulus is unique for each soil.
Specimens prepared with 10% fly ash and compacted 7% wet of optimum
water content had lower resilient modulus as untreated specimens compacted at
optimum water content. However, when the fly ash content was increased to
18%, the resilient moduli of soil-fly ash mixtures was approximately 30% higher,
on average, compared to the modulus of the untreated soil specimen compacted
at optimum water content.
Water content has a strong effect on the resilient modulus regardless of fly
ash type or fly ash content. As the soil water content increases, the resilient
modulus decreases. The moduli of soil-fly ash mixtures prepared 7% wet of
optimum water content with 18% fly ash, ranged from 50 to 106 MPa at a
deviator stress of 21 kPa, whereas similar soil-fly ash mixtures compacted at the
very wet condition ranged from 10 to 42 MPa.
Curing time affects the resilient modulus of soil-fly ash mixtures. For
mixtures prepared 7% wet of optimum water and at 18% fly ash content,
between 7 and 14 days of curing the resilient modulus did not showed any
significant increase (1.0 for Columbia and 1.0 for Dewey). Between 14 and 28
days of curing a larger increase in the resilient modulus occurs (1.44 for
Columbia and 1.10 for Dewey). However, little increase in resilient modulus
occurs after 28 days (1.52 for Columbia and 1.20 for Dewey).
90
Four parameters were used as indicators of soil type (liquid limit (LL),
plasticity index (PI), group index (GI), and liquidity index (LI)). Resilient moduli of
soil-fly ash specimens at a deviator stress of 21 kPa were normalized to the
moduli estimated for an identical untreated specimen compacted 7% wet of
optimum water content. Normalized moduli (Mr ratio) showed an increasing
trend when plotted versus soil index parameters (LL, PI, GI, and LI), indicating
the stabilization effect is greater for poorer subgrades.
Four specimens were prepared with organic Theresa silt loam and two fly
ashes at 10 and 18% fly ash content (Columbia and Dewey). All four specimens
failed during the pre-conditioning stage of the resilient modulus test, indicating
that the untreated organic Theresa silt loam has very low resilient modulus.
However, when 30% Dewey fly ash was added to the organic Theresa silt loam,
the resilient modulus was 80% higher than the moduli of the untreated soil
compacted at the optimum water content. However, the modulus achieved with
30% fly ash is still low compared to inorganic soils prepared with 18% fly ash
content and compacted 7% wet of optimum water content.
91
SECTION 6
UNCONFINED COMPRESSION STRENGTH
Unconfined compression tests were conducted to characterize the
strength of soils and their mixtures with fly ash. Soil-fly ash specimens were
prepared over a wide range of water contents and fly ash contents to determine
how water and fly ash contents affected stabilization.
6.1 UNCONFINED COMPRESSIVE STRENGTH OF FLY ASHES
Unconfined compression tests were conducted on fly ash specimens (i.e.,
not mixed with soil) prepared at a water-to-fly ash ratio of 0.35 as recommended
in ASTM D 5239. Fly ash specimens were cured for 14 days in a 100% relative
humidity room. These specimens were first subjected to a resilient modulus test.
Subsequently, the specimens were capped using sulfur (to alleviate seating
problems) and loaded to failure to determine their unconfined compressive
strength. ASTM D 5102 was used for the unconfined compression tests.
The stress-strain curves of the fly ash specimens tested in unconfined
compression are shown in Fig. 6.1. The unconfined compressive strength was
low (0.2 MPa) for the Dewey fly ash, but it ranged between of 7 to 14 MPa for the
other three fly ashes. Columbia fly ash has the highest compressive strength (14
MPa), followed by King (8 MPa), and Edgewater (7 MPa). The Columbia, King,
and Edgewater fly ashes exhibited brittle behavior, with axial strains at failure in
the range of 0.5 to 0.8%.
92
Compressive Strentgh (MPa)
15
12
9
6
3
0
Fig. 6.1.
Columbia
Dewey
King
Edgewater
0
0.5
1
1.5
Strain (%)
2
2.5
Stress-strain data of unconfined compressive strength tests
performed on fly ash specimens prepared at 35% water-fly ash
ratio for a curing period of 14 days.
93
Turner (1997) reports similar results for Class C, Class F, and offspecification fly ashes for curing periods of 7 and 28 days. The average of the
data reported by Turner (1997) at 7 and 28 days indicates that the Class C fly
ash had a maximum compressive strength that ranged from 9 to 14 MPa. The
Class F fly ash had a maximum compressive strength of 6 MPa, and the offspecification fly ash had a compressive strength of 0.7 MPa. Compressive
strengths of the Columbia, Dewey, King, and Edgewater fly ashes after curing
for 14 days are consistent with those measured by Turner (1997).
6.2 UNCONFINED COMPRESSIVE STRENGTH OF UNTREATED SOILS
Unconfined compressive strengths of six soils compacted at optimum
water content and 7% wet of optimum water content are given in Table 6.1.
Unconfined compressive strength for Red Silty Clay Till, Lacustrine Red Clay,
Brown Silt, and organic Theresa Silt Loam was measured after the resilient
modulus test. Specimens of Joy Silt Loam and Plano Silt Loam were prepared
using a Harvard miniature compactor, and were tested directly for unconfined
compression without performing the resilient modulus test. The specimens
prepared in the Harvard miniature compactor are smaller (33 mm in diameter and
71 mm in height) compared to those used in the resilient modulus tests (102 mm
in diameter and 203 mm in height).
Untreated soil specimens (Red Silty Clay Till, Lacustrine Red Clay, Brown
Silt, and organic Theresa Silt Loam) compacted at optimum water content, and
tested after the resilient modulus test had unconfined compressive strengths
94
Table 6.1. Unconfined compressive strength (kPa) of soil and soil-fly ash mixtures.
Near Soil Optimum Water Content (wOPT)
Soil
Name
Curing
Time
(Days)
7% wet of wOPT
Columbia
*Soil
Alone
*Soil
Alone
Columbia
Very Wet Condition
Dewey
King
Columbia
Fly Ash Content (%)
Fly Ash Content (%)
Dewey
Fly Ash Content (%)
0
10
12
14
16
18
20
0
10
18
10
18
10
18
10
18
10
18
30
7
321
(-0.1)
-
-
-
-
-
-
110.7
(6.7)
-
460
(7.0)
-
445
(7.0)
-
-
215
(9.0)
234
(11)
237
(9.0)
263
(11)
-
14
-
-
-
-
-
-
-
-
304
(7.0)
334
(7.0)
263
(7.0)
467
(7.0)
312
(7.0)
511
(7.0)
-
-
-
-
-
28
-
-
-
-
-
-
-
-
-
466
(7.0)
-
464
(7.0)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
500
(7.0)
-
540
(7.0)
-
-
-
-
-
-
-
RSCT
56
7
328
(-1.0)
-
-
-
-
-
-
132.7
(4.4)
-
-
-
-
-
-
181
(9.0)
203
(11)
208
(9)
225
(11)
-
14
-
-
-
-
-
-
-
-
304
(7.0)
368
(7.0)
283
(7.0)
393
(7.0)
284
(7.0)
425
(7.0)
NT
NT
-
-
-
BS
14
446
(-1.2)
-
-
-
-
-
-
105.5
(7.3)
356
(7.0)
450
(7.0)
312
(7.0)
477
(7.0)
244
(7.0)
348
(7.0)
-
-
-
-
-
OTSL
7
190
(-0.9)
-
-
-
-
-
-
32
(5.4)
-
-
-
-
-
-
60
(11)
90
(14)
62
(11)
100
(14)
270
(9)
130
(18)
7
-
565.8
(1.9)
-
614.1
(2.7)
-
648.6
(3.4)
-
89.7
(7.0)
-
-
-
-
-
-
-
-
-
-
-
7†
-
800.4
(1.9)
-
876.3
(2.7)
-
855.6
(3.4)
-
-
-
-
-
-
-
-
-
-
-
-
-
7
-
-
772.8
(2.4)
-
828
3.2)
-
862.5
(4.0)
62.1
(7.0)
-
-
-
-
-
-
-
-
-
-
-
7†
-
-
966
(2.4)
-
1124.7
(3.2)
-
1214.4
(4.0)
-
-
-
-
-
-
-
-
-
-
-
-
LRC
JSL
PSL
Notes: RSCT = Red silty clay till (wOPT = 13%), LRC = Lacustrine Red Clay (wOPT = 24%), BS = Brown Silt (wOPT = 19%), OTSL = Organic Theresa Silt Loam (wOPT = 29%),
JSL = Joy Silt Loam (wOPT = 19%), PSL = Plano Silt Loam (wOPT = 20%). Number in parenthesis indicates the water content of the soil relative (wSOIL - wOPT) to the soil
optimum water content. NT = Not tested. †Soil-fly ash specimens compacted without delay. *Specimens of soil alone tested immediately after compaction.
95
ranging between 200 to 450 kPa. All untreated soils presented in Table 6.1 that
were prepared at 7% wet of optimum water content were too soft to withstand the
resilient modulus repetitive loads. These specimens were prepared and tested
directly in unconfined compression. Under this condition, their compressive
strength ranged from 30 and 130 kPa.
The unconfined compressive strengths in Table 6.1 need to be interpreted
in the context of the general relationship between unconfined compressive
strength and the quality and consistency of the soils used in pavement
applications. Unconfined compressive strength between 25 and 50 kPa is
considered as “soft” subgrade. Unconfined compressive strength ranging from 50
to 100 kPa is considered as “medium” consistency. Unconfined compressive
strength ranging from 100 to 200 kPa is considered as a “stiff”. Unconfined
compressive strength ranging from 200 to 380 kPa are considered as “very stiff”.
An unconfined compressive strength of 380 kPa or above is considered to be
“hard” (Das 1994). The soils tested were “very stiff” when compacted at the
optimum water content. When compacted at water content similar to their natural
water content (i.e., 7% wet of optimum water content), they are of “soft to
medium” consistency.
6.3
UNCONFINED
COMPRESSIVE
STRENGTH
OF
SOIL-FLY
ASH
MIXTURES
Unconfined compressive strengths of the soil-fly ash mixtures prepared at
different water contents are given in Table 6.1. The unconfined compressive
96
strength was measured after the resilient modulus tests for four soils (Red Silty
Clay Till, Lacustrine Red Clay, Brown Silt, and organic Theresa Silt Loam). For
the other two soils (Plano Silt Loam and Joy Silt Loam), the unconfined
compression test was measured immediately after compaction in the Harvard
miniature compactor without resilient modulus testing (Edil et al. 2000).
Soil-fly ash mixtures were prepared from 10 to 30% fly ash content and
compacted at different water contents: near optimum water content, 7% wet of
optimum water content, and a very wet condition with water contents between 9
and 18% wet of optimum water content. The optimum water content condition
was used as a standard reference condition. The condition of 7% wet of optimum
water content simulates the natural moisture condition observed in situ.
Specimens prepared between 9 and 18% wet of optimum water content
simulates a scenario where very wet conditions are encountered. The specimens
were cured for 7 or 14 days.
6.3.1 General Effect of Fly Ash Stabilization
The general effect of fly ash stabilization for several Wisconsin subgrade
soils is shown in Fig. 6.2. The data presented corresponds specimens
compacted at 7% wet of optimum water content and tested in unconfined
compression after the resilient modulus test. Three soils (Red Silty Clay Till,
Lacustrine Red Clay, and Brown Silt) were used in mixtures with various fly
ashes. These soils represent a wide variety of soft subgrade soils in Wisconsin.
Specimens were compacted two hours after mixing with water to simulate the
97
6
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
quSOIL-FLY ASH/quSOIL
5
4
Columbia
2-Hour Delay
3
2
1
(a)
0
0
5
10
15
Percent Fly Ash (%)
20
qu
SOIL-FLY ASH
/qu
SOIL
6
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
5
4
Dewey
2-Hour Delay
3
2
1
(b)
0
0
5
10
15
Percent Fly Ash (%)
20
quSOIL-FLY ASH/quSOIL
6
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
5
4
King
2-Hour Delay
3
2
1
0
Fig. 6.2.
(c)
0
5
10
15
Percent Fly Ash (%)
20
The unconfined compressive strength of soil-fly ash specimens
normalized to the unconfined compressive strength of the untreated
soil compacted 7% wet of optimum water content.
98
construction delay that typically occurs in the field before subgrade compaction
due to construction operations.
The ratio of the unconfined compressive strength of the fly ash-stabilized
soil to that of the untreated soil (referred to as “strength gain”) is shown in Fig.
6.2 as a function of fly ash content. The strength gain of soil-fly ash specimens
prepared with 10 and 18% fly ash content varies approximately linearly with fly
ash content.
Strength gain for three different soils stabilized with Columbia, Dewey, and
King fly ashes is plotted as a function of fly ash content in Fig. 6.3. The addition
of 10% fly ash causes the unconfined compressive strength to increase by a
factor of 3, on the average, whereas 18% fly ash caused the unconfined
compressive strength to increase by a factor of 4. The largest unconfined
compressive strength was obtained with the Red Silty Clay Till and Brown Silt,
and the smallest with the Lacustrine Red Clay. The final consistency of the
mixtures was “hard”.
A comparison between the specimens prepared with the off-specification
Dewey and King fly ashes, and Columbia as the reference (Class C) is shown in
Fig. 6.4. Unconfined compressive strength of the specimens prepared with the
off-specification fly ashes have been normalized by the unconfined compressive
strength of the corresponding specimen prepared with Columbia fly ash. The
unconfined compressive strength of the soils stabilized with the off-specification
fly ashes (Dewey and King) is similar or larger compared to the specimens
prepared with Columbia fly ash. The soil-fly ash specimens prepared with 10 and
99
quSOIL-FLY ASH/quSOIL
6
5
4
2
1
/qu
SOIL
6
SOIL-FLY ASH
Columbia
Dewey
King
3
0
qu
(a) Red Silty Clay Till
5
4
10
18
Percent Fly Ash (%)
(b) Lacustrine Red Clay
Columbia
Dewey
King
3
2
1
0
quSOIL-FLY ASH/quSOIL
6
5
4
10
18
Percent Fly Ash (%)
(c) Brown Silt
Columbia
Dewey
King
3
2
1
0
Fig. 6.3.
10
18
Percent Fly Ash (%)
Strength gain as a function of fly ash content for red silt clay till,
lacustrine red clay, and brown silt prepared with Columbia, Dewey,
and King fly ashes compacted 7% wet of optimum water content.
100
3
(a) Dewey
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
qu
DEWEY
/qu
COLUMBIA
2.5
2
1.5
1
0.5
0
10
18
Percent Fly Ash (%)
3
Red Silty Clay Till
Lacustrine Red Clay
Brown Silt
(b) King
quKING/quCOLUMBIA
2.5
2
1.5
1
0.5
0
10
18
Percent Fly Ash (%)
Fig. 6.4.
Unconfined compressive strength prepared with (a) Dewey and (b)
King fly ashes normalized to the unconfined compressive strength
prepared with Columbia fly ash as a function of fly ash content.
(Specimens prepared 7% wet of optimum water content).
101
and 18% Dewey and King fly ashes have unconfined compressive strengths
between 0.7 and 1.5 times the unconfined compressive strength using Columbia
fly ash. King fly ash appears less effective with Brown Silt compared to the other
two soils.
6.3.2 Effect of Soil Water Content
The effect of soil water content on the unconfined compressive strength of
soil-fly ash mixtures is shown in Fig. 6.5. Data from three soils (Red Silty Clay
Till, Lacustrine Red Clay, and Brown Silt) are included in Fig. 6.5. Each soil was
mixed with 10% and 18% Columbia, Dewey, and King fly ash.
Scatter exists in the trends because the data are from different soils, and
each fly ash has a unique stabilization effect on each soil. At 10% fly ash content
unconfined compressive strength is fairly constant as the soil water content
increases. However, at 18% fly ash content the unconfined compressive strength
decreases slightly with increasing soil water content.
6.3.3 Effect of Curing Time
Soil-fly ash mixtures were prepared with Red Silty Clay Till at 18% fly ash
content with Columbia, Dewey, and King fly ashes and were cured for periods of
7, 14, 28, and 56 days to evaluate how curing time affects unconfined
compressive strength. Unconfined compressive strength tests were performed on
the specimens after the resilient modulus test. All specimens were prepared at
7% wet of optimum water content.
102
600
10% FA-14 days
18% FA-14 days
550
qu (kPa)
500
450
400
350
300
250
200
18
Fig. 6.5.
20
22
24
26
28
Soil Water Content (%)
30
32
Effect of soil water content on the unconfined compressive strength of
soil-fly ash specimens prepared with 10 and 18% fly ash content and
cured for 14 days.
103
The effect curing time on the unconfined compressive strength of soil-fly
ash mixtures with different fly ash contents is shown in Fig. 6.7. Curing time did
not have a large effect on unconfined compressive strength. After 7 days of
curing, the strength stayed fairly constant. The unconfined compressive strength
of the specimens with 18% Columbia fly ash decreased at 14 days, but increased
and stayed constant from 28 to 56 days.
6.3.4 Effect of Soil Type
The relationship between strength gain and liquid limit (LL), plasticity index
(PI), group index (GI), and liquidity index (LI) is shown in Figs. 6.8 and 6.9. The
scatter in the strength gain data is due to the unique effect of fly ash stabilization
on each soil. The strength gain is not particularly sensitive to any of the four soil
indicators. There is only a slight trend for the soil-fly ash specimens prepared at
18% fly ash content. That is, the strength gain appears to be essentially
independent of the composition of fine-grained soils when 10% or more fly ash is
added.
6.3.5 Effect of Organic Matter
Unconfined compressive strength of the organic Theresa Silt Loam mixed
with two different fly ashes is shown in Fig. 6.10. One of the fly ashes used was
the Columbia Class C fly ash and the other was the off-specification Dewey fly
ash. Organic Theresa Silt Loam originally has low compressive strength (32 kPa)
at 7% wet of optimum water. The addition of fly ash had a small effect on the
104
600
qu (kPa)
500
400
300
200
100
Fig. 6.6.
1 8 % C o lu m b ia
1 8 % D ew e y
1 8 % K in g
0% FA
0
10
20
30
40
T im e (D a y s)
50
60
Effect of fly ash content and curing time on the unconfined
compressive strength of soil-fly ash specimens prepared with
Columbia, Dewey, and King fly ashes at 18% fly ash content.
105
8
(a)
7
10% FA-14 days
18% FA-14 days
5
4
3
qu
SOIL-FLY ASH
/qu
SOIL
6
2
1
0
45
50
55
60
Liquid Limit (LL)
65
70
8
(b)
7
10% FA-14 days
18% FA-14 days
qu
SOIL-FLY ASH/
qu
SOIL
6
5
4
3
2
1
0
20
Fig. 6.7.
25
30
Plasticity Index (PI)
35
40
Effect of (a) liquid limit (LL) and (b) plasticity index (PI) on the
unconfined compressive strength ratio of soil-fly ash specimens.
106
8
(a)
7
10% FA-14 days
18% FA-14 days
qu
SOIL-FLY ASH
/qu
SOIL
6
5
4
3
2
1
0
20
25
30
35
Group Index (GI)
40
45
8
qu
SOIL-FLY ASH
/qu
SOIL
7
(b)
10% FA-14 days
18% FA-14 days
6
5
4
3
2
1
0
-0.25
Fig. 6.8.
-0.2
-0.15
-0.1
-0.05
Liquidity Index (LI)
0
0.05
Effect of (a) group index (GI) and (b) liquidity index (LI) on the
unconfined compressive strength ratio of soil-fly ash specimens.
107
120
Columbia
Dewey
100
qu (kPa)
Organic Theresa Silt Loam
80
60
40
20
0
5
10
15
20
Percent Fly Ash (%)
Fig. 6.9.
Effect of fly ash content on the unconfined compressive strength of
organic Theresa Silt Loam prepared with Columbia and Dewey fly
ashes in very wet conditions and cured for 7 days.
108
strength gain of the organic Theresa Silt Loam compared to the inorganic clays,
even when the fly ash content obtained with 18%. The maximum unconfined
compressive strength reached at 18% fly ash content was 100 kPa, which is
considered as “medium” consistency. Similar results gains were obtained sing
both fly ashes.
The low unconfined compressive strength development of the organic
Theresa silt loam using either Columbia or Dewey fly ashes is believed to be
caused by the inhibition of the hydration by the organic matter in the soil.
Lower strength gains for organic also have also been reported by Arman
and Munfakh (1972). They performed unconfined compressive strength tests with
one inorganic high plasticity clay and two organic high plasticity clays (LOI of 8
and 14%, respectively) mixed with increasing percentages of lime (CaO) (i.e., 0,
2, 4, 6, 8, and 12% by dry weight) and cured for periods ranging between 7 to 28
days. Soil-lime mixtures prepared with the inorganic clay showed a maximum
unconfined compressive strength of 200 kPa when prepared with 6% lime at 7
days curing. Soil-lime mixtures prepared with the organic clays had lower
unconfined compressive strengths of 40 and 80 kPa, respectively, when
prepared with even higher lime contents of 8 and 12% and cured for 7 days.
When the curing time was increased to 28 days the inorganic clay reached a
maximum unconfined compressive strength of 400 kPa and the organic clays
were still below 80 kPa.
Hampton and Edil (1998) describe stabilization of an organic clay using
four different binders to compare the effects of different binders. The first binder
109
(Binder F) consisted of 80% blast furnace slag cement (FSC) and 20% anhydrite.
The second binder consisted of 50% ordinary Portland cement (OPC) and 50%
lime (CaO) and the third binder consisted of 80% ordinary Portland cement
(OPC) and 20% lime (CaO). The fourth binder consisted of 80% (FCS), 14.5%
anhydrite, and 5.5% high aluminum cement. The binder and soil were mixed dry,
and water was added to achieve the natural soil water content. The first and
fourth binder composed mostly of (FSC) caused the largest increase in
unconfined compression of the organic clay. Unconfined compressive strengths
of 1460 and 1827 kPa were reported, compared to unconfined compressions of
40 and 145 kPa for the second and third binders, which were composed mostly
of (OPC) and (CaO). The unconfined compressive strengths for the organic
Theresa Silt Loam at 18% fly ash content (100 kPa) are similar to the strengths
of Hampton and Edil (1998) for binders two and three.
6.4 SYNTHESIS
The effect of fly ash addition on unconfined compressive strength appears
to be unique for each soil and fly ash. A general trend of increasing unconfined
compressive strength with increasing fly ash content or decreasing water content
is evident for all soils that were tested.
The strength gain of soil-fly ash specimens prepared with 10 and 18% fly
ash content varied with soil type and fly ash type. Addition of 10% fly ash caused
the unconfined compressive strength to increase by a factor of 3, on average,
110
whereas 18% fly ash caused the unconfined compressive strength to increase by
a factor of 4.
Soil water content had a small effect on the unconfined compressive
strength of soil-fly ash mixtures for fly ash contents of 10% or more. Curing time
also had a small effect on the strength gain of red silty clay till, prepared 7% wet
of optimum water content. In general, unconfined compressive strength was in
the same range of 450 kPa, on average, for specimens prepared with Columbia
and Dewey ashes and cured for 7, 14, or 28 days. A decrease in unconfined
compressive strength was reported for the soil-fly ash specimen prepared with
18% Columbia fly ash and cured for 14 days. The largest improvement was
achieved at 56 days when the soil-fly ash specimens reached an average
maximum unconfined compressive strength of 520 kPa.
Four parameters were used as indicators of soil type: LL, PI, GI, and LI.
The strength gain of soil-fly ash specimens was insensitive to all the soil index
parameters.
Specimens prepared with organic Theresa Silt Loam and two fly ashes
(Columbia and Dewey) showed lower unconfined compressive strengths
(maximum compressive strength of 100 kPa with 18% fly ash) than those
obtained with the inorganic clays (about 500 kPa). The consistency of the
organic Theresa Silt Loam improved from “soft” to a “medium” consistency when
fly ash was added, whereas it increased from “soft” to “hard” for inorganic clays.
The presence of organic matter in this soil appears to inhibit the pozzolanic
reaction between fly ash and water.
111
SECTION 7
PRACTICAL APPLICATION
7.1 FLEXIBLE PAVEMENT DESIGN
A diagram with a typical layout of a flexible pavement is shown in Fig. 7.1.
A flexible pavement structure is composed of a set of four different layers (i.e.,
asphalt concrete layer, base course layer, subbase course layer, and the native
soil or subgrade). Pavement design requires that coefficients be assigned to
each layer. These coefficients (i.e., a1, a2, and a3) define capacity of each layer to
bear traffic loads within tolerable limits of deflection.
Pavement design consists of selecting layer thicknesses such that the
pavement meets a target structural number. The structural number is related to
the layer thicknesses and layer coefficients by:
SN = a1D1 + a2D2 + a3D3
(7.1)
where:
SN = structural number or structural capacity of the pavement,
a1, a2, and a3 = material coefficients, and
D1, D2, and D3 = thicknesses of the different layers (in., mm).
The required structural number for a pavement system depends on the
traffic load, design period, serviceability, and reliability as well as soil support
value or resilient modulus of the subgrade soil.
112
Layer coefficient (a3) correspond to the subbase course layer in a flexible
pavement, as shown in Fig. 7.1. Typical subbase layer coefficients (a3) for the
soil and soil-fly ash mixtures are shown in Table 7.1. These coefficients were
obtained using the typical CBR and resilient modulus measured at different water
contents and the chart for granular subbase layer coefficients in the AASHTO
guide (1993). In general, subbase layer coefficients increase with fly ash content
and curing time, and decrease with water content. Subbase layer coefficients for
18% fly ash content and 7% wet of optimum water content are similar or higher
than those for untreated soil (i.e., 0% fly ash content) compacted at optimum
water content. Subbase layer coefficients obtained from the resilient modulus test
are more sensitive to water content compared to those obtained from the CBR
test at any given fly ash content.
7.2 BASE THICKNESS DESIGN COMPARISON
The layer coefficients in Table 7.1 were used to design flexible pavement
structures including the subgrade of a stabilized soil and also an untreated
compacted subgrade. The design characteristics described by Edil et al. (2002)
for a secondary highway in Wisconsin were used. The design number of ESALs
was 8,000,000 and the required pavement structural number (SN) was 4.5.
Thickness of the asphalt concrete and subbase layers was kept constant.
The asphalt concrete layer was assumed to be 102 mm thick, which is the
minimum thickness recommended by AASHTO (1993) to withstand at least
7,000,000 ESALs. The asphalt concrete layer structural coefficient (a1) was
113
Asphalt Concrete Layer (a1)
Base Course Layer (a2)
Subbase Course Layer (a3)
Subgrade (Native Soil)
Fig. 7.1. Diagram of a typical flexible pavement structure with its four
components.
114
Table 7.1. Recommended subbase layer coefficients (a3) of soil and soil-fly
ash mixtures from CBR and resilient modulus tests at different
water content
Fly
Ash
Content
(%)
Curing
Time
(Days)
0
Subbase Layer Coefficients (a3)
wOPT
Very Wet
Condition
7% wet of wOPT
CBR
Mr
CBR
Mr
CBR
Mr
-
0.090
0.070
0.030
*0
-
-
10
14
-
-
0.082
0.030
0.076
0.010
18
14
-
-
0.100
0.080
0.080
0.040
18
28
-
-
-
0.100
-
-
*Estimated from CBR tests at 7% wet of wOPT using Eq. 5.2.
115
chosen to be 0.44 (Bahia et al. 1997). A subbase layer 305 mm thick was
assumed, since this is the conventional thickness achieved by construction
equipment for in situ mixing of fly ash. The subbase layer coefficient (a3) was
varied between 0 to 0.10 to simulate different fly ash contents or untreated
subgrade conditions (Table 7.1). The base course layer was assumed to be a
Grade-2 gravel (WisDOT Specification, 2000) with a structural coefficient (a2) of
0.16. The base course thickness (D2) was adjusted to achieve a SN of 4.5.
Thicknesses of the base course that were obtained are shown in Table
7.2. In general, the base course thickness increases as the soil water content
increases, and decreases with increasing curing time and fly ash content. The
base layer thicknesses obtained using the resilient modulus also vary more than
those computed using the CBR.
A thinner base course was required (292 mm), when the subbase layer
was stabilized with 18% fly ash content, compacted 7% wet of optimum water
content, and cured for 14 days, compared to that needed for a subbase of
untreated soil compacted at optimum water content (305 mm). When the curing
time is 28 days, the required thickness of the base layer is even smaller (254
mm). If the subbase consisted of subgrade soil at its typical natural water content
(i.e., 7% wet of wOPT), the required base course thickness would be 432 mm.
116
Table 7.2. Base course thicknesses calculated using CBR and resilient
modulus subbase layer coefficients (a3).
Fly
Ash
Content
(%)
Curing
Time
(Days)
0
Base Course Thickness (mm)
wOPT
7% wet of wOPT
Very Wet
Condition
CBR
Mr
CBR
Mr
CBR
Mr
-
267
305
381
432
-
-
10
14
-
-
279
381
292
419
18
14
-
-
254
292
279
356
18
28
-
-
-
254
-
-
117
SECTION 8
CONCLUSIONS
The objective of the study was to quantify the effect of fly ash stabilization
of different soft subgrades encountered in Wisconsin using locally available selfcementing fly ashes. Soil-fly ash mixtures were prepared at different fly ash
contents (10-30%) and compacted at optimum water content, 7% wet of optimum
water content, and at a very wet condition (9 to 12% wet of optimum water
content). CBR, resilient modulus, and unconfined compression tests were
performed to quantify the effect of fly ash addition on the mechanical behavior of
soils.
8.1 CBR
A general trend of increasing CBR with increasing fly ash content, and
decreasing CBR with increasing soil water content was observed. The gain in
CBR depends on the amount of fly ash in the mixture. Soil-fly ash mixtures
prepared with 10% and 18% fly ash content and compacted at 7% wet of
optimum water content showed CBR gains of a factor of 4 and 8, respectively.
Larger CBR gains were typically obtained when stabilizing wetter or more plastic
(i.e., poorer) subgrade soils.
The presence of larger amounts (10%) of organic matter in one of the
soils inhibited pozzolanic reactions, resulting in a smaller increase in CBR than
was obtained with the inorganic soils. However, use of an off-specification fly ash
118
with higher organic content (16%) resulted in larger (7 times) CBR of the organic
clay relative to that obtained with a typical Class C fly ash.
8.2 RESILIENT MODULUS
Soil-fly ash mixtures prepared with 10% fly ash and compacted 7% wet of
optimum water content had similar or lower resilient modulus than untreated soils
compacted at optimum water content. However, when the fly ash content was
increased to 18%, the resilient modulus was approximately 30% higher, on
average, compared to the resilient modulus of the untreated soil compacted at
optimum water content. The increases in resilient modulus typically were larger
for wetter or more plastic soils (i.e., poorer subgrades).
The effect of curing time on resilient modulus was evaluated for a typical
Wisconsin subgrade soil stabilized with two fly ashes (Dewey and Columbia).
Curing for 28 d caused the resilient modulus to increase between 20 and 50%
relative to the resilient modulus at 7 d.
Four of the soil-fly ash mixtures prepared with the organic Theresa Silt
Loam and two fly ashes (Columbia and Dewey) failed during the pre-conditioning
stage of the resilient modulus test. The high organic content of this soil inhibited
pozzolanic reactions between the fly ash and the water, limiting any
improvements in resilient modulus.
However, appreciable gains in resilient
modulus were obtained when the organic Theresa Silt Loam was stabilized with
the off-specification high carbon fly ash.
119
8.3 UNCONFINED COMPRESSIVE STRENGTH
General trends of increasing unconfined compressive strength with
increasing fly ash content, and decreasing unconfined compressive strength with
increasing water content, were observed. The strength gain that was achieved
varied somewhat with soil and fly ash type, but was not strongly correlated with
soil index properties. Adding 10% fly ash caused the unconfined compression to
increase by a factor of 3, whereas adding 18% fly ash caused the unconfined
compressive strength to increase by a factor of 4. Only marginal increases in
unconfined compressive strength were obtained when curing time was increased
beyond 7 days.
8.4 PRACTICAL APPLICATIONS FOR FLEXIBLE PAVEMENT DESIGN
Results obtained from CBR and resilient modulus tests on soil-fly ash
mixtures were used to develop a set of recommended subbase layer coefficients
(a3) for use in the design of flexible pavements. A comparison analysis also
showed that a reduction in base layer thickness of at least 33% can be obtained
with a fly ash stabilized subgrade relative to that for untreated subgrade soil.
120
SECTION 9
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