TECHNICAL FIELD

This disclosure relates to a foundation for a railway track. The method, system and resultant foundation may have applications beyond railway track construction. BACKGROUND ART

A railway track structure comprises steel rails supported on sleepers (e.g. of timber, pre-stressed concrete, steel, etc.), the sleepers in turn being laid on crushed stone ballast (often called "track ballast").

The track ballast can comprise or overlie sub-ballast (usually a finer grade of crushed stone). In addition, the track ballast may overlie an optional so-called "blanket" or "capping" layer, which can support the track ballast above the so- called sub-grade, as well as restricting the upward migration of wet clay or silt, and providing support for a drainage pathway/structure to capture water passing through the track ballast. The sub-grade is usually constructed over the sub-soil or natural ground. The sub- grade can be formed from native material recovered at the site for the railway track, or may comprise imported material. Typically, the sub-grade is formed into an embankment above the sub-soil or natural ground.

When the native material is unsuitable (e.g. the underlying natural ground is very soft or unstable, such as swamp or marsh land, sandy sub-soil, unstable clay or silt layers, etc.), material for the sub-grade tends to be imported. Alternatively, the native material can be treated prior to use (e.g. by adding grout, cement, or lime stabilisation or other stabilising additives). However, the resultant sub-grade is still supported on very soft or unstable underlying ground. Hence, to ensure adequate load-bearing capacity, there may be a need to excavate deeply and/or widely, and/or employ an excess of imported or treated material. Where the fill material needs to be imported, usually an engineered fill is employed (e.g. a specified, graded and checked fill material, such as a road base or fine crushed rock). In addition, the sub-grade needs to be compacted to a significant extent to withstand the loads it will be subjected to in use (e.g. fully-laden freight trains). Many native materials are therefore not suitable for use in railway track foundations. In addition, for soft and/or poor performing natural ground, considerable engineering works must be employed to achieve a suitable sub-grade.

Once a suitable sub-grade has been constructed (i.e. that is able to generally withstand the usual loads that are applied thereto by rail loadings), the sub-grade will in use periodically be subjected to vibratory and shock loading. Such vibratory and shock loads are transmitted through track ballast, the sub-grade to the natural ground. If the natural ground is soft and/or water-laden the vibratory and shock loads can cause the ground to be (or become) compressed, compacted, and fluids therein can be pumped laterally away from under the track foundation. Over time, this can lead to deformation and degradation of the foundation, or even to failure of the railway line. In either case, there is a requirement for ongoing maintenance of the track at such locations.

Furthermore, as vibratory and shock loadings transfer through the ballast they have the effect of degrading the track ballast over time. This also requires the track ballast to be periodically removed and replaced.

The use of geocells to laterally confine materials in a railway track sub-grade has been investigated. A geocell is a plastic cellular confinement system composed of a honeycomb-like structure which can be filled with sand, soil or the like and is used in soil stabilization applications. The use of geocells in a track sub-grade has been observed to control lateral movements under cyclic loading and hence contribute to enhanced track stability.

Similarly, US 7,470,092 discloses arranging a plurality of cylindrical segment elements in a foundation. More specifically, in relation to Figure 11, US 7,470,092 teaches arranging cylindrical segment elements in the form of reinforced aggregate particle units within the ballast material of a railway track foundation, so as to immediately underlie the rails of the track structure. US 7,470,092 teaches that the cylindrical segment element can be formed from a tubular or ring-shaped component and, in this regard, teaches that such a component can be formed from, inter alia, a tyre that has had both sidewalls removed therefrom.

Globally, there is an ongoing demand to increase the speed and freight capacity of railroad transportation. However, heavier axle loads and higher speeds can exert higher dynamic wheel loads on track structure with more repetitions. As a consequence, a soft subgrade may experience higher repeated stresses which may lead to progressive shear failure and excessive deformation. Moreover, under heavier dynamic loads, the existing railway track may degrade further and faster due to unacceptable track deformation and the lateral spread of ballast, all of which is likely to lead to more frequent maintenance.

It is to be understood that a reference to the background and prior art does not constitute an admission that the background and prior art forms a part of the common general knowledge in the art, in Australia or any other country.

SUMMARY OF THE DISCLOSURE

Disclosed herein is a method of constructing a railway track. The railway track may be a new track, or may replace an existing track. The method is able to provide a foundation for the railway track. The foundation can take the form of or can comprise a sub-grade.

The method comprises preparing existing ground so as to receive a layer thereat. For example, the ground may be cleared, excavated, in-filled, graded, compacted, etc. to make it suitable for having a foundation formed thereat that is suitable for supporting a railway. The preparation of the existing ground may include the formation of an embankment over the existing ground. The method also comprises laying barrier material over the prepared ground. The barrier material to be used may be specified by a design engineer for a given application. The barrier material may act as a barrier to the passage of a certain (e.g. fine) grade of particles through the foundation. Alternatively or additionally, the barrier material may act as a barrier to the passage of water and other fluids through the foundation.

For example, the barrier material may comprise a geotextile material. The geotextile material may comprise a fabric of a woven polymer fibre (e.g. of polypropylene or polyester). Alternatively, the geotextile material may take the form of a

combination of a non-woven geotextile incorporating a geo-grid. Other geotextile materials can be employed. The geotextile material may optionally be formed or treated to have water-resistant, water-repellent and/or waterproof properties.

In another alternative, the barrier material may take the form of a waterproof membrane. Such a membrane may comprise a continuous (e.g. polymeric sheet) or may comprise a tightly woven fabric (e.g. that is formed from a hydrophobic polymeric fibre that may be tightly woven in sheet-form to be made waterproof).

The barrier material may be supplied in the form of a roll of long sheeting (e.g. ~ 80 kN woven geotextile, ~ 5.1 m width; HDPE or polypropylene membrane, etc.). The barrier material may be laid (e.g. rolled) out along and over the prepared ground of the railway track. Depending on the required width of track, adjacent and generally parallel lengths of material may be overlapped (e.g. for a single track two adjacent lengths of overlapped material may be sufficient, with a greater number of lengths being required and overlapped for a multiple rail track/line).

The method further comprises placing a plurality of tyres that have a single sidewall removed therefrom in a laid-flat arrangement on the barrier material. Typically each tyre employed in the foundation has a single sidewall removed therefrom. The tyres may be placed such that a remaining sidewall of each tyre lies over and adjacent to the barrier material. The method as disclosed herein contrasts with the teaching of US 7,470,092, in that only one sidewall is removed from the tyre. In addition, the remaining (non- removed) sidewall of each tyre is placed so as lie over and adjacent to the barrier material. The arrangement of the remaining sidewall of each tyre over and adjacent to the barrier material offers a number of advantages. It can allow the tyres to be infilled with a sub-grade fill material. Further, the arrangement of each tyre over and adjacent to the barrier material can allow the fill material to be compacted within the tyre.

Additionally, the tyre sidewall that is not removed becomes an integral component of the tyre cell. As the base of the tyre cell, the remaining tyre sidewall provides structural stability during construction, uniform load distribution and additional fill confinement at the base of the tyre cell. The resultant additional amount of rubber present in the tyre cell also aids shock attenuating characteristics. Further, the tyre cell can improve the damping properties of a railway track, leading to a reduction of track deflection.

The removal of one tyre side wall converts the tyre into an engineered cellular structure (i.e. it is no longer a tyre). Hereafter, this engineered cellular structure will be referred to as a "tyre cell".

In contrast to geocells and the cylindrical segment elements of US 7,470,092, the use of such tyre cells in the track sub-grade also enables a broader range of fill materials to be employed in the sub-grade (e.g. used/spent track construction material, such as used/spent: track ballast, sub-ballast and other sub-grade materials, as set forth below). In addition, because of the rigours that tyres must endure in their normal use, they are already a highly engineered product. The use of tyre cells in the track sub-grade has been observed to better control lateral movements under cyclic loading and better contribute to enhanced track stability in more extreme environments, such as unstable ground (e.g. swamp, marsh, riverbed, sandy subsoils, etc.). This is especially so when the tyre cells are used in conjunction with an underlying barrier material. In some applications, more than one layer of tyre cells can be employed. For example, in very poor ground conditions, multiple tyre cell layers may be employed to increase load spreading performance. Multiple tyre cell layers may also be employed where larger format tyre cells are either not available or are in short supply. Where multiple tyre cell layers are employed, one layer may be staggered with respect to another layer, i.e. such that a given overlying tyre cell can overlie, and thereby spread and transfer load over, multiple underlying tyre cells.

The tyres for producing the tyre cells may be used (i.e. second-hand) tyres that can be pre-selected so as to be fit for purpose. The tyre sidewall may be removed prior to transporting each tyre cell to the site, or may be removed on site. The method can therefore provide another means of using tyres that would likely otherwise be stockpiled, or disposed of in landfill, or burnt, etc.

As above, the method additionally comprises in-filling the tyre cells with a fill material. The in-filling can be such as to place the underlying barrier material into tension. Thus, the filled tyre cells, together with the underlying tensioned barrier material, can behave as a composite engineered product. In this regard, in use, the filled tyre cells can provide a high degree of load spreading for a comparatively low depth of material. Further, the tensioned barrier material can act in conjunction with the filled tyre cells in spreading this load over a large area, as well as reducing lateral displacement of the filled tyre cells in use. In this regard, the tread of each tyre cell typically has a high degree of hoop strength, which enables each tyre cell to better act as a cellular confinement for the fill material (i.e. much better than geocells), thus enabling spreading of the applied load over a larger base area.

Another benefit of employing filled tyre cells is their ability to attenuate vibratory and shock loads which can be transmitted through the ballast to the sub-grade. This arises from the inherent damping properties of the high performance rubber incorporated in the tyre cell. The vibratory and shock-attenuation characteristics of the tyre cells enables them to receive and accommodate such vibratory and shock loads, which can reduce or slow ground compression, compaction and/or pumping, leading to a reduction in frequency of track maintenance or reconstruction. The tyre cells may also help to extend the life of the track ballast. Thus, each tyre cell can be seen to provide both vibratory/shock- attenuation as well as a high degree of load- spreading cellular confinement. In addition, because of the tyre cells' ability to receive and accommodate vibratory and shock loads, it is understood that noise levels at and within the track ballast can be reduced. In this regard, by receiving and accommodating such vibratory and shock loads, the resultant sound waves that would otherwise be generated are diminished, potentially to a significant extent. Hence, the use of tyre cells in the sub-grade may enable track construction to take place in areas/regions that would otherwise be unsuitable (e.g. high-density living urban areas).

As explained in more detail hereafter, the composite engineered product (in-filled tyre cells on a barrier material) can also reduce the amount of excavation required at a given site, and also the amount of engineered fill required to replace the excavated material. In addition, the amount of compactive effort required to compact the engineered fill can be reduced.

In one embodiment, the fill material for in-filling of the tyre cells may comprise used track construction material. For example, the track construction material can comprise used/spent: track ballast, sub-ballast and sub-grade. This track construction material may, for example, be existing material present at a section of railway track or a railway line to be replaced. Thus, the method can allow for the existing track construction material to be re-used within the foundation, rather than having to be transported away for disposal. This can reduce the amount of new track material required, and hence can reduce construction costs, as well as reducing used/spent material disposal costs.

In one embodiment, when reconstructing a pre-existing railway track, the existing track construction material may be excavated, set aside, optionally divided (e.g. graded into each of track ballast, sub-ballast and sub-grade) and then re-used as the fill material for in-filling the tyre cells. The resultant foundation that is constructed in this way can have railway sleepers (e.g. of timber, pre-stressed concrete, steel, etc.) arranged in relation to the in-filled layer of tyre cells. Typically, the sleepers are laid with respect to the in-filled tyre cell layer via track ballast. The track ballast can in turn be arranged over the sub- grade or capping layer (when present).

In one embodiment, the tyre cells may be laid along the railway track in columns (i.e. each column can generally follow the direction of the railway track - typically in a parallel arrangement to the rail line). A given column of tyre cells may be offset from immediately adjacent column(s) by half a tyre cell. This can provide a type of honeycomb pattern to the laid-flat arrangement of the tyre cells. For a single railway track the tyre cells may be laid out in up to seven columns, with a greater number of columns required for multiple tracks. In other embodiments, the tyre cells may be laid along the railway track in rows.

In one embodiment, the tyre cells may be laid such that a given tyre cell may closely face or touch the two adjacent tyre cells in its column. In addition, the given tyre cell may closely face or touch two adjacent tyre cells in the immediately adjacent column(s). Thus, the given tyre cell may closely face or touch up to six adjacent tyre cells. This close-packed arrangement can enhance the spreading of the load as well as the accommodation of dynamic forces (vibratory and shock) that are transmitted from the overlying railway in use, with adjacent tyre cells able to work together to attenuate such loadings and forces. In other embodiments, the tyre cells may be laid to define a "square" pattern (i.e. where one tyre cell touches four adjacent tyres cells). Other laying patterns are also contemplated.

In one embodiment, at least some of the adjacent tyre cells may be secured to each other. For example, each of the adjacent tyre cells may be secured together, such as by using one or more fasteners (e.g. bolts, rivets, rods, etc.). Alternatively, the adjacent tyres may be secured together such as by using tying elements (e.g. rope, tendons, cable, etc.). This securing together of the tyre cells can be used in certain applications and can enhance lateral stability, with the secured-together tyre cells functioning as a unit in the composite engineered product.

As set forth above, in some applications, the method may further comprise arranging a capping (or blanket) layer over the in-filled tyre cell layer. In one embodiment, the capping layer may be formed from the same fill material as is employed for in-filling of the tyre cells (e.g. the used/spent track construction material). Additionally, or alternatively, the capping layer may be formed from and/or may comprise one or more of: new track ballast, fine crushed rock (e.g. road base), or another stabilised fill material (e.g. basalt or other hard rock, etc.). In some applications, the method may further comprise arranging a further barrier (e.g. geotextile) material, such as a geofabric or geo-grid, waterproof membrane, etc. over the in-filled tyre cell layer. This further layer of barrier material can, for example, function to help prevent overlying ballast, etc. from ingressing or becoming embedded in the fill material of the tyre cells. This further layer of barrier material can also act to further stabilise the sub-base.

In some applications, a layer of new track ballast may be arranged over the in-filled layer of tyre cells, or over the capping layer (when present), or over the further barrier material layer (when present).

Also disclosed herein is a foundation system for supporting a railway track on prepared existing ground. The system comprises barrier material for laying over the prepared ground (e.g. such as set forth in the method above).

The system also comprises a plurality of tyre cells for arranging in a laid-flat arrangement over the barrier material layer. The tyre cells can also be as set forth in the method above. The system further comprises a fill material for in-filling of the tyre cells (e.g. such as set forth in the method above). The system is able to provide a foundation in the form of a sub-grade for a railway track.

Also disclosed herein is a foundation that is constructed by the method or using the system as set forth above. The resultant foundation can support a layer of new track ballast at the in-filled layer of tyre cells, or at the capping layer (when present). The foundation can also support railway sleepers with respect to the in-filled layer of tyre cells, or at the capping layer when present, and can support railway tracks at the railway sleepers. The foundation can take the form of a sub-grade for a railway track. BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a sectional schematic view of a first embodiment of a railway track foundation as disclosed herein. Figure 2 is a sectional schematic view of a second embodiment of a railway track foundation as disclosed herein.

Figure 3 is a sectional schematic view of a third embodiment of a railway track foundation as disclosed herein.

Figure 4 is a plan view of a rail track sub-grade. Figure 5 is a side schematic view of a tyre cell stack for use in the rail track sub- grade.

Figures 6A & 6B respectively show plan and side schematic views of an individual tyre cell for use in the rail track sub-grade.

Figures 7(a) to 7(f) respectively show plots of stress (kPa) against vertical displacement (mm) for six different testing procedures as set forth in Example 3. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

Referring firstly to Figure 1, a foundation for a railway track will be described in the form of an engineered track sub-grade (which can also be referred to as forming part of a "sub-structure" for the railway track). In a basic form, the sub-grade of Figure 1 comprises a barrier material in the form of a geotextile material 10, a single layer 12 formed from a plurality of in-filled tyre cells 14, and a track ballast layer 16, typically of new track ballast material.

As best shown in Figure 6, each tyre cell 14 is produced by removing one sidewall 19 from a tyre. This produces a tyre cell 14 that has a remaining (non-removed) side wall 15 intact. Such a tyre cell has all of the attendant benefits and advantages as outlined above. The plurality of resultant tyre cells 14 are arranged in a side-by- side, laid-flat arrangement over the layer of geotextile material 10, whereby the remaining sidewall of each tyre cell 14 overlies the layer of geotextile material 10.

The sub-grade may be constructed directly over e.g. cleared ground, or may be formed on a pre-existing or newly constructed embankment. In either case, the reference number 18 is used hereafter to indicate the ground at which the sub-grade may be constructed in each of these applications. The tyre cells 14 are typically produced from used (i.e. second-hand) tyres. The tyre cells 14 are typically produced from a large format tyre, such as a truck tyre. In such case, a single tyre cell layer 12 is usually sufficient for construction of the sub- grade. Tyre cells can also be produced from vehicle tyres, such as when truck tyres are not available or are in short supply. When vehicle-derived tyre cells are employed, typically the sub-grade is constructed with a stack of tyre cell layers 12 (e.g. two or more layers). The upper layer is typically staggered with respect to the adjacent lower layer, such that a given overlying tyre cell overlies, and is thereby able to spread and transfer load and forces over, multiple (e.g. 3) underlying tyre cells.

Before forming the tyre cells 14, each tyre is pre-selected (i.e. inspected and checked) so as to be fit for purpose. Because tyres are already a highly engineered and designed product (i.e. having high performance material properties), the tread of each tyre cell is able to provide a high degree of hoop strength in use in the sub- grade. This hoop strength enhances the ability of the tyre cell to laterally confine materials therein (e.g. in comparison to geocells). Used tyres can thus be re- purposed in the sub-grade (i.e. they would likely otherwise be stockpiled, disposed of in landfill, or burnt, etc. - representing non-sustainable or very low sustainability environmental outcomes). As will be explained hereafter, the tyre cells 14 (i.e. that include a remaining, intact side wall 15) provide a number of advantages to the sub-grade in comparison to known sub-grade constructions, including more recent experimental constructions that employ geocells or the cylindrical segment elements of US 7,470,092. One such advantage is the ability of the tyre cells 14 to attenuate dynamic vibratory forces and shock loads which can be transmitted through the ballast to the sub- grade. This ability is attributed to the material properties of the high performance rubber incorporated in the tyre cell. The vibratory and shock-attenuation characteristics of the tyre cells enable them to receive and accommodate such vibratory forces and shock loads. This can reduce or slow ground compression, ground compaction and/or pumping of fluids therein. This may also help to extend the life of the track ballast. In turn, this results in a reduction in the frequency of track maintenance and reconstruction. Moreover, the tyre cells 14 can improve the damping properties of a railway track, leading to a reduction of track deflection.

Another advantage that arises from the use of tyre cells 14 in the track sub-grade is that a broader range/grade of fill materials is able to be employed in the sub-grade (i.e. than can be employed in geocells), such as used/spent track construction material (described in greater detail below). The tyre cells 14 in the track sub-grade can also better resist lateral movement under cyclic loading and can be employed to enhance track stability in extreme environments, such as unstable ground (e.g. swamp, marsh, riverbed, sandy sub-soils, etc.). This is especially so when the tyre cells 14 are used in conjunction with the geotextile (or other barrier) material layer 10.

Further advantages arise from the tyre cell 14 comprising a sidewall 15 that is not removed, but that remains as an integral component of the tyre cell. These include the remaining sidewall 15 acting as the base of the tyre cell 14 in use to provide structural stability during construction. The remaining sidewall 15 can also assist the tyre cell to provide for uniform load distribution in use. The remaining sidewall 15 can further provide for additional fill confinement at the base of the tyre cell in use. The resultant additional amount of rubber that is present in the tyre cell 14 also aids in the cell's shock attenuating characteristics.

The tyre sidewall 19 can be removed prior to transporting each tyre cell 14 to a site. Alternatively, the tyre sidewall 19 can be removed on site using a suitable portable machine (e.g. a purpose-built tyre-cutter). In some applications, the removed sidewall 19 can be re -used in the sub-grade, such as by positioning it within and at the base of its respective cell (i.e. adjacent to remaining wall 15) before the cell is in-filled. This provides further rubber within the cell to enhance the cell's shock attenuating characteristics, composite structural properties, etc.

As shown in Figure 5, the tyre cells 14 can be stacked S on site prior to being arranged over the geotextile material layer 10. The stack height X may be approximately 2 m for ease of handling. Typically, the tyre cells 14 are stacked with their intact side wall 15 facing up (i.e. such that each tyre cell does not collect rain, silt, dust, debris, etc.). This can be particularly useful when constructing the rail line in tropical locations, where insects (e.g. mosquitoes) can breed in stagnant water, or in dusty/dirty locations.

The prepared ground 18 can be ground that is prepared at a "greenfield" site (not previously the site of a railway track). Here, the railway track can be a new track/line. Alternatively, the prepared ground 18 can be ground that is prepared at an existing railway track i.e. that is to be replaced or reconstructed. In either case, the ground can be cleared, excavated, in-filled, graded, compacted, etc. to make it suitable for having the sub-grade formed thereon, and so as to be suitable for supporting an overlying railway.

The geotextile (or other barrier) material 10 is laid out over the prepared ground 18. The geotextile material layer 10 is typically of a woven polymer fibre (e.g. of polypropylene or polyester fibre) that is supplied in the form of a roll of long sheeting. A suitable type of geo-material for the railway track sub-grade is so-called "80 kN woven geotextile". Such a geotextile is supplied in rolls of ~ 5.1 m widths.

Alternatively, the geotextile material 10 can be a combination of a non-woven geotextile and a geo-grid that has a high tensile strength (e.g. a composite product of non-woven polypropylene geo-fabric and high-strength polyester (e.g. PET) fibres). Other geotextile materials can be employed.

In a further alternative, the material 10 can be a waterproof membrane (e.g. a continuous polymer sheet used in construction applications).

The geotextile material 10 is rolled out along and over the prepared ground 18 of the railway track. In the case where two or more rolls are required to cover a given width of track (i.e. depending on the width of track and width of geotextile), the adjacent and generally parallel lengths of material are overlapped. In this regard, and as shown in Figure 4, for a single railway track, two adjacent lengths 10a and 10b of overlapped material are laid out in parallel, but so as to overlap by typically > lm. The two adjacent lengths typically have an overall width that is greater than the layer of tyre cells 12, so that the geotextile material layer 10 extends laterally beyond the tyre cell layer 12 on either side of the sub-grade. This allows the geotextile material layer 10 to be secured over the prepared ground 18 immediately adjacent to the sub-grade, which can also help to secure this ground (e.g. against erosion, etc.) and to facilitate fluid (e.g. water) run-off

It will be understood that a greater number of lengths of ~ 5.1 m width geotextile material 10 will be required for a multiple (e.g. 2+) rail track/line. Multiple overlaps will then be employed (e.g. two overlaps for three adjacent lengths of geotextile, etc.).

As best shown in Figure 4, typically the tyre cells 14 in the layer 12 are laid out over the geotextile material layer 10 and along the railway track in columns (CI, C2, C3, ... C7). For example, for a 7 metre wide sub-grade, and for a single railway track, seven columns of tyre cells are provided. The resultant seven columns C1-C7 have an approximate width of 6.7 m. For a line having two or more tracks, more columns of tyre cells will of course be required.

Each column of tyre cells (CI, C2, etc.) generally follows the direction of the railway track/line, i.e. such that the columns also follow any curvatures of the line. As shown in Figure 4 (i.e. when viewed in plan), each column is also offset from immediately adjacent column(s) by half a tyre cell. This enables the tyre cells in the layer 12 to closely pack, thereby providing the layer 12 with a honeycomb pattern/structure .

In alternative arrangements, the columns can be arranged to extend transversely across the track, in which case they can be referred to as "rows".

As also shown in Figure 4, each tyre cell 14 is arranged to closely face or touch two adjacent tyre cells in its column. The tyre cells 14 in the "outermost" columns (CI & C7) closely face or touch two adjacent tyre cells in the immediately adjacent columns (C2 & C6). The tyre cells 14 in the "intermediate" columns (C2 to C6) closely face or touch two adjacent tyres in the immediately adjacent columns on either side thereof. Thus, edge cells face or touch four adjacent cells, whereas intermediate cells closely face or touch six adjacent cells. This close-packed arrangement also assists with the sub-grade more effectively spreading the load and accommodating the forces transmitted to the sub-grade from the overlying railway in use. In this regard, adjacent tyre cells can work together to share load and attenuate dynamic (including unevenly applied) forces at the sub-grade arising from both moving and stationary trains. In alternative arrangements, the tyre cells 14 can be laid out in a non-"honeycomb" arrangement, such as a "square" pattern. In such a pattern, one tyre cell 14 touches up to four adjacent tyres cells. Other laying patterns are also contemplated including, for example, on curves in the track - i.e. where the tyre cells 14 can be arranged to suit the track formation. This may e.g. result in one tyre cell 14 touching from one to six adjacent tyre cells.

In certain applications, some or all of the adjacent tyre cells 14 in the layer 12 can be secured to each other. For example, securing of the tyre cells can be employed when the underlying ground is particularly soft or unstable, such as swamp or marsh land, mangroves, flood plains, damp or dry riverbeds, sandy sub-soil, unstable clay or silt layers, etc. The adjacent tyre cells can be secured together using fasteners such as bolts, rivets, rods, etc. Alternatively, the adjacent tyres can be secured together using tying elements such as rope, tendons, cable, etc. This securing together of the tyre cells can enhance lateral stability in the layer 12 (i.e. further resisting lateral displacement under load and arising from dynamic forces). In this regard, the secured-together tyre cells can effectively function as a single unit in the composite engineered product.

In construction of the sub-grade, a suitable fill material is employed for in-filling each of the tyre cells 14. The in-filling of each tyre cell 14 is such as to place the underlying geotextile material layer 10 into tension. That is, each filled tyre cell presses down on the geotextile material lying immediately thereunder, and thus pulls at (i.e. stretches) the geotextile material between adjacent cells. Thus, the filled tyre cells, together with the underlying tensioned geotextile, (i.e. the sub- grade), work together as the composite engineered product. In this regard, in use, and when compared to existing sub-grade constructions

(including those employing geocells), the filled tyre cells 14 provide a high degree of load spreading for a comparatively low depth of sub-grade material. The tread of each tyre cell 14 has a high degree of hoop strength, which enables each tyre cell to act as a cellular confinement for the fill material, thus facilitating the spread of the applied load over a larger base area. Further, the tensioned geotextile (or other barrier) material 10 acts in conjunction with the filled tyre cells 14 to spread this load over a large area, whilst at the same time reducing lateral displacement of the filled tyre cells. This performance is regardless of whether adjacent tyre cells are secured to each other or not. Advantageously, the fill material for in-filling each tyre cell 14 can comprise used track construction material. In this regard, the fill material can comprise one or more of: track ballast, sub-ballast and sub-grade materials; or more typically some mixture of these. In this regard, the pre-existing "spent" track material at a railway that is to be replaced or reconstructed can be re-used within the sub-grade, rather than having to be excavated and transported away for disposal. Track construction material becomes spent over time due to wear, weather, wash-away and decomposition, meaning that a railway line periodically needs to be replaced or reconstructed. Alternatively, the used/spent track construction material can be transported from another site to be used in the construction of a new track at e.g. a "greenfield" site.

In either case, by employing used/spent track construction material, this can reduce the amount of new track material required, thereby reducing construction costs. In addition, because the used/spent track construction material does not need to be taken away from site and disposed of, there is a corresponding reduction in both cost and time of transporting and disposing of spent material. In use, the existing (spent) track construction material can be excavated, and set to one side of the sub- grade under construction. Depending on the site requirements, the material may be further separated/divided/graded (e.g. using screening apparatus). The resultant materials can then be blended in various combinations as required, including with a proportion of new material, and then re-used as the fill material for in-filling of the tyre cells 14 and forming the layer 12.

The used/spent track construction material can be combined/blended in various proportions and/or with other (e.g. new) fill materials to alter the properties of the resultant sub-grade. Such other fill materials can include fine crushed rock or another stabilised fill material (e.g. crushed basalt or other hard rock, etc.). The fill material may even comprise a proportion of new track ballast. For some applications, a decision may be made not to employ used/spent track construction material (or some part/component thereof) as the fill material, in which case alternative fill materials can be used.

The fill material is typically deployed over the entire tyre layer 12, in-filling each tyre cell 14, and also filling up the spaces between tyre cells. The fill material is then usually compacted by a suitable machine (e.g. roller, excavator, tracked vehicle, etc.). As a result of such compaction, the tyre tread (sidewall) is placed under tension, as is the geotextile layer 10. Thus, the sub-grade is now ready to receive further materials thereon (e.g. capping, new track ballast), followed by the sleepers, tracks, etc.

In the sub-grade, at periodic intervals along the length of the sub-grade, a transversely extending strip drain 20 is typically positioned over the geotextile (or other barrier) material layer 10. The drain may extend through or under the tyre cell layer 12. Each such drain 20 can allow any water that accumulates in the sub-grade in use to be drained away from the sub-grade (i.e. at either side of thereof). Each drain 20 can feed into larger collection drains or drainage-ways located alongside the sub-grade, as described below. Each drain can also vent to air, or can be vented into a specific drainage system.

Railway sleepers 22 are arranged in relation to the in-filled layer of tyres. In the embodiment of Figure 1, the railway sleepers are embedded into the layer of track ballast 16. The railway sleepers 22 are typically of concrete reinforced with e.g. pre- or post-tensioned steel, or hardwood timber sleepers, or steel sleepers, etc.

Railway tracks 24 are then laid on the railway sleepers 22 in a known manner, such as by pinning. Shock-attenuation materials can also be employed between the rails 24 and the sleepers 22. The simple construction of Figure 1 can be employed when, for example, the underlying ground is one or more of: flat, stable, already compacted (e.g. preexisting rail track), etc. Whilst this construction can also be employed for freight and heavy rail applications, it can be suitably employed for lighter rail applications (e.g. commuter rail, or other non-freight light rail applications). In Figure 1, a rectangular bounded area of the ground is shown by dotted lines 26. This area indicates a volume of land/ground, the excavation of which can be avoided or eliminated during construction by employing the sub-grade as disclosed herein. This reduction in excavation can be attributed to the tyre cells 14 providing a high degree of load spreading for a comparatively low depth of material. In other words, the deeper (and often wider) excavation that is typically employed during railway construction can be reduced or eliminated.

Referring now to Figure 2, where like reference numbers denote like parts, an embodiment of the sub-grade is shown that is suitable for placement over existing ground 18 (e.g. that is not defined as part of an embankment). This sub-grade may, for example, be employed directly over a land base that is stable.

In this embodiment the sub-grade comprises a capping layer 30 that is arranged over the in-filled layer of tyre cells 12. The capping layer 30 can be formed from the same fill material as is employed for in-filling of the tyre cells 14 (e.g. used/spent track construction material). Additionally, or alternatively, the capping layer can be formed from or can comprise one or more of: new track ballast, fine crushed rock or another stabilised fill material (e.g. crushed hard rock, etc). The sub-grade shown in Figure 2 can also comprise, as is typical, a layer of new track ballast 16.

In contrast to Figure 1, in the arrangement shown in Figure 2 the layer of new track ballast 16 overlies a capping layer 30, which in turn caps the tyre cell layer 12. This construction is a more akin to a standard rail construction, such as is used particularly for freight, commuter and heavy rail applications, although again the construction is not limited to such applications.

Figure 2 again shows the volume of land/ground 26, the excavation of which can be avoided or eliminated due to tyre cells providing a high degree of load spreading for a comparatively low depth of material.

Referring now to Figure 3, where like reference numbers denote like parts, an embodiment of the sub-grade is shown for an embankment 40. Such an

embankment may be defined at an existing hill, rise, slope, etc. as part of the existing topography of the land on which the rail line is to be built. Alternatively, the embankment 40 can be constructed to support the rail line in an elevated manner above multiple different land types 18, including land that is stable and land that is unstable or very soft, such as swamp land, marshes, sandy sub-soil, unstable clay or silt layers, etc.

As part of preparing the ground of the embankment 40, the ground can be excavated a short distance down, sufficient for the geotextile and tyre cell layers 10 and 12 and, when employed, sufficient for the capping layer 30. Again, the depicted bounded volume of land/ground 26 is the embankment excavation which can be avoided or eliminated as a result the additional support provided by the composite structure and function of the geotextile layer 10 and tyre cell layer 12. In the arrangement of Figure 3, an additional geotextile (or other barrier material) layer 42 is arranged between the tyre cell layer 12 and the capping layer 30. The additional geotextile layer 42 can help to prevent the overlying track ballast layer 16, etc. from ingressing or becoming embedded in the fill material of each of the tyre cells 14. It can additionally or alternatively function as a waterproof barrier. The additional geotextile layer 42 can further stabilise the sub-base, such as by providing tensile resistance and contributing to the resistance of the sub-grade to lateral displacement, as well as helping to better integrate the capping layer 30, track ballast layer 16, etc. with the geotextile layer 10 and tyre cell layer 12 (i.e. so as to function as a composite engineered product).

In some applications, the additional geotextile layer 42 can comprise a layer of geo- grid. However, it should be noted that the contained movement imparted by the tyre cell layer 12 (whilst still maintaining load distribution) can be greater than the tensile ability of e.g. such a geo-grid layer, which may cause the latter to rupture in practice. To prevent rupture of e.g. geo-grids when placed above the tyre cell layer 12, engineering construction principles of the foundation can take this possibility into account during the design of the sub-grade. In some applications, woven or non-woven geofabric layers may instead be employed, as may other geotextile materials (e.g. that have a tensile capacity similar to the tyre cells). High-tensile waterproof membranes may also be employed.

Figure 3 also depicts excavated water drainage channels 44 running along either side of the sub-grade, each channel comprising a water-collection pipe arranged therein (e.g. an ag-pipe). These pipes 44 are aligned with opposite ends of the strip drain 20 and, at periodic intervals along the sub-grade, have water offtake pipe and air-vent portions 46.

In the arrangement of Figure 3, the embankment 40 is also excavated to accommodate the capping layer 30. Again, the capping layer 30 can be formed from used/spent track construction material and/or one or more of: new track ballast, fine crushed rock or another stabilised fill material (e.g. the same as or different to the tyre cell fill material).

The embankment construction shown in Figure 3 is a more common type of multipurpose foundation construction for a rail line, and is particularly suitable for freight and heavy rail applications, although is not limited to such applications.

As set forth above, instead of employing a geotextile, the barrier material 10 can take the form of a waterproof membrane. For example, the waterproof membrane can comprise a continuous polymeric sheet (e.g. HDPE or polypropylene membrane, etc.) or it may be formed from a tightly woven fibrous (e.g. hydrophilic, polymeric) sheet material, etc.

Non-limiting examples of a method and system for constructing a railway track foundation will now be provided.

Examples

Example 1 Prior to constructing a rail line employing one of the sub-grade constructions as set forth above, the following input parameters were considered as part of the design process:

- geotechnical information concerning the soil underlying the rail line, and the land surrounding the rail line, - slope of land surrounding the rail line,

- typical rainfall in the region of the rail line,

- weight (load) and regularity of trains using the proposed rail line, including rail traffic axle loads and operating speeds,

- track geometry, including: gauge, crossings and transitions profiles, required longevity of the rail line (design life), including the expected service life of the track, taking into account forecasted increase in axle loads and/or speeds, any and all cost limitations/constraints, - materials selection (see below), requirement for and timing of: materials analysis (e.g. of the cellular assembly - see below), experimental testing, field testing, etc., design decision procedure (e.g. decision tree or flow-chart),

- parameters for construction of the sub-grade.

To ascertain what materials to use in construction of the rail line, the following factors were considered: permeability of the sub-grade; cyclic shear strength properties of the sub-grade, including resilient modulus; settlement behaviour of the materials in-filled in the cellular units and of those composing the track

substructure, e.g. of the ballast, sub-ballast and subgrade; tensile stress of the rubber tyre selected for the cellular units.

The following "geocell investigations" of Indraratna et al. (2015) and Biabani et al. (2016) were also analysed:

• Biabani, M. M., Ngo, N. T. Indraratna, B., (2016). Performance evaluation of railway subballast stabilised with geocell based on pull-out testing. Geotextiles and Geomembranes, Elsevier 44 (4), 579-591.

• Biabani, M. M., Indraratna, B., and Ngo, N. T. (2016). Modelling of

geocell-reinforced subballast subjected to cyclic loading. Geotextiles and Geomembranes, Elsevier 44(4), 489-503. • Indraratna, B., Biabani, M. M. and Nimbalkar, S. (2015). Behaviour of geocell reinforced subballast subjected to cyclic loading in plane strain condition. J. of Geotech. & Geoenviron. Eng., ASCE 141(1), 04014081.

In these investigations it was shown that geocells were able to assist in arresting lateral movements as well as contributing to improved track performance under cyclic loading. Further, it was noted that an analytical model was developed that described the additional reinforcement effect that yields from the confinement provided by geocells - detailed in the Indraratna et al. (2015) study.

It was further noted in the course of the design process used when constructing a sub-grade using tyre cells, that some of the analytical procedures developed in the cited prior art studies (i.e. as related to geo-cell investigations) might also be applied to sub-grades comprising tyre cells.

However, it was also noted that the use of the tyre cells had additional advantages, as set forth above, including increased energy attenuation capacity (including shock and vibration attenuation), damping, etc. For instance, it was noted that, not only could the tyre cells provide a high degree of lateral confinement to the fill material (i.e. as computed based on the procedures highlighted in Indraratna et al. (2015) study related to geocells), but they could also contribute effectively to an attenuation of the energy transmitted from rail traffic and hence contribute to a reduction in the stresses applied at the level of the rail sub-grade.

In terms of the required design parameters for construction of the sub-grade, in particular the tyre cellular assembly, these included: the configuration/number of tyre cells 14 per metre, the fill materials selected, the depth of underlying materials replaced, any special conditions on ballast materials and other construction materials, the geometry of the system, the deformation profile under specified cyclic loads, the stress transfer distribution in a track sub-structure incorporating tyre cellular units, optimal ballast layer thickness, optimal sub-ballast layer thickness required in addition to the tyre cellular unit's assembly in relation to the loads considered. Example 2

Experiments were conducted to demonstrate these benefits. The following papers published by Indraratna et. al. (2017; 2018), the relevant contents of which are fully incorporated herein by reference, demonstrate the benefits: Indraratna, B., Sun, Q., Grant, J., "Behaviour of subballast reinforced with used tyre and potential application in rail tracks." Transportation

Geotechnics Elsevier. 12 (2017) 26-36

Indraratna, B., Sun, Q., Heitor, A. & Grant, J. (2018). "Performance of rubber tire-confined capping layer under cyclic loading for railroad conditions." Journal of Materials in Civil Engineering ASCE, 30(3):

06017021. (https://doi.org/10.1061/(ASCE)MT.1943-5533.0002199).

In the Experiments, to confirm the observations that a railway track foundation layer reinforced with rubber tyre cells could help to reduce the thickness of the granular layer (i.e. ballast), improve the track bearing capacity, and reduce the frequency of maintenance, pavement engineering testing principles were adopted. It is known in pavement engineering testing that the bearing capacity is closely linked to plate load tests. Thus, in the experiments, plate load tests were carried out on a single tyre cell filled with sub-ballast material and overlying a geotextile material layer that was subjected to a vertical load. This testing process was then modelled using the Finite Element software ABAQUS to study and quantify the interaction between the tyre cell and the granular medium. The experimental and numerical results revealed that the use of tyre cells can significantly increase the modulus and ultimate bearing capacity of the granular layer. The numerical process was further extended to a finite element track model to demonstrate the expected response of a ballasted railway track with and without tyre cell reinforcement.

The Experiments also set out to test three primary engineering benefits: (i) the confinement provided by the tyre cell would help to increase the stiffness of the contained aggregate, which then reduces vertical strains within the capping and ballast layers;

(ii) the tyre cell/barrier material composite would prevent any lateral movement of the capping material, which would then reduce the deformation of the overlying ballast; and

(iii) the tyre cell/barrier material composite would improve the flexural stiffness of the capping layer which distributes the traffic loads and reduces the maximum vertical stress on the subgrade. Example 3

In the experiments, the sub-ballast (crushed basalt) that was employed was sourced from a local quarry near Wollongong, NSW, Australia. The particle size distribution for the sub-ballast was within the range specified by the rail industry. An 80 kN woven geotextile of 2 mm thickness was placed under the rubber tyre cell for testing purposes. The geotextile was manufactured from durable, high-modulus polypropylene yarns woven into a robust, dimensionally stable geotextile.

Six plate load tests were carried out in a test box (i.e. 800 mm long, 600 mm wide and 400 mm high) to investigate the load transfer mechanisms between the infill and rubber tyre cell. A 200 mm diameter loading plate was employed. Tests with and without the tyre cell were conducted. The sample was prepared (in a dry condition) by placing a 50 mm thick layer of compacted clay-like sand into the test box to simulate the subgrade soil under the track. The sub-ballast layers for different tests were 150 mm and 350 mm thick respectively, and they were compacted to a density of 2100 kN/m ³ in two and four layers, respectively.

Compaction was carried out using a vibratory hammer. The tyre cell unit (when employed) was placed within the test box and then backfilled with gravel. The sub- ballast was present both inside and outside the tyre cell. In some tests, a woven geotextile was placed beneath the tyre cell to prevent dissimilar material layers from mixing and to allow each layer in the test to function as intended. The high tensile strength and low elongation properties of the geotextile were observed to add further reinforcement to the test sample.

With the 350 mm thick sub-ballast, the samples with and without geotextiles were tested and compared. Two 30 mm long strain gauges were attached to the interior wall of the tyre to measure the axial and circumferential strains. The surface of the tyre was brushed lightly with a cleaner and degreaser, and an industrial adhesive was applied before mounting the strain gauge. A stainless steel pressure cell was installed at the interface between the sub-ballast and subgrade to measure the vertical pressure transmitted to the subgrade. Another test was conducted to evaluate the bearing capacity of a unit cell of rubber tyre and gravel composite. In this test the loading plate was 560 mm and the sub- ballast was placed inside the tyre. Lateral spreading of the tyre was recorded by a linear voltage differential transformer (LVDT). After the sub-ballast was in place, the loading plate was placed on top of the test sample and subjected to a vertical load. The centre of the plate coincided with the centre of the rubber tyre. Controlled displacement tests were carried out at a rate of 0.2 mm/min. The vertical load, axial and lateral displacement, and the strain that developed inside the tyre were recorded every second during the test.

Results The results of six plate load tests on unreinforced and reinforced sub-ballast are shown in Figure 7. Fig. 7 (a) shows Test 1 : 150 mm thick sub-ballast without reinforcement; Fig. 7 (b) shows Test 2: 150 mm thick of sub-ballast with tyre cell (only) reinforcement; Fig. 7 (c) shows Test 3 : 350 mm thick of sub-ballast without reinforcement; Fig. 7 (d) shows Test 4: 350 mm thick of sub-ballast with tyre and geotextile reinforcement; Fig. 7 (e) shows Test 5 : 350 mm thick of sub-ballast with tyre cell (only) reinforcement; and Fig. 7 (f) shows Test 6: rubber tyre cell and sub- ballast composite unit cell test. In Figs. 7 (a and b) for a load of 600 kPa, the vertical stress at the centre of the tyre, and at the interface between the sub-ballast and subgrade was observed to be 470 kPa and 264 kPa for Test 1 without reinforcement and Test 2 with reinforcement, respectively. The inclusion of a rubber tyre cell and geotextile reinforcement were observed to reduce the stress transmitted to the subgrade, unlike the subgrade without reinforcement. Similarly, for a 600 kPa load, the stress decreased from 470 kPa to 82 kPa as the sub-ballast increased in thickness from 150 mm to 350 mm in Test 1 and Test 3, respectively (Figs. 7a and 7c). As predicted, the increment of sub-ballast thickness reduced the stress at the interface between the sub-ballast and subgrade. This implied that the rubber tyre cell and geotextile reinforcement effectively reduced the thickness of gravel required for real-life track design.

The apparent vertical stiffness of the sample, as represented by the initial and linear portion of the load-displacement curve (at 2% strain), was calculated from the test data in Figs. 7(c and d). In Test 3, without reinforcement, the apparent stiffness was approximately 51.9 kPa/mm, but in Test 4 with tyre cell and geotextile

reinforcement, the approximate stiffness was 78.4 kPa/mm. This result indicated that with reinforcement, Test 4 showed a 51% gain in stiffness compared to Test 3 without reinforcement. When Test 4 (Fig. 7d) in which geotextile was used, is compared to Test 5 (Fig. 4e), the results indicated that the geotextile provided further reinforcement effect. This was observed to be more beneficial in the case of a thicker layer of sub-ballast.

The strain gauges attached to the inner surface of the tyre cell confirmed that the strain in the tyre was very small; with circumferential tensile strains of 0.015%, 0.061% and 0.060% under vertical loads of 940, 1610 and 1600 kPa for Tests 2, 4, and 5 respectively. The axial compressive strains for the tyre cell were 0.0051%, 0.0085% and 0.0027%, respectively. It was noted that the tyre cell was able to confine the infill materials and maintain its original shape, unlike known geocell reinforcement which deforms substantially under testing.

Example 4 The commercially available finite element analysis software ABAQUS was used to model and analyse the above test results. It was noted that the tests were able to be validated using ABAQUS, with the numerical modelling also observed to add credibility to the simulation of a practical application of a tyre cell -reinforced railway track. Thereafter, a finite element 3D analysis was conducted to model the behaviour of a capping layer composed of in-filled rubber tyre cells.

In the FE 3D analysis, the steel rails were supported on reinforced concrete sleepers spaced at 0.60 m centres. The rail head was 0.075 m wide, the web was 0.018 m wide, and the base was 0.15 m wide. The concrete sleepers were embedded into a layer of coarse granular aggregate (ballast). For a standard gauge track, a sleeper of 2.50 m in width was bevelled to a maximum height of 0.20 m at the ends and 0.15 m at the centre. The ballast layer was 4 m wide at the base, 3 m wide at the crest, and 0.35 m high, with a slope of 1 : 1. The layer of sub-ballast confined by rubber tyre cells was 6 m wide at the base and was 0.25 m thick. A typical passenger car tyre was simulated as being 0.15 m wide, 0.56 m in diameter, and 0.01 m thick.

The analysis was based on a typical modern freight car used by Australian railways to transport heavy bulk materials such as coal, construction materials, and aggregates. Traditionally, these have an axle load of 25 tonnes, which corresponds to a static wheel load of 122.5 kN. A pseudo-static analysis was conducted in which the dynamic effects were considered by multiplying the static load by a dynamic amplification factor (DAF). A 25 t axle load train moving at 100 km/h speed was simulated.

In the FE 3D analysis, a plane strain slice of the cross section of half a ballasted railway substructure was modelled by a finite element mesh refined to observe the track settlement, lateral displacement of the ballast slope, and the subgrade stress of the foundation with or without rubber tyre cells reinforcing the sub-ballast layer. The ballast was modelled as a linearly elastic-perfectly plastic material with a Mohr-Coulomb failure criterion. The material parameters for the sub-ballast, subgrade, and rubber tyre cell were the same as the plate load test simulation referred to above. The tyre cells were modelled as perfect cylinders, and the contact between the tyre cells was considered smooth for simplicity. The sleepers and rails were modelled as a non-yielding linear elastic material, whose significantly greater stiffness compared to the ballast, foundation and rubber tyre cells, replicated a composite structure.

Based on track deflections, a set of three sleepers was noted to be sufficient to represent track response in the longitudinal direction. The transverse width of the plane strain model was assumed to be 1.9 m, and by exploiting centreline symmetry, only half the track was modelled. The FEM mesh with the sub-ballast reinforced with rubber tyres consisted of 22548 elements and 36761 nodes, while the model with unreinforced (no tyres) sub-ballast only consisted of 15050 elements and 24608 nodes. The ballasted track and foundation were meshed with hexahedral 8-noded elements with reduced integration points (C3D8R). Interaction between different layers of gravel was modelled with the same strategy used to simulate the unit cell.

In order to simulate railway field conditions, plane strain condition was applied to the model where the strain in the longitudinal direction was considered insignificant compared to lateral transverse strain. By taking advantage of symmetry, only half of the embankment and foundation was modelled. The vertical planes along the outer edge of the foundation were constrained from lateral displacement in the x- direction, and the same constraint was affixed to the x-y planes to prevent lateral displacement in the z-direction. The base of the model was restricted from any displacement as conventionally required for a FEM mesh (i.e. non-displacement bottom boundary). It was noted that, in railway engineering, design is mainly based on limiting the traffic and load-induced deviator stress in the subgrade to levels that protect the subgrade from progressive shear failure and excessive plastic deformation. Further, in order to maintain the track profile, ballast displacement (vertical and lateral) needs to be controlled within certain limits. This FE 3D modelling demonstrated that the subgrade deviator stress and ballast displacement were positively influenced by the use of rubber tyre cells as reinforcement, in contrast to the unreinforced case.

Results

The FE 3D modelling showed the effect that reinforcement offered by rubber tyre cells had on the deviator stress at the surface of the subgrade. As predicted, the highest deviator stress occurred near the sleeper end and decreased towards the central sleeper. This was consistent with field observations where the largest subgrade depressions usually occurred near the edges of the sleepers. With tyre reinforcement, a train running with the same axle load (i.e. 25 ton) and speed (i.e. 100 km/h), experienced a maximum deviator stress of 46.2 kPa, which was almost a 12% reduction compared to that of an unreinforced section. Intuitively, the tyre cell confining effect caused the tyre cells and gravel infill composite to act as a suffer, flexible "mattress" which allowed a reduced and more uniform stress to be transmitted to the subgrade. The area that the traffic and load-induced subgrade stress was distributed to was also wider than the area without tyre reinforcement.

The FE 3D modelling also showed an improved effect that the tyre cell

reinforcement had on the distribution of deviator stress with the depth of subgrade. The deviator stress in the subgrade decreased with depth. Lateral deformation along the slope of the embankment was also observed to decrease considerably due to tyre cell reinforcement. The contours of lateral displacement for the unreinforced and reinforced layers of sub-ballast, where the largest lateral movement of sub-ballast developed beneath the edge of the sleeper, were also compared. These comparisons showed that the maximum lateral displacement for unreinforced sub-ballast (0.095 m) was considerably more than that of reinforced sub-ballast (0.012 m) under the same load.

Experimental Conclusions

The plate loading tests showed that the inclusion of a rubber tyre cell and geotextile reinforcement helped to reduce the stress transmitted to the subgrade, Further, with an increased thickness of sub-ballast, the reinforcement also reduced stress at the interface between the sub-ballast and subgrade. The inclusion of rubber tyre cells and geotextile reinforcement effectively reduced the design thickness of ballast, while eliminating the need for having a capping layer composed of natural rock aggregates. The experimental results showed that the tyre cell reinforcement could provide more than 50% gain in stiffness of a sub-ballast (capping) layer. Due to its cylindrical structure, the rubber tyre cells can confine the infill material, minimise lateral displacement, and provide a rigid capping with a bearing capacity of up to 6500 kPa with substantial reduction in lateral strains. The measurements confirmed that the strain experienced by the rubber tyre cell was relatively small. An additional confining stress was generated by the hoop stress of the tyre, which was as much as 500 kPa, at the maximum applied load of 6500 kPa.

The FEM study of plate loading test verified that the use of tyres could increase the elastic modulus of reinforced gravel by confinement. The maximum displacement and maximum tension inside the tyre were close to its bottom, and the highest interface shear stresses also developed near the bottom and close to the edge of the tyre. The load-deformation relationships for the reinforced and unreinforced cases agreed reasonably well with the model in view of the experimental and FEM results. The 3D Finite Element track model indicated that the highest deviator stress occurred near the edge of the sleeper, while the confining effect caused the tyre cell and gravel infill composite to act as a stiff but flexible "mattress" that reduced the stress transmitted to the subgrade. It was noted that this would have significant implications on the stability of a soft subgrade that can prematurely yield unless the transmitted stress is reduced by a reinforced capping layer.

As a result of these investigations, each tyre cell 14 was noted to provide for vibratory/shock-attenuation, and a load-spreading cellular confinement. It was further noted that the use of a composite engineered product comprising a geotextile (or other barrier) material layer 10 and one or more tyre cell layers 12 would be particularly advantageous: in rail lines where dynamic forces and/or shock loads tend to be frequent and/or severe; in the case of very soft sub-grade materials, where other ground

improvement methods can be difficult to construct and/or less cost effective; in locations where natural aggregate material for the capping layer is scarce, because the tyre cells 14 can lessen the volume of capping material required and can make use of materials which would otherwise be considered marginal.

Variations and modifications may be made to the embodiments previously described without departing from the spirit or ambit of the disclosure.

For example, the foundation may be configured to support a slab track construction, such as may be employed for intermodal transport or light rail construction. In slab track applications, the ballast/capping layer may be replaced with a concrete slab construction. For example, the concrete slab may be placed or formed directly over the tyre cell layer. The concrete slab would typically be reinforced (e.g. by steel, etc. reinforcing rods/bars), and the rails would be directly embedded and connected into the concrete slab. Additionally, the geotextile layer may be arranged to extend beyond the tyre cell layer so as to lie under and between the concrete slab and the ground.

It should be noted, however, that the use of tyre cells with slab tracks would need to take into account that a slab track is highly rigid and would therefore tend to distribute an overlying load across the sub-grade. This may negate the efficacy of the tyre cells, in that the effective increased elasticity of the sub-grade may contribute to some failure modes of the slab. Engineering construction principles would therefore need to take this into account. In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments.