“Traditional” Embedded Rail

The rail is encased in a resilient elastomer cast into a concrete track slab or steel bridge deck.

Edilon developed an embedding compound which is an elastomeric polymer combined with natural Portuguese cork. Professor Coenraad Esveld of Delft University was involved in development of this system.

A 3 km test track was laid at Best in the Netherlands for acoustic test trains as part of the Silent Freight project. The line came into service in late 1999 with an operational line speed of 160 kph.

Other companies in Europe, e.g. Sika, have developed elastomer embedding compounds.

On light rail systems such as Nottingham Express Transit there has been successful use of rails pre-coated with an elastomeric compound under factory conditions. The coated rail is then cast directly into an in-situ slab.

Balfour Beatty’s BBEST embedded rail system uses a unique rail section fitted into a resilient sleeve, set into a profiled slip-formed slab Successful trial sections were installed on the Crewe-Kidsgrove [photographs] line and in Spain.

No system has currently been developed for mounting turnouts and crossovers in the embedded rail system. Rail expansion joints have been installed at Best and Crewe-Kidsgrove.

System Components

The embedded rail system developed by Edilon, consists of a UIC 54 rail embedded in a trough in a concrete slab or plinth. The rail sits on a continuous resilient pad and is positioned using wedges and shims made from solid elastomeric compound. The surfaces are primed and liquid elastomer is pumped into the trough to embed the rail. The compound sets in typically 30 minutes to 2 hours.

A PVC tube is built into the system to reduce the volume of elastomeric embedding compound required to form a cable duct. Rail is warmed electrically to a neutral temperature prior to pouring the elastomeric compound.

Although embedded rail has been in use in the Netherlands since 1976, its application for heavy rail has been limited to bridges (where reduced track construction weight is an advantage) and level crossings (where the section is adapted for a road crossing).

A section of embedded rail test track was installed at Duerne from which NS RIB drew the following conclusions:

• The track could be built to the required elasticity
• The first corrugation of the rail occurred at 17 years (with no interim maintenance)
• The estimated operational life of the rail was 50 years
• No vibration
• The noise level was the same as that for ballasted track
• Insulation of the track presented no problems
• Renewal of the rail was possible.

Balfour Beatty BBEST Embedded Rail

The BBEST system consists of the rail section seated in a resilient pad. The rail and pad are fitted into a pre-formed shell which has been grouted into the concrete track slab.

Acoustic performance

The web of the rail is contained within the embedding compound and this reduces the amount of air-borne noise emitted from the rail section.

Acoustic specialists have expressed concerns over the emission of ground-borne noise and vibration from traditional embedded rails systems; there is limited opportunity to increase the resilience of the pad beneath the rail.

Construction

A typical estimate of construction speed is approximately 400m/day for slip-forming a profiled single track slab. As with all slip-form operations, construction speed is dependent on concrete supply.

For traditional embedded rail high accuracy is required when setting up the rail prior to pouring the embedding compound; once set in the embedding compound adjustments can only be made to the rail by cutting out the elastomer and recasting.

Historically there have been problems pouring elastomer in damp conditions. This can restrict track-laying to dry days, or require the track to be tented to protect it from the elements.

For the Balfour Beatty BBEST system, the pre-formed rail “shell” is grouted into the slot in the profiled slab. The cover is then removed from the shell and the rail section and pad inserted

Maintenance Issues

Resistance to creep presents problems with the expansion and movement of track-supporting structures. It is unknown exactly how the rail system will perform for long lengths of continuously welded rail.

Visual inspection of the rail web is not possible. Ultrasonic techniques must be used. Inspection of the rail foot is not possible.

According to research, there should be less risk of a rail break since the rail is continuously support and restrained.

There may be less corrugation of the rail because the continuous support will reduce the pin-pin resonance, considered a primary cause of corrugation.

Costs

With traditional embeede rail ,changing rail is a more complex and costly operation than with traditional fasteners; the elastomer has to be cut to release the bond on the concrete before the rail can be removed. The rail must be welded in an elevated position before being replaced in the trough.

Recent cost estimates by Balfour Beatty indicate a capital cost for BBEST equivalent to that of conventional ballasted track.

Conclusion

Embedded rail presents a very elegant solution for track fastening systems.

If integrated into the concrete sub slab, it can prove a very cost-effective solution with low weight and construction height.

The advantages of almost zero maintenance throughout the life of the system are also very attractive..