There are good opportunities for those seeking virtue at Christmas-time. Donate to charity; attend midnight mass; force down a third Brussels sprout without grimacing. Or, if you are responsible for a 53 hour possession starting just when Santa should be doing the rounds, show your face on Christmas morning to see how your site team is getting on.
A high-level visit to the worksite at Horton Bridge on the Great Western main line in Hillingdon therefore felt cheated upon arrival at 9am to find no work in progress. However, the surprise was a pleasurable one, since the team were actually packing up having completed installation of a new footbridge in just 8 hours.
There was plenty of other work going on too. The visit continued to view the demolition of three other bridges, all in the same possession within five miles of each other. These bridge works were carried out by Network Rail on behalf of the Crossrail project, to enable installation of overhead line electrification on the Crossrail West Surface Works section between Paddington and Maidenhead.
The Christmas works were just the first stage. At Easter, the works will reverse at the same sites with three installations and one demolition. And these bridge modifications, contracted to Hochtief, include parapet alterations at a further 12 bridges between Maidenhead and the junction to Heathrow Airport at Stockley flyover.
The project to electrify this section is being delivered by Network Rail. In addition to being enabling works for Crossrail, it will also form part of the Great Western main line electrification scheme. As technical coherency between the Great Western and Crossrail projects is necessary, this section uses the same overhead line equipment as the rest of the Great Western route.
These four bridges were reconstructed because they were all three-span bridges with curved arches which clashed with the required clearances for the new overhead lines.
The new bridge at Horton, which was built alongside the old, was commissioned prior to removal of the old bridge so allowing the diversion of services. For the other bridges constructed on similar alignments, it was necessary to remove the old before the new could be built.
Three bridges at Horton, Trenches and Old Stockley Road were required to carry pedestrians and cycles only. Designers Hyder working for Hochtief developed a single design for installation at all three locations, with a single span of 33 metres.
Geoff Hancox, designated project engineer for Crossrail, explains that the 5.6 metre clearance from rail to soffit is a key feature of the new footbridges. Hancox said: “This allows free-running of the OLE and so, for the electrification, it is almost as if the bridge is not there.” This clearance also allows for the auto-transformer feeder cables and will provide for the largest high-cube container traffic.
Hancox notes an increasing interest in whole life costs for electrification, including an ongoing baselining exercise being carried out by Network Rail. “If we can attribute OLE maintenance costs against individual bridges, we can gain a better understanding of costs associated with reduced clearances. This could assist with future decisions on track lowering compared with reconstruction. It may be that increased costs of reconstruction can be balanced against longer term savings from providing greater clearances for the overhead line equipment.”
The single spans also give flexibility for the track layout. Hancox continued: “We took this design decision with an eye on the future. But, it is already giving unexpected benefits and we are taking advantage of this flexibility. A new crossover is planned at the site of Old Stockley Road bridge, to give operational mitigation during the works at Stockley flyover.”
These will add extra ramps and overbridges, removing conflicts for Heathrow Express trains accessing the Up Relief line.
A downside to increasing the clearances at the bridges is the access from the surroundings. Especially where steep gradients were formerly provided, fitting compliant steps or ramps into the available land became a challenge.
New ramps will be provided to all the footbridges. At Horton, an elevated ramp structure will be tucked into the tight space between the railway and the Grand Union canal. At Old Stockley Road, back-spans to the footbridge will be provided allowing the approach embankment to be graded back to the railway rather than needing an abutment.
At Trenches an 80 metre long approach ramp will be provided, although even this required agreement with the local authority for the continuous ramp gradient of 1 in 12 with a single landing. The ramp gains a maximum height of some 2.5 metres over the adjacent land using Keystone reinforced earth units. The project team are currently looking at using lightweight aggregate to reduce surcharge on the existing embankment.
The fourth bridge at Middlegreen Road is the sole bridge still carrying a road, albeit a single lane with traffic light controls. It would have been prohibitively costly to modify the road approaches to provide the same clearance as the other bridges hence a clearance of 5.2 metres will be provided.
Due to the heavier road traffic and limited construction depth dictated by the approaches, a single span was also not viable. When it is reconstructed at Easter, the bridge will reuse the existing piers and abutments using precast concrete portal units to form a three-span structure. A temporary footbridge provides continuity of access for pedestrians until the new bridge is installed.
The footbridges were designed for efficiency of erection. Jason Hamilton, project manager for Network Rail, explained: “One installation was carried out over Christmas at Horton. It was an extremely smooth operation. We have also realised economies of scale by replicating the same design at the other locations.”
The decks are of steel-concrete composite construction. Weathering steel was used for the main girders to minimise future maintenance requirements. The twin girders and bracing were provided and assembled at ground level by Mabey.
They were lifted as a complete 29 tonne unit by a 500 tonne crane provided by Ainscough.
The concrete deck was formed of 11 precast units each 3 metres long provided by Banghers Precast Concrete. The deck system was designed to be stable in the temporary configuration with the precast units craned into place and supported by the main girders. However, this system requires careful coordination of the pockets within the precast units, the reinforcement projecting into these pockets and the shear studs projecting from the main beams.
Hamilton says, “Hochtief carried out a trial lift of a precast unit the week before the possession. They mapped the precast unit onto the main beams to ensure the shear studs and reinforcement lined up. During the possession, every unit sat right first time and this was a major factor in achieving the eight-hour installation time.”
In-situ concrete will be poured later to stitch the precast units to the main beams, giving the composite action necessary to carry live loads rather than just self-weight. Grout checks were installed prior to lifting the precast units so this work can be carried out in other, shorter possessions.
The precast units were lifted into place with parapet stanchions attached to provide edge protection from the outset. 1.8 metre high solid metal parapets are provided at all the footbridges. In addition, the team consulted the route crime records and attributed trespass incidents to each bridge. With 17 recorded incidents of objects thrown onto the line over the last 10 years, Horton Bridge will be fitted with an overcage to reduce vandalism. Since the same design is used for all the bridges, this could be added at the other bridges in future if required.
Taking the metaphorical wrecking ball to Brunel’s original fabric is not something to be undertaken lightly. English Heritage carried out an extensive consultation on the whole Great Western route following the announcement of the electrification scheme. The consultation concluded in July 2012 with the listing of 35 further structures in addition to the 56 structures previously listed.
The four bridges removed by this project were identified for replacement in the Crossrail Act in 2008. This followed discussion and agreement with English Heritage about which structures could be reconstructed and which could have track lowering.
Typically for railway structures, various alterations had been carried out over the years. The first arch span was a semi-elliptical arch constructed in the late 1830s for Brunel’s broad gauge. The addition of the relief lines in the late 1870s led to a second span to standard gauge. A goods line was added in 1914, leading to the third span, built in a variety of forms including arches and flat metal spans.
Jason Hamilton commented: “Demolitions can be difficult. These structures were in excess of 100 years old, some sections approaching 175 years. No matter how much investigation you do, coring, records, trial holes, you never quite know what you will find until you open up the structure.”
Hamilton added: “We were also conscious of the Beenham Lockside collapse”. This was an uncontrolled collapse of an arch of this three-span form during its demolition in April 2012. An investigation is currently in progress. Hamilton says, “We took care with the demolition sequence, removing the structures evenly in metre-wide strips to keep the structural integrity”.
Demolition was carried out by Gilpin, who used four 30 tonne excavators working simultaneously at each of the three demolition sites. The piers were removed at Trenches and Old Stockley Road bridges. It had been planned to remove the piers to 500mm below sleeper level. However, this proved a little easier than expected as the piers were founded on Terrace Gravels some 200-300mm below the sleepers.
Missing the Mass
All works were completed and the track handed back by 16:00 on 26 December, well within the planned times. So, lots for the project to feel justly virtuous about. But there’s plenty more to consider, with tidying off the Christmas works and preparations for the next phase at Easter. Then there are a couple more potential reconstructions under consideration for next Christmas. Maybe, after all, there are easier ways to achieve virtue.