On the face of it, it seemed like a pretty brave thing to do – to launch a massive 60-metre span temporary bridge into free space above one of our busiest rail routes. What if the sums were wrong, the stresses miscalculated, or the surveys botched?
The result, apart from a massive clang, would be 350 tonnes of scrap steel and travel chaos for thousands. Brunel’s ‘billiard table’ railway would be severely dented and blocked for weeks. Its creator would be spinning in his grave!
That’s the thing about large-scale engineering. Get it wrong and everyone knows about it. Even Brunel would testify to that. His own brave engineering projects didn’t always go to plan after all. But needless to say, on this occasion there were no such occurrences. On his old patch there have been no adverse headlines and no commuter journeys have been disrupted.
Instead, another carefully planned, calculated and executed stage in the Crossrail electrification works has gone ahead without a hitch. Even so, it was a spectacular undertaking, which no doubt Brunel would have enjoyed immensely.
The scheme to electrify and remodel the route between Reading and Paddington as part of the Crossrail project has been broadly divided into the Western Inner and Western Outer schemes. Carillion is the main contractor for the Western Inner section, with responsibility for the installation of OLE between Paddington and Stockley, near West Drayton. The Western Outer contract, headed by Balfour Beatty Rail covers the area from West Drayton to Maidenhead and involves the remodelling of three junctions and the raising of two bridges to give clearance for the OLE and provide W10 loading gauge.
Stoke Poges bridge in Slough (18 miles 69 chains) was constructed with two wrought iron plate girder spans resting on brick piers, whereas Thorney Lane bridge, a Brunel original adjacent to Iver station (14 miles 50 chains), was built as a three-arch masonry construction. Each bridge carries a two- lane highway and each has an adjacent footbridge.
At both sites the heightening work has involved the modification of the existing piers and abutments to support new pre-cast reinforced concrete spans.
The work at Stoke Poges has been made easier because the road traffic could easily be diverted. Unfortunately, at Thorney Lane no diversionary route was available, so the plan has involved the provision of a temporary road bridge adjacent to the original. The problem was how to install this large but temporary structure without disrupting traffic on the Great Western main line.
Balfour Beatty Rail rose to the challenge of Thorney Lane with the assistance of Mabey Hire, a company specialising in large-scale temporary works projects. The devised solution involved use of the unique Mabey Universal Bridging System (MU) – a modular construction technique employing channel steel lattice panels with back-to-back channel section longitudinal girders attached top and bottom. Connecting these are transverse steel I-beam sections (transoms) that support the road deck. The span and loadings required at Thorney Lane dictated a double story triple panel construction and it is noteworthy that the bridge is the largest single span completed on the UK rail infrastructure.
A lift was out of the question so it was decided that, by using a cantilever launch technique, the 350 tonne structure could be rolled into position. Fortunately, a 50 metre by 100 metre area of open land on the south side of the railway was available for building the temporary bridge. Levelling the site was, however, an involved process that included construction of a gabion wall and several thousand tonnes of spoil in-fill.
The bridge itself was assembled like a giant Meccano kit, resting on lines of rollers set into concrete foundations. A nine-metre launch of the partly completed structure took place over the weekend of 13/14 September. This allowed the bridge nose to be set; a ‘launching link’ being inserted to angle the bridge nose upwards. At the same time, further bays were added at the rear of the bridge.
Final launch took place on 27/28 September, two weeks ahead of programme. The launching technique involved pushing the completed structure out across the railway during a four-hour possession using a 45 tonne D8 bulldozer. A specially designed coupling attached the ’dozer to the first transom of the bridge structure.
The maximum deflection of the bridge during launch was calculated to be 1.5-metres – and so it worked out in practice. Guidance and lifting of the bridge nose onto its northern abutment was facilitated by the 9-metre angled nose attachment, whilst the additional tail sections provided a counter-balance. At its furthest unsupported extension, 150 tonnes of bridge in free space was balanced by 200 tonnes on land. Having the bridge descend onto the railway was therefore never a remote possibility!
After a final push from the D8, with the bridge nose engaged on the receiving rollers, the bridge was in place. Hydraulic lifting jacks were then used to raise the bridge so that the rollers could be removed. When finally seated on its bearings, the nose and tail sections were removed to leave a 60-metre span across the railway.
The temporary structure accommodates two lanes of highway on a 7.35-metre carriageway. Cantilevered pedestrian walkways are provided on both sides of the bridge and the structure also has provision to carry essential utilities. Balfour Beatty Rail has constructed the temporary access roadways, including mini-roundabouts on both the north and south approaches. Road traffic was expected to be diverted onto the temporary bridge from 27 October, allowing the preparatory works to progress in relation to the Christmas demolition of the existing structure.
New for old
Demolition of the original Thorney Lane bridge arches will require the construction of steel platforms on the bridge to support long-reach excavators fitted with breaker attachments. With the arches gone, the existing piers and abutments will then be modified to carry the new concrete spans.
The demolition and rebuilding work is scheduled to take place between 3 November and 4 January, with the arch removal being undertaken over the Christmas period. The new bridge is to be completed and reopened to road traffic by the end of March 2015.
In lifting the new bridge spans, site conditions dictate that a 750 tonne capacity crawler crane will be used to achieve the required capacity and radius when positioning the bridge over the railway.
With the new bridge completed, the removal of the temporary bridge will be a reversal of the launch process. Nose and tail sections will be added to guide and counterbalance it and the D8 ‘dozer will again do its stuff. With the bridge dismantled, all preparatory including the gabion wall and infill will be removed and the land surface reinstated.
Four miles away, at Stoke Poges bridge, the situation is remarkably similar. Known locally as Horlicks bridge after the adjacent factory, it too spans five railway tracks, although the distance between the abutments is less at 50-metres.
Although a temporary road bridge is not needed at this location, it has nevertheless been necessary to divert utility services across a temporary bridge structure whilst the original bridge is rebuilt. With limited clearance available over the railway, minimising the deflection of the temporary bridge was crucial. For this reason a 2.4-metre square lattice box construction of MU panels was chosen.
Again both constructed and installed by Mabey Hire, the bridge was lifted into position on 6/7 September.
Site restrictions within this busy urban area presented a further challenge, with limited space to pre-build the structure. Assembly of the 54.5 tonne bridge was completed in just five days, followed by a possession of less than four hours during which it was lifted into position. A 750 tonne capacity mobile crane was provided by Ainscough in order to achieve the required lift radius.
Demolition of the existing bridge at Stoke Poges will run concurrently with the work at Thorney Lane Bridge. The reopening date is slightly more relaxed however at early May 2015.
The hugely impressive tunnelling works of the Crossrail central section has been extensively reported, not least within the pages of The Rail Engineer. Although perhaps over-shadowed by the cavernous excavations under London, the upgrade and electrification of the Great Western main line out of Paddington forms a vital part of the scheme. As such it is of great importance to Network Rail.
Crossrail services will share the Great Western main line between Reading and Paddington with existing West of England services. Some suburban services currently operated by First Great Western will be transferred to Crossrail using new Class 345 electric units. This in turn will free up capacity at Paddington surface-level station. From December 2019, passengers from Reading and other Thames Valley stations will be able to travel right through central London without having to change trains, making it easier and more convenient to get to a range of destinations across the capital and the South East.
And what would Brunel make of the project – his graceful arches being altered to accept the paraphernalia of overhead electrification equipment? It’s been a matter of finding the right balance. And on that subject he would surely be impressed!