ients of natural fibres and fabrics for...
TRANSCRIPT
Chapter 7
REQUIREl.\'IENTS OF NATURAL FIBRES AND
FABRICS FOR ENGINEERING APPLICATIONS
Geosynthetics have established their sway in engineering applications due to
the versatile development, manufacturing methods, testing methods, quality control
and availability of specifications. However in the case of natural fibres and their
products, not much progress has been achieved. The very limited specifications have
been in the case ofjute and coir for erosion control applications, carpets and package.
The great potential of these fibres, the latter in particular has not been taken advantage
of for lack of such studies. Being natural fibres, there is much greater variability
depending on the agricultural inputs, age, processing methods and chemical
composition. In this chapter, a study is made on the properties of coir fibres and their
products in as far as these relate to the stability of slopes. These include tensile
strength, friction and durability, of these fibres and the various fonns by which fabrics
can be made to perfonn functions of drainage, separation, filtration and reinforcement
which can be used in the short tenn or with suitable modifications that can have a
prolonged life of service.
7.1 Properties of coir fibre
In detennining stress for fibres and textiles, the area of cross section,
unfortunately cannot be well defined or easily measured. A quantity, specific stress
can be determined based on the mass per unit length of the specimen and is measured
in Tex or Denier. Detennining the specific stress for a fabric is possible only if the
yam or fibre linear density is known. In the case of natural fibres, the length, diameter
and the rigidity vary and hence it is impossible to work out the area of cross section
and the tensile strength based on this. Changes in cross sectional area during tensile
testing however are very difficult to measure (Richards and Scott, 1986). Tenacity is
the fibre strength expressed as force per unit linear density (gram force per denier).
Tex is defined as the mass in grams of 1000 m of fibre. Denier is the mass in grams of
9000 m of yarn and used more commonly for textile engineering.
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Table 7.1: Properties of coir fibre
PHYSICAL CHEMICALLength
1150
-Water 5.25%
200mm solublesDensity+ I Agmlcc Pectin and 3%
Av.diameter'" O.3mmrelatedcompound
Tenacity+ 10gmltex Hemi 0.25%cellulose
Breaking 30% Lignin 45.84%elongationSwelling in 5% of Cellulose 43.44%water diameter
I Ash 2.22%+ These are only representatIve. Actual values may be vaned.
(Source: Coir The natural geotextile, published by the Kerala State Coir Corporation Ltd.)
7.2 Types of fibre
There are two distinct varieties produced in India. The white fibre extracted
from green husks by an elaborate rerring process for 6 to 10 months and brO\vn fibre
extracted by mechanical means from dry husks.
As discussed earlier, coir mattings and nettings are the most developed
products. The coir yam spun out of the retted fibre on traditional spinning wheels are
woven on hand looms to manufactUre coir products. The decorticated fibre/bristle
fibre spun on machines can also be woven. The mattings are manufactured in widths
of 1 to 4m with thickness varying from 5 to 10 mm. Three dimensional coir mattings
are also produced. Nettings are usually knotted with relatively large meshes. In wet
situations, the coir fibre swells and the mesh size decreases. Nonwoven mats are made
from loose fibres, which are interlocked, by needling or rubberizing. Brown fibre is
exhibiting a large growth as compared to white fibre, the production of which is
decreasing.
. The conversion of coir fibre into yam and further processing has not registered
the required growth. Rubberized coir and rope sector are the major outlets for brown
fibre. Wide width mattings are produced from 4m wide looms and exported mostly
from Kerala.
Production of coir yam using spinning machines is the need of the time and
good quality fibre is essential. Long stapled fibre which adheres to the periphery
(bristle fibre) should be used. Combing drums, which could separate bristle fibre and
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mattress fibre, should find a place in the manufacturing process. It is expected that 25
to 30 % of the fibre will be bristle fibre. Coir yam spun out of the bristle fibre will
have higher strength, both wet and dry, and durability as it has a higher lignin content.
They can last much longer and can be used in place of synthetics largely in
combination with synthetics.
Other products, which are in use, are mattress coir synthetic and organic retted
blankets. In these, mattress coir (short fibre) is sewn into polypropylene nets by
polyester threads (or jute or cotton) lasting for shorter duration and of light weight
(270 to 400 g/m2). Coir rolls made of compressed mattress coir is used for stream
bank stabilization. Coir pillows 50 to 100 mm thick made by stuffing mattress coir
into a woven coir mat pillow case are used for revegetation.
7.3 Scope of work
The potential of natural fibres particularly that of coir for ground improvement
has been proved over the past twenty years by researchers (Henner Schurholz, 1988).
This work attempts to make use of natural fibres and in particular coir for different
aspects of canal slope stability problem. The experimental studies involve the use of
discrete coir fibres, woven coir fabrics and nonwoven coir products, which can
possibly find use in the various aspects of stability. Laboratory studies on the
identification of relevant properties of discrete fibres, fibre soil mixes, coated fibres,
coir yam, coir meshes, needled felts, blended and reinforced fabrics and coated
fabrics have been conducted. Model studies have been carried out to test the utility of
these fabrics for improvement of bearing capacity, reduction of settlements,
deformations of retaining walls and slopes, relief of pore water pressure due to
drainage functions, silt fencing and stability of these products on long term
functioning are presented.
7.4 Experimental studies
7.4.1 Experimental studies on fibre
7.4.1.1 Tension tests on fibres - Stress strain curves for fibres
The stress strain curves for coir fibres of various diameters are shown in
figure 7.1. The gauge length adopted for testing was 10 cm and the rate of strain
100
G 32(073
100 -r--c------
10
0.1 ...., -----,....------,.-------.-----..,------.,
o 10 20
Strain (%)
30 40 50
-+-Plain fibre - CICT Plain fibre· AGL
-.-Cement coated fibre -Q-Bamboo fibre
Figure 7.1: Results of tension test on fibres
101
adopted was 10 mm/minute. Cement coating is seen to impart greater stiffness to the
coir fibres, which is clear from the graph.
7.4.1.2 Studies on shape, size, rigidity and coating of coir fibre
Among natural fibres, coir is the strongest and durable because of its higher
lignin content, but because of the heterogeneity in length and diameter, woven coir
fabrics of fine texture for engineering use are not presently made. In order to improve
the engineering uses of this material, treatment with polyvinyl alcohol and cement
bonding proved useful for strengthening coir. The fibres were also examined. The
effect of diameter and shape factor of discrete coir fibres for their possible use in
strengthening fills and bridging silt fences has been taken up as the first step. The
effect of diameter and curvature increased the strength only to a limited extent, which
was attributed mainly to particle interference during deformation. Segregation of
fibres and surface treatment are recommended for the use of discrete coir fibres for
soil reinforcement.
The use of discrete fibres for reinforcing materials is not new and has been
used for the manufacture of various composites, concrete and soils. The requirements
of the fibres for effective functioning of these composites in relation to the matrices
like quantity, modulus, surface roughness, aspect ratio and orientation had been the
subject of detailed studies. Such studies in soil masses for earth structures, have also
received attention since the advent of reinforced earth and synthetic product
development. A comprehensive ~tate of the art report on randomly distributed fibre
reinforced soil was presented by Ranjan and Charan (1998) both in the case of both
synthetic and natural fibre in respect of laboratory and field applications. 'Texsol' is a
patented application of the use of the mixing of continuous polymer yam and soil
(mostly sands) to produce earth structures of various types and has been described by
Leflaive (1988). In view of the encouraging results of such applications, a choice of
available natural fibre becomes an imperative necessity for sustainable development,
but the effective application should be preceeded by a scientific knowledge of the
interaction of soil with the fibre.
Coir fibres extracted out of dry coconut husk exhibit such a wide range of
dimensions as length varying between 50 and 300 mm and diameter between 100 to
600 ~m. Natural coir is an admixture of fibres varying widely in properties such as
bending, rigidity and breaking strength. Jute on the other hand is fairly uniform.
102
Another difference is that jute has a higher modulus, tenacity and low failure strain as
compared to coir. But jute has a substantially lower longevity and high swelling. In
addition, coir processing involves twisting and the effect of this and natural crimps
affect the bond between soil and the fibre. Studies on the use of discrete fibres and
broken meshes have shown that the friction and confining pressure of soil especially
sand improves the stiffness of fibn: and can strengthen the soil especially fills (Dixon,
1997). Earth fills using either discrete fibres or broken meshes can increase the
friction angle and enable steepl~r slopes to be formed than is possible with
unreinforced soil. The same applies to the ground improvement of foundation of loose
soils. But in all these cases, the durability of the fibre should be maintained so that the
strength does not decrease due to degradation of the fibres in a short time. This aspect
has been examined and studies have shown that cement coating with a prior treatment
of PVA (poly vinyl alcohol) on coir fibres increases the resistance to fungal attack on
them and the stability is improved. In fact current research has shown that flax fibres
coated with polypropylene coating have been successfully tested for automobile
components.
In view of these findings and the necessity to modify manufacturing processes
of coir to make the product more useful for geotextile applications, the factors
affecting the bond strength (friction) between fibre and soil should be properly
investigated. As far as is known, only a very limited study particularly the aspect ratio
has been reported on reinforced earth (Ranjan et aI, 1994). In the case of fibre
reinforced concrete the shape of the steel fibre has been studied (Singh and Singhal,
1998). This study is therefore undertaken to highlight fibre configurations as far as
coir fibre is concerned in view of the peculiar nature of the product and the effect of
lignin on the outer shell of the fibre for rigidity and friction. Lignin is a random three
dimensional network i.e., a natural polymer composed predominantly of phenyl
propane units and this may account for the greater rigidity of thicker fibres.
One of the earliest studies on reinforced sand using fibres was by Gray and Al
Refeai (1986) who showed that for equal weight of reinforcement and aspect ratios,
reed fibres gave higher deviator stress in triaxial tests on sand than glass fibres even
though the latter had a higher modulus (more than 40 times) than the former. It was
therefore concluded by them that stiffness was not as important as friction, \vhich is
higher in reed. Ranjan and Charan extensively reviewed the effect of different factors
on reinforcing effect of fibres on soils particularly sand and concluded that the fibre
103
parameters of importance are the weight fraction, aspect ratio and surface friction.
However in all these studies, the fibre shape was not considered and the resistance
was presumed to be due to the restraint on account of the modulus. Singh and Singhal
found that for the same aspect ratio, M-20 concrete showed greater bond strength with
wavy steel fibres at low fibre contents as compared to plain fibres. In view of the fact
that coir fibres show greater rigidity due to lignin and larger diameter, the effect of
fibre shape may be a factor of significance. Boben et al used I % discrete coir fibres
both plain and cement coated on loose dry sand (Dr = 25 %) for detennination of
penetration resistance in CBR type test. The results are shown in figure 7.2. Cement
coated fibres showed substantial increase in penetration resistance of sand beds.
Hence it was concluded that the coating besides giving durability increased the
frictional resistance. From the above, it is apparent that coir fibres behave differently
from glass fibres or low friction synthetic fibres such as polypropylene as pointed out
by Gray and AI Refeai.
7.4.2 Materials used
Coir fibres collected from Kaniyapuram, Thiruvananthauram were used in this
study. The diameter of coir fibres varied in the range 0.1 to 0.5 mm with larger
Table 7.2
Material Properties
Soil properties Coir fibre
CementPercent Untreated
coated
Sand I 62 Diameter (mm) 0.3 0.5
Silt I 29 Length (mm) 25 25
Clay I 9 Tenacity (N/Tex) 0.08 O.04TI
WL I ... - E (N/Tex) 0.8 OATI
oJ/I
PercentWp 30 54 41
elongation
or-.rc I IS Density (k.t"J/mJ) 0.88 1.1
i
Ydry (max) Proctor ! 16.5k.."\".m3
I+ Tex calcubled using weight of coating in addition to fibre.
104
8000
7000 .
6000
"..-...5000
I'Qa..0,VI 4000VI2!~
3000
2000 -
1000
0
0 5
Penetration (mm)
10 15
-+- Unreinforced·
__ Plain fibre
--.-Cement coated fibre
Figure 7.2: Effect of discrete fibre on sand bed in strain controlled penetration
test
105
diameter showing greater rigidity. The fibres were cut to 25 mm length and mixed
with dry soil, which was silty sand (SM). The properties of the soil and the coir fibres
used are given in Table 7.2.
The soil sample was compacted with and without the fibre at different
moisture contents using Proctor compactive effort. It was noticed that the O. M. e.was nearly constant for soil mixed with fibres. To study the effect of fibre rigidity and
stiffness, tests were conducted using segregated fibres choosing different diameters
0.1, 0.3 and 0.5 mm and also fibres of different shapes of these diameters. The shape
factor was defined as the ratio of curved length to straight length. A shape factor of
1.2 was chosen and the segregation \vas made. To study the effect of coatings, fibres
were pretreated using polyvinyl alcohol and cement. Different percentages of fibres
0.1, 0.3, 0.5 and 1 % by weight of soil were used and compacted samples of fibre
reinforced soil were made.
7.4.3 Experimental work
Cylindrical samples of the compacted soil were prepared in a split mould 72
mm diameter and 144 mm length to facilitate easy removal by hydraulic extractor.
The samples were tested in a unconfined compression test with a strain rate of 1.25
rom/min. The water contents of the specimens were measured both initially and at the
end of the test.
7.4.4 Effect of fibre content on ~trength
The results of the V.e.e. test are shown in figure 7.3 for the composite coir
fibre. A plot of failure stress (log scale) against failure water content is very revealing.
The failure stress increases with percentage increase in fibre for the same failure
water content both for plain and coated fibres. The increase is marked for cement
coated fibre as compared with plain fibre. In fact the actual fibre weight will be less
since cement also is counted as the part of the fibre. Hence cement coating seems to
contribute greatly to the stiffness of the fibre as seen by the low failure strain. Though
it may appear that cementation may contribute to greater failure strength of soil in the
fonn of cohesion, the quantum of cement seems to be so small as to cause this change.
If this result is taken along with the increased penetration resistance of cement coated
fibres on dry sand, (Boben et aI., 1999), it appears that cementation of soil is not
likely to be a major factor since there can be no cementation with dry sand.
106
10000
~
(D 1000a..:.::.....,
~cQ)...t? '-'-:.E-,_Q)
.~......... - .
100(f)Q)...Q.
§"0Q)C
't=C
~10
1
15 20
Failure water content (%)
25
• Unreinforced sand
• 0.3% plain
_. -to _. 0.3% cement coated
-' iF -·0,5% cement coated
• 0.1% plain
_.~. -'0.1 % cement coated
• 0.5% plain
--e-1 % plain
Figure 7.3: Relation between unconfined compressive strength und fuilure wuter
content for fibre reinforced soil
107
7.4.5 Effect of fibre diameter and shape
Figure 7.4 shows the stress strain for the case of soil reinforced with 0.5%
fibre content for different segregated plain fibres. It is clear that larger size fibres
(O.5mm) give greater strength as compared to smaller diameter for the same weight.
Hence it is not the total area of cross section of the fibres but stiffness, which has
more influence. Figure 7.5 shows the same trend for cement coated fibre. Figures 7.4
and 7.5 also show that curved fibres (shape factor 1.2) gave a higher resistance
particulariy with larger diameter as compared to straight fibres. But though it is a
distinct trend, it appears that this is linked with the interference of the fibres with the
soil in the form of a dilatancy component of friction and is connected with failure
strain, which increases with higher fibre content. Further studies using larger diameter
samples may be required to clarify the position. It can safely be inferred that while
cement coating increases the cohesive component of strength at low strains, the
diameter, rigidity and shape of fibre are likely to influence the resistance due to
prolonged strain.
7.4.6 Effect of fibre content on strain
\Vith increase of fibre particularly above I % the failure strain is markedly
increased in plain fibre as seen from figure 7.6. It appears that up to 0.5 % fibre
content, the failure strain is low (within 5 %) for both plain and cement coated fibres.
When the fibre content increases further, the strength and strain increase
simultaneously.
1\aturalcoir fibres occur over a wide range of sizes and shapes and vary in
composition as regards lignin and cellulose content. It is clear that the lignin content
is quite large and gives rigidity and durability to the fibre. Because of the variation,
woven products are difficult to make from coir fibre and there should be a selective
basis for manufacture of woven geotextiles using coir fibre. The study has shown that
large diameter and curved fibres contribute to the greater reinforcement effect in soils
mixed with random fibre. It is clear that rather than the total cross sectional area, the
stiffness and friction of the fibre imparted naturally or by coating is more significant.
The behaviour of these thicker fibres is similar to PVA treated cement coated fibres,
which impart much greater strength. It is apparent that this treatment can impart
108
400
350
,.....lX5
300a..~.....,s:;
~ 250Q)...'t?Q)
.~ 200Q)...Q.E8 150
ic;0:::c 10000
:S50
0
a 2 4 6 8 10
Axial strain (%)
• Unreinforced sand
_·~·-·0.1mm dia curved
_.~. -. 0.3mm dia curved
_...t- .. ·0.5mm dia curved
• 0.1 mm dia straight
o 0.3mm dla straight
• 0.5mm dia straight
o cement coated
Figttre 7.4: Effect of fibre diameter and shape factor on W1confined compressivestrength offibre reinforced soil (plain coir fibre 0.5%)
109
500
400,...,«Ia.~'oJ
.£:.Olc
300oJ.)...'ti.ioJ.)>'(;)o:noJ.)...Q.
~ 2000"0oJ.)C
'+=50c 100:::J
12108642
Oa----,------,-----.------......------r------J
o
Axial strain (%)
• 0.1 mm - straight _. "0' -·0.1 mm - curved
• 0.3mm - straight _·-t,;-·0.3mm - curved
---.- Random • 0.5mm - straight
_. -<T ... 0.5mm - curved
Figw-e 7.5: Effect of fibre diameter and shape factor on lU1confined compressivestrength offibre reinforced soil (cement coated fibre 0.5%)
110
500
....... 400'"Q.~"-J
jQ)....
300"t')
,~Q)....Q.E 2008"0Q),l;;-c00
100c;:)
o .-----r-----.-------.-----r----....-----.------l
o 2 4 6 8 10 12 14
Axial strain (%)
-.-O.1mm - straight ~O.1mm - curved -.-O.3mm - straight
-o-O.3mm - curved --.-0.5mm - straight ~O.5mm - curved
Figure 7.6: Effect offibre diameter and shape factor on lUlconfined compressive
strength offibre reinforced soil (plain coir fibre I %)
III
greater stability and durability to fills at low strain. The usefulness of coir fibres for
increased critical height of embankment is given in appendix III. Increasing fibre
content imparts greater flexibility to soil by yielding which may involve problems of
increased permeability, a factor which has to be taken into account in water retaining
structures even though stability may not be a problem. As a factor of fundamental
importance the study shows that the effect of PVA - cement coating is similar to
lignin bonding on fibres and could throw open furthe~ avenues for exploitation of low
lignin fibres for engineering uses. The future of coir geotextiles thus lies in
segregating coir fibres and using them selectively for rigidity and durability rather
than using them as a heterogeneous blend for making relatively weaker coir yarns. A .
typical example is the use of such fibres for silt fences (Figure 7.7).
Figure 7.7: Coir being utilised around the shoreline of a marunade lake site in an urbansetting (Courtesy Sotir et. ai, 1991)
112
7.5 Studies on coir geotextiles
7.5.1 Coir ropes and woven fabrics
In view of the inadequate information on the engineering characteristics and
biodegradability of natural fibre fabrics some studies on jute and coir were undertaken
at Indian Institute of Technology, Delhi (Rao and Balan, 1995). The work included an
evaluation of the physical and engineering characteristics of these materials, and the
biodegradability behaviour of coir/jute geotextiles in different soil environments and
the resuIt~ are given in Figures 8.55 and 8.56. A study on bearing capacity of sands
using coir ropes and the factors, \vhich influence the improvement of the bearing
capacity was reported by Ramanatha Ayyar et al (1988). Model tests were performed
on uniformly graded sand of effective size 2.3 mm at a density index of 50 %. A steel
plate of 20 cm square and 18 mm thickness was used as a footing in a tank of 160 cm
square and depth 90 em. Single coir ropes of average diameter 4.3 mm were used and
the length, horizontal spacing and number of layers were varied. The test arrangement
is shown in figure 7.8. The load was applied by hydraulic jack against dead weight
reaction. I m long bamboo strips of average cross section, depth 3.5 cm and width 0.5
cm were used to tie the coir ropes and serve as anchorages.
The results showed increase in bearing capacity up to 2.5 times for reinforced
sand and the anchorages were very effective when the area of confinement is reduced
and a length of reinforcement of .'3 B' is optimum, where B is the width of the plate.
A vertical spacing of 0.25 B is very effective in increasing the bearing capacity
(Figure 7.9). A horizontal spacing of x/B = 0.5 was found to be the optimum spacing,
where x is the horizontal spacing. Mandai and Manjunath (1990) reported bearing
capacity improvements with geosynthetics as well as coir rope mat without
anchorages using model studies on strip footings. The values reported a ratio of 1.50
for vertical spacing of 0.25 B with a single mat, which was slightly lower than for
geogrids and nonwoven geotextiles.
Three types of tests were conducted in the present study. One is the tensile
tests on coir ropes \vith different treatments to find the effect of these treatments on
the stiffness and tensile strength. The second set of tests is penetration tests on coir
rope reinforced sand beds to find the effect of the treatments and configuration of coir
fabrics on bearing of loads through static penetration of a plunger at constant rate. The
113
-.
L·t-------t-B
+~--+-
( )( )( )( )
160 +
]......\
- ~.::~ 10/~
Figure 7.8: Test arrangement (Schematic),
Load (tJm2)
o 10 20 30 40 50
10r-..
120CQJ
5 30E Expt.QJ['/J No: ~M-C) 0.. ~
40 ............. NN -N: """,,""00 00 00
50 p::~-o..N C) ""0 <"l-.::J:"!~ - "!
~ --- ................. N N
Figure 7.9: Verical spacing effect
114
third type of tests is a plate bearing test on a large bed using a 30 cm diameter plate
and static loading on a sand deposit reinforced with coir ropes of different treatments.
7.5.2 Materials
The soil used is uniformly graded sand with G = 2.72, DIO = 0.23 mm,
Cu = 1.34, emin = 0.51 and ema.'( = 0.72. Coir mesh using coir rope of diameter 2.90 mm
and of Anjengo variety with 240 m per kg and with opening size lOx 15 mm was
used in the investigation. Coir mesh fabricated on the pattern of chicken mesh with
opening of 50 x 50 mm was also used for the study. These meshes were of density
500 glm2°and 260 g/m2 respectively.
To measure the stiffness and durability of the coir rope, trials were made with
the application of the chemicals given below.
I. G. P. resin with catalyst and accelerator
2. Wood protector (Berger products)
3. Bituseal roofing compound diluted with kerosene
4. Cement with poly vinyl alcohol (1,25,000 LR)
(The binding matrix is expected to give a coating of hydrated polymer making coir
more durable and some cementation as in cement based particle boards).
7.5.3 Penetration test
These were carried out in the CBR moulds 15 cm diameter with the sand at a
relative density of 50 %. The s<\nd was kept for a thickness of 12.5 cm under this
density in the dry state. The reinforcements of the various forms of coir fabrics were
kept at a depth of2.5 cm below the top of the sand except for one study where the two
layers of coir rope were used one at 1.25 cm depth and another at 2.5 cm. A surcharge
of 2 kPa was used for the test. The load was applied by a motorized frame through a
plunger of 5 cm diameter at a strain rate of 1.25 mm/minute and the load penetration
graphs were recorded. This test is intended to give a relative value of the subgrade
reaction as in roads.
7.5.4 Plate bearing test
This test was carried out in a tank of size 1.6 x 1.6 m and 0.9 m depth. The
sand was filled under controlled conditions. The load was applied in increments of 5
kN through a plate 30 cm diameter and 10 mm thick and the deformation was
measured by using four dial gauges. The load was kept constant till the settlement
115
stopped or attained a constant rate slower than 0.02 mmlhour as per 1.S specifications.
The reinforcement is of square size and placed at a depth of IS cm (B/2) below the
loading plate to effect a comparison with the penetration test.
7.6 Results and Discussions
7.6.1 Stress - Strain Curves.
Figure 7.10 shows the stress strain curves of coir rope. It is seen that in all the
cases, the tests were carried to rupture. It is seen that on an average a failure strain of
20 % is obtained on plain rope with a maximum nominal stress of 30 N/mm2. The
secant modulus gives an average of 170 N/mm2• The resin bonded coir rope was
distinct by its high stiffness even though the breaking strength is not so marked. The
average value of strain is 5 % and strength 35 N/mm2, with a modulus of 600 I\lmm2
•
This holds great promise as the resin content is only nominal and hence not
expensive. Bitumen coating is generally used for increasing durability, even though
for slope protection, the application retards plant growth due to its water repellant
nature. In this study it is seen that it reduces stiffness due to slippage and low friction.
The application of bitumen is thus contraindicated. The application of wood protector
was also similar.
On the other hand, poly vinyl alcohol used with cement gave a failure strain of
12 % and strength of 28 N/mm2 and hence is seen to be a positive additive. Being a
reinforcing filler in combination with cement, it imparts stiffness and forms a
protective colloid. The limited studies showed that it is easy to apply and is economic,
and its effectiveness and long term stability are already proved beyond doubt. Further
studies are reported in the subsequent sections.
Penetration tests:
C.B.R type tests have always been an index for subgrade improvement.
Figure 7.11 shows the load penetration curves of the sand subgrade of DR = 30 % with
the coir reinforcements. If the penetration of 2.5 mm is taken as a criterion, coir ropes
show great improvement. One layer of resin bonded coir rope gave a penetration load
of I kN as compared to 0.45 k..'\i for unreinforced sand. This is nearly the same value,
which was observed for two layers of coir rope at the same depth. \Vhen two layers of
coir rope were placed one at B.'4 and another at B/2 the load was 1.5 kN. With a coir
116
40302010 .
O~-----__r_-----_,__-----...,_-----__,
o
40
50 ,---------------~ _
10
Ii' 30E-€z'-"
Strain (%)
-+--High viscosity bitumen Wood protector
~Low viscosity bitumen ~Without treatment
-r-Cement - PVA coated .......... Resin bonded
Figure 7.10: Stress strain curves of single coir rope 2.9 rom ~ under tension
117
2.5 ~-------- _
2
'"' 1.5'N. E-€z'-"VIVIl!! 1ti)
0.5
OF--------,,--------,------.-----..---------.
o 2 4 6 8 10
Penetration (mm)
~Without reinforcement
___ Resin bonded one layer of rope at 8/2
-.-Two layers of rope at 8/2
-G-Two layers of rope one at 8/4 and another at 8/2
~Coirmesh
Figun~ 7.11: Penetration test results on sand reinforced with coir rope
(Dr =30%)
118
mesh at 8/2 (15 x 10 mm size) the load at penetration was 1.4 kN. The coir mesh thus
seems to be more effective than just two layers of rope. Further there is continued
resistance to defonnation. This is in confonnity with earlier studies on the mesh
pattern, but the change is not significant.
7.6.2 Plate load tests
The plate load tests are more realistic and the conclusions are of greater
practical importance. Since the tests were carried out only to limited strain, a
settlement of 10 % of width of plate i.e. 30 mm is taken as the criteria for the
improvement. The failure loads are shown in figure 7. 12. All the reinforcements are
located at a depth of 8/2. The results are in confonnity with earlier trends. For the
load of 135 kPa for unreinforced sand, single layer of rope gave 155 kPa and two
layers 180 kPa. With resin bonding a single layer of rope gave 184 kPa. With cement
PVA treatment the value is seen above 200 kPa. A coir mat of 15 x 10 mm, gave 252
kPa while knotted mesh of 50 x 50 mm gave 240 kPa. A comparison of various
reinforcements shows that the knotted coir mat is most efficient. The resin bonding
and cement PVA bonding also rank very promising as single reinforcement for giving
greater strength.
On the basis of the tests reported in this investigation and earlier work, the
following conclusions can be drawn.
1) It is possible to use coir fabrics for improving the bearing capacity of the
sand bed to twice the value economically.
2) Treating the coir rope by application of resin or coating with cement and
polyvinyl alcohol (PVA) will increase the stiffness of the reinforcement.
But it is difficult to effect unifonn coating unless the process is refined.
The long tenn durability of this application needs further study. However
the potential use for foundations on sands is great.
3) Mesh pattern of coir ropes with knitting similar to chicken mesh seems to
be very effective in mobilizing the stiffness of the reinforcement than coir
mesh manufactured by the usual weave.
119
400
350
300
250,....IQ
I:Le.III 200III
!~
150
100
50
0
0 10 20 30 40 50 60 70
Settlement of plate (mm)
-+-Sand___ Two layers at B/2
-.-Single layer
~Resin bonded
--G- Cement - PVA coated
--.- Knotted coir net - 50 x 50 mm
-t-Coir net - 15 x 10 mm
Figure 7.12: Plate load test results on sand reinforced with coir rope
120
4) Restraining the coir reinforcement by anchorage of split bamboo or any
other stiff natural material \vill increase the bearing capacity significantly.
5) Stiffened coir mats in sand can be very efficient reinforcement and can
provide an effective alternative to use of thick sand blankets and serve as
temporary foundations for roads on soft clay deposits.
7.7 Laboratory tests conducted on nonwoven coir products
Thickness is one of the basic physical properties used to control the quality of
many geo~extiles. Correct thickness values are required in the computation of various
parameters related to filtration and drainage. The thickness of coir needled felt varies
considerably depending on the confining pressure. The pressure range recommended
for nonwoven is in the range of 3.43 x 10 -3 to 3.43 kPa. For moderate high retaining
walls, embankments and slopes, the overburden pressure may go up to 20 kPa. Hence
needled felt specimens were subjected to large confining pressures and the variation
of thickness with pressure is plotted in figure 7.13 and beyond a pressure of 20 kPa
the reduction in thickness is practically nil.
The diameter of loading plate also affects the thickness. For larger specimens
larger diameter loading plates should be used. Otherwise there will be puffing of the
specimen in an area beyond the loaded area. Additional loading pads of various
diameters are required to test specimens of larger diameter. The mechanism to apply
foot pressure in the case of thic~ess gauge used for geosynthetics should also be
improved for making it suitable for coir geotextiles.
Engineering application in general and as reinforcement in particular depends
on the stress-strain characteristics of coir needled felt just as for any other geotextile
and the soil felt interaction behaviour. Indian standards (IS: 1969-1985) recommends
the use of wide width tensile test for evaluation of tensile strength of nonwoven
fabrics, felts etc.
7.7.1 Principal features of wide width tensile strength test
Method particulars:
Specimen size(mm) - 200
gauge length(mm) - 100
clamp size (mm) - 200
strain rate (mm/min)( I0% elongation) - 10mm/min
121
16~-------- _
14
12
~10
EgVI
8VIQ,Ic:
.'::It.()
~ 6
4
2
0
0 5 10 15 20 25
Pressure (kPa)
___ Cement coated AGL 101 (5.6 em) -tr-AGL 101 (10.1em)
--.-AGL 101 (5.6 em)
-G--Cement coated AGL (5.6 em)
-t-Plain felt - AGL (10.1 em)
-4-Plain felt - clcr (10.1 em)
-G-Cement coated AGL 101 (10.1 em) -+-Cement coated AGL (10.1 em)
Figure 7.13 : Average thickness vs pressure curve for needled felt of
diameter 5.6 and 10. Icm both plain and cement coated
122
The specification for testing in different countries shows that the defonnation
rate varies from 2 to 300 mm/min and number of specimens from 10 to 20. The length
and width varies from 100 to 300 mm and 50 to 500 mm respectively.
For coir needled felt of different modifications and treatments, both confined
and in air tests were conducted. The size of specimen was 500 x 200 mm and
markings were made on the needled felt. Clamps were fixed at both ends of the felt.
Glass plates were used on both sides of the test set up longitudinally and sand layers
of 2.5 cm thickness was placed on the top and bottom faces of the needled felt in
which the needled felt was sandwiched. Planks of 500 x 200 mm was placed on the
sand bed and different confining loads were placed over it and a wire was clamped on
one side of the felt which was taken down through a pulley to the bottom to an
arrangement where different loads were kept at the bOttOill end of the lever and hence
tensile loads were applied to the specimen. One end of the felt specimen is fixed
without allowing for any sliding of it. The stress strain graphs were plotted and the
confined moduli were found out for different specimens. The in-air test was
conducted by hanging the specimens vertically. Clamps were fixed at the ends of the
specimen and a lever was used for placing the loads causing elongation. The
deformations were measured accurately and the stress-strain graphs were ploned. The
confined modulus was found to be many fold more than the unconfined or in-air test. .modulus, which is a behaviour usually, associated with nonwovens. The results are
given in figure 7.14.
7.7.2 Soil- fabric friction and pull out
In all soil reinforcement applications, geotextile soil interface-friction is a
major factor governing the satisfactory perfonnance of reinforced earth structures. In
modified direct shear test the needled felt was held securely between sand and dummy
and a load was applied to move the upper block. In the pullout test, a strip was placed
freely between two halves of direct shear box filled with soil. The force required to
pull out the needled felt was obtained (figure 7.15). The pull out resistance was also
determined by using an arrangement (Figure 7.16) used for confined or in-place
tension test described earlier except that no fixing of the specimen was done so as to
make the specimen slide freely.
123
3.5 1 ------- _
I
iI
I
1412108
....,,, I''''....
"" .. "" ...........~.
",'
6
.. '
'".'
4
."I
2
.,
..
3
2.5
0.5
....... • 2
.€zo"tl.s 1.5
Strain (%)
~clcr ~ =0 . __AGL ~ = 0
-A-Coir grid backec'~ =0 --G-- Netlon ~ =0
-+--clcr ~ =2.5 kPa __._AGL 101 ~ =2.5 kPa
--*-Coir grid backec.~ =2.5 kPa. --tr-Netlon ~ = 2.5 kPa
-G-AGL 101 ~ = 10 kPa _·.···Cement coated AGL ~ = 2.5 kPa
- ...... Cement coated AGL ~ =10 kPa
Figure 7.14: Effect ofconfining pressure on the deformation characteristics of feltunder tension
124
Geotextile strip
Tomotor
TensionP.RJaw
Lower halfContainer
Normal stress
I Upper half
.'Qfl2'.'fo/i "l'?/z!'??;';2(7'~:;..;'j:'\'~~Qlt";;·!::'i"
To motor +---i'-..
Figure 7.15: Pullout test apparatus
125
PulleyGlass plate \r---------.. arrangementCoir felt )
ELEVATION
Table
Figure 7.16: Test setup for pullout
126
/Glass plateCoir felt
END VIEW
., .~
1~--------- ----;
pulled out
0.2
0.8
0.6,.....€ze.~IQ • AGL 101 Felt.s
0.4
0 CICTFelt
o 1 2 3 4
Displacement of segments oil (%)
Figure 7.17: Results ofpull out test on 50 x 20 cm felt specimen(cr =2.5 kPa)
127
Normal siressGeotextile
SECTIONAL ELEVATION
Figure 7.18: Modified direct shear apparatus
Also the pull out test with segmental deformations on two different types of
felt shows two facts.
1. The plain needled felt (\vithout pp reinforcement) shows a very low load and
deformation. Also the different segments show nearly the same segmental
deformations at failure indicating that the capacity of this material to transfer
pulling force is limited and only slightly bener than discrete fibres.
2. AGL 101 (pp reinforced) shows higher failure loads and the differential
movements are gradual showing a gradual transfer of force. Also the interior
segment shows about 50 % load at no pull out (Figure 7.17). Hence this type of
reinforcement is more suitable for reinforcement in walls, slopes etc.
The modified direct shear test (figure 7.18) provides a local shear stress shear
displacement relationship whereas pullout resistance integrates the variation in the
shear stress and displacement along the needled felt inclusion. The results of modified
shear test box and pullout test are given in figures 7.19 to 7.22. For pull out tests, for
large confining (normal) pressures of the order of 100 kPa, lOx 10 em needled felt
specimens were used.
128
12....-------------------------
8
.-.. .E.g1: 6Q.I
EQ.I>~
4
o 200 400 600 800 1000 1200 1400
Pull out load (N)
-+-Plain felt
-If-AGL101
-.-Cement coated
Figure 7.19: Results ofpull out test (0' = 150 kPa)(10 x 10 em specimen)
129
14...-----
10
,....E 8 •.sc41
E41 6>~
4
o 200 400 600 800 1000 1200 1400 1600 1800
Pull out load (N)
-+-Plain felt
_AGl101
-.-Cement coated
Figure 7.20: Results ofpull out test (cr = 200 kPa)(10 x 10 cm specimen)
130
25002000150010005000+----------......-----..------..------..
o
4
2
6
10
12
14
E.sI 8E..,>~
Pull out load (~)
-+-Plain felt
_AGL101
~Cementcoated
Figure 7.21: Results of pullout test (a =250 kPa)
(lOx 10 cm specimen)
131
140 ,---------
~
IVC-
800.c.0c~t\ 60L.IVGI
.c.(f)
40
20
OF'------,------,-------,-----r-----r----,-------,
o 20 40 60 80 100 120 140
-+- Heavy bitumen coated
--.- Light bitumen coated
-*-AGL101
-tr- Plain felt - AGL
Normal stress (kPa) .
___ Netlon
-Cement coated felt - Clcr
-e-Coir mesh backed felt
-G-Plain felt - clcr
Figure 7.22: Results of modified direct shear test
I ").)~
7.7.3 Hydraulic properties
Nonwovens are needed both for sandy and clayey soils. In sandy soils, it
contributes to restraint and friction. In clayey soils it provides drainage and dissipates
excess pore pressures. Sheeba et al. (2000) have shown the improvement in clays due
to non woven reinforcement. For coir needled felt to be used in water front structures,
it must satisfy certain requirements for water permeability and soil tightness. These
hydraulic properties must be functional during the useful service of the structure. The
important hydraulic properties are apparent opening size, inplane permeability, across
plane permeability, silt retention etc.
7.7.4 Apparent opening size
Coir needled felt can be applied as filter cloth for the protection of canal
slopes and as reinforcement for sand fills and as silt fence. In order to avoid the build
up of excess pressure under the filter, the material of filter should be sufficiently
water permeable to allow ground water to pass through, which depends on the pore
size and the number of pores per unit area. The top load will also compress the
geotextile, reducing the thickness and also particularly for nonwoven, the size of
opening and consequently the water permeability with increase in stress. In general,
coir needled felt is soil tight relative to woven in specific condition. They will only be
completely soil tight when the largest opening is smaller than the smallest particle of
the subsoil. The soil tightness of a geotextile can be characterized by a relationship
between the pore size of the geotextile and the particle size of sand in the subsoil. The
apparent opening size is determined by sieving spherical glass particles of a known
size through a geotextile. AOS also referred to as 0 95 is defined as the standard sieve
in mm, for which 5 % or less of the glass particles passes through the geotextile. The
needled felt fabric (instead of a wire mesh) was placed in a sieve frame by using a
special clamping arrangement. Different confining pressures were also used to study
the variation of AOS with pressure. The equipment used for geosynthetics was
modified as the needled felt was found to be sagging under the weight of soil and
pressure. So it was necessary to place the felt over a wire mesh of large opening size
so that the mesh acts as a support and at the same time did not prevent the falling of
beads or sand particles on the pan. The results are given in figures 7.23 to 7.27.
133
100100.1
II I
II
I
1 I II !I I III !
I,I
\I
i
I II
• r
I i II1~\I\
I : II I
II
II
I:~
I
I '%M I I! 1"1 II
II I ':.IIi!o
0.01
20
80
40
100
120
oc'VilQ 60a..~
Size of beads (mm)
-+-Surcharge 10.24 kPa Surcharge 5.12 kPa
-tr-Surcharge 2.66 kPa --*,-Surcharge 1.44 kPa
Figure 7.24: Size ofbeads \"s % passing curves of Aspinwall plain felt lll1dervarious surcharge pressures
134
1001010.1
I
i
i
I !f
I
I III1
. ! ,I
I t!!
~I
I
~,I
r-. II :?f IIo
0.01
20
40
80
100
120
0)c:.~ 60IQa.~
Size of beads (mm)
-Q-Surcharge 10.24 kPa ____ Surcharge 5.12 kPa
-fr-Surcharge 2.66 kPa 4-Surcharge 1.44 kPa
Figure 7.25: Size ofbeads vs % passing curves for AGL 101 tmder variouspressures
135
4
1\
80 +-_I--+-+-H~;+----4---4-+-+++-++-I---+--+-l-+-+-l-++t--+-+--+-t-Hr++-l
1\
Size of beads (mm)
-+-Surcharge 10.24 kPa -8-Surcharge 5.12 kPa
-...-Surcharge 2.66 kPa -..r-Surcharge 1.44 kPa
Figure 7.26: Size ofbeads vs % passing curves for cement coated plain feltunder various pressures
136
1001010.1
~ I.
~~ i
I~l\ !
~. i ! I~o0.01
40
60
20
80
100
Size of beads (mm)
-+-Surcharge 10.44 kPa -e-Surcharge 5.12 kPa
-.-Surcharge 2.66 kPa -*-Surcharge 1.44 kPa
Figure 7.27: Size ofbeads vs % passing curves for cement coatedAGL 101
137
7.7.5 Retention test
Retention tests were also conducted for needled felt specimens under
hydraulic loading conditions. But under dynamic hydraulic loading conditions there is
a danger of the soil particles being washed out from the fill. The. AOS of plain
needled felt of both the varieties (C.I.C.T and AGL) is 1.1 mm and I mm respectively
and for cement coated needled felt it is I mm and 0.9 mm respectively. Hence
particles finer than the AOS may get eroded or dislodged through the pores of the felt
especially under draw dO\vn conditions along with flooding. Hence a constant water
level was maintained at the front and top of the sand fill and silt fill constructed using
needled felt, both treated and untreated. Sudden draw down conditions were simulated
along with repeated loading and unloading waye action and fluctuating water levels.
The quantum of the washed out material was measured by collecting the particles
eroded for one metre high wall of both sand and silt fill. The erosion of particles was
high only for the first instant and the quantity collected was less than 10 gm. This may
be due to the formation of a natural filter cake in the earth bag, adjacent to the felt
layer in addition to the reduction in AOS due to the confining pressures.
7.7.6 Inplane permeability
The inplane permeability or transmissivity vanes with the thickness and
density of the fabric. The available testing equipment is meant for geosynthetics
(Figure 7.28). The application of various loads did not cause any change in the
thickness of the fabric, due to the presence of rigid connections. So in order to transfer
the load to the felt, the loading unit provided along with the equipment was replaced
with the loading frame of consolidation apparatus (figure 7.29). The results of inplane
permeability tests are given in figure 7.30.
7.7.7 Cross plane permeability
Both constant head and falling head permeameters are generally used for
measuring across permeability. ASTM D4491-85 specifies permittivity test using
constant head and falling head permeameters. The equipment used for geosynthetics
could be adopted for the testing of natural geotextiles without any modification
(Figure 7.31). The results are given in Figure 7.32.
138
......------------@
tt/J---------@
--!-l---------~
r---------{ 6
1. Base
4. Dial gauge7. Guide Ring
10. Guide Bracket
2. Loading Pad
5. Plunger8. Geotextile
1----0
3. Anvil6. Release Valve9. Water SUpply
Figure 7.28: rnplane permeability apparatus
139
5 )I
2
3 _-C:-::-.i
1. Load disc2. Geotextile3. OUtlet4. Inlet5. Load
Figure 7.29: Modified testing equipment for inplane permeability
1.+0
0.08 .,..-------------~-~--------~
>-.t::l>,-.,.- ()..... Q,I
.~~..... E 0.06~8
IF
0.04 +----...,----..,.----...,.----...,.------r------,
o 20 40 60
Pressure (kPa)
80 100 120
-+-Plain felt - AGL
-.-Cement coated AGL
-Q-Plain felt - clcr
-4-AGL 101
""*-Cement coated AGL101
-s-Cement coated clcr
Figure 7.30: Transmissivity vs pressure curves for different materials
l-ll
-+- ---.1 7
-----------@
I
III
I. ;®I.
®10
(jJ)
1. Base4. Perforated plate7. Plunger
10. Steel ball
2. Perforated plate5. Anvil8. Water inlet
11. Geotextile
3. Porous stone6. Dial gauge9. Guide ring
12. Water outlet
Figure 7.31: Cross plane permeability apparatus
142
0.016 .".-------------------------~
0.014
0.012
r-.0GI
~ 0.01Ee>-=:0 0.008nJGI
EGI
D-
III 0.006IIIe0«
0.004
0.002
O-t----~----,,----___,_----_r__---_._---___,
o 20 40 60
Pressure (kPa)
80 100 120
-+-Plain fel- AGl
~Cementcoated - AGL
,'f-Cement coated clcr
-A-AGL 101
~Cement coated AGL 101
_ Plain felt - clcr
Figure 7.32: Penreabilityvs pressme curves for various tested materials
1-13
7.7.8 Endurance property
Since coir getotextiles are relatively new and of low strength when compared
to synthetic products, questions are always asked as to their behaviour with time.
Various tests to study endurance are creep test, abrasion test, long term flow
(clogging) test and gradient ratio (clogging) test. Endurance characteristics of
geotextiles related to hydraulic structures are discussed here.
The most asked question in the use of coir geotextiles in hydraulic related structure is:
"will it clog?" Obviously, some soil particles will embed themselves within the
structure of the fabric, but the question is whether the fabric wiII completely clog such
that the flow of water through it will be completely shut off. The question can be
directly answered by taking a soil sample and the candidate geotextile
and testing them in the laboratory in
1. gradient ratio test to see that the GR < 3, or
2. long-tenn flow test to see that the tenninal slope of the flow rate versus time
curve is essentially zero.
Another approach to a suitable answer to the clogging question posed is
simply to avoid situations that have known to lead to serve clogging problems. From
available literature it is seen that time conditions are necessary to have a high
likelihood of complete clogging.
If the conditions are present, one should a\'oid the use of geotextile altogether.
I. Cohesionless sands and silts.
2. Gap-graded particle size distributions.
3. High hydraulic gradients
In such cases it is better to use a soil filter, or open up the geotextile to the
point where some soil loss will occur. This of course can be done only if the upstream
condition will permit such soil loss. Recommended values for such situations are
Woven fabrics: POA (percentage open area) > 6 %
Nonwoven fabrics: Porosity> 40 % under the actual stress conditions under which
geotextile is serving.
7.7.9 Long-term permeability
Geotextiles are increasingly being used as a filter layer in place of graded filter
in a variety of civil engineering situations. They retain erodable soil particles and
provide sufficient hydraulic conductivity to permit the free flow of water. It is
1.+.+
established that the hydraulic properties of geotextiles alone differ markedly from that
of soil- geotextile system. In addition to the opening size and penneability of the
geotextile, the hydraulic behaviour of combined soil-geotextiles influence the filter
ability of geotextiles in the long tenn flow situation. The flow rate of the soil
geotextile system decreases as the pores of geotextile get clogged. It is therefore,
imperative that the clogging resistance of geotextiles be evaluated to ensure adequate
long tenn filtration perfonnance.
The most direct testing approach is to take a sample of soil at the site and
place it aeove the candidate geotextile, which is fixed in position in a test cylinder. It
is then evaluated under constant head flow over a long period of time. At a transition
time of approximately 10 hours (for granular soils) to 200 hours (for fine grained
soils) the soil-fabric system will begin its field-simulated behaviour. If the slope of the
response curve becomes zero after this transition time, the fabric is compatible with
the soil (at least the slope continues to be negative). However increased clogging is
being indicated and eventually complete clogging would occur, in such case the fabric
is not suited for that soil and test conditions present. The results of long term flow
tests on soil-fabric system and typical response curves are given in figure 7.33
(Koerner, 1994).
7.7.10 Long term flow test apparatus (figure 7.34)
Under the steady state and reversed flow condition the fabric gets clogged and
this causes certain decrease in water flow rate and an increase in hydraulic head loss
through the geotextile. When the potential for fabric clogging is significant, clogging
resistance of filter fabric should be evaluated to ensure its long-term flow
perfonnance as a filter. Since the clogging depends on soil composition, soil fabric
penneability test was carried out to detennine the long term flow capacity of a
geotextile. The results of discharge vs time curves for different needled felt specimens
are given in Figure 7.35.
A wire mesh of sufficiently large openings is provided below the needled felt
to prevent its defonnation under the overburden.
145
125 ------------~ ._----------------
100
:? 75
i ..e.GI
1!9 50oi:i:
25
0+------,.--------,.-------.---------1
0.1 1 10
Time (hours)
100 1000
-7f- Nonwoven needled
-.-Nonwoven heat set
~Knit monofilament
____ Woven monofilament
Figure 7.33: Long term flow tests on soil-fabric system and typical response
cwves - mica silt soil and various fabrics (after Koerner, 1994)
146
Water level-----
fa _.-::.....-_-_-t-_-_.!..~_'-. ~::::O:O_I ------l(,40:----- --~-.- ------- --- -- -- -- -~~Water ~----- -- -- -- -- -- -
---~(3"~- .
- - --------- -
- - --------- -- - -~--
----- ----~--
---Geotextile
___---'J-2:
----~~--~~r777 •• ••• ~~ . .. .. ~
..... : . :.: f" ::. : ::."#'.. ~.. ..., --' ..... '.. .. .. .....
. , ~o~r"s~pli:. .. ..• •• • " • #'.. " .. . ... . .,: :.:..:,:..:,-.'.~
1. Receiver2. Mould3. Water reservoir
4. Overflow nipple5. Inlet nipple6. Outlet nipple
Figure 7.34: Long term permeability apparatus
147
140.,..-----------------------------
120
100
,.....()Q.I
~ 801.,£.Q.I
E?III 60.c()VI
5
40
20
1001010.1
0+----------,-------..,....-------.....--------0.01
Time (hours)
____ Cement coated AGL 101 ~Cement coated AGL
-+--Cement coated clcr -A-AGL 101
_Plain feltAGL ~Plain felt - clcr
Figure 7.35: Discharge vs time curves for different tested materials
148
7.7.11 Gradient ratio
A test that can be perfonned in 24 hours and is similarly aimed at detennining
the hydraulic compatibility of a soil-fabric system is the U.S.Anny Corps of
Engineers Gradient Ratio Test CW 00215. The most direct testing approach is to take
a sample of the soil at the site and place it above the candidate geotextile, which is
fixed in position in a test cylinder. It is then evaluated under constant head flow over a
long period of time. The hydraulic head at various locations in the soil- fabric column
is measured. Head differences are then converted to hydraulic gradients and the
gradient Fatio, as defined below, is calculated.
G.R = b.hf~is I b.Sf~l.O
b.h ls I b.S1.0
Where
hr +is =head change in centimeters from bottom of fabric to 2.5 cm of soil above the
fabric
Sf~1.0 = fabric thickness plus 2.5 em soil.
b.h2s =head change in cm between 5 cm of soil above the fabric
b.S2.0 =5 cm of soil
The corps of engineers suggests that gradient ratio values greater than 3
indicate non acceptable fabrics for the type of soil under. test. Figure 7.36 gives data
illustrating various combinations 'Of soil types and fabrics.
Gradient ratio test apparatus (Figure 7.37)
Gradient ratio test was conducted to detennine the long tenn filtration
perfonnance of coir geotextiles. In this test a suitable wire or dummy base to support
the natural fibre felt should be used so that the felt will not bend or break. Since the
thickness of natural fibres was more than 0' rings provided which are meant for
geosynthetics, washers were required to be placed between circular plates of the
apparatus, so that the leakage of water was eliminated
Equation to calculate gradient ratio;
G.R = H 1 -HI(H) -HJ/2
The above equation is not considering the thickness of the felt. Since the
thickness is more it has some significance in gradient ratio. So the equation is to be
modified as
149
- 14
- 12
- 4
G.R =L\h f+1S / L\Sf+1 a
L\h 2s / L\S 2.0
The results of the test are given in figure 7.38.
7.7.12 Mechanical properties
7.7.12.1 Puncture resistance
The geotextile during its placement must survive the installation process. Puncture
test, CBR push through test and cone drop test come under the survivability
characteristics of geotextile. Sharp stones, tree stumps, roots, miscellaneous debris
and so on on the ground beneath geotextile present a problem for puncturing through
the fabric after stone base and traffic loads are imposed above it (figure 7.39). In road
structure, when the base course is loaded the aggregate layer is pressed against the
geotextile and the geotextile has to absorb this load in the form of tension. Similarly
needled felt when used for canal bank protection, a layer of riprap may be placed over
it. The sharp edges of the stone revetment or blocks may damage it. Under this
situation thus the chance of geotextile puncturing is very high. Such puncture is an
important consideration because it occurs after the geotextile/geomembrane is
covered and cannot be detected until the purpose for which it is provided is defeated
or a leak from the complete system becomes obvious. Repair cost at that time is
enormous. In fact this chance is even more during the installation phase, either during
the placement or during the compaction of fill/aggregate material. It is therefore the
coir geotextile which must withstand such stresses without puncturing itself. For road
structure application and canal bank protection the geotextile thus should be
dimensioned accordingly to relate its resistance to objects like rocks, sticks etc. under
quasi-static conditions.
Puncture test
In this test, developed by corps of engineers, USA the specimen of geotextile
is clamped to an inside diameter of 44.45 mm and force is applied through a 8 mm
diameter plunger till failure. The modified ASTMD - 3787 provides for a
hemispherical plunger (figure 7.40). The test results are gIven in figure 7.41.
Percentage increase in puncture resistance due to cement coating-
For Bangalore C.LC.T. needled felt
For Aspinwall plain
For Aspinwall felt with polypropylene netting
ISO
10
9
8
7
6
0.~
t:OJ'6~ 4Cl
3
2
Ottawa sand andVicksburg silt loesssoil mixture
.. _ .. _.0;-00_00- .. _.o_.o_ ..~
0+---......--------,----,.----...,------.------.----.-------,
o 5 10 15 20 25 30 35 40
Soil silt content (%)
-+-Woven, monofilament
__Woven, monofilament
~Woven, monofilament
-x---- Non'NOven, needled
--G-Non'NOven, heat set
____ Woven, slit film
_. + ... U.S.Army Corps of Engineers max. acceptable value
Figme 7.36: Gradient ratio test data used to illustrate fabric clogging potentia](after Koerner, 1994)
lSI
j. - WatElI"JElYE!l
__-+_~_....'l"--fr-<"'l _.•....._. ._(~~)
----
-~--- ------- ------ -- -- -~~water. ~----- -- -- -- -- -- -
--.--- ------- -- -- -- -- -- -- -- --- ------- -
",,, .. ..., ...
.. .. '" ....... , :-.- ~
.~. ~. ".'.. .... .-' ..
/
... .... : ....... -.0' Soil sample • N-.loJ-V.. ". .. ... ... . '" .' ..,. -'" .. . ..
o " 0' • 1-'J--.loJ-V, '. ~ .". . -, ~
Geotextile
. ' ..... •
~§) ....(1)
1. Receiver2. Mould3. Water reservoir
4. Overflow nipple5. Inlet nipple6. Outlet nipple
Figure 7.37: Gradient ratio test apparatus
152
10 ...---------------'---~-----..:.--~----
8
6o.~
t:01'i5
5 4
2
0.01 0.1
~Plain felt - clcr
-+-AGL 101
-.-Cement coated clcr
10
Time (hours)
___ Plain felt - AGL
~Cement coated AGL
--6- Cement coated AGL 101
100
Figure 7.38: Gradient ratio YS time for different materials tested
153
Th
__L
t<E--- dp--~
~---- ds -----~
Figure 7.39: Visualisaton of a stone punching a geotextileas pressure is applied from above
----·········-{.D
I 2 ~
······450rrun-:~~~~~ 1. Plunger2. Geotextile
Figure 7.40: Puncture test apparatus
154
Table 7.3: Puncture test result - comparison among tested materials.
Material tested Puncture resistance (kPa)
C.I.C.T felt (Bangalore) 6200
Cement coated C.I.C.T felt 7060
Aspinwall plain 6350
. Aspinwall plain cement coated 6880
Aspinwall felt with polypropylene netting 7310
Aspinwall felt with polypropylene netting-cement7570
coated
7.7.12.2 CBR push through resistance
CBR push through resistance is defined as the ratio of push through load at
failure divided by the cross sectional area of the plunger.
CBR push through test
In this test developed in Germany, fabric is clamped to give an inner diameter
of 15 cm and a standard CBR plunger is pushed centrally through the fabric at a. .
specified rate (Figure 7.42).
The load at failure is the push through load and when divided by the cross
sectional area of plunger gives CBR push through resistance. This gives the resistance
of fabric to withstand localized pressure. The results of CBR push through resistance
are given in Figure 7.43.
Percentage increase in CBR push through resistance due to cement coating:
For C.I.C.T. felt - 31
For Aspinwall plain - 27
For Aspinwall felt with polypropylene netting - 20
155
8000 -------.---.---
7000
6000
5000.....III •
C-
O 4000VIVI2!
ti)
3000
2000
1000
O....=---.....,------.----.,..----.....,------r---...,------l
o 5 10 15 20 25 30 35
Penetration (mm)
___ Plain felt- CICT
-.-Cement coated AGL
___ AGL 101
~Plain felt - AGL
~Cement coated CICT
"*-Cement coated AGL 101
Figure 7.41: Results ofpuncture test
156
Push-slampUpper stretch ring
m /
/'I \ I
~ ~Specimen .I I I· \,(j
\LW(' C>
""r"'"
i
1< 050 )10150
Figure 7.42: CBR push through apparatus
Table 7.4: Results of CBR push through resistance
:\Iaterial tested Puncture resistance (kPa)
C.LC.T. 260
Cement coated CLCT. 340
Aspinwall plain 275
Aspinwall plain cement coated 350
! Aspinwall felt with polypropylene nening 288IAspinwall felt with polypropylene nening Cement 347
coated
400 ~----------'-----------------
8070605040302010O~~~--.--------.-----;---r----r----,--~----,
o
50
350
Q.l 300uc:co-enen 250Q.l"-
.c:Cl:J_ 200o co"-a.=~.c:en
150:JCl.
0::co(j
100
Penetration (mm)
~Cement coated AGL 101 -AGL 101
.......- Cement coated clcr - Plain felt - AGL
Figure 7.43: Results ofCBR push through test
158
7.7.12.3 Cone drop resistance
This test is conducted for evaluating the resistance of geosynthetics/geotextiles
to damage during installation due to dropping of sharp edged or pointed stones on the
geosynthetics/geotextile directly. The puncture resistance is the average size of the
hole (in mm) made by dropping the fall cone on the fabric.
Cone drop test
This test was developed by l\orwegian Road Research Laboratory C\RRL)
and is \videly used in Europe. In this test a geosynthetic/geotextile is clamped to yield
a clear didmeter of 150 mm and a brass cone of 45° included angle having a weight of
I kg is dropped through a height of 500 mm (Figure 7.44). The diameter of the
resultant hole is measured with a graduated conical device. The smaller the hole, the
greater the resistance of geosyntheticlgeotextile to damage. When used in
combination with other direct tensile test results, it provided a convenient means of
qualitative comparison.
Table 7.5: Cone drop test results
Material testedDiameter of cone formed I Remarks(mm) I
IFull penetration and I
C.LC.T. 65 Idistortion II
C.I.C.T. felt cement coated II I No full penetration !
-I
Full penetration and t
Aspinwall plain ! 60I,
distortionI
!
Aspinwall felt with i40
IFull penetration
polypropylene netting I
Aspinwall plain cement ,10 I No full penetration;
coatedAspinwall felt with
i No full penetration, singlepolypropylene netting 5
cement coatedI reboundI
7.8 Testing and standardization
Testing and evaluation is one of the most important aspects for the use of
geotextiles in civil engineering projects. The properties of geotextiles/geosynthetics
provide a means of communication between products, users and designers. They are
required for
159
~--®
L-r--r--- ..-r------.---.-l-----(J)
I-----f-+-------{]Q
1-----(1)81----+-1----1
~~m==s:~~---CD
Geotextiles
-----@
1. Casted ring4. Tie rods7. Cone saving device
2. Feet 3. Clamping ring5. Circular plate 6. Cone release mechanism8. Protection cover 9. Knob 10. Cone
Figure 7.44: Cone drop test apparatus
160
- Quality control of products both during production and during installation.
- Determination of the suitability of the material for the specific application!
functional requirement.
- Specification of the products.
The equipments available are for the testing and standardization of geosynthetic
fabrics. There is hardly any literature available for testing standards for evaluation of
natural geotextiles. Hence the testing equipments available for geosynthetic materials
are adapted and modified to coir geotextile (needled felt). Plain needled felt
manufactUred by C.1.C.T. Bangalore and Aspinwall Geotextiles Limited and cement
PVA coated felt of these varieties were tested in the laboratory for determining
various properties which are of relevance in geotechnical applications.
7.9 Summary of findings
Thickness measurement of coir needled felt has to be carried out using a
modified thickness gauge of larger diameter and provision should be made for
applying large confining pressures. Because of the large porosity of the material,
considerable reduction in the thickness is obtained for the initial pressures in the range
of 2 kPa to 20 kPa. Just as for any other nonwoven geotextile the stiffness and the
elastic modulus also increase considerably under confinement. The inplane and across
permeability of the needled felt is of such an order that ~he drainage capabilities can
be fully utilized when put to use for the consolidation process and excess pore
pressure dissipation. Even under buried condition where large surcharge pressures can
be expected, there is no danger of impaired drainage as seen from the experimental
results. The apparent opening size has not been reduced under confinement and in the
case of fine particles, sand blankets can also be used along with the felt layers so that
the migration of the fines is mitigated. This can also be effected by changing cement
content while using braided rope conduits as for geonets. Blocking and clogging
criteria are also satisfied by the coir needled felt, which is conspicuous by the gradient
ratio test and the long term flow test. Sufficient mechanical resistance is achieved by
means of cement PVA coating which is obvious from the results obtained for cone
drop tests, CBR push through and puncture tests or rather improvement is felt. Pull
out test results indicate that the chances of wedge failure or pull out failure are
161
practically nil by virtue of the pull out resistance developed due to friction under large
confining pressures.
Discrete coir fibres can be utilized for stabilization of slopes in canals below
the water line fluctuations. where erosion control measures by vegetation cannot be
used. Protection of slopes using coir meshes can be used only for natural slopes where
landslides and other slope failures are the main criteria since natural vegetation may
grow in due course of time as the material of coir mesh may degrade biologically thus
forming a mulch which aids vegetation growth. But this philosophy cannot be
employed in the case of irrigation and navigational canals since vegetation may hinder
the flow of water and vessels and also where maintenance of vegetation cannot be
ensured.
Where there is likely to be deep-seated failures, the reinforcement function can
be performed by fabrics of high friction and strength. Discrete fibres will not be
suitable in such situations. Coir fabrics have good frictional characteristics and their
stiffness improves manifold under confinement. But the tenacity of discrete coir fibres
is very low and hence cannot be suitable for improving the bearing capacity.
For steeper slopes, coir needled felt reinforcements could be used with suitable
treatments and modifications. By using fibres of varying thickness, size and rigidity
and by suitable interlocking or needle punching, the AOS can be suitably modified in
accordance with the requirements at the site. The rigidity of the fibres can also be
enhanced by blending with synthetic fibres or by proper impregnations using cement
and PVA coating.
One of the points to be considered is the method of manufacture whereby a
proper geotextile choice is made. Available data shows a wide variation of tensile
strength cN/tex within a narrow range of unit weight. In fact ropes are not good
choice as compared with flat products. Twill woven, braided rope or reinforced
fabrics can be efficiently used for reinforcement functions, while highly permeable
and frictional products can be used to disperse loads and restrain soil without
increasing pore pressures. A combination of nonwoven and woven can also be tried
giving high permeability like geonets with other composites like bamboo strips to
achieve specific purposes. Cost is one of the factors which govern the choice.
The perminivity and transminivity of coir felt, both treated and untreated do
not change considerably under confining pressures and are significantly high. Hence
back filters of coir felt can be used to ensure excess pore pressure dissipation without
162
particle removal. Cement coated coir needled felt or fabrics can be used to withstand
high stresses and ensure greater durability. It is also possible to use geocells made of
felt with coir mesh backing which can be filled with metal or coarse aggregates for
increasing the bearing capacity, where foundation is weak. For large height retaining
walls bamboo with coir interface may be more useful as the elastic modulus of
bamboo is comparable with that of steel and hence the frictional characteristics of
braided rope can be combined with the stiffness of bamboo. On the basis of the above
possibilities, model studies were made to examine these, which are discussed in detail
in the en~uing chapter.
163