It seems incredible – to be able to disconnect a section of double track main line, lift it, turn it through 90 degree and then return it to its original position with millimetre precision; and all this within the space of minutes. It would tax the ingenuity of most modern engineers and yet this has been happening on our national rail network, day in and day out, since the 1880s.
Selby Swing Bridge, which until 1983 formed part of the East Coast main line, is an heroic and iconic marvel of Victorian engineering excellence. Now, after 125 years of operation, it has undergone the most extensive refurbishment in its history.
Constructed for the North Eastern Railway in 1889 by Nelson and Co of York and the Cleveland Bridge Company, Selby Swing Bridge was a marvel of its day. It has five spans, with Span 1 being at the south end. Four of the spans are conventional, but the 40-metre long Span 4 can be rotated through 90° to lie parallel with the northern bank of the River Ouse.
Mounted on a massive bearing and turntable at the river’s edge, its weight is taken by 24 conical cast steel rollers running in an 8.8-metre diameter cast-steel race. The swing span is of an asymmetrical ‘hogback’ plate girder construction.
Its shorter landward section is loaded with 93 tonnes of cast iron weights to counter-balance the longer river span. The adjacent fixed Span 3 has a similar plate girder construction, but is symmetrical.
Originally, to operate the swing span, a mixture of water and glycerine (to prevent freezing) was pumped by steam engines and double-acting force pumps into two hydraulic accumulators. Each accumulator had a 10-inch diameter plunger and a 17-foot stroke with sufficient weight to produce a pressure of 900lb per square inch. This high-pressure fluid powered two three-cylinder Armstrong hydraulic engines housed inside the control cabin directly above the swing span bearing.
Partial modernisation came in the 1960s when electric powered hydraulic pumps replaced the Armstrong equipment, again housed within the control cabin. Hydraulic hoses fed two seven- cylinder ‘Staffa’ motors mounted at bridge deck level. New control panels were installed at this time too.
Vertical shafts from the motors drive bevelled pinion gears around a circular rack fixed to the lower roller path. The operating sequence involves, firstly, the withdrawal of proving bolts by mechanical rodding. Hydraulic action is then used to pull back heavy tapered nose bolts at each end of the swing span and movable resting-blocks are also withdrawn.
A knuckle gear then raises each end of the bridge by a half inch to give clearance for the swing.
All of these operations, including the starting and stopping of the swing movement, have been manually controlled, involving some considerable operator skill. The control panel, looking like something out of Flash Gordon, had an array of levers and lamp indicators for each individual part of the bridge operating sequence. With no braking system provided, stopping the bridge exactly in the right place to engage the nose bolts was something of an art. Imagine the clang from 500 tonnes of moving metal hitting the end stop if there was an error of judgement!
Change of plan
Things were set to change during 2013, when a refurbishment scheme costing £4 million was to have taken place, but then the Hatfield colliery slip happened. With the Doncaster – Goole route closed to all traffic, trains to and from Hull were diverted via Selby. It was, however, a blessing in disguise for the Selby Swing Bridge scheme, as Network Rail scheme project manager Darryl White explained.
“The original plan would have involved a six week blockade during the summer of 2013, during which we intended to reconstruct Span 1 of the bridge and strengthen Spans 2 to 5. We were also to replace the waybeams on Spans1, 2 and 5 and install steel rail bearers and Pandrol Vipa resilient rail fixings on Spans 3 and 4. In addition we would have grit blasted and repainted the entire bridge.”
With up to 130 trains a day now using the bridge a rethink was clearly required. “It is to Network Rail’s great credit that the extra time that became available was used to such good effect,” continued Darryl. “What was a £4 million refurbishment scheme became a £14 million project that has not only allowed us to raise the speed and weight restrictions on the bridge, but also to ensure that no further major maintenance will be required for decades.”
Darryl’s amended remit now included the reconstruction of Spans 1, 2 and 5 and the strengthening of Spans 3 and 4. In addition, all of the timber waybeams were to be dispensed with in favour of steel rail bearers and the Vipa rail fixing system. On Span 3 this would also require the replacement of three cross girders which carry the rail bearers.
Significantly, the entire hydraulic system and its controls were also to be replaced. The time window for the scheme was, however, to remain the same as before at just six weeks. Multi-discipline main contractor Kier Group was appointed, having responsibility for all aspects of the project including the signalling and track works.
The short timescale also involved a rethink on the methods to be used in repainting the bridge structure. Grit blasting would have required the use of a full EnviroWrap system and it also carried the high risk of grit and paint debris contaminating the bridge operating mechanism. The solution has been to use an innovative system produced by Canadian company Termarust Technologies. Rather than using grit, a water jet-wash operating at a pressure of 8000 PSI and a temperature of 90°C is used to remove the old paintwork. The paint debris takes the form of flakes which can be collected in a Terram membrane, allowing uncontaminated water to pass through.
“There are substantial health benefits to utilising this system,” Darryl expained. “Lead in the paint isn’t atomised, therefore the painting sub contractor does not require full air tight suit, face fit mask and breathing apparatus, or blood testing. This means that the operatives can be utilised more frequently, as there is no exposure to lead.”
Normal weather protection is sufficient for the process, rather than a complete environmental cocoon. The new coating applied to the bridge involved a wet-on-wet procedure with no curing time between applications, which again saved time. Whereas grit blasting and conventional painting would have taken six weeks the Termarust process took just two. In fact, this was crucial to the success of the entire project in its limited time window.
Preparatory work began as long ago as October 2013, but the main works commenced at 23:00 on Saturday 26 July. The bridge was then to remain closed to rail traffic until 05:25 on Saturday 8 September. Rather an exact and tight timescale, but such is the nature of modern rail engineering projects.
To facilitate access from dry land, Span 4 was swung to lie parallel with the riverbank throughout the blockade period. This necessitated work sites to be set up both north and south of the bridge. As part of the strengthening works, the swing span required jacking at both ends in order to lie exactly as it does when in its closed position – even this massive structure can flex a little under its own weight!
The riverbank is too soft to carry such a loading, so the preparatory works involved piling 24-metres down to the sandstone bedrock in order to support the lifting jacks. This in itself created a complication, because the exact location of under-bridge ducting wasn’t known. Fortunately, of the three ducts one was empty, allowing underground mapping specialists Infotec Consulting to use their ‘PipeTrack’ gyro- based technology to produce a route and depth survey, accurate to within 5mm.
During the blockade, Selby station, immediately south of the bridge, was transformed into a terminus station. This involved the installation of buffer stops and alterations to the signalling equipment. A temporary footbridge to facilitate disabled access was also constructed. On the north side of the river, the Potters Group rail freight depot has daily traffic, so here too a buffer stop was required. For freight trains to access the terminal it was necessary to employ top and tail working, using only the Up line from Gilberdyke Junction.
The preparatory works also included the strengthening of Span 3 to allow scaffolding to be under-slung from it. Because the river has a very strong tidal current there was perceived to be a risk of flotsam striking the scaffolding at high water. Protective booms were therefore deployed on both sides of the span. Once the scaffolding was in place, the cross girders could be assessed for deterioration and condition- led repairs. This was accomplished by visual inspection and ultrasonic testing.
In the event, only 10-15% of the cross girders required strengthening. With many crevices inaccessible for painting, the Termarust surface treatment in this area included the use of an oil- based penetrant to provide protection against corrosion.
During the first week of the blockade, Spans 1 and 2 were lifted out using a 500 tonne crane. On the north bank of the river, Span 5 was similarly lifted using a 150 tonne crane. Cracking and other defects within the brick piers were repaired before new plated spans were installed. Ousegate, a town thoroughfare, passes beneath spans 1 and 2 with very low headroom. As a result, these spans have carried a red bridge bash rating. The opportunity has now been taken to install concrete collision protection beams. This will reduce the bridge bash rating to double amber – a significant improvement.
Kier Group has subcontracted most of the mechanical, electrical and hydraulic engineering work to AMCO. Entirely new electrical and control equipment cubicles, twin hydraulic pumps and a standby generator have been housed within the ‘hydraulic accumulator’ building adjacent to the north end of the bridge. A one-metre high mezzanine floor will protect the equipment in the event of flooding. Stainless steel hydraulic pipe work reaches the bridge via an original stone-lined culvert.
Mechanically, the operation of the bridge follows the original principle. The two Staffa hydraulic motors have been refurbished and re-installed. As before, one motor alone is sufficient to operate the bridge. The gearing itself has been well maintained over the years and required little attention, but the bridge pintle (main bearing) was found to have been moving. It has been strengthened and fixed in place. Repairs were also required to cracks found in the pintle shroud.
The positioning of the moveable span is now sensed using opto-electronics and a soft start/stop feature means that the bridge automatically glides to an exact stop – no more clangs! The bridge operator now has a touch screen monitor rather than an array of clunking levers. It’s not all armchair operation, however, as the proving of the bridge position for signalling purposes still makes use of bolts that are worked by rodding from a 2-lever ground frame within the cabin. For the signalling releases, the existing IFS panel has been retained.
The track across the bridge has been entirely replaced using high performance rail. At each end of the swing span the rails now have chamfered ends in order to reduce the gaps. As well as reducing maintenance, this also causes less noise as trains pass over the bridge.
So much for the nuts and bolts of the onsite work but, as with any large project, detailed consultation and planning was crucial. With such a limited timescale for the blockade, site constraints and the complex technical work involved, there could be no room for error. Three years of detailed planning, coupled of course with the close cooperation and efficiency of the on-site contractors, has now come to fruition.
It is a great credit to all concerned that the project was completed exactly to plan and the railway was ready for business right on cue. In fact, the blockade was given up early at 22:45 on 7 September and the first train to pass over the reopened bridge was an ECS (empty coaching stock) move at 06:44 the following morning.
The speed limit over the bridge has been raised to 25mph, this being the line speed at this location. After a four-week assessment period, weight restrictions are to be lifted and an OPPOSS Restriction, which has prevented freight trains passing each other on the bridge, will be removed.
Another benefit is the reduction in maintenance costs. Network Rail estimates that the steel work of the strengthened Spans 3 and 4 will be maintenance free for 60 years. The new Spans 1, 2 and 5 will be good for 120 years, the hydraulics for 20 years and the paintwork for 25 years.
Darryl White is justly proud, not only of the work itself, but also of his colleagues and contractors. A local man himself, Darryl was keen from the outset to involve the town community. Organised jointly between Network Rail, Kier Group and the train operating companies, drop in events kept the public informed.
Network Rail and Kier have also liaised closely with Barlby Bridge Community Primary School, which is located close by the bridge on the north bank. They gave a presentation on the project and organised a safety poster competition in which the pupils were asked to find the most creative way of getting across site safety messages to residents and visitors in Selby. The competition was won by 8-year old Wiki Czech who had her poster displayed on the worksite hoardings in Ousgate.
As a gift to the school, Network Rail and Kier have also built an outdoor learning centre with seating, a gazebo and new fencing. “It is great to be able to give something back to the local community that will make a difference for years to come,” Darryl commented. “We were also very impressed with the standard of the entries of the poster design competition, and the winner is a bright and colourful way of helping to remind people to stay safe near to the site.”
Perhaps this interaction will inspire at least some of those young people to take up engineering as a career. They could have no better role models than Darryl and his team. Resplendent in its new finish, Selby Swing Bridge stands as a monument to Victorian ‘can do’ mentality, now dovetailed with modern innovation and engineering excellence.