METHOD

FIELD

[01] Example embodiments relate to methods for strengthening a railway track bed, and to related track beds and track bed strengthening verification methods.

BACKGROUND

[02] A railway track is designed to withstand the combined effects of traffic and the environment, which includes consideration of protecting the subgrade - the material on which the track is constructed. Examples of subgrade materials typically include soft materials, including but not limited to soils, peats and clays, which may be compacted to improve their load-bearing capacity. The thickness of the ballast, sub-ballast and/or slab track elements of typical railway track constructions are designed to ensure that stresses from trains’ dynamic loading are reduced to acceptable levels for the subgrade. However, insufficient ground investigation may result in railway track being built on unforeseen soft spots in the ground, or the ground itself may change under variable conditions, e.g. with a change in water content, or develop voids or sinkholes. Railway tracks subject to subgrade changes in this way may experience additional wear, leading to a reduced operational life span and requiring increased maintenance interventions and potential speed restrictions.

[03] To address this issue, track reconstruction with deep excavations is needed, which can be expensive and disruptive to the railway network. As an alternative, techniques to improve support for railway tracks attempt to strengthen the track bed by admixing the ballast material with some form of binding material, therefore improving the load-bearing capacity of the ballast layer. This process is time-consuming, require significant effort in removal and replacement of the ballast layer. This cannot be performed in-situ, and as such also leads to significant disruption of usual railway operations.

[04] Example embodiments of the present invention aim to address at least one problem associated with known techniques, and may thereby provide a method capable of strengthening a railway track bed in-situ, stabilizing the railway track bed structure by strengthening the underlying subgrade in order to increase its load-bearing capacity.

SUMMARY

[05] In one example, there is provided a method for strengthening a railway track bed in-situ. In one example, the method comprises strengthening a railway track bed comprising a track of rails and sleepers, also known as railroad ties, supported on a track bed which is supported above a subgrade. This method comprises: inserting an injector into the subgrade; and injecting a chemically expanding geopolymer material into the subgrade.

[06] Geopolymers are a class of chemically expandable resins that can be injected into the ground for the purpose of void filling and soil stabilisation. Typically, geopolymers are injected using specialist equipment, following a drilling procedure to provide access for insertion of an injector. Typically, geopolymers have the capacity to expand in volume by three times, or more on injection. Geopolymers have several properties suitable for use in soil stabilisation, including: rapid hardening, with hardening times typically of less than one minute giving the benefit of the geopolymer almost instantly on use; variable final densities of geopolymer material possible during injection, ranging between 10 kg/m ³ to 1500 kg/m ³ , with a recommended range between l OOkg/m ³ and 300kg/m ³ , which are lower than the typical densities of concrete and grout. Suitable choice of material, injection site, pressure and amount of injected material allow for optimised designs to achieve desired levels of subgrade strengthening to achieve a track stiffness with high resistances to fatigue damage under cyclic loading due to traffic, with long fatigue life under railway traffic.

[07] In one example, the injected geopolymer spreads into the subgrade via the hydraulic fracturing phenomenon, reinforcing the subgrade and improving its load-bearing capacity. In this way the subgrade is strengthened, to enable it to withstand a greater load due to traffic without problematic deformation of the railway track or other elements of the track bed structure. In this way the fatigue resistance of the railway track is improved. The geopolymer material used may be selected according to its properties, to deliver the desired density and strength of subgrade material required for railway track bed repair.

[08] In one example, the track bed comprises ballast, and inserting the injector comprises inserting the injector to the subgrade through the ballast. This method can be performed without the need to remove any part of the railway track bed structure, such as the rails, sleepers or ballast layer. Due to the short hardening time of geopolymer materials this method can therefore be performed with minimal disruption of usual railways operations.

[09] In one example, the subgrade comprises a layered structure, with a graded upper layer above a subgrade base, and the method comprises inserting the injector through the graded upper layer into the subgrade base, and injecting geopolymer material into the subgrade base. In one example, the injection of the geopolymer material into the subgrade base continues until the geopolymer material reaches through the subgrade base and into the graded upper layer.

[10] Voids under railway track bed structures may develop, for reasons such as material washout or geological composition. This may include erosion of the subgrade layer. In one example, the injector may be inserted into a void and the geopolymer material may be injected into the void. During this injection process, or any of the processes described herein, the track level may be monitored to ensure that the geopolymer injection into the void achieves a required degree of lift of the railway track bed structure for a smooth railway track geometry.

[11] The formation of sinkholes under a railway track bed structure may cause the structure to continuously lose ballast, deforming the railway track geometry and leading to safety concerns. This is expensive to treat with conventional means such as grouting, which require considerable reconstruction of the railway track bed structure. In one example, a sinkhole can be injected and rapidly filled by geopolymer. In one example, the injector is inserted into a sinkhole and the geopolymer material is injected to fill the sinkhole.

[12] In one example, geotextile material is introduced into the subgrade to provide the reinforcement of subgrades in particularly soft soils, such as but not limited to peat. Examples of suitable geotextile materials include woven and non-fabrics made from plastics, e.g. from polyester or polypropylene. Providing an envelope of geotextile material of a desired shape into the subgrade allows for the shaping of the injected geopolymer, such that particular support structures such as supporting pillars may be created in the subgrade. This allows for the design of the geopolymer to support most of the track bed and railway traffic loads, bypassing weaker elements of the subgrade. In one example, the method comprises injecting geopolymer material to fill an envelope of geotextile material which is provided in a subgrade.

[13] In one example, the geotextile envelope is columnar, allowing the formation of a geopolymer pillar within the envelope. These pillars act in a similar fashion to micro-piles in their support of the railway track bed structure. In one example, the geotextile envelope is generally circular in cross section, but other non-circular cross sections are envisaged. In one example, the method comprises injecting chemically expanding geopolymer material to fill an envelope of geotextile material which is provided in a subgrade where the envelope is substantially columnar.

[14] This subgrade material may extend downwardly below the railway track bed, until a stiff competent layer is found beneath the subgrade. In one example, the subgrade may extend downwardly from the upper layer, and the method may comprise filling an envelope which extends to the lower boundary of the subgrade, to form supporting pillars of geopolymer material within the subgrade. These pillars carry track bed and traffic loads, to the stiff competent layer. In this way the pillars significantly reduce stresses on the subgrade, avoiding track settlement.

[15] In one example, the injector may be inserted such that geopolymer material is introduced at an injection site directly beneath a sleeper. In one example, the injector may be inserted at an angle relative to the railway track bed structure, such as an angle to the vertical, from an insertion point offset from a sleeper. In this way the geopolymer may be injected beneath a load bearing area provided by the sleepers, without the need to remove sleepers or other elements of the railway track structure.

[16] Under-track crossings, for example signal cables, telecommunication cables or other services typically occupy a duct or other conduit in the ballast layer, crossing underneath a railway track bed. These conduits result in variations in the stiffness of the railway track bed structure compared with their immediate surroundings, producing variations in the dynamic load on the railway track bed caused by passing trains. Left unchecked, this may cause track damage. In one example, the geopolymer is injected in the subgrade adjacent to the under-track crossing to locally strengthen the subgrade around the under-track crossing, enabling a smooth stiffness transition and sufficient protection of the under-track crossing.

[17] Abrupt variations in stability or load-bearing capacity of the railway track bed structure from one position along the track to another may lead to maintenance problems, particularly considering the effect of trains travelling at high speeds. Variations may lead to increased dynamic loading and the frequent occurrence of track faults. In one example, different injection methodologies, such as with different types or quantities of geopolymer may be injected into the subgrade at two or more positions in order to provide a gradual change in the strength of the subgrade, introducing a transition zone between a relatively weaker and relatively stronger underlying structure.

[18] In one example, the method comprises providing a first injection of geopolymer material into the subgrade according to any preceding example at a first position, and providing a second injection of geopolymer material at a second position which lies along a railway track bed relative to the first position.

[19] In one example, the injection at a first position along the railway track bed is at a different depth to that of a second position along the railway track bed.

[20] In one example, the injection at a first position along the railway track bed is of a different material to that at a second position along the railway track bed.

[21] In one example, the injection at a first position along the railway track provides a different degree of strengthening to the subgrade to that at a second position along the railway track bed, for example through use of different material properties of different types of geopolymer composition.

[22] In one example, the first position is relatively closer to a hard support for the railway track bed than the second position, and the first injection provides a relatively more strengthened subgrade than the second injection, such that a transition zone in which the subgrade is relatively stiffer is provided between the second position and the hard support.

[23] After the railway track bed has been strengthened using any of the methods described herein, it may be advantageous to verify that the strengthening effect has been achieved, and to confirm the associated improvement on the railway track. In one example, a railway track bed strengthening verification method is provided, the method comprising: performing an initial measurement to determine track bed strength; performing track bed strengthening according to any preceding method; performing a further measurement to verify the extent of track bed strengthening. In one example, the initial measurement and/or the further measurement comprise at least one of track deflection measurement and track vibration measurement.

BRIEF DESCRIPTION OF DRAWINGS [24] For a better understanding of the invention, and to show how example embodiments of the same may be carried into effect, reference will now be made, to the accompanying diagrammatic drawings, in which:

[25] Figures 1 to 3 are schematic a side cross-sectional views of example railway track bed structures, strengthened according to an example embodiment;

[26] Figure 4 is an end cross-sectional view of another railway track bed structure, strengthened according to an example embodiment;

[27] Figures 5 to 9 are schematic side cross-sectional views of example railway track bed structures, strengthened according to an example embodiment; and

[28] Figures 10 and 1 1 shows steps of methods according to example embodiments.

DESCRIPTION OF EXAMPLE EMBODIMENTS

[29] Example embodiments are described for strengthening a railway track bed structure in- situ. Such embodiments involve injecting geopolymer material into the subgrade layer, thereby enabling various types of cost-effective track maintenance to be performed, reducing the need to excavate and reconstruct the railway track bed structure, with minimal disruption caused to usual railway operations.

[30] Figure 1 shows a schematic side cross section view of a railway track bed structure 10. The railway track bed structure 10 comprises a track of rails 11 and sleepers 12, supported on a track bed 13 of ballast 17. These elements of the railway track bed structure 10 are supported by a subgrade 14. Also shown in Figure 1 are an injector 15 for geopolymer material 16, with the geopolymer material 16 present in the injector 15, pre-injection, and in the subgrade 14 after having been injected into the subgrade 14 and having undergone a chemical expansion process. In Figure 1 the injector 15 is shown inserted through the ballast 17 and into the subgrade 14 in order to deliver the geopolymer material to a suitable injection site below the railway track bed structure 10.

[31] In order to perform strengthening of the railway track, such as to stabilize the rails 11 and sleepers 12, thereby improving fatigue resistance, the injector 15 is inserted into the subgrade 14. As a preparatory step, drilling of a suitable bore may be performed. After the injector 15 has reached the injection site, a chemically expanding geopolymer material 16 is injected under pressure into the subgrade 14. There, the geopolymer material 16 expands and provides reinforcement to the subgrade 14.

[32] Figure 2 shows schematic side cross section view of a second railway track bed structure 10. In Figure 2, the subgrade 14 comprises a layered structure, with a graded upper layer 20 above a subgrade base 21. To provide strengthening in this instance, the injector 15 is inserting into the subgrade base 21 , and the geopolymer material 16 is injected into the subgrade base 21 . The injection of geopolymer material 16 into the subgrade base 21 is performed such that geopolymer material 16 reaches through the subgrade base 21 into the graded upper layer 20, bridging the two layers and 20,21 , and reinforcing them to strengthen the track bed structure 10. With a strengthened layered structure, relatively less grading material is needed, and maintenance intervals reduced due to the stronger underlying structure provided by the described methods.

[33] Figure 3 shows a schematic side cross section view of another railway track bed structure 10. In Figure 3, the method comprises an injection of a geopolymer material 16 to fill a void 30 within a subgrade 14 supporting a railway track bed structure 13. This type of maintenance to the railway track bed structure 10 prevents further deterioration of the subgrade 14 and the development of cracking in the track bed 13 at the void 30. In the process of performing a method as described, indeed any of the methods described herein, during the injection process the track level may be monitored to achieve sufficient support and/or lift for smooth track geometry.

[34] Figure 4 shows a schematic end cross section view of another railway track bed structure 10. In Figure 4, a method is performed to inject of a geopolymer material 16 and fill a sinkhole 40 within a subgrade 14 supporting a railway track bed structure 13. This method provides stabilization to the track bed structure 10, which would otherwise continuously lose ballast and eventually require significant repair.

[35] Figures 5 shows a schematic side cross section view of another railway track bed structure 10. In Figure 5, there is shown a plurality of envelopes of geotextile material 50 provided in a subgrade 14. The strengthening method to arrive at the Figure 5 arrangement comprises injecting chemically expanding geopolymer material 16 to fill the envelopes of geotextile material 50. .These envelopes 50 of geopolymer material 16 provide areas of greater strength and loadbearing capacity when compared to the surrounding subgrade 14.

[36] In the embodiment shown in Figure 5 the envelopes of geotextile material 50 are substantially columnar in shape and generally circular in cross-section, allowing the formation of a geopolymer pillar within the envelope 50. In other embodiments, non-columnar envelopes 50 and non-circular cross sections are envisaged.

[37] In the embodiment shown in Figure 5, the subgrade material 14 extends downwardly below the railway track bed 13, until a stiff competent layer 51 is found beneath the subgrade 14. In an embodiment the method may comprise providing an envelope 50 which extends to the lower boundary of the subgrade 14 and reaches the stiff competent layer 51 , and filling the envelope 50 with geopolymer material 16. In this embodiment the pillars of the geopolymer material 16 are formed in the subgrade 14, which will transfer load-bearing from the subgrade to the stiff competent layer 51 and reduce stresses on the subgrade 14, reducing track settlement and deterioration of the subgrade 14.

[38] Figure 6 shows a schematic side cross section view of another railway track bed structure 10. In Figure 6 the geopolymer material 14 is injected adjacent to an under-track crossing 60. The geopolymer material 14 surrounds the conduit containing the under-track crossing 60, but other positions of the geopolymer injection 15 relative to the under-track crossing 60 are envisaged. Injecting geopolymer material 14 around or adjacent to an under-track crossing 60 provides a smooth stiffness transition in the railway track bed structure 10, preventing high dynamic loading which can erode the track bed structure 13 and lead to faults.

[39] Figure 7 shows a schematic side cross section view of another railway track bed structure 10. Figure 7 shows an injection of geopolymer material 16 into the subgrade 14 at a first position 70; and a second, different injection of geopolymer material 16 at a second position 71 which lies along the railway track bed 10 relative to the first position 70.

[40] In related embodiments, third, fourth or more different injections are provided at a plurality of positions along the railway track bed, 10 is at a different depth to that at a second position 71 along the railway track bed 10 to provides different degrees of strengthening to the subgrade 14

[41] Figure 8 shows a schematic side cross section view of another railway track bed structure 10. In Figure 8, the injection of geopolymer material 16 at a first position 70 along the railway track bed 10 is relatively closer to a hard support 80 for the railway track bed 10 than the second position 71 , and a first injection of geopolymer material 16 provides a relative more strengthened subgrade 14 than a second injection of geopolymer material 16, thereby forming a transition zone 81 in which the subgrade 14 is relatively stiffer between the second position 71 and the hard support 80. This transition zone 81 reduces the dynamic loading of the railway track bed structure 10 caused by abrupt variations in track support due to variations in the stiffness of the subgrade 14, which can lead to frequent track faults. This embodiment creates a transition zone 81 without the need to perform track removal.

[42] Figure 9 shows a schematic side cross section view of another railway track bed structure 10, wherein a railway track bed 13 has been strengthened according to the methods described herein. This embodiment comprises a railway track bed structure 10 including a track of rails 11 and sleepers 12 supported on a bed of ballast 17, the ballast 17 being supported above a subgrade 14, combined to provide an in-situ injected geopolymer reinforced subgrade 90. This railway track bed structure 10 is designed to provide improved load-bearing capacity and resistance to fatigue caused by railway operations, in comparison to a railway track bed structure 10 which is not supported by an in-situ injected geopolymer reinforced subgrade 90.

[43] In the examples given above, the types of subgrade material 14 typically include soft materials such soils, peats and clays, which may be compacted to improve their load-bearing capacity. It is understood that other subgrade materials 14 suitable to support a track bed structure 10 may also be strengthened using the embodiments provided.

[44] Whereas the Figures above show the injector 15 performing track bed injection through a ballast layer 17, related embodiments may provide track bed injection without penetrating the ballast by inserting the injector 15, for example when strengthening an embankment on which a railway track structure 10 is supported. This stabilisation may prevent embankment failure, with more freedom in the injection location.

[45] Figure 10 shows steps of a method according to an example embodiment, by which a railway track bed structure 13 such as those described herein may be strengthened. This method comprises, at step 101 inserting an injector into a subgrade; and at step 102 injecting geopolymer material into the subgrade. As described above with reference to Figures 1 to 9, the method can be applied to various engineering challenges specifically related to railway track beds.

[46] Figure 11 shows steps of a method according to an example embodiment. The. The method comprises: performing an initial measurement to determine track bed strength at step 111. Thereafter, the method comprises performing track bed strengthening as described herein, at step 112. To complete the verification, the method further comprises performing another measurement to verify the extent of track bed strengthening, at step 113. In the method, either one of, or both of the initial measurement at step 111 and further measurement at step 113 are indirect measurements of the degree of strengthening achieved. For example, the amount of strengthening may be recognised from a comparison of the measurements of track deflection measurement and/or track vibration at steps 111 and 113.

[47] As described herein, the example embodiments enable a railway track bed structure of rails and sleepers to be strengthened by an in-situ injection of geopolymer into a subgrade, to thereby achieve reinforcement, defect repair or account for changes in the underlying support structure strength, without the need for disruptive track removal and replacement. Furthermore, once strengthening has been performed, methods of verification are described which are also efficient in the context of railway subgrade and track bed support.

[48] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[49] All of the features disclosed in this specification including any accompanying claims, abstract and drawings, and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[50] Each feature disclosed in this specification including any accompanying claims, abstract and drawings may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[51] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.