volume change behaviors of expansive soils stabilized with recycled ashes and fibers

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Volume Change Behaviors of Expansive Soils Stabilizedwith Recycled Ashes and Fibers

Koonnamas Punthutaecha1; Anand J. Puppala, P.E.2; Sai K Vanapalli3; and Hilary Inyang4

Abstract: In this paper, class F fly ash, bottom ash, polypropylene fibers, and nylon fibers were evaluated as potential stabilizers inenhancing volume change properties of sulfate rich expansive soils. As a part of the research evaluation, a comprehensive laboratoryexperimental program was designed and conducted on two different subgrade soils from two locations in Texas. Four dosage levels ofeach stabilizer, two compaction moisture content levels, and 14 days curing period were investigated. Volume change behavioral testsincluding volumetric free swell, volumetric shrinkage strain, and vertical swell pressure tests were conducted on both isolated stabilizertreated and combined ash-fiber stabilized soils. Ash stabilizers showed improvements in reducing swelling, shrinkage, and plasticitycharacteristics by 20–80% whereas fibers treatments resulted in varied improvements. In combined treatments, class F fly ash mixed withnylon fibers was the most effective treatment on both Dallas and Arlington soils, where the soil property enhancements were consideredaverage-to-moderate. Possible mechanisms that resulted in the soil property improvements are discussed along with the recommendedstabilizers and their dosages for expansive soil treatments.

DOI: 10.1061/�ASCE�0899-1561�2006�18:2�295�

CE Database subject headings: Expansive soils; Fly ash; Bottom ash; Shear strength; Shrinkage; Heaving; Volume change;Recycling.

Introduction

Expansive soils are predominant in many parts of the world. Ex-pansive soils undergo large amounts of heaving and shrinking dueto seasonal moisture changes. These movements lead to crackingof the infrastructure built on them and these distress problemshave resulted in billions of dollars of repair costs annually�Nelson and Miller 1992�. The State of Texas alone has spentmore than 1 billion dollars to rehabilitate the foundations ofresidential buildings, lightly loaded structures, buried utilities,highways and airfield pavements, and embankments built on ex-pansive soils.

Over the past three decades, significant research has been per-formed to develop various treatment methods to stabilize soft andexpansive soils. Based on the mechanisms of soil modification,

1Civil Engineer, Bureau of Maintenance and Traffic Safety, Ministryof Transport, Anusawaree, Bang-Khen, Bangkok, Thailand. E-mail:[email protected]; formerly, Lecturer, Mahanakorn Univ. ofTechnology, Bangkok, Thailand. E-mail: [email protected]

2Professor, Dept. of Civil and Environmental Engineering, The Univ.of Texas at Arlington, Arlington, TX 76019. E-mail: [email protected]

3Assistant Professor, Dept. of Civil Engineering, Univ. of Ottawa,Ottawa, Canada K1N6N5. E-mail: [email protected]

4Duke Energy Distinguished Professor and Director, Global Institutefor Energy and Environmental Systems �GIEES�, The Univ. of NorthCarolina at Charlotte, Charlotte, NC 28223-0001. E-mail: [email protected]

Note. Associate Editor: Hilary I. Inyang. Discussion open untilSeptember 1, 2006. Separate discussions must be submitted for individualpapers. To extend the closing date by one month, a written request mustbe filed with the ASCE Managing Editor. The manuscript for this paperwas submitted for review and possible publication on February 11, 2005;approved on July 29, 2005. This paper is part of the Journal of Materialsin Civil Engineering, Vol. 18, No. 2, April 1, 2006. ©ASCE, ISSN
0899-1561/2006/2-295–306/$25.00.
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J. Mater. Civ. Eng. 20

stabilization methods can be classified into physical, mechanical,and chemical stabilization �Mitchell and Katti 1981; Hausmann1990�. Among these, mechanical and chemical stabilization meth-ods are most frequently used since they provide fast, efficient,repeatable, and reliable improvements to soil properties�Hausmann 1990�. However, the majority of these methods do notaddress shrinkage movements in dry environments and somechemical treatments do not provide effective treatment for expan-sive soils containing large amounts of soluble sulfates �Hunter1988; Puppala et al. 2001�. Hence new methods are still needed tosolve or reduce volumetric strain related expansive soil move-ments. In addition to reductions in heave and shrinkage volumechanges, the costs of the new stabilizing materials should begiven some consideration in the overall assessments of new treat-ment methods. Recycled materials were considered attractivechoices provided they result in effective treatment of expansivesoils.

In this research, two types of recycled materials, ashes andfibers, were used to treat sulfate-rich expansive soils from Texas.These ashes were considered since they contain low to moderateamounts of calcium ions, which will not be influenced by thesulfates present in soils. Fibers were considered since these areinert plastic materials and hence provide tensile strength to thenatural soils. Physical and mechanical engineering tests were con-ducted on both control and treated soils to study macrobehaviorialrelated aspects of volume changes in expansive soils. This paperpresents a brief description of test procedures and a summary oftest results, which were conducted to address ash and fiber treat-ments of two types of expansive soils.

Two expansive soils used in this research were rich in sulfatesand were sampled from Dallas-Fort Worth International Airportand South Arlington areas in Texas. Materials evaluated here wereclass F fly ash and bottom ash in the case of ash stabilizers, and
polypropylene and nylon fibers in the case of fiber materials.
RIALS IN CIVIL ENGINEERING © ASCE / MARCH/APRIL 2006 / 295

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Three different dosage levels of each stabilizer and one curingperiod �14 days� were studied. Test results were analyzed to as-sess the effectiveness of stabilization methods in reducing swelland shrinkage characteristics to levels that are considered non-problematic for supporting structures. Test results from individualor isolated stabilizer treatments were analyzed to determine opti-mum dosage levels of both ashes and fibers. Combined stabiliza-tions using the optimized ash and fiber combinations wereperformed on control soils and these results confirmed the en-hancements to soil properties. Future research directions are alsomentioned.

Background and Research Objectives

Expansive soils have high plasticity characteristics and are rela-tively stiff. These soils exhibit swell and shrinkage movementsdue to environmental and seasonal moisture changes. The expan-sive nature of the soil is most obvious near the ground surface dueto very low confining stresses, which are lower than the swellpressures exerted in the expansive soils. As a result, soils tend tolift the infrastructure built on them. In a study conducted for theNational Science Foundation, it was reported that the expansivesoils’ damage to structures, particularly light buildings and pave-ments, is much more than the damage caused by other naturaldisasters, including earthquakes and floods �Jones and Holtz1973�. These damages are estimated to cost several billions ofdollars annually �Nelson and Miller 1992�. Several countries inthe world, including the United States, Israel, India, South Africa,and Australia, have reported infrastructure damage problemscaused by the movements of expansive soils �Nelson and Miller1992�.

Several methods have been investigated to mitigate expansivesoil movements. Chemical additives such as lime and cement areused to reduce potential heave distress in expansive soils. Re-cently, recycled materials have become competitive chemical sta-bilizer materials since they are inexpensive, and their usage canreduce landfill space and costs. Hence they are considered in sev-eral research studies for their potential applications to stabilizeexpansive soils. Some of these recycled materials include coalcombustion products and fiber materials. Recycled coal combus-tion products �CCP� have already been evaluated in several re-search studies on the potential modification of soils �Basma andAl-Sharif 1994; Chimenos et al. 1999; Ferguson and Levorson1999�. A few of these research studies explored the potential of“Class C” fly ash stabilizers for treating expansive soils �Basmaand Al-Sharif 1994; Zalihe and Emin 2002�. Most of these studiesaddressed the volume change behaviors of soils in swell test con-ditions and none of them addressed shrinkage strain environ-ments. Fu et al. �1996� showed that the utilization of class F flyash stabilizers resulted in minimized shrinkage cracking in con-crete materials �Fu et al. 1996�. Since the mechanical behavior ofconcrete materials and soils under shrinkage environment is simi-lar; it is assumed that the class F fly ash can be used to reduceshrinkage strains of expansive soils. This assumption is evaluatedin this paper.

Other secondary materials that are potentially useful in soilstabilizations include plastics and carpet materials, which couldbe recycled into fibers. Both plastic and nylon fibers could then beused to strengthen construction materials. Plastic fibers have al-ready been used to increase the shrinkage crack resistance ofconcrete materials �Alhozaimy et al. 1996�. Hence it is possible to
use recycled plastic-related fiber products to enhance shrinkage
296 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / MARCH


resistances of soils, which was evaluated in the present research.Unfortunately, limited studies have been conducted in which bothfibers and ash products could be used combined to enhance ex-pansive soil properties. The present research also focused on thecombined treatment with coal combustion ash products and re-cycled plastic type waste materials to solve expansive soil distressproblems. Hence the main objectives of the present research were• Evaluate the effectiveness of recycled ash and fiber products

for the stabilization of two natural expansive clays; and• Investigate swell and shrinkage properties of untreated and

stabilized soils using optimized ash and fiber materials in com-bination for soil modifications.This paper presents findings on isolated or individual and com-

bined treatment studies conducted on two expansive soils fromTexas. Research results and relationships presented in this paperare essentially valid for soils and materials similar to those usedin this research. For other soil types, further research and a fewvalidation studies are needed.

Experimental Program

Physical experiments were performed in two phases. The firstphase involved isolated stabilizer studies in the experimental pro-gram, which focused on testing of two soils from Texas �termedhere as Dallas-Fort Worth �DFW� and Arlington soils�, three typesof ashes �two types of class F fly ash and bottom ash�, two typesof polymeric fibers �polypropylene and nylon fibers�, and threedosage levels of each stabilizer. Physical and mechanical proper-ties related to soil tests that addressed expansive soil behaviorwere first conducted on both treated and untreated �control� soilspecimens. Test results were analyzed to establish optimum dos-age levels for each of the ash and fiber stabilizers. These dosageswere used in various combinations in the second phase of thisresearch in combined stabilization studies.

The second phase studies were primarily conducted to evalu-ate the effectiveness of combined treatment methods using bothoptimum ashes and fibers. Both soils were again stabilized withash and fiber materials. Similar physical and mechanical testswere performed on both control and treated soils. The effective-ness of the combined stabilization methods were comprehensivelyevaluated in the second phase.

Table 1. Physical Soil Properties

Soil property DFW Arlington

Specific gravity 2.65 2.46

Liquid limit �%� 50 44

Plastic limit �%� 18 22

Plasticity index �%� 32 22

USCS classification CL CL

Table 2. Chemical Composition of Ashes

Chemical composition Fly ash, F1 Fly ash, F2 Bottom ash

Silicon dioxide �SiO2� 54.8 63.0 21–60

Aluminum oxide �Al2O3� 22.3 19.7 10–37

Calcium oxide �CaO� 9.8 7.4 0–22

Sulfur trioxide �SO3� 0.6 0.1 N/A

Ferric oxide �Fe2O3� 5.1 4.9 5–37

Magnesium oxide �MgO� — 1.6 0–4

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Stabilizer Materials

Materials used in this research are categorized into three groups.The first group of material comprises two types of control soilssampled from Dallas-Fort Worth �DFW� and Arlington areas inTexas. The second group covers recycled ash materials, which areclass F fly ash and bottom ash stabilizers. The last group coversfiber materials, nylon and polypropylene fibers. Both soils weresubjected to physical tests including Atterberg limits and specificgravity tests, these results are reported in Table 1. As per unifiedsoil classification system �USCS�, these soils were classified aslow compressible clays �CL�. These soils are regarded as expan-sive plastic clay with medium plasticity index values.

Ashes tested are products from different coal combustionplants in the United States. Fly ash stabilizers used here weretermed as fly ash 1 or F1, fly ash 2 or F2, and bottom ash or B.Chemical properties of these ashes were analyzed as per ASTMC 311-77 and are shown in Table 2. Both fly ashes are slightlydifferent with respect to chemical composition and have lowamounts of calcium ions. The selection of low calcium type classF fly ash in this research was mainly due to the fact that suchmaterial can be used to stabilize soils containing sulfates withoutforming Ettringite and inducing sulfate heaving in soils, whichwas typically observed when calcium-rich stabilizers were used totreat sulfate-rich soils.

Polymeric fibers including fibrillated polypropylene �PP� andhydrophilic nylon fibers �N� were used in the investigation. Bothpolypropylene and nylon fibers can be produced from the wasteplastic and polymeric materials. Physical characteristics of bothPP and nylon fibers are given in Table 3. As shown in Table 3,fibers are not affected by the presence of salts in soils, biologicaldegradation, and ultraviolet degradation. Additionally, they ex-hibit high tensile strength, which could be used to reinforce soiland enhance their strength.

Table 3. Physical Characteristics of Fibers

Properties Polypropylene fibers Nylon fibers

Tensile strength 97 ksi 130–140 ksi

Young’s modulus 580 ksi 750 ksi

Melt point 330°F 435°F

Chemical resistance Excellent Good

Alkali resistant Excellent Excellent

Acid and salt resistance High Good

Ultraviolet resistance — Excellent

Thermal conductivity — Low

Absorption No 4 to 5%

Specific gravity 0.91 1.1

Fig. 1. Schematic of three-dimensional free swell tests

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Testing Procedures and Specimen Preparation

Laboratory tests were performed on both control and stabilizedsoil specimens to measure changes in physical and engineeringcharacteristics with respect to stabilizer reactions with the controlsoil. Physical tests, Atterberg and standard Proctor compactiontests, were first conducted on both control �or untreated� andtreated soils to measure both plasticity and compaction character-istics. Standard Proctor test results were also used to compact andprepare soil specimens for engineering tests. The engineeringtests performed are three-dimensional free swell tests, three-dimensional shrinkage strain tests, and modified consolidation�pressure swell� tests. These test procedures are briefly describedin the following.

Three-Dimensional Free Swell TestsSpecimens used in this test were prepared by mixing soil withrequired chemicals or fiber stabilizers at targeted moisture contentlevels. Both control and treated soil specimens were mixed withadditives and then compacted in three layers by using standardproctor devices at two moisture content levels. In the case of theisolated stabilizer material investigations, soil specimens werecompacted at the optimum and at 95% dry-of-optimum moisturecontent of a standard proctor test curve. Four stabilizer dosagelevels of 0, 10, 15, and 20% by weight of dry soil were used forclass F fly ash and bottom ash stabilizers. On the other hand,dosage percents of 0, 0.2, 0.4, and 0.6% by weight of dry soilwere used for the polymer fibers. Soil was mixed with fibers byhand and an attempt is made to ensure proper distribution offibers in the soil mix. Then the mix was compacted in three layersto prepare soil specimens. After compaction, soil specimens weretransferred to a 100% humidity-controlled room to cure for 14days. After curing, soil specimens were subjected to three-dimensional swell tests.

The three-dimensional free swell test allows evaluation of bothtreated and untreated soil specimens under unconfined conditions.Fig. 1 shows a schematic of the test setup used in this research.Soil specimens of 105 mm in diameter and 115.5 mm in height�4.1 in. diameter�4.5 in. height� were first compacted and thencovered by a rubber membrane. Porous stones were placed at thetop and bottom of the specimens, which facilitated the movementof water to the soil specimen. The specimen was fully inundatedwith water supplied at both ends. The amount of heave in bothvertical and diametral directions was monitored until there was nosignificant swell for 24 h. The test results were expressed as thepercent swell versus time. The percent values were calculatedbased on the original dimensions of the compacted soil specimen.

In the case of the combined stabilizer material investigations,the optimum stabilizer dosage levels were determined from testresults of isolated stabilizer material studies. The change of dry ofoptimum compaction moisture level in isolated stabilizer studiesto wet of optimum moisture level in combined stabilizer studieswas due to the fact that no considerable improvements were notedon the dry of optimum side in isolated stabilizer studies. Also,higher saturation level at wet of optimum moisture content con-dition promotes chemical reactions in chemical treated soils.Hence wet of optimum moisture level was considered in com-bined fiber-ash stabilization studies.

Three-Dimensional Shrinkage Strain TestsLinear shrinkage strain bar tests are usually performed on com-pacted soil specimens to measure shrinkage strain potentials.
However, due to the sizes of fibers, the linear bar specimen tests

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were not considered in this research. Cylindrical soil specimenswere prepared and tested. Soil slurries were prepared by mixingsoils with and without stabilizers at moisture contents equal tothose corresponding to liquid limit state �Table 4� and slowlytransferred to cylindrical molds, which are 63.5 mm in diameterand 152.4 mm in height �2.5 in. diameter�6 in. height�. Slurryspecimens in the molds were kept at room temperature for 4 hoursand then transferred to ovens at temperature equal to 100°C.

After 48 h, soil specimens were transferred from ovens andtheir vertical and diametral dimensions were measured at threedifferent locations. These measurements were averaged to calcu-late the volumetric shrinkage strains. These average values wereused along with the original dimensions to compute volumetric,vertical, and diametral shrinkage strains.

Modified Consolidation (Pressure Swell) TestsModified consolidation test or pressure swell test was conductedper ASTM D-4546 specification. This test is commonly used tomeasure the maximum swelling pressure of the soil specimens atwhich no volume change of the specimens is anticipated. Thespecimens were compacted in a ring of 63.5 mm in diameter and25.4 mm in thickness �2.5 in. diameter and 1 in. thickness� at aknown moisture content to reach the design dry unit weight. Thesame Proctor hammer was used to compact the soil specimens.After compaction, all specimens were kept in the 100% humidity-controlled room for 14 days. After curing, specimens were sub-jected to the pressure swell tests.

In this test, both control and treated soil specimens were fullysoaked in the standard consolidation setup. Two porous stoneswere placed at the top and bottom of the soil specimen. The topporous stone eliminated the point load effect. A dial gauge wasplaced on the top of the specimen to monitor changes in thespecimen due to water absorption. Load was imposed in incre-ments and recorded in order to maintain original soil volume.Testing was discontinued when the dial gauge no longer indicatedany swell movements for more than 2 days. The accumulated loadand weight of the cap were used to compute the swell pressurepotential of each soil specimen.

Analyses of Isolated Material Test Results

This section summarizes test results of isolated stabilizer studiesand provides analyses of test results on three-dimensional free

Table 4. Liquid Limit Compaction States of Soil Mixtures forThree-Dimensional Shrinkage Strain Studies

Soil typeDosage level

�%�

Liquid limit�%�

Fly ash, F1 Fly ash, F2 Bottom ash

DFW 0 50.5 50.5 50.5

10 49.9 49.9 49.6

15 48.4 47.9 48.8

20 47.7 45.9 46.8

Arlington 0 43.7 43.7 43.7

10 40.1 39.7 41.6

15 38.6 37.4 40.3

20 38.0 35.1 35.5

swell tests, three-dimensional shrinkage tests, and modified con-



solidation tests �pressure swell tests� from the entire testingprogram.

Three-Dimensional Free Swell Test Results

Fig. 2 presents an example of repeatability of test results of con-trol DFW soil specimens. This figure depicts percent free swellstrains against elapsed time intervals. Results showed excellentrepeatability with low standard deviations. The standard devia-tions of the test results are less than 0.5% with a majority of themin the range of 0.1 to 0.2%, indicating that the tests were repeat-able and provided reliable results.

Free swell test results are reported by plotting the averagevalues of free swell strains from three identical soil specimenswith the same conditions. Figs. 3–6 present average volumetricswell strain potentials from three identical soil specimens for eachcondition. These specimens were compacted at optimum moisturecontent and dry of optimum at 95% of maximum dry unit weight.The majority of free swell in soils was recorded within the first 8h �480 min�. Subsequent swell rates were slow and no swellingwas recorded when specimens were soaked for more than 4,000min. The following observations are noted from Figs. 3–6:• The free volumetric swelling potentials of both control soils

decreased with an increase in the percentage of ash stabilizersas shown in Figs. 3 and 4. The majority of volumetric swellingstrains are contributed from vertical swell strains, which de-creased with an increase in ash stabilizers. Diametral swellstrains ranged from 0 to 2%, and they also decreased with thestabilizer treatment.

• Increase in swell behavior due to fiber treatments or inclusionsare observed in polypropylene treated soils when dosage levelsare higher than 0.2% �Figs. 5 and 6�. This phenomenon can beexplained by the fact that polypropylene fibers at higher dos-age levels are difficult to compact and hence may have re-sulted in the large void distribution in the fiber treated soils.Such void distribution results in large swell strains comparableto those of untreated soils.

Fig. 2. Free vertical, diametrical, and volumetric swell strains ofDFW soils

• Nylon fibers decreased the swell behavior of control soils

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�Figs. 5 and 6�. This decrease was proportional to an increasein percentage dosages of nylon fibers. Swelling potentials in-creased slightly at high dosage levels and at dry of optimumcompaction moisture state.Swell strain reductions due to ash stabilizer treatments are at-

tributed to flocculation and possible strength enhancements due tocementation and pozzalonic types of reactions in the soils. Fibersresulted in slight reductions of swell strains, probably due to the

Fig. 3. Volumetric swell strains of ash-treated soils �DFW soils�: �a�of optimum dry unit weight

Fig. 4. Volumetric swell strains of ash-treated soils �Arlington soils�:side of optimum dry unit weight

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reinforcement effects of the fibers. When comparing all treatmentmethods, the class F fly ash treatment resulted in lesser free swellstrains of soils than other treatment methods. Overall, in thepresent research, class F fly ash treatment is considered a bettertreatment method over bottom ash and fiber treatment methods.Both bottom ash and fiber treatments provided moderate to lowimprovements by slightly reducing swell strain properties of con-trol soils.

cted at optimum dry unit weight; and �b� compacted at 95% dry side

mpacted at optimum dry unit weight; and �b� compacted at 95% dry

compa

�a� co


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Three-Dimensional Shrinkage Test Results

Tables 5 and 6 present average three-dimensional shrinkage testresults of both control and treated soils. Results of both DFW andArlington soils are included in these tables and figures. Shrinkagestrains in diametral and vertical directions were first calculatedand were used to determine volumetric shrinkage stains. Thevolumetric shrinkage strain test was considered here since thistest is conducted on soil specimens of considerable volumes.

Fig. 5. Volumetric swell strains of fiber-treated soils �DFW soils�: �a�of optimum dry unit weight

Fig. 6. Volumetric swell strains of fiber-treated soils �Arlington soils�side of optimum dry unit weight



From the tables, the test results demonstrate that the volumet-ric shrinkage strains of both soil types decreased with an increasein the percentage of ash and fiber stabilizers. Bottom ash de-creased volumetric shrinkage strains of control soils by magni-tudes ranging from 4 to 12% for DFW soils and 6 to 10% forArlington soils. A decrease in shrinkage strains of DFW soil wasalso recorded. This reduction ranges from 9 to 14% and from 14to 19% for class F fly ash �F1� and class F fly ash �F2� stabiliza-

acted at optimum dry unit weight; and �b� compacted at 95% dry side

ompacted at optimum dry unit weight; and �b� compacted at 95% dry

comp

: �a� c

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tions, respectively. For Arlington soils, the decrease was from 10to 15% and from 10 to 16%, respectively. The decrease in shrink-age strains are attributed to a decrease in plasticity characteristicsfrom ash treatments, which can be seen from the decrease inliquid limit values of ash treated soils as presented in Table 4.

Both polypropylene and nylon fibers also decreased shrinkagestrains. The maximum decrease is observed at 0.2% dosage levelof fibers. At higher dosage levels, the shrinkage strains reachedplateau conditions. Hence the optimum percentage of fiber dos-ages for the present fibers researched was 0.2%. The decrease inshrinkage strains in fiber treatments is due to strength, in particu-lar, cohesion property enhancements that resulted from the inclu-sions of fibers. The apparent cohesion intercept increase isindirectly related to tensile strength enhancements in soils whichresists tensile forces exerted on soils’ specimens during shrinkagetest conditions.

Also, Arlington soils were better stabilized with fibers thanDFW soils, possibly due to larger amounts of percent finer par-ticles in the Arlington soil, which allowed better attraction withfibers. Also, when comparing decreased magnitudes of shrinkagestrains of fiber and ash treatment methods, it can be mentionedthat ash treatments provided a larger decrease in original shrink-age strains than fiber treatment methods.

Table 5. Average Three-Dimensional Shrinkage Strains of AshTreatment in percent

0%

Class F fly ash, F1 Class F fly ash, F2 Bottom ash

10% 15% 20% 10% 15% 20% 10% 15% 20

DFW 33.4 23.7 20.6 19.7 19.1 18.7 15.0 29.5 23.4 21

Arlington 30.6 21.0 18.7 15.7 20.2 18.1 14.7 24.7 21.2 21

Fig. 7. Swell pressure of ash-treated soils �DFW soils�: �a� compaoptimum dry unit weight

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Pressure Swell Test Results

The pressure swell test results of DFW and Arlington soils stabi-lized with ashes and fibers are presented in the form of bar chartsin Figs. 7–10. Soil specimens with no stabilizers exhibited largeswell pressures, while stabilized specimens yielded low swellpressures. For example, swell pressures of DFW soils stabilizedwith 0, 10, 15, and 20% of bottom ash were 2.12, 1.26, 1.02, and0.96 ksf �1,000 lbs/ ft2�, respectively. Similar decreases in swellpressures were noticed with other stabilizers. The amount of re-duction in swell pressure magnitudes with fiber reinforcementwas small when compared to ash treated soils. At best, it can bementioned that both nylon and polypropylene fibers are slightlyeffective in reducing swell pressures. In summary, ash amend-ments resulted in moderate to high reductions of swell pressures,and fibers resulted in low reductions.

The low reductions in swell pressures due to fiber treatmentcan be attributed to the presence of fibers, which create drainagepaths for the dissipation of pore pressures in a loaded soil speci-men. Another reason could be that the fibers, being tensile ele-ments, restrain the swell pressures generated during soaking.Fiber reinforcement effects on swell pressures are a positive con-tribution, though the contributions can be small to moderate inmagnitude.

Table 6. Average Three-Dimensional Shrinkage Strains of FiberTreatment in percent

0%

Polypropylene Nylon

0.2% 0.4% 0.6% 0.2% 0.4% 0.6%

DFW 33.4 31.4 30.8 30.7 30.4 29.7 29.6

Arlington 30.6 22.0 21.5 21.3 25.6 22.8 21.0

t optimum dry unit weight; and �b� compacted at 95% dry side of
cted a

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Determination of Optimum Dosages of Stabilizers

This section covers a simple optimization analysis of measuredtests parameters in this research. Fig. 11 presents the flowchart ofthe present approach for the determination of the optimum dosageof stabilizers. Three-dimensional free swell, three-dimensionalshrinkage, and pressure swell test results were analyzed to deter-mine optimum dosages of both ash and fiber stabilizers. Thisoptimization was based on the critical criteria set forth by Chen�1988� and other researchers regarding the swell and shrinkageproperties of soils. The outcome of this optimization analysis is

Fig. 8. Swell pressure of ash-treated soils �Arlington soils�: �a� comoptimum dry unit weight

Fig. 9. Swell pressure of fiber-treated soils �DFW soils�: �a� compaoptimum dry unit weight



the determination of final optimum dosage levels for ashes andfibers to be used in the combined stabilization studies.

Problematic Levels Criteria

Expansive soils can be classified using different methods includ-ing mineralogical based identification methods, indirect testmethods, and direct measurement approaches �Chen 1988�.Mineralogical identification methods using x-ray diffraction andscanning electron micrograph studies are useful in the indirectevaluation of the presence of expansive clay minerals such as

at optimum dry unit weight; and �b� compacted at 95% dry side of

t optimum dry unit weight; and �b� compacted at 95% dry side of

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montmorillonite in soils. Indirect methods such as the index prop-erties �Atterberg limits�, potential volume change �PVC� methods,and activity properties provide valuable means to identify swell-ing clays �Chen 1988�. However, none of the indirect methodscan be used independently. Another method of direct measure-ments of swell, shrink, and matric suction properties of soils of-fers the most useful data for practicing engineers to classifyexpansive soils by performing these tests. These direct methodsalso provide results that can be used to assess the severity ofexpansive soils.

In this research, the direct measurement approach utilizing freeswell, swell pressure, and volumetric shrinkage tests was fol-lowed to measure the volumetric expansions of soils. The nextstep is to determine optimum stabilizer dosages that result in non-problematic soil swell and shrink movements. It is important toestablish these swell and shrink test magnitudes at which soils areconsidered as nonproblematic. Such selection criteria depend oninfrastructure types that the site soils support in the field. Lowoverburden structures including pavements and embankments aredistressed by moderate volumetric soil movements. Based on theexisting literature information, volumetric swell expansion of 5%,swelling pressure of 1 ksf �1,000 lb/ ft2�, and volumetric shrink-age strain of 17% were selected as properties for noncritical ornonproblematic levels �Chen 1988; Nelson and Miller 1992; Pun-thutaecha 2002�. These values were used for the determination ofoptimum dosage levels of isolated stabilization methods to reduceuntreated expansive soils from problematic to nonproblematiccharacterization.

Optimum Dosage Levels Based on Criteria

In order to determine optimum dosage levels of each stabilizer, itis necessary to plot average results of volumetric swell strains,swell pressures, and shrinkage strains versus stabilizers dosages.The optimum dosage levels were determined based on the levelsat which swell and shrinkage characteristics reach levels that cor-

Fig. 10. Swell pressure of fiber-treated soils �Arlington soils�: �a� comoptimum dry unit weight

respond to noncritical conditions. Using this linear approach, the

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optimum percentage values for free volumetric swell strain, volu-metric shrinkage strain, and pressure swell properties of each sta-bilizer were determined. Then, the final optimum dosage levelwas calculated by using the following equation:

optimum dosage �%� = a1Ffs + a2Fsh + a3Fps �1�

where Ffs�recommended dosage of stabilizer from free swell testresults; Fsh�recommended dosage of stabilizer from volumetricshrinkage test results; Fps�recommended dosage of stabilizerfrom pressure swell test; and a1 ,a2 ,a3�constants depending onthe weightage factors for swell and shrinkage tests.

The values of constants a1, a2, and a3 are the weightage factorsthat take into account the importance of free swell, swell pressure,and shrinkage strain tests. These values were taken as 0.33 �or33%�, implying that equal consideration is given to all three swelland shrinkage test methods. However, in this research, the im-pacts of swell and shrinkage strains are considerable when com-pared to swell pressures. Hence these constant parameters, a1 anda2, were set to 0.4 �40%� and a3 was set to 0.2 �20%�. Therecommended dosages in percentages of each stabilizer were de-termined. Typical examples of this analysis for class F fly ash

d at optimum dry unit weight; and �b� compacted at 95% dry side of

Fig. 11. Flow chart to determine optimum dosage of stabilizers

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stabilizer were shown in Figs. 12–14. These recommended valueswere then used in Eq. �1� to determine the optimum dosage levelof each stabilizer. The optimum dosage levels of each stabilizerwere presented in Table 7.

Combined Stabilizer Test Results

This section summarizes test results of combined fiber-ash stabi-lizer studies. Swell strain results of stabilized soils are reported interms of percent swell strains corresponding to initial dimensionsof the soil specimens. Tables 8 and 9 present free swell strainresults at optimum and wet of optimum moisture content condi-tions, respectively. From the tables, it can be noted that all soilsstabilized with combined stabilizers yielded better and lowerswell strains than soils treated with isolated stabilizers. This trendis noted on the test results of both DFW and Arlington soils. Thevolumetric free swell strains of combined stabilizer treated soilsranged from 1 to 3% on the wet side of optimum moisture con-ditions, and from 2 to 4% at optimum moisture conditions. This isexpected since soil specimens compacted wet-of-optimum condi-tions have high saturation levels and hence exhibit low swellstrains. Overall, this research proved that soils stabilized withcombined stabilizers experienced lower heave movements thansoils stabilized with either ashes or fiber treatment methods. Swell

Fig. 12. Volumetric swell strains of fly ash treated soils �DFW soil�

Fig. 13. Volumetric shrinkage strains of fly ash treated soils �DFWsoil�



reductions are caused by several reactions, including flocculationand a few chemical strengthening related reactions between soilsand ash stabilizers. The presence of fibers also enhanced tensilestrength and shear strength of soils and this increased strengthoffered resistance to dispersive forces generated during swell testconditions. This resulted in lower swell movements in the fiber-ash treated soils.

Table 10 presents average values of three-dimensional shrink-age strain test results of both control and combined materialtreated soils. These results show that all combined stabilizer-treated soils yielded lower shrinkage strains �13–20%� than thoseof isolated stabilizer-treated soil specimens. The volumetricshrinkage strain test results of control soil specimens ranged from33 to 34% with an average value of 33.4% for DFW soils and thesame ranged from 29 to 31% with an average value of 30.6% forArlington soils. These results indicate “severe” problematic char-acterization levels for control expansive soils. In comparison, thecombined stabilizer treated same soils show a considerable de-crease in the volumetric shrinkage strain potentials.

From Table 10, it can be mentioned that the combined stabili-zations utilizing F2-polypropylene fibers and F2-nylon fibers pro-vided the best performance in reducing volumetric shrinkagestrains of control soils. It should be noted here that compactionmoisture content was not a variable in shrinkage strain test resultsas soil slurries prepared at liquid limit state are used in the shrink-age tests to measure shrinkage strain potentials.

Swell pressure results are presented in terms of ksf�1,000 lb/ ft2� and are shown in Table 11. These results show thatthe swell pressures of combined stabilizer-treated soil specimensranged from 0.6 to 1.2 ksf �600 to 1,200 lb/ ft2� at optimum mois-ture content conditions, and from 0.5 to 1.0 ksf �1,000 lb/ ft2� at95% wet of optimum moisture content conditions. Soil specimenscompacted at optimum moisture content exhibited more swellingthan soil specimens compacted at wet of optimum conditions due

Table 7. Optimum Dosage Levels in Percent of All Stabilizers and SoilTypes

Stabilizer DFW soil Arlington soil

Class F fly ash, F1 19 14

Class F fly ash, F2 15 14

Bottom ash 19 18

Nylon 0.5 0.4

Polypropylene 0.3 0.3

Fig. 14. Swell pressures of fly ash treated soils �DFW soil�

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to differences in saturation levels at the compaction moisture lev-els. Thus soils compacted at wet of optimum conditions experi-enced low swell pressures than those at optimum moisture contentconditions. This behavior is in agreement with the observationsnoted from volumetric swell strain tests. Higher moisture pres-ence at wet of optimum moisture conditions also facilitateschemical reactions that result in reduced soil heaving andshrinking.

Overall, these results indicate that the best reductions in swellpressures were achieved when “bottom ash and nylon fibers”were used to stabilize DFW soils and “class F fly ash �F2� andnylon fibers” were used to treat Arlington soils. In the case ofsoils compacted at 95% wet side of optimum moisture content,the combination of “bottom ash and polypropylene fibers” pro-vided the best stabilization enhancements.

Table 8. Average Free Swell Test Results of Soil Samples Compacted atOptimum Dry Unit Weight

Sample

Ashstabilizers

�%�

Fiberstabilizers

�%�

Verticalswellstrain�%�

Diametralswellstrain�%�

Volumetricswellstrain�%�

D-F1-P 19 0.3 0.87 1.1 3.2

D-F2-P 15 0.3 0.5 1.1 2.9

D-B-P 19 0.3 2.2 0.9 4.1

D-F1-N 19 0.5 1.3 0.9 3.1

D-F2-N 15 0.5 1.0 1.0 3.0

D-B-N 19 0.5 1.7 0.9 3.4

A-F1-P 14 0.3 0.8 1.0 2.9

A-F2-P 14 0.3 0.1 1.1 2.3

A-B-P 18 0.3 1.1 1.3 3.8

A-F1-N 14 0.4 1.1 0.5 2.1

A-F2-N 14 0.4 0.9 0.4 1.8

A-B-N 18 0.4 1.0 1.1 3.2

Note: D�DFW soil; A�Arlington soil; F1,F2�class F fly ash;B�bottom ash; P�polypropylene fibers; and N–nylon fibers.

Table 9. Average Free Swell Test Results of Soil Samples Compacted atWet of Optimum �95% of Optimum Dry Unit Weight�

Sample

Ashstabilizers

�%�

Fiberstabilizers

�%�

Verticalswellstrain�%�

Diametralswellstrain�%�

Volumetricswellstrain�%�

D-F1-P 19 0.3 0.7 0.7 2.0

D-F2-P 15 0.3 0.3 0.8 1.9

D-B-P 19 0.3 0.5 0.9 2.4

D-F1-N 19 0.5 0.8 0.5 1.9

D-F2-N 15 0.5 0.4 0.7 1.8

D-B-N 19 0.5 0.7 0.9 2.5

A-F1-P 14 0.3 0.3 0.9 2.1

A-F2-P 14 0.3 0.2 0.5 1.3

A-B-P 18 0.3 0.6 1.0 2.6

A-F1-N 14 0.4 0.8 0.4 1.6

A-F2-N 14 0.4 0.3 0.6 1.5

A-B-N 18 0.4 0.6 1.0 2.6

Note: D�DFW soil; A�Arlington soil; F1,F2�class F fly ash;
B�bottom ash; P�polypropylene fibers; and N�nylon fibers.
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Summary and Conclusions

The major conclusions obtained from the laboratory tests, optimi-zation studies, and analyses of test results are summarized asfollows.1. All five stabilizers, class F fly ash �F1 and F2�, bottom ash,

polypropylene fibers, and nylon fibers, improved soil proper-ties including volumetric swell strain, swell pressure, andshrinkage strain potentials, of both DFW and Arlington soils.In most of the cases, these improvements reduce expansivesoil behavior from problematic characterization levels tononproblematic levels.

2. Class F fly ash provided the maximum enhancements to con-trol soil properties. The next best improvements are obtainedwhen bottom ash was used for soil treatments. Ash treatedsoils showed improvements in reducing swell, shrinkage, andplasticity characteristics. These improvements can be attrib-uted to well-established reactions with ash stabilizers such asion exchange and flocculation as well as cementitious andpozzalonic reactions to bind soil particles. Ion exchange ef-fects could be indirectly explained from the decrease in liq-uid limit values measured in this research.

3. Reduction in severity levels from “high” of control soils toeither “medium” or “low” severity levels of treated soils wasobserved in class F fly ash treated soils. For bottom ashtreated soils, the severity levels reduced from “high” to “me-dium” severity levels. Severity levels were characterizedbased on established classification methods listed by Chen�1988� and Nelson and Miller �1992�.

Table 10. Average Three-Dimensional Shrinkage Strains of CombinedStabilizer Treated Soils

Soil typeControl

�%�

Combined material treatment�%�

F1-P F1-N F2-P F2-N B-P B-N

DFW 33.4 19.1 18.6 18.5 16.6 20.9 20.5

Arlington 30.6 14.7 13.9 14.4 13.6 19.1 18.5

Note: F1,F2�class F fly ash; B�bottom ash; P�polypropylene fibers;and N�nylon fibers.

Table 11. Pressure Swell Test Results of Combined Material Treatment

Soil type Soil specimen

Swell pressure �1 ksf: 1,000 lb/ ft2�

Optimummoisture

95% wet-side ofoptimum moisture

DFW D-F1-P 1.20 1.02

D-F2-P 0.78 0.67

D-B-P 0.69 0.56

D-F1-N 1.18 0.96

D-F2-N 0.86 0.64

D-B-N 0.65 0.58

Arlington A-F1-P 1.11 0.97

A-F2-P 0.73 0.57

A-B-P 0.85 0.76

A-F1-N 0.95 0.85

A-F2-N 0.68 0.60

A-B-N 0.87 0.77

Note: D�DFW soil; A�Arlington soil; F1,F2�class F fly ash;
B�bottom ash; P�polypropylene fibers; and N�nylon fibers.

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4. It is interesting to note that the highest improvements of fibertreated soils were noted at dosage levels close to 0.2% dos-age level. At higher dosages, these improvements were de-creased. This was attributed to poor compaction at higherdosage levels and also due to development of large voidratios at those compacted densities. Both nylon and polypro-pylene fiber treated soils showed severity levels of soils inthe “medium” severity level, which are considerably impor-tant from “high” level of control soils.

5. Optimization analysis based on dosage levels, determinedfrom nonproblematic soil swell and shrinkage conditionsprovided optimum dosage levels for each stabilizer. Com-bined stabilizations using these optimum dosage levels of ashand fibers provided considerable improvements to many soilproperties when compared to the properties of isolated stabi-lizer treated soils.

6. Among various combined stabilizer related soil conditionsstudied in this research �class F fly ash combined with poly-propylene fibers, class F fly ash combined with nylon fibers,bottom ash combined with polypropylene fibers, and bottomash combined with nylon fibers�, the combined stabilizertreatment with class F fly ash and nylon fibers provided themaximum improvements to control soil properties. Thiscombined treatment reduced swell and shrinkage character-izations of control soils from “high” problematic levels to“low” levels. Hence, for field applications, combined class Ffly ash and nylon fibers treatments are recommended.

Acknowledgments

This study was financially supported by the Advanced Technol-ogy Program �ATP� of Texas Higher Education CoordinatingBoard, Austin, Tex. under Grant No. 1407610-50. The writerswould like to acknowledge this financial support. The writerswould also like to thank Boral Material Technologies, Inc. for
providing fly ash and bottom ash stabilizers used in this research.


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volume change behaviors of expansive soils stabilized with recycled ashes and fibers

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