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Pandrol - Pandrol VIPA-SP track-support for leven viaduct in Cumbria

Pandrol - Pandrol VIPA-SP track-support for leven viaduct in Cumbria

30/09/2006 | Channel: Infrastructure

Pandrol has provided an innovative track-support system that protects the bridge piers from excessive vibration whilst reducing the structure's total weight

Network Rail decided to replace the steel deck structure of the multi-span Leven viaduct in Cumbria, which has been exposed to the harsh environment at the estuary of the River Leven for over 100 years. The steelwork had severely corroded and could no longer be maintained economically.

Network Rail appointed Carillion to undertake the contract, and the novel method proposed by Carillion has achieved a dramatic shortening of the programme conceived by Network Rail. The original programme utilised two separate blockade
periods in two financial years. Carillion has undertaken the replacement of the entire
deck structure within a single 16-week blockade.

The deck structure carries a double-track railway on the route between Barrow-in-
Furness and Lancaster, and Network Rail had planned to replace a single track in each of two years. Carillion developed and implemented a dramatic acceleration in the programme by removing the inspection walkways on either side of the bridge and replacing them with load-carrying structural walkways on which they mounted two gantry cranes.

This simple expedient allowed Carillion to use the gantry cranes to both remove the
old bridge units and place the new deck units. 48 of the 49 bridge spans were
replaced with 96 deck units and accompanying walkways and the trackwork was completed and opened for traffic on time at the end of 16 weeks.

Pandrol was able to provide an important innovation in the steel bridge design, which reduces the total weight of the structure, protects the original bridge piers from excessive vibration, and the local environment from re-radiated secondary
noise from the new steel structures. The Pandrol VIPA-SP system was chosen to meet the exacting demands of the renovated bridge, and assist to reduce the dead
weight to be carried by the gantry cranes.

This new track support system was chosen to fix directly to the steel structure. The traditional method previously used by Network Rail has been based upon the use
of the longitudinal timbers, fixing normal baseplates to the timbers with screwspikes.

Network Rail reports that it is increasingly difficult to obtain timber of the correct
quality and price, and that this track form also leaves a significant maintenance liability for future years, especially in such an exposed coastal location. It was decided to base the new design upon Pandrol VIPA-SP, and marginally adapt the deck design.

The Pandrol VIPA-SP rail fastening system is a highly resilient baseplate that gives a
vertical stiffness of only 20kN/mm whilst controlling dynamic gauge widening. This
high resilience effectively isolates the bridge structure from high vibration levels, which as well as protecting the 150-year-old piers, will also reduce the re-radiated noise emitted from the bridge. A similar installation in Trondheim, Norway, reduced wayside noise levels by 14dB(A). The low stiffness will also spread the train loads over a longer length of track, which permits a lighter weight bridge design. VIPA-SP reproduces the resilience of ballasted track, which alleviates the need for stiffness
transitions on the embankments, and the high electrical resistance of the assemblies
minimises the chance of circuit problems - essential for an all-steel bridge in an exposed, wet environment.

The coastal environment of the Leven estuary on the northern shore of Morecambe Bay suffers from the very fastmoving tidal flows, which make the Bay notorious – and extremely dangerous. The bridge decks were designed with longitudinal camber, in part to shed seawater, which can reach the deck in very high spring tides.

A major design parameter was for easy and rapid replacement of individual VIPA
baseplates should it become necessary. It was decided to use an arrangement of steel \'stools\' which would reduce the effects of the longitudinal camber, and ensure that the stools were co-planar by altering the leglengths. This provided access to the holddown assembly for the baseplates. The longitudinal camber was further accommodated by the resilience of the VIPA assemblies. The throughbolting system for the VIPA could use nuts and bolts to secure the baseplates and the nuts could be easily located on the underside of the steel \'stools\'.

The VIPA system has been designed, recognising the need for very close tolerance on the head of the rail during train operations. This tolerance is very much tighter than normally achieved by structural tolerance, and many of the discussions on the project centred on the need to achieve vertical tolerance of the rail to ±1mm. The
steel bridge design became a \'bottom-up\' construction on site, requiring the use of
height-adjusting shims to achieve tolerance, rather than pourable grouts. It was feared that grouts on steel would perform badly in this coastal environment.

New methods
Such construction methods are relatively new in this country, but Pandrol has experience of many worldwide installations of its products using various construction methods. Thus, Pandrol worked closely with Carillion and Network Rail from the design phase and gave advice about construction methods. Pandrol also gave hands-on training and support on site, and provided a comprehensive Operations and Maintenance Manual to the maintainer.

The individual steel deck structures were delivered by road to the site depot outside Ulverston station and transported by rail the three miles to the northern end of the viaduct. The \'stools\' were used as lifting points to attach the gantry cranes upon
arrival at the northern end of the viaduct. The deck units were installed on the new
bridge bearings as accurately as possible, and then the trackwork commenced. The
VIPA-SP assemblies were delivered to site in pre-assembled units, which, combined with the use of Pandrol FASTCLIP within VIPA meant that installation time of the assemblies and rail was minimised.

The vertical height was adjusted using a variety of shims packed to fill the measured
gap. Pandrol supplied a range of three differing shims of 3, 5, and 10mm thickness. By combining these shims, the vertical accuracy was achieved. Carillion expected the worst-case construction tolerance to be ±22 mm, hence the bridge was designed with a nominal 22mm of Pandrol\'s height adjustment shims, which could be increased or decreased after a survey once the decks were in place. Thus a maximum 44mm of shims and the thick steel stools required bolts lengths of up to
210mm. Pandrol worked closely with Carillion to gain approval for such long bolts, and performed fatigue tests and theoretical calculations to prove the strength of the bolts to Network Rail.

The VIPA-SP baseplate provides slotted holes for lateral adjustment, which allow
± 20mm for lateral alignment. The final track tolerance was achieved using laser
equipment, which was aligned using traditional survey techniques. The photographs show the accuracy of the line and level of the rail, and Network Rail Territory office and the local maintenance department have been quoted as being delighted with the end results.

Network Rail has expressed the desire to adopt Pandrol VIPA-SP as the standard
solution for steel bridges to eliminate the need for longitudinal timber and reduce the
maintenance liability, and many other steel bridges are currently being specified to use Pandrol VIPA-SP. Thus these bridges will also benefit from reduced noise and vibration emission, enhanced ride quality, significant vertical and lateral adjustability, easy installation and high electrical resistance provided by Pandrol VIPA-SP.

The Leven viaduct has proved a successful bridge replacement contract for Carillion and a major move to change the way that Network Rail specifies the track support system on steel structures.