GB2339803A - Weft-knit fabric with straight laid-in natural fibre yarns - Google Patents

Description

2339803 DIRECTIONALLY STRUCTURED FIBRE _G EOTEXTI LES This invention

relates to directionally reinforced fabrics, that is, to textile fabrics constructed so as to provide desired mechanical properties in one or more selected directions. The invention is particularly, although not exclusively, concerned with such fabrics which are of the kind referred to as geotextile or geosynthetic fabrics.

Geotextile fabrics are used in civil engineering in contact with dry, moist or water-immersed soil and other particulate earth or ground materials. The fabric provides reinforcement in its plane and its porosity and absorption characteristics may also be important in relation to its drainage, filtration and separation capabilities.

Typical uses of geotextiles are to retain soil particles whilst allowing through flow of water for example in the stabilisation of embankments or temporary earth road surfaces; or to protect grass seeds or planted areas; or to hold water to limit evaporation or overflow; or to facilitate drainage from moist or wet surfaces.

Generally, geotextiles are required to have sufficient tensile strength and low elongation to allow the fabric to be laid on site, and to have sufficient durability, with respect to degradation when buried in soil, to provide adequate reinforcement and ground protection over the required design life of the civil engineering constructions, or land project, in which the fabric is used (which may be a limited period only). Dimensional stability sufficient to give adequate ease of handling and laying on site is also an important factor. Strength, bulk, porosity and stiffness or flexibility are determined by selection of the textile fibres and the fabric structure.

It can be desirable to use natural fibres, particularly for ecological reasons and also for reduced cost and convenience of local supply especially in developing countries.

Nonwoven structures made from jute, coir and flax have been used for mulching applications. These are however not suitable for applications where appreciable mechanical strength is required.

Plain weave structures made from jute or coir are also known for geotextile applications and these can have greater strength than nonwoven structures. However, in order to obtain high levels of tensile strength it is necessary to use relatively massive or coarse yarns which are not suitable for power machine weaving. Hand or machine weaving has to be used and inexpensive, convenient mass production is not possible. There is also the problem of achieving good dimensional stability. Open woven structures are prone to slippage or distortion.

Knotted net structures using relatively massive or coarse natural yarns are also a possibility but these also are not suited to mass production and present dimensional stability problems. These structures also exhibit lower strength and higher elongations.

Ease of mass production and considerable dimensional stability can be attained with warp knitted structures but the complexity of needle movements on warp knitting machines requires fine filament yarns whereby for adequate strength it is necessary to use synthetic materials; relatively massive or -coarse natural yarns are not suitable.

An object of the present invention is to provide a dimensionally stable textile fabric having good strength in at least one direction, low elongation, good soil/fabric interaction and which can be conveniently mass produced using relatively massive or coarse natural fibre yarns.

According to one aspect of the invention therefore, there is provided a textile fabric having a weft knitted structure, characterised by the provision of straight laid-in natural fibre yarns in at least one direction.

With this arrangement considerable strength in at least one direction is attainable from the straight yarns, and the weft knitted structure can ensure good dimensional stability by locking these yarns in position. Since -these yarns are laid-in straight they can be readily incorporated on a mass production basis even in the case of relatively massive or coarse fibrous yarns. The highest possible strength in one or more directions with low elongation combined with dimensional stability giving ease of handling and laying on-site is achievable in so far as the highest possible number of straight high strength laid-in yarns can be provided in the required direction or directions.

It would not be feasible simply to lay separate high strength yarns -4 side by side for example on a ground surface- In effect the present invention permits this by providing a fabric skeleton structure to hold high strength yarns in position during laying of the fabric.

The weft knitting structure may also be formed from natural fibre yarns which may be the same as or different from the straight yarns.

Alternatively, if desired, and as appropriate, the weft knitting structure may be formed wholly or partially from synthetic yarns.

The natural fibre yarns used in the invention are preferably so called long vegetable fibre yarns. However any other suitable natural fibre yarns whether of vegetable, animal, fruit or leaf origin may be used.

Most preferably, yarns of the weft knitting structure are thinner and more flexible than the laid-in straight yarns and may be weaker.

Suitable national materials are flax, sisal, jute, hemp, coir, abaca, although no restriction to these is intended. The laid-in yarns may have a linear density of the order-of 1 000s say 1000 to 10,000 tex, f or example 6700 tex. The knitting structure yarns may be of the order of 1 00s, say less than 1000 tex, for example 400 tex.

The laid-in yarns may run longitudinally and/or transversely of the f abric.

The distribution of the laid-in yarns and the structure of the weft knitting can be selected as desired to meet requirements for use. Indeed it is a particular advantage of the invention that the fabric can be designed to have desired characteristics within a wide range, as is the case with synthetic textiles, despite the use of natural fibre yarns.

In this respect, where the laid-in yarns extend longitudinally these may be continuous such that a fabric of any desired length, having appreciable strength in the direction of its length, can be achieved. By appropriate selection of the number, spacing, weight, thickness and tensile strength of the laid-in yarns, a desired longitudinal strength characteristic can be obtained.

Where laid-in yarns extend transversely of the fabric their length will be limited by the knit width. In other respects however transverse strength characteristics can be selected in the same manner as for longitudinal yarns as mentioned above.

Where the fabric of the invention is used as a geotextile fabric to stabilise a particulate ground surface, soil or other ground particles interlock with the fabric, by penetration of its interstices, so that the soil/fabric interface exhibits greater shearing resistance than the surrounding soil, i.e.

the soil/fabric coefficient of interaction (a) is greater than unity. The fabric is designed with abutments and apertures to produce high shear resistance and this gives rise to an increased likelihood of wear or damage to the fabric during and subsequent to installation and during backfilling. With the invention, the locking weft knitting structure can act as a relatively inexpensive encapsulating or protective sacrificial structure whereby the effects of shear are accommodated by the weft knitting structure leaving the strength-providing laid-in yarns wholly or completely undamaged and their disposition unchanged.

In effect, the laid-in yarns provide the desired strength characteristics, and the weft knitting structure acts to hold the yarns in position relative to each other whilst the fabric is being installed; and after installation, the weft knitting structure provides a sacrificial protective effect. Accordingly, the laid-in yarns can be selected for mechanical properties whereas the yarns of the weft knitting structure can be selected for cheapness and may be relatively weak and degradeable.

As a consequence of the use of the weft knitting structure to lock the laid-in yarns in position it is possible to arrange for the knitted loops to form the same configuration on both sides of the fabric thereby presenting the same surface on both sides of the fabric for contact with sand or soil which can contact not only the knitted loops, but also the laid-in yarns whereby shear stress is transmitted directly to both the laid-in yirns and the knitted skeleton.

As mentioned, the distribution and other characteristics of the laid-in yarns can be selected to meet stoructural requirements and thus, the yarns may be closely packed e.g. to give a solid fabric of the nature of a dense mat. Alternatively the yarns may be spaced to give an apertured fabric or may be relatively widely spaced individually, or may be arranged in groups which are spaced from each other, thereby to define a net or grid conformation with relatively wide apertures therein, such as a so-called geogrid. Geogrid fabrics are used on surfaces having large gravel particles.

The gravel penetrates into the grid structure where it is 'locked' in position so as to be forced to shear against gravel above and below the fabric, rather than just relying on the surface characteristics. The pattern of the net or grid, with regard to the size and distribution of the apertures, can be selected as desired.

With regard to the mode of production of the fabric of the invention, preferably a weft knitting machine is used which is modified to permit feed of the laid-in yarns. Indeed, it is a further ob ject of the invention to provide for the production of the fabric using existing machinery, with modifications, rather than requiring wholly new machinery, thereby to permit start up and production of the fabric, possibly using local resources, quickly and inexpensively.

Thus, and in accordance with a second aspect of the present invention there is provided a flat bed knitting machine, for use in the production of a fabric as described above, the machine having opposed rows of needles and weft yarn feed devices having an interlink therebetween so as to be movable together along the respective rows thereby to form the said weft knitting structure, characterised in that the interlink between the feed devices is remote from the needles and a guide arrangement for the said straight laid-in yarn or yarns is provided at the needles clear of the said interlink.

With this arrangement, the fabric of the invention can be produced using a flat bed knitting machine vvhich can be conveniently obtained by modification of an existing standard such machine.

In the case where the straight laid-in yarn or yarns run longitudinally of the fabric, the guide arrangement may comprise one or more straight tubes extending in the warp direction e.g. vertically in the case where the needle rows or beds extend horizontally.

In the case where the straight laid-in yarn or yarns run transversely of the fabric, the guide arrangement may comprise a presser foot adapted to apply the yarn between the needles in a weft direction e.g- horizontally in the case where the needle rows or beds extend horizontally.

With regard to the interlink between the cam systems for producing appropriate weft knitted structure, this may comprise separate chains driven in synchronism with each other. In this respect it is to be understood that 1he conventional arrangement is to provide a link bar (or lock bar, or bow) bridging over the needles. In accordance with the invention this can be wholly or partially removed and replaced with the aforesaid chains or with any other suitable interlink providing adequate clearance for the laid-in yarn or yarns.

The weft knitting arrangement may be such as to provide a double jersey knit with the weft knitting yarns. Other constructions may also be possible.

The machine of the invention may also be used to produce a tubular structure. Such tubular structure may be filled with packing and used as a geotextile structure particularly in wet conditions e.g. to stabilise river banks or to provide a water drainage path.

Thus, and in accordance with a further aspect of the present invention there is provided a geotextile structure which comprises knitted tubing containing and retaining therewithin separate packing material.

The knitted tubing is preferably formed from natural fibre yarns (as discussed above), although synthetic yarns are also possible. The packing material preferably comprises natural fibrous'material such as raw vegetable material although waste synthetic fibres or rags or any other suitable waste material may be used.

The tubing can be laid along river banks to protect newly planted reeds.

The tubing can also be used as a consolidation drain.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which:

Figs. 1-5 show diagrammatically a land embankment with and without geotextile reinforcement; Figs. 6-12 show geotextile fabrics constructed in accordance with three _10 embodiments of the invention; Figs. 13-16 show parts of a knitting machine in accordance with the invention for use in making the fabrics of Figs. 6-12; Fig. 17 shows in diagrammatic perspective view a further geotextile structure in accordance with the invention.

Figs. 1-5 show an embankment 1 of soil, soft clay or the like which is typically reinforced with geotextile fabric.

Fig. 1 shows the desired shape of the embankment. Fig. 2 shows how the embankment can fail and split apart, into two parts 2, 3, if not reinforced. Fig. 3 shows.how part 4 of the embankment can fail and collapse due to foundation failure, with a circular failure movement, where one part 5 of the embankment is reinforced with geotextile fabric but the other part 4 is not. Fig. 4 shows how the unreinforced embankment can fail and collapse due to basal failure.

Fig. 5 shows how the embankment 1 maintains its shape, with failure forces being resisted as shown, when reinforced with embedded geotextiles having longitudinal strength in the direction of the indicated forces 6.

Figs 6-6 show one example of a geotextile fabric in accordance with the present invention.

The fabric has a weft knitted structure with laid-in weft strength yarns 7. The strength yarns 7 are laid-in on a 1x1 rib basis.

The width of the fabric, and hence the length of the strength yarns 7 depends on the knitted width which may be 1.75m, depending on the knitting machine as discussed hereinafter. The fabric is produced on a continuous basis and its length can therefore be as desired.

The laid-in yarns 7 are relatively large, strong and stiff compared to the weft knitting yarns 8.

The laid-in yarns 7 are straight, i.e. without any crimp, and parallel to each other so that the fabric is essentially flat.

By way of example, the knitted loops 8 may be formed from a flax yarn of linear density 400 tex, and the laid-in yarns 7 may be sisal yarns of linear density 6700 tex.

The fabric has low extensibility in the direction of the laid-in yarns 7 and the knitted loops 8 hold the laid-in yarns 7 in parallel configuration.

Figs. 9-11 show a further example of a geotextile fabric formed from the same or similar materials to the fabric of Figs. 6-8, however, in this case the laid-in yarns 9 extend longitudinally in the warp direction. Thus, the width of the fabric is limited to the width of the machine (e.g. 1.75m), but the fabric and hence the laid-in yarns 9 may be of any desired length in so far as the fabric is made on a continuous basis.

As shown, the warp laid-in yarns 9 are straight and parallel to each other and held in position slightly spaced apart by the weft knitting loops 10.

With the fabrics of Figs. 6-11, the laid-in yarns are all evenly spaced.

-12 With the embodiment illustrated in Fig. 12 weft laid-in yarns 11 are disposed in spaced-apart groups, As also are the weft knitting loops 12, thereby defining a grid structure.

Modifications and combinations of the above structures e.g. with laid-in yarns in both directions are possible.

Fig. 13-15 show a flat bed weft knitting machine modified to make the f abrics of Figs. 10- 11.

In conventional manner, the machine has two rows or beds of upwardly inclined needles 13 to which weft yarn is fed, on opposite sides from front and back feeder carriages 14, 15 which move backwards and forwards along the needle rows 13.

The machine may be a 1.75m wide powered industrial weft knitting machine such as a Dubied DC-2 (double cam 5 gauge). Conventionally the front and back feeder carriages are interconnected so as to move together by means of a rigid bow which extends above and across the needle rows.

This conventional arrangement is modified by removal. of at least the central portion of the bow, thereby to clear the edge, at the centre of the bed, between the needle rows 13. In its place, the carriages 14, 15 are linked by two endless chains 16, 17 running on a series of single and double sprockets.

At the front, there is an upper endless chain 16 running along the front needle row between two upper end sprockets 18. The front carriage 14 is connected to this.

There is also a lower endless chain 17 which runs around two sprockets 19 mounted below the upper end sprockets 18 and extending around further sprockets 20 across the opposite ends of the machine to run at the rear along the rear needle row. The rear carriage 15 is connected to this.

Angle irons 21 run along the front and back of the machine behind the chain runs 16, 17 which face the needle rows to guide the chains.

Adjustable seating plates 22 are fitted on the knitting machine frame to seat the sprockets 18, 19, 20.

As shown in Figs. 14 and 15, an angle iron 23 is fixed above and along the needle rows and this supports a series of closely equally spaced vertical open-ended brass tubes 24 terminating at their lower ends between the needles 13.

A series of guide bars (not shown) draw inlay yarns from a creel for feed downwardly into the tubes 24.

The machine as described can also be used to make the fabrics of Figs. 6-8 and 12 using a modified feeder, as shown in Fig. 16, to feed the weft laid-in yarns.

The feeder has two presser feet 25 which act to push the laid-in yarn into the gap between the two needle rows, so as to prevent this yarn becoming entangled with the needles.

-14 A sprung wire 26 is attached to the top of the inlay feeder to take up slack at the end of every course. As an alternative, a dead weight lever arm can be used, the weight being adjusted to take up different degrees of slackness giving a more positive motion than the sprung wire which may go slack with use.

A plastics tube 27 runs down the inlay feeder to encapsulate the yarn and avoid tangling with the weft knitting yarn feeder when the carriages reverse their direction to form the next course.

By appropriate modification of the machine described above it is also possible to make a weft knitted continuous tube 28 (with or without any laid-in yarns). As shown in Fig. 17 such a tube, preferably made from natural yarns, may be filled with raw fibrous waste material 29 for geotextile applications, particularly to define water channels along the tube or to act as absorbent rope like materials.

The machine described above can be used with existing pattern cards for the production of appropriate knitting structure. The needle beds can be set for a rib structure so that all four cams knit on every course.

The flat fabrics with laid-in yarns as described above present considerable advantages for geotextile applications.

Main factors which make weft knitted directionally structured fibre geolextiles superior or distinct from other fabric forms are that they have:

1. maximum strength in at least one direction with no crimp - in one or both of the width and length direction; 2. ridges/abutments to resist shear failure; 3. integrity and stability of the structure which is superior to woven and nonwoven structures; 4. a large or small cover factor so the load-bearing yarns can be achieved either to provide more protection or to produce a cheaper f abric; 5. the possibility of eliminating warp yarns: by W reducing density of structure and hence reduced cost, and (ii) providing a drainage path for excess water to dissipate through; 6. ability to produce grid structures that can be used in certain design specif ications.

7. a structure, which as vertical stress increases, can cause the fabric to deform slightly in the cross-strength direction forming a 3D microstructure which locks up particle movement more; 8. a higher shearing resistance than smoother textured synthetic geotextiles; 9. no sizing of warp yarn requirement; and 10. ability to change fabric sizes (width and length) and pattern without redrawing the yarns.

Typical properties of one of the novel knitted directionally structured vegetable fibre geotextiles compared to a mid-range warp knitted synthetic polyester geotextile, tested under identical conditions are:- Geotextile Displacement Breaking Breaking Breaking Breaking Initial Area Coefficient Type at Break Load Strain Strength Load/Width Modulus Density of (mm) (kN) (Nmm") (Knm-') (Nmm") (gm'2) interaction Vegetable

Length direction 16.35 10.33 8.18 38.98 206.6 878.80 1753.2 1 Cross direction 80.04 1.03 40.02 3.74 20.57 9.59 Synthetic Length direction 55.78 2.32 27.89 27.31 46.42 262.6 430.13 0.9 Cross direction 102.87 0.11 51.43 1.31 2.23 41.55 These new structures can as easily be produced from other types of yarns i.e. of a synthetic origin, whilst still offering the advantages gained from both the base structures and new manufacturing techniques.

One of the major features of the geotextiles is that the high strength yarns are incorporated within the structure absolutely straight and at relatively low tensions without suffering any form of abrasion or wear during processing. This technique enables the utilisation of the maximum potential strength and stiffness of the high strength material in the structure.

In the case of the production of reinforced knitted continuous tubes as discussed above, these tubes may be made from vegetable or synthetic fibre yarns which can encapsulate raw vegetable or waste synthetic fibres or waste material. They can be laid along river banks to protect the newly planted reeds from the ebb and flow of the tide or from wave movements by boats until the reeds have become established and are able to stabilise the river bank themselves. Also, they offer the potential of forming a new style of consolidation drain, the knitted fabric acting as a filter and raw fibres as a drainage path. The knitted structure is designed to facilitate initial wash through of fine particles, enabling a natural graded filter to form adjacent to the drain. Uniformly grade sand which normally forms the drainage path is lacking in developing countries, however, they have abundant supplies of raw natural fibres which can provide an adequate drainage path. After the pore water pressure has dissipated the fibres deteriorate without leaving any trace of drainage, consequently making the land suitable for excavation and reuse in years to come.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only. Thus, for example, although reference is made to geotextile applications, the fabrics of the invention may also be used as industrial fabrics or for other purposes.

Claims (23)

-18CLAIMS

1 A textile fabric having a weft knitted structure, characterised by the provision of straight laid-in natural fibre yarns in at least one direction.

2. A fabric according to claim 1 wherein the weft knitted structure is also formed from natural fibre yarns.

3. A fabric according to claim 2 wherein the natural fibre yarns of the weft knitted structure are the same as the laid-in yarns.

4. A fabric according to any one of claims 1 to 3 wherein the natural fibre yarns are long vegetable fibre yarns.

5. A fabric according to any one of claims 1 to 4 wherein the weft knitted structure is formed from yarns which are thinner and more flexible than the laid-in yarns.

6. A fabric according to any one of claims 1 to 5 wherein the laid-in yarns have a linear density of 1000 to 10000 tex.

7. A fabric according to any one of claims 1 to 6 wherein the weft knitted structure is formed from yarns which have a linear density of less than 1000 tex.

8. A fabric according to any one of claims 1 to 7 in use as a geotextile fabric to stabilise a particulate ground surface.

9. A flat bed knitting machine for use in the production of a fabric according to any one of claims 1 to 8, the machine having opposed rows of needles and weft yarn feed devices having an interlink therebetween so as to be movable together along the respective rows thereby to form the said weft knitting structure, characterised in that the interlink between the feed devices is remote from the needles and a guide arrangement for the said straight laid-in yarn or yarns is provided at the needles clear of the said interlink.

10. A machine according to claim 9 for producing fabric with a straight laid-in yarn or yarns running longitudinally of the fabric wherein the guide arrangement comprises one or more straight tubes extending in the warp direction.

11. A machine according to claim 9 for producing fabric with a straight laid-in yarn or yarns running transversely of the fabric wherein the guide arrangement comprises a presser foot adapted to apply the yarn between the needles in a weft direction.

12. A machine according to any one of claims 9 to 11 wherein the interlink is between cam systems and comprises separate chains driven in synchronism with each other.

13. A machine according to any one of claims 9 to 12 wherein the weft knitting arrangement is such as to provide a double jersey knit with the weft knitting yarns.

14. A machine according to any one of claims 9 to -13 adapted to produce a tubular structure.

15. A geotextile structure which comprises knitted tubing containing and retaining therewithin separate packing material.

16. A structure according to claim 15 wherein the knitted tubing is formed from natural fibre yarns.

17. A structure according to claim 15 or 16 wherein the packing material comprises natural fibrous material

18. A structure according to any one of claims 15 to 17 when laid along river banks.

19. A structure according to any one of claims 15 to 17 when used as a consolidation drain.

20. A fabric according to claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

21. A machine according to claim 9 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

22. A fabric when formed using the machine of any one of claims 9-14 or 21.

23. A geotextile structure according to claim 15 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.