When complete, Crossrail will be a three-legged railway. The western leg will emerge from under London at the Royal Oak portal and run out as far as Reading with a connection off to Heathrow.

To the North East, the portal is at Pudding Mill Lane and the Crossrail route will then go through Stratford and out to Shenfield in Essex.

In the South East, after a couple of open sections at Custom House, the line finally emerges from under the Thames at Plumstead but then runs for only about a mile to the terminus at Abbey Wood.

Currently, Abbey Wood is a simple station on the North Kent line. It has two platforms, set each side of the twin-track railway. Trains to London run into both London Bridge/Cannon Street (six per hour) and Charing Cross (two per hour), while outbound services go to Barnehurst, Dartford, Gillingham and Hither Green.

New islands

Under Crossrail, the two existing platforms will both be rebuilt as island platforms by having a track each installed on the ‘back side’ of them. The northerly island, currently the Down platform to Kent, will be the Crossrail terminus while the current Up platform will become the one for the North Kent line.

Passengers coming in from Kent, which could be from as far away as Dover, Ramsgate and Rochester, will change at Abbey Wood onto Crossrail services.

Consideration was given to having both North Kent and Crossrail lines on each island, so that passengers would only have to cross the platform to catch their connecting trains. However, that would have forced city-bound Crossrail trains to cross the North London line to reach the tunnel portal, so delaying services. Passengers will therefore have to cross the new footbridge (with lifts and escalators) to make their connections.

There is an added complication, and that revolves around the electrification systems. The North Kent is a third-rail DC line, while Crossrail will have an overhead 25kV AC supply. Keeping the two railways apart on their own island platforms will eliminate a lot of potential problems.

The northernmost Crossrail track, on the new platform face, will terminate at buffer stops at the end of the station. The more southerly one, on the existing face, will continue on beyond the platform. Although primarily intended to be a ‘dead train park’ where defective trains can be left until they can be recovered, this track will eventually connect with the North Kent Down line, so giving access for engineering trains.

The approach to that junction will not be electrified at all, so removing any electrical crossover problems, as it is primarily intended for diesel or battery-powered engineering movements.

So that’s the plan. Turn a two-track station into a four track one, and run the two twin-track routes alongside each other for the mile or so to the Plumstead portal. Simple.

Complication

Oh – and what about constructing the new Crossrail inside the tunnels? To do that, access will be needed to the portal to bring work trains in. But they will be coming from the North, not from Kent. And, until the station is complete, the connection between the North London line and Crossrail won’t exist anyway.

To make that connection, it was decided to use the Plumstead Sidings that are just at the London-side of the new portal. One of those would be extended, actually running on top of the concrete box behind the portal, and curve round to join the new Crossrail lines between the portal and the station. Work trains could then reverse into the tunnel.

The station would, in any case, have to be built in stages to avoid disrupting existing North Kent traffic.

And the need for the work train route from Plumstead sidings dictated the order in which things would have to be done.

86699_Abbey-Wood-Station-Design-Architects-Impression [online]

Crossrail overground works are being delivered by Network Rail, and it in turn brought in Balfour Beatty as principal contractor under a fixed- price design and build contract worth £132 million. A collaborative team of both Network Rail and Balfour Beatty employees was set up in offices a short walk from Abbey Wood station and two and a half years of design and planning commenced.

Soft going

The team was fortunate in that, although the North Kent line is only a twin track railway, and has always been so, when it was built a generous amount of land was fenced off as railway property. At the time there was little local building and the land quality was quite poor. Although the area is now extensively developed, there was still quite a bit of space available adjacent to the tracks.

This would allow the North Kent lines to be slewed to line up with the ‘new’ southern island platform. But, until that platform was built, there would be no point. Except, it would generate the space for the construction of the tunnel access lines.

So the decision was made that the first phase would be to slew the North Kent line over by about 10 metres, but only for half of the distance between the portal and the station. The rest of the slew would be done later once the platform works had been completed.

As was mentioned earlier, the ground conditions were poor. Aluvia, interspersed with layers of peat, made for soft, damp ground that would rapidly collapse if any weight was applied to it. Evidence showed that the existing embankment had been reworked several times in the past and, although it was now stable, any new work to the side would have the same problems.

Scheme project manager Nick Wilcox outlined the options that were considered. The first was to surcharge the area and deal with the settlement over time. Temporary fill would be placed on the area concerned, forcing it to settle and then topping up as required. The process could be accelerated by installing band drains. However, this would be very time consuming and require a lot of temporary fill.

The second option would be to excavate the uncertain ground and use lightweight fill. If the weight of that fill was half that of the ground removed, then twice the height would only put the same pressure on the substrate as before. It was a neat idea, but the excavation would require temporary works to support the existing embankment and the cost of the fill would be high.

Improving the ground using minipiles with layers of geo- grid trapped between them could form a stable structure when tied into the underlying strata. However, to develop good tension across the geo-grids the embankment would really need to be higher.

So the chosen solution was to go for a series of driven pre-cast concrete piles supporting a concrete slab. In effect, the soft ground would be bridged, albeit at ground level.

Slewing

Work commenced in September 2013. Signalling cables were diverted and equipment cabinets moved from the south side of the tracks to the north side. Once the site was clear, the civil engineers could move in to first of all stabilise the ground and then to build a new line, around 800 metres long, alongside the existing North Kent Up line. This could be done during daylight as the new track was about 10 metres away from the live railway.

While this was going on, an existing footbridge had to be removed and replaced as it would no longer span the enlarged track layout.

Once the track was in place, it was connected into the existing Up line during a 52 hour possession one weekend. The ‘zig-zag’ deviation at the London end would be permanent, the tighter one at the station end would be only temporary until the line could be extended into the station later in the project.

With the Up line displaced, the Down line could be slewed over by the same process, more or less onto the line of the old Up line. However, that old track was lifted and completely replaced – again being connected over a weekend.

Now that the North Kent line had been moved, the first stretch of Crossrail could be built. The old North Kent Down line was lifted and the embankment extended using the concrete slab and piles method mentioned above. Two new lengths of track, again about 800 metres long, were then installed along with a turnout for the line coming in from Plumstead Sidings. This will be installed later by Crossrail’s C610 contractor ATC as part of the tunnel fit-out work.

This first stretch of dedicated Crossrail line was handed over, as planned, on Monday 11 May. Balfour Beatty and Network Rail will now move on to the next phase of this South East Section (SES) project while Crossrail fit-out contractors ATC (Alstom-TSO-Costain joint venture) will complete the link to Plumstead Sidings and take the tracks down into the tunnel itself.

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But that’s not all…

Work on phase two of the project has already started, and a lot of that revolves around the station. Once again it is quite a complicated plan, and Balfour Beatty site agent Simon Swaby took care in his explanation.

The station building itself is exactly on the path of the realigned North Kent Lines, so that will be demolished. It is already closed and boarded off. It will be replaced by a completely new concourse which will span the tracks, with passengers accessing the platforms by stairs, escalators and lifts. Access will be from ground level and also from the adjacent road bridge. The bridge itself has room for the three through-tracks, and the piers will be reinforced with collision protection.

Meanwhile, the existing station footbridge has been replaced by a new, steel-framed structure. The bridge connecting the two platforms will be permanent, although it does not yet have its final cladding, and there is a temporary extension to an equally-temporary station building and gate line which will disappear when the main building opens in 2017.

Construction of the concrete rafts that will take the two ‘outside’ tracks is well underway, and the platforms themselves will also be constructed in phases.

First, the new platform face on the current Up platform will be completed, along with its track and an extension from the slew. Trains to London will be rerouted into the new platform face from February 2016.

This will allow the existing Up platform face to be demolished and rebuilt, completing the island platform that will service the North Kent lines.

Next, the Up line will be rerouted from the slew, eliminating the temporary ‘kink’ and completing the North Kent platform. This will free up the other existing platform which will be demolished and rebuilt as a second island platform ready to take Crossrail. At the same time, the Crossrail lines will be extended from their current position through the station.

The end of the line – for now

So that will complete the Abbey Wood terminus for Crossrail. The end of the line in the South East, although an extended route has been safeguarded as far as Ebbsfleet. The width of the existing railway land was convenient, although three properties had to be demolished to maintain signal sight lines and a few local gardens lost four or five metres off their lengths. Three footbridges will be demolished, including the one on the station, and a whole new station building constructed. Two platforms will have become islands and there will be a fence between the two railways, in part to stop maintainers wandering from AC to DC electrified zones.

When Crossrail opens in 2018, 12 trains per hour will run from Abbey Wood in peak periods. These will be in addition to the existing North Kent line services which will themselves be increased as passengers travel from all over North Kent and the Medway to change to Crossrail at Abbey Wood. It is estimated that the number of passengers using the station will treble over the next 15 years, reaching over 10,000 in the morning peak by 2026.

And that’s just from Crossrail’s south- eastern leg.