Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

RE

GIN

A L

IBR

AR

Y o

n 09

/07/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Shear Strength Behavior of Fiber-Reinforced SandConsidering Triaxial Tests under Distinct Stress Paths

Nilo Cesar Consoli, Ph.D.1; Karla Salvagni Heineck, D.Sc.2; Michele Dal Toé Casagrande, D.Sc.3; andMatthew Richard Coop, D.Phil.4

Abstract: The results of drained triaxial tests on fiber reinforced and nonreinforced sand �Osorio sand� specimens are presented in thiswork, considering effective stresses varying from 20 to 680 kPa and a variety of stress paths. The tests on nonreinforced samples yieldedeffective strength envelopes that were approximately linear and defined by a friction angle of 32.5° for the Osorio sand, with a cohesionintercept of zero. The failure envelope for sand when reinforced with fibers was distinctly nonlinear, with a well-defined kink point, so thatit could be approximated by a bilinear envelope. The failure envelope of the fiber-reinforced sand was found to be independent of thestress path followed by the triaxial tests. The strength parameters for the lower-pressure part of the failure envelope, where failure isgoverned by both fiber stretching and slippage, were, respectively, a cohesion intercept of about 15 kPa and friction angle of 48.6 deg. Thehigher-pressure part of the failure envelope, governed by tensile yielding or stretching of the fibers, had a cohesion intercept of 124 kPa,and friction angle of 34.6 deg. No fiber breakage was measured and only fiber extension was observed. It is, therefore, believed that thefibers did not break because they are highly extensible, with a fiber strain at failure of 80%, and the necessary strain to cause fiberbreakage was not reached under triaxial conditions at these stress and strain levels.

DOI: 10.1061/�ASCE�1090-0241�2007�133:11�1466�

CE Database subject headings: Fiber reinforced materials; Triaxial tests; Shear strength; Sand.

Introduction

The general characteristics of granular soils reinforced with dis-crete fibers has been reported in previous studies by several in-vestigators �e.g., Gray and Ohashi 1983; Gray and Al-Refeai1986; Maher and Gray 1990; Consoli et al. 1998, 2002, 2003,2004, 2005, Michalowski and Cermák 2003; and Heineck et al.2005�. This review shows that fiber inclusion clearly provides anincrease in material strength and ductility. The composite behav-ior is governed by the fiber content and the mechanical and geo-metrical properties of the fiber. The failure mechanism of thefiber-reinforced soil has been found to be dependent on the nor-mal stress. Up to a threshold value referred to as the criticalnormal stress, the fibers slip during deformation. For stresses

1Associate Professor, Dept. of Civil Engineering, Federal Univ. of RioGrande do Sul, Av. Osvaldo Aranha, 99, 3. Andar, CEP 90035-190 PortoAlegre, Rio Grande do Sul, Brazil �corresponding author�. E-mail:consoli@ufrgs.br

2Associate Professor, Dept. of Civil Engineering, Federal Univ. of RioGrande do Sul, Av. Osvaldo Aranha, 99, 3. Andar, CEP 90035-190 PortoAlegre, Rio Grande do Sul, Brazil.

3Associate Professor, Dept. of Transportation, Federal Univ. of Ceará,Campus Universitário do Pici, Bloco 710, CEP 60455-760 Fortaleza,Ceará, Brazil.

4Professor, Dept. of Civil Engineering, Imperial College, Univ. ofLondon, London, SW7 2AZ, UK.

Note. Discussion open until April 1, 2008. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this technical note was submitted for review andpossible publication on March 31, 2006; approved on May 2, 2007. Thistechnical note is part of the Journal of Geotechnical and Geoenviron-mental Engineering, Vol. 133, No. 11, November 1, 2007. ©ASCE,

ISSN 1090-0241/2007/11-1466–1469/$25.00.

1466 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

greater than the critical stress, failure is governed by stretch oryield of the fibers �Maher and Gray 1990�.

The influence of fiber inclusion on the mechanical behavior ofsoils, however, has not been extensively investigated in terms ofthe influence, if any, of the triaxial test stress path direction on thefailure envelope of the fiber-reinforced soil. In the present work,results of a series of triaxial tests conducted on a sand reinforcedwith polypropylene fibers are presented, demonstrating that thefailure envelope of the fiber-reinforced soils was found to be in-dependent of the stress path followed.

Experimental Program

A variety of different triaxial tests were carried out in this experi-mental program using fully saturated samples, at effective confin-ing pressures ranging from 20 to 680 kPa. Different stress pathswere used in order to define the failure envelopes of the Osoriosand, reinforced and nonreinforced with polypropylene fibers.

Materials

A uniform quartzitic sand �Osorio sand� from southern Brazil wastested in this experimental program. The Osorio sand wassampled from the region of Osorio near Porto Alegre. The soil isclassified as nonplastic uniform fine sand and the specific gravityof the solids is 2.62. The grain size distribution is entirely finesand �0.075 mm�diam�0.42 mm�, with an effective diameterof 0.09 mm and uniformity and curvature coefficients of 2.1 and1.0, respectively. Mineralogical analysis showed that sand par-ticles are predominantly quartz. The minimum and maximumvoid ratios are 0.6 and 0.9, respectively.

Polypropylene fibers were used throughout this investigation

EERING © ASCE / NOVEMBER 2007

2007.133:1466-1469.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

RE

GIN

A L

IBR

AR

Y o

n 09

/07/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

to reinforce the soil. Their average dimensions were 24 mm inlength and 0.023 mm in diameter, with a specific gravity of 0.91,tensile strength of 120 MPa, elastic modulus of 3 GPa, linear den-sity of 3 denier, and linear strain at failure of 80%. The fibercontent used in the experiments was 0.5% by weight of soil.

Sample Preparation

The compacted soil and fiber-reinforced specimens used in thetriaxial tests were prepared by hand mixing dry soil, water, andpolypropylene fibers �when used�. During the mixing process, itwas found to be important to add the water prior to adding thefibers, to prevent floating of the fibers. Visual and microscopeexamination of exhumed specimens showed the mixtures to besatisfactorily uniform. The undercompaction process �Ladd 1978�was used to produce homogeneous specimens that could be usedfor a parametric study in the laboratory-testing program. Thespecimens were statically compacted in three layers into a 50 mmdiam by 100 mm high split mould, to a moisture content of 10.0%and dry unit weight of 15.0 kN/m3 �equivalent to a relative den-sity of 50%�. Each sample was compacted in a mould on thetriaxial pedestal by applying a static load via the loading platen.The final height of the sample was controlled to ensure a relativedensity of 50%.

Triaxial Tests

The triaxial tests were conducted using a computer controlledstress path cell �Bishop and Wesley 1975�. This apparatus allowedthe tests to be conducted with different drained stress paths incompression �constant p�, constant radial stress �loading tests�,and a path with dq /dp�=−3 �unloading tests��, on samples ofOsorio sand, both reinforced with polypropylene fibers and non-reinforced. The samples were saturated under back pressure andthe effective confining pressures ranged from 20 to 680 kPa, cov-ering the range of stresses applied in most engineering applica-tions. Saturation was monitored in each test, ensuring B values ofat least 0.97 for all specimens. The axial strains were monitoredinside the triaxial cell using an inclinometer type of local straintransducer �Burland and Symes 1982� and outside the cell using astandard type of displacement transducer. The volumetric strainwas measured by an Imperial College volume gauge �Maswoswe1985� connected to the drainage outlet. The triaxial tests were runat a sufficiently low axial strain rate to ensure full drainage withinthe sample �0.017% per minute�. Drainage was also monitored bymeasuring the excess pore pressure at the opposite end of thespecimen to the drainage. The membrane and area correctionsfollowed recommendations proposed by La Rochelle et al.�1988�.

Results and Analysis

The shear strength envelopes obtained from triaxial tests withpolypropylene fiber-reinforced and nonreinforced samples ofOsorio sand are shown in Figs. 1�a and b�, respectively, where thedeviator stress �q= ��1�−�3���, is plotted against the correspondingmean effective stress �p�= 1

3 ��1�+�2�+�3���. The shear strength en-velope for the fiber-reinforced sand was taken at 20% shear strain��s=�a−�v /3, where �a�axial strain, and �v the volumetricstrain�. This was because the tests on the fiber-reinforced soilwere generally experiencing slight strain hardening until the

maximum strain that the apparatus could reach, as can be seen in

JOURNAL OF GEOTECHNICAL AND GEOE

J. Geotech. Geoenviron. Eng.

the stress-strain data in Fig. 2, so that a “strength” had to bedefined at a particular strain.

Neither the reinforced nor the nonreinforced Osorio sand has astrength envelope that is dependent on the stress path. In the caseof fiber-reinforced sand, a bilinear failure envelope was found,with the strength parameters c�=15 kPa and ��=48.6 deg atlower stress levels and c�=124 kPa and ��=34.6 deg at higherpressures, taking shear strain at failure as 20%. If the shear stressat 28% axial strain were used as the shear strength, which wouldcoincide with the end of the tests for all the stress paths used, thebilinear failure envelope parameters would change to be c�=22 kPa and ��=50.9 deg at lower stress levels and c�=151 kPa and ��=33.2 deg at higher pressures. For the nonrein-forced Osorio sand, the �� obtained was 32.5 deg �for both 20%

Fig. 1. Shear strength envelopes �a� nonreinforced Osorio sand; �b�fiber-reinforced Osorio sand

and 28% shear strain�, which leads to a nonreinforced strength

NVIRONMENTAL ENGINEERING © ASCE / NOVEMBER 2007 / 1467

2007.133:1466-1469.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

RE

GIN

A L

IBR

AR

Y o

n 09

/07/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

envelope being almost parallel to the higher pressure part of thefiber-reinforced shear strength envelope, as expected �e.g., Maherand Gray 1990; Zornberg 2002; and Casagrande 2005�.

Typical stress-strain data for tests on polypropylene fiber-reinforced samples of Osorio sand are shown in Figs. 2�a and b�.These tests were specifically performed and chosen to show thatdifferent stress paths reach the same failure state. Fig. 2�a� pre-sents tests that reach the same point on the strength envelope forcases where the failure occurred in the lower-pressure part of thebilinear failure envelope and Fig. 2�b� shows tests that reach thestrength envelope at a point that represents failure in the higher-pressure part. Again, it is clear that in both cases, the same failurepoint is reached no matter what stress path is followed. For allstress paths, the fiber-reinforced specimens gave a bulging type offailure.

After completing the two constant radial stress �loading� tri-

Fig. 2. Stress-strain-volumetric response of fiber-reinforced CID tri-axial tests comparing loading, unloading, and constant p� stress path�a� p�=20 kPa �loading�, 100 kPa �constant p��, and 200 kPa �un-loading�; �b� p�=100 kPa �loading�, 400 kPa �constant p��, and680 kPa �unloading�

axial tests on the fiber-reinforced specimens with normal stresses

1468 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

of 20 kPa �representing failure in the lower-pressure part� and400 kPa �representing failure occurring in the higher-pressure partof the bilinear failure envelope�, the fibers were recovered fromthe central zone of the specimens and their final lengths measuredgiving the distributions in Fig. 3. Around 500 fibers were mea-sured after each test. These data indicate that none of the fiberschecked in the present study broke in tension; they mostly suf-fered plastic tensile deformations. For failure occurring at meanstress values below the kink point separating the bilinear failureenvelope, it could be suggested that the failure is a composite ofslippage and yielding of fibers, as the fibers show only limitedstretching, and so there is possibly slipping occurring between thefibers and the soil particles because of the low confining stress.On the other hand, it can be seen that for a failure occurring in thehigher-pressure part of the failure envelope, there is a muchgreater amount of fiber stretching. Possibly, the fibers have notbroken because they are highly extensible with a fiber strain atfailure of 80%, and the necessary strain to cause fiber breakagehad not been reached under triaxial conditions at these strains.

Consoli et al. �2007� carried out ring shear tests on the samematerial and molding conditions used in the present research. Thefiber-reinforced Osorio sand specimens were sheared in the ringshear apparatus under normal stresses ranging from 20 to 400 kPaup to 700 mm horizontal displacement �details of the test proce-dures for fiber-reinforced materials are also given by Casagrandeet al. �2006��. It was observed that the fibers were both extendedand/or broken under all the normal stresses studied, as can beseen in Fig. 4. From these data, it is clear that even for failure inthe lower-pressure part of the bilinear failure envelope, for the

Fig. 3. Final fiber lengths after constant radial stress triaxial tests atconfining stresses of 20 and 400 kPa �initial fiber length of 24 mm�

Fig. 4. Final fiber lengths after ring shear tests at normal stressesranging from 20 to 400 kPa �initial fiber length of 24 mm� �adaptedfrom Consoli et al. �2007��

EERING © ASCE / NOVEMBER 2007

2007.133:1466-1469.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

RE

GIN

A L

IBR

AR

Y o

n 09

/07/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

thin polypropylene fibers with an aspect ratio of about 1,000 usedhere, some fiber yielding or stretching takes place and that break-age occurs if sufficient strains are achieved.

Conclusions

An extensive laboratory-testing program was carried out in orderto investigate the influence of fiber reinforcement on a uniformsand. To do so, drained triaxial tests were conducted, consideringa variety of stress paths. The observations and conclusions can besummarized as follows:• The failure envelope of the fiber-reinforced Osorio sand tested

is independent of the stress path for triaxial compression tests;• From the final fiber length distribution, it could be suggested

that for failure occurring at mean stresses below the kink pointseparating the bilinear failure envelope, there is a composite ofslippage and yielding of fibers, with some fibers showing lim-ited stretching and others possibly slipping due to the lowconfining stress. For specimens failing on the higher-pressurepart of the bilinear failure envelope, there is more pronouncedfiber stretching but no breakage because the fibers are highlyextensible and the fiber strains necessary to cause breakagewere not reached under triaxial conditions at these strain levelsin the sample.

Acknowledgments

The writers wish to express their gratitude to PRONEX/FAPERGS �Process No. 04/0841.0� and CNPq—Brazilian Coun-cil of Scientific and Technological Research �Projects Produtiv-idade em Pesquisa No. 300832/2004-4, Edital Universal 2004 No.472643/2004-5 and Pós-Doutorado no Exterior No. 200957/2005-8� for their financial support to the research group.

References

Bishop, A. W., and Wesley, L. D. �1975�. “A hydraulic triaxial apparatusfor controlled stress path testing.” Geotechnique, 25�4�, 657–660.

Burland, J. B., and Symes, M. �1982�. “A simple axial displacementgauge for use in triaxial apparatus.” Geotechnique, 32�1�, 62–65.

Casagrande, M. D. T. �2005�. “Behavior of fiber-reinforced soils under

JOURNAL OF GEOTECHNICAL AND GEOE

J. Geotech. Geoenviron. Eng.

large shear strains.” Ph.D. thesis, Federal University of Rio Grande doSul, Porto Alegre, Brazil �in Portuguese�.

Casagrande, M. D. T., Coop, M. R., and Consoli, N. C. �2006�. “Behaviorof a fiber-reinforced bentonite at large shear displacements.” J. Geo-tech. Geoenviron. Eng., 132�11�, 1505–1508.

Consoli, N. C., Casagrande, M. D. T., and Coop, M. R. �2005�. “Effect offiber reinforcement on the isotropic compression behavior of a sand.”J. Geotech. Geoenviron. Eng., 131�11�, 1434–1436.

Consoli, N. C., Casagrande, M. D. T., and Coop, M. R. �2007�. “Perfor-mance of a fibre-reinforced sand at large shear strains.” Geotechnique,57 �accepted for publication�.

Consoli, N. C., Casagrande, M. D. T., Prietto, P. D. M., and Thomé, A.�2003�. “Plate load test on fiber-reinforced soil.” J. Geotech. Geoen-viron. Eng., 129�10�, 951–955.

Consoli, N. C., Montardo, J. P., Donato, M., and Prietto, P. D. M. �2004�.“Effect of material properties on the behavior of sand-cement-fibrecomposites.” Ground Improvement, 8�2�, 77–90.

Consoli, N. C., Montardo, J. P., Prietto, P. D. M., and Pasa, G. S. �2002�.“Engineering behavior of a sand reinforced with plastic waste.” J.Geotech. Geoenviron. Eng., 128�6�, 462–472.

Consoli, N. C., Prietto, P. D. M., and Ulbrich, L. A. �1998�. “Influence offiber and cement addition on behavior of a sandy soil.” J. Geotech.Geoenviron. Eng., 124�12�, 1211–1214.

Gray, D. H., and Al-Refeai, T. �1986�. “Behavior of fabric-versus fiber-reinforced sand.” J. Geotech. Engrg., 112�8�, 804–820.

Gray, D. H., and Ohashi, H. �1983�. “Mechanics of fiber reinforcement insand.” J. Geotech. Engrg., 109�3�, 335–353.

Heineck, K. S., Coop, M. R., and Consoli, N. C. �2005�. “Effect of microreinforcement of soils from very small to large shear strains.” J. Geo-tech. Geoenviron. Eng., 131�8�, 1024–1033.

Ladd, R. S. �1978�. “Preparing test specimens using undercompaction.”Geotech. Test. J., 1�1�, 16–23.

La Rochelle, P., Leroueil, S., Trak, B., Blais-Leroux, L., and Tavenas, F.�1988�. “Observational approach to membrane and area corrections intriaxial tests.” Proc., Symposium on Advanced Triaxial Testing of Soiland Rock, ASTM, Louisville, Ky, 715–731.

Maher, M. H., and Gray, D. H. �1990�. “Static response of sands rein-forced with randomly distributed fibers.” J. Geotech. Engrg., 116�11�,1661–1677.

Maswoswe, J. J. �1985�. “Stress path method for a compacted soil duringcollapse due to wetting.” Ph.D. thesis, University of London.

Michalowski, R. L., and Cermák, J. �2003�. “Triaxial compression ofsand reinforced with fibers.” J. Geotech. Geoenviron. Eng., 129�2�,125–136.

Zornberg, J. G. �2002�. “Discrete framework for limit equilibrium analy-sis of fibre-reinforced soil.” Geotechnique, 52�8�, 593–604.

NVIRONMENTAL ENGINEERING © ASCE / NOVEMBER 2007 / 1469

2007.133:1466-1469.