Dow

nloa

ded

from

asc

elib

rary

.org

by

Sask

atch

ewan

Uni

vers

ity o

n 09

/25/

12. F

or p

erso

nal u

se o

nly.

No

othe

r us

es w

ithou

t per

mis

sion

. Cop

yrig

ht (

c) 2

012.

Am

eric

an S

ocie

ty o

f C

ivil

Eng

inee

rs. A

ll ri

ghts

res

erve

d.

Effect of Fiber Reinforcement on the Isotropic CompressionBehavior of a Sand

Nilo Cesar Consoli1; Michéle Dal Toé Casagrande2; and Matthew Richard Coop3

Abstract: High-pressure isotropic compression tests were carried out on samples of a uniform fine sand at different initial specificvolumes, reinforced with randomly distributed polypropylene fibers �0.5% by weight, 24 mm in length and 0.023 mm thick�, as well ason a nonreinforced sand specimen. Analysis of the results identified changes in the isotropic compression behavior due to the randominclusion of fibers into the sand. Two distinct and parallel normal compression lines were observed for the fiber-reinforced and nonrein-forced sand. The fibers were exhumed after testing and it was found that fibers had both extended and broken, indicating that the fiberstend to suffer large plastic tensile deformations before breaking and that the fibers act under tension even in isotropic compression.

DOI: 10.1061/�ASCE�1090-0241�2005�131:11�1434�

CE Database subject headings: Fiber reinforced materials; Compression; Anisotropy; Sand.

Introduction

Past research has demonstrated that the inclusion of fibers signifi-cantly improves the engineering response of soils under a varietyof stress paths �e.g., Gray and Ohashi 1983; Gray and Al-Refeai1986; Maher and Gray 1990; Morel and Gourc 1997; Consoli etal. 1998, 2002, 2003, 2004; Zornberg 2002; and Michalowski andCermák 2003�. A number of factors such as the fiber characteris-tics �content, length, thickness, modulus, tensile strength, and fail-ure strain� and the soil characteristics �grain size distribution andmean particle size� influence the behavior of the soil–fibercomposite.

For nonreinforced sands, among others, Coop and Lee �1993�showed that specimens with different initial specific volumes con-verged to a unique normal compression line �NCL� in the v ln p�plane at high pressures. The plastic volumetric strain that occurson compression down the NCL was found to be associated withparticle breakage. This pattern of behavior was seen for sands ofa variety of geological origins, although the NCL varied in itslocation.

The present work is aimed at a fundamental understanding ofthe behavior of a sand reinforced with polypropylene fibers underisotropic compression, when compared with a nonreinforcedsand. The results presented are part of a comprehensive testingprogram that is being carried out at Imperial College London

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

2Research Assistant, Federal Univ. of Rio Grande do Sul, 90035-190Porto Alegre, Rio Grande do Sul, Brazil.

3Reader, Imperial College, Univ. of London, London, U.K.Note. Discussion open until April 1, 2006. Separate discussions must

be 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 November 1, 2004; approved on February 10,2005. This technical note is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 131, No. 11, November 1, 2005.

©ASCE, ISSN 1090-0241/2005/11-1434–1436/$25.00.

1434 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

�United Kingdom� and the Federal University of Rio Grande doSul �Brazil� which is investigating the effect of fiber addition onthe triaxial stress–strain response of the composite material, in-cluding the effects of confining stress and fiber characteristics.

Experimental Program

Materials

The Osorio Sand was sampled from the region of Osorio, insouthern Brazil. The soil is classified as a nonplastic uniform finesand �SP� and the specific gravity of the solids is 2.62. The grainsize is entirely fine sand with an effective diameter of 0.16 mm,and uniformity and curvature coefficients of 1.9 and 1.2, respec-tively. Mineralogical analysis showed that sand particles are pre-dominantly quartz. The minimum and maximum void ratios are0.57 and 0.85, respectively.

Polypropylene fibers were used throughout this investigationto reinforce the soil. Their average dimensions were 24 mm inlength and 0.023 mm in diameter, with a specific density of 0.91,a tensile strength and elastic modulus of 120 MPa and 3 GPa,respectively, and a linear strain at failure of 80%.

Sample Preparation

The specimens tested in the high-pressure apparatus were pre-pared by hand mixing dry soil, water, and polypropylene fiberswhen used. Visual examination of exhumed specimens proved themixtures to be satisfactorily uniform. The undercompaction pro-cess, as proposed by Ladd �1978�, was used to produce homoge-neous specimens that could be used for a parametric study in thelaboratory testing program. The specimens were compacted inthree layers into a 50 mm diameter by 100 mm high split molddirectly on the high-pressure triaxial pedestal, at initial specificvolumes ranging from 1.58 to 1.77 and at a moisture content of10%.

High Pressure Triaxial Cell

The isotropic compression tests were conducted using a computer

controlled high-pressure hydraulic stress path apparatus with a

EERING © ASCE / NOVEMBER 2005

2005.131:1434-1436.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Sask

atch

ewan

Uni

vers

ity o

n 09

/25/

12. F

or p

erso

nal u

se o

nly.

No

othe

r us

es w

ithou

t per

mis

sion

. Cop

yrig

ht (

c) 2

012.

Am

eric

an S

ocie

ty o

f C

ivil

Eng

inee

rs. A

ll ri

ghts

res

erve

d.

cell pressure capacity of 70 MPa, described by Cuccovillo andCoop �1999�. Two pressure multipliers deliver high-pressure oilto the cell chamber and axial loading cylinder at a ratio of 100times the input air pressure. The cell pressure and axial stress arethen controlled by means of a feedback loop from the cell pres-sure and deviatoric force transducers, the computer adjusting theair pressure input to the multipliers by means of pressure convert-ers. The cell chamber is oil filled because water-submersible in-ternal instrumentation would be crushed at these pressures, soinstead the internal axial strains are measured locally on thesample with standard nonsubmersible LVDTs drilled with pres-sure relieving holes to allow the oil into and out of the transducerbody, as described by Cuccovillo and Coop �1997�. The apparatusis also fitted with a LVDT to measure axial strains outside thecell, although here, for isotropic compression, only the measure-ments of volume change are used. These were made with anImperial College type volume gage, working at standard pressures�0–700 kPa� and attached to the back-pressure line.

Test Description

At the beginning of each test the samples were saturated. This isdone by applying a small positive effective stress and then raisingsimultaneously both the cell and back-pressures, keeping the ef-fective stress constant. The back-pressure was increased typicallyto around 800 kPa, where the pressures were kept constant andthe sample was allowed to saturate for at least 24 h or longer ifrequired, until a B value of 95% was achieved. The high perme-ability of the sand allowed the isotropic compression to be carriedout rapidly, and rates of 500 kPa/h were used up to a maximumof 50 MPa.

Test Results and Analysis

The isotropic compression data for the nonreinforced and fiber-reinforced sand specimens are given in Fig. 1. The key feature toobserve is that both the loose and dense fiber-reinforced sandspecimens tend toward a unique NCL when plotted in specificvolume �v�: ln p� space, exactly as Coop and Lee �1993� hadfound for nonreinforced sands. It also can be observed that inser-tion of fibers into the sand changes its behavior significantly.There are two distinct and parallel normal compression lines forthe fiber-reinforced and nonreinforced sand. Coop and Lee

Fig. 1. Isotropic compression data for sand and fiber-reinforced sand

showed that the diversity of geological origins and consequent

JOURNAL OF GEOTECHNICAL AND GEOE

J. Geotech. Geoenviron. Eng.

variety of particle sizes, shapes, and strengths gave a wide rangeof normal compression line locations. In the present case the in-sertion of fibers can also be seen to change its location.

The normal compression line may be characterized by

v = N − � ln p� �1�

where N�volume intercept of isotropic normal compression lineat p�=1 kPa; ��gradient of normal compression line; and thevalues of these parameters are N=2.98 and �=0.156 for the non-reinforced sand and N=3.09 and �=0.156 for the fiber-reinforcedsand.

After completing the isotropic compression of the fiber-reinforced sand sample with the initial specific volume of 1.77,the fibers were exhumed from the sample and it was found thatthey had both extended and broken, as can be seen from the finaldistribution of fiber lengths in Fig. 2�a�. The data in Fig. 2�a� arescattered, and so in Fig. 2�b� the data have been smoothed, first byplotting points representing the average of three successive datapoints and then by putting a trend line through these points. Thesedata indicate that even if the sample is undergoing isotropic com-pression the fibers tend to suffer large plastic tensile deforma-tions, and some of them reach breaking point. Relatively fewfibers are found at the original length of 24 mm, and most havebeen extended and/or broken. It is not known how many of thosewith final lengths around 24 mm have undergone a combinationof extension and breakage that results in a final fragment lengththat coincidentally is similar to the initial length.

A mechanism is suggested in Fig. 3 to explain these observa-tions, in which the fibers both extend and break under tensilestress, even if the overall applied stress is isotropic. The initialfiber shape is represented here as straight, which is simplistic. The

Fig. 2. �a� Final fiber length after isotropic compression test; and �b�final fiber length after isotropic test—smoothed points

isotropic compression causes relative movement among particles

NVIRONMENTAL ENGINEERING © ASCE / NOVEMBER 2005 / 1435

2005.131:1434-1436.

Dow

nloa

ded

from

asc

elib

rary

.org

by

Sask

atch

ewan

Uni

vers

ity o

n 09

/25/

12. F

or p

erso

nal u

se o

nly.

No

othe

r us

es w

ithou

t per

mis

sion

. Cop

yrig

ht (

c) 2

012.

Am

eric

an S

ocie

ty o

f C

ivil

Eng

inee

rs. A

ll ri

ghts

res

erve

d.

and consequently produces tensile stresses in the fibers locatedamong them. There is also the possibility of an additional mecha-nism of fiber breaking during testing by squeezing and crushingof the sand particles, cutting the fibers trapped between them.However, this certainly is not the principle mechanism; otherwisethe fibers would not be extended.

Fig. 4 shows the grain size distribution curves of the fiber-reinforced sand before and after the high-pressure isotropic com-pression test. It can be observed that, as for the nonreinforcedsand, the plastic volumetric strain as the soil is compressed downthe NCL is associated with significant particle breakage.

Concluding Remarks

The following observations and conclusions can be made fromthe high pressure isotropic compression tests:1. The insertion of fibers randomly into the sand changes not

only its shearing behavior as previously observed but also itsisotropic compression behavior. Two distinct and parallelnormal compression lines can be seen for the fiber-reinforcedand nonreinforced sand.

2. For the fiber-reinforced sample, the fibers found in thesample after testing were found to have been both extendedand broken, indicating that the fibers act in tension evenwhen the sample is undergoing large compressive volumetricstrains and that the fibers suffer large plastic tensile deforma-tions before breaking.

Fig. 3. Illustration of proposed mechanism of fiber breakage underisotropic loading

Fig. 4. Grain-size distribution curves

1436 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

3. Some more work at lower isotropic stress levels �less than500 kPa� shall be carried out in a future work, in order toverify if plastic or elastic fiber deformations predominate atsuch stress levels.

Acknowledgments

The writers wish to express their gratitude to CNPq, the BrazilianCouncil of Scientific and Technological Research �ProjectsProdutividade em Pesquisa 300832/2004-4 and Edital Universal472643/2004-5�, and PRONEX-FAPERGS for the financial sup-port to the research group.

Notation

The following symbols are used in this technical note:e � voids ratio;N � volume intercept of isotropic normal compression

line at p�=1 kPa;p� � mean normal effective stress �=��1�+�2�+�3�� /3�;v � specific volume �=1+e�; and� � gradient of normal compression line.

References

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 behaviour 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.

Coop, M. R., and Lee, I. K. �1993�. “The behaviour of granular soils atelevated stresses.” Predictive soil mechanics, Thomas Telford,London, 186–198.

Cuccovillo, T., and Coop, M. R. �1997�. “The measurement of local axialstrains in triaxial tests using LVDTs.” Geotechnique, 47�1�, 167–171.

Cuccovillo, T., and Coop, M. R. �1999�. “An automated triaxial apparatusfor elevated pressures.” Non-Destructive and Automated Testing forSoil and Rock Properties, Proc., ASTM Symp., STP 1350, W. A. Marrand C. E. Fairhurst, eds., ASTM, West Conshohocken, Pa., 231–246.

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

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

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

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

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

Morel, J. C., and Gourc, J. P. �1997�. “Mechanical behavior of sandreinforced with mesh elements.” Geosynthet. Int., 4�5�, 481–508.

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

EERING © ASCE / NOVEMBER 2005

2005.131:1434-1436.