CHAPTER 6

DURABILITY OF WOVEN COIR GEOTEXTILE AND HANDKNOTTED COIR NETTING

6.1 General

There are numerous ground engineering situations where the critical period for

stability is immediately, or very shortly after construction. As soon as the loading acts

on the ground, excess pore pressure is created within the foundation. Subsequently,

with time, pore pressure in the foundation will dissipate from beneath the loaded area

and the shear strength of the foundation will increase. In other words, stability of the

system will improve with passage of time. After a certain time (typically between a

few months and a few years) the whole system will be stable with little or no

assistance from reinforcement. In such situations, the use of a natural reinforcement,

which has a limited, but predictable working life, is sound engineering practice.

Therefore as part of the present study, it was considered worth investigating the

variations in the strength characteristics of woven coir geotextiles and hand knotted

coir nettings with time (when subjected to different environmental conditions).

Behaviour of one type of coir geotextile (H2M6) and two types of hand knotted

nettings(NAl and NA2) subjected to alternate wetting and drying conditions as well

as complete wetting conditions has been analysed in the present study.

The strength of natural geotextile is known to reduce rapidly on interaction

with wet soil or water, possibily under the action of micro-organisms or soil chemical

action. This decay of natural fibers commonly known as biodegradation is an

important factor which needs to be taken into account before these materials can be put

to any engineering use. Early studies on biodegradation of natural fibres reveal that it

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is very complex in nature. Some of the earlier studies were reported by Schurholz

(1991) on the trials in the Institutions of Germany and Netherlands using incubator

specs with injections of micro organisms at 28-30°C and upto 90% humidity.

According to Schurholtz, coir geotextile retained 20% of their original tensile strength

after one year in incubator tests with high fertile soil. It was also observed that when

natural fabric were put in a shower room and kept wet for 167 days with conditions to

simulate the traction effect while flooding, coir had almost no damage while jute and

cotton completely fallen apart before the trial was completed. Rao and Balan (1996)

studied the durability of two varieties of coir yarn (white and brown) in different soil

environments. The rate of degradation of coir in sand and water at different pH

environments were also studied. The rate of degradation was predominant in the early

periods ranging from 4 to 8 months and later it was slower. In sand, coir retained its

initial strength for upto one month and in clay for about three months. Coir was also

found to degrade at a faster rate in water than in soil environments of the same pH

value. The subsequent sections details of the experimental programme carried out to

examine the durability of woven coir geotextile and hand knotted coir netting.

6.2 Experimental programme

Durability of the coir geotextiles and hand knotted coir nettings were examined

by testing the samples after subjecting them to continuous wetting as well as alternate

wetting and drying cycles. Both reduction in tensile strength and loss of weight of coir

samples were investigated.

6.2.1 Alternate wetting and drying of coir geotextile and hand knotted coirnetting samples

The reinforcement samples (coir geotextile H2M6; coir netting NAI& NA2)

were cut to sizes 20cm x 20cm and each sample was weighed by an electronic

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weighing balance 1200 gram capacity x 0.01 gram accuracy. The samples were then

immersed in a tub of water for complete saturation. The weight of each sample was

recorded at regular intervals and the weighing process continued till the consecutive

weights showed negligible variation (less than 1%). This stage indicates complete

saturation of samples. The specimens were taken out and were subjected to air drying;

Herein also, weight was taken at regular intervals and continued till the samples were

completely dry. The whole process completed one cycle of alternate wetting and

drying. Different samples were subjected to different cycles of alternate wetting and

drying (1, 2, 4,8, 16 and 32 cycles).

6.2.2 Continuous wetting of coir geotextile and hand knotted coir netting samples

The required reinforcement samples of size 20cmx 20cm were cut and weights

were taken. Then samples were immersed in a tub of water. Different samples were

taken out and the weights were taken after 30, 180,365 and 548 days).

6.2.3 Tension tests on coir geotextile and hand knotted coir netting samples

Tension tests were also carried out on coir geotextiles and nettings after 1, 2, 4,

8, 16 and 32 cycles of alternate wetting and drying as well as after 90, 180, 365 and

548 days of continuous submergence. Tests were performed using Universal Tension

Testing Machine for geotextiles. Fig. 6.1 shows the test set-up for tension tests on coir

geotextiles and hand knotted coir nettings. The test specimen was placed centrally

between the jaws with a gauge length of 100 mm with approximately same length of

specimen extending beyond jaws at both ends. Loading was done with the help of

three-phase AC power supply. The test was carried out at the rate of lOmm/min. The

strain was measured by means of a dial gauge. The machine was switched on and the

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load was obtained on the load indicator at various strains till failure. The strain was

measured and load at failure was determined. Load versus strain graphs were plotted.

6.3 Results and discussion

Results of the durability tests on coir geotextile and hand knotted coir netting

subjected to alternate wetting and drying conditions as well as continuous wetting

conditions are presented in the form of load versus axial strain curves. All the results

reported are the average of identical tests on three samples. The variation of weight of

each coir sample with passage of time period is also presented in the subsequent

sections.

6.3.1 Variation of strength characteristics of coir geotextile and hand knotted coirnettings with passage of time

Tensile strength characteristics of woven coir geotextiles subjected alternate

wetting and drying conditions are shown in Fig. 6.2. It is evident that the tensile

strength decreases with increase in number of cycles. Further, the axial strain at failure

also showed decrease, in general, with increase in number of cycles. The percentage

reduction in tensile strength after various cycles of alternate wetting and drying is

presented in Table 6.1. It is seen that the percentage loss of tensile strength ranged

from 21% for one cycle (12 days) of alternate wetting and drying and to about 73%

after 32 cycles (220 days). Tensile strength characteristics of hand knotted nettings

subjected to alternate wetting and drying conditions are presented in Figs 6.3 and 6.4

(and the corresponding tensile strength values and percentage reduction in tensile

strength are presented in Tables 6.2 and 6.3). Trend similar to that observed for woven

coir geotextile is observed herein also. The loss of tensile strength ranged from about

16% for one cycle of alternate wetting and drying to about 61 % for 32 cycles, for the

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netting NAl. The corresponding values for NA2 netting were about 10% and 56%.

Alternate wetting and drying results in loss of tensile strength of both geotextile and

nettings at a faster rate. The above findings can be better understood from Fig. 6.5.

Tensile strength of both the geotextile and nettings shows a much slower rate

of reduction when subjected to continuous wetting (Figs. 6.6 through 6.8). Tables 6.4

through 6.6 (and Fig. 6.9) present the tensile strength values and percentage reduction

of the same after continuous wetting for 90, 180, 365 and 548 days. Of all the coir

products studied, the woven coir geotextile H2M6 exhibited only a marginal reduction

in tensile strength even after 548 days (l Yz years) of continuous wetting. It can be

inferred from the results presented in this section that none of the coir products are

durable under alternate wetting and drying, while H2M6 variety of coir geotextile is

durable when it is permanently under submerged condition.

6.3.2 Loss of weight of coir geotextile and hand knotted coir nettings with passage

of time

The percentage loss of weight of geotextiles and nettings subjected to alternate

wetting and drying condition and of continuous wetting are also summarized in Tables

6.1 through 6.6. These are presented in Figs. 6.10 and 6.11 as well. It is seen that

alternate wetting and drying results in loss of weight of the geotextile and nettings at a

fast rate to appreciable values whereas, coir geotextile-H2M6 shows only a negligible

reduction in weight with passage of time, when permanently kept under water. This,

again, suggests that woven coir geotextile H2M6 can be used in situations where they

are likely to be permanently submerged under water, even without any treatment of

COlr.

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6.4 Concluding remarks

The strength of natural geotexile is known to reduce rapidly on interaction with

wet soil or water due to various reasons such as action of micro-organisms or soil

chemical action. This decay of natural fibres commonly known as biodegradation is an

important factor which needs to be taken into account before these materials can be put

to any engineering use. The results of a study made to examine the durability of the

coir geotextile and hand knotted coir netting samples after subjecting them to

continuous wetting as well as alternate wetting and drying cycles have been presented

in this Chapter. Based on the preliminary qualitative data obtained, the following

conclusions are drawn:

(1) The tensile strength of both woven coir geotextiles and hand knotted coir

nettings significantly decreases with increase in number of cycles of

alternate wetting and drying.

(2) The tensile strength of hand knotted coir nettings significantly decreases

with increase of time, when kept permanently under water.

(3) Of all the coir products studied, woven coir geotextile (H2M6) exhibited

only a marginal reduction in tensile strength even after 548 days 1liz

years) of continuous submergence.

(4) Alternate wetting and drying results in loss ofweight of the geotextile and

nettings at a fast rate where as coir goetextile H2M6 shows only a

marginal reduction in weight with passage of time, when permanently

kept under water.

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(5) None of the coir products are durable under alternate wetting and drying

condition; however, woven coir geotextile (type: H2M6) can be used in

situations where they are exist permanent likely under water.

136

Table 6.1 Reduction in tensile strength and weight of woven coir geotextilesubjected to alternate wetting and drying: type of geotextile - H2M6

Number ofPercentage

Tensile Percentage loss incycles

reduction in strength(kN/m) tensile strengthweight

0 - 4.851 -

1 1.84 3.581 21.00

2 3.58 3.70 31.29

4 6.34 3.25 33.00

8 9.64 2.60 46.46

16 22.22 2.45 49.49

32 34.31 1.32 72.70

Table 6.2 Reduction in tensile strength and weight of coir netting subjected toalternate wetting and drying: type of netting - NAI

Number of Percentage Tensile Percentage loss incycles reduction in weight strength(kN/m) tensile strength

0 - 4.65 -

1 1.80 3.92 15.77

2 6.24 3.82 17.87

4 11.96 3.77 18.93

8 16.21 3.53 24.20

16 25.31 2.74 41.04

32 35.53 1.81 61.05

137

Table 6.3 Reduction in tensile strength and weight of coir netting subjected toalternate wetting and drying: type of netting - NA2

Number Percentage reduction Tensile Percentage loss inof cycles in weight strength(kN/m) tensile strength

0 - 6.13 -

1 1.30 5.54 9.60

2 9.20 5.19 15.20

4 15.54 5.00 18.40

8 22.70 4.26 30.40

16 30.94 4.12 32.80

32 40.82 2.70 56.00

Table 6.4 Reduction in tensile strength and weight of woven coir geotextilesubjected to continuous wetting :type of geotextile - H2M6

Number Percentage reduction Tensile Percentage loss inof days in weight strength(kN/m) tensile strength

0 - 4.85 -

90 0.52 4.56 5.92

180 0.65 4.51 7.03

365 0.79 4.41 9.10

585 1.24 4.17 14.14

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Table 6.5 Reduction in tensile strength and weight of coir netting subjected tocontinuous wetting: type of netting - NAt

Number Percentage reduction Tensile Percentage loss inof days in weight strength(kN/m) tensile strength

0 - 4.65 -

90 17.86 3.38 27.30

180 20.24 3.01 35.22

365 25.11 2.65 43.12

585 28.35 2.45 47.33

Table 6.6 Reduction in tensile strength and weight of coir netting subjected tocontinuous wetting: type of netting - NA2

Number Percentage Tensile Percentage loss inof days reduction in weight strength(kN/m) tensile strength

0 - 6.13 -

90 29.13 4.11 32.00

180 31.06 3.82 37.80

365 33.00 3.92 36.00

585 35.00 3.48 43.00

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Fig. 6.1 Test setup for tensile streagtlt tnt

140

5

---)K--- 0

~l-cycle

-x-2-cycle

--f:r- 4-cycle

---..- 8- cycle

4

1

.,JK,-.'

, *---------.;:!E',.x-

"""~,','

/",,/

/I,

I

",I,,I

/

l,,I,,

I

II,

I

I,I

// ~ 16-cvcle

/ ',,: --+- 32 cycleo J!l.1.~ __I.. __J....____===~======~

o 10 20A.xial strain(%)

30 40

Fig. 6.2 Tensile strength characteristics of reinforcement subjected to alternatewetting and drying -reinforcement type: H2M6

141

5

---)1(--- 0

~l-cyc1e

-x-2-cyc1e

-fs-4-cyc1e

--+- 8- cycle

--~-'--..;::+t'""""••------)lE----_.-.-;K.,-

"",l'~

;1',I

/,I

/I,,

/I,

I

/l

II

I,,I

/I

II

/I

II

II

/ -0-16- cycleI

I

/ ~32cyc1eo 11-'::::...- --..L- ..l.-__-=====c:=:::::::::::::::=:::::::::=..J

4

"a'~

g3

~-~1.l!\)

~CI.I

!::l 2:!i....~

1

o 10 20A.'\jal strain(%)

30 40

Fig. 6.3 Tensile strength characteristics of reinforcement subjected to alternatewetting and drying -reinforcement type: NAl

142

7

6

~5~

~t 4

jrn<l) 3;,::l:!a....~

2

1

0

0 10 20 30 40 50 60

---)1(--- O-clays

--0-1 cycle

-x-2cycle

---"-4cycle

-+-8 cycle

----16 cycle

~32cycle

70 80

Axial strain(%)

Fig. 6.4 Tensile strength characteristics of reinforcement subjected to alternatewetting and drying -reinforcement type: NA2

143

100

~ 80....Q)

.t:l'nQ)

;,::l~

60:0-.s'n'n0-",......

~ 400'-'"

~1::Q)

-+-H2M6g

20Q)

P.. -'-NAI

~NA2

0

0 4 8 12 16 20 24 28 32

Nmuber ofcycles

Fig. 6.5 Loss of tensile strength of coir reinforcement subjected to alternatewetting and drying cycles

144

~~

~<,~~~~........- ,.",,-.~-------)K_.-- .- ... .--

"~:-------~.-;;;;: ---------)(I' ~ .- ...-

;,",,' "-",,,,: ...--------)(".",' .J:j-_# ,-,,#',' '- "X; ;''-'>(,;, ", ,,#,.~.-

fo ,'". "";;;5", ,, ,

---::i(--- 0 days,','.-','",','~.~"",

lj.',''' --....-- 90 days, /'

I, " ---/1--- 180 dayst' '•• X.• " --0-- 365 days.,... "#.,~' ---x--- 548 days

l~'.'~-

7

6

5

2

1

oo 10 20

Axial st.raill(~1»

30 4.0

Fig. 6.6 Tensile Strength characteristics of coir reinforcement subjected tocontinuous wetting -reinforcement type: H2M6

145

5

---::1(--- O-days

--+-- 90 days

--tr-180 days

--.--365 days

---e--- 548 daysI'

4

1

oo 10 20 30 40 50

Axial strain (%)60 70 80

Fig. 6.7 Tensile strength characteristics reinforcement subjected to continuouswetting - reinforcement type: NAt

146

7

---x--- O-days

---x--- 90 days

---/).---180 days

--+-- 365 days

---A--- 548 days

6

5

1

oo 10 20 30 40 50 60 70 80

Axial strain(%)

Fig. 6.8 Tensile strength characteristics of reinforcement subjected to continuouswetting -reinforcement type: NA2

147

400 500 600

Fig. 6.9 Loss of tensile strength of coir reinforcement subjected to.continuouswetting cycles

148

50

40

10

oo 4 8 12 16 20 24

NlUl1ber ofcycles

-'-NAI

-fr-NA2

-+-H2M6

28 32

Fig. 6.10 Loss of weight of coir reinforcement subjected to alternate wetting anddrying cycles

149

50~H2M6

--NA140

~NA2

~ ~'bh• po< A

<L>f:: 30 ~A,...::lrJJrJJ0-~l.':: 20~d>01-4<L>p..

10

oo 100 200 300

Nlmlber ofdays

400 500 600

Fig. 6.11 Loss ofweight of coir reinforcement subjected to continuous wettingcycles

150