DB’s experience with Y-steel sleepers

Permanent way is required to absorb the static and dynamic forces resulting in vertical (z), lateral (y), and longitudinal (x) directions from railway traffic loads, to effectively distribute them, and to transfer them into the sub-grade at reduced magnitudes. Superimposed on these external forces are additional internal forces arising from temperature loads, which can influence track-position stability on straight as well as curved track, and which must be taken into consideration in design dimensioning of the permanent way.

The following permanent-way properties are crucial for main-track and turnout applications:

• Great track-position stability
• Long life cycle
• Great cost effectiveness

Track-position stability is a prerequisite for the safe interaction of rolling stock and track systems, and it has an influence on vehicle dynamics. Durability is essential for great reliability and availability, and for long service life. In addition to the costs for construction of the permanent way, long-term behaviour plays a key role in determining cost effectiveness. Advanced permanent-way structures must therefore demonstrate long service life and low maintenance.

Significance of the Y-steel sleeper in the equipment standard of DB Netz AG

In railtrack technology, DB Netz AG pursues the strategy of optimising the cost-effectiveness of its facilities by application of standardised and low-maintenance components. The most extensively used form of permanent way is cross-sleeper track, installed in ballast, with concrete mono-block sleepers at intervals of 60cm to 67cm. At speeds above 120km/h and loads over 30,000 Lt/d, the use of UIC 60 rails on B70 concrete sleepers, 2.60 m long, has become standard. The sleepers are ballasted to 40cm beyond their ends. The thickness of the ballast bed extends to at least 30cm below the sleeper support surface. In case of poor sub-grade conditions, additional support is provided by a sub-grade protection layer approx. 30cm thick. This type of track system, also called ‘heavy permanent way’, has proved its effectiveness both for passenger traffic up to 200km/h and for freight traffic with axle loads up to 25 metric tonnes. Ballasted permanent way has also been successfully employed for higher speeds on DB Netz AG systems. For tracks with lighter loads, and under conditions of restricted space, shortened B70 sleepers 2.40m long are used, as well as rail types S54 and S49. Endless welded rails are standard all over the DB network.

The benefits of permanent way with concrete sleepers include great track-position stability, as well as low production and maintenance expense. The great intrinsic weight and the large support surface of the concrete sleepers contribute to great resistance to lateral displacement, and to put fewer burdens on the ballast. The track can be installed and maintained without restriction by means of mechanised work procedures.

Since the mid-1980s – and especially under condition of restricted space – the Y-steel sleeper has proved effective for relatively light loads as an alternative to the shortened concrete sleeper. Until now, around 400-600km of track has been installed with Y-steel sleepers in the DB AG network.

The Y-steel sleeper consist of two IB 100 S primary beams that have been curved outward by 185mm (for 60cm sleeper interval) or by 210mm (for 65cm sleeper interval). For one Y-steel sleeper, one primary beam curved to the right, and one curved to the left, are connected by L-profile sections at the bottom chords, and by square cross-members at the upper chords. This produces the characteristic forked form of the Y-steel sleeper. A double supporting surface results on one side by the connection of the primary beams. In order to provide a comparable supporting-surface situation for the rails in the forking zones, the primary beams are supplemented in this area by one short secondary beam.

The Y-shaped sleepers are installed in an alternating, right-left pattern. This produces a track panel with a lattice pattern, which is characterised by great frame stiffness and great resistance to lateral shifting.

The primary and secondary beams are bevelled at their ends and have an upper chord 2.032.00m long. Measured at the lower chord, the total length of the sleepers is 2.30m. The sleeper length at its upper chord is used to plan installation of the ballast at the ends of the sleepers. The great frame stability means that this ballast zone can be reduced from 40cm to 30cm, with reduction in turn of the ballast crest from 3.20m (for B70-2.4m sleepers) to 2.632.60m. The 10cm lower structural height of the Y-steel sleeper furthermore reduces the ballast cross-section and enables observing standard ballast-bed thickness: especially beneficial for restricted structural heights.

For its track systems, DB Netz AG has generally approved the use of St 98 Y-type Y-steel sleepers with S 15 401/402 track fastenings, for speeds ≤ 120 km/h and loads < 20,000 Lt/d on straight track, and for curves with r > 350 m. Y-steel sleepers are not used for turnouts. Owing to the greater acquisition costs of Y-steel sleepers in comparison to concrete sleepers, evidence of cost effectiveness in the form of an LCC analysis is a prerequisite for their application1.

Special measures necessary in installation and maintenance of Y-steel sleeper tracks

Various construction procedures are available for installation of track with Y-steel sleepers. Individual-sleeper emplacement with an excavation machine is possible, as is installation by track panels and assembly-line methods with a track-renewal train. Progress of 100 to 200m/h is possible. At sleeper intervals of 60cm, a total of 803 Y-steel sleepers will be required per kilometre.

Owing to the great stiffness of the lattice-like structure, special requirements are placed on the production of Y-steel sleepers in assembly-line processes. For installation of a Y-steel sleeper track with a track-renewal train, the components are laid onto a compacted ballast foundation. Before installation, expendable tamping procedures produce track positioning that does not deviate in the desired direction by more than ± 20mm from the future required position.

After completion of work by the track-renewal machine, the track should not lie deeper than -60mm below the required elevation, and within a tolerance range of ± 20mm from the required direction.

If ballast cleaning is required as part of track renewal, this is necessary before installation of the Y-steel sleepers.
To assure exact placing of track panels in the vicinity of obstacles such as at station platforms or on bridges, conformity with tolerances is necessary before work by the track-renewal train. If such conformity is not possible, manual laying of the individual sleepers is required. Immediately after installation of the track, the track position is re-measured and documented, with reference to the required height and direction. Before tightening of the fastenings, sleeper intervals are measured and adjusted as necessary.

Special tamping machines are used for tamping and alignment work with Y-steel sleepers. Since the end of each Y-sleeper lies staggered half a sleeper with respect to its other end, and not directly across as with conventional cross-sleeper tracks, tamping machines with dual-line packing are used (staggered systems).

Owing to their great frame stiffness, a completely clamped Y-sleeper track cannot be aligned like a cross-sleeper track. Practice has shown that existing directional errors, after being corrected, re-occur. In order to provide the best-possible track position during track-renewal work, the tightening of rails and sleepers must be reduced before compaction and stabilisation of the track. For the compacting step, in any event, the reference rail line for the track direction (on a curve, always only the upper rail line) must be tightened so as to be highly secure. The opposite rail must be tightened to the sleeper with a reduced torque: approximately 80 … 100 Nm. Alternatively, both rail lines can be tightened with reduced torque.

The compaction step should be limited to 30mm lifting and 20mm shifting (directional work). After compaction, the fastenings are completely tightened with full torque (up to 200 Nm). For initial stabilisation, the lifting values should not be more than 15mm, and the shifting, not more than 10mm. If greater shifting is finally required, full torque is applied only after initial stabilisation. For the second stabilisation, the lifting values should not exceed 15mm, and the shifting, 5mm. After second stabilisation, the stipulated ballast cross-section must be provided with at least 0.3m of ballast at the ends of the sleepers.

Before the individual tamping and alignment steps, the track must be filled with sufficient ballast. The compaction machines for the zones at the end of the sleepers must operate at the same stage as the tamping machines, in order to present the ballast from sliding away laterally.

While filling and grading the ballast, it is important not to shift the sleepers.

After the second stabilisation step, the continuous track is finished by the steps of tightening compensation and seamless welding of the rails. After completion of track renewal, the maximum permissible speed will be limited analogously to conventional cross-sleeper track, as a function of track loading from total train loads.

Experience gained

Deutsche Bahn has studied the track-position change of five sections of Y steel sleeper track in comparison to tracks with B70 sleepers. Inspection results from the past years have been analyzed, with the following summarised results:

• During several years of operation under various loading conditions, no significant difference was determined between the track-position change of Y-steel sleeper tracks on ballast and of conventional B70 concrete-sleeper tracks
• Until now, the two permanent-way systems investigated have required little or no maintenance expense
• The existing position of the Y-steel sleeper tracks has been rated as good to very good; all investigated track sections were in satisfactory condition. In individual cases, the longitudinal height and opposed-height position have especially demonstrated increasingly less favourable measured values, with deviations from the horizontal up to 10mm
• In all test sections, the parameters of gauge and direction demonstrate uniform measured values, as well as exact geometry of the track curves and straight sections
• The transport and installation of Y-steel sleepers has been automated over the past years and tamping performance has been enhanced. The installation and maintenance costs have consequently further approached those for concrete sleepers

The advantages of 2.30m Y-steel sleepers are apparent in:

• Track sections with locally restricted installation conditions, since the costs are less for right-of-way expansion and provision of edge paths, owing to the smaller ballast cross-section in comparison to track with concrete sleepers 2.40m long.
• Track sections in which grade rise owing to unavoidable points (e.g., grade crossings, platforms, road overpasses, and catenary lines) is not possible, or prohibitively expensive owing to semi-variable costs
• Track sections in which it is not cost-effective to preserve tamp layers and sensitive mixed zones, or to only clean the ballast during track renewal, since excavation at deep layers for installation of concrete sleepers would lead to damage to soft, unstable sub-grade.
• The complete recycling capability of Y-steel sleepers.

Summary and outlook

In addition to the concrete sleeper, the Y-steel sleeper has proved a cost-effective alternative, for avoidance of semi-variable costs, for speeds up to 120km/h and operational loads up to 20,000 tonnes per day.

Still not fully possible at present are the smooth installation and endless welding of Y-steel sleepers in radii less than 170m, especially in seamless-welded curves with unavoidable obstacle points. Further studies and measurements of curve breathing in the relevant sections, as well as the required calculated evidence, will be necessary.

In the context of cost-effective procurement of components and their maintenance, Deutsche Bahn will systematically pursue further standardisation of the permanent way. Continually increasing demands placed by the transport market on railway tracks will encourage ongoing further improvements of individual components of the permanent way.

References

1. DB Directive 820.2010 “Grundlagen des Oberbaus, Ausrüstungsstandard für Gleise und Weichen”, valid since 1 Jan 2007
2. DB Directive 824.2060 “Oberbauarbeiten durchführen; Neubau oder Umbau von Gleisen; Y-Stahlschwellengleise einbauen,” valid since 1 Jan 2007
3. Test report of DB AG, “Beurteilung der Gleislageentwicklung der Y-Stahlschwellengleise auf Schotter im Vergleich zum konventionellen Betonschwellengleis B 70,” dated 7 Aug 2007

About the authors

Andreas Beck

Mr. Andreas Beck graduated from the Technical University of Munich with a degree in civil engineering in 1997. Between 1997 and 2001, Mr. Beck worked for DE-Consult – Deutsche Eisenbahn-Consulting GmbH and has been a Specialist in Track Technology Management at DB Netz AG headquarters since November 2001.

Dr. Thomas Hempe

Dr. Thomas Hempe has a degree in structural engineering from the University of Hanover, gained in 2000. Between 2000 and 2006, Dr. Hempe was a Scientific Assistant at the Institute for Transport Railway Construction and Operation (IVE) at the University of Hanover. Dr. Hempe was awarded a Doctorate in Engineering in 2006, and he started as a trainee in Engineering/ Procurement at DB AG. Dr. Hempe has been a Subject Specialist in Track Technology Management at DB Netz AG headquarters since June 2007.