CONTRACTORS working for Railtrack will carry out one of the biggest tunnel box jacking operations in the UK at Shortlands Junction near Bromley this weekend.
A 70m long, 12m wide concrete box will be hauled through an existing railway embankment using six 55t hydraulic strand jacks.
The strands are anchored into the far end of a concrete guide raft cast underneath the box's launch area, while the strand jacks are mounted on the outer end of the box. As the jacks pull on the strands, they will move the box forward.
When the operation is complete, the new junction layout will increase capacity at a key pinch point on the rail network.
It will allow four Eurostar trains an hour to travel between Waterloo International and the Channel Tunnel without being caught behind slow moving freight services.
Railtrack has employed contractor Geoffrey Osbourne to carry out the box jacking operation.
From Saturday Railtrack will close the junction for 76 hours and remove the existing track to allow excavation ahead of the box jack. The track will be re-laid for re-opening on Tuesday.
Geoffrey Osbourne project director Barry Elswood said the reinforced concrete box has been built in situ in a cofferdam excavation next to the track.
'We've got 20,000 tonnes of muck to excavate ahead of it, ' he said. 'If the excavation moves ahead of the box it will be fairly straightforward.
'But it will be hard dealing with such a large volume of soil over such a narrow face.'