GB2356880A - Rail support structures and geosynthetics - Google Patents

Abstract

A geocomposite (50) for a rail support structure includes at least a geotextile (32) and a geomembrane (34). Additionally, the geocomposite may include a second geotextile (42) to provide drainage of pore water, and a geonet (52) to provide increased drainage of surface water. The geonet preferably has a drainage characteristic to drain water at least predominantly in a transverse direction.

Description

2356880 RAIL SUPPORT STRUCTURES AND GEOSYNTHETICS This invention relates

to the field of geosynthetic materials (also known as geosynthetics). Aspects of the invention focus on the use of geosynthetics in rail support structures, but the invention is not limited exclusively to this.

The function of ballast in a railway support structure is to provide a firm base for the track, able to absorb vibrations, and support the extreme loads when a train passes overhead in order to maintain the alignment of the track. The use of geosynthetics in the support structure has been known for many years. In general, the material is used as a separator between a layer of ballast and an underlying subgrade to prevent intermixing.

The field of geosynthetics is divided into different material types. As used in the art (and for the avoidance of doubt hereby defined for the present specification), the meanings of the various terms are as follows:

Geosynthetic - a general term including one or more synthetic materials (and combinations of synthetic and natural materials), suitable for use on or in the ground.

Geotextile - a woven or non-woven permeable geosynthetic textile; Geomembrane - an impermeable geosynthetic sheet; Geogrid - a geosynthetic sheet in the form of a grid (i.e. having holes therethrough). The grid can interlock with soil to create tensile reinforcement; Geocore - a geosynthetic member having, or defining, one or more open channels for providing water flow or drainage.

Geonet - a grid-like, or net-like, geocore.

Geoweb - walls of geosynthetic material defining compartments with open tops and bottoms intended to be placed on the ground and filed with granular soil.

Geocomposite - a combination of any two or more geosynthetics.

Fig. 1 illustrates one example of a geotextile 10 used to separate a ballast layer 12 supporting railway track (rails and sleepers) 14, from an underlying subgrade 16.

The purpose of the geotextile 10 is to prevent soil from the underlying subgrade 16 from tending to rise upwardly into the voids between the stones in the ballast thus contaminating the ballast. This erosion of soils from the subgrade tends to cause 2 subsidence of the ballast 12. The contamination of the ballast by the soil can also cause the ballast to lose its structural integrity, as the most fine soil can "lubricate' the ballast stones thereby allowing them to move more easily in relation to each other.

However, a problem with this arrangement is that rainwater draining downwardly through the ballast 12 can pass through the permeable geotextile 10 into the underlying subgrade 16 weakening certain soil types e.g. soils containing a significant proportion of clay and/or silt sized particles, i.e. "fines". Also, when a train travels overhead along the rails, the weight of the train causes extreme pressures in any ground water in the underlying subgrade material 16. The resultant pumping action is known to force water in the underlying subgrade through the geotextile 10, carrying with it fines from the underlying subgrade 16, and thus causing erosion of the subgrade and contamination of the ballast 12. This "erosion pumping" results from the extreme forces experienced by the supporting structure, especially when a train is travelling at speed.

A further problem is that the geotextiles may have a rough surface which tends to abrade subgrade material comprising typically silty/clayey soils and weak clay rich mudstone rocks. This is as a result of the mechanical forces and vibrations experienced by the support structure in use. Such an abrading effect increases the loose fines at the interface between the geotextile and the underlying subgrade, which in turn adds to the "erosion pumping" problem mentioned above.

To try to counter the above problems, Fig. 2 illustrates a known arrangement in which an impermeable geomembrane 18 is used in place of the geotextile 10.

However, in contrast to geotextiles, geomembranes tend to have low puncture resistance. This means that an upper layer 20 of graded sand has to be employed between the ballast 12 and the geomembrane 18 to provide a protection layer which prevents the sharp stones of the ballast from puncturing the geomembrane 18.

Puncture prevention is extremely important, as a puncture in the membrane is likely to cause more severe problems locally (due to concentrated hydraulic pumping) than if a permeable sheet were used instead.

In addition, a second layer of graded sand 22 has to be employed beneath the geomembrane 18 to provide a drainage channel for so-called "pore" water which tends 3 to be expelled (relatively slowly) from the underlying silty/clayey subgrade 16 over a period of time. Unless a drainage outlet for this expelled pore water is provided, the water will tend to collect at the interface between the underlying subgrade 16 and the geomembrane 18, causing the subgrade soil 16 to become very soft, and unable to provide adequate support for the ballast 12 and the track 14.

Although the structure in Fig. 2 has been used to overcome the problems associated with water, the multiple layers in the structure make it very time consuming, labour intensive, and therefore expensive, to construct. Moreover, the geomembrane 18 needs to be handled very carefully in order to prevent punctures. It is possible to include reinforcing nets of stronger fibres in a geomembrane, but this merely increases the tensile strength of the membrane; it does not prevent the rough surfaced ballast abrading and/or puncturing the geomembrane in the spaces between the net fibres.

Reference is also made to GB-A-2243108 (Smith) which describes a geocomposite component in which a layer of geotextile is wrapped around a double cuspated core. The core provides channels for rainwater draining downwardly, and pore water rising upwardly, to drain. The core also provides a solid imperTneable wall to act as an impermeable barrier in the same way as the geomembrane of Fig. 2.

However, such a component is relatively expensive to manufacture. Furthermore, it is very difficult to join individual components longitudinally, or transversely while ensuring that there will be no leakage of water at the joint. As explained above, the leakage of water in a small area can cause worse water damage locally than if a permeable geosynthetic had been used, by virtue of the more concentrated "pumping erosion" caused.

The present invention has been devised to overcome, or at least mitigate, some of the above problems.

Broadly speaking, in a first aspect, the invention provides a geocomposite component for a rail track support structure, the component being structurally integral and comprising a geotextile and a geomembrane secured directly or indirectly to each other. When used in a rail track support structure, the geotextile may be used uppermost.

4 By using a geotextile in a structural laminate with a geomembrane, the geomembrane can be protected from puncturing by the ballast, thereby avoiding the need to use a layer of sand between the ballast and the geocomposite. In contrast to a fibre net-reinforced geomembrane, the geotextile can provide the necessary abrasion and puncture protection for the geomembrane, because the geotextile provides a complete textile covering over the geomembrane.

If the geocomposite comprises only the above layers, then it is envisaged that should this geocomposite be used for example over silty/clayey soils or weak clay rich mudrocks, then a layer of filter sand would be required between the geocomposite and the subgrade to allow pore-water expelled from the subgrade to drain away.

However, in a preferred form, the geocomposite further comprises a second (lower) geotextile on the opposite side of the geomembrane to the first (upper) geotextile. The second geotextile is intended to aid the drainage of pore water by providing a drainage path, and to provide a slightly compressible and/or compliant surface for bearing against the underlying subgrade thus minimising the abrasion of the subgrade. Therefore, it is preferred that the second geotextile has a different characteristic from the first geotextile. In the preferred embodiment, the second geotextile is a needle punched textile, or is a mechanically bonded textile (these terms sometimes being used synonymously).

In some forms of the invention, the first geotextile may be secured directly to the geomembrane. However, in a particular form, a geocore, for example a geonet, may be used between the first geotextile and the geomembrane. The geocore can improve the lateral drainage within old contaminated ballast. Surface water (for example, rainwater) draining downwardly through the ballast and through the first geotextile is then drained laterally out of the pennanent way structure. This can reduce the problem of lack of drainage capacity of old contaminated ballast. The first geotextile inhibits the debris washed down from above from entering, and clogging, the geocore.

It is preferred that the geocomposite component has a directional drainage characteristic such that water tends to drain transversely, rather than longitudinally.

This can reduce the effect of water flowing longitudinally within the drainage core or being pumped longitudinally when a train passes overhead. It helps to ensure that water is drained transversely and from under the track support structure, rather than allowing it to move easily longitudinally and thus remain under the track where it might still contribute towards damage.

Although the above techniques are especially suitable for track support applications, they are not limited exclusively to such use. Nevertheless, it is believed that, due to the extreme mechanical and natural conditions experienced by track support structures, the problem of water handling is most extreme in such an application.

In a closely related second aspect, the invention provides a geocomposite component for a rail track support structure, the component being structurally integral and comprising a first geotextile having a first characteristic, and a second geotextile having a second characteristic different from the first characteristic.

In a closely related third aspect, the invention provides an elongate geosynthetic component comprising means for providing drainage of water within the component, the component having a drainage characteristic such that drainage occurs at least predominantly in a transverse direction relative to the component.

In a closely related fin-ther aspect, the invention provides a rail track support structure comprising means for draining water at least partly through the structure, the drainage means having a characteristic such that water is drained at least predominantly in a transverse direction.

Embodiments of the invention are now described with reference to the accompanying further drawings, in which:

(Fig. I is a schematic sectional view of a geotextile used in the prior art); (Fig. 2 is a schematic sectional view of a geomembrane used in the prior art);

Fig. 3 is a schematic section view though a first embodiment of a geocomposite; Figs. 4a and 4b are schematic sections showing an enlarged view of the non- woven filaments of the geotextile welded together; Fig. 5 is a schematic sectional view showing the geocomposite of Fig. 3 in a track support structure; 6 Fig. 6 is a schematic section showing a second embodiment of a geocomposite; Fig. 7 is a schematic sectional view showing the geocomposite of Fig. 6 in a track support structure.

Fig. 8 is a side view showing how two pieces of the geocomposite in Fig. 8 can be joined together; Fig. 9 is a plan view of the geocomposite of Fig. 6; Fig. 10 is a schematic section showing a third embodiment of a geocomposite; Fig. I I is a schematic sectional view showing geocomposite of Fig. 10 in a track support structure; Fig. 12 is a schematic perspective view of a track support structure; Fig. 13 is a schematic perspective view of a geonet providing directional drainage characteristics; Fig. 14 is a schematic section of the geonet of Fig. 13; and Fig. 15 is a schematic plan view showing a repair of a rail track support structure.

It will be appreciated that the drawings are merely schematic, and are not meant to depict the relative sizes or thickness of the different materials.

The geocomposite of the preferred embodiments were developed with the aim of achieving at least some of the following performance characteristics:

(a) the ability to be able to lay the geocomposite directly under ballast without having to employ a layer of covering sand between the geocomposite and the ballast; (b) an impermeable characteristic to prevent water from moving upwardly or downwardly through the geocomposite (e.g. to prevent rain water from passing into the underlying subgrade, and to prevent hydraulic pumping of underlying water into the ballast); (c) to enable the geocomposite easily to be joined to an adjacent section of geocomposite in a substantially liquid tight manner when required; (d) to provide drainage for pore water to enable the geocomposite to be laid on an underlying subgrade without having to employ a layer of sand between the geocomposite and the subgrade; 7 (e) to reduce the effect of abrading or wearing the underlying subgrade caused by track vibrations and sharp edges of ballast pressing against the subgrade; to provide for improved lateral drainage of rainwater which has drained downwardly through the ballast, to reduce the effect of loss of permeability due to contamination of the ballast; (g) to provide for transverse directional drainage of rainwater.

The different embodiments can achieve at least some, but not necessarily all, of the above performance characteristics, and will find application depending on ground and track conditions and the surrounding environment.

Referring to Figs. 3 to 5, a first embodiment of a geocomposite 30 comprises a layer of geotextile 32 laminated to a layer of geomembrane 34 to form an integral structure. The geotextile 32 may be woven, but in this embodiment is a non-woven, and include fibres of polypropylene or polyethylene-coated-polypropylene. Fig. 4a shows how polypropylene fibres 36 fuse to each other to form the textile at their points of mutual contact, and Fig. 4b shows the fusing of polypropylene fibres 36 coated with polyethylene 38. In the latter case, the polyethylene flows together to form a more comprehensive, and strong, weld between the two fibres. Either type of fibres can provide a strong non-woven structure, but the polyethylene-coated- polypropylene is believed to offer excellent tensile strength and puncture resistance. It will, of course, be appreciated that other materials may be used, as desired.

The geomembrane will typically be polymeric (i.e. non-bituminous), for example, of polyethylene. The geomembrane may either be formed as a separate sheet which is then laminated to the geotextile, or it may be applied as a coating to one side of the geotextile. Particularly where polyethylene-coated-fibres are used in the geotextile, an extremely good heat bond can be formed between the geotextile and the geomembrane at their interface, although it will be appreciated that the polyethylene can alternatively be bonded to non-coated fibres.

Referring to Fig. 5, the geocomposite 30 is used with the geomembrane 34 face downwardly, and is laid between the ballast 12 and the underlying subgrade 16. The geotextile 32 provides puncture protection for the geornembrane 34, so that it can be laid directly under the ballast 12. Moreover, the lamination of the geotextile 32 and 8 the geomembrane 34 as a combined structural unit means that there will be no risk if slippage between the two, and thus avoids introducing a weak shear boundary in the permanent way structure.

In this embodiment, a layer of sand 22 below the geocomposite 30 is still used to provide adequate drainage for pore water expelled from the subgrade. However, depending on the ground conditions, the amount of pore water will normally be very small, and only a thin layer of sand is required.

By avoiding the need for the upper layer of sand, and strengthening and protecting the geomembrane, the geocomposite can be laid much more easily than, for example, the geomembrane of the prior art Fig. 2. Therefore, the laying procedure, whether for new track or for track repair, can be less time consuming, and hence less expensive.

Where it is necessary to join to separate sections of the geocomposite, this can be achieved easily by overlapping the edges of the two sections. If required the overlapping joint can be sealed e.g. by welding or gluing.

In contrast to the prior art arrangement discussed hereinbefore, this embodiment can achieve the performance advantages (a), (b) and (c).

Figs. 6 and 7 illustrate a second embodiment of geocomposite 40 which is a development of the first embodiment. In the second embodiment, a second geotextile 42 is laminated to the undersurface of the geomembrane 34. The second geotextile 42 has a different characteristic from the first geotextile 32. The first geotextile 32 is required to have a high tensile strength, and be puncture resistant, whereas these characteristics are not needed for the second geotextile 42. Instead, the second geotextile 42 is desired to be relatively non-abrasive, and to provide a drainage path for pore water expelled from the subgrade material. It is preferred that the second geotextile be needle punched, or mechanically bonded, to provide these characteristics.

The second geotextile may be of the same material as the first geotextile, if desired. However, in the present embodiment, the second geotextile is of a different material, for example, polyester.

Referring to Fig. 7, the geocomposite 40 can be laid directly between the ballast 12 and the underlying subgrade 16, without the need for any underlying layer of sand.

9 This is because the second geotextile 42 provides a drainage path illustrated by 45 to allow pore water expelled from the underlying subgrade 16 to drain within the second geotextile 42. Moreover, the needle punched second geotextile is somewhat compliant and will not tend to abrade the underlying subgrade. The abrasion is believed to be less because needle punched textile is not thermally bonded and can better absorb vibration shear stress. The needle punch material does not need to move over the subgrade with vibrations as does a thermally bonded non-woven which is less able to absorb vibrations. This is significant because the vibrations transmitted downwardly through the ballast 12 might otherwise tend to abrade the underlying subgrade 16, resulting in a greater quantity of loose fines, which could potentially clog the drainage path formed by the second geotextile 42.

In order to join two individual pieces of the second geocomposite 40, a joining piece of the first geocomposite 30 can be used if desired. The joining piece would simply be welded or glued across the join in contact with the upper surface of each second geocomposite 42.

However, as shown in Figs. 8 and 9, it is envisaged that the second geotextile 42 be produced to have a projecting flap 44 (of the first geotextile 32 and the geomembrane 34) along two adjacent edges 46 and 48. As best seen in Fig. 9, this would enable two pieces of the second geotextile 42 to be joined by using the flap 44 of one piece to overlap the edge of the second piece. The flap can be welded or glued to the uppermost surface in the same manner as described above. Typically the flap has a projecting width of about 30 cm, although any suitable size can be employed as desired.

As with the first embodiment, the provision of the geocomposite 40 as a structurally integral laminate avoids the problems of slippage between the different layers of the geocomposite. However, the second embodiment can provide the additional advantage of not having to lay an underlying layer of sand before the laying the geocomposite, thus simplifying yet further the work required to lay the track support structure.

In contrast to the prior art arrangements discussed hereinbefore, this embodiment can achieve the performance advantages (a), (b), (c), (d) and (e).

Figs. 10 and I I illustrate a third embodiment of geocomposite 50 which_ is a further development of the second embodiment. In the third embodiment, a geocore 52 is arranged between the first geotextile 32 and the geomembrane 34. The geocore 52 may be any suitable core providing drainage paths, but in this embodiment it is preferred that the geocore is a geonet (as it is believed that-a net structure can provide sufficient strength capable of supporting the weight of the ballast and the weight of trains passing overhead, without crushing).

The purpose of the geocore 52 is to provide drainage of water (e.g. rainwater) draining downwardly from the ballast 12 through the first geotextile 32. As illustrated by the arrows 54 in Fig. 11, the water is able to drain freely (e.g. laterally) in the drainage paths of the geocore. The first geotextile 32 acts as a filter to prevent small debris in the ballast from being washed into the geocore 52 where it might otherwise risk clogging of the geocore.

In contrast to the prior art arrangements discussed hereiribefore, this embodiment can achieve the performance advantages (a), (b), (c), (d), (e) and (f).

Referring to Fig.12, it has been appreciated during the development work for this invention that a railway support structure designed to provide directional transverse drainage of water may be extremely advantageous. For example, Fig. 12 shows schematically a railway support structure 60 arranged in the elongate direction of a track (shown by arrow 62). When water drains into the support structure 60 it is preferable for the water to drain in a transverse direction (indicated by arrows 64) rather than in a longitudinal directional (indicated by arrows 66). This is because the movement of water in the longitudinal direction would otherwise_allow the water to remain under the track area where it has potential to cause damage.

When a geomembrane (or a geocomposite containing a geomembrane) is used in the support structure 60, the direction of water drainage is of even more concern because the rainwater will collect above the geomembrane. If the geomembrane is included in a section of the track as a repair (e.g. to repair a soft spot caused by water damage), then it is important that the water be drained transversely. If the water is able to move longitudinally along the track to the end of the repaired area with the geomembrane, and back into contact with the subgrade soil, there would then be the risk that ftirther water damage to the subgrade may be caused at this position ftirther along the track at the end of the repair.

Accordingly, it is preferred that the geocore 52 be configured to drain water preferentially in a transverse direction, rather than in the longitudinal direction of the track.

Figs. 13 and 14 illustrate one possible configuration of a geonet 70 for forming the geocore 52. The geonet 70 comprises transverse members 72 interconnected by longitudinal members 74. The longitudinal members 74 are relatively shallow in depth compared to the transverse members 72 which occupy the entire height (depth) of the geonet 70. Therefore, water entering the geonet 70 is substantially unable to move in a longitudinal direction, but is guided to move transversely over the longitudinal members 74.

It will be appreciated that many other designs of directional drainage material may be used. In many embodiments, it will not be essential to totally prevent longitudinal movement of the water, but merely to enable transverse drainage preferentially. This might be achieved by relatively more material being used for filaments or walls extending predominantly transversely than for filaments or walls extending predominantly longitudinally.

The principles of directional drainage are not limited only to the geocomposites discussed above, but may be used with any track support structure, particularly those containing a geosynthetic. For example, Fig. 15 illustrates schematically a technique for repairing ballast damaged by water. In this technique, at least some ballast between and/or under the sleepers 80 is removed, and geosynthetic mats 82 are placed in position before re-laying the ballast over the mats 82. In this embodiment, the mats 82 are configured to drain water preferentially in a transverse direction, to reduce the chances of further water damage being caused by water moving longitudinally in the ballast. The mats may be dimensioned to be narrower or shorter than the spacing between ad acent sleepers, or (as shown by the dotted line 82a) the mats may be at least the spacing dimension such that adjacent mats meet. The mats may be butted together, or overlapped. If desired, the mats may be joined by welding or gluing together.

12 Although the above description focuses on the use of geosynthetics in railway support structures, it will be appreciated that the techniques may be employed in other fields (although it is believed that the problem of water damage is most acute in railway support structures, where extreme forces and vibrations are encountered, and very specialised solutions are required to solve the problem).

It will be appreciated that the above description is non-limiting and refers to preferred forms of the invention. Many modifications may be made within the scope of the invention. Although, features believed to be of particular significance are identified in the appended claims, the applicant claims protection for any novel feature or idea described herein and/or illustrated in the drawings, whether or not emphasis has been placed thereon.

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Claims (42)

1. A geocomposite component for a rail track support structure, the component being structurally integral and comprising a geotextile and a geomembrane secured directly or indirectly to each other.

2. A component according to claim 1, further comprising a second geotextile having a different characteristic from the first geotextile.

3. A geocomposite component for a rail track support structure, the component being structurally integral and comprising a first geotextile having a first characteristic, and a second geotextile having a second characteristic different from the first characteristic.

4. A component according to claim 3 ftu-ther comprising an impermeable barrier between the first and second geotextiles.

5. A component according to claim 2, 3 or 4, wherein the second geotextile has a greater transmissivity than the first geotextile.

6. A component according to claim 2, 3, 4 or 5, wherein the second geotextile is more compressible than the first geotextile.

7. A component according to any of claims 2 to 6, wherein the second geotextile has a characteristic such that it is better able to absorb vibrations than is the first geotextile.

8. A component according to claim 2 or 3 or any claim dependent thereon, wherein the second geotextile is needle punched.

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9. A component according to claim 2 or 3 or any claim dependent thereon, wherein the second geotextile is mechanically bonded.

10. A component according to any preceding claim, wherein the first geotextile is thermally bonded.

11. A component according to claim 2 or 33 or any claim dependent thereon, wherein the second geotextile is on the opposite side of the geomembrane to the first geotextile.

12. A component according to claim 2 or 3 or any claim dependent thereon, wherein the first geotextile is on a first face of the component, and the second geotextile is on a second face of the component.

13. A component according to any preceding claim, wherein at least one of the geotextiles is non-woven.

14. A component according to any preceding claim, wherein at least one of the geotextiles comprises a polymeric material.

15. A component according to claim 14, wherein at least one of the geotextiles comprises polypropylene.

16. A component according to claim 15, wherein at least one of the geotextiles comprises polyethylene-coated-polypropylene.

17. A component according to claim 14, 15 or 16, wherein at least one of the geotextiles comprises polyester.

18. A component according to claim I or any claim dependent thereon, wherein the geomembrane comprises a polymeric material.

19. A component according to claim 18, wherein the geomembrane comprises polyethylene.

20. A component according to claim 2 or any claim dependent thereon, wherein the second geotextile is attached directly to the geomembrane.

21. A component according to claim 1 or any claim dependent thereon, wherein the first geotextile is attached directly to the geomembrane.

22. A component according to claim I or 2, or to any of claims 5 to 20 dependent thereon, ftirther comprising a geocore between the first geotextile and the geornembrane, the geocore providing one or more open channels for water drainage.

23. A component according to claim 22, wherein the geocore has a directional drainage characteristic.

24. A component according to claim 23 wherein the geocore drains predominantly in a transverse direction.

25. A component according to claim 22, 23 or 24, wherein the geocore is a geonet.

26. A component according to any preceding claim, wherein the component is in roll form or in sheet form.

27. An elongate geosynthetic component comprising means for providing drainage of water within the component, the component having a drainage characteristic such that drainage occurs at least predominantly in a transverse direction relative to the component.

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28. A component according to claim 27, wherein the component is a geocomposite.

29. A component according to claim 27 or 28, wherein the component comprises a geotextile.

30. A component according to claim 27, 28 or 29, wherein the component comprises an impermeable barrier.

31. A component according to claim 27, 28, 29 or 330, wherein the component includes at least one water drainage channel.

32. A rail track support structure comprising a component as defined in any preceding claim.

33. A structure according to claim 32, comprising ballast arranged above the component, the ballast being in direct contact with the component.

34. A structure according to claim 33, further comprising ballast arranged below the component.

35. A structure according to claim 32, 33 or 34, flirther comprising a layer of sand arranged below and in direct contact with the component.

36. A structure according claim 32, 33 or 34, wherein the component is arranged directly on a subgrade.

37. A structure according to claim 36, wherein the subgrade comprises or contains one or more of.. clay; silt; and clay-rich mudstone.

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38. A rail track support structure comprising means for draining water at least partly through the structure, the drainage means having a characteristic such that water is drained at least predominantly in a transverse direction.

39. A structure according to claim 38, wherein the structure comprises a geosynthetic component.

40. A structure according to claim 39, wherein the geosynthetic component is a geocomposite component.

41. A method of laying or repairing a rail track support structure, the method including laying a component as defined in any of claims 1 to 3 1.

42. A component for a rail track support structure, or a rail track support structure, being substantially as hereinbefore described with reference to any of Figs. 3 to 15 of the accompanying drawings.