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: sca1@bolton.ac.uk

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