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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
130
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
131
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
133
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.
134
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.
135
(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
138
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
139
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
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