Over the recent Easter period, Spencer Rail completed a first in this country by successfully moving an 800-tonne pedestrian subway into its final position on a bed of compressed nitrogen gas. Writes Collin Carr

The work took place at Rochester station as part of the second phase of the overall East Kent Resignalling Programme which will concentrate control of a significant portion of Network Rail’s Kent route into the East Kent Signalling Centre (EKSC) at Gillingham.

Network Rail’s prime objective for this project is to improve asset condition, provide capacity and capability enhancements and deliver operational savings within the project area. As a consequence, the scheme includes the total renewal of life- expired signalling on a section of the Chatham main line between the end of the Phase 1 project near Sittingbourne and the Victoria Signalling Centre control area near Longeld. This is in addition to the re-control and partial renewal and modification of existing signalling on the connecting routes.

Assessing asset condition

To progress these objectives, Network Rail procured the services of Vertex Systems Engineering to carry out an initial assessment of asset condition and undertake a whole life- cycle cost assessment comparing the various technologies considered appropriate for the scope of the East Kent Phase 2 project. This programme of work was completed by March 2013.

A package of work, valued at £147 million, was put together by Network Rail as a collaborative contract compliant with BS11000. The collaborative partners for the project were Atkins for the signalling works and for the overall project design work, Balfour Beatty Rail for the trackwork and Spencer Rail for all the civils work which was valued at approximately £25 million.

This project, to upgrade the 33-mile stretch of East Kent’s railway network by replacing the old signalling equipment and increasing the capacity and capability of the region’s rail network, must be completed by Spring 2016.

Earlier this year Spencer Rail successfully handed over one of the most challenging aspects of the project which was the installation of the 800 tonne pedestrian subway mentioned earlier. The subway forms part of the new station that Spencer Rail is building at Rochester.

Designing a solution

Spencer Rail was responsible for early contractor involvement in engineering a solution for constructing and installing a new subway as part of its contract to rebuild Rochester station. To facilitate the planning, Spencer’s team was initially co-located with the designers at the Atkins offices in Croydon.

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There was plenty of room to construct the reinforced concrete subway, which is 28 metres long, 7.6 metres wide and 4.0 metres high, alongside the railway embankment at Rochester and it was completed well in advance of the 96-hour Easter blockade.

So, all that was required was to determine how Spencer Rail, after excavating through the embankment the previous day, would slide the 800 tonne structure approximately 36 metres into its final resting place. A number of propulsion systems capable of sliding the subway into position were considered and, after careful evaluation, Spencer Rail decided to procure the services of Freyssinet. This was a big decision and Tom Kerins, operations director for Spencer Rail, was very pleased with the result. He emphasised that they made the right choice with Freyssinet which had been an excellent company to work with.

Remember your history?

Now then, can you remember your engineering history? Eugene Freyssinet was the person who invented and patented prestressing which, as we all know, helped to revolutionise the art of building – so clearly Spencer Rail knew that they had put their subway slide in good hands.

This slightly more modern Freyssinet Autoripage® technique has been designed for use when a structure is built to one side of the railway formation, after which the embankment is removed exposing the newly created ground level ready for the structure to be slid into its final position. Freyssinet has used this technique in Europe more than fifty times over the last ten years but it is the first time it has been used in this country.

Clearly, the weather was a key issue, especially when excavating the embankment and re-establishing the formation around the subway afterwards. C J Pryor was the subcontractor that supplied the earthworks plant and expertise and, although there was some rain, this phase of the work went well.

Continuous skidway installed

Freyssinet installed 10-metre-long steel skidways two weeks prior to the slide. These were lined, levelled and bolted together to form a continuous track either side of the subway. In addition, the construction of the subway box was modified by Spencer Rail in liaison with the Freyssinet design department to accommodate the Autoripage technique by adding 10 cantilever corbels, five on each side.

The corbels measured 1.6 metres by 0.7 metres wide by 2.5 metres high. They formed the lifting points for ten 385-tonne capacity hydraulic lifting jacks which, complete with the Air Pad Sliding (APS) feet, were placed on the skidway and bolted to the underside of each of the corbels. In order to keep the support pressure uniform, the jacks were linked to a central control system.

This assembly of jacks created a lifting capacity of 3850 tonne, which was significantly greater than the required 800 tonne. However, the redundant capacity ensured that if one jack should fail, the others would cope with the added weight. In addition, it reduced the bearing pressure on the ground for each jack position by spreading the load over 10 points.

A floating subway

Each jack was fixed to a 1125mm square APS foot. Nitrogen, an inert gas that is less susceptible to temperature change than air and therefore behaves more predictably, was then injected under pressure and contained within the footplate by a rubber seal. This meant the subway was effectively floating and that the coefficient of friction was reduced to 1% as it was pushed along the carefully laid skidways.

The push-pull jacks had a 1200mm stroke and they automatically clipped onto the skidway, retracted, re-clipped and repeating this movement throughout the sliding process. The jacks were able to achieve a speed of 20 metres per hour which saved vital time during a blockade operation.

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To make the process a little more challenging there were a number of cables on the embankment that could not be moved so they had to be supported on a temporary service bridge spanning the embankment excavation. In addition, a sheet piled box had to be constructed to accommodate a lift shaft that ran alongside an opening in the subway wall.

These all meant that the slide had to be carried out with a minimum of deviation. In fact, the maximum tolerance allowed was 10mm. This proved to be quite a challenge for Spencer Rail’s senior engineer, Anuk Perera, and site engineer Ryan Hughes. There was no hiding place for them and they had to get it right which they did with the subway sitting in its final resting place just 8mm off line. It might be easier to take the first penalty kick in a World Cup Final than to have this responsibility.

New Technology

There were no accidents or incidents throughout the work and the views of the team involved are best expressed by Tom Kerins, operations director at Spencer Rail, who said: “Installing the subway was an extremely important and challenging aspect of the works at Rochester and it is testament to the expertise, forward-thinking and hard work of our team that we have completed the installation successfully. Moving such a large structure into place presented us with a number of logistical challenges but, by using innovative technology, it was completed without incident and ahead of schedule. It was the first subway slide of this type carried out in the UK using this technology.”

Now that the subway slide can be stored in the annals of engineering history, Spencer Rail is able to concentrate on the many other aspects of this project including the creation of a new ‘Access for All’ three platform station at Rochester. This, of course, includes the pedestrian subway and the decommissioning of the old Rochester station which is 500 metres away.

Over the next two years, apart from building the new station at Rochester, Spencer Rail will also be extending the platforms at numerous other stations, increasing their capacity from six to 12- car trains, and decommissioning eight signalling boxes throughout the East Kent region once the new signalling system has been installed.

This is all interesting stuff, but moving 800 tonnes of concrete structure a distance of 36 metres with less than 10mm deviation on a bed of compressed nitrogen has got to be a highlight in anyone’s career. It’s nearly as good as inventing prestressing – but not quite.