The broad sleeper (BS)

A new sleeper design from Heinrich Cronau GmbH i. L., Homburg

Heinrich Cronau GmbH i. L. began work on the development of a broad sleeper track in the spring of 1996 with the support of the executive board and the Network division of the German railway operator Deutsche Bahn AG, the objective being to reduce the scope and costs of maintenance work. The concrete broad sleeper (CBS) is a newly developed design of sleeper which is characterised by the following features (Fig. 1):
- Twice the width (57 cm) of concrete sleeper B 70.
- Length 2.40 m as opposed to 2.60 m, i.e. 20 cm shorter.
- Two water channels on the outsides, whereby the high point is located in the track axis
- A rigid foam insert in the unsupported zone
- Non-prestressed reinforcement, in addition to the prestressing steel reinforcement
- Design characteristics (height and attachment) otherwise identical to conventional concrete sleepers (e.g. B 70 W).

The sleeper design results in flat, continuous laying without the typical "sleeper pockets". Only a 3 cm wide laying gap arises (in the straight line), and this is closed by a gap cover (Fig. 2).
Tamping is essential at the sides of the rails, i.e. in front of the sleeper ends, when constructing and maintaining the broad sleeper track.
For the purposes of the tamping work, a tamping machine was modified such that the tamping unit is turned by 90 degrees and installed on a frame which can be moved at the side. To enable inspection of the tamping work, plexiglass sleepers were incorporated into the test track, in addition to the broad sleepers (Fig. 3).
An initial test section was put into operation on the Rhine valley line in Waghäusel after a development period of only 7 months, length 450 m, track load 65,000 LT/day, Vmax.=160 km/h, r=2.650 m, camber=60 mm.
After 50 million load tonnes (Sept. 1998), the quality status index stands at 28, denoting good to very good positional stability for the track. Following the good experience with the test section, a pilot project between Homburg/Saar and Bexbach was commissioned and went into operation in October/November 1997.
A section of 6 km was modified with one track featuring broad sleepers and the opposite track in standard design as a reference line. Section data: Max. permissible speed 120 km/h, track load 18,500 / 21,000 LT/day, radii up to 350 mm, with 135 mm camber.

The virtually identical positioning and load for the broad sleeper track and the standard reference track provide an ideal basis for comparative measurements. The measurements carried out to date have provided the following results for the broad sleeper track:
- high positional stability of the track (frequent measurements with track measuring train set),
- improved resistance to transverse displacement (+ 15 %),
- 2 dB (A) increase in airborne noise, which can be halved by means of a sound insulation variant,
- very low structure-borne noise and
- very good subsidence behaviour (less than 50 %). In August/September, a further 5 km section of the pilot project was fitted out with the broad sleeper track between Neunkirchen/Saar and Bexbach. This section features varying subgrade conditions and includes five bridge constructions. A long-term maintenance contract for the pilot project has been concluded with the local operational control centre.
The broad sleeper track is expected to involve a reduced scope of maintenance work, on account of the following factors:

• Reduced surface pressure The surface pressure between sleeper and ballast is reduced by 36 %. According to ORE studies and information from the Technical University of Munich, this leads to a manifold improvement in positional stability.
• Increased resistance to transverse displacement As yet, it has only been possible to carry out transverse displacement resistance measurements in the test section. Measurements in the 60 mm camber have been carried out for the first time. The resistance to transverse displacement improves by 15 % (73 % with sound insulation).
• Less surface water The two water channels in the broad sleeper and the gap cover keep approx. 50 % of surface water away from the bearing subgrade located directly below the track. This prevents softening of the subgrade and the danger of frost damage.
• More effective tamping process The newly developed sleeper-end tamping process (Figs. 4 and 5) also results in advantages. In the customary sleeper pocket tamping process, insertion of and working with the tamping pick impairs the ballast quality and causes damage to the underside of the sleepers and the subgrade. The ballast structure is additionally loosened up when the pick is removed. These disadvantages are largely avoided with the sleeper-end tamping process.
• Ballast cleaning The flat mode of installation for the broad sleeper and the additional gap cover prevent soiling of the bearing ballast structure. Ballast cleaning is limited to the area in front of the sleeper ends. The absence of ballast soiling has a positive effect on positional stability
• Vegetation control The bearing part of the ballast cross-section is protected by the broad sleeper and the gap cover. No plant growth is possible in this area. Relevant aspects of vegetation control:
  - Observation of the bracing and fastening devices and
  - prevention of dirt from infiltrating the ballast under the sleeper. The problem of vegetation control is substantially reduced and simplified.

Costs: The expectations and verifiable advantages are accompanied by higher direct investment costs. The additional costs relating to the sleeper, the requirement for double the cargo space and the additional gap cover amount to total costs which are between 10 and 20 % higher than those for the classic superstructure. These additional costs are offset in the medium term by lower maintenance costs, however. Conversely, direct savings arise as a result of avoidable investment in the following cases, for example:
- increased ballast pressure, no subballast matting or bases for the sleepers,
- structure-borne noise problems
- avoidable additional costs,
- height problems (e.g. relating to the aerial contact line) (reduction in the required quantities of ballast) and
- problems in the subgrade.
Interesting scope for potential applications also arises, for example,
- for level crossings; the level crossing has already been installed in Karlsruhe (CBS-BÜ),
- for locomotive sidings on ecological grounds; concepts have been evolved with the environmental protection authorities, licensing pending (CBS-LOK), and
- a wheel control system to combat derailing (Eschede) (CBS-RLS).

Heinrich Cronau GmbH i. L. is a partner in Eurobalt II, a working group comprising 7 railway authorities, 5 research institutes and 2 superstructure contractors on the subject of "The limits of the sleeper track".

Further information is available from: Heinrich Cronau GmbH i. L. ...