Slab track trials

Just over a year ago (issue 116, June 2014), Rail Engineer reported on trials of two slab track systems installed in Asfordby Tunnel, on the Network Rail test track near Melton Mowbray. The PORR and IVES slab system trials were accompanied by a trial of the VTRAS transition system between slab and ballasted track.

All three systems were installed by Rhomberg Sersa, the company marketing and installing these systems in the UK. The installation had gone very well, and seemed to confirm the claims of Rhomberg Sersa that it was possible to install slab track very quickly to an extremely high standard on an existing railway using either PORR or IVES.

The success of the trials at Asfordby has attracted a great deal of positive attention from many parties, particularly infrastructure owners, contractors and designers. This has culminated in the recent use of PORR in Winchburgh tunnel (issue 130, August 2015).

It was therefore an opportune time to visit Rhomberg Sersa’s headquarters in the UK near Doncaster for an update on the Asfordby trials and to hear of the company’s plans for the future.

Technology revisited

First, it is timely to review the three systems and what they offer. A more comprehensive explanation appears in the earlier article.

IVES, a Rhomberg Rail development, simply stands for Intelligent, Versatile, Effective and Solid. It is not strictly slab track, since it consists of individual prestressed concrete units with rail support assemblies for each rail, separated by a small gap.

The system is ballastless though, the units being laid on asphalt paving, and it does behave like a slab since the 250mm deep concrete units are heavy (one tonne each) and a stainless steel dowel pin is used to restrain the units from moving relative to the tarmac.

Not every unit needs to be dowelled and, on the Asfordby trial site where the curves are relatively flat, one dowel every fifth unit was the requirement. Where sharper curves, faster speeds or heavier loads dictate, dowels would be installed more often by design.

The PORR system, jointly developed by Austrian Railways (ÖBB) and Allgemeine Baugesellschaft A. Porr AG, also relies on an accurately machine-laid asphalt base on a 100mm Type 1 sub-base. However, the concrete modules of this system are significantly different. Each slab is five metres in length and weighs about five tonnes. They are only 150mm thick, however, and so they are slightly flexible.

Five jacking screws are incorporated into each unit, one in the centre and one towards each of the four corners. These are used to level the units approximately 80mm above the asphalt base. The flexibility of the modules allows them to twist and bend to accommodate some of the variation in rail alignment that may be required in track transitions, for example. Greater variations in alignment are dealt with by designing and casting modules with the necessary curvature and cross-level variation built into them.

In each module, there are two large windows, one towards each end. Once the module has been aligned and levelled correctly, self-levelling concrete is poured in through these windows to fill the void between the unit and the supporting asphalt. At Asfordby, the rails are supported and fastened in Vossloh assemblies, as with the IVES modules. However, plans are in hand for production of units using Pandrol Vipa housings as an alternative.

The VTRAS transition module (Vertical Transition System) is designed to avoid problems caused by a sudden change in track stiffness where ballasted and ballastless track meet. The module is essentially a steel ladder structure consisting of two steel beams with transverse support plates between them at intervals corresponding to the sleeper spacing. These carry resilient pads for the sleepers to sit upon. The plates have upturned ends to restrain the sleepers from lateral movement.

The installation at Asfordby has a cast-in-situ concrete support block under the track at the start of the slab track. The one end of the VTRAS module is supported on the outer end of this block whilst the remainder sits on the bottom ballast, under the sleepers it is to support. The track is packed or tamped through the VTRAS unit in the normal way, but the stiffness of the ladder of steel ensures a gradual increase in the overall track stiffness through the length of the transition assembly.

Asfordby outcomes

Rhomberg Sersa has learned many things from the trial, which has given the company the opportunity to further improve the system and to adapt it specifically for implementation in the UK. Managing director Carl Garrud and project manager Chris Kearns, joined by consultant John Lyall of Addo Partnership, explained how the trials had gone.

To start with, there is a far wider variety of rail sections in common use in the UK than in mainland Europe. This could have been an inhibiting factor in the adoption of PORR or IVES here. However, Vossloh and PORR developed a universal rail baseplate that could be moulded into any PORR unit regardless of the rail type to be installed.

The appropriate standard Vossloh clips may be fastened into this plate, avoiding the need to cast special PORR units for each different type of rail. Not only that, but should the decision be made in future to re-rail the line with a different rail section, the PORR units need no alteration, a simple change of certain components of the fastening system will do the job.

A second peculiarity of the UK environment, compared with the rest of Europe, is the available aggregate gradings within the SCC (self compacting concrete). Simplifying somewhat, British aggregate producers do not produce such fine gradings. This has consequences when trying to produce the necessary self-levelling concrete that will be able to find its way right under the PORR units without leaving voids.

This issue became clear at Asfordby. Rhomberg Sersa spent about five months working with many different suppliers of concrete additives, looking for a way to create concrete with the right flow characteristics using standard aggregates available in this country. Success was achieved in partnership with Hope Construction Materials, which came up with a recipe that is now being marketed as ‘Hopeflow Rail’ alongside the existing ‘Hopeflow’ from which it was derived.

For many, the greatest positive attribute of the PORR system is the speed at which slab track may be installed in an existing rail track – Chris claims that 100 metres can be installed in a 12 hour shift. Once slab track is installed, there are huge benefits to be enjoyed in terms of longevity, durability and cost reduction by comparison with ballasted track.

The cost and disruption of converting ballasted track to ballastless has been the great barrier to the wider introduction of slab track on existing railways (as opposed to its use in new construction).

Engineering teams at Network Rail and elsewhere have shown great interest, having seen at Asfordby the reduction in disruption and cost that the PORR and IVES systems can offer in comparison with other ballastless systems when they are to be installed in an existing rail line.

There is still an extra cost, of course, compared with the conventional renewal of ballasted track, and the need to fund that cost is an issue.

Carl and his colleagues are confident that the reduced whole-life cost and improved quality and reliability of ballastless track are great enough to make the conversion viable. However, until infrastructure owners see this for themselves, which will take time and experience of the track form, there is still a tendency to choose these systems only where there is another factor involved.

Currently this extra factor is often the need to increase clearances, such as for electrification or to allow the carriage of larger freight containers. This has meant that tunnels are frequently the locations chosen for the conversion to ballastless track.

Infrastructure owners have not been the only ones to learn from the Asfordby trials. Babcock Rail visited the site to see what had been done. As a result, Rhomberg Sersa was included as a sub contractor in its bid for EGIP (Edinburgh Glasgow Improvement Project).

PORR in Winchburgh tunnel

Babcock saw the PORR system as the ideal solution to the requirement to increase clearances in Winchburgh tunnel on the route. Early engagement of the contractor in this project resulted in the adoption of the PORR system with all its advantages.

In fact, in the end the PORR system and the versatility of Rhomberg Sersa saved even more time than originally anticipated. When carrying out preparatory work on the Up line, the first line to be renewed, the Babcock construction team encountered serious unexpected challenges prior to installing the 92 PORR units. The ground conditions were significantly worse than envisaged and there was a major problem with water ingress. This situation required a massive team effort from all the project partners.

The delays had been kept down to five days by the time that Rhomberg Sersa was handed the site to begin the installation of the PORR units. Two VTRAS transition units were also to be installed, at each end of the resulting slab track. The installation team was able to pull back significant time, including being able to demonstrate that the self-levelling concrete was sufficiently strong to carry traffic only 15 hours after pouring it. The result was that the Up was handed back to the first engineering train on time, a terrific effort by all parties and a great demonstration of the capabilities of the PORR system.

The lessons learned from the Up line allowed the JV to de-risk the Down line works, allowing another 92 PORR units and 2 more VTRAS to be installed in that line without incident.

Glasgow Queen’s Street and beyond Network Rail needs to provide electrification clearances in other tunnels on the EGIP project, and perhaps the most important of these are the tunnels at Queen’s Street station.

The challenges of this site may make Winchburgh look simple. Obviously, there will be major logistical challenges as a result of the location at one end of a very busy station in the centre of a major city.

Access will be a major issue, especially as, when the tunnel works are to occur, there will be contractors hard at work on the station itself. Bringing out several thousand tonnes of spoil as the existing track and slab are removed will take some doing, even before considering importing the materials for the new works.

The tunnels lie on a 4% gradient down towards the station. As at Winchburgh, water ingress is to be expected and will have to be carefully managed.

All of this means that the PORR system is very much in contention for the slab track installation for the plain line, with Sonneville’s LVT system under consideration for the S&C.

Beyond Queen’s Street, Network Rail and others are considering the application of the PORR system and VTRAS for several other projects. Carl was emphatic about the benefits of early contractor engagement. He said that to get the full benefit from a system like PORR, it has to be planned into the project programme in detail and in a way which is possible only if the decision to use it is made as early as GRIP 2 or 3.

He illustrated this by saying that Rhomberg Sersa had been able to cut the programme of a tenderer for the GOBE (Gospel Oak to Barking) project by 30 days, just by incorporating the PORR system into the plan.

The tenderer was unsuccessful for unconnected reasons, but the point was made.

IVES improvements

The IVES system is also under active consideration for some future projects, but no definite decision has yet been made on any of them.

In the meantime, Rhomberg Sersa has been actively refining the methodology to optimise its application in the UK. The Asfordby experience highlighted a logistical challenge around the application of standard highways technology to lay the asphalt base for the IVES units in a rail environment.

This did not prove to be a serious problem in Asfordby, which is a relatively short tunnel, but the trial did alert the team to the requirement to modify its approach to avoid problems in longer tunnels. This it has done successfully and Rhomberg Sersa is now confident that it can manage installations in tunnels of any length.

Other developments

Carl and his team are keeping an eye on developments with the use of asphalt bases for rail tracks. The accuracy of laying asphalt is now so good that it would be feasible to lay sleepers directly on an asphalt bed.

This could have many advantages, giving the sleepers a stiff but resilient base that could be ‘tuned’ to the application, and waterproofing the trackbed. It would be repairable with technology that is in common use outside the rail industry, but should last longer than ballasted track before needing repair.

Rhomberg Sersa UK is a small company that is working with other larger organisations to provide specialist knowledge and techniques to assist them to be more efficient and effective. Examples of this include involvement with Track Partnership (London Underground and Balfour Beatty), direct consultancy work with LUL and development work with Amey Consulting for Heathrow Express.

On the Midland Metro in Birmingham City centre, the company is a sub-contractor to Balfour Beatty, installing the rails on the metro extension between Snow Hill and New Street stations. It has also played a role on the Metro works further up the line in Wolverhampton.

The company is actively involved as part of the Amey Sersa JV (Amey Rail – Rhomberg Sersa) in the Northern S&C Alliance with Network Rail, introducing the ‘Swiss’ method for the renewal of S&C using bespoke machines and their operators.

The Northern S&C Alliance is also regularly using the Sersa Second Life System, or SLS. This is a life extension system for S&C, the benefits of which may be understood by considering the outcome of its first UK application at Oxhays in 2005. Designed to provide a life extension of five years, in practice it far exceeded that and the treated S&C has only recently been renewed, ten years later.

As a final thought, it is worth noting Carl’s statistic that Rhomberg Sersa installs about 45km of ballastless track per annum worldwide. That’s a lot of concrete slabs!