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Cover story - Making order from chaos

CTRL - Getting the Channel Tunnel Rail Link to St Pancras Station demands a mind-boggling array of civil engineering. Andrew Mylius tries to comprehend the site.

Russell Lang's description of Channel Tunnel Rail Link (CTRL) Contract 103 is taken at full gallop, with barely a pause for breath.

Even so, it lasts for a good 20 minutes. He concludes: 'This is a fascinating job. There are lots of interfaces; lots of individual bits of work all going on at the same time.

Logistically it's a challenge.

'But from a technical point of view there's nothing out of the ordinary. We've got some big structures, but they're conventional designs.'

Lang is contractor Kier-Nuttall's project manager for the £107M Contract 103, which lies just north of London's St Pancras Station. He is passionate about making the complex and challenging job work.

Only a detailed map of the site can really illustrate the complexity of the project - which is in fact a combination of many individual projects, each a technical or logistical challenge in its own right. Access is a major problem, with main lines and the Regent's Canal in close proximity. Tunnels, bridges and viaducts have to be fitted into this confined space.

Progress is good, despite all.

Two major steel bridges have been placed since construction got under way in 2001. In the north-west corner of the site a major steel arch truss bridge was launched over the Midland Main Line (MML) in Christmas 2002, to carry the CTRL and North London Incline. In the northeast corner, the CTRL emerges from tunnel directly onto a steel truss tube that carries it over the East Coast Main Line (ECML).

Two more modest 19th century brick and plate girder structures, carrying lines into St Pancras Station over the Regent's Canal and busy Camley Street, have been demolished and rebuilt to take the higher rail loadings required for the CTRL.

New access to a materials handling depot used by construction materials firms Castle Cement, Tarmac and Hanson have been constructed, as the CTRL alignment scythes across their old access road. Materials are already being offloaded from a siding off the ECML and delivered to the firms' shared aggregates and cement hoppers across the MML via new conveyors and blowers.

Construction of the materials transfer system involved excavating a 6m deep shaft a mere 2m from the live railway. 'We used absolutely massive strutting - we couldn't allow any movement at all, ' Lang says.

Right now, Kier-Nuttall is finishing off two major concrete flyover structures which will enable the CTRL lines to cross each other without needing points and signals.

These are imposing but structurally straightforward reinforced concrete boxes. They have been built to enable unimpeded flow of trains, which is critical to achieving high capacity on the new CTRL lines into and out of St Pancras, and on to the West Coast Main Line (WCML) and ECML.

The contractor is also in the process of waterproofing the completed viaducts connecting to the WCML. A huge muck shift is still needed to raise the North London Incline onto its new viaduct, which will fly over the realigned and lowered York Way.

Realignment of the Incline and York Way will take place early next year during concurrent blockades lasting two months and one month respectively. York Way realignment work involves lowering the road, which at present runs on a brick arch viaduct, to ground level, so it can duck under the CTRL and North London Incline.

And far off, on the other side of the site in the north-west corner, a bridge referred to as RT13 is being partially rebuilt to take European rail loading. RT13 carries the North London Line, new North London Incline and WCML connection over a road. Strengthening involves removal of the steel girder deck and the tops of the brick piers under the tracks to be used by European trains. Here, the old structure is being replaced with reinforced concrete.

However, carrying out the work while allowing North London Line services to continue running and providing road access for rail maintenance contractors requires a phased approach to demolition and construction. The bridge is being sliced away longitudinally, one slow stage at a time. Piled retaining walls are required to maintain track bed stability on the MML as each long, thin slice is removed.

With the south abutment of the bridge now replaced, two further phases of replacement remain to be carried out. At present, reinforcement for a central strip is being laid. Once this is finished, the completed parts of the bridge will be handed over to the maintenance contractors, allowing Kier-Nuttall to move onto the section of bridge immediately next to the North London Line.

The only section of C103 that Lang concedes is technically difficult is excavation of a new tunnel, one of two being delivered as part of CTRL works for Network Rail. These, it is hoped, will one day be used to link the Thameslink line to the ECML.

Using a partially automated open face backhoe tunnel boring machine, Kier-Nuttall has already completed one of the tunnels.

But work on the second, which runs north of the first, is on temporary hold.

The more northerly of the new Thameslink tunnels briefly crosses a critical point under the original 19th century brick Thameslink tunnel, which in turn runs beneath the MML, itself perched right on the edge of Regent's Canal yacht basin. Ground movement caused by tunnelling could trigger a catastrophic sequential collapse.

Ground anchors have already been placed to retain the canal basin wall. However, the pause in tunnel excavation is needed while design of further temporary works is considered. Lang expects no more than 20mm of movement, which in a 7m diameter brick structure is not a big deal.

While Lang and Kier-Nuttall get to grips with their geotechnical challenge, the many disjointed parts of the site above ground will be fused together by degrees, forming a logical whole out of the present apparent chaos.

Completion is due late next year.

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