6th National Congress on Civil Engineering, April 26-27, 2011, Semnan University, Semnan, Iran

INFLUENCE OF FIBER REINFORCEMENT ON TRIAXIAL SHEAR BEHAVIOR OF CEMENTED SANDY SOILS

Mehdi Hooresfand1, Amir Hamidi2 1Graduate student, School of Engineering, Tarbiat Moallem University, Tehran, Iran

2Assistant Professor, School of Engineering, Tarbiat Moallem University, Tehran, Iran

mehdihooresfand@yahoo.com hamidi@tmu.ac.ir

Abstract A number of triaxial tests were conducted for evaluation of randomly distributed fiber reinforcement effects on the behavior of cemented sand. Cemented samples were prepared by addition of Portland cement up to 3% by weight and were cured for about seven days after mixture. Polypropylene fibers with a length of 12 mm were added and mixed in three different weight percentages of 0%, 0.5%, 1%. Specimens were compacted in relative densities of 50% and 70%. Consolidated drained compression triaxial tests were performed under confinements of 100, 300 and 500 kPa. Tests results indicated that addition of polypropylene fiber to cemented sandy soils increases peak and residual strength. However, ultimate dilation decreases with enhancement of fiber content in cemented soil. Keywords: Fiber, Cemented sand, triaxial test, shear strength, volume change behavior.

1. INTRODUCTION Shear strength characteristics of artifically cemented sandy soils has been studied in the past by many researchers, such as Saxena et al. (1978), Clough et al. (1981), Leroueil and Vaughan (1990), Airey (1993) and Coop and Atkinson (1993) and Hamidi and Haeri (2005). Also, experimental studies on influence of reinforcement on shear strength of uncemented and cemented sands using fiber inclusions have also been reported by different researcgers, e.g., Gray and Alrefeai 1986; Gray & Maher, 1989, Maher and Gray 1990; Al-Refaei, 1991, Maher and Ho 1993; Omine et al. 1996, Consoli et al., 1998, 2002, 2003. The results showed that fiber-reinforced soil is a potentially composite material which can be advantageously employed in improvement of the structural behavior of soils. One of the main advantages of randomly distributed fibers is the maintenance of strength isotropy and the absence of potential planes of weakness that can develop parallel to the oriented reinforcement. The fiber reinforcement causes significant improvement in shear strength of sand. More importantly it exhibits greater extensibility and small loss of post-peak strength (i.e. greater ductility in the composite material) as compared to sand alone or to sand reinforced with high modulus inclusions. However, more research is necessary to evaluate the influence of fiber reinforment on the mechanical behavior of cemented soils. 2. EXPERIMENTAL STUDIES A number of 18 conventional triaxial compression tests were carried out in this research on fiber-reinforced cemented sand. Fiber contents were selected as 0%, 0.5% and 1% and cement content was 3% by weight of dry sand. Specimens prepared in two different relative densities of 50% and 70% at confining pressures ranging from 100 to 500 kPa. Fibers with length of 12 mm and diameter of 0.023 mm were used in sample preparation process. 2.1 MATERIALS 2.1.1 SOIL The soil used in the present study obtained from Babolsar shores near Caspian Sea in Mazandaran province, Iran. Specific Gravity of Solids Gs is 2.74, with a Uniformity Coefficient Cu and Curvature Coefficient Cc of 1.75 and 0.89 respectively. The minimum and maximum void ratios of soil were determined as 0.5 and 0.75 respectively. Grain size distribution curve of base soil is shown in Fig. 1.

6th National Congress on Civil Engineering, April 26-27, 2011, Semnan University, Semnan, Iran

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Fig. l. Grain size distribution curve for base soil

Table 1- Physical and mechanical characteristics of fibers

Characteristic Polypropylene

Type Monofilament

Fiber length 12 mm

Thickness 23 μm

Tensile Strength 400 MPa

Young modulus 6000-10000 MPa

Specific density 0.91gr/cm3

Elongation 12%

2.1.2 FIBERS Polypropylene fibers used in sample preparation. Physical and mechanical characteristics of fibers are presented in Table 1. 2.2 Specimen Preparation and Testing Procedures Specimen preparation was conducted by first determining the required weight of sand, cement, water, and fiber for mixture. The mixture proportions were determined according to a weight-fraction relationship, where fibers were considered to be a part of the solids with different specific gravity, Gs, from that of soil and cement. The amount of water added during the mixing process was 10% by weight. This water content had no influence on stiffness and stress deformation properties of the sand (Maher and Gray 1990) and was enough to achieve hydration of cement in the mixture (Lade and Overton 1989; Clough et al. 1981). The compacted soil and fiber-reinforced specimens used in triaxial tests were prepared by mixing dry sand, cement, water and polypropylene fibers. During the mixing process, it was found to be important to add the water prior to addition of fibers, to prevent floating of the fibers. Visual and microscope examination of exhumed specimens showed the mixtures to be satisfactorily uniform. The under compaction process (Ladd,

0 10 20 30 40 50 60 70 80 90

100

0.01 0.1 1 10 Particle size (mm)

Perc

ent F

iner

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1978) was used to produce homogeneous specimens that could be used for a parametric study in laboratory testing program. Specimens were statically compacted in eight layers into a 100 mm diameter by 200 mm height split mold, to relative densities of 50% and 70% at a moisture content of 10%. Finally, the molds were wrapped in moisture-proof bags and stored in a humid room (at a temperature of oo 222 ± and a relative humidity over 90%) to cure for 7 days before testing. Static drained triaxial compression tests were carried out at confining pressures of 100, 300 and 500 kPa. As several researchers like Lambe and Whitman (1979) observed, the stress–strain behaviors of dry and saturated granular soils are analogous provided that pore fluid can flow freely into or out of pores and no excess pore pressure can develop. Therefore consolidated drained triaxial tests on cemented soil were performed on completely dry samples. Therefore volume change gauge was placed on cell pressure's line. Triaxial tests were performed at a sufficiently low axial strain rate of 0.3 mm/min. 2.3 ANALYSIS OF TEST RESULTS Curves for deviatoric stress: axial strain and volumetric strain: axial strain curves for triaxial tests under confining pressures of 100, 300, 500 kPa on fiber-reinforced cemented sand with 0.5% and 1% monofilament polypropylene fibers are shown in Figs. 2-7 for sand with 3% cement content in a relative density of 70%.

Fig. 2. Deviatoric Stress: axial Strain curve for reinforced cemented sand in confinement of 100kPa

Fig. 3. Volumetric Strain: axial Strain curve for reinforced cemented sand in confinement of 100kPa

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Fig. 4. Deviatoric Stress: axial Strain curve for reinforced cemented sand in confinement of 300kPa

Fig. 5. Volumetric Strain: axial Strain curve for reinforced cemented sand in confinement of 300kPa

Fig. 6. Deviatoric Stress: axial Strain curve for reinforced cemented sand in confinement of 500kPa

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Fig. 7. Volumetric Strain: axial Strain curve for reinforced cemented sand in confinement of 500kPa

Effect of fiber reinforcement on stress-strain response of sand can be investigated by Figs. 2, 4 and 6. According to these figures, brittle behavior of non reinforced sand changes to a ductile one with enhancement of fiber content in soil. It can be concluded that the most advantage of fiber reinforcement application in cemented specimens is obvious improvement of ductility of the soil. An absolute measurement of this behavior may be explained by brittleness index ( BI ) according to the following equation:

(1)

Where, qf and qu are peak and ultimate deviatoric stresses respectively. As the brittleness index decreases, the behavior of soil changes to a ductile one. Figure 8 shows variation of brittleness index with fiber content for different confining pressure. The brittleness index decreases with more fiber inclusion and decreases with increase in confining pressure.

Fig. 8. Brittleness index versus fiber content curves for reinforced sand

As a result, it can be concluded that fiber reinforcement can increase the peak shear strength of cemented soil and decreases its dilative behavior. Figure 9 indicates increase in peak shear strength of cemented soil with increase in fiber content.

1qq

Iu

fB =

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Fig. 9. Peak shear strength versus fiber content for reinforced sand The axial strain corresponding to failure and post-peak response is dependent on fiber inclusion to the composite. Both the axial strain at failure and the ultimate strength are greater for the fiber-reinforced material. In Figs. 3, 5 and 7, volumetric strain versus axial strain plots show a difference between the non-reinforced and the fiber-reinforced cemented sand. The non-reinforced cemented sand behavior is initially compressive, followed by a strong expansion with a maximum dilation rate taking place immediately after the peak strength. A similar result has been observed by Coop and Atkinson (1993) for cemented carbonate sands. Subsequently, the dilation rate decreases as the sand approaches a final steady condition. Inclusion of fiber to cemented sand increases the initial compressive volumetric strain and also strains at which the peak strength is reached. Similarly to cemented sand, the maximum dilation rate of the fiber-reinforced cemented sand occurs at a higher axial strain than strains to failure. For the cemented specimens, fiber inclusion changes the marked brittle behavior of the cemented specimens to a more ductile one. 3. CONCLUSIONS According to the test results, the following conclusions can be reported:

1. Fiber inclusion to cemented soil enhances the peak and residual shear strengths. 2. Addition of fiber to cemented sand increases the initial compressive volumetric strains and also strains

at which the peak strength is reached. However, ultimate dilation of cemented samples decreases by increase of fiber content.

3. Increase in fiber content reduces the brittleness index which results in the change of behavior frome brittle to ductile.

4. REFERENCES 1. McGown, A., Andrawes, K. Z. & AI-Hasani, M. M. “Effect of inclusion properties on the behavior

of sand.” Geotechnique, 28 (3) (1978), pp. 327-46. 2. Saxena, S. K., and Lastrico, R. M. (1978). “Static properties of lightly cemented sand.” J. Geotech.

Eng. Div., Am. Soc. Civ. Eng., 104(12), pp 1449–1464. 3. Clough, G. W., Sitar, N., Bachus, R. C., and Rad, N. S. (1981). “Cemented sands under static

loading.” J. Geotech. Eng. Div., Am. Soc. Civ. Eng., 107(6), pp 799–817. 4. Leroueil, S., and Vaughan, P. R. (1990). “The general and congruent effects of structure in natural

soils and weak rocks.” Geotechnique, 40(3), pp 467–488. 5. Airey, D. W. (1993). “Triaxial testing of naturally cemented carbonate soil.” J. Geotech. Eng.,

119(9), pp 1379–1398.

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6. Coop, M. R., and Atkinson, J. H. (1993). “The mechanics of cemented carbonate sands.” Geotechnique, 43(1), pp 53–67.

7. Hamidi, A., Haeri, S.M. (2005). “Critical state concepts for a cemented gravely sand.” Electronic Journal of Geotechnical Engineering 10 (E), pp 1–12.

8. Gray, D.H., Al-Refeai, T., (1986). “Behavior of fabric versus fiber-reinforced sand.” Journal of Geotechnical Engineering, ASCE 112 (8), pp 804–820.

9. Gray, D. H. & Maher, M. H. (1989). “Admixture stabilization of sand with discrete, randomly distributed fibers.” Proc. XIIth Int. Conf. on SMFE, Rio de Janeiro, Brazil, pp. 1363-6.

10. Maher, M.H., Gray, D.H., (1990). “Static response of sand reinforced with fibers.” Journal of Geotechnical Engineering, ASCE 116 (11), pp 1661–1677.

11. Al Refeai, T.O., (1991). “Behavior of granular soils reinforced with discrete randomly oriented inclusions.” Geotextiles and Geomembranes 10, pp 319–333.

12. Maher, M.H., Ho, Y.C., (1993). “Behavior of fiber-reinforced cement sand under static and cyclic loads.” Geotechnical Testing Journal 16 (3), pp 330–338.

13. Omine, K., Ochiai, H., Yasufuku, N., and Kato, T. (1996). “Effect of plastic wastes in improving cement-treated soils.” Proc., 2nd Int. Congr. on Environmental Geotechnics, Balkema, Rotterdam, The Netherlands, pp 875–880.

14. Consoli, N. C., Prietto, P. D. M., and Ulbrich, L. A. (1998). “The influence of fiber and cement addition on behavior of a sandy soil.” J. Geotech. Geoenviron. Eng., 124(12), pp 1211–1214.

15. Consoli, N.C., Montardo, J.P., Prietto, P.D.M., Pasa, G.S., (2002). “Engineering behavior of a sand reinforced with plastic waste.” Journal of Geotechnical and Geoenvironmental Engineering 128 (6), pp 462–472.

16. Consoli, N.C., Vendruscolo, M.A., Prietto, P.D.M., (2003). “Behavior of plate load tests on soil layers improved with cement and fiber.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE 129 (1), pp 96–101.

17. Lade, P. V. and Overton, D.C., (1989). “Cementation Effects of Frictional Materials.” Journal of Geoteehnical Engineering, ASCE, 115(10), pp 1373-1389.

18. Clough, G.W., Sitar, N., Bachus, R.C., Rad, N.S., (1981). “Cemented sands under static loading.” Journal of Geotechnical Engineering Division 107 (6), pp 799–817.

19. Ladd, R. S., (1978). “Preparing Test Specimens Using Under compaction.” Geotechnical Testing Journal, 1(1), pp 16-23.