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Ballastless track ensures a high-quality ride

01 Apr 2007

Rheda 2000 ballastless track has been used in all station areas and pointwork on Taiwan's high speed line. The track design was adapted to suit the elevated structure and seismic protection standards. Special slab designs had to be developed to meet specific alignment constraints

Dipl-Ing Torsten Bode
Senior Sales & Project Manager for Rail.One International GmbH.

Dipl-Ing Arnold Pieringer
Senior Consultant for Ballastless Track at Rail.One GmbH Pfleiderer Track Systems

LINKING 14 major cities along Taiwan's west coast, the recently-opened Taipei - Kaohsiung high speed line looks set to become the island's most important transport mode. A fleet of 300 km/h Series 700T Shinkansen trainsets will provide a mix of fast and semi-fast services over the 346 km route, with an end-to-end journey time of 90 min envisaged for the headline expresses making just one intermediate stop.

Construction of the line has been an engineering challenge in many respects, not least the trackwork. Strict requirements were laid down by Taiwan High Speed Rail Corp in respect of reliability, maintainability and availability of the permanent way throughout the specified life cycle. The final trackwork is a complex mix of Japanese and European ballastless track technology, with J-slab Shinkansen track used for the plain line sections and Rheda 2000 in the station areas and the cross-capital tunnel. In total there is more than 75 km of Rheda 2000 trackwork and 120 ballastless turnouts.

As it lies above a junction between the Eurasian and Philippine tectonic plates, Taiwan experiences a minor earthquake every other day on average. Because of this, and the low-lying land along the route, about 282 km of the line has been built on elevated structures. Foundations for the viaducts and bridges were installed over drilled piles, some of which are up to 60 m deep. Additional piles were needed at locations where seismic vibrations cause the soft alluvial soil layers to 'float'.

Even though ballastless track has been used successfully for decades, many projects are still designed for ballasted track, and this was the case with THSRC. After the decision was taken to switch to a ballastless trackform, it was necessary to ensure compatibility with civil engineering structures.

Rail.One was responsible for detailed design of the Rheda 2000 trackwork, which involved rail stress analyses, dynamic track-structure interaction evaluations and structural calculations for standard and customised trackwork elements. To ensure efficient just-in-time delivery of more than 118 000 concrete twin-block sleepers for the plain line sections, as well as monobloc sleepers for special trackwork and turnouts, Rail.One established a local office and installed a mobile sleeper manufacturing unit in co-operation with a domestic concrete manufacturer.

Elevated plain track

Although ballastless track is widely used on bridges and viaducts in Germany, the Federal Railway Office has not issued a general approval for these installations, preferring to consider each application separately. There are various design guidelines which have to be modified to suit the selected track type - for example, the Rheda Berlin design adopted for the Köln - Frankfurt line.

On plain track in Taiwan, the slab segments are separated from the concrete protection layer by an intermediate protection layer (Fig 1). The load-transferring cam plates are surrounded by elastomeric bearings to protect the structure from live loading forces and other constraints. To simplify construction, a standard slab length of 7 500 mm was adopted, although special slabs were developed for specific areas these range from 4 500 mm to 9 000 mm, supported by three or five cam plates.

Turnouts on bridges

In general, the slab design for the turnouts followed the same arrangement as the plain track, with cam plates attached to the protection layer. However, these are linear cam plates which support the turnout slab in both the longitudinal and transverse directions. This design had previously been adopted for the reconstructed S-Bahn ring line in Berlin, which also has turnouts located on viaducts.

The turnouts for Taiwan were manufactured in Germany by BWG (p223), and have individual point machines at each turnout drive in the switch rail area, plus separate drives for the swing-nose crossing. These drives require a recess approximately 170 mm deep in the supporting slab, which precludes any arrangement of cross cam plates in the same location. Consequently, the cams are arranged behind the recesses. The same situation applies to the area around the crossing, where conflicts between the recesses and the cross cam plates necessitate the use of slab segments up to 80 m long.

Further influencing the slab segmentation are special restrictions on the relative displacement between the slab and rails within the turnout fasteners. Fig 2 shows the basic schematic arrangement of cam plates for a crossover turnout.

The number of cross cam plates per turnout was varied to suit the segment length and the forces to be applied by live loads and temperature fluctuations. Rail stress calculations and load cases for the bridges influenced the detailed design of the slabs for single and double crossovers, as well as for standard turnouts. For all the Rheda 2000 trackwork monobloc sleepers were used to ensure a monolithic connection between the sleepers and the concrete track slab.

Rail expansion joints

As part of the civil engineering design, efforts were made to avoid the necessity for rail expansion joints, which were largely successful. However, in two locations special provision for expansion was needed in the Rheda 2000 track sections.

One location was to the south of Taichung station, where the line is carried on three parallel steel bridges for a total length of 410 m (right). The second was only a few km further south, where there was a continuous hollow box girder bridge 220 m long. Owing to their thermal behaviour and general geometry, these bridges required the provision of expansion joints at both ends of each structure - 16 in total. Based on the calculated displacement of the steel bridges, it was decided to use BWG's Type 600 joints, which allow for a maximum expansion of 600 mm.

Whereas concrete bridges are normally designed with their thermally-neutral axis in the middle of the bridge, the steel bridges here have fixed bearings on one of the two middle piers, resulting in a maximum expansion length of approximately 300 mm. The bridge REJs had to allow for movement cycles resulting from temperature, shrinkage and creeping, train acceleration and braking, as well earthquake forces. The seismic load case was a major factor, as this represented a considerable proportion of the bridge movements. The bridges are equipped with dampers to protect against fast movements such as earthquake displacements or structural movements caused by train acceleration and braking. But the dampers do not provide any security against slow displacements such as daily or seasonal thermal movements.

The middle bridge of the three-span steel structure carries the two main tracks, whilst the outer spans each accommodate the platform loop tracks and a diverging connection to Wujih depot. Thus both of the outer spans support two turnouts. Guard rails are provided throughout the steel bridge trackwork, including the expansion joints.

Rotating and floating slabs

The track design required the development of two new elements, known as rotating and floating slabs.

The rotating slabs are installed on the outer steel bridges south of Taichung, and were necessary because of alignment constraints in combination with limiting topographical features - the bridge location was determined by the crossing of a road and a river. Because the depot tracks must diverge at this point, it was necessary to position two turnouts on each of the steel bridges. This in turn led to tracks crossing the bridge civil expansion joint at an angle.

When a track crosses a CEJ at an angle other than 90°, any longitudinal bridge movements lead to lateral offsets in the track. These lateral offsets occurred at eight locations (two tracks at each end of each bridge span). At four of these locations, the magnitude of the offset exceeded the allowable limit, which in turn required the use of specially-developed rotating slabs. These 9 m long slabs are supported at one end by a circular and elastomerically-buffered cam plate, and at the other by plier devices (photo p220).

The rotating slabs were designed to distribute the local lateral offset over a longer length, so as to assure an acceptable dynamic ride as the trains arrive or depart from the station platforms at the planned operating speed for the area. The circular cam plates were manufactured from stainless steel, and fitted with a vulcanized elastomeric bearing between the positive and negative cam plate frames. They were supplied by a German company specialising in custom-made bearings, but because of the complex technology had to be delivered as complete elements.

Half of each cam was installed in the concrete protection layer of the bridge, and half in the track slab supporting the rail expansion joint, with shear studs providing the required connection to the surrounding concrete. Two stiff iron I-beams were installed at the end of the CEJ opposite the next track slab, and enclose the rotating slab at the opposite end to the circular cam plate, whilst maintaining the required track alignment. The rotating slabs were installed on pre-tested slip sheeting to reduce the friction between the slabs and the supporting concrete protection layer.

The so-called floating, or turnout extension slab was needed to reduce the relative displacements between the bridge and the turnout crossing area. Because of track alignment constraints, some turnouts had to be located near CEJs. The effect of thermal deformation of the bridge on the turnout rails, and consequently on the rail fasteners, subsequently increased to such an extent that special countermeasures became necessary.

The thermal effects could only be restricted by the use of an artificial extension to the track slab over the bridge expansion joint. This made it possible to increase the number of fasteners, and thus to reduce the relative displacements between rail and fastener. In turn, this cut the slippage of the rail in the fasteners to acceptable levels.

In the area of the CEJ, the turnout slab had to be physically interrupted to ensure access to the expansion joint, so the adjacent track slabs had to be connected by auxiliary rails between the running rails. To ensure that the extended slab would move along with the main turnout slab, the extended slab was supported on slip sheeting. Lateral guidance of the extension slab is ensured using a longitudinal cam plate in the axis of the straight track. The longest of these extension slabs is 12·5 m long.

Dynamic calculations

Dynamic CWR and uplift force calculations were undertaken to assess in greater detail the interaction between the trackwork and the supporting structures. Dynamic effects primarily result from the interaction between the moving train and the bridge, whilst CWR and uplift forces arise from interaction between the track and the bridge itself. Whereas the first two types of calculations are normally necessary for ballasted track, the uplift force calculation is only required for slab track.

The uplift force calculation is used to establish a service limit state for the uplift and compressive forces in the fastenings at the bridge deck ends. These forces result from end rotation under train loading in the single bridge spans used for typical applications. Their influence is only minor compared to the dynamic behaviour of a highly elastic pad in the rail fastening system, installed according to European standards with a stiff intermediate layer. The track primarily transfers the train loading to the bridge, which in turn produces dynamic interaction in relation to the speed of the train, the length and stiffness of the bridge girder, as well as the masses of both elements.

The CWR calculation inherently provides a check on rail stressses. Although calculations for a ballastless trackform reveal a slightly different slip friction curve than that for the fastening system used on European standard ballasted tracks, the principles and results for the two systems are analogous. The limiting values for additional rail stresses are likewise slightly different, but only in terms of compression stresses, and in fact the ballastless designs perform more favourably.

The calculations showed clearly that the results are primarily influenced by the characteristics of the civil engineering structures, and that the choice of track system has only a minor impact on the results. The CWR results could be influenced slightly by the use of low-friction clamps in certain areas, and uplift force calculations could be affected by the introduction of special fasteners. The small number of expansion joints, low-friction clamps and special fasteners are evidence that the elevated structures in Taiwan have been well designed.

Guard rails

Guard rails are provided along the track to limit the consequences of a derailment and the impacts on the trackwork or the civil structures. Rail.One's experience suggests that guard rail specifications are subject to fundamental debate on every high speed railway project. This also applied in Taiwan, although THSRC included its specifications for derailment protection features in the original tender documents. These specifications stipulated guard rails on tracks over steel bridges, which correlates with German standards.

Guard rails were installed on all four steel bridges laid with Rheda 2000, and on specific transition lengths to the adjacent viaduct sections. The UIC33 profile guard rails are installed with a clearance of 180 mm to the rail head, held in place by welded or cast steel trestles mounted on special sleepers, or using stainless steel anchors drilled and glued into the slab concrete.

Another challenging design task was to clarify the relationship between the guard rails and rotating slabs which act in conjunction with the expansion joints.

  • CAPTION: Aerial view of steel bridges south of Taichung Station, showing connecting line branching off to Wujih depot
  • CAPTION: Rotating slab under construction on one of the steel bridges at Taichung, showing the fixed 'plier' end in the foreground
  • CAPTION: Fig 1. Schematic arrangement of slab segments and single cam plates on the concrete protection layer for plain track sections
  • CAPTION: Fig 2: Crossover turnout cam plate design (not to scale)

 

 

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