Crossrail – Europe’s biggest building site. 26 miles of new railway being built in tunnels under the centre of London that is due to go into service in 2018. But it isn’t. Crossrail is, in fact, a 73-mile long railway that will stretch from Reading (Berkshire) to Shenfield (Essex) and Abbey Wood (Kent). So nearly twice as much of it will be outside those new tunnels as in them.

And parts are already in service. In fact, the first train ran on New Year’s Eve.

OK – maybe we have stretched a point there, but it’s true that the first piece of Crossrail infrastructure carried its first fare-paying service on 31 December 2014.

The original arrangement

The location was the new Stockley viaduct just to the west of Hayes & Harlington station on the Great Western main line (GWML). At this point, the GWML consists of four tracks. Running almost exactly East-West they are, from the south, the Down Main, Up Main, Down Relief and Up Relief.

Heathrow Express trains come out of Paddington on the Down Main and simply turn left just after Hayes & Harlington to curve round onto the Down Airport line.

On their return, trains emerge from Heathrow on the Up Airport, cross over the GWML Up and Down Mains which run through a concrete box installed in the 1990s, and then join the Up Main just before Hayes & Harlington station.

That’s all fairly simple

But there is also a Heathrow Connect service. This comes out of London on the Down Relief line so, to turn left for Heathrow, it has to cross the Main lines. To do this, up until the end of last year, the train would turn left onto the Up Airport line, travel the wrong way along that until it had traversed the concrete box over the Main lines, and then transfer over to the Down Airport using a crossover just before the portals to the tunnels under the airport.

On it’s return, the Heathrow Connect service would cross the main lines on the Up Airport in the usual way, but instead of swinging right onto the Up Main it would swing left, across the Down Relief using a ladder crossing onto the Up Relief.

So the stopping service that is Heathrow Connect ends up going the wrong way on two tracks of this complex junction with all the inherent risks of delay that this can cause.

Given that the Heathrow Connect service will eventually be absorbed into Crossrail, which will be running a high-frequency service both to Heathrow and to Reading on the Relief lines, this is one bottleneck too many.

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Solution for the future

Network Rail is responsible for engineering and improving the overground sections of Crossrail, so, working with designer Jacobs, it came up with a plan to rearrange this junction completely.

A new two-span flyover would be built to the west of the concrete box, crossing all four GWML lines (and a freight line servicing the nearby Hanson terminal) and carrying the Up Airport track onto a high-level viaduct. This would run alongside and parallel to the existing lines until it cleared the (shortened) freight siding and would then drop down to filter into the Up Relief line.

This work was completed over Christmas 2014. In the short term, Down services will also use this new link in the reverse direction until Christmas 2016 by which time a second connection up onto the flyover from the Down Relief will be completed.

Carillion was awarded the contract to construct the new viaduct in December 2012, but the company was no stranger to this area. Indeed, it was Carillion which constructed the original flyover interchange and the cut and cover tunnel (the first section of the tunnel to the airport) in 1994/5.

In addition, Carillion had an ‘advanced works’ contract, immediately prior to the current main award, to construct a section of the approach ramp on the south side of the mainline and demolish 17 existing structures, some as close as three metres from the existing lines.

Before construction could commence, some challenging environmental aspects had to be addressed. There was historic contamination of the site from Victorian landfill sites – the area was mined for sand and gravel, which was taken to London via the Grand Union Canal, and it was then infilled with waste brought by returning barges.

There was also an area of wildlife interest in the form of ponds inhabited by amphibians. And the Crossrail Bill, which authorised the construction of all aspects of the works, has strict requirements for noise, vibration and dust emissions – all of which required a complex process of monitoring and control to ensure compliance.

As for the main construction works, the two main elements were the viaduct, with access ramps along the north side of the site, and the bridge itself.

The viaduct

Running for 450 metres along the north edge of the site, and containing a single track, the viaduct is a massive structure. It is made up of 19 concrete spans resting on concrete piers, which in turn are resting on in-situ concrete bored piles. At the western end it curves south to access the new bridge, at the eastern end it drops down a 280 metre-long in-situ concrete box and reinforced earth ramp and connects with the Up Relief line just short of the Station Road bridge.

The piers were cast in-situ. Precast beams were brought in from Shay Murtagh while the robust kerbs, designed to retain a derailed train, house cable ducts and provide a safe walled walkway, were from Concast.

WFO landed reduced [online]

Each arch was assembled by lifting in the beams and stabilising them on an RMD Megashor falsework. After casting the in-situ infill decks, the robust kerbs were lifted into place. 500 to 1,000 tonne cranes were used for these lifts and, due to the very restrictive site, each lift had to be undertaken from a specific (and different) location in the yards of the adjacent warehouses – a time-consuming process.

Top ballast was added and then the track (rail from Tata Steel secured by Pandrol Fastclips to Tarmac concrete sleepers) installed on top.

The lower sections of the ramp down to the Up Relief line consist of an in-situ ‘U’ section filled with a lightweight aggregate and a tapered approach ramp constructed using a reinforced-earth panel to bring the tracks to ground level. The seven- metre-high in-situ walls were constructed using a rolling gantry. This section was very challenging to construct within the tight gap between the freight line and adjoining properties.

Part-way along the viaduct is half of a turnout, supplied by Vossloh. The concrete wall alongside is missing – instead there is a temporary steel structure. This will become the access to the second ramp which will connect with the Up Main and Down Relief tracks. It will be constructed throughout 2015 whilst maintaining an operational railway with the tracks (and the rest of the turnout) installed and the connection made during the Christmas 2016 closure.

A sprayed-on waterproof membrane, jointing and anti graffiti protection across all the structures were applied by VolkerLaser.

Bridging the gap

The bridge itself is a weathering-steel structure. This is a high-strength, low-alloy material which rapidly forms an impermeable oxide layer, similar to rust. However, once the surface layer is fully formed, then no further degradation occurs. The structure is protected by the oxide layer and will never need painting, which significantly reduces the whole-life cost of the structure, although it does make it look a bit like a rusty bridge.

To ensure a consistent protective layer, the same weathering steel has to be used for everything. Bolts, nuts and welding rods all have to be manufactured from the same material.

The bridge itself was constructed by Portuguese company Martifer using Italian steel. Manufacture was carried out in Romania and the 275 component parts shipped to site.

Meantime, the embankment alongside the existing Heathrow rail access to the south of the main lines was widened and reinforced. This would both act as an assembly site for the new bridge and as the base of the rail link to it. Reinforced-concrete bridge abutments were constructed at the edge of this embankment and at the end of the new viaduct opposite, and a central pier was built between the Main and Relief lines.

The bridge components were reassembled into two connected bridges, the 40-metre-long West Span and the other 70-metre East Span, by the company’s own largely-Romanian workforce. A disposable launching nose and tail were added and the concrete deck of the West Span was poured to act as a counterweight during the bridge launch.

When ready, the structure was launched over the GWML tracks using strand jacks mounted on a temporary tail unit. First target was the central pier which had been fitted with Teflon skid blocks to receive it.

“We had to be careful,” Ewen Morrison, Network Rail’s civils project manager, explained. “Not only is the bridge curved, it is on a 305 metre radius, but we were also launching it downhill.”

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After a few anxious moments, the nose contacted the pier as planned, stabilising the structure. Then it was just a question of resetting the jacks and relaunching the bridge over the Relief lines. Once that had been achieved, and the joint between the two bridge sections was supported by the central pier, the bridge could be separated, the skid blocks removed, bearings installed and the nose and sections removed.

Then the tracklayers could move in. Ballast and track went down, along with some heavy-duty check rails in the centre of the track ‘just in case’. For the same reason, there were also concrete retaining walls and cable troughs, but the walkways, safety fences and handrails are all fibre- reinforced plastic, purchased in the USA or China.

Overhead wiring and signalling kit went in throughout the structures – 10,000 metres of signalling cable and 50 OLE structures.

Finishing off over Christmas

It was all ready just before Christmas. During the holiday closedown, when trains on the main lines weren’t running, the final connections could be made and preparatory work undertaken for the following phases of the work.

The Hanson freight siding (Dawley goods loop) was shortened and a new connection made with the Up Relief line west of Dawley Road Bridge. This allowed the previous siding junction to be used for the new viaduct.

A brand new crossover was installed between the Up and Down Relief lines, in the gap left by the shortening of the goods loop, to allow Down trains to access the viaduct. The old Up Airport connection with the Up Main was also moved, and the redundant track lifted to make way for a concrete box to be built across the Relief lines over Christmas 2015. Some preliminary piling also took place to lay its foundations.

A new link between the two Relief lines was installed west of the entire junction under the old Stockley footbridge, and a temporary connection made between the Up Airport and the line over the new bridge.

It’s all very complicated, but Toufic Machnouk, Network Rail’s project manager for the rail systems, managed to explain it all using several plans, a whiteboard and various coloured pens.

So that was it – phase one complete. In a year’s time, the concrete box which will carry the connection between the viaduct and the Up Main line will be constructed, along with the ramp to carry the track down to ground level, and some more trackwork undertaken. Then in 2016 the turnout on the viaduct will be completed and the final track layout alterations made. All of which will have been facilitated by the work done this last Christmas.

After two years of hard work, the first Heathrow Connect train came over the bridge on 31 December. Now there will only be two more years of work before the new bridge, viaduct and junction are fully integrated with the network and what will soon become Crossrail.