Barrett Byrd Associates

Preparatory works for launch of the massive CTRL bridge over the East Coast Main line at King's Cross was taking a small but critical step this week.

Piling for a key element of temporary works is currently taking place quietly and efficiently just below where cross-Channel high speed trains will emerge from tunnel at King's Cross on their way to St Pancras. The piles will support trestles critical to the safe launch of a new steel bridge which will carry the trains over four existing East Coast Main line tracks.

Timing – with the piling and all else – is crucial: the 2060t bridge is scheduled to slide into place during a 55 hour possession at Christmas. "The bridge has to move at that time, not least to provide access to the CTRL tunnel portals where breakthrough will be imminent," says project manager of Kier Nuttall joint venture Russell Lang.

Kier Nuttall is responsible for building the ECML overbridge as part of its £107M CTRL Contract 103. This contract is for the infrastructure which links the western end of the CTRL tunnels to St Pancras station, a major project in its own right with over 30 substantial structures including bridges, tunnels and retaining walls. Project manager and designer is Rail link Engineering (RLE).

"It's a highly complex and challenging contract, which includes almost all elements of civil engineering," Lang says.

Adding to the complexity is having to build alongside, over and under some of the busiest railway tracks in the country, not just those of the ECML but the Midland Main Line and the North London Line too. The rules on site are incredibly strict, wholly directed at ensuring construction operations are carried out in the safest of manners.

This applies equally to the very biggest jobs and those at the other end of the spectrum, such as the ECML bridge temporary works piling mentioned above, being carried out by foundations specialist Roger Bullivant Ltd (RBL). "It would be easier if we could bring in a really big piling rig and deliver ready mixed concrete to the piles by truck, but it is not possible because of the safety rules," says RBL's piling foreman Jason Hartman.

"The access is there but it is very close to live track, onto which there can be no possibility of anything toppling. Consequently we are using a small rig out of which we're having to get the very best; and pumping the concrete in quite a distance, which has given us a few difficulties. Despite this, we are well up in quality terms and within programme."

RBL is installing twelve 600mm diameter cast in situ concrete piles, four of them 24m deep and eight 27m, a not inconsiderable diameter and depth for the little Klemm 703 doing the augering. It is having to be operated with particular skill.

The ground is London Clay overlain by a metre or two of made material containing – surprisingly after the hot summer – an amount of water. Coping with this by sleeving the top 3m or so of bore has slowed progress, according to Hartman, but not by much. Just the top 15m of each pile contains reinforcement, the first 4m of T32 bar, the remaining 11 m of T25. Keeping to the 'small but safe' rule, the cages are being delivered in 3m sections.

Steel trestles founded on RBL's piles will provide a vital element of support when the ECML overbridge is launched at Christmas. At the moment, the massive structure is nearing completion on the line of the CTRL to the west of the ECML.

It looks very impressive, a 75m long steel through truss with large section booms and diagonal members, and almost equally large curved anciliary steelwork to carry acoustic cladding. Once in position, the totally enclosed bridge will effectively extend the CTRL tunnels westwards by a short way to ensure Eurostar trains stay out of earshot of nearby residential properties.

The bridge has been designed by RLE and fabricated by Watson Steel Structures. It will be jacked longitudinally into position, fronted by a 27m long temporary nosing to help achieve reach.

Even so, and even with its western end 'ballasted' by having the concrete deck there installed, the dynamics are such that the structure cannot be pushed over its western abutment to reach the eastern one without intermediate support. The stretch over the ECML is just too great. Hence RBL's temporary piles and the trestles.

"We had a major bridge launch on this contract last Christmas which went well," Russell Lang says. The launch this December is a more complicated one, however. "The ECML bridge deck tapers from 12.75m wide to 9.9m wide, which means the skate equipment is more complex. Nevertheless, everything is running smoothly at the present time and I have no reason to believe activities this Christmas will be any different."

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