New Bailey Street bridge, Salford, carried two tracks on timber waybeams, supported on a traditional wrought iron deck (see below). This deck consisted of deck plates supported on rail bearers, with cross girders spanning between side beams and a central box beam. Part of the deck was no longer in use and had been capped in concrete.
The revised track layout of the Ordsall Chord scheme no longer requires the bridge to carry two tracks. Instead, it will carry a single line, with a raised vertical rail alignment and altered horizontal curvature (radius 280 metres), transitioning back to straight at the Western end of the bridge. The bridge had to be preserved, but also strengthened to meet modern standards, and the primary initial challenge was how to fit a track system to the new geometry without over-loading the bridge. Ballasted track was thereby ruled out and the concept envisaged was some form of baseplated track, with concrete plinths combined with a concrete deck slab.
Waybeams were not feasible due to the curvature and space available, but the idea of having something that could still be removed/replaced more easily than a fully integrated in-situ concrete plinth was of interest to both track and structures maintainers. With the geometry involved, and to meet modern standards, the track would of course still need to be securely fixed.
The ideas and aims were passed to the project team and the designer for the bridge, Tony Gee & Partners, to consider alternatives. This led to Rhomberg Sersa being approached for proposals based on their extensive experience of precast modular track forms.
As a part of the Northern Hub Project team (with Network Rail and Skanska/BAM as well as the Northern S&C Alliance with AmeySersa), Rhomberg Sersa Rail Group was then employed to develop a low height, low weight slab solution for New Bailey Street bridge.
Working with Tony Gee and supported by Vienna Consulting Engineers (VCE), the Rhomberg Sersa team developed a variant of the IVES Slab Track Plinth System that is intended for bridge decks. In this case, the plinth had to be slimmed down to meet the height constraints and deal with the planned tolerances.
A plinth could still have been created in-situ but, to maximise off-site works, allow the crossfall on the bridge deck to work easily, and provide for simple removal and replacement, a precast solution was adopted.
The use of Vossloh DFF304 rail supports, in combination with the precast system, provided a second level of tolerance absorption – a key factor in achieving the desired construction accuracy. This meant that the accuracy of the IVES Plinths positioning could be to civil engineering tolerances, while the DFF304 and rails can be adjusted in three dimensions above the IVES Plinths to deliver the required accuracy in rail positioning – an intelligent and versatile solution!
To meet the aspiration of simple removal and replacement of the plinths, a method of fixing was developed that uses vertical reinforcement as anchor dowels. Combined with under-grouting, the dowels ensure secure fixing of the precast elements to the bridge deck. Removal can be achieved either by over-coring these anchors and shearing/splitting, or by wire cutting through the grout layer under the blocks followed by core drilling to remove the fixings. Either approach would allow one or more plinths to be removed and replaced if they were to be damaged.
During construction, the key challenge was core-drilling the deck slab since it was heavily reinforced – a replacement process would have been more straightforward! Each aperture had to be accurately located, with the aid of drilling templates, to match the plinth geometry. For future sites, cast-in pockets or slots could be considered. Once positioned, the IVES plinths were then lifted to height using spindles and, after installation of shutters, they were secured with a high specification grout, poured through the plinths, to fill the voids under them and surround the spindles and anchors.
The Vossloh DFF304 baseplates and rail were then placed onto the plinths and lifted/aligned using the Rhomberg RhoFAS rail alignment system which also holds track gauge and rail inclination.
Once the rail was to its final design position, grout was poured to fix the supports.
For one of the transition zones, there was sufficient depth for a Rhomberg V-TRAS steel transition module to be fitted. These modules ease the change in effective track stiffness that occurs between ballasted track and ballastless track on a structure. This reduces track geometry deterioration rates, thereby minimising the need for future unplanned ballast packing adjacent to the bridge. Tony Gee designed a sill beam onto which the V-TRAS bearing was fixed and, because of the formation width and curvature, gabion ballast retaining walls were used to stabilise the track on its new tighter radius. Combined with measures specified by WSP for the rest of this now CWR curve, such as restraint plates, this track system should minimise the maintenance needed on this important zone.
All UK slab track projects contracted to Rhomberg Sersa in the UK (see past issues of Rail Engineer that covered Asfordby Tunnel, Winchburgh, QST, GOBE) are delivered by Rhomberg Sersa UK and Rhomberg Bahntechnik; a department specialising in slab track technology and installations. Its Austrian teams have extensive installation experience which allows Rhomberg Sersa UK to bring best practices and technology from Continental Europe to the UK infrastructure, increasing productivity and quality while reducing costs.
As ever on a project of this scale (Northern Hub), interfaces were a challenge; WSP (formerly Parsons Brinckerhoff) had been contracted to undertake the p-way design and Tony Gee the bridge and civils designs. BAM/Skanska was undertaking the civils works, Northern S&C Alliance (AmeySersa/Network Rail) the pway works with Rhomberg Sersa tackling the slab track that interfaced across all packages.
The need for low construction height solutions that are capable of resolving tolerance challenges is clear.
Combined with the need for keeping added weight to a minimum, the prospect of using this form of solution on refurbished bridges brings an option to the network’s bridge engineers and asset managers. The interface with the bridge deck can easily be adapted to suit a range of scenarios. Furthermore, the need for low height, robust, fast-to-build solutions for slab track replacement or refurbishment can also lead to solutions such as these.
This article was written by Barnaby Temple, head of engineering at Rhomberg Sersa UK.
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