Indian Journal of Fibre & Textile Research
Vol. 33, September 2008, pp. 339-344
Designer natural fibre geotextiles—A new concept
Subhash Ananda
Centre for Materials Research and Innovation, The University of Bolton, Bolton, U.K.
This paper reports the development in flat weft knitting technology; the design and production of the novel natural fibre
geotextiles along with their interactive behaviour in different soil types and conditions. These natural fibre products are
found to be much more environment-friendly than their synthetic equivalents; the fibres themselves are a renewable resource
and are biodegradable. Such structures would also offer economical benefits to a number of developing countries, where
vegetable fibres are grown and cultivated in large quantity. These structures have been designed to provide the highest
possible strength in one direction , combined with the ease of handling and laying on site; soil particles interlock with the
fabric to such an extent that the soil/fabric interface exhibits greater shearing resistance than the surrounding soil, i.e. the
soil/fabric coefficient of interaction is greater than one; a degree of protection to the high strength yarns during installation;
a tensile strength in the range of 100-200 kN m-1; and ease of manufacture on conventional textile machines.
Keywords: Flat weft knitting technology, Geotextile, Knitted fabric, Nonwovens, Soil reinforcement
1 Introduction Geotextiles are used in numerous civil engineering
applications for reinforcement, filtration, separation,
drainage and erosion control. Mainly, there are four
types of polymers used as raw materials, viz.
polyester, polyamide, polypropylene and
polyethylene. They can be woven, nonwoven, knitted,
knotted, grids, membranes and even composite
materials. They represent one of the fastest growing
markets of all technical textiles. In a recent document
entitled “A competitiveness analysis of the UK
technical textiles sector” commissioned by the DTI,
UK, amongst the areas highlighted as having above
average growth potential over the next decade
especially within the UK and wider European
markets, the nonwoven and woven geotextiles show
5.9% and 5% growth per annum in value terms
respectively.1
In many ground engineering situations, e.g.
temporary roads over soft land, basal embankment
reinforcement, geotextiles are only required to
function for a limited time period. Furthermore,
synthetic geotextiles are normally prohibitively
expensive for developing countries. Many of these
countries have abundant supplies of cheap, indigenous
natural fibres (jute, sisal, coir, etc) and textile
industries capable of converting them into geotextile
fabrics. This paper reports the development in flat
weft knitting technology; the design and production of
the novel natural fibre geotextiles along with their
interactive behaviour in different soil types and
conditions.
2 Properties Required in Geotextiles
As stated earlier that the goetextiles can perform
several functions either individually or simultaneously,
this versatility relies upon their structural, physical,
mechanical and hydraulic properties. The emphasis of
the use of geotextiles in this paper is on short-term
reinforcing applications. The general properties
required to perform this function are given in Table 1
____________________________ aTo whom all the correspondence should be addressed.
E-mail: [email protected]
Table 1Functional requirements for reinforcing geotextiles2
Properties Requirement
status
Properties Requirement
status
Tensile strength HI Creep HI
Elongation HI Permeability NA-MI
Chemical resistance I-HI Resistance to
flow MI
Biodegradability HI Properties
of soil HI
Flexibility MI Water HI
Friction properties HI Burial HI
Interlock HI UV light I
Tear resistance MI Climate NA
Penetration MI QA & control HI
Puncture resistance MI Costs HI
HI – Highly important, I – Important, MI – Moderately important,
NA – Not applicable.
INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2008
340
(ref. 2). Figure 1 illustrates the stress-strain properties
of various vegetable fibre yarns. It is observed that the
vegetable fibre yarns have high strength, high
modulus, low breaking extension and low elasticity.
These properties make them ideal to form reinforcing
geotextiles.2 It is also important to note that vegetable
fibre yarns and fabrics possess low levels of creep
during use.
Geotextiles provide an invaluable solution to the
problem of constructing embankments over soft
compressible ground where water fills the pores
between soil particles under the embankment. The
load from the embankment fill increases the tendency
for the embankment to fail.3 Figures 2(b)-(d) illustrate
three typical modes of failure that may be
encountered (splitting, circular and basal), caused
because the underlying soft soil does not have
sufficient strength to resist the applied shear stresses.
The use of geotextiles at vertical increments in an
embankment and/or at the bottom of it between the
underlying soft soil and embankment fill [Fig. 2(e)]
would provide extra lateral forces that either prevent
the embankment from splitting or introduce a moment
to resist.
3 Production of Novel Structures The novelty of this invention lies in the fact that a
standard mechanically operated flat weft knitting
machine, which could be second-hand or
reconditioned and normally used to produce jumpers
and knitwear, is modified and redesigned to enable
extremely coarse and absolutely straight (without any
crimp or waviness as in a woven structure) natural
fibre yarns to be incorporated into a geotextile
structure. It is feasible to introduce at selected
intervals the high strength and low extension yarns in
the machine or length direction or in the cross or
width direction (Fig. 3) or in both major directions.
Suitable natural high strength materials are sisal, coir,
jute, flax and abaca, whilst the loop forming or
knitting yarn can be cotton, viscose, jute and flax. The
reinforcing yarns can be between 1000 tex and 10,000
tex, e.g. 6,700 tex sisal has been used in these
materials. The knitting yarn may be of the order 100-
1000 tex, e. g. 400 tex flax was used to produce
fabrics as illustrated in Fig 3. The distribution of the
laid-in yarns and the structure of the knitted loops can
be selected as desired to meet the requirements for
use. It is possible to produce solid fabrics (Fig. 4)
with high strength yarns being placed at desired
distance from one another either in the machine or in
the cross machine direction. It is also feasible to
design grid structures by omitting needles at
predetermined intervals and the sisal inlay yarns left
out to produce large apertures in the geotextiles,
similar to Tensar Geogrids, as shown in Fig. 5.
It is also possible to alter both the knitting yarn and
knitted structure to design and produce geotextiles with
specific characteristics for soil reinforcement. A further
Fig. 1—Stress-strain properties of various vegetable fibre yarns2
Fig. 2—Short-term applications for geotextiles in embankments3
[(a) primary embankment (b) splitting failure, (c) circular failure,
(d) basal failure and (e) no failure]
Fig. 3—High strength yarns in the (a) length and (b) width
directions
ANAND : DESIGNER NATURAL FIBRE GEOTEXTILES
341
novelty of this invention is that the tubular natural fibre
structures of any desired width can also be produced on
these machines, which can be further filled with straw,
paper and any other type of material and used as a
geotextile structure, particularly in wet conditions, e.g.
to stabilize river banks or to provide a water drainage
path. Figure 6 illustrates a view of a flat weft knitting
machine as delivered before various modifications
have been carried out. It is not feasible to introduce
warp yarns into the knitted structure due to the
connecting bow which synchronizes the movement of
the cam systems placed on the two opposing inclined
needle beds, where the knitting needles are housed.
Figure 7 illustrates the top view of the modified and
redesigned machine with chain and sprocket
arrangement, which facilitates the insertion of warp
threads via tubes placed vertically across the whole
machine width (Fig. 8). A close-up view (end-view) of
the machine showing the needles in opposing needle
beds, knitting feeders and knitting yarns and thick warp
yarns introduced vertically between the two needle beds
across the entire machine width is shown in Fig. 9. The
modified knitting machine also requires a creel to house
the warp yarns as well as a modified and continuous
fabric take-up system to cope with a heavy and thick
fabric. This novel technology and associated inventions
have been protected through a patent application and are
currently being commercially exploited by a UK knitting
company.4
Figures 10 and 11 show the fabric structures with
reinforcing yarns laid-in the width direction and the
strength yarns incorporated in the warp direction are
shown in Fig. 12.
Fig. 4—Sisal/flax novel knitted geotextile with reinforcement in
the width direction
Fig. 5—Sisal/flax novel knitted geotextile in grid structure
Fig. 6—Flat weft knitted machine with a conventional bow
Fig. 7—Modified and redesigned flat weft knitted machine
without bow
Fig. 8—Warp insertion tubes
INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2008
342
4 Performance Assessment In a study
5 at the University of Bolton, eleven
different geotextile fabrics with different fibre types
and/or fabric structures were systematically
investigated for a wide range of performance criteria
of geotextiles for soil reinforcement. Some properties
of all eleven types of geotextiles tested and analyzed
are presented in Table 2 (ref. 2). The list consists of
two fabrics designed and produced using the novel
knitting technique developed during this work (fabrics
1 and 2); 3 woven fabrics using vegetable fibre yarns
also produced during this study (fabrics 3-5); and
fabrics 6-11 obtained from external sources for
comparison with the vegetable fibre geotextiles. It is
observed from Table 2 that all natural fibre
geotextiles have high strength, low breaking extension
and high modulus in the strength direction. It is also
expected that these structures would also possess low
elasticity and creep values, which are essential for soil
reinforcement purposes.
4.1 Measurement of Shearing Interaction
Eleven geotextiles were tested in a 300 × 300 mm
partially fixed direct shear box. The relative
horizontal displacement of the two halves of the shear
box, the change in sample height during shearing and
the vertical displacement of the top four corners of the
upper half of the shear box were monitored by linear
dial gauges. The tests were conducted with dry
Leighton Buzzard sand and limestone gravel (average
particle diameter 0.8 mm and 6 mm respectively).
Nominal normal stresses of 50, 100, 150 and 200
kNm-2
were applied to the samples to represent the
likely range of soil pressures which would apply to
field situations.
The upper and lower halves of the shear box were
completed each in three layers of equal thickness
using a vibrating hammer and tamping plate to a
predetermined thickness to produce a normal unit
weight of 96% and 94% of the maximum nominal dry
unit weight for the sand and gravel respectively.
These figures were chosen to represent the density
likely to be achieved on site, whilst maintaining an
accuracy of ± 01 mg m-3
from the mean dry density in
subsequent shear box tests. The leading side of the
bottom half of the shear box has the geotextile
clamped to it.2
4.2 Coefficient of Interaction
The efficiency of geotextiles in developing
shearing resistance at the soil-fabric interface is
indicated by the coefficient of interaction (á) defined
Fig. 9—Warp insertion viewed from the end of machine
Fig. 10—Reinforcing yarns in width direction
Fig.11—Knitted grid construction
Fig 12—Different structures of reinforcing yarns in length
direction
ANAND : DESIGNER NATURAL FIBRE GEOTEXTILES
343
INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2008
344
as the ratio of the friction coefficient between soil and fabric (tan ä) and the friction coefficient for soil sliding on soil (tan ö). Values of peak (ö’max) and residual (ö’r) shearing angles together with their coefficient of interaction (á) are shown in Table 3 (ref. 2), some of these values are above one for the
sand (e.g. in case of knotted coir geotextile), indicating that by introducing the geotextile in the sand it actually strengthens the ambient sand. This could possibly be due to the surface texture of some of these geotextiles, in that the sand grains can interlock with the fabric and reduce movement. The
main properties required for reinforcing geotextiles for short-term applications can be generalized in that they must possess high tensile strength with low breaking extension and provide a good shear resistance in the fill used for the construction works. Table 3 shows that for overall performance the
nonwoven natural fibre geotextiles (fabrics 8 and 9) are found to be the least suitable for reinforcing application. The geotextiles made with the knitted and woven vegetable fibre (fabrics 1-5) show the best performance; all of these structures having been designed and produced at the University of Bolton.
5 A
further research programme investigated other natural fibre yarns, e.g. cotton; biodegradable manufactured yarns such as staple-fibre viscose rayon and filament viscose rayon; and other novel structures such as those based on single-jersey structures and incorporating high strength yarns in the machine
direction.6 It has been demonstrated that this novel
technology could be adopted for the manufacture of natural fibre geotextiles at a reasonable cost and would provide efficient and high performance geotextiles for short-term solutions for construction and civil engineering applications.
5 Conclusions A wide range of new and novel knitted structures
have been designed and developed using a modified
and redesigned flat knitting equipment. These natural
fibre geotextiles are found to have superior properties
in comparison to the mid-range of synthetic geo-
textiles for soil reinforcement, when considering
strength and frictional resistance. The high degree of
frictional resistance of the vegetable fibre geotextiles
is probably developed from both the coarseness of the
natural fibre yarns and the novel structures. These
vegetable fibre geotextiles will be much more
environment-friendly than their synthetic counterparts
and the fibres themselves are a renewable resource
that is biodegradable.
Acknowledgement The author wishes to express his gratitude to Dr
Martin Pritchard and Professor Bob Sarsby for the
significant contributions right from the conception of
this novel invention.
References 1 Anand S C, Devlopments in technical fabrics–Part 2, Knitting
Int, August (2000)53.
2 Pritchard M, Anand S C & Sarsby R W, Novel vegetable fibre
geotextile structures for soil reinforcement, Proceedings,
Conference on Textiles Engineered for Performance, UMIST,
20-22 April 1998.
3 Pritchard M, Sarsby R W & Anand S C, Textiles in civil
engineering: Part 2–Natural fibre geotextiles, Handbook of
Technical Textiles; edited by A R Horrocks and S C Anand
(Woodhead Publishing Ltd, Cambridge, UK), 2000, 388.
4 Anand S C et. al, Directionally structured textile fabrics, U K Pat
GB 2339803 27 November 2002.
5 Pritchard M, Vegetable fibre geotextiles, Ph.D thesis, Bolton
Institute, UK, 1999.
6 Aziz W, Further development of natural fibre geotextiles for soil
reinforcement purposes, M.Phil thesis, Bolton Institute, UK,
2000.
Table 3Shearing interactive values of vegetable fibre geotextiles compared to two synthetic geotextiles2
[Fill vs Fill: ϕmax (sand), 40.5o; α ϕmax, 1.00; ϕr (sand), 33.1o; α ϕr, 1.00; ϕmax (gravel), 54.7o; and α ϕmax, 1.00]
Fabric
No.
Geotextile type Load
kNm-1
Strain % ϕmax sand
deg
α
ϕmax
ϕr
sand, deg
α
ϕr
ϕmax
gravel, deg
α
ϕmax
1 Knitted flax sisal inlay 207 8 40.9 1.01 33.0 1.00 50.5 0.86
2 Knitted grid flax sisal 144 7 38.8 0.94 32.5 0.98 50.9 0.87
3 Woven sisal warp flax weft 180 10 40.0 0.98 32.4 0.97 49.8 0.84
4 Woven sisal warp coir weft 113 16 42.1 1.06 33.1 1.00 53.4 0.95
5 6×1 woven weft rib sisal coir 171 8 42.0 1.05 33.2 1.00 50.9 0.87
6 Woven coir geotextile 20 28 41.9 1.05 33.1 1.00 51.2 0.88
7 Knotted coir geotextile 18 53 43.5 1.11 36.7 1.21 51.8 0.90
8 Nonwoven hemp 2 56 39.3 0.96 34.8 1.07 44.6 0.70
9 Nonwoven coir latex 4 6 34.7 0.81 - - 36.4 0.52
10 Woven polyester 41 8 40.4 1.00 31.9 0.95 46.6 0.75
11 Warp knitted grid polyester 46 28 38.4 0.93 31.8 0.95 51.3 0.88