Network Rail’s Crossrail South East Section Project (CSESP) is the most significant piece of above ground work anywhere in the £15bn scheme.
If you were to ask a non-engineer to talk about the Crossrail project, it’s fairly inevitable that tunnelling would feature prominently. The media focus and a very welcome BBC documentary - at least as far as the engineering profession was concerned - have understandably turned the photogenic tunnel boring machines excavating miles of tunnels under London into the most readily recognisable symbols of the project.
Such a focus, however, ignores the £2.3bn that Network Rail is investing to upgrade the existing rail network east and west of London to make sure it is ready for the arrival of the new 118km line.
The most significant element of this above-ground work is the £132M project to connect Crossrail into the existing National Rail network in south east London. The Crossrail South East Section Project (CSESP) covers a 3km stretch running from east to west from the Crossrail tunnel portal at Plumstead in south east London to the south eastern terminus at Abbey Wood station.
The project is being delivered by a joint venture between Balfour Beatty Rail and Balfour Beatty’s civil engineering, building and mechanical & electrical division Construction Services UK (CSUK). It includes the installation of two new overhead electrified Crossrail lines between the portal and Abbey Wood station; a major remodelling of the station itself and modifications to several bridges.
One of the main challenges has been the sequenced repositioning of the existing North Kent lines that run east and west through the 3km railway corridor. These must be moved to a more southerly position to make way for the new Crossrail lines, which will eventually run parallel to them between Abbey Wood and the Plumstead portal.
The track repositioning has been planned in phases with minimal railway possessions.
“The whole project staging is about minimising disruption to the operational railway while meeting the key handover dates,” says Simon Swaby senior site agent, Balfour Beatty Construction Services UK.
“The whole project staging is about minimising disruption to the operational railway while meeting the key handover dates”
Simon Swaby, Balfour Beatty
The first part of the project, which came to an end in early May, consisted of moving an 800m section of the North Kent line close to the Plumstead portal to create the space for the new Crossrail track.
“The significance of that section of the track and its completion is that the [Crossrail] system-wide team needs to use it to fit out the new Crossrail tunnels,” says Crossrail head of communications, Nick Mann.
“They’ve built a railhead at Plumstead where they keep the multi-purpose gantries to fit out the tunnels. The gantries need to come out of that railhead, go onto the [800m] track section that Network Rail has installed and from there go into the tunnel.”
To begin with, signalling and telecoms cables and a high voltage cable which sat to the south of the North Kent lines had to be moved to the north side of the site. This freed up space to widen the embankment, making room for the first of the Kent lines to be moved.
This embankment widening was complicated, however, by the fact that underlying Abbey Wood is 1m to 2m of made ground overlying between 4m and 8m of alluvium silt.
“From a geotechnical point of view it is nothing more than soup and will not support anything of weight,” says Swaby.
For that reason, the decision was taken to build a reinforced ballast slab which sits on driven piles. “It’s quite low tech but as an engineering solution it’s incredibly efficient,” Swaby explains. “You dig down to those driven piles, build your slab, put your fill material on top and put the ballast on top. Driven piles go in at such a speed that we can put in 50m of ballast slab per week.”
“We had a series of 52-hour weekend possessions where we connected the new north Kent line in at either end and then went on to slew the down line”
Simon Swarby, Balfour Beatty
With the embankment widening complete, the Balfour Beatty team could then relocate the first of the sections of North Kent Line track, the up line to London. This allowed work to start on the next stage of the project: the slewing of the down Kent line.
“We had a series of 52-hour weekend possessions where we connected the new north Kent line in at either end and then went on to slew the down line,” says Swaby.
“It [the slewing of the down line] required engineering trains to come in, excavate down 600mm from the top of the existing ballast and then back come up with new sub base material and then [add] new ballast and new track.”
In parallel with the works to move the North Kent lines and prepare the way for the Crossrail lines, the project team also had to construct a temporary station and build a new footbridge above the platforms and track.
The temporary bridge on the south edge of the site was built so that the existing Abbey Wood station could be demolished and rebuilt to accomodate new Crossrail platforms as well as the North Kent Lines.
In the existing configuration, the North Kent lines are served by flank platforms but in the reconfigured station the trains will arrive at either side of a single island platform.
During a series of possessions, the objective will be to build this new platform in two stages and bring the remaining section of the Kent lines to the south of the site so they arrive either side of this island platform.
When NCE visited the site in middle of May, the project team was beginning to construct the first half of this platform and had commenced earthworks to slew the North Kent upline to the south side of it.
“The plan is that the track alignment of the up line will be in place by February next year and then between February and August next year, during a series of railway possessions, we’ll then build the other half of the North Kent line platform,” says Swaby. “We build the [first half of the] new platform, move the track over, demolish the old platform clearing the site and then build the second half of the [new] platform, joining them together eventually to build one brand new island platform.”
Swaby thinks the interim station has been delivered to an unusually high standard. “Normally you’re lucky if an interim station is fit for purpose,” he says. “The one we’ve delivered here probably betters the existing infrastructure.”
In addition to this, the construction team has also had to construct a new footbridge over the station. The main span of the bridge is new permanent works with a temporary side spans on it and a temporary set of stairs down to the interim station. Construction of the new footbridge again enabled the demolition of the existing station infrastructure, including a footbridge at the western end of the site at a point just to the west of where the Harrow Manor Way dual carriageway passes over the station.
“A key requirement from our client Network Rail was that we maintained some form of unpaid access over the railway at Harrow Manor Way, because to use the new station overbridge you’d have to have a valid ticket,” says Swaby. “So we closed the north bound bus lane on the Harrow Manor Way dual carriageway and turned that into a temporary pedestrian route.”
Once both of the North Kent Line tracks have been moved to their new positions on either side of the new island platform, there will be room to build the new Crossrail lines and their island platform on the north side of the station.
Track for westbound trains will be on the northern edge of the station and terminates at there. Immediately parallel to it, east bound track will continue for a further kilometre past the station. “This is to act as a crippled train berth and to provide a line for engineering trains accessing from the Network Rail network,” says Swaby.
Box culvert diversion
During the construction of the Abbey Wood station section of the Crossrail lines and the slewing of the parallel section of the North Kent lines, a further complication will be added by the need to divert an existing concrete box culvert crossing the railway from north to south and then back from south to north further east along the tracks. The diversion work starts next month and will move the culvert away from the eventual footprint of the Crossrail tracks.
Rather than sitting the box culvert on top of a slab or building it insitu, the team developed a precast solution with the box culvert resting on a pair of sheet pile walls.
The culvert will be built up in three stages around the relocation of the North Kent lines and the eventual construction of the Crossrail lines. “As soon as your sheet pile is in [the ground] and you’ve excavated [the area] between, you can crane your culvert units into position, grout them in place and you’re finished,” says Swaby. Concurrently, work will also begin on the distinctive batwing-shaped glue laminated station roof linking them to an entrance on Harrow Manor Way above and to the west of the site. The use of precast structural elements will speed up construction, keep down costs and help to minimise railway possessions.
Precast beams and columns
“The majority of the station makes use of precast concrete beams for the platforms followed by a series of precast columns that support the new main station building,” says Swaby. “And then the new main station building makes use of precast concrete planks to form the deck and then finally on top of that you’ve got the main station roof which makes use of glulam [glue laminated] timber beams.”
The structure of the building has been split into three parts to allow phased construction that will be coordinated with the railway possessions booked for completion of track work. The aim is to keep the tracks live once the first precast floor slab has been dropped into place. “We’ll work on top of that [the precast floor slab], constructing the Glulam roof and all of the fit out and mechanical and electrical works in the station with live trains running beneath,” explains Swaby.
Ultimately, the North Kent Line part of the station is scheduled to be complete in late 2017 for North Kent line, while Crossrail trains will begin to arrive at the end of 2018.
In the meantime, the CSESP project has already triggered an important transition in the larger Crossrail scheme with the completion of the first 800m section of the Crossrail reception line in early May.
“The tunnelling for the Crossrail project is pretty much finished now,” says Mann. “The 800m section at Plumstead allows the focus to shift from creating the space underground to fitting it out.”
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