WO2013004242A1

WO2013004242A1 - A substructure system of a railway track

Info

Publication number: WO2013004242A1
Application number: PCT/DK2012/050244
Authority: WO
Inventors: Hans Henrik Gylov; Kevin Bo GATZWILLER
Original assignee: Rockwool International A/S
Priority date: 2011-07-06
Filing date: 2012-07-03
Publication date: 2013-01-10
Also published as: EP2729616A1

Abstract

The substructure system comprises a resilient bottom plate in the form of a mineral fibre plate having a Young's modulus of 0.5 MPa to 1 MPa. A high bending strength top plate is positioned on top of the bottom plate with one major surface of the respective plates abutting each other to support the ballast layer for distribution of local pressures from the ballast layer and protection of the resilient bottom plate, the top plate having a Young's modulus of at least at least 2.6 MPa. Further, a ballasted substructure, use of such a substructure system, and methods of manufacturing and positioning of such a substructure system are disclosed.

Description

A SUBSTRUCTURE SYSTEM OF A RAILWAY TRACK

The present invention relates in a first aspect to a substructure system for being positioned as a subballast layer below a ballast layer of a railway track according to the introductory part of claim 1. In its other aspects the invention relates to a ballasted substructure positioned below a railway comprising a substructure system according to the first as- pect, use of a substructure system according to the first aspect for preventing subgrade overstressing, mitigating dynamic forces and providing for effective load distribution in a substructure beneath a railway, a method of manufacturing a substructure system for being positioned as a subballast layer below a ballast layer of a railway track, and a method for positioning a substructure system according to the first aspect and/or a substructure system as provided by means of the latter method as a subballast layer below a ballast layer of a railway track.

A ballast layer is typically provided as a top portion of the substructure beneath railway tracks, sleepers of the track system having rails attached to them by means of a suitable attachment system. The track system is typically positioned on, i.e. partly embedded in, the upper parts of the ballast layer. The ballast layer typically consists of crushed granular material, specifically crushed hard stones and rocks. The ballast layer performs a number of important functions of which one is to resist, damp and mitigate forces exerted on the tracks by passing train traffic. Often, it is desirable to further position a subballast layer between the ballast layer and the subgrade, e.g. in order to ensure suitable separation of the ballast layer from the subgrade soils so that they are not mixed with each other over time.

The most commonly applied subballast layer consists of broadly- graded naturally occurring or processed sand-gravel mixtures or crushed natural aggregates. The inherent characteristics of this material, such as the durability, make the subballast fulfil the desired functions in many instances, including separation of ballast layer and subgrade. The soils of the subgrade form the platform on which the track structure, including ballast and potentially subballast, is constructed, the subgrade's main function being to provide a suitable foundation of the track structure above it. In use, the traffic- induced stresses stemming from passing trains typically extend downwards as much as five meters below the sleepers, i.e. considerably below the usual depth of ballast and subballast. Hence, the subgrade is an important foundation component, the type and properties of the subgrade soils having significant influence on track performance and maintenance requirements. In the usual case the subgrade soils have suitable properties to allow for the subgrade to form a major component of the superstructure support resiliency and hence contribute substantially to elastic deflection of the rail under wheel loading from passing train traffic.

However, in areas with dramatic differences in the climate such as long periods of rain followed by long periods of drought, e.g. in areas of Southern Europe or Northern Africa, clayey and like climate-sensitive soils will tend to crack and compact under the dynamic stresses and forces from passing traffic. When railroads are positioned in such areas there is thus an increased risk of the soil cracking along and beneath the track structure when climate variations over time affect the subgrade soils. This may cause soil, and thus the ballast layer and railroad tracks supported by the subgrade, to sink or otherwise being deformed, which again may cause the tracks to be deformed or destroyed.

Generally speaking, for the subgrade to be able to continuously serve as a satisfactory foundation for the track structure, the following phenomena should be avoided :

• subgrade attrition,

• excessive progressive settlement stemming from repeated traffic loading,

· consolidation settlement and massive shear failure under the combined weights of train, track structure and soils or earth,

• progressive shear failure from repeated wheel loading, and

• significant volume change (swelling and shrinking) from repeated wheel loading. These phenomena may lead to subgrade failure and consequentially track de-alignment, excessive wear on the tracks, greatly increased maintenance costs, and also, in extreme cases, trains being derailed from the tracks.

Simply positioning the ballast layer to extend farther into the ground typically would not solve this problem to a sufficient degree. Even large downward extensions of the ballast layer would not successfully alleviate the problems related to attrition of the subgrade soils since high contact stresses from the ballast particles will to a large part not be effectively load-distributed before reaching the subgrade soils. As quality ballast materials are difficult to acquire, a ballast layer with a sufficient downward extension to satisfactorily solve the problem would be so expensive that it would not be realistic to deploy.

Instead, today it is a normal procedure to replace the subgrade soil on site with fill of more suitable characteristics. However, even when such fill can be provided locally, this is often an extremely high-cost procedure.

Particularly with the purpose of vibration damping, i.e. not with the specific object of protecting the subgrade, it has in the prior art been suggested to incorporate layers of elastic material, especially made from rubber, PUR-foams and cork, respectively, as well as combinations thereof in substructures of train lines. It has also been suggested to apply plates of mineral wool in order to damp vibrations (or noise) caused by such heavy traffic on the tracks. Vibration damping is generally carried out to protect the surroundings from vibrations (or noise) generated when for example a train passes by on the rails. Vibrations spread to the surroundings through the rails, rail fasteners, sleepers, ballast, and ground surface layers. Vibrations spreading to the surroundings can cause damages to buildings and are often experienced as unpleasant to people living in surrounding buildings. One example of such a vibration damping layer is disclosed in applicant's EP patent publication no. 1 444 400 Bl, which suggests applying a dual density mineral wool plate for vibration damping. To minimize stiffness variations of the plate over its life-span, the plate has undergone an artificial pre-aging treatment on its upper and lower surfaces before being positioned in the ground.

On this background it is the object of the present invention to avoid or minimize the risk of failure of the subgrade beneath railway tracks in geographic locations experiencing dramatic differences in the climate combined with unfavourable or climate-sensitive soil conditions.

In the first aspect of the invention this object is achieved by means of a substructure system as described above, which is characterized by the characterizing features of claim 1.

This substructure system generally provides not only an effective separation of ballast layer and subgrade, but also adds a well- defined, creep-free, climate insensitive and constant degree of resilience and ductility to the track structure whereby subgrade failures may be avoided even in geographic locations experiencing dramatic differences in the climate combined with unfavourable or climate-sensitive soil conditions.

The inventors of the present invention have discovered that a track structure comprising a substructure system according to the invention entails much less movement or deflection in the subgrade due to the combination of a high bending strength top plate and an elastic or resilient bottom plate. The result is that the mechanical energy flow generated in the contact between wheels and rail from passing trains is minimized when reaching the subgrade, thus minimizing the dynamic loading of the subgrade soils. The decreased stresses in the subgrade soils lead to a reduced risk of subgrade failure. Although this may also lead to increased deflections in the track structure itself, the inventors have discovered that this rarely or never poses a practical problem.

Furthermore, due to the elastic bottom plate the mechanical energy exerted on the subgrade and the ballast can be virtually eliminated, the substructure system at lower frequencies and the rail pad at higher frequencies absorbing a major part of the energy exerted on the rail structure. Also, the mechanical pressure on at-grade level is greatly reduced. The mechanical pressure can indeed be almost halved, which has a very positive influence on the bearing capacity. As mentioned, the resilient bottom plate is protected by means of the top plate. In practice, local peak contact pressures under the track's sleepers may easily reach contact pressure values that are at least three to four times higher than the average contact pressure. The inventors have discovered that a top layer realized according to the invention provides an extremely fine distribution of these local contact pressures so that they can easily be handled by the resilient bottom layer such as to be mostly absorbed before reaching the subgrade. This is mainly due to the high Young's module of the top plate.

The substructure system according to the invention thus prevents subgrade overstressing, mitigates dynamic forces and provides for effective load distribution. Vertical, transverse and longitudinal forces transmitted through rails, fastening system and sleepers dissipate before reaching the subgrade so that the subgrade is protected from excessive stresses, and mechanical shocks are attenuated.

The substructure system may be easily installed, provides efficient drainage, can be recycled, has a low pollution effect and can be manufactured at competitive price. It has further been found that the effect of the substructure system is only slightly or not at all influenced by the temperature of the surrounding environment, which means that the system works effectively under a wide range of temperatures. Also, it is highly durable.

In terms of this specification it is generally noted that if the top plate comprises one or more anisotropic materials, the relevant Young's modulus (often denoted "E") is that as measured in a direction or orientation perpendicular to the major surfaces of the top plate, thus providing for a high bending strength. Similarly, if the bottom plate comprises one or more anisotropic materials, the relevant Young's modulus is that as measured in a direction or orientation perpendicular to the major sur- faces of the bottom plate.

Claims 2 to 8 define preferred embodiments of the first aspect of the invention.

In one preferred embodiment the bottom plate has an average density of 190-260 kg/m³, preferably 210-250 kg/m³, and/or a thickness of 20-100 mm, preferably 25-65 mm, most preferred 28-32 mm. Such associated density and thickness values may provide suitable bottom plate properties, e.g. strength and elasticity.

In another preferred embodiment the bottom plate has been subjected to a pre-aging treatment on its opposite major surfaces lowering the static and/or dynamic stiffness of the bottom plate. Similarly if the bottom plate is comprised of two or more plate-shaped, preferably substantially similar layers, the layers before being put together to make up the bottom plate having preferably been subjected to a pre-aging treatment on opposite major surfaces of each layer, lowering the static and/or dynamic stiffness of each layer. Alternatively, the layers could be pre-treated after having been joined, i.e. on respective top and bottom surfaces of the plate only. In this regard it has been realized that there is a risk that the load from passing trains could cause an ageing of the bot- torn plate over time. Such ageing is characterized by the change in static and dynamic stiffness of the anti-vibration plate of the vibration damping system. For instance, the static and the dynamic stiffness of the anti- vibration plate may decrease significantly during the first 5 to 10 years of use. By subjecting the bottom plate to a pre-aging step as described, fluctuations in stiffness properties can largely be avoided.

In another preferred embodiment the top plate is in the form of a mineral fibre plate, optionally a carded stone wool plate, comprising mineral fibres and a binder and having an average density of 320-400 kg/m³, preferably 350-380 kg/m³, and/or a thickness of 10-50 mm, preferably 18-30 mm, most preferred 22-25 mm. For a mineral wool plate these values will provide a top plate with suitable characteristics, which may further be manufactured with ordinary production equipment. The skilled person will realize that a top plate with a Young's modulus of a desired value or within a desired interval can be manufactured using suitable production parameters, such as amount and type of mineral fibres and binder. A carded stone wool top plate may be made by subjecting a collected web of stone wool fibres to a disentanglement process, the compacted stone wool fibres being opened up, the stone wool fibres and binder being more evenly distributed in the plate produced. This in- creased homogeneity in the element results generally in an increased level of mechanical strength and a higher Young's modulus relative to elements made by methods which use other mixing methods.

As used herein, the term "collected web of mineral wool fibres" is intended to include any mineral wool fibres that have been collected together on a surface, i.e. they are no longer entrained in air, e.g. granulate, tufts or recycled web waste.

The binder used can generally be any material suitable for bonding mineral wool fibres in a matrix. In particular, it is preferably a mate- rial that dries, hardens, or becomes cured under defined conditions. Generally, such processes (generally referred to as "curing") are irreversible and result in a cohesive composite material.

Inorganic as well as organic binders can be employed. Organic binders are preferred. Further, dry binders as well as wet binders can be used. Specific examples of binder materials include, but are not limited to, phenol formaldehyde binder, urea formaldehyde binder, phenol urea formaldehyde binder, condensation resins, acrylates and other latex compositions, epoxy polymers, sodium silicate, hot melts of polyure- thane, polyethylene, polypropylene and polytetrafluoroethylene poly- mers etc.

One suitable binder is a binder composition comprising a sugar component, and a reaction product of a polycarboxylic acid component and an alkanolamine, the amount of sugar component being within the range of 30 to 80 percent by weight, based on the total weight (dry mat- ter) of the binder components.

In an embodiment a dry binder could be used. Any suitable dry binder could be used, but is is preferred to use a phenol formaldehyde binder, as this type of binder is easily available and has proved efficient.

The amount of binder employed for the bottom plate is between 3 to 5 wt%, preferably 3.8 wt%. The amount of binder employed for the top plate is between 4 to 6 wt%, preferably 5 wt%.

The mineral fibres (also known as man-made vitreous fibres or MMVF) used according to the present invention could be any mineral fibres, including glass fibres or stone fibres, but preferably stone fibres are used. Stone wool fibres generally have a content of iron at least 3% and alkaline earth metals (Calcium oxide and magnesium oxide) from 10 to 40%, along with the other usual oxide constituents of mineral wool. These are silica, alumina, alkali metals (sodium oxide and potassium ox- ide), which are usually present in low amounts, and can also include ti- tania, phosphorus and other minor oxides. Fibre diameter is often in the range of 2 to 10 microns, in particular 3 to 6 microns.

Referring also to the above discussion it is preferred that the top layer has essentially not been subjected to any pre-aging treatment and/or any other treatment having the purpose of reducing stiffness. The high bending strength top plate has such characteristics that the stiffness properties will only be marginally affected or aged by forces caused by passing trains or other in situ loads. In the intended use any pre-aging or like treatment is thus essentially unnecessary and un- wanted because it can be expected to lower Young's modulus and thus negatively influence the functional properties.

The top layer could be manufactured from other suitable materials, such as steel or high-strength plastics materials. In such cases Young's modulus of the material will typically be chosen at high values.

Advantages similar to those described above in connection with the first aspect of the invention are achieved in the following further aspects of the invention.

In one further aspect the invention provides a ballasted substructure positioned below a railway, as defined in claim 9.

Another aspect of the invention, as defined in claim 10, provides for use of a substructure system according to the first aspect of the invention for forming a subballast layer positioned between a ballast layer and subgrade soils.

Claim 11 provides yet another aspect of the invention in the form of a method of manufacturing a substructure system according to the first aspect of the invention in which the bottom plate has been subjected to a pre-aging step as described above. It is preferred that this pre-aging step comprises subjecting an area of the opposite major surfaces of the bottom plate to a compression pressure in the interval from 50 to 250 kN/m², preferably from 80 to 200 and more preferably from 100 to 150 kN/m², whereby the static and/or dynamic stiffness of the plate measured according to the method defined in Deutsche Bahn-Norm BN 918 071-1 is reduced compared to the static and/or dynamic stiff- ness prior to the compression treatment.

Yet another aspect of the invention, as defined in claim 13, provides a method for positioning a substructure system according to the first aspect of the invention as a subballast layer below a ballast layer of a railway track.

The invention will in the following be explained in more detail with reference to an exemplary embodiment as shown in the schematic drawing's single figure, Fig. 1, as well as variations thereof.

Fig. 1 shows an embodiment of a substructure system according to the first aspect of the invention as positioned to form a subballast layer 1 beneath a railway track comprising two rails 2. The two rails 2 are positioned on top of and by means of suitable rail fasteners attached to sleepers, of which one sleeper 3 is shown in Fig. 1. The sleeper 3 is embedded in a ballast layer 4. A ballasted substructure positioned below the railway track thus comprises the ballast layer 4, surrounding soils of the subgrade 6 comprising the natural ground soils and/or embankment fill, and the subballast layer 1. The entire track system consisting of the subballast layer 1, rails 2 and sleepers, including sleeper 3, is positioned in a depression or furrow 5, which has been dug in the surrounding sub- grade 6 to extend in the direction of the railroad track.

The ballast layer 4 comprises crushed granular material, specifically crushed hard stones and rocks, and provides for separation of the ballast layer from the subgrade 6. The subgrade 6 forms a platform or foundation on which the track system is constructed. The soils of the subgrade 6 may be of a clayey type, which is sensitive to climate changes, but can in principle be of any type, including comprising fill material.

The subballast layer 1 forms a lower support of the above ballast layer 4 and comprises a parallelepiped, resilient bottom plate 7 and a parallelepiped, bending-stiff top plate 8. The bottom plate 7 has an up- per major surface 7a and an opposite, lower major surface 7b as well as four smaller side surfaces. Similarly, the top plate 8 has upper and lower major surfaces 8a, 8b and four smaller side surfaces. The top plate 8 is positioned on top of the bottom plate 7 so that the surfaces 7a, 8b abut each other. Alternatively, a third material may be positioned between the top and bottom plates 8, 7, such as a further plate, e.g. manufactured from mineral wool. Also, both top and bottom plates 8, 7 may each comprise more than one material layer or more than one plate, such as for example two or three layers each.

The bottom plate 7 is in the form of a mineral fibre plate comprising mineral fibres and a binder and having a Young's modulus of 0.84 MPa. This provides suitable properties of the bottom plate 7 for it to fulfil its purpose according to the invention, i.e. to absorb dynamic and static forces from trains or like vehicles passing on the railway track to protect the surrounding subgrade 6. The bottom plate 7 further has an average density of about 230 kg/m³, and a thickness of about 30 mm. Hereby, a bottom plate 7 with suitable strength properties is provided. The bottom plate 7 should not be too weak since it will need to be able to effectively absorb the forces from the upper parts of the track system without being deteriorated so much as to lose its properties related to its purpose.

Before being positioned in the ground, more specifically after the bottom plate 7 has been manufactured at a factory, on the factory the bottom plate 7 has been subjected to a pre-aging treatment on its opposite major surfaces 7a, 7b, lowering the static and dynamic stiffness of the bottom plate 7. In the alternative, in which the bottom plate 7 is comprised of two or more plate-shaped, preferably substantially similar layers, the layers have before being put together to make up the bottom plate 7 been subjected to a pre-aging treatment on opposite major surfaces of each layer, similarly lowering the static and dynamic stiffness of each layer.

A number of suitable methods may be applied for subjecting the opposite major surfaces 7a, 7b of the bottom plate 7 to a compression treatment. A simple and economical method involves subjecting the bottom plate 7 to a compression treatment by passing it through one or more pairs of rollers, a nip being provided between the one or more rollers making contact with the major surfaces 7a, 7b. The one or more pairs of rollers could have same or different diameters, but it is preferred that on one side the roller diameter is relatively large and on the other relatively small, e.g. diameters substantially larger and substantially smaller, respectively, than the thickness of the bottom plate 7, thereby exerting different pressures over the surfaces of the bottom plate 7.

The pre-aging step generally preferably comprises subjecting an area of the opposite major surfaces 7a, 7b of the bottom plate 7 to a compression pressure in the interval from 50 to 250 kN/m², preferably from 80 to 200 and more preferably from 100 to 150 kN/m², whereby the static and/or dynamic stiffness of the plate measured according to the method defined in Deutsche Bahn-Norm BN 918 071-1 is reduced compared to the static and/or dynamic stiffness prior to the compression treatment. Hereby, fluctuations in stiffness can typically be avoided to a suitable degree.

The top plate 8 is also in the form of a mineral fibre plate, more specifically a carded stone wool plate, comprising mineral fibres and a binder, preferably a cured binder, and having a Young's modulus of about 10 MPa in a direction perpendicular to its major surfaces 8a, 8b, providing a suitably high bending strength for it to fulfil its purpose according to the invention, i.e. to support the ballast layer for distribution of local pressures from the ballast layer and protection of the resilient bottom plate. The top plate 8 further has an average density of about 360 kg/m³, and a thickness of about 23 mm. Alternatively, the top plate can be manufactured from steel, another metal or metal alloy, a stiff and strong plastics material, such as a reinforced plastics material, or rubber with a suitably high Young's modulus. As examples, for a steel top plate a suitable Young's modulus would be 150 to 250 MPa, typically about 200 MPa, for a reinforced plastic top plate suitably 40-45 MPa, and for a rubber top plate suitably 8 to 12 MPa, typically about 10 MPa.

It is important to note that the top plate 8 has not been subjected to any pre-aging treatment and/or any other treatment having as its specific purpose to reduce stiffness. As already mentioned, this is not necessary with regard to maintaining proper stiffness properties during lifetime of the plate and would only lower the overall stiffness of the top plate 8, which would work against its purpose.

Generally, the composition and manufacture of the plates 7, 8 should be adapted to achieve the properties and characteristics as mentioned above. A person skilled in the art will generally understand that the plates 7, 8 can be composed in a large number of different ways to achieve the desired characteristics. Typically, each of the plates 7, 8 comprises at least 20 %, preferably at least 50 % and more preferably at least 80 % or 90 % by weight of one or more types of mineral fibres, e.g. rock, slag, glass and similar vitreous materials, as well as a binder in a suitable amount. If each plate consists solely or primarily of mineral wool, the amount of mineral fibres is typically at least 90 %, the binder amount typically being 1 to 10 %, more typically 4-6 % by weight. Also, a small amount of oil has typically been added. In order to provide a strong and resistant surface of the plates, the plates may further be covered on one or more of its surfaces with a layer of surfactant-free geo- textile.

The bottom plate 7 can for example comprise one or more anti- vibration plates as marketed by the applicant under the tradename RockBallast™, which is a dual density plate, i.e. having two layers of mineral wool of different density positioned on top of each other with major surfaces facing each other. Alternatively, the bottom plate could in princi ple be made up of one or more of the embodiments of multi- layered anti-vibration plates as described in above-mentioned EP 1 444 400 Bl.

Similarly, the top plate 8 can comprise one or more of the plates as marketed by the applicant under the tradename RockGuard™, which is a high density stone wool plate of high bending stiffness typically used as a force-absorbing and protecting layer between a railway track structure and a non-soil, harder substructure, such as between a railroad track structure and a load-bearing structure of a bridge. However, a standard RockGuard™ plate has been subjected to a pre-aging treatment similar to the treatments as described above. When used as a top plate in a substructure system according to the present invention, the plate should preferably be provided in a form, which has not been pre-aged.

The entire track system including the subballast layer 1 may be positioned in the subgrade 6 as shown in Fig. 1 according to the follow- ing method. Generally, as will be appreciated by a person skilled in the art of mounting rail tracks, the positioning and interrelations, such as mounting, embedding and fastening, between components such as rails, sleepers, ballast, subballast and subgrade are carried out in a conventional manner.

Before positioning of the track system in the subgrade 6, typically geotechnical investigations and considerations relating to the climate in which the railway track is to be placed have been carried out, leading to the conclusion that subgrade attrition or other subgrade failure modes might form a risk if conventional track systems were to be applied.

A depression or furrow 5 of suitable dimensions is buried in the ground at the area on which the track system is to be positioned. The soil surfaces of the subgrade 6 at the sides of the furrow 5 may then be prepared, e.g. by levelling the ground in the furrow 5. The subgrade 6 may also be stabilised, e.g. by covering the inner sides of the furrow 5 with a material selected from the group consisting of water pervious foil, granulates of rubber, gravel or mixtures thereof. In principle, natural ground soils can furthermore be replaced to some degree with embankment fill.

Then the subballast layer 1 is positioned, typically by first placing the bottom plate 7 with its bottom surface 7b abutting the bottom of the furrow 5, the top plate 8 then being positioned on top of the bottom plate 7 with its bottom surface 8a abutting the top surface 7a of the bottom plate 7. Alternatively, the top plate 8 and bottom plate 7 may be suitably joined with each other beforehand, e.g. at the factory or on site, respective major surfaces 8b, 7a abutting each other from the outset. The ballast layer 4 is then positioned, typically poured, from the opening of the furrow 5 onto the top surface 8a of the top plate 8 to substantially fill the furrow 5 substantially to the top of the surrounding elevated ground surface level 9. Finally, sleepers, including sleeper 3, are positioned on and embedded into a top portion of the ballast layer 4, and the rails 2 are attached to the sleepers.

In use of such a track system, the often relatively high local pressures (up to three to four times higher than average pressure), typically exerted by train traffic, pass through the rails 2, fastening system and sleeper 3 into the ballast layer 4. The forces are distributed downwards in the ballast layer 4 in inhomogeneous load lines between mutual contact points of particles of its granular material, the load-bearing oc- curring along more or less arbitrarily extending chains of particle-to- particle contact forces. This results in a non-uniform load distribution pattern towards the subgrade 6. The non-uniformity of the load distribution entails that a major part of the forces or pressures are often exerted locally on relatively small areas of the upper surface 8a of the top plate 8, providing to a large extend non-uniformly distributed relatively high pressures on the top plate. Due to the high Young's modulus of the top plate 8, the resulting high bending stiffness causes distribution of these locally exerted forces to extend substantially over the entire top plate 8, the force distribution being substantially equal over the entire bottom major surface 8b when the forces reach the upper major surface 7a of the bottom plate 7. The forces are then distributed substantially equally into the resilient bottom plate 7, which due to the superior force distribution caused by the top plate 8 is able to absorb the forces to such a degree that essentially only relatively small, evenly distributed forces reach the subgrade 6. The end result is that overstressing of the sensitive soils of the subgrade 6 is prevented. The skilled person will appreciate that the latter considerations also apply generally to other embodiments of the substructure system according to the invention as applied in any suitable track system.

The substructure system, ballasted substructure, use of the substructure system as well as the methods for manufacture and positioning of the substructure system as explained by means of examples in the above can be varied further within the scope of the appended claims. For example, several separated ballast layers or ballast layers of other suitable compositions may be applied, and transitions or connections between rails and ballast other than sleepers can be imagined. The top and bottom plates are not necessarily parallelepipeds (although preferred), but may rather take any other suitable shapes as long as the purpose is fulfilled.

Finally, under suitable conditions the substructure system may in principle be used in other applications than railway track systems where similar or like problems may arise, for example as a substructure for other types of track-based transport systems or traffic lines, such as trolley systems, rail bus or rail car systems, tramlines and the like.

Claims

C L A I M S

1. A substructure system for being positioned as a subballast layer below a ballast layer of a railway track,

characterized by comprising

a resilient bottom plate having two opposite, major surfaces and being adapted to absorb dynamic and static forces from trains or like vehicles passing on the railway track, the bottom plate being in the form of a mineral fibre plate comprising mineral fibres and a binder and having a Young's modulus of 0.5 MPa to 1 MPa, preferably 0.6 MPa to 0.9 MPa, most preferred 0.62 MPa to 0.84 MPa, and

a high bending strength top plate having two opposite, major surfaces and being adapted to be positioned on top of the bottom plate with one major surface of the respective plates abutting each other to support the ballast layer for distribution of local pressures from the bal- last layer and protection of the resilient bottom plate, the top plate having a Young's modulus of at least 2.6 MPa, preferably at least 3 MPa, more preferred at least 4 MPa, more preferred at least 5 MPa, more preferred at least 7 MPa, most preferred at least 10 MPa.

2. A substructure system according to claim 1, wherein the bot- torn plate has an average density of 190-260 kg/m³, preferably 210-250 kg/m³, and/or a thickness of 20-100 mm, preferably 25-65 m m, most preferred 28-32 mm.

3. A substructure system according to claim 1 or 2, wherein the bottom plate has been subjected to a pre-aging treatment on its oppo- site major surfaces lowering the static and/or dynamic stiffness of the bottom plate.

4. A substructure system according to any one of the previous claims, wherein the bottom plate is comprised of two or more plate- shaped, preferably substantially similar layers, the layers before being put together to make up the bottom plate preferably having been subjected to a pre-aging treatment on opposite major surfaces of each layer, lowering the static and/or dynamic stiffness of each layer.

5. A substructure system according to any one of the previous claims, wherein the top plate is in the form of a mineral fibre plate com- prising mineral fibres and a binder, optionally a carded stone wool plate, and preferably has an average density of 320-400 kg/m³, preferably 350-380 kg/m³, and/or a thickness of 10-50 mm, preferably 18-30 mm, most preferred 22-25 mm, and/or a Young's modulus of less than 15 MPa, more preferred less than 12 MPa, most preferred less than 10 MPa.

6. A substructure system according to claim 4, wherein the top layer has essentially not been subjected to pre-aging treatment and/or any other treatment having the purpose of reducing its stiffness.

7. A substructure system according to any one of the previous claims positioned as a subballast layer below a ballast layer of a railway track.

8. A substructure system according to any one of the previous claims, wherein the top plate is comprised of two or more plate-shaped, preferably substantially similar layers.

9. A ballasted substructure positioned below a railway, comprising a ballast layer and surrounding subgrade soils, such as the natural ground soils and embankment fill, and a substructure system according to any one of the previous claims forming a subballast layer positioned between the ballast layer and the subgrade soils such as to support the ballast layer, the top plate being positioned on top of the bottom plate with one major surface of the respective plates abutting each other for distribution of local pressures from the ballast layer and protection of the resilient bottom plate.

10. Use of a substructure system according to any one of claims 1 to 8 for preventing subgrade overstressing, mitigating dynamic forces and providing for effective load distribution in a substructure beneath a railway, the substructure system forming a subballast layer positioned between a ballast layer and subgrade soils such as to support the ballast layer, distribute local pressures from the ballast layer and protect the re- silient bottom plate, the top plate being positioned on top of the bottom plate with one major surface of the respective plates abutting each other.

11. A method of manufacturing a substructure system for being positioned as a subballast layer below a ballast layer of a railway track, comprising the steps of:

a) providing a substructure system according to any one of claims 1 to 8,

b) pre-aging the bottom plate of the substructure system by subjecting its opposite major surfaces to a compression treatment in one or more steps, which compression treatment is sufficient to reduce the static and/or the dynamic stiffness of the plate by at least 10 %, preferably at least 15 %, more preferably at least 20 % compared to the static and/or dynamic stiffness prior to the compression treatment.

12. A method according to claim 11, wherein the pre-aging step b) comprises subjecting an area of the opposite major surfaces of the bottom plate to a compression pressure in the interval from 50 to 250 kN/m², preferably from 80 to 200 and more preferably from 100 to 150 kN/m², whereby the static and/or dynamic stiffness of the plate meas- ured according to the method defined in Deutsche Bahn-Norm BN 918 071-1 is reduced compared to the static and/or dynamic stiffness prior to the compression treatment.

13. A method for positioning a substructure system according to any one of claims 1 to 8 and/or a substructure system as provided by means of the method according to any one of claims 11 to 12 as a sub- ballast layer below a ballast layer of a railway track, comprising the steps of:

i) providing surrounding subgrade soils, such as natural ground soils or embankment fill,

ii) positioning the substructure system on a portion of the sub- grade soils, preferably in a furrow in the ground, to form a subballast layer, the top plate being positioned on top of the bottom plate with one major surface of the respective plates abutting each other to support the ballast layer for distribution of local pressures from the ballast layer and protection of the resilient bottom plate, and

iii) positioning a ballast layer on top of the substructure system.

14. A method according to claim 13, further comprising the step of:

iv) laying or embedding sleepers and rails on and/or into the ballast layer.