On today's modern railways there are many key economic drivers forcing a change in thinking for track infrastructure:

* greater gross annual tonnages

* higher axle loads

* higher speeds

* increase in train pathways

* reductions in noise and vibration

* more stringent

* reductions in maintenance resource and costs

* the need to mix freight and passenger traffic.

Balfour Beatty Rail Projects has developed a new embedded slabtrack system that improves safety, performance and availability while reducing maintenance and whole-life costs. The new 74kg/m rail is continuously supported in an elastomeric pad and a fibre-reinforced plastic shell. The pad and shell are manufactured to the required tolerances and delivered to site in suitable lengths. The system uses an efficient low-profile steel-fibre-reinforced concrete track slab.

Design concept

The design is adaptable, suiting any railway's local standards and specifications. The principal active element is a Polyurethane polymer pad, which supports the rail in position. The pads are extremely durable and can be manufactured to a variety of densities to suit specific requirements. Their lightweight composition makes them easy to handle and transport.

The pads sit in a glassfibre reinforced plastic shell, which acts as the permanent interface between the concrete slab and removable pads and rails. Again the emphasis is on weight reduction and durability, facilitating handling and storage.

It is important to allow the rails and rail pads to be removed for maintenance and replacement. The new rail profile, designated BB14072, makes this a straightforward process of lifting out the old and fitting the new without affecting the shell and slab. As the rails are continuously supported by the slab, they do not require the same vertical bending strength as traditional rail.

Rail installation begins with the fitting of the pads and shells to a section of rail. This assembly is then located by lifting, inclination and alignment (LIAN) supports, allowing precise positioning of each rail in its slot during grouting. Vertical and horizontal rail alignment can be adjusted independently while held by the supports.

Installation

A key advantage of the system is the separation of civil construction tolerances from the higher accuracy requirements of rail positioning. The system is installed in a 'top-down' process into a pre-formed track slab (see Figure 1). Rails are accurately positioned to line, level, inclination and gauge.

The system allows track slab installation to proceed ahead of track laying, reducing the programme dependencies of each activity (see Figure 2). Unlike most other systems, the whole concrete track slab can be constructed in one pour, for example by slipforming.

The track slab, which uses a 50MPa concrete containing 35kg/m^sup 3^ of Dramix steel fibres, is cast in-situ, slipformed or precast with slots for the sub-system. Rail is supplied in long or short lengths for alumino-thermic or flash butt welding. The rail, pad and shell are held by special alignment frames and grouted into position in the slots. Storm water is directed away from the slab and formation by a drainage system, while electrical details can be provided to suit third and fourth rail power and signalling equipment requirements.

Key benefits:

* accuracy of finished installation

* flexibility of installation to suit a variety of situations

* easy on-site training of locally available labour

* suitable for restricted areas such as tunnels

* track slab programme separated from rail fixing.

Slab production

The slotted concrete slab is produced to standard construction industry tolerances of + or -5mm. The rail, elastomeric pad and supporting shell are then fitted into the slots, then adjusted to the tighter railway tolerances (+ or -1mm). This simplifies accurate rail location and adjustment.

The cast slab should provide a solid, durable base with a potential service life of up to 100 years. The concrete is reinforced with steel fibres to improve cracking resistance, with a first crack strength of 6MPa and a high degree of ductility. Water and debris drain into central gullies 100m apart to direct the water into a main carrier drain alongside the track bed.

Lower maintenance costs

With only four components and no fastenings or fittings to come loose or corrode, little inspection is required. The full rail section can be ultrasonically scanned for defects. The seals, pad and shell resist grease and acid and have a 30-year design life. The system allows increased rail side wear.

Performance

Mott MacDonald handled the principal design studies, and tests on the system were undertaken by AEA Technology Rail and Technische Universitat Munchen. The tests showed exceptional static and dynamic spring characteristics and biaxial load fatigue performance between -20[degrees]C and 50[degrees]C. The system gives superior gauge retention with no pad deterioration or performance loss after 4.7 million cycles of fatigue loading. The resilience meets the current Deutsche Bahn AG requirements for slabtrack and can be tuned to meet each client's requirements for ride quality, noise and vibration for any rail application.

Safety

The system is designed to deliver the highest safety performance. The continuously embedded support restricts rail movement under load and thus reduces susceptibility to lateral buckling, crucial rail wear and fatigue problems such as corrugations and rolling contact fatigue. If a rail breaks, it is retained, with traffic able to pass until a repair can be made (see Figure 3). The unique design can also include a guard that considerably reduces the risk of derailment (see Figure 4). With little maintenance, the system presents less accident risk to track workers and trains. The simple design reduces vandalism risk, allows easy inspection and cleaning of the slab and safer access or escape during any incidents.

Increased availability and clearance

Lower maintenance increases track availability, reducing both unplanned delays and speed restrictions. The system depth (top of rail to bottom of slab) is only 370mm, saving 300-400mm over a ballasted system, and may reduce to 270mm in tunnels. This gives extra clearance for larger vehicles or smaller tunnels, for new overhead electrification or for reducing weight on viaducts and bridges. The stability, durability and gauge retention of the system reduces the kinematic envelope for larger vehicles, tilt technology, higher speeds or avoidance of structure gauge clashes.

Reduced whole life costs

System maintenance costs can be as low as 20% of that for ballasted track. Taking capital and maintenance costs into account, this can mean an internal rate of return over 10% more than for a main-line ballasted system. There are additional technical, safety, environment and availability benefits.

System specification

The current design specification, including the rail and pad characteristics, can be adjusted to meet clients' requirements.

Track slab

The system provides a stable, durable, cost-effective and continuous support for the rail system (see Figures 5 and 6). In its basic 1435-gauge main line configuration, the slab is:

* 2200mm wide at the base

* 370mm from top of rail to underside of slab

* continuously reinforced with a single layer of steel bar

* constructed from 50MPa steel-fibre-reinforced concrete

* suitable for sub-base moduli of 80MPa and above

* suitable for in-situ, precast or slipform construction

* low-maintenance with 100-year design life.

Putting slab track to the test

The dual-gauge test track between Medina del Campo and Olmedo in Spain includes a 120m section of Balfour Beatty embedded slabtrack (see Figures 7-10). The installation in Spain has validated the theoretical results, and the track is performing exactly as predicted.

Since May 2002, a Talgo infrastructure monitoring and test train has been operating over the slabtrack section at speeds of up to 240km/h. Detailed monitoring recorded a rail deflection under a 17.5-tonne axle-load of 1.6mm vertically and 0.1mm horizontally.

Concluding remarks

As there are fewer components, track inspection is greatly simplified. Eliminating track problems associated withh ballast, such as wet spots or weak shoulders, will reduce maintenance requirements. The absence of easily removed components reduces the susceptibility of the track to parts working loose, or to vandalism. As the BB14072 rails do not have a thin web, ultrasonic testing of the whole cross-section is simplified.

Although the initial cost of installed slabtrack is 30% higher than conventional ballasted track, it offers major savings in long-term maintenance costs. Over a 30-year period, Balfour Beatty estimates that the cumulative savings give an internal rate of return 12% higher than that of ballasted track. To date, Deutsche Bahn has now tested some 14 different designs of ballastless track.