Ref#36.pdf

Stabilization of Organic Soils with Fly Ash
Erdem O. Tastan1
; Tuncer B. Edil, F.ASCE2
; Craig H. Benson, F.ASCE3
; and Ahmet H. Aydilek, M.ASCE4
Abstract: The effectiveness of fly ash use in the stabilization of organic soils and the factors that are likely to affect the degree of stabilization
were studied. Unconfined compression and resilient modulus tests were conducted on organic soil–fly ash mixtures and untreated soil spec-
imens. The unconfined compressive strength of organic soils can be increased using fly ash, but the amount of increase depends on the type of
soil and characteristics of the fly ash. Resilient moduli of the slightly organic and organic soils can also be significantly improved. The
increases in strength and stiffness are attributed primarily to cementing caused by pozzolanic reactions, although the reduction in water
content resulting from the addition of dry fly ash solid also contributes to strength gain. The pozzolonic effect appears to diminish as
the water content decreases. The significant characteristics of fly ash that affect the increase in unconfined compressive strength and resilient
modulus include CaO content and CaO=SiO2 ratio [or CaO=ðSiO2 þ Al2O3Þ ratio]. Soil organic content is a detrimental characteristic for
stabilization. Increase in organic content of soil indicates that strength of the soil–fly ash mixture decreases exponentially. For most of the
soil–fly ash mixtures tested, unconfined compressive strength and resilient modulus increased when fly ash percentage was increased. DOI:
10.1061/(ASCE)GT.1943-5606.0000502. © 2011 American Society of Civil Engineers.
CE Database subject headings: Fly ash; Soil stabilization; Stiffness; Organic matter.
Author keywords: Organic soil; Fly ash; Stabilization; Strength; Stiffness; Stabilization.
Introduction
Construction of roadways on soft organic soils can be problematic
because organic soils typically have low shear strength and high
compressibility (Edil 1997). Current practice for construction of
roadways over organic soil subgrades mostly involve the removal
of the organic soil to a sufficient depth and replacement with
crushed rock (referred to as “cut and replace”) or preloading to
improve engineering properties. Chemical stabilization with
binders such as cement, lime, and fly ash can be undertaken rapidly
and often at low cost, and therefore chemical stabilization is
becoming an important alternative (Keshawarz and Dutta 1993;
Sridharan et al. 1997; Kaniraj and Havanagi 1999; Parsons and
Kneebone 2005).
Chemical stabilization of soft soils involves blending a binder
into the soil to increase its strength and stiffness through chemical
reactions. The binder is intended to cement the soil solids, thereby
increasing strength and stiffness. The binders are generally added
as dry solids. In practice, reducing the water content of high-water-
content soils to the optimum water content (OWC) is difficult and
time-consuming. Therefore, addition of dry solids and cementitious
materials is preferable. Thus, addition of a binder reduces both the
water content and binds the soil particles, which results in an
increase in strength and stiffness. Common binders include cement,
lime, fly ash, or mixtures thereof. The use of fly ash as a binder is
attractive because fly ash is an industrial by-product that is
relatively inexpensive, compared with cement and lime (Federal
Highway Administration 2003). Additionally, using fly ash for soil
stabilization, particularly fly ashes that otherwise would be land-
filled, promotes sustainable construction through reduction of
energy use and reduction of greenhouse gases.
Fly ash has been shown to effectively stabilize soft inorganic
soils (Ferguson 1993; Acosta et al. 2003; Prabakar et al. 2004;
Bin-Shafique et al. 2004; Trzebiatowski et al. 2005), but little is
known regarding the effectiveness of stabilizing soft organic soils
with fly ash. Organic soils are known to be more difficult to sta-
bilize chemically than inorganic soils (Hampton and Edil 1998;
Janz and Johansson 2002). The objectives of this study were
(1) to determine if fly ashes can stabilize organic soils, and, if
so, (2) to quantify the improvement in the unconfined compressive
strength (UCS, qu) and resilient modulus of the organic soil as
admixed with fly ash, and (3) to investigate potentially important
factors affecting the stabilization process, such as fly ash and
soil characteristics, fly ash percentage in the mixture, and water
content.
Background
Chemical Stabilization
When binders such as lime, cement, and fly ash are blended with
soil in the presence of water, a set of reactions occur that result in
dissociation of lime (CaO) in the binders and the formation of ce-
mentitious and pozzolanic gels [calcium silicate hydrate gel (CSH)
and calcium aluminate silicate hydrate gel (CASH)]:
CaO þ H2O ⇒ CaðOHÞ2 ð1Þ
1
Assistant Project Engineer, Paul C. Rizzo Associates, Inc., Monroe-
ville, PA 15146.
2
Professor, Geological Engineering Program, Dept. of Civil and Envir-
onmental Engineering, Univ. of Wisconsin, Madison, WI 53706. E-mail:
edil@engr.wisc.edu
3
Wisconsin Distinguished Professor and Chairman, Dept. of Civil and
Environmental Engineering, Univ. of Wisconsin, Madison, WI 53706.
E-mail: benson@engr.wisc.edu
4
Associate Professor, Dept. of Civil and Environmental Engineering,
Univ. of Maryland, 1163 Glenn Martin Hall, College Park, MD 20742
(corresponding author). E-mail: aydilek@eng.umd.edu
Note. This manuscript was submitted on September 17, 2010; approved
on January 6, 2011; published online on January 8, 2011. Discussion period
open until February 1, 2012; separate discussions must be submitted for
individual papers. This paper is part of the Journal of Geotechnical
and Geoenvironmental Engineering, Vol. 137, No. 9, September 1,
2011. ©ASCE, ISSN 1090-0241/2011/9-819–833/$25.00.
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / SEPTEMBER 2011 / 819
J. Geotech. Geoenviron. Eng. 2011.137:819-833.
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CaðOHÞ2 ⇒ Ca2þ
þ 2½OH
ð2Þ
Ca2þ
þ 2½OH þ SiO2 ⇒ CSH ð3Þ
Ca2þ þ 2½OH þ Al2O3 ⇒ CASH ð4Þ
These reactions are referred to as cementitious and/or pozzo-
lanic reactions that result in the formation of cementitious gels.
The increase in strength was found to be roughly related to the type
and quantity of possible reaction products (i.e., cement reaction
product, CSH for short-term strength and pozzolanic reaction prod-
uct, CASH for long-term strength gain).
The source for the pozzolans (a siliceous or aluminous material)
is either the soil or the binding agent. These reactions contribute to
stabilization of soils in two ways. First, plasticity of the soil is re-
duced by the exchange of calcium ions in the pore water with
monovalent cations on clay surfaces and by compression of the ad-
sorbed layer because of the elevated ionic strength of the pore water
(Rogers and Glendinning 2000). Second, the CSH or CASH gels
formed by cementitious and pozzolanic reactions bind the solid par-
ticles together, and this binding produces a stronger soil matrix
(Arman and Munfakh 1972). For organic soils, reactions are
expected to be inhibited or delayed by the existence of organic
compounds (Hampton and Edil 1998; Tremblay et al. 2002).
Mechanisms of organic matter interference with strength gain in
chemical stabilization are not fully understood, but the following
mechanisms are suggested (Hampton and Edil 1998; Axelsson et al.
2002; Janz and Johansson 2002): (1) organic matter can alter the
composition and structure of CSH gel, a cementing compound that
forms bonds between particles and also the type and amount of
other hydration products, e.g., ettringite; (2) organic materials often
contain materials such as humus or humic acid, which retard
strengthening reactions; (3) organic matter holds 10 or more times
its dry weight in water and may limit water available for hydration;
and (4) organic matter forms complexes with aluminosilicates and
with metal ions, and such complexes interfere with hydration.
Some fly ashes contain lime and pozzolans, such as Al2O3 and
SiO2, and therefore are self-cementing. The effectiveness of a given
fly ash is expected to depend on the relative abundance of CaO and
oxides providing pozzolans. For example, Class C fly ashes (i.e.,
fly ashes meeting the requirements in ASTM C618 (ASTM 2008)
for use in ready-mix concrete) have a CaO content 20% (by
weight) and a Al2O3 þ Fe2O3 þ SiO2 content of 50–70%. In con-
trast, Class F fly ashes have 10% CaO. Consequently, Class C
ashes generally are more effective at forming CSH and CASH gels
than Class F ashes (Sridharan et al. 1997).
Janz and Johansson (2002) indicate that the CaO=SiO2 ratio,
which stands for relative abundance of CaO and SiO2, is an indi-
cator of the potential for pozzolanic reactions and that binders
with larger CaO=SiO2 ratios are likely to be more effective stabi-
lizers. For example, C3S clinker, which is a strong binder, has
a CaO=SiO2 ratio = 3. Similarly, the ratio of CaO=ðSiO2 þ
Al2O3Þ can also be used as an indicator of the potential to form
CSH and CASH gels (Odadjima et al. 1995). However, binders
with a high CaO=SiO2 or CaO=ðSiO2 þ Al2O3Þ ratio can still
be ineffective if pozzolanic reactions are limited by the availability
of CaO pozzolans (e.g., too little CaO, SiO2, and/or Al2O3)
Inhibition of Cementing Reactions by Organic Matter
Fly ash specifications for concrete applications usually include an
upper bound on the organic carbon content of the fly ash. This
upper bound is normally characterized by the loss on ignition
(LOI) measured with ASTM C311. Clare and Sherwood (1954)
indicated that the organic matter in organic soils adsorbs Ca2þ
ions.
When cement, lime, or fly ash (any source of Ca2þ
ions) is added to
organic soils, following the hydration of lime [Eqs. (1) and (2)],
released Ca2þ ions are likely to be exhausted by the organic matter,
which limits the availability of Ca2þ
ions for pozzolanic reactions.
Thus, the amount of CaO in fly ash should be large enough to com-
pensate for the consumption of Ca2þ
ions by the organic matter in
the soil. The possible interactions of organic compounds with poz-
zolanic minerals (Ca2þ or Alþ3) or CaðOHÞ2 are summarized as
follows (Young 1972): (1) calcium ions can be adsorbed by the
organic matter instead of reacting with pozzolanic minerals; (2) or-
ganic compounds react with CaðOHÞ2 and precipitate, which forms
insoluble compounds and limits the availability of Ca2þ ions for
pozzolanic reactions; (3) alumina can form stable complexes with
organic compounds, and calcium ions can also complex with or-
ganic compounds, but Young (1972) stated that complexes formed
by Ca2þ
ions were not stable and would not affect the calcium ion
equilibria; and (4) organic compounds can adsorb on CaðOHÞ2
nuclei, which inhibit the growth of nuclei and formation of
CSH. Hampton and Edil (1998) indicated that the organic matter
in soils can also retain large amounts of water, which can reduce the
amount of available water for hydration reactions when a cementi-
tious additive is blended with soil.
Similarly, organic matter in soil is known to affect stabilization
using cements or fly ashes. For example, Tremblay et al. (2002)
evaluated how cement stabilization of an inorganic soil [a clay with
plasticity index (PI) = 26] was inhibited by organic content by add-
ing organic compounds to the soil, such as acetic acid, humic acid,
tannic acid, ethylenediaminetetraacetic acid (EDTA), and sucrose.
Tremblay et al. (2002) also suggested that pozzolanic reactions are
likely to be inhibited if the pH of the soil-cement mixture is less
than 9.
Materials and Methods
Soils
Three soft organic soils with different organic contents were used in
the study: Markey (silty, sandy peat), Lawson (low plasticity or-
ganic sandy clay), and Theresa (moderately plastic organic clay).
All soils were collected within 1 m of the ground surface and are
typical of organic soils encountered as a subgrade during roadway
construction in Wisconsin. Index properties of the soils (and com-
paction parameters) are summarized in Table 1. All three soils had
bell-shaped compaction curves, but the maximum dry unit weight
of these soils is less than the typical for soils from Wisconsin
with similar plasticity (Edil et al. 2006). An inorganic silt from
Boardman, Oregon (Boardman silt) was also used in the testing
program. Index properties of the silt are summarized in Table 1.
This silt, which has similar particle-size distribution as the fly ashes
in the study, was used as a nonreactive binder in some of the mix-
tures to separate the effects of cementing and reduction in water
content by adding dry solid.
Fly Ashes
Six fly ashes and Type I portland cement were used as binders in the
study. The fly ashes were obtained from electric power plants in the
upper Midwestern United States and were selected to provide a broad
range of carbon content (0.5–49%), CaO content (3.2–25.8%),
and CaO=SiO2 ratio (0.09–1.15). General properties of the fly ashes
are summarized in Table 2.
The Stanton and Columbia fly ashes classify as Class C ash
and the Coal Creek fly ash classifies as Class F ash, according
to ASTM C618 (ASTM 2008). The remainders are referred to
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as off-specification fly ashes because they do not meet the require-
ments for either Class C or Class F fly ashes in ASTM C618. In
addition, the fineness of the Dewey and Columbia ashes exceeds
the maximum for Classes C and F, and the pozzolanic activity at
7 days of the Presque Isle ash does not meet the minimum for
Classes C and F. The Dewey, King, and Columbia fly ashes are
derived from subbituminous coals, the Presque Isle fly ash is
derived from bituminous coal, and the Coal Creek and Stanton
fly ashes are derived from burning lignite. All of the fly ashes,
except for the Presque Isle fly ash, which was collected by fabric
filters, were collected by electrostatic precipitators and stored dry
in silos.
Among the six fly ashes, Dewey has the highest carbon content
(LOI ¼ 49%) and Coal Creek has the lowest carbon content
(LOI ¼ 0:5%). King has the highest CaO content (25.8%) and Pre-
sque Isle has the lowest CaO content (3.2%). Dewey and King have
the highest CaO=SiO2 ratios (1.15 and 1.08), Stanton and Colum-
bia have midrange CaO=SiO2 ratios (0.5 and 0.7), and Presque Isle
and Coal Creek have the lowest CaO=SiO2 ratios (0.1 and 0.2). All
of the fly ashes have less CaO and a smaller CaO=SiO2 ratio than
the Type 1 portland cement (CaO content = 62%, CaO=SiO2 ratio =
2.9). The fly ashes generally are comprised of silt-size particles
( 75 μm and 2 μm), with a coarse fraction between 5% and
50% and a 2 μm fraction between 10% and 67%. Dewey
and Columbia fly ashes have similar grain-size distributions and
are somewhat finer than King, Coal Creek, and Stanton, which have
similar grain-size distributions. Presque Isle fly ash has mostly
uniform size particles (∼0:03 mm).
pH
The pH of each soil was measured using both ASTM D4972
(ASTM 2007e, for inorganic soils) and ASTM D2976 (ASTM
2004b, for peats). These methods differ in the ratio of dry solid
to distilled water that is used (1∶1 for D4972, 1∶16 for D2976).
All three soils had near-neutral pH, and both test methods yielded
a similar pH.
The pH of each fly ash was measured using ASTM D5239
(ASTM 2004a) and the procedure described in Eades and Grim
(1966). ASTM D5239 uses a solid to distilled water ratio of 1∶4
and a 2-h lag between mixing and pH measurement. The Eades
and Grim method uses a solid to distilled water ratio of 1∶5, a
lag of 1 h, and requires the use of CO2-free water. The pH of
the each fly ash was also measured at 1, 2, 6, 24, 48, and 96 h after
mixing to assess the pH change over time; however, the pH did not
vary significantly with time. All pH results at 1 h after mixing are
given in Table 2.
Unconfined Compression Testing
Unconfined compression tests were conducted on specimens pre-
pared from the soils and soil–fly ash mixtures following ASTM
D5102 (ASTM 2009b). The strain rate was 0:21%= min, which
is the same rate used by Edil et al. (2006) for evaluating soil–
fly ash mixtures prepared with inorganic soils. Test specimens were
prepared by first mixing the dry soil and the dry fly ash at the speci-
fied fly ash content on dry weight basis. Subsequently, the amount
of water required was added, and after a wait of 2 h (to simulate
field conditions), the mixture was compacted in a steel mold with a
diameter of 33 mm and height of 71 mm. The compactive effort for
specimen preparation was adjusted in such a way that the same
impact energy per unit volume, as in the standard Proctor effort
[ASTM D698 (ASTM 2007a)], was applied. After the compaction,
the specimens were extruded with a hydraulic jack, sealed in
plastic, and cured for 7 days in a room maintained at 100% rela-
tive humidity and 25°C. Although the tests were performed on
Table
1.
Index
Properties
and
Classifications
of
Soils
Tested
Soil
name
LL
PI
Fines
content
(%)
Active
clay
content
(
2
μm)
(%)
OC
(%)
Gravel
content
(
4:75
mm)
(%)
G
s
Classification
pH
w
N
γ
d
(kN=m
3
)
w
opt
USCS
AASHTO
ASTM
D4972
ASTM
D2976
Markey
peat
53
1
25
15
27
8
2.23
Pt
A-8
(0)
5.9
6.3
57
10.3
47
Theresa
soil
31
8
75
36
6
—
2.57
OL
A-4
(5)
7.6
7.1
20
15.2
21
Lawson
soil
50
19
97
55
5
—
2.58
OL-OH
A-7-5
(23)
6.9
6.8
28
13.3
28
Boardman
silt
22
1
79
12
1
—
2.67
ML
A-2-4
(0)
—
—
11
17.3
17
Note:
LL
=
liquid
limit;
PI
=
plasticity
index;
OC
=
organic
content
[ASTM
D2974,
(ASTM
2007b)];
G
s
=
specific
gravity;
w
N
=
natural
water
content;
γ
d
=
maximum
dry
unit
weight
(ASTM
D698);
w
opt
=
optimum
water
content
[(ASTM
D6698,
(ASTM
2007c)];
USCS
=
unified
soil
classification
system;
AASHTO
=
AASHTO
classification
system
(numbers
in
parentheses
indicate
the
group
index).
Fines
content
and
grain
size
diameters
(for
C
u
calculations)
are
based
on
ASTM
D422
(ASTM
2007d).
Active
clay
content,
specific
gravity,
and
liquid
and
plasticity
index
were
determined
following
the
procedures
in
ASTM
C837,
D854,
and
D4318,
respectively
(ASTM
2009a,
2010b,
2010a).
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Table 2. Properties and Classifications of Fly Ashes Tested
Parameter Dewey King Presque Isle Coal Creek Columbia Stanton Typical Class C Typical Class F
SiO2 (%) 8.0 24.0 35.6 50.4 31.1 40.2 40.0 55.0
Al2O3 (%) 7.0 15.0 18.0 16.4 18.3 14.7 17.0 26.0
Fe2O3 (%) 2.6 6.0 3.5 7.2 6.1 8.7 6.0 7.0
CaO (%) 9.2 25.8 3.2 13.3 23.3 21.3 24.0 9.0
MgO (%) 2.4 5.3 1.0 4.3 3.7 6.6 5.0 2.0
CaO=SiO2 1.15 1.08 0.09 0.26 0.75 0.53 0.60 0.16
pH 9.9 10.9 11.3 11.9 12.8 11.7 — —
Specific gravity 2.00 2.66 2.11 2.59 2.63 2.63 — —
Fineness, max (%) 57 18 26 28 58 23 34 34
Strength activity at 7 days, min (%) 83 78 49 83 96 111 75 75
Loss on ignition, max (%) 49.0 12.0 34.0 0.5 0.7 0.8 6 6
Classification Off-spec Off-spec Off-spec Class F Class C Class C Class C Class F
Note: Off-spec = off-specification. Loss on ignition was measured per ASTM C311 (ASTM 2011) at 550°C.
Fig. 1. Unconfined compressive strength (qu) of mixtures prepared with various fly ashes, Type I portland cement, and Boardman silt at very wet
water content: (a) Markey peat; (b) Lawson soil; (c) Theresa soil
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specimens cured 7 days to simulate the early curing conditions dur-
ing construction, both inorganic and organic soils are expected to
have significant strength gains with increasing curing time for
calcium-based additives (Edil et al. 2006; Sakr et al. 2009).
Resilient Modulus Test
The resilient modulus is a widely used property in flexible pave-
ment design, as explained in the AASHTO Guide for Design of
Pavement Structures (AASHTO 1993), and it indicates the stiffness
of a soil under a confining stress and a repeated axial load. Resilient
modulus, Mr, is calculated based on the ratio of deviator stress and
the recoverable strain. Different confining and deviator stresses are
applied on the test specimens to cover the range of expected in situ
stresses.
Specimens for the resilient modulus test were prepared in a pol-
yvinyl chloride (PVC) mold with a diameter of 102 mm and a
height of 203 mm in the same manner as the unconfined compres-
sion test specimens were prepared. Compactive effort was adjusted
in such a way that the same compaction energy per unit volume as
the one specified in the standard Proctor compaction method
(ASTM D698) was applied (i.e., 600 kN=m3
). Required compac-
tive effort was obtained when the number of blows with the stan-
dard Proctor hammer was 22 and the number of compacted layers
was 6. After compaction, specimens were cured for 7 days in a wet
room, maintained at 25°C and 100% humidity. Specimens were ex-
truded from the PVC molds after curing and tested according to
AASHTO T292 (AASHTO 1991). Side friction during extrusion
was minimized by applying a very thin grease layer between the
Fig. 2. Resilient moduli (Mr) of soil–fly ash mixtures prepared with various fly ashes and Boardman silt: (a) Markey peat; (b) Lawson soil; (c) Theresa
soil (FA = fly ash, wet = wet of optimum, opt = optimum water content)
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PVC mold and the soil. The loading sequence for cohesive soils
was followed, and the conditioning stress was applied as 21 kPa
instead of 41 kPa because some specimens were too soft to with-
stand 41 kPa conditioning stress. Confining stress was 21 kPa for
all loading sequences, and the deviator stress was increased in steps
of 21, 34, 48, 69, and 103 kPa and applied 50 repetitions at each
step. The reported Mr is the modulus obtained at the initial state of
stress, i.e., at 21 kPa confining and deviator stress because the
stresses are relatively small in the subgrade level, and the modulus
of the stabilized material does not depend strongly on stress level.
Results and Analysis
Soil–fly ash mixtures were prepared with fly ash contents (based on
dry weight) of 10, 20, and 30%. Most of the tests were conducted
on specimens prepared at a very wet condition, corresponding to
6–14% wet of the OWC for the Lawson soil, 5–22% wet of the
OWC for the Theresa soil, and 5–18% wet of the OWC for the
Markey peat. This very wet condition is intended to simulate
the natural water contents of soft subgrades in the upper Midwest-
ern United States (Edil et al. 2006). Additional tests were con-
ducted with the soil fraction at OWC per standard Proctor.
These tests were conducted as well-defined control conditions
and to assess the effect of water content. For the specimens
prepared at OWC, fly ash contents were only 10% and 20%
(the specimens were unrealistically dry for reactions with 30%
fly ash). Soil-cement mixtures were prepared at the very wet
condition with 10% cement, and only unconfined compression tests
were conducted on these mixtures. The cement dosage chosen
(10%) is greater than the typical dosage for inorganic soils because
of the organic content and also to provide a direct comparison with
10% fly ash content.
General Effectiveness of Fly Ash Stabilization
Unconfined compressive strengths (qu) of the soil–fly ash mixtures
prepared at the very wet condition are shown as a function of fly ash
type in Fig. 1. The qu of mixtures prepared with organic soil and
Boardman silt (nonreactive additive) or Type 1 portland cement
(a highly reactive binder) are also included in Fig. 1 for comparison.
Also shown in Fig. 1 are qu of each soil alone (without fly ash)
when compacted at the very wet condition. Triplicate specimens
were tested for unconfined compressive strength as quality control,
and the averages of these tests are reported as results. Addition of
fly ash to the organic soils resulted in significant increase in qu
relative to that of the unstabilized soil in the very wet condition.
Once stabilized with fly ash, both the Lawson and Theresa
soils classify as at least stiff subgrade [qu between 100 and
200 kPa (Bowles 1979)], instead of soft (25–50 kPa) or very soft
(0–25 kPa) in their unstabilized very wet conditions. qu exceeding
100 kPa was not always obtained for the Markey peat in the very
wet conditions, but adding fly ash to the Markey peat did increase
the qu by a factor of up to 10. It is clear from Fig. 1 that the final qu
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35
Lawson Soil, OWC+ (8-14)%
Dewey
King
P. Isle
Coal Creek
Columbia
Stanton
Unconfined
Compressive
Strength
(kPa)
Fly Ash Percentage
(a)
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30 35
Lawson Soil, OWC + (9-14)%
Dewey
King
P.Isle
Coal Creek
Columbia
Stanton
Resilient
Modulus
(MPa)
Fly Ash Percentage
(b)
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35
Theresa Soil, OWC+ (8-22)%
Dewey
King
P. Isle
Coal Creek
Columbia
Stanton
Unconfined
Compressive
Strength
(kPa)
Fly Ash Percentage
(c)
0
20
40
60
80
100
120
0 5 10 15 20 25 30 35
Dewey
King
P. Isle
Coal Creek
Columbia
Stanton
Resilient
Modulus
(MPa)
Fly Ash Percentage
(d)
Fig. 3. Engineering properties of organic soil–fly ash mixtures as a function of fly ash percentage in the mixture: (a) qu of stabilized Lawson soil;
(b) Mr of stabilized Lawson soil; (c) qu of stabilized Theresa soil; (d) Mr of stabilized Theresa soil
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achieved varies depending on the organic soil and the fly ash. This
is in contrast to the findings reported for inorganic soils stabilized
with different fly ashes by Edil et al. (2006), for which final
strengths were comparable, although strength factors varied.
Fig. 2 shows the resilient moduli of soil–fly ash mixtures as a
function of binder type and content. The Mr for Markey peat and
Lawson and Theresa soils are reported at their OWCs because these
soils were too soft to be tested at very wet conditions, i.e., 13% wet
of OWC for Markey peat and 10% wet of OWC for Lawson and
Theresa soils. The resilient modulus of Markey peat, even with
30% fly ash, never reached 35 MPa at very wet conditions, meaning
that Markey peat can be considered a very soft subgrade, i.e.,
Mr 35 MPa (Asphalt Institute 1999). Markey peat–Boardman
silt mixtures were too soft at very wet conditions to withstand
the conditioning stress, indicating that the addition of fly ash is
more effective than the addition of silt at very wet conditions.
Lawson soil admixed with 20% Dewey or Columbia fly ashes at
very wet conditions was medium-stiff, i.e., Mr ∼ 85 MPa (Asphalt
Institute 1999). When stabilized with 30% Dewey, King, Stanton,
or Columbia fly ash, Lawson soil had a resilient modulus as high
as 110 MPa at very wet conditions. At OWC, the resilient modulus
of stabilized Lawson soil was always, even with 10% fly ash,
higher than 50 MPa. Theresa soil admixed with 20% Dewey, King,
Stanton, or Columbia fly ashes at very wet conditions had resilient
moduli of 50–70 MPa. When the percentage of these fly ashes was
increased to 30% at very wet conditions, the resilient modulus
0
100
200
300
400
500
600
0 10 20 30 40 50
30% Fly ash
Markey Peat, OWC+ (10-14)%
Unconfined
Compressive
Strength
(kPa)
LOI (%) of Fly Ash
(a)
r= -0.20
t= -2.47
0
20
40
60
80
100
120
140
0 10 20 30 40 50
30% Fly ash
Resilient
Modulus
(MPa)
LOI of Fly Ash (%)
(b)
r= -0.03
t= -0.20
0
100
200
300
400
500
600
9.5 10 10.5 11 11.5 12 12.5 13
Unconfined
Compressive
Strength
(kPa)
pH of Fly Ash
(c)
r= 0.11
t= 1.33
0
20
40
60
80
100
120
140
9.5 10 10.5 11 11.5 12 12.5 13
Resilient
Modulus
(MPa)
pH of Fly Ash
(d)
r = -0.08
t = -0.64
0
100
200
300
400
500
600
10 20 30 40 50 60
Unconfined
Compressive
Strength
(kPa)
Fineness of Fly Ash (%)
(e)
r= -0.12
t= -1.47
0
20
40
60
80
100
120
140
10 20 30 40 50 60
Resilient
Modulus
(MPa)
Fineness of Fly Ash (%)
(f)
r = 0.1
t = 0.76
Fig. 4. Engineering properties of soil–fly ash mixtures: (a) qu as a function of LOI of fly ash; (b) Mr as a function of LOI of fly ash; (c) qu as a function
of pH of fly ash; (d) Mr as a function of pH of fly ash; (e) qu as a function of fineness of fly ash; (f) Mr as a function of fineness of fly ash
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varied between 65 and 105 MPa, indicating that the stabilization
process produced significant improvement in resilient modulus
(i.e., medium-stiff subgrade consistency) considering that untreated
soil (no fly ash) was too soft to be tested. At OWC, the resilient
modulus of stabilized Theresa soil varied between 50 and 130 MPa,
depending upon the fly ash type and percentage used. Admixing
10% fly ash with any of the three soils at very wet conditions failed
to yield a resilient modulus greater than 50 MPa.
Comparison of the qu or resilient moduli obtained with different
fly ashes indicates that the criteria used to define fly ashes for con-
crete applications (Class C) are not necessarily indicative of the
effectiveness for soil stabilization. For example, in some cases
Dewey and King fly ashes (both are off-specification fly ashes)
resulted in comparable or greater strength and stiffness gain than
Columbia and Stanton fly ashes, which are Class C ashes and
qualify for use as concrete additives.
0
100
200
300
400
500
600
0 0.2 0.4 0.6 0.8 1 1.2
30% Fly ash
Unconfined
Compressive
Strength
(kPa)
CaO/SiO
2
of Fly Ash
(a)
Coal Creek
Fly Ash
Dewey
Fly Ash
r= 0.38
t=4.84
0
20
40
60
80
100
120
140
0 0.2 0.4 0.6 0.8 1 1.2
30% Fly ash
Resilient
Modulus
(MPa)
CaO/SiO
2
of Fly Ash
(b)
r= 0.30
t=2.33
0
100
200
300
400
500
600
0 0.2 0.4 0.6 0.8 1
Unconfined
Compresive
Strength
(kPa)
CaO/(SiO
2
+Al
2
O
3
)
(c)
r=0.43
t=5.60
0
20
40
60
80
100
120
140
0 0.2 0.4 0.6 0.8 1
Resilient
Modulus
(MPa)
CaO/(SiO
2
+Al
2
O
3
)
(d)
r= 0.31
t=2.45
0
100
200
300
400
500
600
0 5 10 15 20 25 30
Unconfined
Compressive
Strength
(kPa)
CaO Content (%) of Fly Ash
(e)
Dewey
Fly Ash
Coal Creek
Fly Ash
r= 0.46
t= 6.10
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30
Resilient
Modulus
(MPa)
CaO Content of Fly Ash (%)
(f)
Coal Creek
FA
Dewey
FA
r= 0.22
t=1.69
Fig. 5. Engineering properties of soil–fly ash mixtures, (a) qu as a function of CaO=SiO2 ratio of fly ash; (b) Mr as a function of CaO=SiO2 ratio of fly
ash; (c) qu as a function of CaO=ðSiO2 þ Al2O3Þ ratio of fly ash; (d) Mr as a function of CaO=ðSiO2 þ Al2O3Þ ratio of fly ash; (e) qu as a function of
CaO content of fly ash; (f) Mr as a function of CaO content of fly ash
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The effect of reactivity of the binder can be evaluated by com-
paring the qu of the soil–fly ash mixtures to the qu obtained using
cement or nonreactive Boardman silt as the additive in Fig. 1. qu
obtained with 10% cement at the very wet conditions was always
higher than those obtained with 10% fly ash at the same water con-
tent, and in many cases 10% cement resulted in higher qu than ob-
tained with 30% fly ash. In contrast, the mixtures prepared with
Boardman silt had lower qu and resilient moduli than comparable
soil–fly ash mixtures. Thus, the increase in strength or resilient
modulus obtained by fly ash stabilization generally is attributable
to chemical reactions and the reduction in water content obtained
by adding dry solids, but the significance of the reactions depends
on the type of fly ash and the soil.
The importance of reactivity is also illustrated through the effect
of fly ash content. For most of the mixtures, the qu and resilient
modulus increased as the fly ash content increased (Fig. 3). The
exceptions are the mixtures prepared with the less reactive fly ashes
(Presque Isle and Coal Creek). Additionally, qu and resilient modu-
lus do not increase linearly with fly ash content. In most cases, the
increase in qu and resilient modulus obtained as the fly ash content
increased from 0–10% or 10–20% was larger than those obtained
when the fly ash content was increased from 20–30%. Thus, the
benefits accrued by adding more fly ash diminish as the fly ash
content increases.
Effects of Fly Ash Characteristics
Graphs relating qu and resilient modulus to properties of the fly ash
(LOI, pH, fineness, CaO=SiO2 ratio, CaO=ðSiO2 þ Al2O3) ratio,
and CaO content,) were prepared to identify characteristics of
the fly ashes that have an important role in improving the strength
and stiffness of the organic soils (Figs. 4 and 5). qu and resilient
moduli of mixtures prepared at the very wet condition are shown
because this condition is of practical interest for field situations
(Edil et al. 2006). The resilient modulus data from both cells were
compared using a paired t-test at significance level of 0.05, corre-
sponding to tcr ¼ 1:96 for unconfined compression test results and
tcr ¼ 2:01 for resilient modulus test results.
Fig. 4 suggests that qu and resilient modulus are not affected by
LOI, pH, or fineness (percentage retained on 45 μm sieve) of the fly
ash. This observation is consistent with the statistical analysis,
which shows that qu and Mr are not correlated with LOI, pH, or
fineness (t 1:96 for qu and t 2:01 for Mr). In contrast, qu
and resilient modulus suggest a correlation with CaO=SiO2 and
CaO=ðSiO2 þ Al2O3), and the statistical analysis supports this ob-
servation (Fig. 5). Relatively strong relationships exist between qu
or resilient modulus and these parameters for the Lawson and
Theresa soil, whereas weaker relationships exist for the Markey
peat, thus the Markey peat data are excluded for calculation of cor-
relation coefficient (r) and t. The relationships between qu and
CaO=SiO2 and CaO=ðSiO2 þ Al2O3Þ for the Lawson and Theresa
soils are illustrated with second-order nonlinear regressions, shown
as solid lines in Figs. 5(a) and 5(c). Statistically, CaO content as-
sociated with qu but not with resilient modulus.
Fig. 5 suggests that both CaO and CaO=SiO2 or CaO and
CaO=ðSiO2 þ Al2O3Þ are important variables affecting the qu of
the soil–fly ash mixtures prepared with the Lawson and Theresa
0
100
200
300
400
500
600
0 5 10 15 20 25 30
10% Dewey FA
10% King FA
10% Columbia FA
20% Dewey FA
20% King FA
20% Columbia FA
Unconfined
Compressive
Strength
(kPa)
OC of Soil (%)
(a)
r= 0.02
t= 0.19
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30
Resilient
Modulus
(MPa)
OC of Soil (%)
(b)
r= 0.17
t=1.32
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35 40
(c)
Unconfined
Compressive
Strength
(kPa)
Pl
r= -0.02
t= -0.19
0
20
40
60
80
100
120
140
0 5 10 15 20 25 30 35 40
Resilient
Modulus
(MPa)
PI
(d)
r= -0.17
t=-1.32
Fig. 6. Engineering properties of soil–fly ash mixtures prepared at very wet conditions, (a) qu as a function of OC of soil; (b) Mr as a function of OC
of soil; (c) qu as a function of PI soil; (d) Mr as a function of PI soil
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soils. The highest qu and resilient moduli were obtained when the
CaO content was at least 10, CaO=SiO2 ratio was between 0.5 and
1.0, and CaO=ðSiO2 þ Al2O3Þ ratio was between 0.4 and 0.7.
A similar conclusion can be drawn for the Markey peat, although
the trends in qu and resilient modulus for the Markey Peat are
modest. As illustrated in Figs. 5(e) and 5(f), CaO content alone
is not sufficient to evaluate whether fly ash will cause an increase
in qu or resilient modulus. The circled data in Figs. 5(e) and 5(f)
correspond to mixtures prepared with the Lawson and Theresa soils
and Coal Creek (CaO content = 13.3%) or Dewey fly ash (CaO
content = 9.2%). Appreciably higher qu and resilient moduli are
obtained with Dewey fly ash. This is attributed to its significantly
higher CaO=SiO2 ratio (1.15 versus 0.26 of Coal Creek) even
though Coal Creek fly ash has greater CaO than that of Dewey
fly ash. The results indicate that CaO of 10% by weight is needed
as a threshold value for strength gain, and CaO content and
CaO=SiO2 ratio play a combined role on qu and resilient modulus
of soil–fly ash mixtures.
Effects of Soil Type
The influence of organic soil type was evaluated by graphing qu and
resilient modulus against organic content (OC) and PI (Fig. 6). Soil
pH was not included in the analysis because the pH varied over a
narrow range (6.1–7.3). As in the analysis of fly ash properties, the
qu and resilient moduli of mixtures prepared shown in Fig. 6
correspond to the very wet condition.
Data for soil–fly ash mixtures from the study conducted by Edil
et al. (2006) were also included in the analysis to increase the gen-
erality of the findings. Edil et al. (2006) used Dewey, King, and
Columbia fly ashes that were obtained from the same source as
the fly ashes used in this study. Edil et al. (2006) used a variety
of soils with OCs ranging from 1–10% and PIs ranging from
15–38, and they mixed these soils with the three fly ashes. qu data
were adopted from their study for different mixtures, each having one
of the following soils: inorganic clay (OC = 2%, PI = 38), slightly
organic clay (OC = 4%, PI = 35) and organic clay (OC = 10%
and PI = 19).
As shown in Fig. 6(a), qu decreased significantly as the OC in-
creased to 10%, and then leveled off for higher OCs. A sharp de-
crease in resilient modulus in response to an increase in OC of soil
was also observed in Fig. 6(b). This inverse relationship between
qu or resilient modulus and OC may reflect the inhibition of
pozzolanic reactions by organic matter. Alternatively, the inverse
relationship between qu or resilient modulus and OC may
reflect the weakness of organic solids relative to mineral solids.
0
100
200
300
400
500
0 100 200 300 400 500
Markey, OWC+ (10-18)%
Markey, OWC± 5%
Lawson, OWC+ (8-14)%
Lawson, OWC± 5%
Theresa, OWC+ (7-22)%
Theresa, OWC± 3
Unconfined
Compressive
Strength
of
Mixtures
with
Boardman
Silt
(kPa)
Unconfined Compressive Strength of
Mixtures with Fly Ashes(kPa)
(a)
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Markey Soil, OWC± 5%
Lawson Soil, OWC± 5 %
Theresa Soil, OWC± 2%
Resilient
Modulus
of
Samples
with
Boardman
Silt
(MPa)
Resilient Modulus of Samples with Fly Ashes (MPa)
(b)
Fig. 8. Comparison of engineering properties of mixtures prepared
with Boardman silt and fly ashes at the same binder content and similar
water content: (a) qu; (b) Mr
0
100
200
300
400
500
600
0 100 200 300 400 500 600
Markey 10% FA
Markey 20% FA
Lawson 10% FA
Lawson 20 % FA
Theresa 10% FA
Theresa 20% FA
Unconfined
Compressive
Strength
(kPa)
at
around
OWC
Unconfined Compressive Strength (kPa)
at Wet of OWC
1:1 Line
(a)
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Markey Peat
Lawson Soil
Theresa Soil
Resilient
Modulus
(MPa)
at
around
OWC
Resilient Modulus (MPa)
at Wet of OWC
1:1 Line
(b)
Fig. 7. Engineering properties soil–fly ash mixtures prepared at opti-
mum and wet of optimum water contents with the same binder type and
percentages: (a) qu; (b) Mr
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Regardless, the trend in Fig. 6(a) suggests that the effectiveness
of fly ash stabilization is significantly reduced when the OC
exceeds 10%.
The effect of PI on qu and resilient modulus is shown in
Figs. 6(c) and 6(d). Greater qu and resilient moduli are obtained
when the PI is eight or more. However, the apparent effect of
PI in Fig. 6(c) is probably spurious. The trend is more likely related
to OC because the Markey peat had the highest OC and the lowest
PI of the soils that were tested. A broader range of soils is needed to
adequately assess the effect of PI.
Effects of Water Content
The effect of water content on the stabilization was investigated by
plotting the qu and resilient modulus of the soil–fly ash mixture
prepared at very wet of OWC condition against the qu and resilient
modulus of the soil–fly ash mixture prepared at OWC (Fig. 7).
The resilient moduli of soil–fly ash mixtures prepared at OWC
were almost always higher than those of prepared at very wet of
OWC. When the fly ash percentage was 10%, the soil–fly ash mix-
tures prepared at OWC usually had higher qu, as opposed to those
prepared at wet of OWC. On the other hand, as the fly ash percentage
0
100
200
300
400
500
7 8 9 10 11 12 13 14
Lawson Soil
(a)
10% FA, OWC+ (8-13)%
20% FA, OWC+ (8-12)%
30% FA, OWC+ (8-14)%
Unconfined
Compressive
Strength
(kPa)
pH of the mixture
0
20
40
60
80
100
120
140
7 8 9 10 11 12 13 14
10% FA, OWC+ (9-13)%
20% FA, OWC+ (6-14)%
30% FA, OWC+ (6-12)%
Resilient
Modulus
(MPa)
pH of the mixture
Lawson Soil
(b)
0
100
200
300
400
500
7 8 9 10 11 12 13 14
Theresa Soil
(c)
10% FA, OWC+ (8-12)%
20% FA, OWC+ (7-22)%
30% FA, OWC+ (8-11)%
Unconfined
Compressive
Strength
(kPa)
pH of the mixture
0
20
40
60
80
100
120
140
7 8 9 10 11 12 13 14
10% FA, OWC+ (8-11)%
20% FA, OWC+ (5-10)%
30% FA, OWC+ (8-14)%
Resilient
Modulus
(MPa)
pH of the mixture
Theresa Soil
(d)
0
50
100
150
7 8 9 10 11 12 13 14
Markey Peat
(e)
10% FA, OWC+ (13-18)%
20% FA, OWC+ (11-17)%
30% FA, OWC+ (10-14)%
Unconfined
Compressive
Strength
(kPa)
pH of the mixture
5
10
15
20
25
30
7 8 9 10 11 12 13 14
10%FA
20%FA, OWC+ (5-8)%
30%FA, OWC+ (7-13)%
Resilient
Modulus
(MPa)
pH of the mixture
Markey Peat
(f)
Fig. 9. Engineering properties of soil–fly ash mixtures prepared at very wet water content as a function of mixture pH after 1 h: (a) qu for stabilized
Lawson soil; (b) Mr for stabilized Lawson soil; (c) qu of stabilized Theresa soil; (d) Mr of stabilized Theresa soil; (e) qu for stabilized Markey peat;
(f) Mr for stabilized Markey peat
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increased to 20%, soil–fly ash mixtures prepared at OWC usually
had lower qu than mixtures prepared at very wet of OWC, unlike
the 10% fly ash case. The shear strength of a cohesive soil generally
is inversely related to water content (Seed and Chan 1959; Khoury
and Zaman 2004). On the other hand, observed increase in qu as
water content increases can be attributed to the use of more water
in hydration that can increase the amount of cementitious products.
The qu values of mixtures prepared with Boardman silt (non-
reactive binder) are plotted against qu of mixtures prepared with
the fly ashes in Fig. 8. In nearly all cases, the soil–fly ash mixtures
had higher qu and resilient moduli than the mixtures prepared with
Boardman silt for the very wet condition. However, at OWC, the qu
tended to be more similar for the soil–fly ash mixtures and the
mixtures prepared with Boardman silt. That is, the reactivity effect
appears to diminish as the water content decreases. It appears that if
initial water content is less than a critical amount needed for
hydration reactions, strength gain may be limited.
Effects of pH of the Soil–Fly Ash Mixture
For soils stabilized with cement and blast furnace slag, if organic
content is below 15%, in general there is significant strength gain
only if humic acid is less than 0.9% or pH higher than 5 (Kitazume
2005). For organic soils, it is well known that humic acids consume
the calcium ions in the binder. When the acids are neutralized, the
remaining binder quantity contributes to strength gain. Tremblay
et al. (2002) mixed 14 different organic compounds with the
soil-cement mixture (the two soils were a clay and a silt, and
the two cements were ordinary portland Type 10 and sulfate-rich
geolite 20) and investigated the effect of organic compound on the
soil stabilization. They reported that if an organic compound
caused a pore solution pH of less than 9, no strength gain was
noted. However, they also mentioned that a pore solution pH of
more than 9 did not always indicate significantly high strengths.
pHs measurements conducted 1, 2, 24, 48, and 96 h after mixing
were not significantly different. When Lawson and Theresa soils
were mixed with a fly ash with CaO content higher than 10, the
pH of the mixture reached above 9, which indicates that cementi-
tious reactions are not likely to be inhibited (Tremblay et al. 2002).
The pH of the mixtures involving Markey peat were also above 9 as
the percentage of fly was increased to 30%. The effect of pH on the
qu and stiffness of the soil–fly ash mixtures is shown in Fig. 9.
There is no apparent relationship between qu or resilient modulus
and mixture pH. Fig. 9 seems to verify Tremblay et al.’s conclusion
that pH higher than 9 does not necessarily indicate higher qu.
Correlations between UCS and Resilient Modulus Test
Results
The relationships between unconfined compressive strengths and
resilient moduli at 21 kPa deviator stress for organic soil–fly
ash mixtures with the same fly ash type and percentage, prepared
at the same water content, and cured for the same length of time are
given in Fig. 10. Fig. 10 includes qu data from two different tests:
(1) tests on small-size specimens (33 mm in diameter and 72 mm in
height) that were not subjected to resilient modulus testing, and
(2) tests on large specimens (102 mm in diameter and 203 mm
in height) that were previously tested in a resilient modulus test.
However, only one set of resilient modulus test data was used
in correlation with both sets of unconfined compression test data
for a given soil–fly ash mixture. According to Fig. 10(a), which
includes qu for small-size specimens, the conversion factor for
qu (kPa) to obtain resilient modulus (kPa) varies from 70–570,
and the best fit is 270. In Fig. 10(b), in which qu testing is per-
formed on larger samples subjected to resilient modulus testing
prior to testing, there is much less dispersion of the data, and
the conversion factor from qu (kPa) to resilient modulus (kPa) is
213 and close to the best fit given in Fig. 10(a). The coefficients
corresponding to the slope of curve fit in Figs. 10(a) and 10(b)
are close.
The secant modulus at 50% (E50) was obtained by dividing half
of the peak strength (qu=2) with the strain observed at that stress
level in the unconfined compression test. Comparison of E50 with
resilient modulus is given in Fig. 11. Fig. 11(a) shows the compari-
son of E50 obtained from the unconfined compression tests per-
formed on small specimens and resilient moduli obtained from
the tests performed on large specimens. In Fig. 11(a), resilient
modulus varies between 1:6E50 and 20E50. Fig. 11(b) depicts
the comparison of E50 and resilient moduli results that were ob-
tained by using the same specimens (larger specimens) in uncon-
fined compression and resilient modulus tests. In this case, resilient
modulus varies between 1:8E50 and 12E50. In both cases, resilient
modulus is higher than E50.
Model for Stabilization of Organic Soils with Fly Ashes
The important factors in stabilization of organic soils with fly ash
can be summarized as follows: (1) fly ash properties: CaO content
and CaO=SiO2 ratio; (2) soil properties: OC; and (3) mixture char-
acteristics: fly ash content and water content. Each of these vari-
ables was included in a nonlinear regression analysis to find an
0
50000
100000
150000
200000
250000
300000
0 100 200 300 400 500
Markey Soil
Lawson Soil
Theresa Soil
Resilient
Modulus
(kPa)
Unconfined Compressive Strength (kPa)
(a)
Mr=570 q
u
Mr=270 q
u
R
2
=0.54
Mr=70 q
u
0
20000
40000
60000
80000
100000
120000
140000
0 100 200 300 400 500 600 700
(b)
Markey Peat
Lawson Soil
Theresa Soil
Resilient
Modulus
(kPa)
Post-Mr Unconfined Compressive Strength (kPa)
Mr=213q
u
R
2
=0.84
Fig. 10. Relations between Mr and qu of soil–fly ash mixtures: (a)
unconfined compression tests performed on 133-mm-diameter and
72-mm-high specimens and resilient modulus tests performed on
102-mm-diameter and 203-mm-high specimens; (b) 102-mm-diameter
and 203-mm-high specimens for both unconfined compression and
resilient modulus tests
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equation that can be used to predict the qu of organic soil–fly ash
mixtures. The equation was derived statistically as follows: (1) trials
of linear and quadratic curvilinear regression models between sig-
nificant characteristics and unconfined compressive strength were
preformed to see which model best described the relation based on
F-test; and (2) a multiple regression model including all significant
characteristics. Multiple regression models included second-order
or transformed functions of significant characteristics investigated.
Possible correlations between independent variables were also
checked, and highly correlated variables were dropped from the
model. Only data for the very wet condition were included because
this condition is of practical importance. The qu of the soil alone
was also included in the analysis. The following regression model
was developed:
qu-treated ¼ 320 þ 795ðCaO=SiO2Þ 573ðCaO=SiO2Þ2
125;673ðeOC
Þ þ 6ðFApercÞ þ 25ðqu-untreatedÞ
33ðpHmixtureÞ ð5Þ
where FAperc = fly ash percentage; pHmixture = pH of the soil–fly ash
mixture after 1 h; qu-treated = stabilized unconfined compressive
strength of soil (kPa) after 7 days of curing; qu-untreated = unconfined
compressive strength of untreated soil (kPa); and OC = organic
content of soil (%). The developed model was intended to represent
the data obtained from a wide range of organic soils and fly ashes,
but it has not been validated on independently obtained data. A
comparison of the predicted versus measured unconfined compres-
sive strength is shown in Fig. 12. According to Fig. 12, the regres-
sion model represents the qu data reasonably well, with R2
= 0.71.
According to Eq. (5), the following inferences can be made:
(1) there is an optimum ðCaO=SiOÞ2 ratio that maximizes the sta-
bilized strength of the soil; (2) increase in the fly ash percentage
increases the qu of the soil–fly ash mixture; and (3) higher organic
content of the soil indicates less qu of the soil–fly ash mixture. The
model does not include CaO because it is highly correlated with
other terms in the model. However, the physical effect of CaO
content is still reflected in the model by mixture pH term, which
is controlled by the ½OH
ions liberated after disassociation of
CaðOHÞ2 (formed by hydration of CaO). The unconfined compres-
sive strength can be correlated to resilient modulus within a range
of uncertainty. Alternatively, resilient modulus tests can be per-
formed on promising mixtures.
Conclusions
The objective of this study was to determine if unconfined com-
pressive strength and resilient modulus of soft organic soils can
be increased by blending fly ash into the soil. Tests were conducted
with three organic soils and six fly ashes. Portland cement and an
inorganic silt were also used as a stabilizer for reference purposes.
Fly ashes were mixed with soils at three different percentages and
two different water contents (OWC and 9–15% wet of the OWC).
The following conclusions are advanced:
1. Unconfined compressive strength of organic soils can be in-
creased using fly ash, but the amount of increase depends
on the type of soil and characteristics of the fly ash. Large in-
creases in qu (from 30 kPa without fly ash to 400 kPa with
fly ash) were obtained for two clayey soils with an OC less
than 10% when blended with some of the fly ashes. More mod-
est increases in qu (from 15 kPa without fly ash to 100 kPa
with fly ash) were obtained for a highly organic sandy silty
peat with OC ¼ 27%. Resilient modulus tests could not be per-
formed on organic soils without fly ash stabilization at wet
conditions because the specimens were too soft. The addition
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Markey Peat
Lawson Soil
Theresa Soil
Resilient
Modulus
(MPa)
E
50
(MPa)
M
r
=1.6E
50
M
r
=20E
50
(a)
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Markey Peat
Lawson Soil
Theresa Soil
Resilient
Modulus
(MPa)
E
50
(MPa)
M
r
=1.8E
50
M
r
=12E
50
(b)
Fig. 11. Relations between secant modulus at (E50) and Mr of soil–fly
ash mixtures: (a) unconfined compression tests performed on 133-mm-
diameter and 72-mm-high specimens and resilient modulus tests
performed on 102-mm-diameter and 203-mm-high specimens;
(b) 102-mm-diameter and 203-mm-high specimens for both uncon-
fined compression and resilient modulus tests
0
100
200
300
400
500
600
0 100 200 300 400 500 600
Predicted
Unconfined
Compressive
Strength
(kPa)
Measured Unconfined Compressive Strength (kPa)
1:1 Line
Fig. 12. Predicted versus measured unconfined compressive strength
of soil–fly ash mixtures
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of fly ash, at wet conditions, to the slightly organic soils,
Lawson and Theresa (OC = 5 and 6%, respectively) produced
Mr varying from 10–100 MPa, depending on the type and per-
centage of the fly ash. At OWC, Mr for these soils could be
improved up to 120 MPa with the addition of fly ash. However,
for Markey peat (OC ¼ 27%), stabilization with fly ash never
produced Mr 30 MPa no matter which fly ash type and per-
centage (up to 30%) was used.
2. The significant characteristics of fly ash affecting the increase
in qu and Mr include CaO content and CaO=SiO2 ratio [or
CaO=ðSiO2 þ Al2O3Þ ratio]. The highest qu and Mr were
obtained when the CaO content was greater than 10% and
the CaO=SiO2 ratio was 0.5–0.8. Comparable increases in
qu and Mr were obtained with the Class C ashes, normally used
in concrete applications, and the off-specification fly ashes
meeting the aforementioned criteria for CaO content and
CaO=SiO2 ratio. However, much lower qu and Mr were
obtained with one off-specification fly ash primarily because
of its low CaO content and CaO=SiO2 ratio. Carbon content of
the fly ash (i.e., loss on ignition) seemed to have no bearing on
the qu and Mr of the soil–fly ash mixtures.
3. For most of the cases qu and Mr increased when fly ash per-
centage was increased. Exceptions were mixtures with the less
reactive Presque Isle and Coal Creek fly ashes (CaO 10%
and CaO=SiO2 0:5)
4. The reactivity effect appears to diminish as the water content
decreases, i.e, improvement in the qu of the soil due to the ad-
dition of fly ash or inorganic silt to the soil was approximately
the same for the mixtures prepared at OWC. When the fly ash
percentage in the mixture was 10%, the expected trend of high-
er qu when water content decreased was observed. On the other
hand, as the fly ash percentage increased to 20% (more reduc-
tion in water content compared to 10% fly ash case), soil–fly
ash mixtures prepared wet of OWC usually had greater qu than
the ones prepared at OWC. The trend of stronger mixtures at
wet conditions as opposed to the mixtures prepared at OWC is
attributable to the requirement for more water for hydration
reactions of the higher amount of fly ash.
5. Soil organic content is a detrimental characteristic for stabili-
zation. An increase in the organic content of soil indicates that
the strength of the soil–fly ash mixture will decrease exponen-
tially. No effect of soil pH and plasticity could be discerned on
resilient modulus of the soil stabilized with fly ash. However,
more research on the effect of these characteristics is required
because the variation in pH and plasticity of the soils in this
study was not sufficient.
6. Fly ash stabilization of soils at OWC always resulted in greater
resilient moduli than at wetter conditions. Resilient modulus
can be estimated from unconfined compressive strength using
a multiplication factor between 70 and 570. Estimation of re-
silient modulus based on static E50 obtained from the uncon-
fined compression test can be made using a multiplication
factor in the range of 1.6–20, which shows that the lower-strain
resilient modulus is always higher than the high-strain E50.
Acknowledgments
Financial support for this study was provided by the National
Science Foundation (NSF) and the U.S. Federal Highway Admin-
istration Recycled Materials Resource Center (RMRC). Fly ashes
used in the study were provided by Alliant Energy, Xcel Energy,
We Energy, Great River Energy, and LaFarge North America. The
findings and opinions in this report are solely those of the authors.
Endorsement by NSF, RMRC, or the fly ash suppliers is not
implied and should not be assumed.
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