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Strength Characteristics of Sand Reinforced with Coir Fibres and Coir Geotextiles
G. Venkatappa Rao Professor, Department of Civil Engineering, Indian Institute of Technology, New Delhi, India gvrao@civil.iitd.ernet.in Senior Lecturer, Department of Civil Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh, India rkd@recham.ernet.in and Ujwala D. Former Student, Department of Civil Engineering, Indian Institute of Technology, New Delhi, India u_damarashetty@yahoo.com |
ABSTRACT
Laboratory triaxial compression tests were carried out in order to determine the strength characteristics of sand reinforced with coir fibres and coir geotxtiles. The mechanical behaviour of the composite material was investigated through varying four confining pressures (24.5 kPa to 196 kPa) two types of coir fibres in a random arrangement as well as in a layered arrangement with percentage varying from 0.5% to 1%. For oriented reinforcement, two types of woven coir geotextiles of different mass per unit area and aperture size and one non-woven coir geotextile were used. Tests were performed on 100 mm diameter and 200 mm high specimens. The results indicated that inclusion of coir fibres and coir geotextiles improves the performance of sand specimens. The admixtures can be used in rural roads and for ground improvement
Keywords: Sand, Coir fibres, Coir Geotextiles, Ground improvement, Behaviour
INTRODUCTION
India is the largest producer (66% of world production) of coir fibre from the husk of coconut fruit. From one million coconut husks, 80 tons of fibre can be extracted. Though about 13000 million nuts are annually harvested in India, less than 25% are used industrially. The resultant mutilated husk either becomes garbage or are dried and burnt as fuel. This rather indiscriminate destruction of a potentially useful material owes much to the lack of alternate end uses of coir fibres.
Many civil engineering problems in costal areas in India need stabilization of soft soil. Some of these structures may be temporary for establishing roads and rail communication links and in some accessibility by itself possesses a major problem. To tackle such problems in civil engineering, geosynthetics emerged as a good solution. To improve the mechanical properties of soils, a variety of materials are used for reinforcement e.g. metallic elements, geosynthetics and others. Majority of geosynthetics used in civil engineering application are polymeric in composition. These products generally have a long life and do not undergo biological degradation, but are liable to create environmental problem from its manufacture till the end use. In effecting this, the use of biodegradable natural fibres are gaining popularity.
This paper presents the results of drained triaxial tests carried out on Yamuna sand reinforced with coir fibres in random arrangement as well as a single layer of coir fibres, two woven coir geotextiles and one non-woven coir geotextile placed at the mid height of the specimen. In view of the extensive availability of coir fibres in the coastal areas of India, it is planned to investigate their efficacy for possible use in ground improvement, and in building rural roads.
BACKGROUND
Banerjee et al. (2002) investigated the dimensional and mechanical properties of coir fibres as a function of fibre length. A sufficient quantity of retted coir fibres was collected from one particular pit in Kerala and subjected to certain tests to find out fibre length distribution, determination of thickness, distribution of coir fibres with in a husk, tensile properties and flexural rigidity. From their studies it was concluded that the length, thickness and linear density of fibres obtained from this particular type of husk range from 50 mm to 250 mm, 130 µ to 325 µ and 19 tex to 60 tex respectively. The longer fibres are, in general, thicker than the shorter ones. The fibre thickness is highly variable along its length. The breaking load and work of rupture increases perceptibly for fibres longer than 149 mm. However the fibres shorter than 150 mm exhibit very similar values. The flexural rigidity of coir fibres longer than 149 mm exhibit sharp increase with the increase in fibre length.
Venkatappa Rao and Balan (2000) after conducting Drained triaxial test on specimens of sand reinforced with coir fibres (25 mm and 50 mm) upto 1% reported a significant gain in strength parameters and stiffness.
Varghese et al. (1989) investigated the possibility of increasing the bearing capacity of cohesionless soils by reinforcing with coconut fibres through model studies. It has been observed that the bearing capacity of foundation soil will be maximum when the reinforcement is kept at a depth of 0.41 times the width of the foundation.
Guha (1995) reveal that coir fibre differs from jute fibres in an aspect other than durability, jute fibres exhibit moderately high modulus as well as high tenacity and very low elongation at break whereas coir fibres behave exactly in the opposite manner, namely moderately low modulus, low tenacity and very high elongation at break. This difference persists irrespective of the length of coir fibre.
From the literature presented above it can be concluded that limited work has been carried out in the past to determine the strength characteristics of coir fibre and coir geotextiles reinforced soil to understand the role played by these coir fibres and coir geotextiles and to determine the field of their application.
EXPERIMENTAL WORK
To investigate the effects of test parameters on the mechanical behaviour of unreinforced and reinforced sand, a total of 48 triaxial compression tests were performed. The test parameters included: four confining pressures (24.5 kPa to 196 kPa) two types of coir fibres in a random arrangement with percentage varying from 0.5% to 1%. For oriented reinforcement, two types of woven coir geotextiles of different mass per unit area and aperture size and one non-woven coir geotextile were used in this study.
Test Material
Sand
The investigation was carried out on locally available Yamuna sand, which is fine-grained uniformly graded sand. It had a specific gravity of 2.67, maximum particle size of 0.52 mm, minimum particle size of 0.04 mm, mean particle diameter (D50) of 0.24 mm, coefficient of uniformity (Cu) of 1.76 and coefficient of curvature (Cc) of 1.09. Minimum and maximum void ratios were 0.64 and 1.04 while the corresponding dry unit weights were 16.30 kN/m3 and 13.10 kN/m3 respectively.
Coir Fibres
To establish the natural scatter in length, sufficient quantity of coir fibre was collected and mixed by hand. Representative sample were selected at random from main mass. Fibres, which were quite wavy in configuration, were taken out individually straightened out and then measured for length against a scale to the accuracy of 1mm. With in a husk a wide spectrum ranging between 50 mm and 250 mm was found. Accordingly, the fibres were divided into four groups: 50-99 mm, 100-149 mm, 150-199 mm and 200-250 mm. The longer fibres, in general were found to be stiffer than the shorter one, as their thickness is more than the shorter one. In order to study the effect of stiffness of fibre on the behaviour of sand, the range of fibres between 150-199 mm designated as Type A1 (Figure 1) and fibres < 100 mm designated as Type A2 (Figure 2) were selected for experimental work. These fibres were later cut into lengths of 25 mm as shown in Figs.3 and 4 and their properties as reported by Banerjee et al, (2002) are tabulated in Table 1.
Figure 1. Figure 1 Coir Fibre Type A1
Figure 2. Coir Fibre Type A2
Figure 3. Coir fibres Type A1 after cutting them into 25 mm length.
Figure 4. Coir fibres Type A2 after cutting them into 25 mm length.
Table 1. Properties of coir fibres (After Banerjee et al., 2002)
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| Property | Type A1 | Type A2 |
| ||
| Breaking load, N | 487.3 | 217.8 |
| Tenacity (cN/tex) | 12.4 | 11.5 |
| Modulus (Initial) (cN/tex) | 113.7 | 85.9 |
| Modulus offset) (cN/tex) | 13.8 | 9.5 |
| Breaking extension,% | 44.8 | 41.7 |
| Energy to break (Joules) | 0.0158 | 0.0062 |
| Thickness in 1/100th mm | 20.42 | 13.57 |
| Linear density (tex) | 39.4 | 18.9 |
| ||
Coir Geotextiles
Two different varieties of woven and one non-woven coir geotextiles designated as Type B, C and Type D was used in the present study. The tensile strength and tensile elongation at failure in machine direction of these geotextiles are tabulated in Table 2. More details of the properties of these coir geotextiles are available from Venkatappa Rao and Dutta (2005).
Table 2. Properties of coir geotextiles
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| Coir geotextile | Mass per unit area (gsm) | Tensile strength (kN/m) | Tensile strain at failure (%) |
| |||
| Type B | 610 | 11.45 | 25.42 |
| Type C | 1335 | 31.5 | 42 |
| Type D | 750 | 2.76 | 31.67 |
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EXPERIMENTAL PROCEDURE
The specimens have been prepared by a procedure similar to that adopted for preparing specimens of saturated cohesionless soil for conventional consolidated drained triaxial tests. Saturated sand was deposited in 5 layers into the rubber membrane inside a split mould former. Each sand layer was compacted to the required unit weight by giving 25 blows with tamping rod. Fibres by dry weight of soil were mixed thoroughly with sand. Such a mixture was then placed in layers into a split mould to obtain 100 mm diameter and 200 mm high specimen with care being taken to ensure uniform distribution and random orientation of fibres. The unit weight of sand specimen was maintained at 16 kN/m3. In case of woven and nonwoven coir geotextile reinforcement, circular discs of 100 mm diameter were cut from the fabric. Figs. 5 to 7 show a view of woven and Nonwoven geotextile discs. Such reinforcement discs were placed on the already compacted/densified and leveled sand layer of 100 mm height. The procedure was repeated till the full height of the specimen was reached. Thus the reinforced sand specimens were built up layer by layer with circular discs of reinforcement placed almost at the mid height of the specimen. Accordingly, the ratio of specimen radius (r) to reinforcement spacing (DH) works out to be 0.5 as illustrated in Figure 8. Similarly coir fibres of Type A1 and Type A2, with a fibre content of 0.5% and 1% by dry weight of soil are placed as a single layer maintaining the r/DH ratio equal to 0.5. Conventional consolidated drained triaxial tests were then conducted on these specimens. The specimens, were tested at a deformation rate of 0.66 mm/min.
Figure 5. View of woven coir geotextile of type B
Figure 6. View of woven coir geotextile of type C
Figure 7. View of nonwoven coir geotextile type D
Figure 8. Position of reinforcement in triaxial specimen.
RESULTS AND DISCUSSION
Stress Strain Behaviour
Unreinforced sand
Tables 3 and 4 present the values of deviator stress & the corresponding axial failure strain at various confining pressures. It can be generally seen from these tables that the peak deviator stress and corresponding axial strain increases with the increase in confining pressure. For example, sand at s3= 49 kPa exhibits a peak stress of 289.8 kPa and a failure strain of 2.9%, whereas these values at s3= 196 kPa are 790.7 kPa and 5.6% respectively.
Table 3. Deviator stress at failure
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| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
Deviator stress (kPa) at confining pressure of (kPa) |
|||
| 24.5 | 49 | 98 | 196 | |||
| ||||||
| Sand | 37.2 | 289.8 | 478.3 | 790.7 | ||
| A1 | Random | 0.5 | 230.2 | 477.5 | 721.6 | 940.7 |
| A1 | Random | 1.0 | 442.4 | 653.2 | 1080.5 | 1389.0 |
| A2 | Random | 0.5 | 294.4 | 533.5 | 696.2 | 982.2 |
| A2 | Random | 1.0 | 459.4 | 815.5 | 974.6 | 1319.9 |
| A1 | Layered | 0.5 | 278.9 | 643.3 | 695.7 | 1058.7 |
| A1 | Layered | 1.0 | 305.5 | 666.9 | 750.9 | 1237.0 |
| A2 | Layered | 0.5 | 196.6 | 467.0 | 634.7 | 1163.2 |
| A2 | Layered | 1.0 | 225.7 | 494.8 | 679.0 | 1166.2 |
| B | Layered | - | 368.0 | 570.5 | 859.4 | 1222.1 |
| C | Layered | - | 463.5 | 620.5 | 933.3 | 1178.9 |
| D | Layered | - | 279.6 | 516.2 | 824.0 | 1149.9 |
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Table 4. Strain at failure
| ||||||
| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
Deviator stress (kPa) at confining pressure of (kPa) |
|||
| 24.5 | 49 | 98 | 196 | |||
| ||||||
| Sand | 37.2 | 289.8 | 478.3 | 790.7 | ||
| A1 | Random | 0.5 | 230.2 | 477.5 | 721.6 | 940.7 |
| A1 | Random | 1.0 | 442.4 | 653.2 | 1080.5 | 1389.0 |
| A2 | Random | 0.5 | 294.4 | 533.5 | 696.2 | 982.2 |
| A2 | Random | 1.0 | 459.4 | 815.5 | 974.6 | 1319.9 |
| A1 | Layered | 0.5 | 278.9 | 643.3 | 695.7 | 1058.7 |
| A1 | Layered | 1.0 | 305.5 | 666.9 | 750.9 | 1237.0 |
| A2 | Layered | 0.5 | 196.6 | 467.0 | 634.7 | 1163.2 |
| A2 | Layered | 1.0 | 225.7 | 494.8 | 679.0 | 1166.2 |
| B | Layered | - | 368.0 | 570.5 | 859.4 | 1222.1 |
| C | Layered | - | 463.5 | 620.5 | 933.3 | 1178.9 |
| D | Layered | - | 279.6 | 516.2 | 824.0 | 1149.9 |
| ||||||
Sand reinforced with randomly distributed coir fibres
Typical values of deviator stress and corresponding axial failure strains for unreinforced and reinforced sand with 0.5% and 1% of coir fibres (Type A1 & A2) of 25 mm length are also tabulated in Tables 3 & 4 at different confining pressures. In general, it is observed that the inclusion of coir fibres increased the deviator stress developed at any strain level, which confirms the ability of coir fibre to strengthen the sands. For example, at s3= 98 kPa, the unreinforced sand has a deviator stress at failure as 478.7 kPa and the corresponding axial strain at failure of 5.2%, the sand reinforced with coir fibres with a content of 0.5% (A1 type) shows a deviator stress of 721.6 kPa and the corresponding axial failure strain of 8.1%. At higher pressures the increase in deviator stress is very less but strain at failure increased. The results (Table 3) further indicate that the deviator stress at failure increases with the increase in fibre content for Type A1 and Type A2 fibres. For example, sand with 0.5% fibre Type A1 and Type A2 at 49 kPa exhibit a deviator stress of 477.5 kPa and 533.5 kPa respectively whereas, this value for the sand is 289.8 kPa. Similarly for the sand with 1% fibre Type A1 and Type A2 at a confining pressure of 196 kPa, the values of deviator stress are 1389.0 kPa and 1319.9 kPa respectively whereas for the sand this value is 790.7 kPa. On comparing the values of deviator stress in Table 3, It is found that the increase in deviator stress is more for sand reinforced with Type A2 fibre compared to Type A1 fibre at lower confining pressures, whereas at higher confining pressures sand reinforced with Type A1 and A2 fibres show more or less similar increase in deviator stress. This increase in strength at lower confining pressure may be attributed to increase in contact area between soil and fibre as Type A2 fibre has more surface area.
Sand reinforced with single layer of coir fibres
Typical values of deviator stress and corresponding axial failure strains for sand reinforced with 0.5% and 1% (single layer) of coir fibres (Type A1 & A2) of 25 mm length are tabulated in Tables 3 & 4 at different confining pressures. In general, it is observed that the inclusion of coir fibres as single layer of reinforcement increased the deviator stress developed at any strain level. For example, at s3= 49 kPa, the unreinforced sand has a deviator stress at failure as 289.8 kPa and the corresponding axial strain at failure of 2.9%, the sand reinforced with coir fibres as single layer of reinforcement with a content of 0.5% (Type A1 and Type A2) shows a deviator stress of 643.3 kPa & 467.0 kPa and the corresponding axial failure strain of 6.8% & 10.3% respectively. Similar trend is observed for sand reinforced with coir fibres as single layer of reinforcement with a content of 1.0% (both Type A1 and Type A2) at the same confining pressure. Table 3 further indicate that for sand reinforced with Type A1 & A2 coir fibre as a single layer of reinforcement, there is marginal increase in deviator stress with increase in percentage of coir fibres as a single layer of reinforcement. For example, at s3= 49 kPa, sand reinforced with coir fibres (Type A1 & A2) as a single layer of reinforcement and a content of 0.5% exhibit a deviator stress of 643.3 kPa and 467.0 kPa respectively whereas, these values for the sand reinforced with coir fibres (Type A1 & A2) as a single layer of reinforcement and a content of 1% are 666.9 kPa and 494.8 kPa respectively. So it can be concluded that increase in content of coir fibres, as single layer of reinforcement does not have much effect in increasing the value of deviator stress. On comparing the values of deviator stress in Table 3, it is found that the increase in deviator stress is more for sand reinforced with Type A1 fibres as a single layer of reinforcement compared to Type A2 fibres at lower confining pressures. The behaviour is unlike of random inclusion. For example, at s3=24.5 kPa, sand reinforced with coir fibres (Type A1) as a single layer of reinforcement and a content of 0.5% show a deviator stress at failure as 278.9 kPa and the corresponding failure strain of 5.4% and sand reinforced with coir fibres of Type A2 at the same content show a deviator stress of 196.6 kPa and the corresponding failure strain of 6.3% respectively whereas these values for the normal sand at the same confining pressure are 37.2 kPa and 1.9% at failure.
Sand reinforced with oriented coir geotextiles
Typical values of deviator stress and corresponding axial failure strains for sand reinforced with coir geotextiles Type B, C and Type D are tabulated in Tables 3 and 4 at different confining pressures. It can be generally seen from Tables 3 & 4 that for sand reinforced with coir geotextiles both peak stress and the corresponding axial failure strain increases with increase in confining pressure. For example, sand reinforced with one disc of coir geotextile Type B, C & D at 24.5 kPa exhibits a peak deviator stress of 368 kPa, 463.5 kPa and 279.6 kPa and a failure strain of 6.7%, 8.6% & 4.2% respectively whereas, these values for unreinforced sand are 37.2 kPa and 1.9% respectively. For sand reinforced with coir geotextiles of Type B, C and D, at 98 kPa, exhibits a peak deviator stress of 859.4 kPa, 933.3 kPa and 824.0 kPa and a failure strain of 7.5%, 12.1% & 6.9% respectively whereas, these values for unreinforced sand are 478.3 kPa and 5.1% respectively. Similar behaviour could be noticed at other confining pressures also.
Volumetric Strain
Table 5 present the values of volumetric strain at failure corresponding to different confining pressures for sand reinforced with coir fibres and coir geotextiles. A study of this table indicate that the tendency of samples to dilate is restricted by the coir fibres or coir geotextiles. It is also believed that coir fibres and coir geotextiles restrain lateral deformation and consequently, effectively restricts the dilation of the samples. This effect becomes further apparent when the percentage of coir fibres increases.
Table 5. Volumetric strain at failure
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| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
Volumetric strain (%) at confining pressure of (kPa) | |||||
| 24.5 | 49 | 98 | 196 | |||||
| ||||||||
| Sand | 1.52 | 0.97 | 1.04 | 1.05 | ||||
| A1 | Random | 0.5 | 3.06 | 1.53 | 1.02 | 0.84 | ||
| A1 | Random | 1.0 | 3.51 | 2.12 | 1.06 | 1.21 | ||
| A2 | Random | 0.5 | 2.28 | 1.99 | 2.56 | -0.12 | ||
| A2 | Random | 1.0 | 3.66 | 1.38 | 0.64 | -0.24 | ||
| A1 | Layered | 0.5 | 1.98 | 1.10 | 1.15 | 1.23 | ||
| A1 | Layered | 1.0 | 1.82 | 1.76 | 0.81 | 0.67 | ||
| A2 | Layered | 0.5 | 0.95 | 1.77 | 0.57 | -0.94 | ||
| A2 | Layered | 1.0 | 2.60 | 1.25 | 0.28 | 0.37 | ||
| B | Layered | - | 3.07 | 3.11 | 1.97 | 0.12 | ||
| C | Layered | - | 3.25 | 3.69 | 2.98 | 2.50 | ||
| D | Layered | - | 2.01 | 2.31 | 1.85 | 0.57 | ||
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Table 6. Strength characteristics
| ||||||
| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
s3<scrit | s3>scrit | c’ (kPa) | f’ | c’ | f’ |
| ||||||
| Sand | 0 | 43.060 | 0 | 43.060 | ||
| A1 | Random | 0.5 | 0 | 55.960 | 88.12 | 37.500 |
| A1 | Random | 1.0 | 0 | 61.490 | 95.07 | 45.650 |
| A2 | Random | 0.5 | 0 | 57.960 | 97.13 | 37.070 |
| A2 | Random | 1.0 | 0 | 63.540 | 152.40 | 39.250 |
| A1 | Layered | 0.5 | 0 | 59.880 | 112.47 | 37.070 |
| A1 | Layered | 1.0 | 0 | 60.470 | 90.77 | 42.390 |
| A2 | Layered | 0.5 | 0 | 55.320 | 41.05 | 45.120 |
| A2 | Layered | 1.0 | 0 | 56.340 | 52.58 | 44.320 |
| B | Layered | - | 0 | 59.490 | 82.81 | 43.330 |
| C | Layered | - | 0 | 61.330 | 109.56 | 40.820 |
| D | Layered | - | 0 | 57.430 | 74.86 | 42.870 |
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Strength Characteristics
The effective stress strength parameters f' and c’are summarised in Table 6. A study of this table reveals the following:
The strength parameters for sand without coir fibres are f= 43.06o and c’ = 0.0.
Up to a confining pressure of 49 kPa, for sand reinforced with 0.5% coir fibres of Type A1 in a random arrangement, the f increased from 3.06o to 55.96o and for sand reinforced with coir fibres of Type A2 in a random arrangement the increase in f was from 43.06o to 57.96o. The value of c’ was equal zero in both the cases. However for sand reinforced with 1.0% coir fibres in a random arrangement of Types A1 and A2, the increase in f was from 43.06° to 61.49° and from 43.06° to 63.54° respectively. The value of c’ was equal zero in both the cases. For confining pressure greater than 49 kPa, the value of c’ increased from zero to 88.12 kPa and 97.13 kPa for sand reinforced with 0.5% coir fibres of Type A1 and A2 respectively. Similarly the increase in c’ at 1.0% coir fibre content in sand is from zero to 95.07 kPa and 152.40 kPa for Type A1 and A2 coir fibres respectively.
Upto a confining pressure of 49 kPa and a coir fibre content of 0.5% in sand as a single layer of reinforcement, the value of f increased from 43.06° to 59.88° and 43.06° to 55.320 for both the types (Type A1 & A2) of coir fibres respectively. The value of c’ was equal zero in both the cases. Similarly at 1% coir fibres (Types A1 & A2) in sand as a single layer of reinforcement, the f increased up to 60.47° and 56.34° for both the types of reinforcement and c’ was equal to zero in both the cases. For confining pressure greater than 49 kPa and coir fibre content of 0.5%, the value of c’ increased from zero to 112.47 kPa for A1 type coir fibres and 41.05 kPa for A2 type coir fibres as a single layer of reinforcement in sand. Similarly the increase in c’ at 1.0% coir fibre content in sand as a single layer of reinforcement, is from zero to 90.77 kPa and 52.58 kPa for Type A1 and A2 coir fibres respectively.
Upto a confining pressure of 49 kPa, sand reinforced with woven coir geotextile Type B & C, the value of f’ increased from 43.06° to 59.49° & 61.33° respectively. On the other hand for sand reinforced with non-woven coir geotextile Type D, f’ increased to 57.43° and c’ remained zero in all the above cases. For confining pressure greater than 49 kPa, sand reinforced with woven coir geotextile Type C, c’ increased to 109.56 kPa and the value of f’ decreased to 40.82°. Similarly when sand reinforced with woven coir geotextile Type B, c’ increased to 82.81 kPa and f’ was 43.33°. For sand reinforced with non-woven coir geotextile of Type D, c’ increased to 74.86 kPa and f’ decreased to 42.87°.
The strength parameters of sand reinforced with Type A2 fibre at 1.0% content in a random arrangement is greater than that reported for any of the coir geotextiles (Type B, C and D) reinforced soil.
The strength parameters of sand reinforced with Type A1 coir fibres at 1% content are comparable with Type C and greater than Type B and Type D reinforced sand.
Sand reinforced with Type A1 fibre at 0.5% content exhibited the same strength as that of Type D.
The strength parameters of sand reinforced with Type A2 at 0.5% are comparable to that of both Type B and Type D reinforced sand.
The strength parameters of sand reinforced with Type A1 fibre in a random arrangement at a content of 1% and sand reinforced with Type A1 fibres at content of 1% as layered reinforcement are comparable to that of Type B and Type C reinforced sand. Hence, they can be used in place of geotextiles.
Initial Tangent Modulus
The typical variation of initial tengent modulus with confining pressure and percentage inclusion of coir fibres is presented in Table 7. Table 7 also contains the variation of initial tangent modulus with confining pressure for sand reinforced with woven and non-woven coir geotextiles. A study of this table reveals:
For sand, unreinforced and reinforced with coir fibres and coir geotextiles, the initial tangent modulus increases with increase in confining pressures.
For sand reinforced with randomly distributed as well as single layer of coir fibres of Type A1, the initial tangent modulus in general increases marginally with the increase in coir fibres content from 0.5% to 1.0%.
For sand reinforced with randomly distributed as well as single layer of coir fibres of Type A2, the initial tangent modulus in general decreases with the increase in coir fibres content from 0.5% to 1.0%.
Secant Modulus
The typical variation of secant modulus with confining pressure and percentage inclusion of coir fibres is presented in Table 8. Table 8 also contains the variation of secant modulus with confining pressure for sand reinforced with woven and non-woven coir geotextiles. A study of this table reveals:
For sand, unreinforced and reinforced with coir fibres and coir geotextiles, the secant modulus increases with increase in confining pressures.
For sand reinforced with randomly distributed as well as single layer of coir fibres of Type A1, the secant modulus in general increases marginally with the increase in coir fibres content from 0.5% to 1.0%.
For sand reinforced with randomly distributed as well as single layer of coir fibres of Type A2, the secant modulus in general decreases with the increase in coir fibres content from 0.5% to 1.0%.
Table 7. Initial tangent modulus
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| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
Initial tangent modulus (Mpa) at confining pressure of (kPa) | |||
| 24.5 | 49 | 98 | 196 | |||
| ||||||
| Sand | 12.55 | 71.27 | 63.72 | 145.85 | ||
| A1 | Random | 0.5 | 4.63 | 11.19 | 22.76 | 73.33 |
| A1 | Random | 1.0 | 10.93 | 18.35 | 35.50 | 76.43 |
| A2 | Random | 0.5 | 11.05 | 14.14 | 25.98 | 30.70 |
| A2 | Random | 1.0 | 7.59 | 10.41 | 13.29 | 29.01 |
| A1 | Layered | 0.5 | 11.87 | 21.65 | 27.84 | 29.62 |
| A1 | Layered | 1.0 | 17.76 | 27.88 | 28.37 | 30.57 |
| A2 | Layered | 0.5 | 7.24 | 8.52 | 12.84 | 42.22 |
| A2 | Layered | 1.0 | 3.20 | 8.32 | 10.89 | 22.37 |
| B | Layered | - | 11.67 | 24.47 | 32.01 | 40.81 |
| C | Layered | - | 10.59 | 15.54 | 26.35 | 64.93 |
| D | Layered | - | 39.29 | 33.07 | 29.53 | 47.84 |
| ||||||
Table 8. Secant modulus
| ||||||
| Reinforcement type |
Reinforcement arrangement |
Percentage reinforcement |
Initial tangent modulus (Mpa) at confining pressure of (kPa) | |||
| 24.5 | 49 | 98 | 196 | |||
| ||||||
| Sand | 19.18 | 41.42 | 43.52 | 80.43 | ||
| A1 | Random | 0.5 | 4.23 | 11.28 | 16.18 | 45.26 |
| A1 | Random | 1.0 | 8.09 | 16.95 | 22.93 | 45.66 |
| A2 | Random | 0.5 | 7.77 | 11.85 | 16.95 | 19.77 |
| A2 | Random | 1.0 | 5.10 | 7.63 | 8.83 | 19.82 |
| A1 | Layered | 0.5 | 8.48 | 18.72 | 19.46 | 23.83 |
| A1 | Layered | 1.0 | 10.79 | 17.10 | 16.64 | 20.19 |
| A2 | Layered | 0.5 | 4.59 | 7.38 | 9.82 | 29.86 |
| A2 | Layered | 1.0 | 2.37 | 6.39 | 8.56 | 12.24 |
| B | Layered | - | 8.27 | 17.29 | 20.16 | 27.64 |
| C | Layered | - | 7.94 | 9.17 | 15.31 | 34.41 |
| D | Layered | - | 26.57 | 20.66 | 20.78 | 33.04 |
| ||||||
CONCLUSION
The following conclusions are drawn from the experimental studies
The results are encouraging in that the behaviour is similar to that observed with synthetic fibres and meshes. Considering that coir has a longer life compared to other natural fibres that degrade much faster, it is possible to use these fibres in rural roads and ground improvement.
REFERENCES
