Journal of Scientific & Industrial Research Vol.59, January 2000, pp 55-62

Studies on the Potential of Coir Reinforced Structures

K P Sheebha, S Sobha, Santha Leah Paul and D Rema Devi Department of Civil Engineering, College of Engineering, Trivandrum 695 016

Received : 21 December 1998; revised received: 07 October 1999

Geotextiles made of natural fibres are increasingly finding a place in erosion control, but not for soil reinforcement application, in spite of the fact that strong fibres like coir which have a very high lignin content can be effectively made use of provided they are given suitable treatment. This paper deals with potentialities of coir products like coir ropes, coir felt and grids for improving weak subsoil. The effectiveness of coir n.eedled felt on clay beds were examined using CBR type penetration tests on a kaolinite clay bed at fast and slow rates of strain with a view to comparing them with sand blankets used for accelerating consolidation. The needled felt was also strengthened by nylon netting and coir grids. Limited cyclic wetting and drying tests performed on these products showed that the strength is not significantly reduced by these tests. It is concluded that needled felt is a good seating layer to economise on sand blankets for clay formation on which embankments can be constructed at a fast rate as there is little possibility of undrained failure.

Light bituseal and cement-PYA coating help to improve both the resistance and durability of the coir composites.

Introduction Geotextiles have made an extensive impact on soil

stabilisation following the concept of soil reinforcement. This has been possible due to a number of functions of these products unlike matallic reinforcement which pro­vides only restraint. The filtration, separation and drain­age functions make these products more useful in deal­ing with fine grained soils and improve their properties . The advent of polymeric materials for this use has been due to the stability, strength and manufacturing ease of these products while the use of natural fibres used in geotextiles has been confined only to soil bio-engineer­ing problems on the basis that these fibres are far too weak to compete with synthetic fibres. On this account, the use of geotextiles made of natural fibres presently accounts for a very insignificant amount of the world market. The potential of natural fibres particularly that of coir for ground improvement has been proved over the past 20y by researchers 1• Coir is a renewable fibre resource and is more durable than jute though both these fibres grow abundantly in India. Coir consisting of ap­proximately 50 per cent lignin and 50 per cent cellulose is one of the strongest natural fibres and has the poten­tial for more lasting functions than for its present use for making low grade products such as mattings and car­pets.

The concept of soil reinforcement in resisting defor­mation due to load is the restraint imposed on the soil mass due to the relatively high modulus of the reinforce-

ment and effective frictional transfer of load between them. Though coir fabrics have lower modulus as com­pared with metallic and synthetic fabrics, they come under the category of extensible reinforcements for use, i.e., ply soil, most durable among natural fabrics. Their beneficial role has been shown in earlier studiesl.2

• A comparison of the properties of natural fibres and other reinforcement materials is given in Table 1 (ref. 3 and 4). Mechanical properties of fabrics made from both natu­ral and synthetic materials commonly used for reinforce­ment in soils are given in Table 2

While this applies for reinforcement in cohesion less soil masses, it is different for ground improvement in a soft clay sub grade. In this case the reinforcement mate­rial placed between the fill and soft clay has to perform several functions like separation, drainage and anchor­age besides reinforcement. In this concept the role of coir needled felt has been considered to be a potential resource for ground improvement. The cross and in plane permeabilities of coir felt are of the order of 0 .005 and 0.08 em/sec respectively. The latter is much higher than the value for synthetic fabrics and hence this is suitable for dissipation of pore pressures in soft clays.

Materials and Methods

Two aspects were considered in this study. One was to examine the strength deformation behaviour of need led felt in clay soils, and in particular the possibility of us-

56 J SCI IND RES VOL.59 JANUARY 2000

Table I -Typical strength properties of fibres used for reinforcements

Fibre Tensile strength N/mm2 Modulus of Elasticity kN/mm2 Strain at failure per cent

Glass 1250-2500 70-80 2-3

Polypropylene 30-40 1.1-1.6 20

HOPE 20-30 0.4-1.2 30-40

Nylon 50-80 1.0-2.8 25

Coir 90-140 2-3 15~40

Reed 30-40 1.0-2.0

Elephant Grass 180 5.0 4

Table 2 -Typical mechanical properties of fabrics

Fabric type Unit weight gm/m2 Tensile strength kN/m Secant modulus at Elongation at failure

Geolon 400 (Woven) 220 43

Typar (Non woven) 136 25

CE 121 Netlon (HOPE) 730 7.5

Coir rope, 2.9mm diameter 250m/kg 30 N/mm2

Coir Matting (a) 915 25

Coir Board (b) 440 10

Plain needled felt unconfined (CJ = 0) 1000 0.06

Plain needled felt (CJ = 2.5 kPa) sand embedded I 000 0.8

Aspinwall coir felt AGL I 00 (CJ = 2.5 kPa) sand embedded 400 1.3

ing this product to replace thick sand blankets for which sand is becoming scarce. The second was to examine the durability of coir in sandy subgrades.

Soil Standard China clay processed from English India

Clays Ltd ., Trivandrum was chosen for the clay bed and river sand was used for the sandseam in clay beds and durability studies. Properties of these materials are given in Table3.

I 0 per cent strain kN/m per cent

200 15

6 62

70 20

0.1 kN/ mm2 30

100 53

40 40

0.6 10

8 15

10 17

Coir Products Needled coir felt of two types and needled felt with

coir mesh and nylon fishing net stitching were used in the study. Photographs of the fabrics are given in Figure 1. Table 4 gives particulars of the fabrics.

' I

Experimental Procedure Penetration Tests

The objective of the study is to examine the utility of needled felt and other composites to improve the bear-

SHEEBHA et al.: POTENTIAL OF COIR REINFORCED STRUCTURES 57

Figure I (a) - A spin wall coir needled felt

Figure I (b)- Coirgrid stitched plain needled felt

ing capacity of clay beds to provide for accelerated con­struction of embankments or structures with a high fac­tor of safety against failure as a temporary foundation. The long term stability of these reinforcements is not essential in these cases, as the clay foundation will con­solidate and support the structures later on without rein­forcement. Only in cohesion less soils, the effect of the restraint5 and the durability is a more important crite­rion, for long term performance.

Preparation of Test Specimen

Kaolinite clay was mixed thoroughly with water. A moulding water content of 40 per cent was chosen for easy remoulding. The soil was carefully filled in layers giving compaction so a? to eliminate the air voids com-

pletely. The prepared clay sample had a thickness of 10 em and a diameter of 15 em. A surcharge load of 3 kPa was placed over this and the entire assembly was im­mersed in water to attain equilibrium of water content.

The tests were conducted by varying the rate of pen­etration and type of reinforcing coir composite. Slow rate, 0.24 mrnlmin of penetration and a fast rate of I .2 mrnlmin were used. To increase the tensile strength against failure of reinforcement by rupture, it was se-

.1

curely stitched with nylonmesh fishing ne.t on either side or with coirmesh on one side. Coirfelt free of any modi­fication was also tested. The penetration tests were also conducted on nonreinforced clay samples with 3 em and .1.5 C!U thick sand layer and that reinforced with coir rope laid in the form of a mesh of spacing 2 em in either direction. In all cases, the' layers were placed at a depth of 2.5 em (D/2) below the top of soil. The typical stress vs. penetration/diameter rati_os are shown in Figure 2 and Figure 3.

Durability Tests

To make astudy of the coir composites as reinforce­ment, the durability of the coir in field situations must be known. Very limited data are available in this direc­tion . Rajagopal and Ramakrishna6 studied the degrada­tion of the coir geotextiles in clay soils in a variety of environments. Their results indicated that though the wet strength of coir ropes was much less, Joss of strength was due to absorption of water and on drying the strength was nearly restored. Alkaline environment also contrib­utes to the decrease of strength . Coating on the samples by polymer was seen to improve the performance. In this study, only cohesionless sand which is least- corro­sive and. freely draining was used. This is the soil for which reinforcement is most effective. To start with, the durability studies were limited to the effect of wet­ting and drying only.

The importance of this study lies in the use of the reinforcement below railtracks, road subgrades and dam filters which are subjected to constant changes in ground water level. More exhaustive studies on environment and impact are in progress. One cycle of wetting and drying involved keeping the coir product under water for one day and drying in open. The study was carried out for three months with 40 cycles of repetition. The properties studied were Joss of unit weight and change of penetration resistance of the sand bed of density in-

58 J SCI IND RES VOL.59 JANUARY 2000

250

. 200

l ~ 150 1T

100

50

0 0.05

--ctay

0.1

-+-clay+ sand

0.15

s/D

0.2 0.25 0.3

--clay+felt

""*"" clay+felt+sand seam ........ clay+felt+coir mesh+sand seam -+- clay+felt+nylon+sand seam

Figure2 - Penetration resistance vs deformation at the rate of 1.2 mm/min

dex 25 per cent with the reinforcement before and after wetting and drying. The results are given in Table 5.

Results Discussion

Penetration Tests

The resistance vs penetration curve is similar to a stress deformation curve in compression test. Figure 2 and 3 indicate that at a water content of 40 per cent, the strain rate has a decisive influence on the strength of the clay. The clay exhibits essentially a strain softening trend with a decrease in the mobilized shear resistance. This means that there is no significant effect of strain rate and the clay exhibits only undrained failure, the faster loading rate indicating a higher viscous resistance. This is in accordance with strain rate effects on undrained tests on clays7• However when a sand layer was used, there is dissipation of pore water pressure due to the higher permeability and the shear resistance increases with greater mobilization of friction through incre~sed effective stress. A partial drained type of failure is s~en

for both rates of strain in the presence of a sand layer. With needled felt, the results are more marked and the clay bed undergoes an increasingly drained deformation with higher resistance even at higher rates of strain. This is due to the higher permeability of coir felt as com­pared to sand, particularly in-plane permeability. The effect of stiffening layers like nylon and coir mesh along with felt reinforcement is less marked at slow strain as compared to higher rates of strain. With increasingly drained failure the viscous effect gradually weakens. This suggests that coir felt mainly provides drainage and the stiffening layers provide additional temporary support of giving further resistance in quick loading caused by construction.

Durability Tests From careful analysis of these data, it is seen that

needled felt decreases in density after wettingand dry­ing unlike coir rope. It is clear that the decrease in density of the former is probably due to the disintegra­tion of the fabric and some loss of fibre in the process. Untreated felt thus loses tensile strength as a fabric even

SHEEBHA et al.: POTENTIAL OF COIR REINFORCED STRUCTURES 59

Table 3 - Properties of soil

Clay Sand

Liquid limit (per cent ) 62 Minimum void ratio 0.77

Plastic limit (per cent ) 31 Maximum void ratio 0.55

Plasticity Index (per cent ) 31 Angle of internal friction (degrees) 31

Specific gravity 2.66 Specific gravity 2.61

Clay fraction (per cent ) 69 Effective size d10

(mm) 0.27

Table 4- Details of coir reinforcements used in the study

Type of Manufacturer Fibre Fabrication Unit Other features

reinforcement process Weight

g/ "m2

Needled felt Central Institute of Coir Coir fibre Non woven 940 Plain and

Technology, Coir Board Needle punched stiffened felt with

Ban galore coir mesh and

nylon fishing net

stitching

Aspinwall coir Aspinwall Geotech Coir fibre Non woven 620 P.P net 35 x

felt AGL 101 Ltd, Alleppey reinforced with Needle punched 20mm mesh

polypropylene

enet

Table 5- Properties of coir composites on wetting and drying

Materials Unit weight Secant modulus (s/D= 0.1) Penetration test (kN/m2

)

Initial cycles Initial After After40 40 cycles

Needled felt 12mm (grn/m2) 940 780 7500 9000

Aspinwall felt (grn/m2) 620 570 9250 9250

Pl ai n coir rope untreated I

3.6 mm diameter (rn!kgj . 200 200 7000 7000

Cement and PYA treate coir

rope (mlkg) I 40 40 13000 13000

Light bituseal treated coir

rope (mlkg) 135 135 8750 8750

Heavy bituseal treated coir

rope (m/kg) 90 90 7500 7500

60 J SCI IND RES VOL.59 JANUARY 2000

250

200

"ii ~ 150 ....... cr

50

0 0.05 0.1 0.15

s/D

0.2 0.25 0.3

--clay -+- clay+sand -- clay+felt

-M- clay+felt+sand seam -11- clay+felt+coir mesh+sand seam -+- clay+felt+nylon+sand seam

Figure 3 - Penetration resistance vs deformation at the rate of 0 .24 mm/min

though the discrete fibres may still offer restraint espe­cially if a synthetic net is integrated in the manufacture as in Aspinwall type felt. In penetration tests, it is seen that the penetration resistance is more in the case of ce­ment-PYA coated coir rope with the secant modulus of the bed getting nearly doubled. It is also seen that 40 cycles of wetting and drying do not affect its efficiency (Figure 4 and 5) .

Light bituseal treatment seems to be suitable as the dense bituseal lowers friction of coir reinforcement. Figure 6 shows that wetting and drying does not signifi­cantly effect the secant modulus of the sand bed rein­forced with coir ropes of different treatments.

Conclusions From the available data the following conclusions

are drawn. The behaviour of clay formations to loading changes from one of undrained to drained with the use of coirfelt. Thus it is clear that needled coirfelt can be successfully used as a bedding layer to minimise the sand layer for drainage for consolidation of clay foundations. If the loading on these formations is not too sudden, even

at low strain levels, significant improvement in resis­tance can be obtained . But if faster rate of loading is to be used, coirfelt needs to be stiffened.

Coir composites for long term reinforcement effect need treatment. In sandy beds which are not very vul­nerable to degradation, needled felt gives increased re­sistance due to large area of exposed surface. Untreated felt is prone to disintegration and cement-PYA coating gives higher modulus and greater resistance to deforma­tion. This is seen to be true even for coir ropes. Bituseal treatment with high concentration of bitumen can lead to loss of friction which lowers the efficiency of the re­inforcement. The studies also showed that needled felt of smaller thickness is more efficient as reinforcement from the point of view of cost-effectiveness, as the sur­face area for restraint is more for the same volume and the compressibility of the fabric is smaller, but for higher transmittivity as a filter, thicker felt may be required .

Further studies on durability, particularly plate bear­ing tests , are needed for confirmaticrff-of these conclu­sions .

t'

ii"

SHEEBHA et al.: POTENTIAL OF COIR REINFORCED STRUCTURES

0 2 4

--Sand alone

-11- Light Bituseal treated

6

s(mm)

-+- Dense Bituseal treated

-11- Cement PVA Treated

8 10

.....- Coir rope with out treatment

Figure 4- Penetration curves for sand reinforced with coir rope before wetting and drying

1800

1600

1400

1200

~ 1000 c:

800

600

400

200

2 4

--Sand alone

-11- Light Bituseal Treated

6

s(mm)

8

-e-Coir rope without treatment .....-Dense Bituseal Treated

--cement PVA., Treated ·

Figure 5 - Penetration curves for sand reinforced with coir rope ll,fter 40 cycles Of wetting and drying

'

61

'\

12

62 J SCI IND RES VOL.59 JANUARY 2000

1800

1600

1400

1200

.. 1000 a.. ~ ,. 800

600

400

200

0 0 2 4 6

s(mm)

8 10 12

I-+- SandSro;;;-· --Coirfelt (Coir Board) before .40 cycles --.- Coirfelt (Coir Board) after 40 cycles - .,._ Aspinwall coirfelt( after 40 cycles)

--Aspinwall coirfelt( before 40 cycles)

Figure 6.- Penetration curves for sand reinforced with coir felt before and after wetting and drying

Acknowledgements /

The study has been made possible through the assis­tance of the research scheme on coir composites funded by the State Committee of Science, Technology and Environment, Kerala. The materials were supplied by the Centre for Development of Coir Tec hnology, Trivandrum and the authors are thankful to N Balakrishnan Nair, Director, and T S Ramanatha Ayyar, Consultal)J to this centre.

References Henner Schurholz, Uti lisation Potentials for Coir Fabrics as Geotextiles, Proc Semin Coir Ceo text, Coimbatore , (Coir Board, Cochin). September 1988, pp 1-9.

2 Ramanatha AyyarT S, Joseph J & Beena K S, Bearing Capacity of Sand Rein forced with Coir Rope, Proc Indian Geotext Conf, Bombay, 1988.

3 Rehsi S S, Use of Natural Fibre Concrete in India, Natural Fi­bre Reinforced Cement and Concrete, edited by R N Swamy (B lackie, Glassy) 1998.

4 Me Gown A, Andrawes K Z & Al-Husani M A, Effect of Inclu­sion Properties on the Behaviour of Sand, Geotechnique, 28 (3 ) (1978) pp 327-346.

5 Dixon J & Langley P, Geogrids for Slope Stabilisation, Civ Eng, UK, (May 1990), pp 47-54.

6 Rajagopal K & Ramakrishna S, Degradation Behaviour of Coir Geotextiles with in Clayey Soils, Proc Geosynth Asia'97, Ban· galore (Central Board oflrri gation and Power, New Delh i) 1997.

7 Whitman R V, Some Considerations and Data Regarding the Shear Strength of Clays, Proc ASCE, Res Conf Shear Strength

Cohesive Soils, Colarado (June 1960) pp 581-614.