A record-breaking box push is at the heart of a £65M junction improvement in Kent. Alan Sparks reports from the Shortlands 'dive under'.
As Railtrack strives to cut the Eurostar travelling time between London and Paris to two hours 20 minutes, bottlenecks on the network that threaten this goal are systematically under improvement.
One such pinch point is at Shortlands, near Bromley in Kent.
Here the main twin track line to Waterloo carries the Eurostar and Connex services and is on the western twin tracks at Shortlands station. To the north of the station a twin track freight line to Blackfriars leaves the main line at Shortlands junction.
These lines must cross at some point between Shortlands and where the CTRL joins it at Swanley. Once the CTRL connection is complete the Eurostar will instead run on the eastern twin tracks at Shortlands.
'To eliminate this bottleneck we opted for a grade separated junction, so the freight lines can cross beneath the Eurostar line via an underpass, ' explains Railtrack senior programme manager Peter Ellis.
'This large investment is not just for Eurostar though, as the projected increase in demand for commuter and freight rail services make solving this junction snarl-up crucial for the long term.'
A further £35M is being spent between Waterloo and Swanley, where the Eurostar links up with the Channel Tunnel Rail Link section 1. By 2007, most Eurostar journeys will run along the CTRL to St Pancras, but until then the Waterloo line will continue to carry this traffic.
So once complete next October, there will be four years of speedier cross channel travel thanks to the improvements at Shortlands junction. Main design and construct contractor for the project, and for the massive box push at its heart, is Osborne, supported by design consultant Mott MacDonald.
Osborne is the sole UK licensee for the French developed Autoripage box push system, preferred by Railtrack because of its perceived low inherent risk factor. But what was dubbed the Chord Line Box was bigger and more challenging than anything ever undertaken in France - or anywhere else in the world, for that matter.
On a severe skew, the 70m long, 12m wide and 7.56m deep concrete box weighing 4,500t would have to be pushed a record 61m. 'Box pulling is perhaps a better term, ' says Mott MacDonald engineer, Tony Wilkins. 'The motive power for the move actually comes from strand jacks mounted on the rear of the box pulling on cables which run in channels along the underside of the base slab to anchors on the front of a thrust slab on which the box is actually constructed (see diagram p35).
'Before the box is poured the thrust slab is covered with grease, which then has a layer of polythene laid over it. For extra lubrication during the actual move a special bentonite mix is pumped under the box through pipes cast into the walls and base slab.'
No other treatment of the ground is necessary - with the major threat unpredictable groundwater. Here the project struck lucky. 'Throughout the site we have encountered a complete variation in ground conditions, with erratic bands of clay, sand and shale, ' says Osborne director Barry Elswood.
'But for the box push we struck a band of strong, consistent sand which, because there was no groundwater problems, was absolutely ideal.'
This, though, was not the real battle against the clock, as Ellis reveals: 'After all the public consultation that we undertook, we only secured planning permission on the Wednesday before the August bank holiday boxpush possession, which was booked years in advance. If we had missed this slot the next available time was at Christmas - which would have cost the project an extra £6M.'
Preparation began back at the end of last year, with the construction of a 10m deep cofferdam alongside the rail line. Value engineering by Mott MacDonald and Osborne resulted in a switch from contiguous piled walls in every location to a mixture of slope regrading, soil nailing, and steel 'H' piles with precast concrete planks spanning between.
At the bottom the thrust slab was poured, acting initially as a prop to the piled walls.
Piling subcontractor May Gurney installed 1050mm diameter flight auger piles up to 15m deep to stabilise the Eurostar line during construction. Some 20,000m 3of earth had to be excavated to create construction space for the concrete box, which was located as close to the existing line as possible and separated from it by a temporary headwall.
The big push was scheduled for a 76 hour possession over the August bank holiday. Balfour Beatty Rail Infrastructure Services removed around 70m of track in front of the box as excavation began. With the headwall removed, a team of up to 20 excavators working in three shifts began shifting another 10,000m 3to clear the box's path.
Most of this was dug out from the opposite side of the line using an unconventional 'hay making' technique. This involved the excavators at the face tipping their loads immediately behind them, for the earth to be picked up by a second line of machines which again tipped it behind them without actually tracking backwards, and so on.
In all, the actual muckshift involved up to 12 separate excavation and tipping operations.
Elswood says this method was adopted to minimise the risk from extreme weather conditions - something of a feature during much of the summer. 'We had seen haymaking used on an Autoripage project in Brittany, and it seemed the perfect option for this job, as it cut down dumper movements.
'But it did involve a lot of coordination and backup. In the end it went far better than we ever could have expected.'
As the V-shaped cutting was dug out the box push began. At full throttle the Autoripage system is said to be capable of hitting 6m an hour forward movement, but the limiting factor is usually how fast the earth in front of the box can be dug out. At Shortlands the restricted access cut progress to a maximum of just over 3m an hour.
Nevertheless the move was completed in 27 hours, giving plenty of time to complete the whole operation well within the 72 hour possession.
'At every turn we needed to consider what the impact would be on the public - always going for the quietest option. And over the whole scheme only one compulsory purchase was needed, ' says Elswood. 'Of major concern over the summer was controlling dust, but over the past couple of weeks our main worry has been mud.'
Newsletters have been published and a 24 hour seven day a week helpline has been set up, to help inform the public of why any disruption has been caused. And to avert further interruption, park and ride schemes have been provided for up to 360 workers at the height of the project.
Bromley council, aware of the possible issue over noise pollution, has a mobile phone link up with noise monitors on site, so that it is instantly alerted when noise levels are breached.