Customising ballastless track to suit different conditions
TRACK:?Contractors can now choose from a variety of ballastless trackforms to suit the requirements of different projects ranging from urban tunnels to high speed lines.
Dipl-Ing Ralf Kowalski, Senior Project Manager, Heitkamp Rail GmbH
Considerable experience with different forms of ballastless track has been built up over the last three decades. Although the capital cost is higher than ballasted track, maintenance costs are much lower, and a range of designs has been developed that offer specific advantages for different applications.
Over the past decade, Heitkamp Rail has been actively involved with a number of major projects requiring the installation of different ballastless track forms. Based on this experience, we developed our own design of compact slab track, which has been in use since 2004. We are now introducing a further design known as HIso-Track, which is specifically intended for urban applications where the transmission of noise and vibration can be a critical issue.
One especially demanding application was in the cross-city railway that opened in Berlin in May 2006. This 9 km north-south link includes a 3·5 km tunnel which features a four-track station for regional trains at Potsdamer Platz as well as the eight-track underground part of Berlin's spectacular Hauptbahnhof (RG 7.06 p386).
It was essential to prevent noise and vibrations being transmitted to the structures and buildings above and adjacent to the railway. The answer was to install ballastless track on mass spring systems in the tunnel (RG 1.05 p41). This required the mass of the trackbed layer and the stiffness of the elastomer to be matched in such a way that vibrations caused by passing trains would not be passed to the adjacent structures.
Heitkamp Rail was awarded the contract to design and build the entire superstructure for the north-south link, which included no less than eight different mass spring systems arranged in an alternating sequence.
Transporting and installing the large masses was a major logistical exercise that was made more difficult by the short construction period of only 21 months. Combined with local restrictions on construction work and other constraints, the project posed an extraordinary challenge in terms of choosing the best construction methods and procedures. To ensure the effectiveness of the mass spring systems, a very extensive quality assurance programme was drawn up and implemented.
High speed test section
One of the earliest designs of slab track to be used extensively in Germany was the Rheda system. The Rheda 2000 version has been developed for applications on high speed lines, including HSL-Zuid in the Netherlands (RG 4.05 p204). Rheda 2000 KH has also been selected for use on around 95% of the 968 km Wuhan - Guangzhou Passenger Dedicated Line in China (RG 8.07 p484). When this opens in 2010 journey times between the two cities will be cut from 11 h to 4 h.
A 10 km long test section of ballastless track is now being installed near Wuhan. This section of the alignment includes approximately 8 km of earth structures, including a station, 35 passages in concrete culvert form, and a total of 1·2 km of bridges. The contractor for the line is Wuhan-Guangzhou PDL Ltd, which awarded the contract to build the test track to a consortium formed by the Eighth China Railway Bureau and Heitkamp Rail. As part of this consortium Heitkamp Rail is responsible for planning the project and for checking the substructures. It also was in charge of site management during the track construction phase, which began in the autumn of 2007. Work on this section is due for completion at the end of this month.
In the middle of France, north of the Central Massif on the border between the Rhône-Alpes and Auvergne regions, is the 1·4 km St Martin d'Estréaux tunnel. Built in 1857, it is located near Roanne on the line between Moret, south of Paris, and Lyon. The tunnel passes through an area with difficult geology, and it was constantly subject to water inflow and ballast wash-out. Therefore a permanent speed restriction required trains to slow from 110 km/h to 80 km/h.
To deal with the problem, RFF decided in 2002 to replace the track through the tunnel using a ballastless design. Heitkamp Rail was awarded the contract to install reinforced concrete slab track with individual Pandrol-Vipa supports. The drainage system had to be modified to include one central and two lateral drainage channels, and to allow for later electrification of the line, the gradient had to be reduced.
The opportunity was taken to create a test section for the Stedef design. This type of track involves the construction of a reinforced concrete foundation on which the track is laid, aligned and the embedded in concrete. In terrain with good physical and mechanical properties, the Stedef track is embedded into the supporting structure and a fibre-reinforced concrete is used to reduce the conventional reinforcement.
Throughout the construction period, services continued under single line working at reduced speed. Since the work was completed in 2003, normal traffic has resumed to the satisfaction of the client.
In order to cope with rising demand, the single-track section of Austria's Tauern main line between Schwarzach-St Veit and Spittal is being doubled. This project includes the construction of a new Birgl tunnel, which has been equipped with slab track.
The construction project included installation of heavy and lightweight resilient mass spring systems on elastomer full-surface supports, as well as a newly-developed heavy resilient mass spring system on strip supports. In combination with traversable sound-absorbing material, an even surface has been built onto which emergency vehicles could be driven to gain access to the tunnel. The overall length is around 3 km.
The transition zones between the slab track in the tunnel and the ballasted track on the main line have been relaid using concrete sleepers with resilient pads on the underside, and the ballast glued to provide a progressively stiffer track structure.
Since its completion in 1884, the 10·4 km Arlberg tunnel has been a feature of the main line between Innsbruck and Bludenz in Austria. The line is being comprehensively upgraded, with reconstruction of the tunnel to meet today's safety standards. As with other projects, the work is being carried out without closing the railway, and completion is due in 2009.
In the initial phase of work escape and rescue routes were planned. These included special passageways linking the rail tunnel with the road tunnel 400 m away. Once this phase is complete, the tunnel will be enlarged and a shotcrete shell installed. Ballastless track, in this case an elastically-supported track/base plate system, will then be laid, ensuring a less maintenance-intensive future for the line.
Backed by extensive experience with different types of track superstructure, Heitkamp Rail developed its own design of compact slab track (Fig 1).
A concrete trough supported by a 300 mm thick hydraulic bonded base course accommodates a rail and sleeper assembly with embedded sleepers. The first step is to install ballast, rail and sleeper inside the trough using conventional methods. The track becomes 'slab track' by grouting the ballast with a special cement paste after the rails and sleepers have been located in their final position by lifting and compaction with standard tamping machines.
Use of conventional levelling and alignment methods ensures precise positioning of the track structure immediately after it has been laid. This position remains constant even after several years of operation under load - this was confirmed by measurements performed on a test section near Waghäusel on the Deutsche Bahn main line between Mannheim and Karlsruhe. In 2004 the design was approved by the Federal Railway Authority in Germany for use on earth structures, in tunnels and on short bridges.
Taking the process a stage further, Heitkamp has now developed its own mass-spring system (Fig 2) for use on railways in urban areas where transmission of noise and vibrations to buildings becomes critical. HIso-Track is suitable for applications where the level of insulation performance has to be very high, and the design allows for corrective measures to be taken later. The system is simple to install and level, and the use of elastomer bearings allows replacement when required.
The crucial element lies in the design of the support points, which are constructed in such a way that special equipment can be used to check the load and deformation and hence the insulation performance of each elastomer bearing. In addition, the insulation performance can be adjusted by removing or adding spacer plates. If required, one bearing can be exchanged for another.
Installation of HIso-Track involves several stages. Once the foundation base is ready for the assembly, a separating layer of standard underlay membrane is laid out without leaving any gaps; this is then cut out in the area around the support points. Instead of shuttering the sidewalls - quite an elaborate process - pre-fabricated elements can be used. The next stage involves reinforcement of the trackbed plate. As soon as this has hardened sufficiently, the bearings can be installed.
Each elastomer element bearing is lowered into the bearing shafts with the installation wrench and turned into the final position. The upper load transfer element does not turn and is locked for the load transfer only after compression of the bearings with the lifting and measuring devices. The forces resulting from this preloading are used to lift the trackbed plate, and the air gap required below the trackbed plate can be adjusted by installing spacer rings using special equipment.
The load-displacement behaviour is recorded graphically and saved for each bearing preload. The information required includes the verification of the spring rigidity as well as the actual bearing load.
To make a proper comparison with ballasted track, the effectiveness of HIso-Track was studied under realistic conditions at Heitkamp's own test site. The results show that:
- the eigenfrequency of the system is arbitrary by choosing different bearing stiffnesses starting from 7 Hz;
- from 12 Hz onwards there is a significant reduction of vibrations;
- above 28 Hz, the isolation is greater than 20 dB;
- the isolation efficiency within the important frequency range of secondary noise (60 to 100 Hz) lies between 25 dB and 40 dB;
- the system demonstrates full effectiveness for all load levels.