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Experimental investigations on tensile and pullout behaviour of woven coir geotextiles
ARTICLE IN PRESS Geotextiles and Geomembranes 26 (2008) 384– 392
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Experimental investigations on tensile and pullout behaviour of woven coir geotextiles E.A. Subaida, S. Chandrakaran , N. Sankar Department of Civil Engineering, National Institute of Technology, Calicut, Kerala 673 601, India
Available online 21 April 2008
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a b s t r a c t
Keywords: Woven coir geotextile Yarn Fibre Tension Shear Pullout
Results of an experimental programme to investigate the tensile and interface properties of woven coir geotextiles are presented. Tension tests were conducted on coir fibres, yarns and woven geotextiles at different gauge lengths and strain rates. Based on statistical analysis, a gauge length of 150 mm and a strain rate of 5%/min was adopted for the purpose of characterization of tensile properties. Tensile strength of woven coir geotextile was expressed as a function of fibre strength, yarn properties and weaving pattern. Pullout test and modified direct shear test were conducted on geotextiles in granular soils of different grain sizes. At low normal stress, bond resistance of coir geotextile–sand interface obtained was more than shear strength of soil. But consistent values of bond resistance were not obtained at higher normal stresses. The opening size of mesh relative to the soil grain size influenced the pullout interaction between soil and geotextile. For closely woven geotextiles, the pullout resistance did not vary much in soils of different grain sizes, whereas for geotextiles with open meshes, pullout resistance obtained was more in fine sand compared to coarse sand. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction Increase in the environmental awareness and sustainability along with high cost of petroleum-based geosynthetics have lead the researchers in developing countries to investigations for substitutes using natural products (e.g., Lekha and Kavitha, 2006; Rawal and Anandjiwala, 2007; Sarsby, 2007). Coir fibre obtained from the husk of coconut is among the strongest and most durable natural fibres, due to high lignin content (Girish and Ramanatha Ayyar, 2000; Rao and Balan, 2000). Presently, coir mesh mattings are extensively used for erosion control along steep slopes (Balan and Rao, 1994; Lekha, 2004). Geotextiles made from coir fibres have tremendous possibilities in various other geotechnical applications also. Good performance of coir geotextiles has been reported in drainage installations as vertical drains and blanket drains in embankments (Ramanatha Ayyar et al., 2002; Rao and Balan, 2000). Coir geotextile serves as an effective reinforcing material for temporary clay structures in the wetland environment (Lekha and Kavitha, 2006). Laboratory model test on sand reinforced with coir rope resulted into a considerable increase in
Corresponding author. Tel.: +91 495 2286209; mobile: +91 9447102147; fax: +91 495 2287250. E-mail address: [email protected] (S. Chandrakaran).
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the bearing capacity (Ramanatha Ayyer et al., 1988), which is beneficial for strengthening the subgrade soil. Rate of degradation of coir vary depending upon the medium and depth of embedment and climatic conditions (Rao and Balan, 2000). These products are found to be feasible for applications where they are meant to serve only in the initial stage and final strength is attained by vegetation or soil consolidation. Extensive research work is being carried out to increase the life span of coir geotextiles by various treatments (Girish and Ramanatha Ayyar, 2000) to extent their fields of application. Tensile strength is the most important design parameter to be established when reinforcement is embedded in soil. Though many types of coir geotextiles are produced by coir industry, a standard method for characterization of their tensile properties is not yet developed. Balan (1995) initiated the works for studying the engineering properties of coir geotextiles. The tensile properties of woven and non-woven coir geotextiles under different deformation rates and specimen dimensions were investigated in the study. Following the findings of this study, Rao and Dutta (2005) conducted tensile tests on both woven and non-woven coir geotextiles and reported wide variations among tensile strengths of similar specimens. Coir fibres exhibit a wide range of dimensions and strength. Coir yarns are found to be quite uneven when twisted in the conventional spinning process. Because of this heterogeneity, coir yarns and geotextiles show wide variations in tensile
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characteristics especially under varying testing parameters. The yarn tenacity of spun yarn is greatly influenced by gauge length and strain rate (Anindya et al., 2005; Das and Neckar, 2005). Tensile properties of woven coir geotextiles are directly related to the properties of yarns in warp and weft directions. Hence, it is essential to arrive at the testing parameters in a systematic way, taking into account the yarn and fibre properties. A correlation between yarn strength and geotextile strength enables the manufacture of geotextiles with specific requirements through proper selection of yarns and geometrical parameters. The behaviour of reinforced soil structures is largely governed by interaction mechanisms that develop between the reinforcement inclusions and the fill material. Interface interaction of planar reinforcements varies with material properties and soil parameters (Abramento and Whittle, 1995). Woven coir geotextiles are available with wide ranges of apertures characterized by a combination of longitudinal and transverse yarns (warp and weft yarns). Dense coir geotextiles, in which yarns are arranged very closely, is a sheet like material in which interface mechanism is governed by frictional resistance. Modified direct shear test provides information regarding the shearing (bond) interaction between soil and geotextile. Ajitha and Jayadeep (1997) reported a high value of shear parameters for sand–coir geotextile interface than geosynthetics. The size of box and boundary conditions has only a slight effect on the bond strength (Degoutte and Mathieu, 1986). Fourie and Fabian (1987) and Mitchell et al. (1990) drew important conclusions on the interface shear strength between soil and various geosynthetics using direct shear tests conducted on small size geosynthetic samples. Based on these findings, direct shear test on coir geotextile–soil composite was conducted in the present study using a conventional direct shear box. When the mesh size is large, the structure of coir geotextile is similar to that of geogrids. The interaction mechanism between soil and geogrid includes interface shear resistance that takes place along the longitudinal ribs, particle interlocking within the apertures and passive thrust against transverse elements (Jewell, 1990). Since, reinforcement is subjected to pullout forces under operational conditions, at least partly, pullout test is considered to be a more realistic model than direct shear test to study interface interaction. A number of studies had been reported on soil–geogrid interaction by various researchers (Alfaro et al., 1995; Ochiai et al., 1996; Palmeira and Milligan, 1989; Palmeira, 2004; Sidnei et al., 2007; Sugimoto et al., 2001). Pullout resistance of a reinforcement material is directly related to its structural composition and material specific characteristics and is a function of the relative density and the particle size of soil (Wilson and Koerner, 1993). The studies reported so far for the characterization of tensile behaviour of coir geotextiles did not take into account the fibre and yarn properties. Also, no significant study has been reported in the literature on interface behaviour of coir geotextiles. In this background, attempt has been made to conduct a comprehensive experimental study on the tensile and pullout behaviour of coir geotextiles and the results so obtained are presented in this paper. A procedure for conducting tension tests on woven coir geotextiles has been developed based on the observations and statistical analysis. Direct shear test and pullout tests have been conducted using granular soils of different grain sizes to study the influence of geotextile properties on interface behaviour.
industries within Kerala, India, were used for the study. Yarns and fibres constituting these geotextiles were also tested under tension. Fibres for tension test were separated out of the yarns. Figs. 1 and 2 show the photographs of coir fibres, coir yarns and coir geotextiles. The physical properties of yarns and geotextiles determined as per Indian Standard specifications are given in Tables 1 and 2, respectively. Coir fibres are named after the places of production as Anjengo, Aratory, Beach and Vycome. Coir yarns are designated by the type of fibre and runnage. Runnage is the length of yarn in metres required to make one kilogram (m/kg). Coir geotextiles are designated as mesh mattings based on the type of warp yarn. Locally available dry sand with a specific gravity of 2.65 was used for direct shear and pullout tests. To study the interaction of geotextiles with soils of different grain sizes, uniform samples of fine, medium and coarse sand as per Indian Standard classification were prepared from this sand. Table 3 shows the properties of soils used in the study.
2.2. Tension test Tension tests were conducted on coir fibres, yarns and woven geotextiles on a strain controlled tensile testing machine. Separate jaws were used to hold the fibres, yarns and geotextiles to provide firm gripping, and proper pads were used to avoid damage of specimens between the jaws. The specimens that failed at the grip during testing were discarded. The number of specimens tested for determination of coefficient of variation was 25 as per Indian
2. Experimental programme 2.1. Materials Five types of woven coir geotextiles manufactured from yarns of different origins and runnages, procured from various coir
385
Fig. 1. Photographs of (a) coir fibres and (b) coir yarns.
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Table 1 Properties of coir yarns Type of yarn
Runnage (m/kg)
Scorage
Thickness (mm)
Density (kN/m3)
Anjengo Anjengo Vycome Vycome Vycome Beach Aratory
275 220 240 220 200 240 240
11.8 9.5 10.8 10.6 9.7 8.5 11.0
4.840 5.705 5.954 5.816 4.980 5.840 5.016
9.7 9.8 9.7 9.7 9.8 9.4 9.3
Table 2 Properties of woven coir geotextiles Thickness No. of No. of Mass/ picks ends unit area (mm) (dm) (dm) (g/m2)
Designation Warp yarn
Weft yarn
MMA1
Vycome 758.18 6.42 240 Beach 864.80 6.79 240 Aratory 1286.56 8.39 240 Vycome 727.14 7.03 200 Vycome 401.44 6.46 200
MMB1 MMA2 MMV1 MMV2
Anjengo 275 Beach 240 Anjengo 220 Vycome 240 Vycome 220
Aperture size (mm mm)
8.9
7.6
9 12
7.9
7.2
8 17
18.9
8.9
12
8.9
6.8
8 10
4.7
3.9
21 25
Table 3 Properties of soil Type of sand
Uniformity coefficient
Average particle size Relative (mm) density (%)
Friction angle (1)
Fine Medium Coarse
1.4 2.4 1.0
0.145 0.750 3.375
41 42 49
90 90 90
temperature and humidity conditions. To get reproducible results, the specimens were conditioned at a temperature of 27 1C and a relative humidity of 65%. 2.2.1. Tension test on coir yarns Coir yarn specimens were randomly selected for tension test and fixed to the jaws kept at the required gauge length. A uniform pretension of 2 N was applied and the yarns were subjected to tensile force at the desired strain rate. Considering practical feasibility, four out of seven types of yarns were used to fix up the testing parameters. The gauge lengths used were 50, 100, 150, 200 and 300 mm. Due to practical difficulties still higher, gauge lengths could not be used. The deformation rate was varied from 0.3%/min to 20%/min.
Fig. 2. Photographs of woven coir geotextiles used for the study. (a) MMA1, (b) MMA2, (c) MMB1, (d) MMV1 and (e) MMV2.
Standards IS: SP15 (part 2): 2000 and for other tests the number of samples was decided as 10 for a limit of error of 10%. Tensile strength of coir yarns and geotextiles vary depending upon the
2.2.2. Tension test on coir fibres Tension tests were conducted on coir fibres to study the influence of fibre strength on yarn strength. Considering convenience of fixing in the jaws and availability of sufficient number of specimens, fibres were tested at a gauge length of 100 mm at a strain rate of 5%/min. 2.2.3. Tension test on woven coir geotextiles Tension tests were conducted on woven coir geotextiles at a strain rate of 5%/min as decided from the tests on yarns. The specimens were subjected to a pretension of 10 N. The sizes of the
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specimens were varied to study the effect of gauge length and width of specimen on tensile behaviour. Gauge lengths of 100, 150, 250 and 300 mm were used. Changing the number of yarns parallel to the direction of load as 4, 6, 8, 10 and 12 varied the width of specimen. The effect of length to width ratio was studied by keeping one of the dimensions constant while the other changed. 2.3. Direct shear test A conventional direct shear box of size 60 60 50 mm was used to study the frictional behaviour of various geotextiles with fine, medium and coarse sand at normal stresses of 100, 200 and 300 kPa. Geotextile specimen was placed at the middle of two halves filled with soil at 90% relative density. Required normal stress was applied and the specimen was sheared at the rate of 1.25 mm/min. 2.4. Pullout test The pullout test device consisted of a box of 450 mm length, 450 mm width and 600 mm depth, a vertical load application system, a horizontal force actuator device, a clamp and other required instrumentation. Fig. 3 shows the arrangement for pullout test. The pullout box consisted of steel plates welded at the edges. The back end of the tank was sealed, while the front end was provided with a slot at the mid-height to allow the geotextile to pass through. Sponge tapes and plywood pieces were mounted on the slot to prevent the loss of soil during the pullout operation. Friction acting between the soil and all over the inside walls of the box was minimized by means of greased rubber membranes. The normal load was applied through a rigid plate. The facility for rotating the loading cap during pullout operation was not provided in the testing equipment and this might have caused a non-uniform distribution of vertical stress. An electric jack operated by a motor applied the pullout force, which was measured using a proving ring placed between the motor and the clamping system. The pullout displacement at the clamp was measured by means of a displacement dial gauge. Sand was placed in layers of about 50 mm thick by sand raining technique in order to prepare a homogeneous sand sample. Weighed quantity of sand was allowed to fall from a height of 500 mm through a sieve that covered the plan area of box and tamped to get a relative density of 90%. After placing sand up to the pullout opening level, geotextile specimen of width 200 mm was placed on the top of the levelled sand to get an embedment
Rigid plate
300
Normal load
600
Dial gauge Coir geotextile Clamp
Proving Ring
Sand
450 Fig. 3. Pullout test setup (dimensions in mm).
387
length of 400 mm. The front end of the geotextile was firmly connected to the loading shaft by the clamp. Sand was then placed in layers up to the top level. The upper plate was then fixed and the desired normal load was applied. Each type of geotextile was tested under three normal pressures of 10, 20 and 30 kPa. The lower normal stresses were adopted to ensure the pullout failure minimizing the possibility of tension failure. The displacement dial gauge was fixed to measure the pullout displacement at the clamp. Then the geogrid was pulled out at a constant rate of 1.25 mm/min until the failure of specimen was reached.
3. Results and discussions 3.1. Tension test on fibres Table 4 shows the result of tension test on fibres. It is observed that tensile properties of coir fibres vary with their origin. Beach fibre has the maximum stiffness among the four types of fibres tested. Anjengo type fibres undergo large deformation before failure. The failure stress and strain of Vycome and Aratory fibres are comparable. 3.2. Tension test on coir yarns The tensile strength of coir yarn is generally termed as yarn tenacity which represents the maximum tensile force taken by the yarn in centiNewton or milliNewton per unit tex. Tex is the linear density of yarn expressed in gram per kilometre. 3.2.1. Effect of gauge length and strain rate on tensile behaviour of yarns To study the effect of gauge length on yarn tenacity, the gauge length was increased from 50 to 300 mm keeping the strain rate constant. An increase in yarn tenacity is observed when gauge length is increased from 50 to 150 mm and beyond that yarn tenacity be decreased. The coir fibres in yarns vary widely in length and diameter. The lengths of fibres vary from 10 to 250 mm and the diameters vary from 0.1 to 0.7 mm. Most of the strong fibres are of length between 100 and 150 mm. During spinning, short fibres are wound by longer fibres. At shorter gauge lengths, absence of strong and long fibres results into a reduction in yarn tenacity. The percentage of fibres longer than 150 mm is very less in yarns. The distribution of flaws increases the probability of slippage at higher gauge lengths and leads to localization of stress. Thereby yarn tenacity is low at gauge lengths above 150 mm. The same behaviour is observed at all strain rates. Fig. 4 shows the relationship between gauge length and yarn tenacity at a strain rate of 5%/min. Relationship between failure strain and gauge length shows that at lower lengths, more elongation takes place due to gripping of fibres at both ends. The variation of yarn tenacity with respect to deformation rate is shown in Fig. 5. An increase in yarn tenacity is observed as deformation rate is increased. The effect of strain rate over failure strain is not consistent for different types of yarns. Anjengo 275 shows a Table 4 Results of tension tests on coir fibres Fibre type
Breaking stress (MPa)
Breaking strain (%)
Breaking modulus (MPa)
Anjengo Vycome Beach Aratory
33.18 24.50 18.69 23.50
16.2 13.0 8.7 12.0
204.81 188.50 214.80 195.83
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or less constant thereafter. The influence of fibre type on the failure strain of yarn is clear from the two sets of values (Tables 4 and 5).
60
Yarn tenacity (mN/tex)
50
3.2.2. Selection of testing parameters Statistical analysis on tension test results shows the influence of gauge length and deformation rate on coefficient of variation of yarn tenacity and failure strain (Figs. 6 and 7). At shorter gauge lengths, the failure of yarn takes place due to breakage of fibres and at higher lengths slippage failure dominates. Wide variations in fibre strengths and mass variations are reflected in the corresponding yarn tenacity values at shorter gauge lengths. Increase in variation is also observed beyond a gauge length of 150 mm, because of non-uniform slippages of shorter fibres. At a gauge length of 150 mm, the coefficient of variation is found to be a minimum for both yarn tenacity and failure strain, which may be due to a balance between fibre breakage and slippage failures.
40
30
20
Anjengo-275 Beach-240 Vycome-240
10
Anjengo-220
40
0 0
100
200 Gauge length (mm)
300
400
30 Coefficient of variation (%)
Fig. 4. Effect of gauge length on yarn tenacity.
Yarn tenacity (mN/tex)
60
40 Anjengo-275 Anjengo-220 Vycome-240 Beach-240
20
20
Beach-240 10
Anjengo-275 Anjengo-220 Vycome-240
0 100
0 0 0
4
8
12
200 Gauge length (mm)
300
400
Fig. 6. Gauge length vs. coefficient of variation in yarn tenacity.
Strain rate (%/min) Fig. 5. Effect of strain rate on yarn tenacity.
30
Type of yarn
Anjengo 275 Anjengo 220 Beach 240 Vycome 240 Vycome 220 Vycome 200 Aratory 240
Breaking load (N)
Failure strain (%)
166.96 200.73 206.76 177.47 188.65 207.50 180.16
27.85 29.05 21.17 22.47 24.47 15.30 24.72
Yarn tenacity (mN/tex) 47.76 42.80 43.98 43.22 39.66 42.54 42.02
steady rate of increase of failure strain with strain rate. For beach 240, no significant effect is observed in failure strain with strain rate while for Anjengo 220 and Vycome 240 failure strain increases up to a strain rate of 5%/min and then remains more
Coefficient of variation (%)
Table 5 Yarn tenacity and failure strain of coir yarns
Anjengo-275
Anjengo-220
Beach-240
Vycome - 240
20
10
0 0
2
4 6 Strain rate (%/min)
8
10
Fig. 7. Strain rate vs. coefficient of variation in failure strain.
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Similarly a minimum variation of yarn tenacity and failure strain is observed at a strain rate of 5%/min. Based on these observations, it is decided to select a gauge length of 150 mm and a strain rate of 5%/min for the purpose of characterization of tensile properties of coir yarns. Results of tension tests on yarns conducted at these testing parameters are given in Table 5. Fig. 8 shows the load vs. elongation curves for different types of coir yarns. It is observed that all of the curves follow more or less similar pattern with an initial concavity and thereafter a non-linear behaviour. 3.2.3. Yarn strength as a function of fibre properties The fibre properties such as type, distribution, strength and yarn parameters namely thickness and method of spinning are found to be the important variables affecting yarn tenacity. The proportion of fibres with similar aspect ratio is more or less the same in all types of yarns. Hence, fibre strength is considered as the major factor controlling the yarn tenacity. Since it is very difficult to quantify the variations in spinning among yarn types, the only yarn parameter considered in this study is the thickness. For a particular runnage, thickness of yarn is indicated through scorage, which is a measure of coarseness or fineness of a yarn. Scorage is a number obtained by dividing 20, the number of strands that could be laid close to each other without overlapping in a length of 0.914 m. The following relationship between yarn tenacity, fibre modulus and scorage could be developed through regression analysis of test data. YT ¼ 0:1596Ef þ 1:1246C 0:0811,
(1)
389
less compared to that of yarns. At a gauge length of 150 mm and a width of specimen containing 10 or more number of yarns, consistent values of loads and failure strains are observed among similar specimens. Hence, it is decided to fix up the length and width of specimen as 150 mm and not less than 10 yarns, respectively. In order to avoid narrow strips for closely spaced geotextiles, a minimum width of 150 mm was used for all types. Fig. 9 shows the stress–strain curves for the tested coir geotextiles in warp direction. The curve is non-linear for geotextiles of low stiffness, whereas for MMA2 which is a dense type geotextile the curve is almost linear. The results of tension tests on geotextiles are given in Table 6. 3.3.1. Prediction of geotextile strength from yarn properties The spacing of yarns in the geotextile and yarn strength is the key parameters controlling the tensile strength of woven coir geotextiles. The following empirical relationship provided good correlation between test results and predicted values of tensile strength of geotextiles (Fig. 10): St Tg ¼ YT LD N (2) 106 , Sp St Tg ¼ ð0:1596Ef þ 1:1246C 0:0811Þ LD N 106 , Sp
(3)
where Tg is tensile strength of geotextile in kN/m, YT is tenacity of yarn in mN/tex, LD is linear density in tex, N is number of yarns/m
where YT is yarn tenacity in mN/tex, Ef is fibre modulus in N/mm2 and C is scorage of yarn.
40
MMA1
3.3. Tensile behaviour of woven coir geotextiles
MMA2 MMV1
Weaving selected yarns in warp and weft directions in a specific pattern makes woven coir geotextiles. The influence of gauge length on tensile properties of geotextiles was studied in the same way as that for yarns. As length of the specimen is increased keeping the width as constant, the tensile strength is found to be decreased. At the same time, no specific trend is observed in breaking load or failure strain while adopting different length to width ratios. The coefficient of variation of tensile strength and failure strain of geotextiles varied from 1.5% to 39% and 1.2% to 29% for different tests, which are considerably
Load (kN/m)
30
MMB1 20
10
0
250
10
0
200
Load (N)
MMV2
20 Strain (%)
30
40
Fig. 9. Results of tension tests on geotextiles in warp direction.
150 Anjengo-275
Table 6 Results of tension tests on geotextiles
Anjengo-220 100
Vycome- 240 Vycome-220 Vycome-200
50
Beach-240
Type of Warp direction geotextile Load Failure Secant (kN/m) strain (%) modulus at 10% strain (kN/m)
Weft direction Load (kN/m)
Failure Secant strain (%) modulus at 10% strain (kN/m)
MMA1 MMB1 MMA2 MMV1 MMV2
11.00 13.26 12.60 8.80 6.46
20.00 22.00 20.70 24.00 15.00
Aratory-240 0 0
10
20 Elongation (%)
30
Fig. 8. Results of tension tests on yarns.
40
13.25 15.60 38.00 14.00 7.47
23.20 20.00 36.12 23.00 17.00
82.00 87.25 88.00 90.00 44.17
81.70 82.00 92.00 55.30 48.30
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parallel to the direction of tensile force, St is spacing of yarn in the direction of test in millimetre, Sp is spacing of yarns perpendicular to the direction of test in millimetre.
3.4. Direct shear test The results of direct shear test on sand–coir geotextile composite are given in Table 7. A small displacement of 3–5 mm is only required for the development of interface friction as in the case of other extensible reinforcements (Fig. 11). The bond ratio varies from 0.68 to 1.72 for various coir geotextiles in different soils. Values of bond resistance obtained at 100 kPa are reasonable. The contact efficiency increases when the opening size of the geotextile increases and it appears to be higher than unity when the opening size is slightly larger than the particle size. The values of shear parameters at 200 and 300 kPa do not show a consistent behaviour. Higher normal stresses may cause an appreciable deformation of geotextile pushing it beneath the predetermined shear plane. This may result into inconsistent values of bond resistance.
3.5. Pullout test The pullout behaviour of coir geotextile with closely woven yarns is found to be different from that with open meshes in soils of different grain sizes. The pullout resistances of MMA2, with very close yarn spacing, are almost same in fine, medium and 120
Shear stress (kPa)
390
80
Fine sand +MMA1 +MMB1
40
+MMA2 +MMV1 +MMV2
0 1
0
40
2 3 4 Shear displacement (mm)
5
Fig. 11. Direct shear test results of geotextiles in fine sand at 100 kPa.
Pullout force (kN/m)
Predicted value (kN/m)
18
20
12
6
Fine sand Medium sand Coarse sand
0
0
0
10 20 30 Experimental value (kN/m)
40
0
Fig. 10. Predicted vs. experimental values of tensile strength of geotextiles.
5
10 15 20 Pullout displacement (mm)
25
Fig. 12. Pullout resistance of MMA2 at 10 kPa.
Table 7 Results of modified direct shear tests Type of sand
Normal stress (kPa)
Shear strength of sand (kPa)
Bond strength (kPa) MMA1
MMB1
MMA2
MMV1
MMV2
Fine sand
100 200 300
57.99 153.46 288.36
96.14 156.95 266.45
99.69 192.31 280.79
86.46 145.60 248.04
75.79 189.55 237.37
63.93 176.98 246.91
Medium sand
100 200 300
86.16 171.77 250.85
106.20 194.72 255.64
83.27 175.89 221.43
86.81 180.22 261.34
90.34 191.94 261.6
100.71 148.74 227.36
Coarse sand
100 200 300
97.77 234.30 355.34
112.87 180.87 213.16
57.61 233.22 299.41
84.09 169.75 302.22
83.79 154.21 347.67
69.53 243.56 256.28
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20 Fine sand Medium sand Coarse sand
Pullout force (kN/m)
15
10
5
0 0
5
10 15 20 Pullout displacement (mm)
25
Fig. 13. Pullout resistance of MMB1 at 10 kPa.
coarse sand (Fig. 12). For geotextiles with open meshes, pullout resistance is maximum in fine sand and minimum in coarse sand (Fig. 13). The influence of bearing against weft yarns and interlocking is reflected in this observation. This is in confirmation with the findings of Palmeira and Milligan (1989) that the pullout resistance is strongly influenced by the bearing member space ratio, S/D, in which S is the bearing member spacing and D is the bearing member thickness. Higher bearing resistance from an individual bearing member is expected when the S/D ratio is higher. Jewell (1990) identified limiting values of the S/D50 ratio as 10 that characterize either interface shear or full interaction mechanisms. As per this criterion, the interaction between MMA2 and sand may be considered as entirely due to interface friction. For other types of geotextiles, total pullout resistance is a combination of bearing and interlocking together with friction. Hence, the pullout resistance is a maximum in fine sand. Another important observation is that there is an increase in the tensile capacity of coir geotextiles with open meshes, when confined well in soil, than in air tensile strength. The displacement at peak pullout resistance is more in coarse sand compared to fine sand for the same geotextile under the same normal stress. Similar observation was reported by Sridharan and Hans (1988) for rough reinforcement.
4. Conclusions Tension tests were conducted on coir yarns and woven coir geotextiles at different gauge lengths and strain rates. The results indicate that the tensile properties of coir yarns and woven coir geotextiles vary significantly based on gauge length and strain rate. When gauge length and strain rate were adopted as 150 mm and 5%/min, consistent values of tensile strength and failure strain were obtained and hence these values are suggested as testing parameters for characterization of tensile properties of coir yarns and woven coir geotextiles. The test results demonstrate that the width of the geotextile specimen for tension test would have a minimum of 10 force bearing yarns to get reliable results. Tensile properties of coir fibres are found to vary depending upon their origin. Beach fibre has the highest breaking modulus. Breaking stress as well as failure strain is large for Anjengo fibres compared to other types. Vycome and Aratory fibres show identical behaviour under tension. Tensile strength of coir yarn
391
can be represented as a function of fibre modulus and scorage of yarn. Tensile strength of woven coir geotextiles is dependent on the type of fibre, thickness of yarn and the weaving pattern. Tensile strength of most of the tested mesh mattings lies in the range of 10–20 kN/m. To get satisfactory performance in the function of temporary reinforcement, closely woven coir geotextiles of high tensile strengths are to be used. Modified direct shear tests were conducted on sand–coir geotextile composite using fine, medium and coarse sand. At lower normal stress, the bond resistance of woven coir geotextiles–sand interface is more than the shear strength of soil, while at higher normal stresses the values obtained are not consistent. Pullout tests on coir geotextiles were conducted in fine, medium and coarse sand. Tensile strength of geotextile and the relative size of mesh opening to particle size of fill material govern the pullout resistance of woven coir geotextiles. Closely woven geotextiles offer good pullout resistance in fine, medium as well as coarse sand due to high interface friction. Pullout resistance of open meshed geotextile is more in fine-grained soil compared to coarse-grained soil because of good interlocking and bearing resistance. It should be noted that the results presented in this paper are limited to the short-term tensile properties of coir geotextiles. Coir being biodegradable material losses its tensile strength due to in situ deterioration. However, this material is eventually converted into organic matter which in turn aggregates the soil particles and tries to retain its improved properties for a certain period. It is presumed that the construction carried out get stabilized by this period. If coir geotextiles are to be proposed for long-term reinforcement applications, there is a need to suggest reduction factors while using the reported values, which is not attempted in this paper due to limitations in the available data. It is recommended that future studies include durability and long-term performance aspects of coir geotextiles.
References Abramento, M., Whittle, A.J., 1995. Analysis of pullout tests for planar reinforcement in soil. Journal of Geotechnical Engineering ASCE June, 476–492. Ajitha, B., Jayadeep, T., 1997. Interfacial frictional properties of geotextiles and bio-mats. In: Proceedings of Indian Geotechnical Conference, Vadodara, pp. 287–290. Alfaro, M.C., Hayashi, S., Miura, N., 1995. Pullout interaction mechanism of geogrid strip reinforcement. Geosynthetics International 2 (4), 679–698. Anindya, G., Ishtaque, S.M., Rengaswamy, R.S., 2005. Analysis of spun yarn failure: Part-I, tensile failure of yarns as a function of structure and testing parameters. Textile Research Journal 75 (10), 731–740. Balan, K., 1995. Studies on engineering behaviour and uses of geotextiles with natural fibres. Ph.D. Thesis, Indian Institute of Technology, Delhi, India. Balan, K., Rao, G.V., 1994. Erosion control with natural geotextiles. In: Proceedings of the International Seminar and Techno Meet on Environmental Geotechnology with Geosynthetics, New Delhi, pp. 317–325. Das, D., Neckar, B., 2005. Yarn strength behaviour at different gauge lengths. Indian Fibre & Textile Research 30 (December), 414–420. Degoutte, G., Mathieu, G., 1986. Experimental research of friction between soil and geomembranes or geotextiles using 300 mm shear box. In: Proceedings of the 3rd International Conference on Geotextiles, Vienna, pp. 791–796. Fourie, A.B., Fabian, K.J., 1987. Laboratory determination of clay–geotextile interaction. Geotextiles and Geomembranes 6 (4), 275–294. Girish, M.S., Ramanatha Ayyar, T.S., 2000. Improvement of durability of coir geotextiles. In: Proceedings for the Indian Geotextiles Conference, Bombay, India, pp. 309–310. Jewell, R.A., 1990. Reinforcement bond capacity. Geotechnique 40 (3), 513–518. Lekha, K.R., 2004. Field instrumentation and monitoring of soil erosion in coir geotextile stabilized slopes—a case study. Geotextiles and Geomembranes 22 (5), 399–413. Lekha, K.R., Kavitha, V., 2006. Coir geotextile reinforced clay dykes for drainage of low-lying areas. Geotextiles and Geomembranes 24 (1), 38–51. Mitchell, J.K., Seed, R.B., Seed, H.B., 1990. Kettleman Hills waste landfill slope failure. I: liner-system properties. Journal of Geotechnical Engineering 116 (4), 647–668.
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Ochiai, H., Jun, O., Shigenori, H., Takao, H., 1996. The pull-out resistance of geogrids in reinforced soil. Geotextiles and Geomembranes 14 (1), 19–42. Palmeira, E.M., 2004. Bearing force mobilisation in pullout tests on geogrids. Geotextiles and Geomembranes 22 (6), 481–509. Palmeira, E.M., Milligan, G.W.E., 1989. Scale and other factors affecting the results of pull-out tests of grid buried in sand. Geotechnique 39 (3), 511–524. Ramanatha Ayyer, T.S., Joseph, J., Beena, K.S., 1988. Bearing capacity of sand reinforced with coir rope. In: Proceedings of the First Indian Geotextiles Conference on Reinforced Soil and Geotextiles, vol. 1, Bombay, December, pp. 11–16. Ramanatha Ayyar, T.S., Ramachandran Nair, C.G., Balakrishnan Nair, N. (Eds.), 2002. Comprehensive Reference Book on Coir Geotextiles. Centre for Development of Coir Technology (Publishers), Kerala, India. Rao, G.V., Balan, K. (Eds.), 2000. Coir Geotextiles—Emerging Trends. The Kerala State Coir Corporation Ltd., Alappuzha, Kerala, India. Rao, G.V., Dutta, R.K., 2005. Characterization of tensile strength behaviour of coir products. Electronic Journal of Geotechnical Engineering, paper 0520, vol. 10 (B), 2111 N, Burdick Street Stillwater, OK, USA.
Rawal, A., Anandjiwala, R., 2007. Comparative study between needlepunched nonwoven geotextile structures made from flax and polyester fibres. Geotextiles and Geomembranes 25 (1), 61–65. Sarsby, R.W., 2007. Use of ‘Limited Life Geotextiles’ (LLGs) for basal reinforcement of embankments built on soft clay. Geotextiles and Geomembranes 25 (4–5), 302–310. Sidnei, T.H.C., Benedito, S.B., Jorge, G.Z., 2007. Pullout resistance of individual longitudinal and transverse geogrid ribs. Journal of Geotechnical and GeoEnvironmental Engineering ASCE January, 37–50. Sridharan, A., Hans, R.S., 1988. Effect of soil parameters on the friction coefficient in reinforced earth. Indian Geotechnical Journal 18 (4), 322–339. Sugimoto, M., Alagyawanna, A.M.N., Kadoguchi, K., 2001. Influence of rigid and flexible face on geogrid pullout tests. Geotextiles and Geomembranes 19 (8), 483–507. Wilson, R.F., Koerner, R.M., 1993. Finite element modeling of soil–eogrid interaction with application to the behavior of geogrids in a pullout loading condition. Geotextiles and Geomembranes 12 (5), 479–501.
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Experimental investigations on tensile and pullout behaviour of woven coir geotextiles E.A. Subaida, S. Chandrakaran , N. Sankar Department of Civil Engineering, National Institute of Technology, Calicut, Kerala 673 601, India
Available online 21 April 2008
article info
a b s t r a c t
Keywords: Woven coir geotextile Yarn Fibre Tension Shear Pullout
Results of an experimental programme to investigate the tensile and interface properties of woven coir geotextiles are presented. Tension tests were conducted on coir fibres, yarns and woven geotextiles at different gauge lengths and strain rates. Based on statistical analysis, a gauge length of 150 mm and a strain rate of 5%/min was adopted for the purpose of characterization of tensile properties. Tensile strength of woven coir geotextile was expressed as a function of fibre strength, yarn properties and weaving pattern. Pullout test and modified direct shear test were conducted on geotextiles in granular soils of different grain sizes. At low normal stress, bond resistance of coir geotextile–sand interface obtained was more than shear strength of soil. But consistent values of bond resistance were not obtained at higher normal stresses. The opening size of mesh relative to the soil grain size influenced the pullout interaction between soil and geotextile. For closely woven geotextiles, the pullout resistance did not vary much in soils of different grain sizes, whereas for geotextiles with open meshes, pullout resistance obtained was more in fine sand compared to coarse sand. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction Increase in the environmental awareness and sustainability along with high cost of petroleum-based geosynthetics have lead the researchers in developing countries to investigations for substitutes using natural products (e.g., Lekha and Kavitha, 2006; Rawal and Anandjiwala, 2007; Sarsby, 2007). Coir fibre obtained from the husk of coconut is among the strongest and most durable natural fibres, due to high lignin content (Girish and Ramanatha Ayyar, 2000; Rao and Balan, 2000). Presently, coir mesh mattings are extensively used for erosion control along steep slopes (Balan and Rao, 1994; Lekha, 2004). Geotextiles made from coir fibres have tremendous possibilities in various other geotechnical applications also. Good performance of coir geotextiles has been reported in drainage installations as vertical drains and blanket drains in embankments (Ramanatha Ayyar et al., 2002; Rao and Balan, 2000). Coir geotextile serves as an effective reinforcing material for temporary clay structures in the wetland environment (Lekha and Kavitha, 2006). Laboratory model test on sand reinforced with coir rope resulted into a considerable increase in
Corresponding author. Tel.: +91 495 2286209; mobile: +91 9447102147; fax: +91 495 2287250. E-mail address: [email protected] (S. Chandrakaran).
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the bearing capacity (Ramanatha Ayyer et al., 1988), which is beneficial for strengthening the subgrade soil. Rate of degradation of coir vary depending upon the medium and depth of embedment and climatic conditions (Rao and Balan, 2000). These products are found to be feasible for applications where they are meant to serve only in the initial stage and final strength is attained by vegetation or soil consolidation. Extensive research work is being carried out to increase the life span of coir geotextiles by various treatments (Girish and Ramanatha Ayyar, 2000) to extent their fields of application. Tensile strength is the most important design parameter to be established when reinforcement is embedded in soil. Though many types of coir geotextiles are produced by coir industry, a standard method for characterization of their tensile properties is not yet developed. Balan (1995) initiated the works for studying the engineering properties of coir geotextiles. The tensile properties of woven and non-woven coir geotextiles under different deformation rates and specimen dimensions were investigated in the study. Following the findings of this study, Rao and Dutta (2005) conducted tensile tests on both woven and non-woven coir geotextiles and reported wide variations among tensile strengths of similar specimens. Coir fibres exhibit a wide range of dimensions and strength. Coir yarns are found to be quite uneven when twisted in the conventional spinning process. Because of this heterogeneity, coir yarns and geotextiles show wide variations in tensile
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characteristics especially under varying testing parameters. The yarn tenacity of spun yarn is greatly influenced by gauge length and strain rate (Anindya et al., 2005; Das and Neckar, 2005). Tensile properties of woven coir geotextiles are directly related to the properties of yarns in warp and weft directions. Hence, it is essential to arrive at the testing parameters in a systematic way, taking into account the yarn and fibre properties. A correlation between yarn strength and geotextile strength enables the manufacture of geotextiles with specific requirements through proper selection of yarns and geometrical parameters. The behaviour of reinforced soil structures is largely governed by interaction mechanisms that develop between the reinforcement inclusions and the fill material. Interface interaction of planar reinforcements varies with material properties and soil parameters (Abramento and Whittle, 1995). Woven coir geotextiles are available with wide ranges of apertures characterized by a combination of longitudinal and transverse yarns (warp and weft yarns). Dense coir geotextiles, in which yarns are arranged very closely, is a sheet like material in which interface mechanism is governed by frictional resistance. Modified direct shear test provides information regarding the shearing (bond) interaction between soil and geotextile. Ajitha and Jayadeep (1997) reported a high value of shear parameters for sand–coir geotextile interface than geosynthetics. The size of box and boundary conditions has only a slight effect on the bond strength (Degoutte and Mathieu, 1986). Fourie and Fabian (1987) and Mitchell et al. (1990) drew important conclusions on the interface shear strength between soil and various geosynthetics using direct shear tests conducted on small size geosynthetic samples. Based on these findings, direct shear test on coir geotextile–soil composite was conducted in the present study using a conventional direct shear box. When the mesh size is large, the structure of coir geotextile is similar to that of geogrids. The interaction mechanism between soil and geogrid includes interface shear resistance that takes place along the longitudinal ribs, particle interlocking within the apertures and passive thrust against transverse elements (Jewell, 1990). Since, reinforcement is subjected to pullout forces under operational conditions, at least partly, pullout test is considered to be a more realistic model than direct shear test to study interface interaction. A number of studies had been reported on soil–geogrid interaction by various researchers (Alfaro et al., 1995; Ochiai et al., 1996; Palmeira and Milligan, 1989; Palmeira, 2004; Sidnei et al., 2007; Sugimoto et al., 2001). Pullout resistance of a reinforcement material is directly related to its structural composition and material specific characteristics and is a function of the relative density and the particle size of soil (Wilson and Koerner, 1993). The studies reported so far for the characterization of tensile behaviour of coir geotextiles did not take into account the fibre and yarn properties. Also, no significant study has been reported in the literature on interface behaviour of coir geotextiles. In this background, attempt has been made to conduct a comprehensive experimental study on the tensile and pullout behaviour of coir geotextiles and the results so obtained are presented in this paper. A procedure for conducting tension tests on woven coir geotextiles has been developed based on the observations and statistical analysis. Direct shear test and pullout tests have been conducted using granular soils of different grain sizes to study the influence of geotextile properties on interface behaviour.
industries within Kerala, India, were used for the study. Yarns and fibres constituting these geotextiles were also tested under tension. Fibres for tension test were separated out of the yarns. Figs. 1 and 2 show the photographs of coir fibres, coir yarns and coir geotextiles. The physical properties of yarns and geotextiles determined as per Indian Standard specifications are given in Tables 1 and 2, respectively. Coir fibres are named after the places of production as Anjengo, Aratory, Beach and Vycome. Coir yarns are designated by the type of fibre and runnage. Runnage is the length of yarn in metres required to make one kilogram (m/kg). Coir geotextiles are designated as mesh mattings based on the type of warp yarn. Locally available dry sand with a specific gravity of 2.65 was used for direct shear and pullout tests. To study the interaction of geotextiles with soils of different grain sizes, uniform samples of fine, medium and coarse sand as per Indian Standard classification were prepared from this sand. Table 3 shows the properties of soils used in the study.
2.2. Tension test Tension tests were conducted on coir fibres, yarns and woven geotextiles on a strain controlled tensile testing machine. Separate jaws were used to hold the fibres, yarns and geotextiles to provide firm gripping, and proper pads were used to avoid damage of specimens between the jaws. The specimens that failed at the grip during testing were discarded. The number of specimens tested for determination of coefficient of variation was 25 as per Indian
2. Experimental programme 2.1. Materials Five types of woven coir geotextiles manufactured from yarns of different origins and runnages, procured from various coir
385
Fig. 1. Photographs of (a) coir fibres and (b) coir yarns.
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Table 1 Properties of coir yarns Type of yarn
Runnage (m/kg)
Scorage
Thickness (mm)
Density (kN/m3)
Anjengo Anjengo Vycome Vycome Vycome Beach Aratory
275 220 240 220 200 240 240
11.8 9.5 10.8 10.6 9.7 8.5 11.0
4.840 5.705 5.954 5.816 4.980 5.840 5.016
9.7 9.8 9.7 9.7 9.8 9.4 9.3
Table 2 Properties of woven coir geotextiles Thickness No. of No. of Mass/ picks ends unit area (mm) (dm) (dm) (g/m2)
Designation Warp yarn
Weft yarn
MMA1
Vycome 758.18 6.42 240 Beach 864.80 6.79 240 Aratory 1286.56 8.39 240 Vycome 727.14 7.03 200 Vycome 401.44 6.46 200
MMB1 MMA2 MMV1 MMV2
Anjengo 275 Beach 240 Anjengo 220 Vycome 240 Vycome 220
Aperture size (mm mm)
8.9
7.6
9 12
7.9
7.2
8 17
18.9
8.9
12
8.9
6.8
8 10
4.7
3.9
21 25
Table 3 Properties of soil Type of sand
Uniformity coefficient
Average particle size Relative (mm) density (%)
Friction angle (1)
Fine Medium Coarse
1.4 2.4 1.0
0.145 0.750 3.375
41 42 49
90 90 90
temperature and humidity conditions. To get reproducible results, the specimens were conditioned at a temperature of 27 1C and a relative humidity of 65%. 2.2.1. Tension test on coir yarns Coir yarn specimens were randomly selected for tension test and fixed to the jaws kept at the required gauge length. A uniform pretension of 2 N was applied and the yarns were subjected to tensile force at the desired strain rate. Considering practical feasibility, four out of seven types of yarns were used to fix up the testing parameters. The gauge lengths used were 50, 100, 150, 200 and 300 mm. Due to practical difficulties still higher, gauge lengths could not be used. The deformation rate was varied from 0.3%/min to 20%/min.
Fig. 2. Photographs of woven coir geotextiles used for the study. (a) MMA1, (b) MMA2, (c) MMB1, (d) MMV1 and (e) MMV2.
Standards IS: SP15 (part 2): 2000 and for other tests the number of samples was decided as 10 for a limit of error of 10%. Tensile strength of coir yarns and geotextiles vary depending upon the
2.2.2. Tension test on coir fibres Tension tests were conducted on coir fibres to study the influence of fibre strength on yarn strength. Considering convenience of fixing in the jaws and availability of sufficient number of specimens, fibres were tested at a gauge length of 100 mm at a strain rate of 5%/min. 2.2.3. Tension test on woven coir geotextiles Tension tests were conducted on woven coir geotextiles at a strain rate of 5%/min as decided from the tests on yarns. The specimens were subjected to a pretension of 10 N. The sizes of the
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specimens were varied to study the effect of gauge length and width of specimen on tensile behaviour. Gauge lengths of 100, 150, 250 and 300 mm were used. Changing the number of yarns parallel to the direction of load as 4, 6, 8, 10 and 12 varied the width of specimen. The effect of length to width ratio was studied by keeping one of the dimensions constant while the other changed. 2.3. Direct shear test A conventional direct shear box of size 60 60 50 mm was used to study the frictional behaviour of various geotextiles with fine, medium and coarse sand at normal stresses of 100, 200 and 300 kPa. Geotextile specimen was placed at the middle of two halves filled with soil at 90% relative density. Required normal stress was applied and the specimen was sheared at the rate of 1.25 mm/min. 2.4. Pullout test The pullout test device consisted of a box of 450 mm length, 450 mm width and 600 mm depth, a vertical load application system, a horizontal force actuator device, a clamp and other required instrumentation. Fig. 3 shows the arrangement for pullout test. The pullout box consisted of steel plates welded at the edges. The back end of the tank was sealed, while the front end was provided with a slot at the mid-height to allow the geotextile to pass through. Sponge tapes and plywood pieces were mounted on the slot to prevent the loss of soil during the pullout operation. Friction acting between the soil and all over the inside walls of the box was minimized by means of greased rubber membranes. The normal load was applied through a rigid plate. The facility for rotating the loading cap during pullout operation was not provided in the testing equipment and this might have caused a non-uniform distribution of vertical stress. An electric jack operated by a motor applied the pullout force, which was measured using a proving ring placed between the motor and the clamping system. The pullout displacement at the clamp was measured by means of a displacement dial gauge. Sand was placed in layers of about 50 mm thick by sand raining technique in order to prepare a homogeneous sand sample. Weighed quantity of sand was allowed to fall from a height of 500 mm through a sieve that covered the plan area of box and tamped to get a relative density of 90%. After placing sand up to the pullout opening level, geotextile specimen of width 200 mm was placed on the top of the levelled sand to get an embedment
Rigid plate
300
Normal load
600
Dial gauge Coir geotextile Clamp
Proving Ring
Sand
450 Fig. 3. Pullout test setup (dimensions in mm).
387
length of 400 mm. The front end of the geotextile was firmly connected to the loading shaft by the clamp. Sand was then placed in layers up to the top level. The upper plate was then fixed and the desired normal load was applied. Each type of geotextile was tested under three normal pressures of 10, 20 and 30 kPa. The lower normal stresses were adopted to ensure the pullout failure minimizing the possibility of tension failure. The displacement dial gauge was fixed to measure the pullout displacement at the clamp. Then the geogrid was pulled out at a constant rate of 1.25 mm/min until the failure of specimen was reached.
3. Results and discussions 3.1. Tension test on fibres Table 4 shows the result of tension test on fibres. It is observed that tensile properties of coir fibres vary with their origin. Beach fibre has the maximum stiffness among the four types of fibres tested. Anjengo type fibres undergo large deformation before failure. The failure stress and strain of Vycome and Aratory fibres are comparable. 3.2. Tension test on coir yarns The tensile strength of coir yarn is generally termed as yarn tenacity which represents the maximum tensile force taken by the yarn in centiNewton or milliNewton per unit tex. Tex is the linear density of yarn expressed in gram per kilometre. 3.2.1. Effect of gauge length and strain rate on tensile behaviour of yarns To study the effect of gauge length on yarn tenacity, the gauge length was increased from 50 to 300 mm keeping the strain rate constant. An increase in yarn tenacity is observed when gauge length is increased from 50 to 150 mm and beyond that yarn tenacity be decreased. The coir fibres in yarns vary widely in length and diameter. The lengths of fibres vary from 10 to 250 mm and the diameters vary from 0.1 to 0.7 mm. Most of the strong fibres are of length between 100 and 150 mm. During spinning, short fibres are wound by longer fibres. At shorter gauge lengths, absence of strong and long fibres results into a reduction in yarn tenacity. The percentage of fibres longer than 150 mm is very less in yarns. The distribution of flaws increases the probability of slippage at higher gauge lengths and leads to localization of stress. Thereby yarn tenacity is low at gauge lengths above 150 mm. The same behaviour is observed at all strain rates. Fig. 4 shows the relationship between gauge length and yarn tenacity at a strain rate of 5%/min. Relationship between failure strain and gauge length shows that at lower lengths, more elongation takes place due to gripping of fibres at both ends. The variation of yarn tenacity with respect to deformation rate is shown in Fig. 5. An increase in yarn tenacity is observed as deformation rate is increased. The effect of strain rate over failure strain is not consistent for different types of yarns. Anjengo 275 shows a Table 4 Results of tension tests on coir fibres Fibre type
Breaking stress (MPa)
Breaking strain (%)
Breaking modulus (MPa)
Anjengo Vycome Beach Aratory
33.18 24.50 18.69 23.50
16.2 13.0 8.7 12.0
204.81 188.50 214.80 195.83
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or less constant thereafter. The influence of fibre type on the failure strain of yarn is clear from the two sets of values (Tables 4 and 5).
60
Yarn tenacity (mN/tex)
50
3.2.2. Selection of testing parameters Statistical analysis on tension test results shows the influence of gauge length and deformation rate on coefficient of variation of yarn tenacity and failure strain (Figs. 6 and 7). At shorter gauge lengths, the failure of yarn takes place due to breakage of fibres and at higher lengths slippage failure dominates. Wide variations in fibre strengths and mass variations are reflected in the corresponding yarn tenacity values at shorter gauge lengths. Increase in variation is also observed beyond a gauge length of 150 mm, because of non-uniform slippages of shorter fibres. At a gauge length of 150 mm, the coefficient of variation is found to be a minimum for both yarn tenacity and failure strain, which may be due to a balance between fibre breakage and slippage failures.
40
30
20
Anjengo-275 Beach-240 Vycome-240
10
Anjengo-220
40
0 0
100
200 Gauge length (mm)
300
400
30 Coefficient of variation (%)
Fig. 4. Effect of gauge length on yarn tenacity.
Yarn tenacity (mN/tex)
60
40 Anjengo-275 Anjengo-220 Vycome-240 Beach-240
20
20
Beach-240 10
Anjengo-275 Anjengo-220 Vycome-240
0 100
0 0 0
4
8
12
200 Gauge length (mm)
300
400
Fig. 6. Gauge length vs. coefficient of variation in yarn tenacity.
Strain rate (%/min) Fig. 5. Effect of strain rate on yarn tenacity.
30
Type of yarn
Anjengo 275 Anjengo 220 Beach 240 Vycome 240 Vycome 220 Vycome 200 Aratory 240
Breaking load (N)
Failure strain (%)
166.96 200.73 206.76 177.47 188.65 207.50 180.16
27.85 29.05 21.17 22.47 24.47 15.30 24.72
Yarn tenacity (mN/tex) 47.76 42.80 43.98 43.22 39.66 42.54 42.02
steady rate of increase of failure strain with strain rate. For beach 240, no significant effect is observed in failure strain with strain rate while for Anjengo 220 and Vycome 240 failure strain increases up to a strain rate of 5%/min and then remains more
Coefficient of variation (%)
Table 5 Yarn tenacity and failure strain of coir yarns
Anjengo-275
Anjengo-220
Beach-240
Vycome - 240
20
10
0 0
2
4 6 Strain rate (%/min)
8
10
Fig. 7. Strain rate vs. coefficient of variation in failure strain.
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Similarly a minimum variation of yarn tenacity and failure strain is observed at a strain rate of 5%/min. Based on these observations, it is decided to select a gauge length of 150 mm and a strain rate of 5%/min for the purpose of characterization of tensile properties of coir yarns. Results of tension tests on yarns conducted at these testing parameters are given in Table 5. Fig. 8 shows the load vs. elongation curves for different types of coir yarns. It is observed that all of the curves follow more or less similar pattern with an initial concavity and thereafter a non-linear behaviour. 3.2.3. Yarn strength as a function of fibre properties The fibre properties such as type, distribution, strength and yarn parameters namely thickness and method of spinning are found to be the important variables affecting yarn tenacity. The proportion of fibres with similar aspect ratio is more or less the same in all types of yarns. Hence, fibre strength is considered as the major factor controlling the yarn tenacity. Since it is very difficult to quantify the variations in spinning among yarn types, the only yarn parameter considered in this study is the thickness. For a particular runnage, thickness of yarn is indicated through scorage, which is a measure of coarseness or fineness of a yarn. Scorage is a number obtained by dividing 20, the number of strands that could be laid close to each other without overlapping in a length of 0.914 m. The following relationship between yarn tenacity, fibre modulus and scorage could be developed through regression analysis of test data. YT ¼ 0:1596Ef þ 1:1246C 0:0811,
(1)
389
less compared to that of yarns. At a gauge length of 150 mm and a width of specimen containing 10 or more number of yarns, consistent values of loads and failure strains are observed among similar specimens. Hence, it is decided to fix up the length and width of specimen as 150 mm and not less than 10 yarns, respectively. In order to avoid narrow strips for closely spaced geotextiles, a minimum width of 150 mm was used for all types. Fig. 9 shows the stress–strain curves for the tested coir geotextiles in warp direction. The curve is non-linear for geotextiles of low stiffness, whereas for MMA2 which is a dense type geotextile the curve is almost linear. The results of tension tests on geotextiles are given in Table 6. 3.3.1. Prediction of geotextile strength from yarn properties The spacing of yarns in the geotextile and yarn strength is the key parameters controlling the tensile strength of woven coir geotextiles. The following empirical relationship provided good correlation between test results and predicted values of tensile strength of geotextiles (Fig. 10): St Tg ¼ YT LD N (2) 106 , Sp St Tg ¼ ð0:1596Ef þ 1:1246C 0:0811Þ LD N 106 , Sp
(3)
where Tg is tensile strength of geotextile in kN/m, YT is tenacity of yarn in mN/tex, LD is linear density in tex, N is number of yarns/m
where YT is yarn tenacity in mN/tex, Ef is fibre modulus in N/mm2 and C is scorage of yarn.
40
MMA1
3.3. Tensile behaviour of woven coir geotextiles
MMA2 MMV1
Weaving selected yarns in warp and weft directions in a specific pattern makes woven coir geotextiles. The influence of gauge length on tensile properties of geotextiles was studied in the same way as that for yarns. As length of the specimen is increased keeping the width as constant, the tensile strength is found to be decreased. At the same time, no specific trend is observed in breaking load or failure strain while adopting different length to width ratios. The coefficient of variation of tensile strength and failure strain of geotextiles varied from 1.5% to 39% and 1.2% to 29% for different tests, which are considerably
Load (kN/m)
30
MMB1 20
10
0
250
10
0
200
Load (N)
MMV2
20 Strain (%)
30
40
Fig. 9. Results of tension tests on geotextiles in warp direction.
150 Anjengo-275
Table 6 Results of tension tests on geotextiles
Anjengo-220 100
Vycome- 240 Vycome-220 Vycome-200
50
Beach-240
Type of Warp direction geotextile Load Failure Secant (kN/m) strain (%) modulus at 10% strain (kN/m)
Weft direction Load (kN/m)
Failure Secant strain (%) modulus at 10% strain (kN/m)
MMA1 MMB1 MMA2 MMV1 MMV2
11.00 13.26 12.60 8.80 6.46
20.00 22.00 20.70 24.00 15.00
Aratory-240 0 0
10
20 Elongation (%)
30
Fig. 8. Results of tension tests on yarns.
40
13.25 15.60 38.00 14.00 7.47
23.20 20.00 36.12 23.00 17.00
82.00 87.25 88.00 90.00 44.17
81.70 82.00 92.00 55.30 48.30
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parallel to the direction of tensile force, St is spacing of yarn in the direction of test in millimetre, Sp is spacing of yarns perpendicular to the direction of test in millimetre.
3.4. Direct shear test The results of direct shear test on sand–coir geotextile composite are given in Table 7. A small displacement of 3–5 mm is only required for the development of interface friction as in the case of other extensible reinforcements (Fig. 11). The bond ratio varies from 0.68 to 1.72 for various coir geotextiles in different soils. Values of bond resistance obtained at 100 kPa are reasonable. The contact efficiency increases when the opening size of the geotextile increases and it appears to be higher than unity when the opening size is slightly larger than the particle size. The values of shear parameters at 200 and 300 kPa do not show a consistent behaviour. Higher normal stresses may cause an appreciable deformation of geotextile pushing it beneath the predetermined shear plane. This may result into inconsistent values of bond resistance.
3.5. Pullout test The pullout behaviour of coir geotextile with closely woven yarns is found to be different from that with open meshes in soils of different grain sizes. The pullout resistances of MMA2, with very close yarn spacing, are almost same in fine, medium and 120
Shear stress (kPa)
390
80
Fine sand +MMA1 +MMB1
40
+MMA2 +MMV1 +MMV2
0 1
0
40
2 3 4 Shear displacement (mm)
5
Fig. 11. Direct shear test results of geotextiles in fine sand at 100 kPa.
Pullout force (kN/m)
Predicted value (kN/m)
18
20
12
6
Fine sand Medium sand Coarse sand
0
0
0
10 20 30 Experimental value (kN/m)
40
0
Fig. 10. Predicted vs. experimental values of tensile strength of geotextiles.
5
10 15 20 Pullout displacement (mm)
25
Fig. 12. Pullout resistance of MMA2 at 10 kPa.
Table 7 Results of modified direct shear tests Type of sand
Normal stress (kPa)
Shear strength of sand (kPa)
Bond strength (kPa) MMA1
MMB1
MMA2
MMV1
MMV2
Fine sand
100 200 300
57.99 153.46 288.36
96.14 156.95 266.45
99.69 192.31 280.79
86.46 145.60 248.04
75.79 189.55 237.37
63.93 176.98 246.91
Medium sand
100 200 300
86.16 171.77 250.85
106.20 194.72 255.64
83.27 175.89 221.43
86.81 180.22 261.34
90.34 191.94 261.6
100.71 148.74 227.36
Coarse sand
100 200 300
97.77 234.30 355.34
112.87 180.87 213.16
57.61 233.22 299.41
84.09 169.75 302.22
83.79 154.21 347.67
69.53 243.56 256.28
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20 Fine sand Medium sand Coarse sand
Pullout force (kN/m)
15
10
5
0 0
5
10 15 20 Pullout displacement (mm)
25
Fig. 13. Pullout resistance of MMB1 at 10 kPa.
coarse sand (Fig. 12). For geotextiles with open meshes, pullout resistance is maximum in fine sand and minimum in coarse sand (Fig. 13). The influence of bearing against weft yarns and interlocking is reflected in this observation. This is in confirmation with the findings of Palmeira and Milligan (1989) that the pullout resistance is strongly influenced by the bearing member space ratio, S/D, in which S is the bearing member spacing and D is the bearing member thickness. Higher bearing resistance from an individual bearing member is expected when the S/D ratio is higher. Jewell (1990) identified limiting values of the S/D50 ratio as 10 that characterize either interface shear or full interaction mechanisms. As per this criterion, the interaction between MMA2 and sand may be considered as entirely due to interface friction. For other types of geotextiles, total pullout resistance is a combination of bearing and interlocking together with friction. Hence, the pullout resistance is a maximum in fine sand. Another important observation is that there is an increase in the tensile capacity of coir geotextiles with open meshes, when confined well in soil, than in air tensile strength. The displacement at peak pullout resistance is more in coarse sand compared to fine sand for the same geotextile under the same normal stress. Similar observation was reported by Sridharan and Hans (1988) for rough reinforcement.
4. Conclusions Tension tests were conducted on coir yarns and woven coir geotextiles at different gauge lengths and strain rates. The results indicate that the tensile properties of coir yarns and woven coir geotextiles vary significantly based on gauge length and strain rate. When gauge length and strain rate were adopted as 150 mm and 5%/min, consistent values of tensile strength and failure strain were obtained and hence these values are suggested as testing parameters for characterization of tensile properties of coir yarns and woven coir geotextiles. The test results demonstrate that the width of the geotextile specimen for tension test would have a minimum of 10 force bearing yarns to get reliable results. Tensile properties of coir fibres are found to vary depending upon their origin. Beach fibre has the highest breaking modulus. Breaking stress as well as failure strain is large for Anjengo fibres compared to other types. Vycome and Aratory fibres show identical behaviour under tension. Tensile strength of coir yarn
391
can be represented as a function of fibre modulus and scorage of yarn. Tensile strength of woven coir geotextiles is dependent on the type of fibre, thickness of yarn and the weaving pattern. Tensile strength of most of the tested mesh mattings lies in the range of 10–20 kN/m. To get satisfactory performance in the function of temporary reinforcement, closely woven coir geotextiles of high tensile strengths are to be used. Modified direct shear tests were conducted on sand–coir geotextile composite using fine, medium and coarse sand. At lower normal stress, the bond resistance of woven coir geotextiles–sand interface is more than the shear strength of soil, while at higher normal stresses the values obtained are not consistent. Pullout tests on coir geotextiles were conducted in fine, medium and coarse sand. Tensile strength of geotextile and the relative size of mesh opening to particle size of fill material govern the pullout resistance of woven coir geotextiles. Closely woven geotextiles offer good pullout resistance in fine, medium as well as coarse sand due to high interface friction. Pullout resistance of open meshed geotextile is more in fine-grained soil compared to coarse-grained soil because of good interlocking and bearing resistance. It should be noted that the results presented in this paper are limited to the short-term tensile properties of coir geotextiles. Coir being biodegradable material losses its tensile strength due to in situ deterioration. However, this material is eventually converted into organic matter which in turn aggregates the soil particles and tries to retain its improved properties for a certain period. It is presumed that the construction carried out get stabilized by this period. If coir geotextiles are to be proposed for long-term reinforcement applications, there is a need to suggest reduction factors while using the reported values, which is not attempted in this paper due to limitations in the available data. It is recommended that future studies include durability and long-term performance aspects of coir geotextiles.
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