BEHAVIOR OF A SANDY SILT REINFORCED WITH DISCONTINUOUS RECYCLED FIBER INCLUSIONS

Transcription

1 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/ , Telefax: 1/ , J.D. Frost, Professor, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA, Telephone: 1/ , Telefax: 1/ , Y. Wang, Associate Professor, School of Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA, Telephone: 1/ , Telefax: 1/

2 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.

3 Murray, Frost, and Wang 3 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

4 Murray, Frost, and Wang 4 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

5 Murray, Frost, and Wang 5 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, D 50, 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.

6 Murray, Frost, and Wang 6 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 D 50 of the sandy silt was 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.

7 Murray, Frost, and Wang 7 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/ft 3 and 19.0%, respectively. Although the global density of each specimen was 100 lb/ft 3, the specimen density was not uniform as a result of non-uniformity inherent in the static load application method.

8 Murray, Frost, and Wang 8 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 deaired 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 ascompacted. 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.

9 Murray, Frost, and Wang 9 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 importantly, 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

10 Murray, Frost, and Wang 10 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

11 Murray, Frost, and Wang 11 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

12 Murray, Frost, and Wang 12 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 soakedtriaxial 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

13 Murray, Frost, and Wang 13 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 ascompacted 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.

14 Murray, Frost, and Wang 14 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.

15 Murray, Frost, and Wang 15 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 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 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 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 Gray, D. H., and Ohashi, H., (1983), "Mechanics of Fiber Reinforcement in Sand," Journal of Geotechnical Engineering, Vol. 109, No. 3, pp 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 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 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 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

16 Murray, Frost, and Wang Maher, M. H., and Gray D. H., (1990), "Static Response of Sand Reinforced with Randomly Distributed Fibers," Journal of Geotechnical Engineering, Vol. 116, No Maher, M. H., and Ho, Y.C., (1994), "Mechanical Properties of Kaolinite/Fiber Soil Composite," Journal of Geotechnical Engineering, Vol. 120, No. 8, pp 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 Crockford, W. W., Grogan, W. P., and Chill, D. S., (1993), "Strength and Life of Stabilized Pavement Layers Containing Fibrillated Polypropylene," 72 nd Annual Meeting, Transportation Research Board, Washington, D.C., Paper no 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 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 Lambe and Whitman, (1969), Soil Mechanics, John Wiley & Sons, Inc., New York, NY. pp. 548.

17 Murray, Frost, and Wang 17 Table 1. Fiber properties. Fiber Type Nominal Fiber Length 1 (mm) Nominal Fiber Width 1 (mm) Specific Gravity One-pass carpet Fibrillated Polypropylene Note 1: Determined using a Quanimet 570 optical image analyzer.

18 Murray, Frost, and Wang 18 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) Control One-Pass Carpet Fibrillated Polypropylene Fiber Conten t (%) Peak Compressive Stress (kpa) Compressive Stress At 10% Axial Strain (kpa) Compressive Stress At 20% Axial Strain (kpa) Peak Stress Increase (%)

19 Murray, Frost, and Wang 19 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 s 3 (kpa) Fiber Content (%) As- Compacted Peak Stress s 1 (kpa) Soaked Peak Stress s 1 (kpa) Decrease in Peak Stress Due to Soaking (%) Change in Peak Stress as Compared to the 0% Fiber As- Compacted Condition (%)

20 Murray, Frost, and Wang 20 Table 4. Moisture condition prior to saturation for soaked tests. Fiber Content (%) Optimum Moisture Content (%) As-Compacted Moisture Content (%) Moisture Condition Relative to Optimum % wet % wet % dry

21 Murray, Frost, and Wang % fiber 16.5 zero air voids line (Gs = 2.65) Dry Unit Weight (kn/m 3 ) % fiber 1% fiber 2% fiber 3% fiber Moisture Content, % Figure 1. Moisture density relationships (standard compactive effort) for sandy silt reinforced with one-pass carpet fiber.

22 Murray, Frost, and Wang % fiber zero air voids line (G s = 2.65) Dry Unit Weight (kn/m 3 ) % fiber 2% fiber 3% fiber Moisture Content, % Figure 2. Moisture density relationships (standard compactive effort) for sandy silt reinforced with fibrillated polypropylene fiber.

23 Murray, Frost, and Wang fibrillated polypropylene (Crockford et. al, 1993) Maximum Dry Unit Weight (kn/m 3 ) short chopped carpet fiber (Jones, 1997) one-pass carpet fiber fibrillated polypropylene fibrillated polyproylene (Nataraj et al., 1997) Fiber Content (%) Figure 3. Variation in maximum dry density with fiber content.

24 Murray, Frost, and Wang one-pass carpet fiber Optimum Moisture Content (%) fibrillated polypropylene (Nataraj et al., 1997) short chopped carpet fiber (Jones, 1997) fibrillated polypropylene fibrillated polypropylene (Crockford et al., 1993) Fiber Content (%) Figure 4. Variation in optimum moisture content with fiber content.

25 Murray, Frost, and Wang one-pass carpet fiber s 3 = 34.5 kpa 3% fiber Principal Stress Difference (kpa) % fiber 0% fiber 1% fiber Axial Strain, % Figure 5. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced as-compacted specimens, confined at 34.5 kpa.

26 Murray, Frost, and Wang polypropylene fiber Principle Stress Difference (kpa) s 3 = 34.5 kpa 1% fiber 0.5% fiber 0.3% fiber 0% fiber Axial Strain, % Figure 6. Stress-strain relationships for unreinforced and fibrillated polypropylene reniforced as-compacted specimens, confined at 34.5kPa.

27 Murray, Frost, and Wang one-pass carpet fiber 900 Major Principle Stress at Failure (kpa) s 3 = 69 kpa polypropylene fiber s 3 = 34.5 kpa s 3 = 69 kpa s 3 = 34.5 kpa Fiber Content, % Figure 7. Strength increase as a function of fiber reinforcement for silty sand reinforced with polypropylene and one-pass carpet fiber.

28 Murray, Frost, and Wang one-pass carpet fiber s 3 = 34.5 kpa Principal Stress Difference (kpa) % as-compacted 1% soaked 2% soaked % soaked Axial Strain, % Figure 8. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 34.5 kpa.

29 Murray, Frost, and Wang one-pass carpet fiber s 3 = 69 kpa Principal Stress Difference (kpa) % as-compacted 1% soaked 2% soaked % soaked Axial Strain, % Figure 9. Stress-strain relationships for unreinforced and one-pass carpet fiber reinforced soaked specimens confined at 69 kpa.

30 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 ascompacted 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