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Scarborough Bridge – Monte Carlo or bust
Graeme Bickerdike
It was a very ordinary structure, comprising two spans across the Ouse to the north of York Station. Its replacement, installed over the February half term, is also pretty average. You might wonder then why I'm wasting your time telling you about it, or indeed why the project cost £6 million. Thing is, this wasn't a typical bridge reconstruction. The key to its delivery – overcoming the site constraints – had been turned in advance of the nine-day blockade which saw the new steelwork craned into place. That's why I can write this beforehand, leaving the project's denouement to be conveyed in a few dramatic pictures. And given the size of the crane, I'm confident they will be very dramatic. Judge for yourself.
Head towards the coast from York and the first structure you'll encounter – just 40 metres off the end of Platform 4 – is Scarborough Bridge, unsurprisingly identified as YMS/1. When the route opened in 1845, trains crossed the river on timber arches, the sockets for which are still visible in the masonry pier and abutments. The latter incorporate archways allowing pedestrians free access along the banks. The spans were rebuilt in 1875 to comprise four 22 metre wrought iron main girders, ornamental lattice girders to both sides and a timber deck. The east side features a footbridge, tied to the main structure but effectively independent.
Corroded ironwork – for which the city's vast pigeon population must bear some of the blame – and rotting timbers are only to be expected after 140 years, but there eventually comes a need for intervention. The bridge is not heavily trafficked (two First TransPennine Express services use it every hour) but its RA0 load rating meant bridge examiners had to be in attendance whenever a steam special crossed. And its movement was not kind – often fatally so – to the bats which roosted in its nooks and crannies. Studies concluded that the baseline effort involved in doing any major work on the structure made reconstruction the most cost-effective option, rather than just strengthening what was already there.
The four new weathering steel decks, manufactured in Chepstow by Mabey Bridge, are U-shaped in section and each is supported by a pair of plate girders. Whilst not aesthetically pleasing, the decks will demand no substantive maintenance for 60 years. In any case, they are screened by newly-fabricated latticework on the west side – emulating the original – and the existing footbridge to the east.
Story Contracting, the principal contractor, pinned down the buildability early by working closely with Network Rail's in-house design team. The decks are swan-necked, sitting on shallow (200mm) cill units from Moore Concrete and thus avoiding the need to break out any stonework. The arrangement brings with it full RU loading. The pier and abutments remain in good condition although they have benefited from some stitching. Added on the west side is a cantilevered walkway to provide a safe crossing route for track staff.
And that's it, except…
York's central area is compact – a function of its historical roots – with tight streets and space at a premium. Valuable railway land has been sold off for housing development, a reality which prevents larger vehicles from reaching the triangle between the East Coast main line and the York-Scarborough. This area could then only act as a secondary compound for the works, with bulk materials driven onto site during possessions of the ECML.
The main focus was on Marygate car park, a council-owned facility north of the river which was taken-over in phases. The railway fortunately forms its western boundary. Network Rail's project team established a 'command room' here last November, cohabiting with Story. Although the car park is fairly generous, access can only be gained via a narrow street off the A19 and involves negotiating a sharp right-hand turn with buildings on each corner.
Separating the south side of the car park from the river is a row of houses and wide pedestrian walkway. Alongside the end-terrace – now a bed and breakfast – is a gap of about 10 metres to the toe of the bridge's approach embankment. It was in this gap that a crane would have to operate, the longest lift being 75 metres over the river to place the cill unit onto the southern abutment.
Forget the Romans; York is a railway city. The train operators and Network Rail have a substantial presence here and those members of their workforce who use the footbridge were no doubt inconvenienced by the need to close it. So the job was very visible, adding to the burden on the project team's shoulders. Arguably, the temporary loss of the footbridge was felt more keenly than that of the train service.
It is a busy thoroughfare, forming a key route to the station and nearby businesses. In mitigation, there had to be a big hit of publicity in the York Press, together with signage on the ground and letter drops.
Also affected from time to time was the Ouse, with a full closure in place during the main works and partial closures for specific activities such as the erection of a scaffold, supplied by the Wood Group (formerly Pyeroy). This comprised a footbridge onto which the lengthened S&T cabling was slewed as well as providing access across the river when the footbridge was blocked off; working platforms were also wrapped around the pier and abutments. To combat the threat of flooding – one of the project's two biggest risks – debris boards were installed around its base, designed to cope with a river level reaching the top of York's flood defences.
For years, trains have being sharing Scarborough Bridge with a 16-inch main owned by Yorkshire Water. Typically, stats companies charge substantial sums for the removal of their services and rarely do so at a pace in sync with a project timeline. However an agreement was already in place here that allowed Network Rail to request the main's prompt removal, at Yorkshire Water's expense. To fulfil this obligation, the sections at either end were disconnected and removed, leaving the remainder to be lifted out with the lattice girder.
In the railway's early days, members of the public accessed the bridge by stairways within the abutments, emerging between the two tracks and then crossing whilst trains passed on either side. The inevitable happened on more than one occasion, eventually prompting the installation of the footbridge. But the stairways left voids within the abutments which have now been filled, not with foam concrete but a structural resin called Benefil, produced on-site from a liquid base. A bit like Heineken, it gets into places other materials cannot reach.
Identified as the right machine to fulfil the big lifts was Ainscough's Demag CC 2500-1, configured with an 84-metre boom and thus reaching 14 metres higher than the central tower of York's nearby Minster.
A crawler crane, it boasts a 500-tonne capacity and superlift attachment which, for some loads, can itself weigh up to 250 tonnes. It comes to site on the back of 17 wagons and takes three days to build with a 200-tonne crane. You can understand then why this process was begun a week ahead of the blockade to minimise the possibility of disruption due to high winds, the project's other big risk.
For the crane to reach its two operating positions, a ramp had to be provided from the corner of the car park, requiring the railway embankment to be cleared of vegetation over a distance of 75 metres. However, as all the trees were covered by preservation orders, consent had to be sought from the Council and a replanting scheme developed in liaison with Bootham 2025, a group which oversees the local conservation area. A crane mat was installed at the top of the ramp comprising 200 piles, sunk 12-18 metres and arranged in two strips to accommodate separate pile caps for each of the crane's tracks.
To ensure optimum efficiency, the sequencing of lifts for the bridge removal and installation was planned methodically by the Story team to minimise the number of changes in the crane's position and loading.
A merry dance
The four bridge decks journeyed to site two days before the blockade got underway. To help this difficult operation, they were jacked up on their trailers, allowing them to sit further forward than would otherwise be the case. Even then, as a result of their 24-metre length, they had to be reversed down Marygate – the narrow street off the A19 – for 300 metres in order to take the right-hand turn as a left-hander, approaching it face first. Marygate was matted out for these manoeuvres, stripped of lighting columns and other furniture, and a dwarf wall dismantled in front of a pub.
Careful consideration had to be given to choreography within the compound. As well as the new decks, the Demag CC 2500-1 and its service crane, there was a need to carry the old decks away to be cut in half and loaded onto wagons for removal from site, involving the use of a third crane. This was added as an insurance measure relatively late in the planning process to ensure nothing impacted on the main crane's activities.
The project's preparedness for the blockade was the subject of regular reviews from a year out. Each element within the programme was analysed using Monte Carlo simulations to calculate the probabilities of different outcomes occurring. From this, a confidence level could be established for the job finishing on time. Scarborough Bridge passed the 95% threshold despite the uncontrollable concern of crane operations being brought to a standstill by wind speeds exceeding 25mph, common features of a British winter.
At the start and end of the blockade – which ran from 23:40 on Friday 13 February to 05:25 on Monday 23rd – Rules of the Route periods were used to make signalling modifications, ensuring the occupation of track circuits by the project's RRVs did not affect movements on the East Coast main line. The points were clipped and scotched.
Beyond that, I'll leave the pictures to tell the story. There was nothing unique to report about the removal and installation sequence: it was a bridge replacement like many others. If all went to plan (and someone turned up with a camera), this story should look rather eye-catching. I will, though, report the high praise of Story by Network Rail's scheme project manager, Darryl White: "The magnitude of this project has brought with it sizeable challenges and the Story team has lived up to its reputation of a superb standard of delivery."
Two other things are worth flagging up. The York-Scarborough meets the line up from Hull at Seamer, 40 miles away; the blockade encompassed all of it. To maximise the opportunity afforded, the call went out to find other projects that could be achieved over the nine days, with around two dozen going ahead: canopy cleaning, culvert relining or reconstruction, point-end replacement, level crossing works, spot resleepering and wholesale vegetation clearance to reduce leaf-fall problems.
The cost of these jobs was significant – around £3 million – but by pulling them into the blockade, this price tag came close to being covered by the £2-3 million worth of efficiency savings that would otherwise have been spent on access arrangements and piecemeal overnight/weekend working. The ambitious intention now is for no additional possessions to be needed on the route during the whole of CP5.
And then we should take our hats off to team member Eamon McAuley who literally built the bridge single-handed…albeit in Lego. It was remarkably detailed – including the track layout and little orange men with chainsaws – and could be deconstructed and rebuilt to follow the lifting sequence. Sitting as a centrepiece in the conference room, it proved more useful than a PowerPoint when explaining the challenges to visitors and stakeholders.
Yes, this really was a model project.
Photos courtesy of Mulholland Media
Graeme Bickerdikehttp://therailengineer.com
SPECIALIST AREAS Tunnels and bridges, historic structures and construction techniques, railway safety Graeme Bickerdike's association with the railway industry goes back to the mid-nineties when he was contracted to produce safety awareness videos and printed materials aimed at the on-track community. This led to him heading a stream of work to improve the way safety rules are communicated and understood - ultimately simplifying them - for which he received the IRSE's Wing Award for Safety in 2007. In 2005, Graeme launched a website to catalogue and celebrate some of the more notable disused railway structures which still grace Britain's landscape. Several hundred have since had their history researched and a photographic record captured. A particular focus has been the construction methods adopted by Victorian engineers and contractors; as a result, the site has become a useful resource for those with asset management responsibilities. Graeme has been writing for Rail Engineer for the past ten years, generally looking at civil engineering projects and associated issues. He has a deep appreciation of the difficulties involved in building tunnels and viaducts through the 19th Century, a trait which is often reflected in his stories.
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Simon Hatch 1st April 2015	At	3:07 pm
Great to see operations underway and another well understood use of the latest technology the bridge was laser scanned and a BIM model created for planning of site operations, the work was carried out by a dedicated team at Bridgeway Consulting.