BEHAVIOUR OF FIBER REINFORCED SOIL Kalpana Maheshwari and C. H. Solanki Applied Mechanics Department, S V National Institute of Technology, Surat – 395 007, Gujarat, India

ABSTRACT The increasing value of land and the limited availability of sites for construction are greatly encouraging engineers to considered in situ soil improvement of weak soil deposits. Geotechnical engineers often encounter problems in designing foundations of structures on soft clayey soil. There may be a need for ground treatment to improve the bearing capacity of the soil. In granular soils in situ the soil may be very loose and indicate potential large elastic settlement. Under these conditions soils need to be densified to increase the unit weight and shear strength. The soil at a construction site or part thereof is not always totally suitable for supporting structures. In practice admixtures with fly ash, lime and geogrids are used frequently to stabilize soils and improve their load carrying capacity. Polypropylene fibers have been extensively used in civil engineering applications for many years. These fibers are used in concrete as a three dimensional secondary reinforcement. The influence of randomly oriented polypropylene fiber on the engineering behaviour of soil has not been reported to the same extent. Ease of application and reduction in cost are making this treatment more popular. The purpose of this investigation is to identify and quantify the influence of fiber variables (content and length) on performance of fiber reinforced soil specimens. In this study fibers were mixed with soft clay in various proportions (0%, 0.5%, 1.0%, 1.5% and 2.0%) to investigate the relative strength gained in terms of compaction, CBR, unconfined compression, etc. This paper presents a review of existing experimental and analytical work in this field and identifies other areas needing attention.

INTRODUCTION The increasing infrastructure growth in urban and metropolitan areas has resulted in a dramatic rise in land prices. The building industry has been forced to look for cheaper land for construction. As a result construction is now carried out on sites which, due to poor ground conditions, would not previously have been considered suitable for development. The soil at a construction site or part thereof is not always totally suitable as a structural support. This greatly encourages engineers to considered in situ soil improvement of weak soil deposits. Geotechnical engineers often enounter problems in designing foundations for structures on soft clayey soil. There may be a need for soil treatment to improve the bearing capacity of the soil. In granular soils the in situ soil may be very loose with potential for large elastic settlement. Under these conditions, soils need to be densified to increase their unit weight and shear strength. Improvement of desireable properties of soil like compaction, CBR, unconfined compression, shear strength and swelling characteristics can be undertaken by a variety of soil improvement techniques. There are many soil improvement techniques either chemical or mechanical. These may be classified as ground reinforcement, ground improvement and ground treatment. Ground reinforcement techniques include stone column, soil nails, micro piles, ground anchors, geosynthetics, lime columns, vibro-concrete columns and mechanically stabilized earth etc. Ground improvement techniques include deep dynamic compaction, drainage / surcharge, elecro–osmosis, compaction grouting, blasting and surface compaction. Ground treatment techniques include soil cement, lime admixture, flyash, dewatering and heating / freezing. All these techniques require skilled manpower and equipment to ensure adequate performance. Recently, the use of fiber reinforced soil has been developed to improve the behaviour of soil in geotechnical engineering. Reinforced soil can be obtained by either incorporating continuous reinforcement inclusions within the soil mass in a defined pattern (i.e. systematically reinforced soils) or mixing discrete fibers randomly within a soil fill (i.e. randomly reinforced soils). The concept of reinforcing soil with natural fiber materials originated in ancient times but randomly distributed fiber reinforced soils have recently attracted increasing attention in geotechnical engineering. In comparison with conventional geosynthetics (strips, geotextile, geogrids, etc), there are some advantages in using randomly distributed fiber as reinforcement. In comparison with systematically reinforced soils randomly distributed fiber reinforced soils have some advantages:

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Firstly, preparation of randomly distributed fiber reinforced soils mimics soil stabilization by admixture. Secondly, discrete fibers are simply added and mixed with the soil much like cement, lime, or other additives. Thirdly, randomly distributed fibers limit potential planes of weakness that can develop parallel to oriented reinforcement.

This technique has become a focus of interest in recent years. Randomly distributed fiber reinforced soil (RDFS) may be used for soil improvement in a variety of applications such as slope, embankment, sub-grade/sub-base, retaining structures and shallow foundations.

LITERATURE REVIEW Hoare (1979) conducted series of laboratory Compression and CBR tests on sand gravel reinforced with randomly distributed synthetic fibers < 2 % by weight and found that the presence of fiber increased the apparent angle of internal friction and ductility of the soil particularly at low confining stress. Andersland and Khattak (1979) carried out triaxial testing under confining stress of 294 kpa to 441 kpa on kaolinite clay reinforced with paper pulp (cellulose) fibers. The addition of fibers increased both stiffness and undrained strength of the clay. The effective angle of friction of reinforced soil was reported to range from 200 for unreinforced clay to 310 for all fibers for a sample under consolidated drained condition. Also, consolidated undrained testing exhibited the value of angle of internal friction ranging from 200 for unreinforced clay to 80.40 for samples of fibers only. Gray and Ohashi (1983) conducted direct shear testing on sand, and reported that increased shear strength, increased ductility (absorbed strain energy) and reduced post peak strength loss due to the inclusion of discrete fibers. Freitag (1986) found randomly distributed fibers in a compacted fine-grained soil could result in greater strength and stiffness. Gray and Al-Refei (1986) performed triaxial compression tests to compare the stress-strain response of sand reinforced with continuous, oriented fabric layers as opposed to randomly distributed, discrete fibers. Test results showed that both types of reinforcement improved strength, increased the axial strain at failure and in most cases reduced post-peak loss of strength. Setty and Rao (1987) measured the soaked and unsoaked CBR of laterite soil reinforced with randomly distributed polypropylene fiber and found that at a constant aspect ration of 25, the soaked CBR increased from 7% to 19.31% and the unsoaked CBR increased from 10.85% to 24.79% due to addition of fiber by up to 2% of dry weight of the soil. The fiber content beyond 2% has no significant improvement in the CBR. Arteaga (1989) found that the inclusion of discrete fibers increased both the cohesion and the angle of internal friction of the specimens. The increase in cohesion of typically cohesionless materials due to the inclusion of discrete fibers was termed the ‘‘apparent cohesion’ of the material. Maher and Gray (1990) showed that the advantages of randomly distributed fibers over continuous inclusions include the maintenance of strength isotropy and the absence of the potential planes of weakness that can develop parallel to continuous planar reinforcement elements. Setty and Rao (1987) carried out triaxial tests, CBR tests and tensile strength tests on black cotton soil reinforced with randomly distributed polypropylene fibers. The test results showed a significant increase in cohesion intercept and a slight decrease in angle of internal friction (i.e. the overall effect is to increase shear strength) with an increase in fiber content up to 3 % by weight. Al-Refeai (1991) proved that fibrillated polypropylene fibers outperformed glass fibers, and the optimum fiber length was 76 mm (3 in.) for sands. Fletcher and Humphries (1991) found that fibrillated fibers at an optimum fiber content of 1.0% dry weight increased the CBR of sandy silt. Lawton and Fox (1992) showed that a section stabilized with randomly distributed fiber provides the highest ultimate strength in terms of CBR. However, the fiber reinforced sand was less stiff than the sand reinforced with multi oriented geosynthetics at a low value of penetration. Maher and Ho (1993) showed that addition of fiber in cement treated sand significantly increased the peak compressive strength, tensile strength and energy absorption capacity. Ahlrich and Tidwell (1994) determined an optimum fiber content of 0.5% dry weight for stabilizing sands with monofilament fibers. Ranjan (1995) conducted soaked and unsoaked CBR tests on sand reinforced with natural and synthetic fiber with an aspect ratio of 100 found that soaked CBR value of sand increased by 2.2 times, whereas unsoaked CBR increased by 2.18 times its unreinforced value due to addition of 2 % polypropylene fiber. Bauer and Oancea (1996) conducted triaxial test results and indicated that the secant modulus as an indication of the stiffness within the initial vertical strain of 2% decreased with increasing polypropylene fiber contents up to 0.5%. They also reported that beyond this vertical strain the secant modulus remains fairly constant. Michalowski and Zhao (1996) conducted triaxial test results which indicated that the steel fibers led to an increase in the peak shear–stress and the stiffness before reaching failure. They also reported that polyamide fibers produced an increase in the peak shear–stress for large confining pressures, but the effect was associated with a considerable loss of stiffness prior to failure and a substantial increase of the strain to failure. Ranjan et al. (1996) carried out regression analysis of test results to develop the mathematical model to bring out the effect of all the factors of fibers on shear strength of reinforced soil. Fiber inclusion increases the

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shear strength of the soil. The shear strength increases linearly with the increasing amount of fiber until 2% by weight beyond which the gain in strength is smaller. Webster and Santori (1997) conducted laboratory experiments to determine the optimum fiber content for a 51 mm (2 in.) monofilament polypropylene fiber in typical concrete sand. The laboratory results were presented using the unconfined compressive strength of each sand-fiber specimen. The laboratory results indicated that 0.8% fibers by dry weight of material were the optimum dosage rates for a 51 mm (2 in.) monofilament polypropylene fiber. Consoli et al. (1998) carried out triaxial compression tests which showed that fiber reinforcement increased the peak and residual strengths, but decreased stiffness. They also indicated that the cohesion intercept was slightly affected by fiber inclusions. Gregory and Chill (1998) presented a slope stabilization application in which fiber reinforcement offers benefits in relation to continuous planar inclusions in projects involving the localized repair of failed slopes. Mo et al. found that the addition of glass fiber in soil– cement mixtures improves the compressive and tensile strength of the mixtures by 30% and 38%, respectively. They also reported that the assessment of the durability for glass fiber modified mixtures is an issue and needs further investigation. Kaniraj and Kavanagi (2001b) conducted unconfined compression tests on fly ash–soil specimens prepared with 3% cement content alone and also with 3% cement and 1% fiber contents after different periods of curing. The study showed that the fiber inclusions increase the strength of the raw fly ash–soil specimens as well as that of the cement stabilized specimens and change their brittle behaviour to ductile behaviour. Santoni et al. (2001) obtained five primary conclusions from their investigation. Firstly the inclusion of randomly oriented discrete fibers significantly improved the unconfined compressive strength of sands. Secondly an optimum fiber length of 51 mm (2 in.) was identified for the reinforcement of sand specimens. Thirdly, a maximum performance was achieved at a fiber dosage rate between 0.6 and 1.0% dry weight. Fourthly specimen performance was enhanced in both wet and dry of optimum conditions. Finally the inclusion of up to 8% of silt does not affect the performance of the fiber reinforcement. Tingle et al. (2002) showed that geofiber stabilization of medium sand improves the CBR by about 6 times over unstabilized sand. When geofiber is mixed into sand, the fiber develops friction at interface points with the particle that resist rearrangement of particles under loading. Zornberg (2002) showed that the critical normal stress is a function of the tensile strength of the fibers, the soil shear strength, and the fiber aspect ratio, but is independent of the fiber content. Consoli and Casagrande (2003) carried out two steel plate load tests (0.3 m diameter, 25 mm thick) on a thick homogeneous stratum of compacted sandy soil reinforced with polypropylene fibers and also on the same soil without the reinforcement. The polypropylene fiber-reinforced specimens showed a marked hardening behaviour up to the end of the tests at axial strains larger than 20%, whereas the non reinforced specimens demonstrated almost perfectly plastic behaviour at large strains. The triaxial test data showed the friction angle to be barely affected by polypropylene fibers inclusion, increasing from 300 to 310. On the other hand, the cohesion intercept increased from 23 kN/m2 to 122 kN/m2. Kumar and Tabor (2004) showed that the relative increase of the residual strength was much higher than the relative increase of the peak shear strength. Gaspard et al. (2003) found that the inclusion of fibrillated-polypropylene fibers (PFs) to the soil–cement mixture significantly increased the indirect tensile strength (ITS), the indirect tensile strain and the toughness index (TI). Furthermore, increasing the curing period as well as the addition of fibers maintained or significantly increased the resilient modulus of the mixture. The addition of fiber to the soil mixture did not improve the unconfined compressive strength (UCS) when compared to similar mixtures without fiber. Michalowschi and Cermak (2003) proved that the addition of a small amount of synthetic fibers increases the failure stress of the composite with a drop in initial stiffness. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. Yetimoglu and Salbas (2003) carried out direct shear test on sands reinforced with randomly distributed discrete fibers. They found that fiber reinforcements could provide smaller loss in post-peak strength and change the brittle behaviour of the sand to somewhat more ductile. Hence the residual shear strength angle of the sand tends to be increased by adding the fiber reinforcements. Gosavi et al. (2003) found that CBR value of jute reinforced black cotton increased by 27.55% and 44.89% due to addition of 2% fiber with an aspect ratio of 25 and 20 respectively. Consoli et al. (2005) showed that the insertion of fibers randomly into sand changes not only its shearing behaviour as previously observed but also its isotropic compression behaviour. For fiber-reinforced samples the fibers found in the sample after testing were found to have been both extended and broken, indicating that the fibers act in tension even when the sample is undergoing large compressive volumetric strains and that the fibers suffer large plastic tensile deformations before breaking. Yetimoglu et al. (2005) conducted laboratory California Bearing Ratio (CBR) tests to investigate the load–penetration behaviour of sand fills reinforced with randomly distributed discrete fibers overlying soft clay. The test results indicated that adding fiber inclusions in sand fill resulted in an appreciable increase in the peak piston load. Khatta and Alrashidi (2006) showed that the durability, UCS, ITS, fracture toughness and resilient modulus values of soil–cement–fiber mixtures either remained the same or were greater than soil–cement mixtures. Heineck et al. 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displacements. At very small strains, the introduction of polypropylene fibers did not influence the initial stiffness of the materials studied. Ibraim and Fourmont (2006) proved that processed cellulose fiber (PCF) modified soil– cement mixtures exhibited superior performance in tensile and compressive characteristics at an optimum fiber dosage. Casagrande et al. (2006) conducted compaction and direct shear tests on sand specimens of different densities unreinforced and reinforced with fibers in different proportions. The presence of reinforcement provides extra resistance to the compaction causing a less dense packing as the quantity of fibers is increased. The results of the compaction tests indicated that the maximum dry density of reinforced sand decreases with increasing fiber content. An optimum moisture content of 10%, independent of the amount of fibers, was also recorded. The results of the direct shear tests indicate that inclusion of fibers increases the peak shear strength and the strain required to reach the peak. The highest fiber content (1.0%) and specimens with 1.0 void ratio gave a gain in strength of about 60%. Kumar et al. (2006) observed that unconfined compressive strength of clay increases with the addition of fibers and it further increases when fibers are mixed in clay sand mixture. The effect of fiber inclusions on the value of OMC and MDD was negligible. The inclusion of 2% of 6 mm plain fibers or 1% of 12 mm plain fibers or 1.5% of 6 mm crimped fibers in the highly compressible clay increases the strength by almost 100%. The inclusion of 2% of 3 mm plain fibers or 0.5% of 6 mm plain fibers along with 10% sand content increases the strength of highly compressible clay by about 150% (over the base strength of clay). The inclusion of 2.0% of 6 mm plain fibers or 1.5% of 6 mm crimped fibers or 1% of 12 mm plain fibers in the highly compressible clay mixed with 10% sand content, increases the strength by almost 180% (over the base strength of clay). Chen (2006) performed a series of CU and CD type triaxial compression tests on comparable unreinforced and fiber-reinforced specimens of Ottawa Sands to evaluate the effective stress strain- pore pressure-volume change behaviour of fiber-reinforced soils. The results show that fibers increase the cohesion and effective friction angle of Ottawa Sands. Cai (2006) tested nine groups of treated soil specimens prepared at three different percentages of fiber content (i.e. 0.05%, 0.15%, 0.25% by weight of the parent soil) and three different percentages of lime (i.e. 2%, 5%, 8% by weight of the parent soil). It was found that fiber content, lime content and curing duration had significant influence on the engineering properties of the fiber–lime treated soil. Puppala and Musenda (2007) indicated that the fiber reinforcement enhanced the UCS of soil and reduced both volumetric shrinkage strains and swell pressures of expansive clays. The fiber treatment also increased the free swell potential of the soils. Sivakumar Babu (2008) discussed several experimental results, different methods for evaluating the strength of fiber-reinforced soil and the application of these methods to predict the strength response of coir-fiber-reinforced soil. Ozkul and Baykal (2007) found that the contribution of rubber fibers to the strength of clay decreases with increasing levels of confinement. A limiting confining pressure exists beyond which the presence of rubber fibers tends to degrade the strength of the clay. For the soil tested this limiting confining stress was between 200 kPa and 300 kPa. Tapkın (2008) observed for fiber-reinforced specimens that the Marshall Stability values increased and flow values decreased in a noticeable manner. The fatigue life of these specimens was also increased. The improvement of the properties of asphalt concrete shows the positive effect of polypropylene fibers. The fiber-reinforced asphalt mixture exhibits good resistance to rutting, prolonged fatigue life and less reflection cracking. It was concluded that the application of polypropylene fibers alters the characteristics of asphalt mixture in a very beneficial way. Tang et al. (2007) investigated the effects of discrete short polypropylene fiber (PP-fiber) on the strength and mechanical behaviour of uncemented and cemented clayey soil The test results indicated that the inclusion of fiber reinforcement within uncemented and cemented soil resulted in an increase in the unconfined compressive strength (UCS), shear strength and axial strain at failure, a decrease in the stiffness, the loss of post-peak strength and changed the cemented soil’s brittle behaviour to more ductile. Kalhor (2008) showed that in clay reinforced with metal fibers,the best ratio of length to diameter of fibers is 20 and, by passing this limit, the level of samples axial strength is decreased. Bent fibers in comparison with straight fibers cause more increase in axial strength. Usually the peak point of axial strength is related to a range from 90 degrees to 120 degrees. Kumar and Singh (2008) have concluded that fly ash is suitable in sub base if it is reinforced with polypropylene fiber. Jadhao and Nagarnaik (2008a) reached four conclusions. Firstly the inclusion of randomly distributed fibers significantly improved the unconfined compressive strength of soil fly ash mixtures. Secondly the increase in fiber length reduced the contribution to peak compressive strength but increased the contribution to strain energy absorption capacity in all soil fly ash mixtures. Thirdly an optimum dosage rate of fibers was identified as 1.00 % by dry weight of soil-fly ash, for all soil fly ash mixtures. Fourthly the maximum performance was achieved with a fiber length of 12 mm as reinforcement of soil fly ash specimens. Chandra et al. (2008) found that 1.5% fiber content and an aspect ratio of 100 were considered as an optimum quantity for Soils A and B, whereas 1.5% fiber content with an aspect ratio of 84 was considered as an optimum quantity for Soil C. For a constant thickness of base and DBM as for the standard section the thickness of the sub base reduced by 38.52%, 26.23%, and 16.67%, respectively for reinforced Soils A, B, and C. Jadhao and Nagarnaik (2008b) showed that for all the proportions of soil fly mixtures increasing fiber content increased UCS, residual strength and absorbed energy. The rate of increase, however, was 68

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reduced with increasing fiber content. Sivakumar Babu and Vasudeven (2007) indicate that the addition of coir (1– 2%) as a random reinforcing material increases both strength and stiffness of clay soil. Marandi et al. (2008) found that with increase in palm fiber length, and fiber inclusion, the ductility increased and the stiffness decreased. The increase in fiber length effectively increased CBR values and this trend was more effective when the fiber inclusion increased. Park (2008) conducted a series of unconfined compression tests to examine the effect of fiber reinforcement and distribution on the strength of fiber-reinforced cemented sand (FRCS). He found that the strength of the FRCS increases as the number of fiber inclusion layers increases. A fiber reinforced specimen, where fibers were evenly distributed throughout the five layers, was twice as strong as a non-fiber-reinforced specimen. Using the same amount of fibers to reinforce two different specimens, a specimen with five fiber inclusion layers was 1.5 times stronger than a specimen with one fiber inclusion layer at the middle of the specimen. Chauhan et al. (2008) suggested that 0.75% of coir fiber and 1% of polypropylene fiber by weight of dry soil appeared to be optimum. Nagu et al. (2008) indicated an improvement in strength parameters of lime stabilized clayey soil with the nylon fiber reinforcement. The fiber addition significantly imparted ductility to the soil which changed the mode of failure from brittle to ductile. Samadhiya and Viladkar (2008) found that the dry unit weight reached a maximum value at a fiber content of 0.3% with 10 mm fiber length for clay specimens. There is a substantial increase in unconfined compression strength of clay at 0.3% fiber content with 20 mm fiber length. At 20 mm fiber with 0.3% fiber content compression index of clay has decreased to the maximum extent. Gupta et al. (2008) concluded that the bearing capacity of the randomly distributed fiber reinforced soil (RDFS) increases with increase in fiber content. In the direct shear test, peak shear stress of RDFS is more than that of unreinforced sand. As the fiber content increases the peak shear stress increases up to 0.2% FC.

MATERIAL USED For the present study, soil sample of blackish colour was collected from Bhal Chandra concrete industry, Dabhoi, Baroda. The soil sample was collected in polythene gunny bags and then air-dried. Testing of the soil sample was carried out by “Geo Engineering Services”, Baroda. The soil was classified as CH according to the Unified Soil Classification System. Engineering properties of this soil are listed in Table 1. Table 1: Engineering properties of the clayey soil used. Sr. No. 1. 2. (i) (ii) (iii) (iv) 3. (i) (ii) (iii) (iv) 4. 5. (i) (ii) 6. (i) (ii)

Properties

Value

Specific gravity Grain size analysis Gravel, % Sand, % Silt, % Clay, % Consistency limit Liquid limit, % Plastic Limit, % Plasticity index, % Shrinkage Limit, % IS Classification Compaction study Optimum moisture content, (Standard Proctor Test )% Maximum dry density, g/cm3 Shear strength parameters Cohesion, kg/cm2 Angle of internal friction

2.444 1 8 61.8 25.0 52.9 27.5 25.4 23.5 CH 16.23 1.65 2.27 1.16o

Polyester fibers of sizes 6 and 12 mm used in this investigation were provided by Relience Industries Limited. The product specifications of the polyester fibers are given below:

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Table 2: Physical and engineering properties of fibers used Type Cut length Cross-section Diameter Tensile elongation Specific gravity Tensile strength Colour

Polyester 12.1 mm Triangular 30-40 µm >100% 1.34 – 1.39 400-600N/m2 Almost Colourless

Special qualities: In spite of less surface friction polyester fibers are better bonded together with clay due to its typical triangular cross-section.

TESTING PROCEDURES The scope of this investigation was to study the effects of adding polyester fibers on the strength characteristics of highly compressible clay soil compacted at maximum dry density and OMC. To quantify the increase in the strength due to addition of fibers a series of unconfined compressions tests and CBR tests were performed on soil specimens prepared at the maximum dry unit weight determined using the standard Proctor test and optimum moisture content. After testing the unreinforced soil samples (0% fibers) a suite of samples was prepared and tested with 0.5%, 1.0%, 1.5% , 2.0% and 3.0% (by weight of dry soil) of polyester fiber. Verification tests were also performed in order to examine the repeatability of the test results. All specimens tested for unconfined compression in this investigation were 38 mm diameter and 76 mm high. To obtain consistency specimens were prepared with a mixture of moist soil and fibers compacted into three equal layers. Before compaction the inside of the mould was coated with a lubricant to lessen the risk of fracturing of the specimens during removal. The compactive effort was applied until the soil filled the mould to the desired height of the untrimmed sample. Between the compaction of each layer the surface of the layers was scoured to provide a reasonable bond between the layers. Following removal of each sample from the mould it was immediately trimmed to the desired height. Any polyester fibers sticking out at the top and bottom of the samples were trimmed with scissors. The CBR tests were conducted inside a modified proctor mould soaked as per ASTM D1883-92. The mould was a rigid metal cylinder with an inside diameter of 152mm and a height of 178 mm. A manual loading machine equipped with a movable base that travelled at a uniform rate of 1.27 mm/min and a calibrated load indicating device was used to force the penetration piston with a diameter of 50 mm into the specimen. The loads were carefully recorded as a function of penetration up to a total penetration of 30 mm to also observe the post-failure behaviour.

RESULTS AND DISCUSSION Figure 1 shows the effect of inclusion of polyester fibers of various cut lengths on UCS of highly compressible clay mixed in different proportion of fibers (1.0%, 1.5%, 2.0%, and 3.0%). It clearly shows that there is a significant increase in peak strengths of samples with 6 mm and 12 mm fibers. The UCS of the original soil, i.e. with 0 %fiber, is 3.443 kg/cm2 and it is at a maximum with 1.5 % fiber at 5.94 kg/ cm2 for 12 mm fiber and 5.66 kg/cm2 for 6 mm fiber. This shows also UCS Value increases with the length of the fiber. The increase in strength is 64.39 % with the inclusion of 1.5 % 6mm size fibers. The increase is 72.52 % with the inclusion of 1.5 % 12mm size fibers.

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Figure 1: Effect of polyester fiber sizes on unconfined compressive strength of clay mixed with different percentage of fibers.

Figure 2: Soaked CBR Value of clay mixed with different percentage of fibers. Figure 2 shows the effect of inclusion of polyester fibers of various cut lengths on the soaked CBR Value of highly compressible clay mixed with different proportions of fiber. The soaked CBR Value of the soil is 0.75 %, that for the 1.5% 6 mm fiber is 5.03 % and 1.5% 12mm fiber is 5.15 %.. The increase in CBR Value is 570.67 % with the inclusion of 1.5 % 6mm length fibers and the increase is 586.67 % with the inclusion of 1.5 % 12mm length fibers. Photographs of loose fibers are shown in Figure.3.

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Figure 3: Photograph showing loose 6 mm and 12 mm polyester fibers.

CONCLUSIONS The following conclusions can be drawn from the study: •

There is significant increase in UCS with the inclusion of polyester fibers in highly compressible clay. The increase in strength is 64% with the inclusion of 1.5 % 6mm size fibers, and that with 1.5 % 12mm fiber is 72%.

•

The increase in soaked CBR Value is 570% with the inclusion of 1.5 % 6 mm size fibers and 586% with 1.5 % 12 mm fiber.

•

The USC and soaked CBR value decrease with increased inclusion of fiber content more than 1.5% polyester fiber. There is thus no significant effect with addition of polyester fiber beyond 1.5%.

REFERENCES Ahlrich, R. C., and Tidwell, L. E. (1994), ‘‘Contingency airfield construction: Mechanical stabilization using monofilament and fibrillated fibers’’, Tech. Rep. GL-94-2, U.S. Army Engr. Waterways Experiment Station, Vicksburg, Miss. Al-Refeai, T. (1991), "Behavior of granular soils reinforced with discrete randomly oriented inclusions ", Geotextiles and Geomembranes, 10, 319 – 333. Amir Kalhor (2008), “Effect of Metal Fibers on Clayey Soils”, EJGE Vol 13, 1-13. Andersland, O. B. and Khattak, A.S. (1979), Mixtures", Proceeding International Conference on Soil Reinforcement, Vol I, Paris, 11-16. Arteaga, C. B. (1989), ‘‘The shear strength of Ottawa sand mixed with discrete short length plastic fibers’’ Thesis, Mississippi State University, Mississippi State, Miss. Bauer, G.E., Oancea, A., (1996), “Triaxial testing of granular soils reinforced with discrete polypropylene fibers.”, Proceedings of the First European Geosynthetic Conference, Maastricht, Netherlands. In: De Groot, M.B., Den Hoedt, G., Termaat, R.J. (Eds.), Geosynthetics: Applications, Design and Construction. A.A. Balkema, Rotterdam, 407–410. Cai, Y., Shi, B., Ng, C. W. W. and Tang, C. (2006), "Effect of polypropylene fiber and lime admixture on engineering properties of clayey soil", Engineering Geology 87, 230–240. Casagrande; M. D. T., Coop, M. R. and Consoli. N. C. (2006), "Behaviour of a Fiber-Reinforced Bentonite at Large Shear Displacements", Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132 (11), 15011508. Chauhan, M.S., Mittal, S. and Mohanty, B.(2008), "Performance evaluation of silty sand subgrade reinforced with fly ash and fiber.", Geotextiles and Geomembranes 26 (5), 429–435. Chen, Cheng-Wei (2006), "Drained and Undrained Behavior of Fiber-Reinforced Sand", Department of Civil and Environmental Engineering, E2509 Lafferre Hall, Columbia, MO 65211.

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