BYLINE: David Rhodes
Director, Technical Development, Pandrol Ltd
INTRO: A better understanding of the whole-life costs, plus innovative technology to deal with noise and vibration, are persuading track designers to adopt low-maintenance ballastless trackforms more widely
NON-BALLASTED tracks on concrete bases are seeing increasing use, especially on urban transit and high speed railways. Several major projects, including the Taipei - Kaohsiung high speed line in Taiwan, HSL-Zuid in the Netherlands, and the second phase of Britain’s Channel Tunnel Rail Link (p619) will incorporate substantial lengths of non-ballasted track.
Despite the high initial costs, non-ballasted trackforms have a significant advantage in requiring little or no maintenance to retain a high quality of geometry over many years. As railway engineers gain a better understanding of the life-cycle costs, they are able to assess more objectively the potential benefits of non-ballasted against ballasted trackwork.
Conventional ballasted track is still used in almost all new construction, because it does have some significant advantages. In particular, the ballast provides a means of adjusting track geometry, during both construction and maintenance operations, and it provides a degree of elasticity in the track structure.
The key to a successful slab track design is providing elasticity and adjustability in a practical and economical way, despite the inherently rigid nature of the concrete slab. The most widely used form of slab track for high speed lines uses prefabricated, pre-stressed units, typically 5m long, laid on a slip-formed base slab with a cement-asphalt mortar interface layer.
Almost all Japanese and Italian high speed lines built since the early 1980s have used this technique. It allows for most of the adjustment that is required when the track is being built, and provides some flexibility in the interface between the pre-cast unit and the concrete poured in situ. With this type of track, sufficient extra resilience can be obtained with a conventional rail pad. In tunnels, only a small amount of maintenance adjustment is required, and this can be achieved with minor modifications to the rail fastenings. On elevated structures, especially in the seismically-active areas where the design is most used, it is usual to mount the rail fastening on a baseplate, which provides almost unlimited vertical and lateral adjustment possibilities (Fig 1).
Another design, used on high speed, conventional main line and urban railways, makes use of twin-block sleepers or sleeper blocks set into rubber ’boots’ recessed into the base slab. This provides additional elasticity, and makes it possible to use a standard concrete sleeper rail fastening. When used on high speed track, some additional adjustability may be needed in the rail fastenings. Most French high speed lines use conventional ballasted track, but where non-ballasted track is used, such as in long tunnels, this has been the preferred system. Other applications include the Channel Tunnel and the Øresund link (Fig 2).
To make the tracklaying process similar to that used for ballasted tracks, a number of techniques have been developed in which the rail is positioned by temporary supports, and sleepers or sleeper blocks suspended from the rails. Concrete is then poured around the sleepers to fix them in position.
This results in a rigid interface between the pre-cast units and the in situ concrete. The interface can fail when subjected to impact loads, and is vulnerable to damage by water ingress. Various ideas have been tried to overcome these problems, including treatment of the pre-cast concrete surfaces and the use of very elastic fastenings (Fig 3). These techniques have been used to provide very effective slab track designs for transit systems, but the proliferation of Rheda track design variants in Germany is an indication of the inherent difficulties encountered in making this design work on high speed lines.
Where railways - whether metros or high speed lines - run on slab track in urban areas, it may be necessary to consider the impact of environmental noise and vibration. With conventional track, the ballast helps to reduce vibration transmission and absorb airborne noise. With concrete track, extra mitigation measures may be needed to compensate for the absence of ballast. This typically requires low-stiffness supports under the rail (Fig 4). The elasticity that can be provided is limited by the amount of rail ’roll’, and hence dynamic gauge widening, that can be accepted on curves. Metro networks typically set this limit between 4mm and 7mm, but high speed lines may have much tighter tolerances.
In general, slab tracks require a low frequency dynamic stiffness of 50 to 100MN/m to ensure good structural performance (or about 30MN/m for Rheda systems), but less than 20MN/m for effective environmental vibration isolation. To prevent transmission of very low frequency vibration, it is necessary to use massive floating slab tracks, or to use less conventional assemblies which hold the rail under the head, and not under the foot (Fig 5). In this way the dynamic stiffness may be reduced to 5 to 10MN/m without the attendant problem of excessive rail roll.
Non-ballasted tracks are here to stay. The challenge to track designers is to make such tracks economical to build, environmentally acceptable, and true to the railway operators’ expectations of minimal long-term maintenance requirements.
CAPTION: Fig 1. Where the Japanese shinkansen run through seismically-active areas, a baseplate-mounted rail fastening allows almost unlimited adjustment
CAPTION: Left Fig 2. The Øresund link tunnel uses a trackform with twin-block sleepers recessed into a base slab
Right: Fig 3. Very elastic fastenings help reduce the risk of impact damage to the interface between pre-cast and in situ poured concrete
CAPTION: Fig 4. Extra mitigation measures may be needed to compensate for vibration that would have been absorbed by ballasted track
CAPTION: Fig 5. Non-conventional rail fastenings, which support the head rather than the foot of the rail, can help to prevent the transmission of very low frequency vibration