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Swelling behaviour of a geofiber-reinforced expansive soil
Geotextiles and Geomembranes 27 (2009) 73–76
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Technical Note
Swelling behaviour of a geofiber-reinforced expansive soil B.V.S. Viswanadham a, *, B.R. Phanikumar b,1, Rahul V. Mukherjee b,1 a b
Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Department of Civil Engineering, Vellore Institute of Technology, Vellore 632014, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2008 Received in revised form 14 May 2008 Accepted 30 June 2008 Available online 15 August 2008
This paper reports the results of laboratory study performed on expansive soil reinforced with geofibers and demonstrates that discrete and randomly distributed geofibers are useful in restraining the swelling tendency of expansive soils. Swelling characteristics of remoulded expansive soil specimens reinforced with varying fiber content (f ¼ 0.25% and 0.50%) and aspect ratio (l/b ¼ 15, 30 and 45) were studied. Onedimensional swell-consolidation tests were conducted on oedometer specimens. Reduction in heave and swelling pressure was the maximum at low aspect ratios at both the fiber contents of 0.25% and 0.50%. Finally, the mechanism by which discrete and randomly distributed fibers restrain swelling of expansive soil is explained with the help of soil–fiber interaction. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Expansive soil Fibers Fiber reinforcement Swelling Swell-consolidation test
1. Introduction Expansive clays swell upon absorption of water and shrink upon evaporation of water (Chen, 1988; Nelson and Miller, 1992). Hence, civil engineering structures found in these soils are severely damaged. The annual cost of damage is estimated at £150 million in the UK, $1000 million in the USA and many billions of pounds worldwide. To ameliorate the problems posed by expansive soils, many innovative techniques have been developed. Belled piers (Chen,1988), granular pile-anchors (Phanikumar,1997; Phanikumar et al., 2004) and chemical stabilization with lime and fly ash (Chen, 1988; Hunter, 1988; Cokca, 2001; Phanikumar and Sharma, 2004) have been suggested for mitigating heave problems. Geosynthetic inclusions (Ayyar et al.,1989; Vessely and Wu, 2002) were also found effective in reducing swelling potential of expansive soils. Very recently, Ikizler et al. (2008) have reported a potential decrease in swelling pressure as a result of the inclusion of expanded polystyrene geofoam placed between an expansive soil and a rigid wall. In recent years, discrete fibers have been added and mixed into soils to improve the strength behaviour of soil (Maher and Ho,
* Corresponding author. Tel.: þ91 22 25767344; fax: þ91 22 25767302. E-mail addresses: [email protected] (B.V.S. Viswanadham), phanikumar_ [email protected] (B.R. Phanikumar), [email protected] (R.V. Mukherjee). 1 Tel.: þ91 416 2202212. 0266-1144/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2008.06.002
1994; Al Wahab and El-Kedrah, 1995; Nataraj and McManis, 1997; Ziegler et al., 1998; Cai et al., 2006; Sivakumar Babu et al., 2008). Maher and Ho (1994) showed that the fiber reinforcement increased the shear strength and ductility of kaolinite. Tension cracking and volume change due to swell/shrink in compacted clays decreased when reinforced with polypropylene fibers (Al Wahab and El-Kedrah, 1995; Nataraj and McManis, 1997). Ziegler et al. (1998) observed that fiber inclusions increased the tensile strength. Reinforcement of expansive soils with discrete geofibers (flexible polymeric fibers) offers an alternative method to chemical stabilization techniques and other methods for reducing swelling potential. Viswanadham (1989) and Ayyar et al. (1989) have reported about the efficacy of randomly distributed coir fibers in reducing the swelling tendency of the soil. The efficacy of combination of fly ash and polypropylene fibers in reducing swelling and shrinkage characteristics was also reported (Puppala and Musenda, 2000; Punthutaecha et al., 2006; Tang et al., 2007). Puppala and Musenda (2000) and Punthutaecha et al. (2006) showed that fiber reinforcements enhanced the unconfined compressive strength and reduced the swelling potential of expansive clays. Cai et al., 2006 reported an increase in fiber content led to reduction in swelling potential of lime stabilized clayey soil. Seda et al., 2007 present about beneficial use of shredded waste tire rubber for swelling potential mitigation in expansive soils. However, the reports on the use of discrete geofibers for swelling potential mitigation in expansive soils have not been seen yet. Thus, this paper presents the efficacy of the use
74
B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
of fiber reinforcement in controlling heave of remoulded expansive soils. A laboratory study was undertaken to evaluate the feasibility of using the fibers in an expansive soil to reduce swell potential. Onedimensional swell-consolidation tests were performed on compacted mixtures of fibers and the expansive soil to assess the optimum fiber content and aspect ratio for maximum heave reduction.
Table 2 Properties of polypropylene fiber (as supplied by the manufacturer) Type and composition
Slit film and polypropylene
Width (mm) Denier Tenacity (gpda) Breaking load (N) Breaking elongation (%) Specific gravity Melting point ( C)
2 890 5.45 48.4 18 0.91 170
a
2. Experimental investigation A laboratory investigation was conducted for studying the efficacy of fiber reinforcement in arresting heave of expansive soils. Swelling behaviour of unreinforced expansive soil specimens was studied and compared with that of specimens reinforced with flexible polypropylene fiber. The experimental investigation was conducted on unreinforced expansive soil specimens and fiberreinforced specimens compacted in oedometer. Swell-consolidation tests were performed. 2.1. Test materials 2.1.1. Expansive soil The soil used in this investigation had a free swell index of 93%. The soil was collected from a depth of 1.5 m from Pune, in Maharashtra state of India. The X-ray diffraction spectra gave the following mineralogical composition – montmorillonite: 48–50%, quartz: 30–32%, calcite: 15–16% and antase: 1–2%. Table 1 shows the index properties of the soil. Based on the plasticity properties, the soil was classified as CH according to USCS classification (see Table 1). 2.1.2. Polypropylene fiber The fiber used for reinforcing the expansive soil specimens was a polypropylene fiber (840-TF15090). It had a width of 2 mm and thickness of about 0.021 mm and specific gravity of 0.91. Table 2 shows various properties of the fibers as supplied by the manufacturer (Techfab, India). The fiber content f was varied as 0.25% and 0.50% by dry weight of expansive soil. The aspect ratio l/b of the fibers was varied as 15, 30 and 45. Aspect ratio is defined as the ratio of length to width of the fiber. For example, l/b ¼ 15 implies 2 mm wide fibers of 30 mm long. Fig. 1 shows the standard Proctor compaction curves of unreinforced expansive soil and soil blended with varying fiber content (f ¼ 0.25% and 0.50%).
Grams per denier (denier is defined as a weight of 9000 m long fiber).
2.2. Tests conducted Swell-consolidation tests were conducted in the conventional oedometer of diameter 75 mm and thickness (H) 25 mm. Swell potential (S%) and swelling pressure (ps) were determined for unreinforced specimens and specimens reinforced with varying fiber content. As the specimens were prepared at the respective OMC and MDD obtained from the compaction curves of soil–fiber blends, the initial void ratio (e) was different for different fiber contents (f ¼ 0.25% and 0.50%). The specimens (soil–fiber blends) were statically compacted in the oedometer in five layers, each of thickness 5 mm, to ensure uniform dry density. The distribution of fiber reinforcement was random in all the specimens. Heave was allowed under a seating surcharge of 3 kPa by free inundation or by allowing water continuously into the soil specimen. After final heave (DH) was attained, the sample was compressed under incremented vertical loads till initial void ratio (e) was attained. Swell potential (S%) was reported as the ratio of the increase in thickness of the sample upon inundation (DH) to its initial thickness (H). Swelling pressure (ps) was determined as the pressure corresponding to the initial void ratio, e, of the specimens obtained from the e–log p curve.
3. Discussion of test results 3.1. Effect of fiber reinforcement on swelling behaviour Figs. 2 and 3 show the variation of heave with time for unreinforced samples in comparison with that for samples reinforced
15
Property
Value
Specific gravity Consistency properties Liquid limit (%) Plastic limit (%) Plasticity index (%) Shrinkage limit (%) Optimum moisture content, OMCa (%) Maximum dry unit weight (gd,max)a (kN/m3) Free swell index, FSI (%) USCS classificationc Cation exchange capacity (meq/100 g) Mineralogical compositionb (%)
2.72
a b c
71 30 41 12 26 15
Dry unit weight (kN/m3)
Table 1 Index properties of the expansive soil
14
13 f = 0 % (Unreinforced soil) f = 0.25 % (l/b = 15) 12
f = 0.25 % (l/b = 30)
93 CH 47.5
f = 0.25 % (l/b = 45) f = 0.50 % (l/b = 15) f = 0.50 % (l/b = 45)
Montmorillonite: 48–50, quartz: 30–32, calcite: 15–16 and antase: 1–2
Standard Proctor compaction. XRD spectra. Expressed in milliequivalents per 100 g.
11
0
10
20
30
40
Water content (%) Fig. 1. Compaction characteristics of soil with and without fibers.
50
B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
1.4
1 0.8
0.06 Unreinforced soil
f = 0.25 %
l/b = 15
0.05
l/b = 45
0.6 0.4 0.2 0 0.1
f = 0.50 %
l/b = 30
Heave (mm)
Heave (mm)
1.2
75
1
10
100
1000
0.04 0.03 0.02 0.01
10000
Time (minutes)
0
Fig. 2. Variation of heave with time (f ¼ 0.25%).
0
10
20
30
40
50
Aspect ratio with fiber contents of 0.25% and 0.50% at varying aspect ratio (l/ b ¼ 15, 30 and 45). The final heave of unreinforced soil sample was 1.35 mm, reached in 3 days. On being reinforced with polypropylene fiber, heave decreased at all aspect ratios, indicating that fiber reinforcement was effective in controlling heave. The reinforced samples also continued by heaving up to 3 days. The fiberreinforced samples exhibited higher reduction in heave at lower aspect ratios of 15 and 30 than did those at higher aspect ratios (see Figs. 2 and 3). For example, when the fiber content was 0.25%, the aspect ratios of 15 and 30, respectively, resulted in a heave of 0.55 mm and 0.50 mm, whereas the aspect ratio of 45 resulted in a heave of 1.2 mm. In the case of fiber-reinforced swelling clays, amount of swelling is reduced through: (i) replacement of swelling clay by non-swelling fiber, and (ii) resistance offered by the fiber to swelling which depends upon the clay–fiber contact area. At lower lengths or aspect ratios of the fiber, the contact between clay and fiber would be more effective resulting in higher resistance to swelling. At higher lengths or aspect ratios of fiber, the fiber would be subjected to bending and folding, which reduces the contact area between clay and fiber, leading to lesser resistance to swelling. Further, at higher aspect ratios, compaction efficiency could decrease. Hence, heave reduction was less at higher aspect ratios for both the fiber contents of 0.25% and 0.50%. Fig. 4 shows the variation of heave (mm) with aspect ratio for varying fiber content (%). Heave decreased with increasing fiber content for all aspect ratios. Heave reduction was rapid up to an
Fig. 4. Variation of heave with aspect ratio.
aspect ratio of 15 for both the fiber contents. At the aspect ratio of 45, reduction in heave was more for the fiber content of 0.50%. This shows that lower aspect ratios were more effective, because heave could be significantly reduced even at smaller fiber dosages. This was because of the effective clay–fiber contact area at low aspect ratios. Moreover, the heave at the aspect ratio of 45 (at f ¼ 0.50%) was higher than that at the aspect ratio of 15 (at f ¼ 0.50%). This establishes the efficacy of lower aspect ratios. Figs. 5 and 6 show the e–log p curves for the unreinforced sample and for samples reinforced with fibers of varying aspect ratio (l/b ¼ 15, 30 and 45). The data shown in the figures pertain to the fiber contents of 0.25% and 0.50%. As all the samples were compacted at the void ratios corresponding to their respective maximum dry densities and optimum moisture contents, which varied with varying fiber content, the initial void ratios (e) of different specimens were different. The data shown in both the figures indicate that swelling pressure of the samples decreased on being reinforced with fibers. As the final heave decreased with introduction of fiber, swelling pressure (ps as shown in Figs. 5 and 6) also decreased. For example, the swelling pressure was 105 kPa for unreinforced oedometer sample, whereas the values of swelling pressure for fiber-reinforced samples at l/b ¼ 15, 30 and 45 were 90 kPa, 85 kPa and 98 kPa, respectively. As the aspect ratio of 45 resulted in a higher amount of heave compared to those of 15 and 30, the corresponding swelling pressure was also higher at 98 kPa. For the type of fibers and soil used in the present study, a marginal
1.4 Unreinforced soil
Heave (mm)
1
1.1
l/b = 15 1
l/b = 45
void ratio (e)
1.2
0.8 0.6 0.4
0.8
Unreinforced soil l/b = 15 l/b = 30 l/b = 45
0.7
0.2 0 0.1
0.9
1
10
100
1000
Time (minutes) Fig. 3. Variation of heave with time (f ¼ 0.50%).
10000
0.6
1
3
10
ps=105 kPa 100
log p Fig. 5. e–log p curves (f ¼ 0.25%).
1000
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B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
unreinforced sample, it was equal to 90 kPa, 85 kPa and 98 kPa, respectively, for fiber-reinforced samples at l/b ¼ 15, 30 and 45.
1.1
void ratio (e)
1
Acknowledgements 0.9
The authors would like to thank the reviewers for their critical review and suggestions for improving the quality of this manuscript. Thanks are also due to M/s Techfab (India), Mumbai for supplying polypropylene tape fibers.
0.8 Unreinforced soil l/b = 15
0.7
l/b = 45 0.6 1
3
References
ps=105 kPa
10
100
1000
log p Fig. 6. e–log p curves (f ¼ 0.50%).
reduction in the swelling pressure was observed by the addition of fibers. Finally, the mechanism by which discrete and randomly distributed fibers restrain swelling of expansive soil is explained with the help of soil–fiber interaction. When swelling of the soil occurs, the flexible polymeric fibers in the soil are stretched and tension in fibers resists the further swelling. Resistance offered by the fibers to swelling depends upon the soil–fiber contact area. Based on the observed results in the present study, when long fibers are mixed randomly, they tend to twist or fold. This reflects in the form of loss of effective soil–fiber contact area for restraining swelling. It can be clearly stated that the technique of fiber-reinforced soil is a very effective method and which helps usage of expansive soils available at the construction sites. However, the construction methodology of the method has not yet been well developed. One of the key issues with the fiber-reinforced soil is in achieving uniformity in mixing discrete fibers with soil. More recently Zhang et al., 2003 developed and demonstrated fiber–soil mixing process in the field for slope test sections between Shaw and Slocum in Concordia Parish in USA. With the development of the construction technology, this improvement technique will have an extensive application prospect and could be employed in many fields of geotechnical engineering (Cai et al., 2006). 4. Conclusions Swell-consolidation tests were performed on oedometer specimens reinforced with polypropylene fiber of varying dosage and aspect ratio. Effect of fiber reinforcement on swell potential and swelling pressure was studied. The chief conclusions are as follows. 1. Reinforcing expansive clay specimens with polypropylene fiber reduced heave. Heave was reduced more at lower aspect ratios than at higher aspect ratios. Reduction in swelling can be attributed to replacement of swelling clay by fiber and resistance offered by fiber to swelling through clay–fiber contact. 2. Swelling decreased with increasing fiber content (f) for all aspect ratios. Reduction in swelling was rapid up to 15 at all fiber contents. When the aspect ratio was low, even small dosages of fiber were also effective. 3. Swelling pressure (ps) also decreased in the case of fiber-reinforced samples. While the swelling pressure was 105 kPa for
Al Wahab, R.M., El-Kedrah, M.A., 1995. Using fibers to reduce tension cracks and shrink/swell in a compacted clay. In: Proceedings of Geoenvironment 2000. Geotechnical Special Publication No. 46, vol. 1. ASCE, New York, pp. 791–805. Ayyar, T.S.R., Krishnaswamy, N.R., Viswanadham, B.V.S., 1989. Geosynthetics for foundations on a swelling clay. In: Proceedings of International Workshop on Geotextiles, Bangalore, India, pp. 176–180. Cai, Y., Shi, B., Ng, C.W.W., Tang, C., 2006. Effect of polypropylene fibre and lime admixture on engineering properties of clayey soil. Engineering Geology 87 (3–4), 230–240. Chen, F.H., 1988. Foundations on Expansive Soils. Elsevier Scientific Publishing Co., Amsterdam. Cokca, E., 2001. Use of class C fly ashes for the stabilization of an expansive soil. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 127 (7), 568–573. Hunter, D., 1988. Lime-induced heave in sulphate-bearing clay soils. Journal of Geotechnical Engineering, ASCE 114 (2), 150–167. Ikizler, S.B., Aytekin, M., Nas, E., 2008. Laboratory study of expanded polystyrene (EPS) geofoam used with expansive soils. Geotextiles and Geomembranes 26 (2), 189–195. Maher, M.H., Ho, Y.C., 1994. Mechanical properties of kaolinite/fiber soil composite. Journal of Geotechnical Engineering, ASCE 120 (8), 1381–1393. Nataraj, M.S., McManis, K.L., 1997. Strength and deformation properties of soil reinforced with fibrillated fibers. Geosynthetics International Journal 4 (1), 65–79. Nelson, D.J., Miller, J.D., 1992. Expansive Soils: Problems and Practice in Foundation and Pavement Engineering. John Wiley & Sons, New York. Phanikumar, B.R., 1997. A Study of Swelling Characteristics of Granular Pile Anchorfoundation System in Expansive Soils. PhD thesis, Jawaharlal Nehru Technological University, Hyderabad, India. Phanikumar, B.R., Sharma, R.S., 2004. Effect of fly ash on engineering properties of expansive soils. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 130 (7), 764–767. Phanikumar, B.R., Sharma, R.S., Srirama Rao, A.S., Madhav, M.R., 2004. Granular pileanchor foundation (GPAF) system for improving the engineering behaviour of expansive clay beds. Geotechnical Testing Journal, ASTM 27 (3), 279–287. Punthutaecha, K., Puppala, A.J., Vanapalli, S.K., Inyang, H., 2006. Volume change behaviours of expansive soils stabilized with recycled ashes and fibers. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 18 (2), 295–306. Puppala, A., Musenda, C., 2000. Effects of fiber reinforcement on strength and volume change in expansive soils. Transportation Research Record 1736, 134–140. Seda, J.H., Lee, J.C., Carraro, J.A., 2007. Beneficial use of waste tire rubber for swelling potential mitigation in expansive soils. In: Schaefer, V.R., Filz, G.M., Gallagher, P.M., Sehn, A.L., Wissmann, K.J. (Eds.), Proceedings of Geo-Denver 2007: New peaks in Geotechnics. Geotechnical Special Publication No. 172. ASCE. Sivakumar Babu, G.L., Vasudevan, A.K., Haldar, S., 2008. Numerical simulation of fiber-reinforced sand behaviour. Geotextiles and Geomembranes 26 (2), 181–188. Tang, C., Shi, B., Gao, W., Chen, F., Cai, F., 2007. Strength and mechanical behaviour of short polypropylene fiber reinforced and cement stabilized clayey soil. Geotextiles and Geomembranes 25 (3), 194–202. Vessely, M.J., Wu, J.T.H., 2002. Feasibility of geosynthetic inclusions for reducing swelling of expansive soils. Transportation Research Record 1787, 42–51. Viswanadham, B.V.S., 1989. Bearing Capacity of Geosynthetic Reinforced Foundation on a Swelling Clay. Master of Technology dissertation, Indian Institute of Technology Madras, India. Zhang, Z., Farrag, K., Morvant, M., 2003. Evaluation of the Effect of Synthetic Fibers and Nonwoven Geotextile Reinforcement on the Stability of Heavy Clay Embankments. FHWA/LA.03/373. Louisiana Transportation Research Center, Baton Rouge, LA, USA, pp. 7–19. Ziegler, S., Leshchinsky, D., Ling, H.I., Perry, E.B., 1998. Effect of short polymeric fibers on crack development in clays. Soils and Foundations 38 (1), 247–253.
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Technical Note
Swelling behaviour of a geofiber-reinforced expansive soil B.V.S. Viswanadham a, *, B.R. Phanikumar b,1, Rahul V. Mukherjee b,1 a b
Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Department of Civil Engineering, Vellore Institute of Technology, Vellore 632014, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2008 Received in revised form 14 May 2008 Accepted 30 June 2008 Available online 15 August 2008
This paper reports the results of laboratory study performed on expansive soil reinforced with geofibers and demonstrates that discrete and randomly distributed geofibers are useful in restraining the swelling tendency of expansive soils. Swelling characteristics of remoulded expansive soil specimens reinforced with varying fiber content (f ¼ 0.25% and 0.50%) and aspect ratio (l/b ¼ 15, 30 and 45) were studied. Onedimensional swell-consolidation tests were conducted on oedometer specimens. Reduction in heave and swelling pressure was the maximum at low aspect ratios at both the fiber contents of 0.25% and 0.50%. Finally, the mechanism by which discrete and randomly distributed fibers restrain swelling of expansive soil is explained with the help of soil–fiber interaction. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Expansive soil Fibers Fiber reinforcement Swelling Swell-consolidation test
1. Introduction Expansive clays swell upon absorption of water and shrink upon evaporation of water (Chen, 1988; Nelson and Miller, 1992). Hence, civil engineering structures found in these soils are severely damaged. The annual cost of damage is estimated at £150 million in the UK, $1000 million in the USA and many billions of pounds worldwide. To ameliorate the problems posed by expansive soils, many innovative techniques have been developed. Belled piers (Chen,1988), granular pile-anchors (Phanikumar,1997; Phanikumar et al., 2004) and chemical stabilization with lime and fly ash (Chen, 1988; Hunter, 1988; Cokca, 2001; Phanikumar and Sharma, 2004) have been suggested for mitigating heave problems. Geosynthetic inclusions (Ayyar et al.,1989; Vessely and Wu, 2002) were also found effective in reducing swelling potential of expansive soils. Very recently, Ikizler et al. (2008) have reported a potential decrease in swelling pressure as a result of the inclusion of expanded polystyrene geofoam placed between an expansive soil and a rigid wall. In recent years, discrete fibers have been added and mixed into soils to improve the strength behaviour of soil (Maher and Ho,
* Corresponding author. Tel.: þ91 22 25767344; fax: þ91 22 25767302. E-mail addresses: [email protected] (B.V.S. Viswanadham), phanikumar_ [email protected] (B.R. Phanikumar), [email protected] (R.V. Mukherjee). 1 Tel.: þ91 416 2202212. 0266-1144/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2008.06.002
1994; Al Wahab and El-Kedrah, 1995; Nataraj and McManis, 1997; Ziegler et al., 1998; Cai et al., 2006; Sivakumar Babu et al., 2008). Maher and Ho (1994) showed that the fiber reinforcement increased the shear strength and ductility of kaolinite. Tension cracking and volume change due to swell/shrink in compacted clays decreased when reinforced with polypropylene fibers (Al Wahab and El-Kedrah, 1995; Nataraj and McManis, 1997). Ziegler et al. (1998) observed that fiber inclusions increased the tensile strength. Reinforcement of expansive soils with discrete geofibers (flexible polymeric fibers) offers an alternative method to chemical stabilization techniques and other methods for reducing swelling potential. Viswanadham (1989) and Ayyar et al. (1989) have reported about the efficacy of randomly distributed coir fibers in reducing the swelling tendency of the soil. The efficacy of combination of fly ash and polypropylene fibers in reducing swelling and shrinkage characteristics was also reported (Puppala and Musenda, 2000; Punthutaecha et al., 2006; Tang et al., 2007). Puppala and Musenda (2000) and Punthutaecha et al. (2006) showed that fiber reinforcements enhanced the unconfined compressive strength and reduced the swelling potential of expansive clays. Cai et al., 2006 reported an increase in fiber content led to reduction in swelling potential of lime stabilized clayey soil. Seda et al., 2007 present about beneficial use of shredded waste tire rubber for swelling potential mitigation in expansive soils. However, the reports on the use of discrete geofibers for swelling potential mitigation in expansive soils have not been seen yet. Thus, this paper presents the efficacy of the use
74
B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
of fiber reinforcement in controlling heave of remoulded expansive soils. A laboratory study was undertaken to evaluate the feasibility of using the fibers in an expansive soil to reduce swell potential. Onedimensional swell-consolidation tests were performed on compacted mixtures of fibers and the expansive soil to assess the optimum fiber content and aspect ratio for maximum heave reduction.
Table 2 Properties of polypropylene fiber (as supplied by the manufacturer) Type and composition
Slit film and polypropylene
Width (mm) Denier Tenacity (gpda) Breaking load (N) Breaking elongation (%) Specific gravity Melting point ( C)
2 890 5.45 48.4 18 0.91 170
a
2. Experimental investigation A laboratory investigation was conducted for studying the efficacy of fiber reinforcement in arresting heave of expansive soils. Swelling behaviour of unreinforced expansive soil specimens was studied and compared with that of specimens reinforced with flexible polypropylene fiber. The experimental investigation was conducted on unreinforced expansive soil specimens and fiberreinforced specimens compacted in oedometer. Swell-consolidation tests were performed. 2.1. Test materials 2.1.1. Expansive soil The soil used in this investigation had a free swell index of 93%. The soil was collected from a depth of 1.5 m from Pune, in Maharashtra state of India. The X-ray diffraction spectra gave the following mineralogical composition – montmorillonite: 48–50%, quartz: 30–32%, calcite: 15–16% and antase: 1–2%. Table 1 shows the index properties of the soil. Based on the plasticity properties, the soil was classified as CH according to USCS classification (see Table 1). 2.1.2. Polypropylene fiber The fiber used for reinforcing the expansive soil specimens was a polypropylene fiber (840-TF15090). It had a width of 2 mm and thickness of about 0.021 mm and specific gravity of 0.91. Table 2 shows various properties of the fibers as supplied by the manufacturer (Techfab, India). The fiber content f was varied as 0.25% and 0.50% by dry weight of expansive soil. The aspect ratio l/b of the fibers was varied as 15, 30 and 45. Aspect ratio is defined as the ratio of length to width of the fiber. For example, l/b ¼ 15 implies 2 mm wide fibers of 30 mm long. Fig. 1 shows the standard Proctor compaction curves of unreinforced expansive soil and soil blended with varying fiber content (f ¼ 0.25% and 0.50%).
Grams per denier (denier is defined as a weight of 9000 m long fiber).
2.2. Tests conducted Swell-consolidation tests were conducted in the conventional oedometer of diameter 75 mm and thickness (H) 25 mm. Swell potential (S%) and swelling pressure (ps) were determined for unreinforced specimens and specimens reinforced with varying fiber content. As the specimens were prepared at the respective OMC and MDD obtained from the compaction curves of soil–fiber blends, the initial void ratio (e) was different for different fiber contents (f ¼ 0.25% and 0.50%). The specimens (soil–fiber blends) were statically compacted in the oedometer in five layers, each of thickness 5 mm, to ensure uniform dry density. The distribution of fiber reinforcement was random in all the specimens. Heave was allowed under a seating surcharge of 3 kPa by free inundation or by allowing water continuously into the soil specimen. After final heave (DH) was attained, the sample was compressed under incremented vertical loads till initial void ratio (e) was attained. Swell potential (S%) was reported as the ratio of the increase in thickness of the sample upon inundation (DH) to its initial thickness (H). Swelling pressure (ps) was determined as the pressure corresponding to the initial void ratio, e, of the specimens obtained from the e–log p curve.
3. Discussion of test results 3.1. Effect of fiber reinforcement on swelling behaviour Figs. 2 and 3 show the variation of heave with time for unreinforced samples in comparison with that for samples reinforced
15
Property
Value
Specific gravity Consistency properties Liquid limit (%) Plastic limit (%) Plasticity index (%) Shrinkage limit (%) Optimum moisture content, OMCa (%) Maximum dry unit weight (gd,max)a (kN/m3) Free swell index, FSI (%) USCS classificationc Cation exchange capacity (meq/100 g) Mineralogical compositionb (%)
2.72
a b c
71 30 41 12 26 15
Dry unit weight (kN/m3)
Table 1 Index properties of the expansive soil
14
13 f = 0 % (Unreinforced soil) f = 0.25 % (l/b = 15) 12
f = 0.25 % (l/b = 30)
93 CH 47.5
f = 0.25 % (l/b = 45) f = 0.50 % (l/b = 15) f = 0.50 % (l/b = 45)
Montmorillonite: 48–50, quartz: 30–32, calcite: 15–16 and antase: 1–2
Standard Proctor compaction. XRD spectra. Expressed in milliequivalents per 100 g.
11
0
10
20
30
40
Water content (%) Fig. 1. Compaction characteristics of soil with and without fibers.
50
B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
1.4
1 0.8
0.06 Unreinforced soil
f = 0.25 %
l/b = 15
0.05
l/b = 45
0.6 0.4 0.2 0 0.1
f = 0.50 %
l/b = 30
Heave (mm)
Heave (mm)
1.2
75
1
10
100
1000
0.04 0.03 0.02 0.01
10000
Time (minutes)
0
Fig. 2. Variation of heave with time (f ¼ 0.25%).
0
10
20
30
40
50
Aspect ratio with fiber contents of 0.25% and 0.50% at varying aspect ratio (l/ b ¼ 15, 30 and 45). The final heave of unreinforced soil sample was 1.35 mm, reached in 3 days. On being reinforced with polypropylene fiber, heave decreased at all aspect ratios, indicating that fiber reinforcement was effective in controlling heave. The reinforced samples also continued by heaving up to 3 days. The fiberreinforced samples exhibited higher reduction in heave at lower aspect ratios of 15 and 30 than did those at higher aspect ratios (see Figs. 2 and 3). For example, when the fiber content was 0.25%, the aspect ratios of 15 and 30, respectively, resulted in a heave of 0.55 mm and 0.50 mm, whereas the aspect ratio of 45 resulted in a heave of 1.2 mm. In the case of fiber-reinforced swelling clays, amount of swelling is reduced through: (i) replacement of swelling clay by non-swelling fiber, and (ii) resistance offered by the fiber to swelling which depends upon the clay–fiber contact area. At lower lengths or aspect ratios of the fiber, the contact between clay and fiber would be more effective resulting in higher resistance to swelling. At higher lengths or aspect ratios of fiber, the fiber would be subjected to bending and folding, which reduces the contact area between clay and fiber, leading to lesser resistance to swelling. Further, at higher aspect ratios, compaction efficiency could decrease. Hence, heave reduction was less at higher aspect ratios for both the fiber contents of 0.25% and 0.50%. Fig. 4 shows the variation of heave (mm) with aspect ratio for varying fiber content (%). Heave decreased with increasing fiber content for all aspect ratios. Heave reduction was rapid up to an
Fig. 4. Variation of heave with aspect ratio.
aspect ratio of 15 for both the fiber contents. At the aspect ratio of 45, reduction in heave was more for the fiber content of 0.50%. This shows that lower aspect ratios were more effective, because heave could be significantly reduced even at smaller fiber dosages. This was because of the effective clay–fiber contact area at low aspect ratios. Moreover, the heave at the aspect ratio of 45 (at f ¼ 0.50%) was higher than that at the aspect ratio of 15 (at f ¼ 0.50%). This establishes the efficacy of lower aspect ratios. Figs. 5 and 6 show the e–log p curves for the unreinforced sample and for samples reinforced with fibers of varying aspect ratio (l/b ¼ 15, 30 and 45). The data shown in the figures pertain to the fiber contents of 0.25% and 0.50%. As all the samples were compacted at the void ratios corresponding to their respective maximum dry densities and optimum moisture contents, which varied with varying fiber content, the initial void ratios (e) of different specimens were different. The data shown in both the figures indicate that swelling pressure of the samples decreased on being reinforced with fibers. As the final heave decreased with introduction of fiber, swelling pressure (ps as shown in Figs. 5 and 6) also decreased. For example, the swelling pressure was 105 kPa for unreinforced oedometer sample, whereas the values of swelling pressure for fiber-reinforced samples at l/b ¼ 15, 30 and 45 were 90 kPa, 85 kPa and 98 kPa, respectively. As the aspect ratio of 45 resulted in a higher amount of heave compared to those of 15 and 30, the corresponding swelling pressure was also higher at 98 kPa. For the type of fibers and soil used in the present study, a marginal
1.4 Unreinforced soil
Heave (mm)
1
1.1
l/b = 15 1
l/b = 45
void ratio (e)
1.2
0.8 0.6 0.4
0.8
Unreinforced soil l/b = 15 l/b = 30 l/b = 45
0.7
0.2 0 0.1
0.9
1
10
100
1000
Time (minutes) Fig. 3. Variation of heave with time (f ¼ 0.50%).
10000
0.6
1
3
10
ps=105 kPa 100
log p Fig. 5. e–log p curves (f ¼ 0.25%).
1000
76
B.V.S. Viswanadham et al. / Geotextiles and Geomembranes 27 (2009) 73–76
unreinforced sample, it was equal to 90 kPa, 85 kPa and 98 kPa, respectively, for fiber-reinforced samples at l/b ¼ 15, 30 and 45.
1.1
void ratio (e)
1
Acknowledgements 0.9
The authors would like to thank the reviewers for their critical review and suggestions for improving the quality of this manuscript. Thanks are also due to M/s Techfab (India), Mumbai for supplying polypropylene tape fibers.
0.8 Unreinforced soil l/b = 15
0.7
l/b = 45 0.6 1
3
References
ps=105 kPa
10
100
1000
log p Fig. 6. e–log p curves (f ¼ 0.50%).
reduction in the swelling pressure was observed by the addition of fibers. Finally, the mechanism by which discrete and randomly distributed fibers restrain swelling of expansive soil is explained with the help of soil–fiber interaction. When swelling of the soil occurs, the flexible polymeric fibers in the soil are stretched and tension in fibers resists the further swelling. Resistance offered by the fibers to swelling depends upon the soil–fiber contact area. Based on the observed results in the present study, when long fibers are mixed randomly, they tend to twist or fold. This reflects in the form of loss of effective soil–fiber contact area for restraining swelling. It can be clearly stated that the technique of fiber-reinforced soil is a very effective method and which helps usage of expansive soils available at the construction sites. However, the construction methodology of the method has not yet been well developed. One of the key issues with the fiber-reinforced soil is in achieving uniformity in mixing discrete fibers with soil. More recently Zhang et al., 2003 developed and demonstrated fiber–soil mixing process in the field for slope test sections between Shaw and Slocum in Concordia Parish in USA. With the development of the construction technology, this improvement technique will have an extensive application prospect and could be employed in many fields of geotechnical engineering (Cai et al., 2006). 4. Conclusions Swell-consolidation tests were performed on oedometer specimens reinforced with polypropylene fiber of varying dosage and aspect ratio. Effect of fiber reinforcement on swell potential and swelling pressure was studied. The chief conclusions are as follows. 1. Reinforcing expansive clay specimens with polypropylene fiber reduced heave. Heave was reduced more at lower aspect ratios than at higher aspect ratios. Reduction in swelling can be attributed to replacement of swelling clay by fiber and resistance offered by fiber to swelling through clay–fiber contact. 2. Swelling decreased with increasing fiber content (f) for all aspect ratios. Reduction in swelling was rapid up to 15 at all fiber contents. When the aspect ratio was low, even small dosages of fiber were also effective. 3. Swelling pressure (ps) also decreased in the case of fiber-reinforced samples. While the swelling pressure was 105 kPa for
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