BEHAVIOR OF A SANDY SILT REINFORCED WITH ...yw6/Fiberrecycling/(soil...on soil reinforced with...
Transcript of BEHAVIOR OF A SANDY SILT REINFORCED WITH ...yw6/Fiberrecycling/(soil...on soil reinforced with...
Murray, Frost, and Wang 1
Technical Paper by J.J. Murray, J.D. Frost, and Y. Wang
BEHAVIOR OF A SANDY SILT REINFORCED WITH DISCONTINUOUS
RECYCLED FIBER INCLUSIONS
Authors: J.J. Murray, QORE Property Sciences, Duluth, Georgia 30097, USA, Telephone:
1/770-476-3555, Telefax: 1/770-476-0213, J.D. Frost, Professor, School of Civil and
Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA,
Telephone: 1/404-894-2280, Telefax: 1/404-894-2281, Y. Wang, Associate Professor, School of
Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA,
Telephone: 1/404-894-7551, Telefax: 1/404-894-9766.
Murray, Frost, and Wang 2
ABSTRACT: Laboratory compaction and triaxial compression tests were performed to assess
the compaction characteristics and load deformation response of a sandy silt reinforced with
randomly oriented recycled carpet fibers. Discrete, randomly distributed fiber inclusions
significantly increase the peak shear strength, reduce the post peak strength loss, increase the
axial strain to failure, and in some cases change the stress strain behavior from strain-softening to
strain-hardening for a sandy silt. Fiber inclusions also impede the compaction process, causing a
reduction in the maximum dry density of reinforced specimens with increasing fiber content.
The strength losses associated with in service saturation are significantly reduced with fiber
reinforcement. This study suggests that large volumes of recycled waste fibers can be used as a
value-added product to enhance the shear strength and load deformation response of soils.
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INTRODUCTION
Earth reinforcement has proven to be a practical technique for improving the shear strength of
embankments, earth retaining structures, and shallow foundations. One method of reinforcement
is the use of woven or non-woven fabrics or grids, interlayered with compacted fill for tensile
strength improvement. Another less widely used method for earth reinforcement is the use of
randomly oriented tensile inclusions. These inclusions consist of natural or synthetic materials
and are typically short (less than 2" long). An advantage of randomly oriented fibers is the
preservation of strength isotropy and the absence of planes of weakness that can develop parallel
to oriented fabrics (1). Previous research studies have shown that manufactured fibers consisting
of glass, steel, copper, and fibrillated polypropylene as well as natural fibers such as reed and
pulp increase the shear strength of sands and clays. This study examines the use of randomly
oriented recycled carpet and apparel fibers for reinforcement of a sandy silt. The influence of the
fibers on the moisture density relationship and load-deformation response is presented.
BACKGROUND
A substantial amount of research has been performed on soil reinforced with oriented arrays of
fiber and randomly oriented discrete fibers. Most of the work in this area has been concentrated
on soil reinforced with manufactured and natural fibers at fiber contents ranging from 0 to 1% by
dry weight of soil. More recently, however, soil reinforcement with manufactured and natural
fibers at higher fiber concentrations (1 to 5%) has been conducted. McGown et al. (2) classified
earth reinforcement with randomly distributed discrete fibers into two categories: ideally
inextensible and ideally extensible inclusions. Ideally inextensible inclusions include high
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modulus metal strips or bars and have rupture strains greater than soil alone. Ideally extensible
inclusions include low modulus natural and synthetic fibers and are characterized as having
rupture strains less than or comparable to soil alone.
Lee et al. (3) reported an increase in the shear strength and rigidity of sand reinforced with
firewood shavings. Andersland and Khattak (4) observed an increase in the stiffness and
undrained shear strength of kaolin clay reinforced with paper pulp fibers. Gray and Ohashi (5)
showed that low modulus inclusions, characterized as ideally extensible inclusions, do not
rupture during shear but rather slip at confining stresses below a "critical confining stress" and
stretch above it. They also reported significant increases in peak strength and reductions in post
peak strength loss with fiber reinforcement. McGown et al. (6) noted an increase in shear
strength and a reduction in post peak strength loss for sand reinforced with polypropylene mesh
fibers. Gray and Al-Refai (7) compared the results of triaxial compression tests performed on
sand reinforced with oriented fabric layers and randomly distributed discrete fibers. Both
inclusion types yielded increases in peak strength and reductions in post peak strength loss.
However, the fabric layers resulted in lower stiffness at small strains, whereas, fibers yielded
increased stiffness at all strain levels. Consoli et al. (8,9) also reported increases in peak strength,
however, the stiffness was observed to decrease with both cemented and uncemented soils. Gray
and Ohashi (5) also observed a decrease in stiffness at low shear displacements with an increase
in fiber modulus or stiffness. Gray and Al-Refai (7) also found that at the same aspect ratio
(length to diameter) and weight fraction, rougher textured fibers and not stiffer fibers proved to
be more effective for increasing strength. According to studies performed by Gray and Al-Refai
(7) and Gray and Maher (1) shear strength increased with fiber content up to an asymptotic upper
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limit for both fabric and fiber reinforced sands. Maher and Gray (10) also observed increases in
shear strength with fiber content up to an asymptotic upper limit and attributed this to the
confining stress and fiber aspect ratio. Additionally, they found that better graded, more angular
sand particles resulted in higher fiber contributions to strength while an increase in soil grain
size, D50 , reduced the fiber contribution to strength. Their study also showed that an increase in
the fiber aspect ratio resulted in a lower critical confining stress and an increase in fiber
contribution to strength. Conversely, Maher and Ho (11) reported the opposite effect for
kaolinite clay reinforced with randomly distributed pulp, glass, and polypropylene fibers. They
found that for increasing fiber lengths, the increase in normalized compressive strength of the
composite actually decreased and verified this finding with tests performed on fiber reinforced
cemented sands. Their results showed that as the cement content increased, the relative
contribution of fiber length to strength decreased. Additionally, they observed that increasing
the fiber content yielded higher hydraulic conductivity for all three fibers tested.
EXPERIMENTAL PROGRAM
Against the background of studies noted above which used predominantly virgin fibers, this
paper discusses the use of randomly oriented discrete carpet fibers for reinforcement of a sandy
silt. In addition, a number of tests were performed using virgin fibrillated polypropylene fibers
to allow for comparison of the results using recycled fibers with those previously reported for
virgin materials. A total of 42 triaxial compression tests were conducted to determine the effects
of fiber type, weight content, soil moisture content history, and confining pressure on shear
strength and stress deformation response of a sandy silt.
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TEST MATERIALS
Soil properties
The soil used in this study was obtained by the Georgia Department of Transportation from the
Washington By-pass project located in Wilkes County, Georgia. A grain size analysis was
conducted in accordance with ASTM D 422: Standard Test Method for Particle-Size Analysis of
Soils. The D50 of the sandy silt was 0.073 mm. Results of Atterberg Limits performed in
accordance with ASTM D 4318: Standard Test Method for Liquid Limit, Plastic Limit, and
Plasticity Index of Soils indicated that the soil was non-plastic. Based on the Unified Soil
Classification System (USCS) the soil was identified as a low plasticity silt (ML) but contained a
significant portion of sand and is better characterized as a sandy silt. The moisture density
relationship for the soil alone as well as mixtures of soil and varying percentages of fibers were
determined according to ASTM D 698: Test Method for Laboratory Compaction Characteristics
of Soil Using Standard Effort.
Fibers
The reinforcing materials used in this study were one-pass recycled carpet fibers and virgin
fibrillated polypropylene fibers. The one-pass carpet fibers consisted of nylon pile fiber,
polypropylene backing and adhesives and were passed once through a shredder. Properties of
the one-pass carpet fibers and fibrillated polypropylene fibers are summarized in Table 1.
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TRIAXIAL TESTING
Preparation of test specimens
A uniform distribution of the fiber and soil was achieved through a consistent mixing procedure.
Prior to mixing, alternating layers of fiber and soil were placed in the mixing pan. Each mixture
consisted of approximately 2500 g (5.5 pounds) of sandy silt and a corresponding percentage of
waste fiber weight based on the dry weight of the soil and the desired fiber dosage. The soil and
fibers were placed in a Lancaster counter-current rapid batch mixer and a predetermined amount
of water was added slowly to facilitate the mixing process and provide a soil- fiber mixture of the
desired moisture content. After removal from the mixer, the samples were further mixed by
hand until it was determined by visual inspection that a uniform distribution of the fiber
throughout the soil had been achieved. The soil fiber mixture was allowed to hydrate for 24
hours. To prepare the specimens for compression tests, the calculated mass of soil and fiber was
placed in a 7.11 cm (2.8- inch) diameter 14.2 cm (5.6- inch) long metal split mold with an
overflow collar. The specimens were hydraulically compressed with a static- loading machine.
The static load was applied only for the length of time required to compress the specimen to the
height of the mold. This time period was short and usually lasted less than one minute. The
resulting specimen diameter and length was slightly larger than that of the mold due to expansion
of the mold during compression and rebound of the specimen after removal of the static
compression load. The density was maintained constant for each specimen by adjusting the mass
of sample used for the calculated specimen volume. The dry density and moisture content was
maintained at 100 lb/ft3 and 19.0%, respectively. Although the global density of each specimen
was 100 lb/ft3, the specimen density was not uniform as a result of non-uniformity inherent in the
static load application method.
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Test procedure
The specimens were encased in a latex membrane and placed in a triaxial cell with filter paper
and porous stones between the specimen and the platens. The specimens were confined with de-
aired water. Additionally, for the soaked tests, a vacuum of 4 inches Hg was applied at the top of
the specimen and a hydraulic gradient of 5 to 10 cm was applied from the bottom to the top of
the specimen. The specimens were allowed to imbibe water for a period of 48 hours.
The triaxial compression tests performed in this study were consolidated undrained triaxial tests.
The specimens were consolidated and were tested undrained, however the specimens were not
saturated prior to testing. Throughout this paper, these specimens are referred to as as-
compacted. Triaxial compression tests were also performed to simulate in service saturation. In
these soaked tests, the specimens were first consolidated to the desired confining stress and then
allowed to imbibe water for a period of 48 hours prior to testing. The behavior of fiber
reinforced samples was measured using automated GeoComp Loadtrac systems equipped with
automated digital data acquisition systems. The specimens were loaded in the same direction as
they were compressed to minimize the effect of the non-uniformity of the specimens. A loading
rate of 0.7% axial strain per minute was applied and the specimens were tested to a maximum
axial strain of approximately 20%. Failure was specified as the peak axial stress or the stress at
20% axial strain where a peak was not evident.
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TEST RESULTS AND ANALYSIS
Effect of fiber inclusion on soil moisture density relationship
Previous studies have primarily examined the influence of fiber dosages in the range of 0 to 1%
by weight. The present study examined the behavior of soil with dosages as high as 3%. At
these higher dosage rates, the variation in properties such as maximum dry density and optimum
moisture content can become quite significant. To determine the effect of higher dosage rates of
reinforcing fibers on the moisture density relationship, standard compaction tests (ASTM D 698)
were conducted for unreinforced sandy silt as well as sandy silt reinforced with either one-pass
carpet fiber or fibrillated polypropylene fiber at fiber contents of 1, 2, and 3% by dry weight of
soil. The moisture density relationships for the one-pass carpet and fibrillated polypropylene
reinforced specimens are shown in Figures 1 and 2, respectively. As shown, the increase in fiber
content decreased the maximum dry density for both the one-pass carpet fiber and fibrillated
polypropylene fiber and increased the optimum moisture content for the one-pass carpet fiber. A
minimal change in the optimum moisture content was noted for the specimens reinforced with
the fibrillated polypropylene fibers. The changes in the maximum dry density can be attributed
to two factors. Firstly, fibers have a specific gravity of about 1 whereas the soil particles have a
value of about 2.7. As the fiber content is increased, the lower density fibers replace the higher
density soil grains resulting in a lower composite density. Secondly and more important ly, the
presence of the fibers decreases the effectiveness of the compaction process at a given energy
level resulting in a lower dry density at a given moisture content. The increase in optimum
moisture content with increasing fiber content is attributed to the absorption of water by the
fibers, thus increasing the amount of water in the soil fiber mixture. It is noted that this
additional water contained in the fibers does not have the same lubricating effect that it would
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have if it were absorbed by the soil particles. The optimum moisture content of the specimens
reinforced with fibrillated polypropylene fibers does not change since these fibers do not have an
affinity to absorb water. The moisture density relationship for the same sandy silt reinforced
with short chopped carpet fibers was determined in a previous study (12). A comparison of the
variation in maximum dry density and optimum moisture content for sandy silt reinforced with
one-pass carpet fiber, fibrillated polypropylene, and short chopped carpet fiber are shown in
Figure 3 and 4, respectively. Also shown in Figures 3 and 4 are the variation in maximum dry
density and moisture content for sands reinforced with fibrillated polypropylene fiber at lower
fiber contents (13,14). As described previously, the maximum dry density decreased with
increasing fiber content for the one-pass carpet and fibrillated polypropylene fiber reinforced
sandy silt. The sandy silt reinforced with short-chopped carpet fiber follows the same trend with
the exception of the 2% fiber content specimen where the maximum dry unit weight increased
over the 1% fiber specimen. A similar response was also observed by Crockford et al. (13) at
fiber contents ranging from 0 to 1%. This behavior was attributed to an optimum response of the
composite material as a result of the interactions of the soil and fibers, the compaction technique,
volume fractions and frictional characteristics of the components. Nataraj and McManis (14)
observed a trend that was very similar to that observed by Crockford et al. (13) at fiber contents
ranging from 0 to 0.4% with the exception of the .4% fiber specimen which showed an increase
in maximum dry unit weight over the 0.3% fiber specimen.
As shown in Figure 4, the optimum moisture content for the sandy silt reinforced with one-pass
carpet fiber increased with increasing fiber content whereas, the fibrillated polypropylene
reinforced specimens showed essentially no change in optimum moisture content with fiber
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content. Nataraj and McManis (14) also observed an increase in optimum moisture content with
increasing fiber content for a sand reinforced with fibrillated polypropylene at fiber contents
ranging from 0 to 0.4%. The short chopped fiber reinforced specimens followed the same trend
as the one-pass carpet fiber reinforced specimens with the exception of the 2% fiber specimen
which decreased over the 1% fiber specimen. Crockford et al. (13) observed a reduction in
optimum moisture content with an increase in fiber content from 0 to 0.6% with a subsequent
increase in optimum moisture content to the maximum fiber content of 1%. This response was
similar to that observed for short chopped fiber reinforced specimens and was also attributed to
an optimum response of the composite material.
Effect of fiber inclusion on compressive strength and stress deformation response
The results of triaxial compression tests for sandy silt reinforced with one-pass carpet fiber and
fibrillated polypropylene fiber, confined at 34.5 and 69 kPa are summarized in Table 2. The
principal stress difference versus axial strain for the one-pass carpet and fibrillated
polypropylene fiber reinforced specimens, confined at 34.5 kPa are shown in Figures 5 and 6,
respectively. As indicated in Table 2 the peak compressive stress increased significantly with
increasing fiber content for specimens reinforced with one-pass carpet and fibrillated
polypropylene fiber. The increase in peak stress ranged from 28.7% to 203.7% for specimens
reinforced with one-pass carpet fiber and from 61.2% to 155.1% for specimens reinforced with
fibrillated polypropylene fiber.
The stress deformation behavior changed from strain-softening for the 0% fiber content
specimens to strain-hardening for the fiber reinforced composites with the exception of the
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specimens reinforced with 0.3% and 0.5% fibrillated polypropylene fiber confined at 34.5 kPa
which exhibited slight strain softening behavior. Other researchers (5,13,15) also observed
increased peak strength for fiber reinforced soils. As observed in unconfined triaxial
compression tests performed for this study, small strain stiffness decreased with fiber
reinforcement for specimens reinforced with both fiber types at confining pressures of 34.5 and
69 kPa. The 1% fibrillated polypropylene reinforced specimens enhanced the strength of the
composite more than the 1% one-pass carpet fiber specimens. However, as shown in Figure 7,
an optimum fiber content or asymptotic upper limit was not observed for the composites
reinforced with one-pass carpet fiber, whereas the composites reinforced with polypropylene
fiber appear to approach an upper limit at around 1% fiber content. Moreover, the one-pass
carpet fiber specimens reinforced at 3% showed greater peak strength increases than 1%
fibrillated polypropylene reinforced specimens.
Effect of fiber inclusion and soil moisture content history on compressive strength
and stress deformation response
The results of soaked-triaxial compression tests for sandy silt reinforced with one-pass carpet
fiber are summarized in Table 3 and shown in Figures 8 and 9. The specimens in the soaked-
triaxial compression tests were allowed to imbibe water for a period of 48 hours prior to testing.
These tests were conducted to simulate in service saturation that can occur during periods of
heavy rainfall or due to other natural or man-made events. As shown in Table 3 the soaked tests
showed reduced strength as compared to the as-compacted conditions at the same fiber contents.
Prior to soaking, the moisture condition of the 0 and 1% fiber content specimens was slightly wet
of optimum whereas, it was 1% dry of optimum for the 2% fiber content specimens as shown in
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Table 4. Unreinforced specimens subjected to saturation show greater decreases in strength if
they are compacted on the dry side of optimum prior to saturation as opposed to the wet side of
optimum (16). This statement suggests that the 2% fiber content specimens, which were
compacted on the dry side of optimum should show greater strength losses after saturation than
the 0 and 1% fiber content specimens, which were compacted on the wet side of optimum.
However, this did not occur and an apparent trend was not observed between moisture condition
prior to saturation and strength loss after saturation. This is most likely a result of the effect fiber
reinforcement has on the soil structure. As stated earlier, fiber reinforcement has the same effect
as reducing the compactive effort. Since, on the dry side of optimum, increasing the compactive
effort tends to disperse the soil, fiber reinforcement, which reduces the effectiveness of the
compactive effort would tend to create a more flocculated structure. Thus, fiber reinforcement
significantly changes the structure of the soil and consequently greatly influences the relationship
between moisture density and strength.
As shown in Figures 8 and 9, the strength of the unreinforced soaked specimens decreased
significantly as compared to the as-compacted condition. However, the 1 and 2% fiber content
soaked specimens exhibited increased strength over the unreinforced soaked specimens at all
strain levels with the exception of 0 to 3% strain for the specimens confined at 69 kPa. More
importantly, the 2% soaked specimens showed increased strength over the unreinforced as-
compacted condition at strains greater than about 9%. Additionally, the soaked reinforced
specimens exhibited strain-hardening behavior. These findings indicate that fiber reinforcement
can significantly reduce the effect of in-service saturation on embankments.
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CONCLUSIONS
Standard Compaction tests were conducted on a sandy silt reinforced with randomly distributed
recycled carpet fibers and manufactured fibrillated polypropylene fibers. The results of
compaction tests indicated that the maximum dry density decreases with increasing fiber content.
Results of the triaxial compression tests performed in this study revealed that reinforcement with
all fiber types increased shear strength and modified the stress deformation response of a sandy
silt. The following conclusions resulted from this study:
1. The inclusion of discrete, randomly oriented fibers impeded the compaction process, reduced
the maximum dry density for one-pass carpet and fibrillated polypropylene fiber reinforced
sandy silt and increased the optimum moisture content for the one-pass carpet fiber
reinforced specimens.
2. Discrete, randomly oriented fiber inclusions significantly increased the peak shear strength,
reduced the post peak strength loss, increased the axial strain to failure (ductility), and in
some cases changed the stress-strain behavior from strain-softening to strain-hardening.
3. An optimum fiber content or asymptotic upper limit was not observed for specimens
reinforced with recycled one-pass carpet fibers. However, the specimens reinforced with
virgin fibrillated polypropylene fibers appeared to approach an upper limit at a fiber content
of 1%.
4. Fiber reinforcement significantly changes the structure of the soil and consequently greatly
influences the relationship between moisture density relationship and strength.
5. The strength losses associated with in service saturation (soaking) are greatly reduced with
fiber reinforcement.
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ACKNOWLEDGEMENTS
The research described in this paper was supported by grants from CCACTI, the Consortium on
Competitiveness of the Apparel,Carpet and Textile Industries. This support is gratefully
acknowledged.
REFERENCES
1. Gray, D.H., and Maher, M.H., (1989), "Admixture Stabilization of Sands with Discrete, Randomly Distributed Fibers," Proceedings, XIIth International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Brazil, Vol. 2, pp. 1363-1366.
2. McGown, A., Andrawes, K.Z., and Al-Hasani, M. M., (1978), "Effect of Inclusion Properties
on the Behavior of a Sand," Geotechnique, Vol. 28, No. 3, pp. 327-346. 3. Lee, K. L., Adams, B. D., and Vagneron, J. J., (1973), "Reinforced Earth Retaining Walls",
Journal of Soil Mechanics and Foundations Division, ASCE, Vol. 99, SM10, pp. 745-764. 4. Andersland, O. B., and Khattak, A. S., (1979), "Shear Strength of Kaolinite/Fiber Soil
Mixtures," Proceedings, International Conference on Soil Reinforcement, Vol. I, Paris, France, pp. 11-16.
5. Gray, D. H., and Ohashi, H., (1983), "Mechanics of Fiber Reinforcement in Sand," Journal of
Geotechnical Engineering, Vol. 109, No. 3, pp. 335-353. 6. McGown, A., Andrawes, K.Z., Hytiris, N., and Mercer, F.B., (1985), "Soil Strengthening
Using Randomly Distributed Mesh Elements," Proceedings, XIth International Conference on Soil Mechanics and Foundation Engineering., III, San Francisco, Calif., pp. 1735-1738.
7. Gray, D.H., and Al-Refai, T.O., (1986), "Behavior of Fabric versus Fiber-Reinforced Sand,"
Journal of Geotechnical Engineering, Vol. 112, No. 8, pp. 804-820. 8. Consoli, N. C., Prietto, D. M., and Ulbrich, L. A., (1998), "Influence of Fiber and Cement
Addition on Behaviour of Sandy Soil," Journal of Geotechnical Engineering, Vol. 124, No. 12, pp. 1211-1214.
9. Consoli, N. C., Ulbrich, L. A., and Prietto, P. D. M., (1997), “Engineering Behaviour of
Randomly Distributed Fiber-Reinforced Cemented Soil,” Proceedings of the International Symposium on Recent Developments in Soil and Pavement Mechanics, Rio de Janeiro, Brazil, pp. 481-486.
Murray, Frost, and Wang 16
10. Maher, M. H., and Gray D. H., (1990), "Static Response of Sand Reinforced with Randomly Distributed Fibers," Journal of Geotechnical Engineering, Vol. 116, No. 11.
11. Maher, M. H., and Ho, Y.C., (1994), "Mechanical Properties of Kaolinite/Fiber Soil
Composite," Journal of Geotechnical Engineering, Vol. 120, No. 8, pp. 1381-1393. 12. Jones, A., (1997), "Laboratory Strength Measurements of a Residual Soil Reinforced with
Recycled Carpet Fiber," Internal Report, Geosystems Group, Department of Civil and Environmental Engineering, Georgia Institute of Technology, pp. 1-40.
13. Crockford, W. W., Grogan, W. P., and Chill, D. S., (1993), "Strength and Life of Stabilized
Pavement Layers Containing Fibrillated Polypropylene," 72nd Annual Meeting, Transportation Research Board, Washington, D.C., Paper no. 930888.
14. Nataraj, M. S., and McManis, K. L., (1997), "Strength and Deformation Properties of Soils
Reinforced with Fibrillated Fibers," Geosynthetics International, Vol. 4, No. 1, pp. 65-79. 15. Fatani, M. H., Bauer, G. E., and Al-Joulani, N., (1991), "Reinforcing Soil with Aligned and
Randomly Oriented Metallic Fibers," Geotechnical Testing Journal, Vol. 14, No. 1, pp. 78-87.
16. Lambe and Whitman, (1969), Soil Mechanics, John Wiley & Sons, Inc., New York, NY. pp.
548.
Murray, Frost, and Wang 17
Table 1. Fiber properties.
Fiber Type
Nominal Fiber Length1
(mm)
Nominal Fiber Width1
(mm)
Specific Gravity
One-pass carpet
17.0 0.45 1.12
Fibrillated Polypropylene
30.7 4.30 0.91
Note 1: Determined using a Quanimet 570 optical image analyzer.
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Table 2. Peak stress, axial stress at 10% and 20% axial strain, and peak stress increase for sandy silt reinforced with one-pass carpet and fibrillated polypropylene fiber.
Fiber Type
And Confining Pressure
(kPa)
Fiber Conten
t (%)
Peak Compressive
Stress (kPa)
Compressive
Stress At 10%
Axial Strain (kPa)
Compressive
Stress At 20%
Axial Strain (kPa)
Peak Stress
Increase (%)
34.5 0 282.5 252.9 235.0 -- Control 69.0 0 339.0 334.9 329.4 --
1 363.9 354.9 341.8 +28.7 2 576.1 488.6 576.1 +103.9 34.5 3 857.9 734.6 857.9 +203.7 1 515.5 445.2 481.0 +52.0 2 665.7 537.5 631.2 +96.3
One-Pass Carpet
69.0 3 869.7 622.3 835.2 +156.5 .3 485.8 473.4 334.2 +72.0 .5 572.7 552.7 493.4 +102.7 34.5 1 720.8 634.7 720.8 +155.1 .3 546.5 543.7 538.9 +61.2 .5 576.1 569.9 558.9 +69.9
Fibrillated Poly-propylene 69.0
1 669.1 587.8 669.1 +97.4
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Table 3. Decrease in peak stress due to soaking and change in peak stress as compared to the 0% fiber as-compacted condition.
Confining Pressure
σ3 (kPa)
Fiber Content
(%)
As-Compacted Peak Stress
σ1 (kPa)
Soaked Peak Stress
σ1 (kPa)
Decrease in Peak Stress
Due to Soaking (%)
Change in Peak Stress as
Compared to the 0% Fiber As-Compacted Condition
(%)
34.5 0 317.0 197.8 -37.6 -37.6 34.5 1 398.3 287.4 -27.9 -9.3 34.5 2 610.6 419.0 -31.4 +32.2 69.0 0 408.0 308.0 -24.5 -24.5 69.0 1 584.4 373.5 -36.1 -8.4 69.0 2 628.0 503.1 -31.5 +23.3
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Table 4. Moisture condition prior to saturation for soaked tests.
Fiber Content (%)
Optimum Moisture Content
(%)
As-Compacted
Moisture Content (%)
Moisture
Condition Relative to Optimum
0 18.0 19.0 1 % wet 1 18.5 19.0 0.5 % wet 2 20.0 19.0 1 % dry
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14.0
14.5
15.0
15.5
16.0
16.5
17.0
12 14 16 18 20 22 24 26 28
Moisture Content, %
Dry
Uni
t Wei
ght (
kN/m
3 )
0% fiber
1% fiber
2% fiber
3% fiber
zero air voids line (Gs = 2.65)
Figure 1. Moisture density relationships (standard compactive effort) for sandy silt reinforced with one-pass carpet fiber.
3% fiber
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14
14.5
15
15.5
16
16.5
17
12 14 16 18 20 22 24 26 28
Moisture Content, %
Dry
Uni
t Wei
ght (
kN/m
3 )
zero air voids line (Gs = 2.65)0% fiber
1% fiber
2% fiber
3% fiber
Figure 2. Moisture density relationships (standard compactive effort) for sandy silt reinforced with fibrillated polypropylene fiber.
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14.5
15
15.5
16
16.5
17
0 1 2 3 4 5 6
Fiber Content (%)
Max
imum
Dry
Uni
t Wei
ght (
kN/m
3 )
fibrillated polypropylene (Crockford et. al, 1993)
short chopped carpet fiber (Jones, 1997)
one-pass carpet fiber
fibrillated polypropylene
fibrillated polyproylene (Nataraj et al., 1997)
Figure 3. Variation in maximum dry density with fiber content.
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10
12
14
16
18
20
22
24
0 1 2 3 4 5 6Fiber Content (%)
Opt
imum
Moi
stur
e C
onte
nt (%
)
one-pass carpet fiber
short chopped carpet fiber (Jones, 1997)
fibrillated polypropylene
fibrillated polypropylene (Nataraj et al., 1997)
fibrillated polypropylene (Crockford et al., 1993)
Figure 4. Variation in optimum moisture content with fiber content.
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0
100
200
300
400
500
600
700
800
900
1000
0 2 4 6 8 10 12 14 16 18 20Axial Strain, %
Pri
ncip
al S
tres
s D
iffe
renc
e (k
Pa)
0% fiber
1% fiber
2% fiber
3% fiber
Figure 5. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced as-compacted specimens, confined at 34.5 kPa.
one-pass carpet fiberσ3 = 34.5 kPa
Murray, Frost, and Wang 26
0
100
200
300
400
500
600
700
800
900
1000
0 5 10 15 20 25
Axial Strain, %
Pri
ncip
le S
tres
s D
iffe
renc
e (k
Pa)
0% fiber
0.3% fiber
0.5% fiber
Figure 6. Stress-strain relationships for unreinforced and fibrillated polypropylene reniforced as-compacted specimens, confined at 34.5kPa.
1% fiber
polypropylene fiber
σ3 = 34.5 kPa
Murray, Frost, and Wang 27
200
300
400
500
600
700
800
900
1000
0 0.5 1 1.5 2 2.5 3 3.5
Fiber Content, %
Maj
or P
rinc
iple
Str
ess
at F
ailu
re (k
Pa)
Figure 7. Strength increase as a function of fiber reinforcement for silty sand reinforced with polypropylene and one-pass carpet fiber.
polypropylene fiber
σ3 = 34.5 kPa
σ3 = 69 kPa
one-pass carpet fiber
σ3 = 69 kPa
σ3 = 34.5 kPa
Murray, Frost, and Wang 28
0
100
200
300
400
500
600
700
800
900
1000
0 2 4 6 8 10 12 14 16 18 20
Axial Strain, %
Pri
ncip
al S
tres
s D
iffe
renc
e (k
Pa)
0% as-compacted
0% soaked
2% soaked1% soaked
Figure 8. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 34.5 kPa.
one-pass carpet fiberσ3 = 34.5 kPa
Murray, Frost, and Wang 29
0
100
200
300
400
500
600
700
800
900
1000
0 2 4 6 8 10 12 14 16 18 20
Axial Strain, %
Pri
ncip
al S
tres
s D
iffe
renc
e (k
Pa)
0% as-compacted
1% soaked 2% soaked
0% soaked
Figure 9. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 69 kPa.
one-pass carpet fiberσ3 = 69 kPa
Murray, Frost, and Wang 30
LIST OF FIGURES
Figure 1 - Moisture density relationships (standard compactive effort) for sandy silt reinforced with one-pass carpet fiber. Figure 2 - Moisture density relationships (standard compactive effort) for sandy silt reinforced with fibrillated polypropylene fiber. Figure 3 - Variation in maximum dry density with fiber content. Figure 4 - Variation in optimum moisture content with fiber content. Figure 5 - Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced as-compacted specimens confined at 34.5 kPa. Figure 6 - Stress-strain relationships for unreinforced and fibrillated polypropylene fiber reinforced as-compacted specimens confined at 34.5 kPa. Figure 7 - Strength increase as a function of fiber reinforcement for silty sand reinforced with polypropylene and one-pass carpet fiber. Figure 8 - Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 34.5 kPa. Figure 9 - Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 69 kPa