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65GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NO. 1

Technical Note by M.S. Nataraj and K.L. McManis

STRENGTH AND DEFORMATION PROPERTIES OF

SOILS REINFORCED WITH FIBRILLATED FIBERS

ABSTRACT: The results of preliminary laboratory tests on a clay and a sand rein-

forced with randomly distributed fibrillated fibers are presented. The results of compac-tion, direct shear, unconfined compression and California Bearing Ratio (CBR) testsare described. The influence of test parameters such as normal stress, the amount of re-

inforcement, specimen size, and moisture content is addressed. The tests show that fi-bers significantly increase the peak compressive strength of clay and sand. The addition

of fiber reinforcement in the sand and clay specimens resulted in substantial increases

in the peak friction angle and cohesion values. The increase in strength is a function of fiber content and moisture content. The Mohr-Coulomb failure envelopes for the clay

specimens are described by a combination of curvilinear and linear sections. The CBRvalues for the clay and sand specimens were also significantly improved using fiber re-

inforcement. This preliminary study suggests that for the soils tested and reinforcedwith 25 mm long polypropylene fibrillated fibers, the optimum fiber content isapproxi-

mately 0.3% of the dry unit weight of the soil.

KEYWORDS: Fiber reinforcement, Shear strength, CBR, Geosynthetic.

AUTHORS: M.S. Nataraj, Associate Professor, Department of Civil and

Environmental Engineering, University of New Orleans, Louisiana 70148, USA,

Telephone: 1/504-280-7044 or 1/504-280-6668, Telefax: 1/504-280-5586; and, K.L.

McManis, Professor and Chairman, Department of Civil and EnvironmentalEngineering, University of New Orleans, Louisiana 70148, USA, Telephone:

1/504-280-6271, Telefax: 1/504-280-5586.

PUBLICATION: Geosynthetics International is published by the Industrial FabricsAssociation International, 345 Cedar St., Suite 800, St. Paul, Minnesota 55101-1088,USA, Telephone: 1/612-222-2508, Telefax: 1/612-222-8215. Geosynthetics

International is registered under ISSN 1072-6349.

DATES: Original manuscript received 22 October 1996, revised version received 27February 1997 and accepted 3 March 1997. Discussion open until 1 November 1997.

REFERENCE: Nataraj, M.S. and McManis, K.L., 1997, “Strength and DeformationProperties of Soils Reinforced With Fibrillated Fibers”, Geosynthetics International,Vol. 4, No. 1, pp. 65-79.

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NATARAJ AND McMANIS D Soils Reinforced With Fibrillated Fibers

66 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NO. 1

1 INTRODUCTION

The use of fiber reinforcement has been suggested in recent years for various geotech-nical applications. Fibergrids are one adaptation of this concept and consist of discrete,fibrillated polypropylene fibers. Upon mixing a predetermined amount of fibergrid

with soil at a particular moisture content, the fibergrids open up to produce a net-likeconfiguration. This configuration provides a mechanical means for reinforcement of

the soil matrix. The mechanical interlock effect of the fibers provides increased tensilestrength and cohesion to the soil matrix. Laboratory tests measured an increase in the

ability of the reinforced soil to resist shear strain; an increase in the post-peak strengthresponse; and an increase in the modulus of the soil. This reinforcement mechanism haspotential in the construction of highways and slopes.

2 REVIEW OF LITERATURE

Research of different types of reinforcement and materials has been conducted by

several investigators; however, the amount of information available on fiber reinforce-ment is still limited. The available research results vary depending on the fiber length

and soil tested, and the type of test performed. The results of compaction tests fora silty,clay soil specimen reinforced with fibers indicate that increasing the volume of fibersin the soil generally causes a modest increase in the maximum dry unit weight, and a

slight decrease in the optimum moisture content (Fletcher and Humphries 1991). Inanother study, the compaction test results for a silty clay soil specimen reveal that at

all fiber contents, the soil specimens containing fibers exhibited the same compactionbehavior as the corresponding unreinforced soil specimens. Furthermore, the dry unitweight was slightly more influenced by fiber content on the dry side of the optimum

moisture content than on the wet side of optimum (Al Wahab and Al-Qurna 1995). Thestrength of a compacted sandy clay soil specimen reinforced with synthetic fibers ex-

hibited higher unconfined compressive strength values than the unreinforced soil speci-mens, and the percent strength gain was most apparent in specimens remolded at mois-

ture contents wetter than optimum (Fretaig 1986). Similarly, unconfined compressiontest results for silty clay soil specimens reinforced with polypropylene fibers indicate

that an optimum fiber content of approximately 1% of the dry unit weight of the soil,

and a fiber length of 25 mm, maximize the strength, workability and homogeneity of

the soil-fiber matrix. The optimum fiber content depends largely on the soil and fiber

types (Al Wahab and Al-Qurna 1995). Reinforcement of micaceous, silt soil specimens

significantly enhanced the California Bearing Ratio (CBR) values, with an increase inCBR values from 65 to 133% over that of the unreinforced soil specimens. The mica-

ceous, silt soil specimens containing fibrillated fibers yielded 16% higher CBR valuesthan soil specimens containing monofilament fibers (Fletcher and Humphries 1991).

Compaction tests conducted on sandy soil specimens reinforced with polypropylene/

nylon fabric sheets resulted in specimens with less dense packing (Hoare 1979). Theresults of direct shear tests conducted on quartz sand specimens reinforced with dry fi-bers show that the fiber reinforcement generally increases the ultimate shear strength,and also the limits reduction in the post-peak shearing resistance of the soil specimen

(Gray and Ohashi 1983). Also, reinforced uniformly graded sand specimens exhibit

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curvilinear-linear Mohr-Coulomb failure envelopes while reinforced well-graded, or

angular, sand specimens exhibit bilinear failure envelopes (Gray and Maher 1989).

3 SCOPE AND METHODOLOGY OF STUDY

The main objectives of this preliminary study were to investigate the strength and de-formation characteristics of soils reinforced with randomly distributed fibrillated fi-

bers. The laboratory tests performed include compaction (ASTM D 698), direct shear(ASTM D 3080), unconfined compression (ASTM D 2166), and CBR tests (ASTM D

1883). Tests were conducted on a cohesive (clay) and a cohesionless (sand) soil. Thetypical properties of the sand and the clay used in this study are shown in Table 1. The

other parameters of this study include fiber content as a percentage of the dry unit

weight of the soil, moisture content, and specimen size. Fibrillated polypropylene fi-

bers approximately 25 mm in length were used as reinforcement and are manufactured

by Synthetic Industries, (Chattanooga, Tennessee, USA). It is known that the length,diameter, roughness, and strength of the fiber influence the behavior of a reinforced

soil; however, in this preliminary study, these factors are not considered, as only one

type of fiber is used in the tests.The fibers in their pre-open form can be mixed with soils either mechanically or by

hand. In this study, both the sand and clay were oven dried, weighed to the nearest 0.01g, and were placed in a large metal pan. The required amount of water was added to the

soil in small increments and hand mixed to ensure uniform distribution. Then an amountof the pre-opened fibers that was predetermined by the dry unit weight of the soil, wasadded in small increments to the soil-water mixture. Further mixing by hand was con-

tinued until the fibers were well dispersed. The fiber content in most of thesoil test spec-imens varied from 0.1 to 0.3% of the dry unit weight of the soil. The prepared specimens

were then stored in a humidity room for 18 hours. The moisture content was measuredbefore and after each test and no measurable discrepancies were observed.

Table 1. Properties of the sand and clay used in this study.

Properties of the sand

D10

(mm)

D50

(mm)

C U C C φ

(_)

Classification

(USCS)

γdmax

(kN/m3)

wopt

(%)

0.11 0.17 1.56 0.95 33.5 SP 15.2 15.2

Properties of the clay

Liquid limit

(%)

Plastic limit

(%)

Plasticity index

(%)

c

(kPa)

φ

(_)

Classification

(USCS)

γdmax

(kN/m3)

wopt

(%)

44 18 26 84.0 19.5 CL 16.6 17.9

Notes: USCS = Unified Soil Classification System; c = soil cohesion;φ = peak friction angle of the soil; D10and D50 = soil particle diameters such that 10 and 50% of the soil particles by weight, respectively, are smaller

than that diameter; C U = coefficient of uniformity; C C = coefficient of curvature; γdmax = maximum dry unitweight; wopt = optimum moisture content; SP = poorly graded sand, gravelly sands, with little or no fines; CL= inorganic clays, silty clays, sandy clays of low plasticity.

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4 ANALYSIS OF TEST RESULTS

4.1 Cohesive Soil (Clay)

4.1.1 Compaction Tests

The relationship between the dry unit weight and moisture content of unreinforcedclay specimens and reinforced clay specimens with fiber contents of 0.1, 0.2 and 0.3%

was studied (Figure 1). The compaction behavior of the reinforced specimens at eachfiber content exhibits similar compaction behavior to unreinforced specimens. An in-crease in fiber content from 0 to 0.2%, increases the maximum dry unit weight slightly,

and decreases the optimum moisture content. The maximum dry unit weight for clay,reinforced with 0.2% fibers is 16.74 kN/m3. Similar results have been obtained in other

studies (Fletcher and Humphries 1991; Al Wahab and Al Qurna 1995; Maher and Ho1994).

4.1.2 Unconfined Compression Tests

The effects of moisture content, fiber content, and specimen size on the unconfinedcompressive strength of the clay were considered. Clay specimens, with and without

reinforcement, were prepared at the maximum dry unit weight and optimum moisturecontent determined from compaction tests. Three specimens were prepared for eachcompression test with the selected fiber content. The specimen sizes used were 33 mm

Figure 1. Compaction test results for fiber reinforced and unreinforced clay specimens.

D r y u n i t w e i g h t ( k N / m 3 )

Moisture content (%)

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by 72 mm, 70 mm by 140 mm, and 100 mm by 117 mm (diameter× height). Observa-

tion of the failed unreinforced clay specimens revealed a shear failure plane; and, with

the addition of reinforcing fibers, the specimens bulged in compression. These failuremodes were observed in all specimens.

The stress-strain relationships for reinforced and unreinforced 70 mm by 140 mm

clay specimens compacted at a maximum dry unit weight and at the optimum moisturecontent areshownin Figure 2. Each curve in Figure 2 represents a best fit curve obtained

from three individual tests. A greater soil specimen fiber content results in an increasein the following parameter values: the compressive strength; the post-peak strength; theinitial tangent modulus; and the secant tangent modulus at peak and at one-half of the

peak compressive strength.The strength test results varied for the different specimen sizes. The strength in-

creased with an increase in specimen diameter from 33 to 70 mm, with or without rein-forcement. However, the 100 mm diameter specimen strength values were slightly low-er than the strength values for the reinforced clay soil specimens. Similar results have

been observed by Al Wahab and Al Qurna (1995).The effect of moisture content on the strength of reinforced and unreinforced clay

specimens was studied using the Harvard Miniature test apparatus and the correspond-ing specimen size was 33 mm in diameter by 72 mm in height. The dry unit weight and

optimum moisture content values obtained using the Harvard Miniature apparatus(ASTM D 4609) are similar to values obtained using the Standard Proctor tests. (Theoptimum moisture contents for fiber contents of 0.1, 0.2 and 0.3% were 17.6, 16 and

17.62, respectively.) The moisture contents of the specimens were varied by 2 to 4%

Figure 2. Unconfined compressive strength test results for fiber reinforced and

unreinforced clay specimens.

Notes: Specimen size = 70 mm diameter× 140 mm height. Each curve is the average result of three tests.

S t r e s s ( k P a )

Strain (%)

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on both the dry and wet side of the optimum moisture content. Three specimens were

prepared in each case for each specimen fiber content. The specimens were compacted

with a 178 N tamp in 3 layers using 30 blows per layer. The results of unconfined com-pression tests that were conducted on these specimens are summarized in Figure 3. Thestrength of the reinforced clay specimens increases with increasing moisture content.

The strengths reach peak values at the optimum moisture content, and with a furtherincrease in moisture content, the strengths in all cases decrease. The strengths of the

reinforced clay specimens on both thedry andwet side of the optimum moisture contentare greater than that of the unreinforced clay specimens at any moisture content. Rein-forced clay specimens with 0.2% and 0.3% fiber content show a substantial increase

in strength compared to specimens with 0.1% fibers. It was also observed in the teststhat with the addition of fibers, the post-peak strength loss was less forspecimens prepa-

red wet of the optimum moisture content when compared to specimens prepared dryof the optimum moisture content.

4.1.3 Direct Shear Tests

Clay specimens with and without reinforcement were tested in 64 mm square and 100

mm square shear boxes (plan dimensions). The specimens were prepared in the shear

box at the maximum dry unit weight and optimum moisture content of the clay. The

Notes: Harvard Miniature apparatus specimen size = 33 mm diameter× 72 mm height. Each point is the

average result of three tests. Three layers prepared with 30 blows per layer.

Figure 3. Variation of unconfined compressive strength with moisture content for fiber

reinforced and unreinforced clay specimens.

U n c o n f i n e d c o m p r e s s i v e

s t r e n g t h ( k P a )


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tests were conducted at a strain rate of 0.178 mm/minute and at normal stresses ranging

from 20.7 to 462 kPa. Figure 4 shows a typical shear-displacement curve for 100 mm

size clay specimens with and without reinforcement (fiber contents of 0.1, 0.2 and 0.3%of the dry unit weight of the soil) at a normal stress of 342 kPa. Fiber reinforcementincreased the peak shear strength of the clay in both the 64 mm and 100 mm size speci-

mens. For some specimens, the results indicated a reduction in post-peak loss of shearstrength with an increase in fiber content.

Figure 5 presents the linear Mohr-Coulomb envelopes for clay with and without rein-forcement for 100 mm size specimens. The peak friction angle and cohesion values forthe unreinforced clay are φ = 19.5_ and c = 84 kPa, respectively. For clay specimens

with a 0.3% fiber content, φ = 32_ and c = 122.5 kPa. This is a 64% increase in the peak friction angle value and a 45% increase in the cohesion value. The results show that

there is a definitive increase in the cohesion value with the addition of fibers, which maynot be true for the peak friction angle value.

The failure envelopes, appear to be slightly nonlinear. At normal stresses of less than

approximately 200 kPa the failure envelopes are steeper than at higher stresses. It canbe seen from Figure 5 that in the lower normal stress range of 20.7 to 200 kPa the failure

envelopes are actually curved. In the 200 to 462 kPa stress range, the envelopes areveryclose to a straight line. These direct shear results are being analyzed using an approach

suggested by Maksimovic (1989). Preliminary results indicate that for a clay specimenreinforced with a 0.3% fiber content, the failure envelope can be expressed as τ = 11.41 σ 0.34, inthe lower stressrange of20 to200kPa, and as τ = 17.78 + σ tan32_, in the higher

stress range of 200 to 462 kPa.

Notes: Specimen size = 100 mm× 100 mm in plan area. Normal stress = 342 kPa.

Figure 4. Shear stress-displacement results for fiber reinforced and unreinforced clay

specimens.

S h e a r s t r e s s ( k P a )

Displacement (mm)

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Figure 5. Mohr-Coulomb failure envelopes for fiber reinforced and unreinforced clay

specimens.

Note: Specimen size = 100 mm× 100 mm in plan area.


Normal stress (kPa)

4.1.4 CBR Test Results

California Bearing Ratio tests were conducted on reinforced and unreinforced clay

specimens at maximum dry densities and moisture contents (ASTM D 698). The pre-liminary test results for specimens with various fiber contents are shown in Figure 6.The CBR value of 8.44 for the unreinforced clay specimen increases to approximately

12.6 for specimens with a 0.3% fiber content. This is a 48% increase in the CBR valuefor the unreinforced clay specimen. Similar results are reported elsewhere (Fletcher andHumphries 1991).

4.2 Cohesionless Soil (Sand)

4.2.1 Compaction Tests

The compaction characteristics of sand specimens with fiber contents of 0.1, 0.2, 0.3,

and 0.4% of the dry unit weight of the soil were determined (see Figure 7 for typical

results). There is no significant difference in the behavior of reinforced and unrein-forced sand specimens in the compaction tests. A fiber content of 0.1% slightly in-

creases the maximum dry unit weight and the optimum moisture content. Increasingthe fiber content from 0.2 to 0.4% has no significant effect on the magnitude of either

the maximum dry unit weight or the optimum moisture content. Similar results are re-

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Figure 6. California Bearing Ratio values for reinforced clay specimens with different fiber

contents.

Figure 7. Compaction test results for fiber reinforced and unreinforced sand specimens.

C B R v a l u e

Fiber content (%)

CBR at 2.54 mm penetration

CBR at 5.08 mm penetration

D r y u n i t w e i g h t ( k N / m 3 )


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ported by others (Maher and Ho 1994). There is a reduction in the unit weight of the

sand-fiber mixtures due to the compaction resistance of the fibers, and the fact that the

fibers have a lower specific gravity than soils. The interactions between the soil and thefiber reinforcement controls the response of the sand-fiber mixture to compaction.

4.2.2 Unconfined Compression Tests

The effect of moisture content and fiber content on the unconfined compressivestrength of reinforced and unreinforced sand specimens was studied using the HarvardMiniature apparatus specimens: 33 mm in diameter by 72 mm in height. The moisture

contents were varied by increments of 2 to 4% on both the dry and wet side of the opti-mum moisture content obtained in the Standard Proctor compaction test. The speci-

mens were compacted in three layers with a compactive effort of 40 blows per layer,which was based on earlier calibration tests. Three specimens were prepared at each

moisture content and for 0.1, 0.2 and 0.3% fiber contents. Extreme care was taken in

preparing the specimens and in conducting the tests, so as to keep the specimens intact.A typical stress-strain curve for the 0.3% fiber content reinforced sand specimens is

shown in Figure 8. The curves represent thebest fitcurve obtained from three individual

tests. It can be seen that an increase in the moisture content from 12 to 16% increases

the peak strength of the reinforced sand up to the optimum moisture content (wopt =16%). The strength decreases with an increase in moisture content beyond the optimum

w

w

w

w

w

Figure 8. Stress-strain relationship at various moisture contents, w, for reinforced sand

specimens with a 0.3% fiber content.

Notes: Harvard Miniature apparatus specimen size = 33 mm in diameter× 72 mm in height. Each curve

is the average result of three tests. Three layers prepared with 40 blows per layer.

S t r e s s ( k P a )

Strain (%)

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moisture content. The results of the tests using different fiber contents suggest that the

strain corresponding to the peak stress increased with an increase in moisture content

and fiber content. The post-peak strength of the unreinforced sand at all moisture con-tents diminished rapidly with respect to strain compared to the post-peak strength of thereinforced soil. This result is also similar to other studies (Fretaig 1986). Results show-

ing the variation of unconfined compressive strength with moisture content are summa-rized in Figure 9 for reinforced and unreinforced sand specimens with different fiber

contents. The strength of the reinforced sand specimens on both the dry and wet sideof the optimum moisture content is greater than the strength of the unreinforced sandat any moisture content. The inclusion of fibers increases the strength of the sand speci-

mens; the maximum strength increase occurs for reinforced sand specimens containing0.3% fibers.

4.2.3 Direct Shear Tests

The shear strength of unreinforced sand specimens and sand specimens reinforcedwith 0.1, 0.2 and 0.3% fiber content was measured. The 64 mm and 100 mm size speci-

mens were tested at the respective maximum dry unit weight and optimum moisture

content. The specimens were prepared in the shear box, and loaded at a strain rate of

1 mm/minute. The normal stresses ranged from 20.7 to 462 kPa. Figure 10 represents

the stress versus displacement response of 100 mm size sand specimens with and with-out reinforcement under a normal stress of 342 kPa. An increase in fiber content in-

Figure9. Variation ofstrength with moisture content for fiber reinforced andunreinforced

sand specimens.Notes: Harvard Miniature apparatus specimen size = 33 mm in diameter× 72 mm in height. Each point

is the average result of three tests. Three layers prepared with 40 blows per layer.

U n c o n f i n e d c o m p r e s s i v e s t r e n g t h ( k P a )


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Figure 11. Mohr-Coulomb failure envelopes for fiber reinforced and unreinforced sand

specimens.Note: Specimen size = 100 mm× 100 mm in plan area.

Figure 12. Effect of normal stress on the peak friction angle of fiber reinforced and

unreinforced sand specimens.

Note: Specimen size = 100 mm× 100 mm in plan area.


Normal stress (kPa)

Normal stress (kPa)

P e a k f r i c t i o n a n g l e ( _

)

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4.2.4 CBR Test Results

California Bearing Ratio tests were conducted on reinforced and unreinforced sandspecimens. The reinforced sand specimens had fiber contents of 0.1, 0.2, and 0.3% of the dry unit weight of the sand (ASTM D 698). In all instances, an increasing fiber con-

tent significantly increased the CBR value when compared to unreinforced specimens.A CBR value of 18 was obtained for the unreinforced sand specimen. The CBR values

of 26.7, 29.3, and 33.0 were obtained for sand reinforced with 0.1, 0.2, and 0.3% fibercontents, respectively. The CBR value at a 0.3% fiber content was approximately 83%

greater than the CBR value for the unreinforced sand specimen. After each CBR test,the fibers were inspected and showed no signs of breakage.

5 SUMMARY

The strength and deformation characteristics, and the optimum fiber content to maxi-mize the strength of clay and sand specimens reinforced with polypropylene fibers was

studied. The compaction characteristics of the fiber reinforced soils were similar to thatof the unreinforced soils. The maximum dry unit weight reached a maximum value at

a fiber content of 0.2% for the clay specimens and 0.1% for the sand specimens.Direct shear test results showed that the addition of fibers to the clay and sand speci-

mens increased the peak shear strength of the soil and reduced, in some cases, the post-

peak strength loss. The addition of fibers to clay and sand specimens results in substan-tial increases in the measured values of the peak friction angle and cohesion. The

inclusion of fibers in the soils significantly increased the compressive strength. Thecompressive strength of the soils increased with an increase in the moisture content upto the optimum moisture content. The CBR values also increase significantly with the

addition of reinforcing fibers.Finally, the test results from this study indicate that by reinforcing the two selected

soils with 25 mm long fibrillated fibers, the optimum fiber content is 0.3% of the dryunit weight of the soil specimen. However, these results are not conclusive and further

studies are required to determine the optimum fiber content for a given or site-specificsoil.

ACKNOWLEDGEMENT

The authors thank Synthetic Industries, Chattanooga, Tennessee, USA for supplyingthe fiber samples for this study.

REFERENCES

Al Wahab, R.M. and Al-Qurna, H.H., 1995, “Fiber Reinforced Cohesive Soils for Ap-

plication in Compacted Earth Structures”, Proceedings of Geosynthetics ’95, IFAI,Vol. 2, Nashville, Tennessee, USA, February 1995, pp. 433-446.

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ASTM D 698, “Standard Test Method for Laboratory Compaction Characteristics of

Soil Using Standard Effort (600 kN-m/m3)”, West Conshohocken, Pennsylvania,

USA.

ASTM D 1883, “Standard Test Method for CBR (California Bearing Ratio) of Labora-

tory - Compacted Soils”, West Conshohocken, Pennsylvania, USA.

ASTM D 2166, “Standard Test Method for Unconfined Compressive Strength of Cohe-

sive Soil”, West Conshohocken, Pennsylvania, USA.

ASTM D 3080, “Standard Test Method for Direct Shear Test of Soils Under Consoli-

dated Drained Conditions”, West Conshohocken, Pennsylvania, USA.

ASTM D 4609, “Standard Guide for Screening Chemicals for Soils Stabilization”, An-nex A1, West Conshohocken, Pennsylvania, USA.

Fletcher, C.S. and Humphries, W.K., 1991, “California Bearing Ratio Improvement of

Remolded Soils by the Addition of Polypropylene Fiber Reinforcement”, Proceed-

ings of Seventieth Annual Meeting,Transportation Research Board, Washington, DC,

USA, pp. 80-86.Fretaig, D.R., 1986, “Soil Randomly Reinforced with Fibers”, Journal of Geotechnical

Engineering, ASCE, Vol. 112, No. 8, pp. 823-826.

Gray D.H. and Maher, M.H., 1989, “Admixture Stabilization of Sands With Random

Fibers”, Proceedings of the Twelfth International Conference on Soil Mechanics and

Foundation Engineering, Balkema, Vol. 2, Rio de Janeiro, Brazil, August 1989, pp.

1363-1366.

Gray, D.H. and Ohashi, H., 1983, “Mechanics of Fiber Reinforcement in Sand”, Jour-

nal of Geotechnical Engineering, ASCE, Vol. 108, No. 3, pp. 335-353.

Hoare, D.J., 1979, “Laboratory Study of Granular Soils Reinforced with Randomly Ori-

ented Discrete Fibers”, Proceedings of International Conference on the Use of Fab-

rics in Geotechnics, Vol. 1, Paris, France, pp. 47-52.

Maher, M.H. and Ho, Y.C., 1994, “Mechanical Properties of Kaolinite/Fiber Soil Com-posite”, Journal of Geotechnical Engineering, ASCE, Vol. 120, No. 8, pp.1381-1393.

Maksimovic, M., 1989, “Non-Linear Failure Envelope For Soils”, Journal of Geotech-

nical Engineering, ASCE, Vol. 115, No. 4, pp. 581-586.

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