Putting slab track to the test
INTRO: GIF's dual-gauge test track between Medina del Campo and Olmedo includes a 120m section of experimental slab track designed by Balfour Beatty. Keith Barrow reports that practical tests show the design is behaving in line with theoretical models
FOR SIX MONTHS trains have been operating at up to 240 km/h over a short section of innovative slab track, which offers significant savings and maintenance compared to conventional track. Developed over the past four years, the Balfour Beatty Embedded Slab Track (Bbest) relies on a new rail section to reduce the complexity of installation and the number of components required.
Since the early 1960s, concrete slab track has increasingly been seen as a viable alternative to traditional ballasted track. Various designs are now in commercial service around the world, notably on Japan's shinkansen network and high speed lines in Germany. Ballastless track has also proved particularly popular for use in tunnels.
Drawing on its experience of installing slab track designed by its railway customers, Balfour Beatty believed that existing designs did not fully exploit the concept's potential. Using similar components to ballasted track requires a large number of parts, making installation, inspection and maintenance expensive and reducing the overall benefits.
In 1998, Balfour Beatty Rail Technologies Engineering & Development Director Charles Penny decided to take the development of slab track a stage further. Bbest employs a symmetrical-section rail continuously supported by a low-profile concrete slab. The rail is carried on an elastomeric pad within a fibre-reinforced plastic shell, and embedded in grout in slots within the slab to ensure accurate positioning. Polymer seals deter water and debris ingress. The continuous support offers improved ride quality, reduced wheel wear and increased rail life.
With a thickness of just 370mm from the slab base to the rail head, Bbest offers scope for loading gauge enhancement, sitting 450mm lower than conventional ballasted track with UIC60 rail. An even smaller slab thickness can be used in tunnels, bringing the height to rail head down to as low as 270mm.
Because the sides of the rail are fully supported, the tendency for the rail to roll is constrained. This improved rail stability also allows the safe operation of wider vehicles where the loading gauge permits. On bridges the smaller track structure and the elimination of ballast reduces the dead weight. The reduction in the exposed surface area of the rail also helps to limit noise emissions, which have proved to be a problem with some types of slab track.
During the development process, AEA Technology Rail carried out a rail fatigue analysis of Bbest. Mott MacDonald handled the principal design studies, and Technische Universität München was responsible for dynamic testing of components. During 2001 ISVR carried out acoustic analysis. The installation in Spain is intended to validate the theoretical results, and so far the track is performing exactly as predicted.
Bottom-up slab design
Previous top-down designs of slab track have worked by accurately positioning the rails, which then have concrete poured around them to fix them into position.
Balfour Beatty's bottom-up design takes a different approach. The slotted concrete slab is produced to standard construction industry tolerances of