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Study on the efficacy of harmful weed species Eicchornia crassipes for soil reinforcement
Ecological Engineering 85 (2015) 218–222
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
Study on the efficacy of harmful weed species Eicchornia crassipes for soil reinforcement S. Bordoloi, S.K. Yamsani, A. Garg ∗ , S. Sreedeep, S. Borah Department of Civil Engineering, Indian Institute of Technology, Guwahati, India
a r t i c l e
i n f o
Article history: Received 8 August 2015 Accepted 30 September 2015 Available online 18 October 2015 Keywords: Natural fiber Eicchornia crassipes Compressive strength Moisture content Fiber content
a b s t r a c t Growing awareness of sustainability in construction has increased attention toward increase in use of natural fibers for soil reinforcement applications. Researchers have explored the utility of natural limited life fibers such as jute, reed and sisal for soil reinforcement. In this study, an attempt was made to demonstrate the use of local weed named water hyacinth (Eicchornia crassipes) as soil reinforcement. A series of unconfined compression strength (UCS) tests was conducted on silty sand reinforced with randomly distributed fiber. These series of tests aim to study influence of different fiber content, soil density and moisture content. The stress–strain response of fiber reinforced soil shows the increase in post peak strength and ductility. This was mainly attributed due to presence of cellulose content. The effect of soil density on increased strength due to inclusion of fiber is significant in case of lower moisture content. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Reinforcement of soil helps to enhance the engineering characteristics of soil; such as shear strength and compressibility thereby increasing the bearing capacity, reduce settlements and lateral deformation in various earthen structures (Vidal, 1969). The use of green technology (i.e., vegetation) for improving soil strength has also received significant attention (Genet et al., 2008; Stokes et al., 2009; Garg and Ng, 2015; Garg et al., 2015a,b,c). Randomly distributed fiber-reinforced soil (RDFS) is one of the diverse reinforcing techniques, in which fibers of desired dimensions and quantity are mixed randomly in soil (Hejazi et al., 2012). Due to growing concern of climate change, cost and easy availability, there has been increase in use of natural fibers as a potential for reinforcing soil (Maher and Ho, 1994; Santoni et al., 2001; Gosavi et al., 2004; Dasaka and Sumesh, 2011; Hejazi et al., 2012; Güllü and Khudir, 2014). Most of studies on using natural fibers as reinforcement focused on utilizing natural fibers extracted from jute, coir and bamboo as soil reinforcement material. On the other hand, relatively very few studies have been conducted to utilize natural fiber extracted from a harmful weed, water hyacinth (WH). WH is an aquatic weed, that causes several problems such as physical interference in fishing,
navigation, irrigation systems, increased sedimentation and blockage in canals due to its fast growth (17.5 metric tons/hectare/day; Shoeb and Singh, 2000). It also invades fresh water habitats and competes almost all neighboring species growing in their vicinity thereby decreasing biodiversity (Crafter et al., 1992). The advantages of using WH as fiber lie in its biochemical composition. The cellulose content present inside WH cells is mainly responsible in providing strength to the fiber. The lignin act minimizes the biodegradation that is caused by microorganisms (Patel et al., 1993). Hemicelluloses are mainly responsible for moisture sorption and biodegradation (Rowell and Stout, 1998; Malik, 2007; Methacanon et al., 2010). Thus, exploring use of fiber from WH will not only help to enhance soil reinforcement but may add value to environment by impeding growth and spread of such invasive species. This study explores the utility of WH fiber as reinforcement material for soil. A series of experimental studies are conducted to investigate unconfined compressive strength (UCS) of reinforced soil with WH under different fiber content (%), soil density and moisture. 2. Material and methods 2.1. Composition of water hyacinth fiber
∗ Corresponding author. E-mail addresses: [email protected] (S. Bordoloi), [email protected] (S.K. Yamsani), [email protected] (A. Garg), [email protected] (S. Sreedeep), [email protected] (S. Borah). http://dx.doi.org/10.1016/j.ecoleng.2015.09.082 0925-8574/© 2015 Elsevier B.V. All rights reserved.
Cellulose, hemicellulose, lignin and ash content of the WH fibers were determined using methods suggested by Jenkins (1930), Goering and Van Soest (1970), TAPPIT222om-88 (TAPPI
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219
Fig. 1. Pictorial description of the water hyacinth fiber in comparison with hair strand.
Test Methods, 1996) and ASTM E1755-01 (2007), respectively. Based on these testing, cellulose, hemicellulose, lignin and ash content constitutes 45.58 + 0.63%, 21.20 + 0.51, 11.13 + 0.16 and 11.20%, respectively. Moisture content of water hyacinth determined using oven drying (at 1050 ◦ C for 24 h) was found to be 11.87%. The average breaking tensile strength of the fiber was determined according to IS-1670-1991 and found to be 250 N. This tensile strength of the WH fibers is comparable to those of similar natural fibers such as coir and sisal reported in the literature (Prabakar and Sridhar, 2002; Dasaka and Sumesh, 2011). The specific gravity of the fibers was determined as per IS-2720-Part 3-1980 and was found to be 0.67. 2.2. Morphology of water hyacinth fiber The morphology of the dried WH fiber (Fig. 1) was investigated using FE-SEM (Field emission scanning electron microscope) using Zeiss (Sigma model) instrumentation available at Central Instrumentation Facility at Indian Institute of Technology Guwahati. The FE-SEM images were obtained at two magnifications highlighting the cross section of the fiber sample. Fig. 2(a) and (b) demonstrates that hollow cavities and complex interweave of tissues in WH fiber respectively. The lightweight of material is mainly because of this structural framework.
Fig. 2. FE-SEM images of water hyacinth fiber showing: (a) hollow cavity like structure and (b) complex interweave of tissues.
2.3. Soil properties
Table 1 Engineering properties of red soil.
The soil used in this study was red soil, commonly found in North Eastern part of India. Grain size distribution of the soil was determined using procedures prescribed in IS-2720-Part 4-1985. It is constituted mainly of coarse sand (22%), medium sand (35%) and fine sand (28%). The silt and clay fraction of the soil are 9% and 6%, respectively. Consistency limit of the soil determined according to IS-2720-Part 5 (1985) gave liquid limit and plastic limit as 40.50% and 24.81%, respectively. The USCS classification of soil is silty sand (ASTM, D2487-11). Standard Proctor’s light compaction technique was used to determine its maximum dry density (MDD) and optimum moisture content (OMC) according to procedures prescribed in Indian Standard code IS-2720 part-7 (1980). MDD and OMC were found to be 1.72 g/cc and 16.92%, respectively. Table 1 summarizes the index and engineering properties of the soil. 2.4. Test program and specimen preparation UCS tests were conducted on both unreinforced and reinforced soil–WH composite at different compaction states corresponding
Soil properties
Values
Specific gravity
2.69
Grain size analysis Coarse sand (4.75 mm–2 mm) Medium Sand (2 mm–0.425 mm) Fine sand (0.425 mm–0.075 mm) Silt (0.075 mm–0.002 mm) Clay (<0.002 mm)
22.00 (%) 34.64 (%) 28.04 (%) 9.83 (%) 6.39 (%)
Consistency limits Liquid limit Plastic limit Shrinkage index Plasticity index
40.50 (%) 24.81 (%) 23.30 (%) 15.69 (%)
Compaction study Optimum moisture content Maximum dry density
16.92 (%) 1.70 (g/cc)
Shear parameters Cohesion Angle of internal friction
16.90 (kPa) 14.20◦
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S. Bordoloi et al. / Ecological Engineering 85 (2015) 218–222
Fig. 4. Stress–strain response of WH reinforced soil–fiber composite (soil + WH(x %)) where x is the percentage of water hyacinth reinforced with respect to dry soil sample.
3. Results and discussion 3.1. Effect of water hyacinth fiber reinforcement on stress–strain response of soil
Fig. 3. Compaction mold.
to three soil densities (0.95 MDD, MDD and 1.05 MDD) and three moisture contents (OMC, OMC −5% and OMC +5%.). In addition, the influence of variation in % fiber content (as 0.5%, 0.75% and 1% by dry weight of soil) on UCS was also investigated. The fiber percentage was restricted to 1% as the fibers tend to stick to each other while mixing thereby forming pockets of low density. For each case, tests were repeated three times (total 108 tests) to check any variability in observed UCS. All UCS tests were conducted at a constant strain rate of 1.25 mm/min as suggested in IS-2720 part-10 (1991). Before preparation of soil specimen with WH fiber, careful procedures were followed to ensure minimization of variability in selected fiber from WH plant. The WH plants used in this study were procured from the same water body to avoid uncertainties related to genetic variation. The plants were air dried for three days until there was no significant change in moisture content (Prabakar and Sridhar, 2002). The stem of these dried plants were separated from the roots and cut into fibers of required dimension (length 28 mm and mean diameter of 0.4 mm). The amount of fibers to be added were weighed and dry mixed uniformly using RDFS technique with an oven dried soil sample The required amount of water (according to test program) was added to the soil–fiber mixture. The mixture was sealed inside a plastic bag and kept inside a desiccator for 24 h for uniform distribution of water. After that, the samples were compacted by static compaction in a specially prepared mold (Fig. 3), where compaction can be applied in opposite direction.
Fig. 4 shows the comparison of stress–strain response between unreinforced and reinforced soil fiber composite (compacted at OMC and MDD). In addition, the influence of variation of fiber content on stress–strain response is also shown. It can be observed from the figure that there is a visible increase in the peak UCS of reinforced soil. The unreinforced soil failed abruptly after attaining peak strength followed by a sharp decline of strength thereafter. However, higher peak strength as well as post peak strength (with no abrupt failure) was observed in fiber reinforced soil. This is attributed due to presence of high cellulose content in WH fiber. The difference between post peak strength and peak strength is quite less for reinforced soil as compared to unreinforced soil. This indicates that the ductility behavior of the soil has increased due to addition of WH fiber. Similar stress–strain response has been witnessed in the case of sand reinforced with polypropylene fibers (Yetimoglu and Salbas, 2003). This behavior can be attributed to rearrangement of soil particles and its interaction with adjoining fiber at the plane of failure. After attaining peak strength, as the strain level increases, rearrangement or rotation of particles will mobilize the tensile strength of the interlocked fibers gradually. This in turn can resist the applied shear forces along the failure plane (Li et al., 2005). 3.2. Comparison of peak unconfined compressive strength between unreinforced soil and reinforced soil fiber composite Fig. 5(a)–(c) shows the mean unconfined compressive strengths (from three repeated tests for each case) of soil samples (both reinforced and unreinforced) at the selected compaction states for 0.5%, 0.75% and 1% fiber content, respectively. Generally, the results portray an increase in soil strength with increase in density of the soil–fiber composite as well as unreinforced soil. This is probably because of higher interlocking between soil particles at a denser state. As observed from the figures, the soil samples compacted at lower moisture content exhibited higher strength as compared to soil with comparatively higher moisture content. This can be attributed to the increase in strength associated with increase in soil suction (lower moisture content) (Khan et al., 2014). Fig. 6 depicts the effect of WH fiber reinforcement on the
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Fig. 6. Bar chart comparison of the tested samples (soil + WH(x %)) where x is the percentage of water hyacinth reinforced with respect to dry soil sample.
(Dasaka and Sumesh, 2011) and also synthetic polypropylene fiber (Cai et al., 2006). 4. Summary and conclusions The present study explores the utility of a harmful weed Eicchornia crassipes, commonly termed as water hyacinth (WH) for application of soil reinforcement. UCS tests were conducted to analyze unconfined compressive strength under different soil densities, soil water contents and fiber contents. Tests were also repeated (three in each case) to discuss any variability in observed UCS. Based on the results and discussion, following conclusions can be drawn.
Fig. 5. (a) 0.5% WH reinforced soil–fiber composite, (b) 0.75% WH reinforced soil–fiber composite and (c) 1% WH reinforced soil–fiber composite.
mean unconfined compressive strength (with standard deviation) of unreinforced soil at different dry densities and moisture content. It can be observed that optimum compaction state (corresponding to maximum UCS) is found to be of soil compacted at 1.05 MDD and OMC-5%. Inclusion of WH fibers resulted in increase in strength for all moisture contents in comparison to unreinforced soil. However, there was reduction in strength of soil corresponding to 1% fiber content. The maximum strength values for fiber content of 1% were found to be between 0.5% and 0.75%. Based on this observation, this study recommends an optimum WH fiber content of 0.5–0.75% by dry weight of the soil for soil reinforcement. The obtained range of unconfined compressive strength using WH fiber is comparable to studies related to inclusion of natural coir fiber
1) The unconfined compressive strength of the unreinforced soil was observed to increase with the addition of WH fibers, attributed to the significant amount of cellulose present in the fiber. 2) Inclusion of WH fiber modified the brittle behavior of soil to ductile behavior of soil–fiber composite. The reinforced soil composite showcases a low drop in post peak strength compared to that of unreinforced soil. 3) At particular moisture content, the increase in soil density increases the unconfined compressive strength. This effect of density is profoundly significant in case of lower moisture content soil–fiber composite. Out of the selected compaction states carried out in the study, soil compacted at OMC-5% and 1.05 MDD showed highest unconfined compressive strength. 4) All percentage inclusion showed increase in the compressive strength as compared to unreinforced state of the soil. However, the strength decreased as the WH fiber content was more than the optimum content. 5) The study recommends optimum WH fiber content as 0.5–0.75% for mobilizing maximum strength corresponding to any compaction state. Acknowledgement The authors would like to thank the department of Central Instrumentation Facility, IIT Guwahati, for providing support in the FE-SEM study of the water hyacinth fiber. References ASTM D2487-11, 2011. Standard Practice for Classification of Soils for Engineering Purpose (Unified Soil Classification System).
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ASTM E1755-01, 2007. Standard method for the determination of ash in biomass. 2003 Annual Book of ASTM Standards, vol. 11.05. ASTM International, Philadelphia, PA. Cai, Y., Shi, B., Ng, C.W., Tang, C.S., 2006. Effect of polypropylene fibre and lime admixture on engineering properties of clayey soil. Eng. Geol. 87 (3), 230–240. Crafter, S.A., Njuguna, S.G., Howard, G.W., 1992. Wetlands of Kenya. In: Proceedings of the KWWG Seminar on Wetlands of Kenya, National Museums of Kenya, 3–5 July 1991, Nairobi, Kenya. Dasaka, S.M., Sumesh, K.S., 2011. Effect of coir fiber on the stress–strain behavior of a reconstituted fine-grained soil. J. Nat. Fibers 8 (3), 189–204. Garg, A., Ng, C.W.W., 2015. Investigation of soil density effect on suction induced due to root water uptake by Schefflera heptaphylla. J. Plant Nutr. Soil Sci., http:// dx.doi.org/10.1002/jpln.201400265. Garg, A., Leung, A.K., Ng, C.W.W., 2015a. Comparisons of soil suction induced by evapotranspiration and transpiration of S. heptaphylla. Can. Geotech. J., http:// dx.doi.org/10.1139/cgj-2014-0425. Garg, A., Coo, J.L., Ng, C.W.W., 2015b. Field study on influence of root characteristics on suction distributions in slopes vegetated with Cynodon dactylon and Schefflera heptaphylla. Earth Surf. Process. Landf., http://dx.doi.org/10.1002/esp.3743. Garg, A., Leung, A.K., Ng, C.W.W., 2015c. Transpiration reduction and root distribution functions for a non-crop species Schefflera heptaphylla. Catena 135, 78–82. Genet, M., Kokutse, N., Stokes, A., Fourcaud, T., Cai, X., Ji, J., Mickovski, S., 2008. Root reinforcement in plantations of Cryptomeria japonica D. Don: effect of tree age and stand structure on slope stability. For. Ecol. Manag. 256 (8), 1517–1526. Goering, H.K., Van Soest, P.J., 1970. Forage fiber analyses (apparatus, reagents, procedures, and some applications). In: USDA Agricultural Handbook. Gosavi, M., Patil, K., 2004. Improvement of properties of black cotton soil subgrade through synthetic reinforcement. J. Inst. Eng. (India) CV: Civil Eng. Div. 84, 257–262. Güllü, H., Khudir, A., 2014. Effect of freeze–thaw cycles on unconfined compressive strength of fine-grained soil treated with jute fiber, steel fiber and lime. Cold Reg. Sci. Technol. 106, 55–65. Hejazi, S.M., Sheikhzadeh, M., Abtahi, S.M., Zadhoush, A., 2012. A simple review of soil reinforcement by using natural and synthetic fibers. Constr. Build. Mater. 30, 100–116. IS-1670, 1991. Determination of Breaking Load and Elongation at Break of Single Strand. Textiles and Yarns. IS-2720-(Part 3), 1980. Methods of Test for Soils. Determination of Specific Gravity. Bureau of Indian Standards Publications, New Delhi.
IS-2720 (Part 4), 1985a. Methods of Test for Soils, Grain Size Analysis. Bureau of Indian Standards Publications, New Delhi. IS-2720 (Part 5), 1985b. Determination of Liquid and Plastic Limit. Bureau of Indian Standards Publications, New Delhi. IS-2720-(Part7), 1980. Determination of Water Content-Dry Density Relation Using Light Compaction. Bureau of Indian Standards Publications, New Delhi. IS-2720 (Part 10), 1991. Determination of Unconfined Compressive Strength. Bureau of Indian Standards Publications, New Delhi. Jenkins, S.H., 1930. The determination of cellulose in straws. Biochem. J. 24 (5), 1428. Khan, F.S., Azam, S., Raghunandan, M.E., Clark, R., 2014. Compressive strength of compacted clay–sand mixes. Adv. Mater. Sci. Eng. 2014. Li, Y., Mai, Y.W., Ye, L., 2005. Effects of fibre surface treatment on fracture-mechanical properties of sisal-fibre composites. Compos. Interface 12 (1–2), 141–163. Malik, A., 2007. Environmental challenge vis a vis opportunity: the case of water hyacinth. Environ. Int. 33 (1), 122–138. Maher, M.H., Ho, Y.C., 1994. Mechanical properties of kaolinite/fiber soil composite. J. Geotech. Eng. 120 (8), 1381–1393. Methacanon, P., Weerawatsophon, U., Sumransin, N., Prahsarn, C., Bergado, D.T., 2010. Properties and potential application of the selected natural fibers as limited life geotextiles. Carbohydr. Polym. 82 (4), 1090–1096. Patel, V.B., Patel, A.R., Patel, M.C., Madamwar, D.B., 1993. Effect of metals on anaerobic digestion of water hyacinth-cattle dung. Appl. Biochem. Biotechnol. 43 (1), 45–50. Prabakar, J., Sridhar, R.S., 2002. Effect of random inclusion of sisal fibre on strength behaviour of soil. Constr. Build. Mater. 16 (2), 123–131. Rowell, R.M., Stout, H.P., 1998. Jute and Kenaf. Handbook of Fiber Chemistry., pp. 466–502. Santoni, R.L., Tingle, J.S., Webster, S.L., 2001. Engineering properties of sand–fiber mixtures for road construction. J. Geotech. Geoenviron. 127 (3), 258–268. Stokes, A., Atger, C., Bengough, A.G., Fourcaud, T., Sidle, R.C., 2009. Desirable plant root traits for protecting natural and engineered slopes against landslides. Plant Soil 324 (1–2), 1–30. Shoeb, F., Singh, H.J., 2000. Kinetic studies of biogas evolved from water hyacinth. In: Proceedings of Agroenviron. TAPPI Test Methods, 1996. TAPPI T222 om-88 klason lignin. Vidal, H., 1969. The principle of reinforced earth. Highw. Res. Rec. 282. Yetimoglu, T., Salbas, O., 2003. A study on shear strength of sands reinforced with randomly distributed discrete fibers. Geotext. Geomembr. 21 (2), 103–110.
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
Study on the efficacy of harmful weed species Eicchornia crassipes for soil reinforcement S. Bordoloi, S.K. Yamsani, A. Garg ∗ , S. Sreedeep, S. Borah Department of Civil Engineering, Indian Institute of Technology, Guwahati, India
a r t i c l e
i n f o
Article history: Received 8 August 2015 Accepted 30 September 2015 Available online 18 October 2015 Keywords: Natural fiber Eicchornia crassipes Compressive strength Moisture content Fiber content
a b s t r a c t Growing awareness of sustainability in construction has increased attention toward increase in use of natural fibers for soil reinforcement applications. Researchers have explored the utility of natural limited life fibers such as jute, reed and sisal for soil reinforcement. In this study, an attempt was made to demonstrate the use of local weed named water hyacinth (Eicchornia crassipes) as soil reinforcement. A series of unconfined compression strength (UCS) tests was conducted on silty sand reinforced with randomly distributed fiber. These series of tests aim to study influence of different fiber content, soil density and moisture content. The stress–strain response of fiber reinforced soil shows the increase in post peak strength and ductility. This was mainly attributed due to presence of cellulose content. The effect of soil density on increased strength due to inclusion of fiber is significant in case of lower moisture content. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Reinforcement of soil helps to enhance the engineering characteristics of soil; such as shear strength and compressibility thereby increasing the bearing capacity, reduce settlements and lateral deformation in various earthen structures (Vidal, 1969). The use of green technology (i.e., vegetation) for improving soil strength has also received significant attention (Genet et al., 2008; Stokes et al., 2009; Garg and Ng, 2015; Garg et al., 2015a,b,c). Randomly distributed fiber-reinforced soil (RDFS) is one of the diverse reinforcing techniques, in which fibers of desired dimensions and quantity are mixed randomly in soil (Hejazi et al., 2012). Due to growing concern of climate change, cost and easy availability, there has been increase in use of natural fibers as a potential for reinforcing soil (Maher and Ho, 1994; Santoni et al., 2001; Gosavi et al., 2004; Dasaka and Sumesh, 2011; Hejazi et al., 2012; Güllü and Khudir, 2014). Most of studies on using natural fibers as reinforcement focused on utilizing natural fibers extracted from jute, coir and bamboo as soil reinforcement material. On the other hand, relatively very few studies have been conducted to utilize natural fiber extracted from a harmful weed, water hyacinth (WH). WH is an aquatic weed, that causes several problems such as physical interference in fishing,
navigation, irrigation systems, increased sedimentation and blockage in canals due to its fast growth (17.5 metric tons/hectare/day; Shoeb and Singh, 2000). It also invades fresh water habitats and competes almost all neighboring species growing in their vicinity thereby decreasing biodiversity (Crafter et al., 1992). The advantages of using WH as fiber lie in its biochemical composition. The cellulose content present inside WH cells is mainly responsible in providing strength to the fiber. The lignin act minimizes the biodegradation that is caused by microorganisms (Patel et al., 1993). Hemicelluloses are mainly responsible for moisture sorption and biodegradation (Rowell and Stout, 1998; Malik, 2007; Methacanon et al., 2010). Thus, exploring use of fiber from WH will not only help to enhance soil reinforcement but may add value to environment by impeding growth and spread of such invasive species. This study explores the utility of WH fiber as reinforcement material for soil. A series of experimental studies are conducted to investigate unconfined compressive strength (UCS) of reinforced soil with WH under different fiber content (%), soil density and moisture. 2. Material and methods 2.1. Composition of water hyacinth fiber
∗ Corresponding author. E-mail addresses: [email protected] (S. Bordoloi), [email protected] (S.K. Yamsani), [email protected] (A. Garg), [email protected] (S. Sreedeep), [email protected] (S. Borah). http://dx.doi.org/10.1016/j.ecoleng.2015.09.082 0925-8574/© 2015 Elsevier B.V. All rights reserved.
Cellulose, hemicellulose, lignin and ash content of the WH fibers were determined using methods suggested by Jenkins (1930), Goering and Van Soest (1970), TAPPIT222om-88 (TAPPI
S. Bordoloi et al. / Ecological Engineering 85 (2015) 218–222
219
Fig. 1. Pictorial description of the water hyacinth fiber in comparison with hair strand.
Test Methods, 1996) and ASTM E1755-01 (2007), respectively. Based on these testing, cellulose, hemicellulose, lignin and ash content constitutes 45.58 + 0.63%, 21.20 + 0.51, 11.13 + 0.16 and 11.20%, respectively. Moisture content of water hyacinth determined using oven drying (at 1050 ◦ C for 24 h) was found to be 11.87%. The average breaking tensile strength of the fiber was determined according to IS-1670-1991 and found to be 250 N. This tensile strength of the WH fibers is comparable to those of similar natural fibers such as coir and sisal reported in the literature (Prabakar and Sridhar, 2002; Dasaka and Sumesh, 2011). The specific gravity of the fibers was determined as per IS-2720-Part 3-1980 and was found to be 0.67. 2.2. Morphology of water hyacinth fiber The morphology of the dried WH fiber (Fig. 1) was investigated using FE-SEM (Field emission scanning electron microscope) using Zeiss (Sigma model) instrumentation available at Central Instrumentation Facility at Indian Institute of Technology Guwahati. The FE-SEM images were obtained at two magnifications highlighting the cross section of the fiber sample. Fig. 2(a) and (b) demonstrates that hollow cavities and complex interweave of tissues in WH fiber respectively. The lightweight of material is mainly because of this structural framework.
Fig. 2. FE-SEM images of water hyacinth fiber showing: (a) hollow cavity like structure and (b) complex interweave of tissues.
2.3. Soil properties
Table 1 Engineering properties of red soil.
The soil used in this study was red soil, commonly found in North Eastern part of India. Grain size distribution of the soil was determined using procedures prescribed in IS-2720-Part 4-1985. It is constituted mainly of coarse sand (22%), medium sand (35%) and fine sand (28%). The silt and clay fraction of the soil are 9% and 6%, respectively. Consistency limit of the soil determined according to IS-2720-Part 5 (1985) gave liquid limit and plastic limit as 40.50% and 24.81%, respectively. The USCS classification of soil is silty sand (ASTM, D2487-11). Standard Proctor’s light compaction technique was used to determine its maximum dry density (MDD) and optimum moisture content (OMC) according to procedures prescribed in Indian Standard code IS-2720 part-7 (1980). MDD and OMC were found to be 1.72 g/cc and 16.92%, respectively. Table 1 summarizes the index and engineering properties of the soil. 2.4. Test program and specimen preparation UCS tests were conducted on both unreinforced and reinforced soil–WH composite at different compaction states corresponding
Soil properties
Values
Specific gravity
2.69
Grain size analysis Coarse sand (4.75 mm–2 mm) Medium Sand (2 mm–0.425 mm) Fine sand (0.425 mm–0.075 mm) Silt (0.075 mm–0.002 mm) Clay (<0.002 mm)
22.00 (%) 34.64 (%) 28.04 (%) 9.83 (%) 6.39 (%)
Consistency limits Liquid limit Plastic limit Shrinkage index Plasticity index
40.50 (%) 24.81 (%) 23.30 (%) 15.69 (%)
Compaction study Optimum moisture content Maximum dry density
16.92 (%) 1.70 (g/cc)
Shear parameters Cohesion Angle of internal friction
16.90 (kPa) 14.20◦
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Fig. 4. Stress–strain response of WH reinforced soil–fiber composite (soil + WH(x %)) where x is the percentage of water hyacinth reinforced with respect to dry soil sample.
3. Results and discussion 3.1. Effect of water hyacinth fiber reinforcement on stress–strain response of soil
Fig. 3. Compaction mold.
to three soil densities (0.95 MDD, MDD and 1.05 MDD) and three moisture contents (OMC, OMC −5% and OMC +5%.). In addition, the influence of variation in % fiber content (as 0.5%, 0.75% and 1% by dry weight of soil) on UCS was also investigated. The fiber percentage was restricted to 1% as the fibers tend to stick to each other while mixing thereby forming pockets of low density. For each case, tests were repeated three times (total 108 tests) to check any variability in observed UCS. All UCS tests were conducted at a constant strain rate of 1.25 mm/min as suggested in IS-2720 part-10 (1991). Before preparation of soil specimen with WH fiber, careful procedures were followed to ensure minimization of variability in selected fiber from WH plant. The WH plants used in this study were procured from the same water body to avoid uncertainties related to genetic variation. The plants were air dried for three days until there was no significant change in moisture content (Prabakar and Sridhar, 2002). The stem of these dried plants were separated from the roots and cut into fibers of required dimension (length 28 mm and mean diameter of 0.4 mm). The amount of fibers to be added were weighed and dry mixed uniformly using RDFS technique with an oven dried soil sample The required amount of water (according to test program) was added to the soil–fiber mixture. The mixture was sealed inside a plastic bag and kept inside a desiccator for 24 h for uniform distribution of water. After that, the samples were compacted by static compaction in a specially prepared mold (Fig. 3), where compaction can be applied in opposite direction.
Fig. 4 shows the comparison of stress–strain response between unreinforced and reinforced soil fiber composite (compacted at OMC and MDD). In addition, the influence of variation of fiber content on stress–strain response is also shown. It can be observed from the figure that there is a visible increase in the peak UCS of reinforced soil. The unreinforced soil failed abruptly after attaining peak strength followed by a sharp decline of strength thereafter. However, higher peak strength as well as post peak strength (with no abrupt failure) was observed in fiber reinforced soil. This is attributed due to presence of high cellulose content in WH fiber. The difference between post peak strength and peak strength is quite less for reinforced soil as compared to unreinforced soil. This indicates that the ductility behavior of the soil has increased due to addition of WH fiber. Similar stress–strain response has been witnessed in the case of sand reinforced with polypropylene fibers (Yetimoglu and Salbas, 2003). This behavior can be attributed to rearrangement of soil particles and its interaction with adjoining fiber at the plane of failure. After attaining peak strength, as the strain level increases, rearrangement or rotation of particles will mobilize the tensile strength of the interlocked fibers gradually. This in turn can resist the applied shear forces along the failure plane (Li et al., 2005). 3.2. Comparison of peak unconfined compressive strength between unreinforced soil and reinforced soil fiber composite Fig. 5(a)–(c) shows the mean unconfined compressive strengths (from three repeated tests for each case) of soil samples (both reinforced and unreinforced) at the selected compaction states for 0.5%, 0.75% and 1% fiber content, respectively. Generally, the results portray an increase in soil strength with increase in density of the soil–fiber composite as well as unreinforced soil. This is probably because of higher interlocking between soil particles at a denser state. As observed from the figures, the soil samples compacted at lower moisture content exhibited higher strength as compared to soil with comparatively higher moisture content. This can be attributed to the increase in strength associated with increase in soil suction (lower moisture content) (Khan et al., 2014). Fig. 6 depicts the effect of WH fiber reinforcement on the
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Fig. 6. Bar chart comparison of the tested samples (soil + WH(x %)) where x is the percentage of water hyacinth reinforced with respect to dry soil sample.
(Dasaka and Sumesh, 2011) and also synthetic polypropylene fiber (Cai et al., 2006). 4. Summary and conclusions The present study explores the utility of a harmful weed Eicchornia crassipes, commonly termed as water hyacinth (WH) for application of soil reinforcement. UCS tests were conducted to analyze unconfined compressive strength under different soil densities, soil water contents and fiber contents. Tests were also repeated (three in each case) to discuss any variability in observed UCS. Based on the results and discussion, following conclusions can be drawn.
Fig. 5. (a) 0.5% WH reinforced soil–fiber composite, (b) 0.75% WH reinforced soil–fiber composite and (c) 1% WH reinforced soil–fiber composite.
mean unconfined compressive strength (with standard deviation) of unreinforced soil at different dry densities and moisture content. It can be observed that optimum compaction state (corresponding to maximum UCS) is found to be of soil compacted at 1.05 MDD and OMC-5%. Inclusion of WH fibers resulted in increase in strength for all moisture contents in comparison to unreinforced soil. However, there was reduction in strength of soil corresponding to 1% fiber content. The maximum strength values for fiber content of 1% were found to be between 0.5% and 0.75%. Based on this observation, this study recommends an optimum WH fiber content of 0.5–0.75% by dry weight of the soil for soil reinforcement. The obtained range of unconfined compressive strength using WH fiber is comparable to studies related to inclusion of natural coir fiber
1) The unconfined compressive strength of the unreinforced soil was observed to increase with the addition of WH fibers, attributed to the significant amount of cellulose present in the fiber. 2) Inclusion of WH fiber modified the brittle behavior of soil to ductile behavior of soil–fiber composite. The reinforced soil composite showcases a low drop in post peak strength compared to that of unreinforced soil. 3) At particular moisture content, the increase in soil density increases the unconfined compressive strength. This effect of density is profoundly significant in case of lower moisture content soil–fiber composite. Out of the selected compaction states carried out in the study, soil compacted at OMC-5% and 1.05 MDD showed highest unconfined compressive strength. 4) All percentage inclusion showed increase in the compressive strength as compared to unreinforced state of the soil. However, the strength decreased as the WH fiber content was more than the optimum content. 5) The study recommends optimum WH fiber content as 0.5–0.75% for mobilizing maximum strength corresponding to any compaction state. Acknowledgement The authors would like to thank the department of Central Instrumentation Facility, IIT Guwahati, for providing support in the FE-SEM study of the water hyacinth fiber. References ASTM D2487-11, 2011. Standard Practice for Classification of Soils for Engineering Purpose (Unified Soil Classification System).
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