Fixed track forms for high-speed lines

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A successful and topical conference about fixed track forms for high-speed lines was held recently in Manchester. Organised jointly by the Permanent Way Institution and Union of European Railway Engineering Associations (UEEIV), it brought together over 100 experts from across Europe.

Selection of a trackform

Niall Fagan, head of track engineering for HS2, gave the keynote address on choosing the right trackform for a high-speed railway. He noted that there are no established rules or processes to follow when trying to determine whether slab track or ballasted track is the most appropriate solution. He highlighted the need to apply an evidence-based structured evaluation process.

Two key issues in the selection process are: is ballasted track sustainable at very high speeds and tonnages in terms of performance, maintenance and renewals? And can the risks associated with installing- slab track on earthworks be mitigated effectively?

A key risk associated with ballasted track is the ongoing settlement of the ballast under loading. The cumulative tonnage is a key input to the degradation of the track system, with speed an input, but to a lesser extent. Since the life span of ballast is a function of the number of tamps, then a higher tonnage leads to more tamping, which leads to lower ballast life and hence more renewals. This is against the background of a line designed for high availability.

A statistical analysis by SNCF and Systra has been extrapolated for the design values of HS2 tonnage and speed to give a prediction of tamping effort. The design criteria for HS2 Phase 1 (London to Birmingham), after completion of Phase 1 and 2, is 18 trains per hour travelling at 360kph. This corresponds to 60 million gross tons per annum (MGTPA). For comparison, HS1 carries 14 MGTPA, the Paris to Lyon TGV line carries 27 MGTPA and the Tokaido Shinkansen between Tokyo and Osaka carries 46 MGTPA.

Slab track has been perceived as less able to cope with ground movements, compared with ballasted track. There are ways to mitigate these issues, such as adjustable rail fastenings to compensate, within limits, for vertical settlement. However, provision of a consistent support platform and consistent stiffness across earthworks, tunnels and bridges is a fundamental requirement.

When the earth moves

Wojciech Nawrat, head of research and development at PCM Rail.One, then gave a presentation about the dependency between the super and substructure in the selection, design, construction and operation of ballastless track systems.

Developing one of the themes highlighted by Niall Fagan, he talked in more detail about the risks of constructing slab track on earthworks.

Movements of earthworks may occur over different timescales – there may be settlement during construction but also residual settlements after the commencement of operations resulting from deformation in the subsoil, in the embankment itself and due to dynamic track loading. Added to this, there may be short and long-term movements of structures due to elastic deformation and creep.

Deformations will vary along the line of route, with settlements under high embankments, localised hard spots under bridges and over culverts, and potential heave in cuttings. There may also be challenges at the interface if separate contracts are let for the substructure and the rail systems.

The construction programme may need to allow hold periods, to allow for proof that deformation conditions match the predictions, prior to construction of slab track. Remedial measures may need to be pre-installed to accommodate for residual movements, which typically should be limited to less than 15mm vertically. Where heave is expected in cuttings, it is possible to pre-install compensation plates that may be taken out later to reduce the track level as the heave occurs.

Proven systems

Dieter Pichler of Vienna Consulting Engineers and Johann Floh of PORR Bau GmbH discussed the ÖBB- PORR ballastless system. The first installation of this proven trackslab system, developed in conjunction with ÖBB (Austrian Federal Railways), was in 1989. Since then, it has been installed for over 300km of high-speed lines and 300km of high- volume lines, including long tunnels such as the Wienerwaldtunnel.

The system comprises precast reinforced concrete slabs, 5.2 metres in length, each containing eight embedded rail fixings. The slabs are set to line and level, then self-compacting concrete is poured to anchor the slabs into position. Settlement compensation can be provided either by adjustment of rail fasteners or, if greater compensation is needed, by lifting and re-grouting the slabs.

Later in the day, Walter Antlauf of Max Bögl and Steve Swain of Tarmac presented the Slab Track Bögl (FFB) system. This system has been developed through field tests in Germany since the 1970s. Since 2004, it has seen extensive use in the Far East, with over 10,000km installed on the Chinese high-speed network. In Europe, over 200km has been installed in applications, including the Katzenberg tunnel in Germany and sections of high-speed railway near Erfurt in Germany.

The system comprises standard slabs 6.45 metres long, 2.55 metres wide by 0.2 metres deep. The slabs are pre-stressed laterally and longitudinally and, after installation, are mechanically connected longitudinally by post-tensioned steel rods. Track curvature is achieved by precision-grinding each rail seat location. This produces slabs which are customised to their location on the line.

Stefan Knittel of Rhomberg Rail Consult discussed track renewal during engineering hours, focusing on proven solutions to optimise slab track replacement in overnight possessions. In particular, he looked at methods which allowed the track to remain in use during the works, in contrast to the traditional approach of carrying out all work stages in a single shift.

Methods included using temporary support to slab tracks in tunnels, whilst concrete was removed and re-cast. In a case study on the Madrid metro, trains were permitted to travel on the temporarily supported track at reduced speed. Other techniques included a determination of the concrete strengths required to run works traffic over freshly cast slab track. A case study was presented of the city tunnel at Malmö, Sweden, where up to 350 metres of track was cast per day, with works trains run with full axle load over the track just 16 hours later.

Rayleigh waves

Prof Peter Woodward of Heriot- Watt University, Edinburgh, presented on the role of peak particle velocity and critical speed. The passage of a train over the track causes vibrations in the ground, and these propagate through the surface of the ground as Rayleigh waves.

On high-speed lines, the train speed may start to approach the Rayleigh wave velocity and this can lead to significant increases in the amplitude of vibrations. As accelerations increase to a critical peak particle velocity of around 20mm/s, then ballast can start to migrate, leading to increased maintenance demand. Once acceleration exceeds 0.7 to 0.8g, ballast starts to destabilise. Away from the track itself, excessive vibrations can be transmitted to adjacent buildings.

Various definitions have been provided for the critical ratio, which is taken as the train speed divided by the velocity of the surface wave, including the effect of the trackform. Research shows that, below a critical ratio of 0.6, the increase in vibration is likely to be small. Once the critical ratio exceeds around 0.6, accelerations start to increase rapidly and non-linearly.

Where the proposed line speed would appear to exceed this critical ratio, there are two mitigating methods: measures to increase the ground stiffness, such as soil stabilisation or piling, which increase the Rayleigh wave velocity, and/or measures to increase the trackform stiffness, such as using slab track, to increase the critical track velocity. The latter is the Rayleigh wave velocity taking into account the stiffness of the track system.

Analytical methods, typically involving 3D finite element modelling of the ground, can be used to predict the wavelength of the Rayleigh waves. This can be used to assess the depth of stabilisation measures, where ground improvement is proposed, given that the peak displacement drops off rapidly below a depth of half the wavelength. Such analysis can also be used to assess the effect of providing minimum values of ground stiffness, or the circumstances in which a stiffer trackform is required, noting that there are a number of inputs to the calculation, many of which include a degree of uncertainty.

Noise and vibration

Dr Julie Dakin and Brian Stewart, both of Mott MacDonald, discussed reducing noise and vibration from high-speed railways. They noted that noise and vibration are increasingly a major concern for people adjacent to transport infrastructure and that people are becoming more active in complaining using the various media channels now available.

At low train speeds, up to around 30km/h, the majority of noise from a train comes from the traction equipment. At higher speeds, above 30km/h, the rolling noise due to the roughness of the wheel on the rail becomes dominant. At high speeds, over 250km/h, aerodynamic noise becomes the biggest contributor. The main sources are airflow around the bogies, around the pantograph, around the front of the train and at the gaps between carriages.

The most obvious mitigation measure is a noise barrier, to avoid straight-line transmission. However, barriers need careful consideration so as not to impede access or train evacuation and can become large structures over four metres high.

Likewise, they do not mitigate for tall buildings such as blocks of flats. Other solutions such as rail dampers may assist in mitigating rolling noise, but this may not provide much benefit for high-speed routes where aerodynamic noise tends to dominate.

Ground-borne vibration is generated by the natural roughness of wheels on the track. It may be transmitted, for example, through tunnel linings into the ground and thence to buildings, where it may be felt as a low- frequency rumble. It typically occurs in the frequency range 30 to 200Hz; above this, the ground tends to damp out higher frequencies.

There are options to mitigate ground-borne vibration by changing the transmission path. Generally, ground-borne vibration and noise can be reduced by reducing the stiffness of the rail support. Many systems exist to do this, including resilient baseplates, booted sleepers and full floating slabs. However, care is needed to consider the various sources and frequencies of vibration, since modifying the transmission path in this manner can push previously non-critical effects into the zone of interest.

Track structure interaction

Dr David Rhodes of D R Squared and Mike Baxter of Track International Consulting talked about the interaction between ballastless track and bridge structures on high-speed lines. This issue relates to the relative movement of a bridge structure beneath a trackform of continuous welded rail.

A bridge may move due to bending under traffic loads, due to thermal expansion and contraction or as a response to traction and braking forces. This results in additional stresses within the rail. Traditionally, this effect is mitigated by installing a rail expansion joint at the structural discontinuity. However, on high-speed lines, such devices are expensive to install and maintain so it is desirable to minimise their use.

Track engineers may find it somewhat strange that there are codified limits for rail stresses in the structural design standards. These permit the use of a non-linear finite element analysis of the combined track and structure system. Work is also in hand at European level to review the limits, particularly with respect to slab track.

The thermal movement of a bridge structure places a limit on the longest structure length that is possible without exceeding the rail stress limits. However, this can be mitigated by using reduced or zero longitudinal restraint (ZLR) track fixings in the vicinity of the structure movement joint. Case studies were presented of measures which could be applied, including a 225-metre- long continuous viaduct without rail expansion joints, but including ZLR fixings within a 16 metre zone at each structure movement joint.

Improved ballasted track

In the last presentation of the day, Dr Klaus Riessberger of the Technical University Graz put an alternative proposal to the conference compared with previous debates about fixed track forms. He presented the results of in-service testing on alternative sleeper designs which could give high performance ballasted track.

The designs were based on the premise that maintenance of ballasted track could be reduced if the stresses on the ballast are more even, compared with the traditional ‘hit and miss’ loading applied by sleepers. A more even stress distribution should in theory reduce the peak stresses on the ballast and hence decrease degradation under load.

A series of proposals was presented, all of which had the common feature of a near-continuous longitudinal member below the rail. It was noted that, in track engineering, there is nothing new and a version of this was used in 1854 on the Semmeringbahn in Austria.

Full-frame and half-frame design sleepers have been installed at trial sites on Austrian railways and on the Union Pacific railroad in the USA. These sites, subject to heavy traffic and on tight curvature, have indicated a reduction in tamping intervals from, typically, three to four years to over 15 years without tamping. At the USA site, over 1,200 million gross tons of traffic had run over the site over five years without any signs that maintenance was required.

It had been a packed day. As Brian Counter, technical director of the Permanent Way Institution, said in his summing up, the partnership with UEEIV had resulted in a successful and sold-out conference. Attendees had been treated to an informative and thought-provoking set of presentations on a topic of immediate relevance.

Written by Mark Phillips