The smallest, but arguably most complex of the three principal elements making up section two is the area around and including St Pancras International station.
Area 100, as it is known, has involved remodelling, refurbishment and extension of the station itself, provision of six new international platforms, three platforms for domestic high-speed services and moving of the existing Midland Main Line (MML) to four new platforms.
Independently of the other project contracts, it is one of the largest and most challenging railway schemes undertaken anywhere in the world in recent times, involving about £600M of work.
Ian Gardner was involved at the inception of the Rail Link Engineering (RLE) team and in setting the Area 100 strategy.
'The idea at St Pancras was always to try to get good railway access into the station, and to generate developable lands to re-energise this part of London, ' he explains.
It was inevitable that major work would be needed both inside and outside the station to accommodate the 18 car Eurostar trains. They measure nearly 400m in length - considerably longer than an intercity train - so right from the start the design team knew that the Grade 1-listed St Pancras train shed would have to be extended and grade-separated approaches would have to be provided to get trains to and from the twin-track CTRL without conict.
The solution has been to take the MML trains out of the shed and dedicate the space to the six new international platforms for Eurostar trains (platforms five to 10).
A 240m long extension - wider than the original - has been built to the north of the old station, giving full cover to the Eurostars and providing new concourses and platforms on the west side for the MML (platforms one to four), and to the east for the new high-speed services to Kent (platforms 11-13).
The extension structure and roof are unashamedly modern, rather than trying to replicate the tied arch structure of the Victorian train shed. It consists of a steel frame with 20m high columns on a very open 30m grid, clad in aluminium louvres and glass. A glass transept measuring 22.5m in length and more than 100m in width links the old building with the new.
'I think it would have been incorrect to try to ape Victoriana, ' says Gardner. 'Also, the sequencing of the work for the extension roof meant it had to be built in two halves, so we needed a design that could cope with that - the existing station's single arch concept would not have worked.' A major turning point in the design development came when the team opted to maximise space beneath the main platform level. 'The original scheme didn't make very good use of the street-level undercroft, ' explains Gardner. 'Part of our scheme was to regenerate it and take it from a liability to an asset.' The undercroft was created when the station was built in 1868. As the railway approaches the station from the north, the track level rises to clear the Regent's Canal, putting the final level of the tracks 6m above ground level. As a result the platforms are supported on a grid of cast iron columns, spaced so that the undercroft could be used to store beer barrels.
It was decided that this should be turned into the international arrivals and departures halls, with retail and catering facilities for the entire station. Power and services for station and railway were to be housed in a network of rooms excavated in the undercroft floor.
The station's main entrance has been moved from its front to the east side, meaning passengers and visitors enter at undercroft level and then head either for the domestic or international areas before going up to the platforms by escalator.
But the condition of the undercroft - along with the rest of the train shed designed by Victorian engineer William Barlow - could not be fully ascertained until the MML trains had been moved temporarily into the eastern part of the station extension and the old station vacated.
'At the start of the project it was still an operational railway station so we weren't able to get access to certain things, ' explains Gardner. 'We did preliminary calculations and worked out that the cast iron columns were strong enough and the foundations were okay, but the horizontal wrought iron deck was de nitely not.' The solution developed was to cast a new concrete deck directly on top of the existing deck, with the load transmitted straight into the columns.
'Columns were generally strong enough, but we had to be very careful how we put load into them. That was more difcult than we thought, ' Gardner says.
The 700 columns were in good condition, confirms Ailie MacAdam, RLE contract manager for the station works.
They sit unrestrained on gritstone pads atop brick pier foundations bearing on London Clay, some of which needed to be consolidated using grout injection. But they could take little lateral movement.
Major alterations to the station building and the excavations immediately next to it meant that the deck and columns were liable to movement. A vast station box was excavated for Network Rail's Thameslink service in the shadow of the Barlow Shed's western wall. Though heave of the box's base and walls was carefully controlled with the aid of deep, contiguous bored pile walls, and propping action from its massive roof and floor slabs, ground movement was still in the order of 20mm to 25mm.
And within the station, large longitudinal slots were cut in the deck slab to flood the undercroft with light and accommodate 70m long travelators.
The transverse deck beams, originally integral to the roof's tied arch structural system, had to be severed to create the openings. Though this system was replaced by the new concrete deck, the new slots meant that the undercroft would now be subject to a far wider thermal range than it had ever been before.
'To accommodate these movements we introduced a new bearing at every column head to cope with lateral loads, ' explains MacAdam. 'Temporary works, which had always been extensive to construct the new deck, had to be re-looked at to incorporate this design development.' In the shed's spectacular vault, reglazed to Barlow's original design, extensive corrosion was found on some of its wrought iron members. Work started off with a full survey, carried out by abseilers, who checked for loss of section and for damage to rivets. Secondary members were then removed for further checks before being grit-blasted back to sound metal, repaired where necessary, and repainted.
Principal arches, longitudinal beams and bracing was cleaned, repaired and painted in situ.
Worst corrosion was found in the north gable, which was to support one end of the glazed transept linking to the shed extension. The wrought iron 'boots' connecting the northernmost arch to its brick piers were in very poor condition and the piers themselves were degraded. RLE with contractor for works at St Pancras, a joint venture of Costain, O'Rourke, Bachy and Emcor Rail known as CORBER, and subcontractor Shepley was forced to devise an alternative method for refurbishing the north gable.
Over the rest of the roof it was able to hang a lightweight aluminium scaffold-cum-crash deck from its arches. This enabled high-level refurbishment, reslating and reglazing to be carried out while simultaneously the work on the station deck and undercroft was in full swing.
But for the arch supporting the north gable an access platform had to be built off the deck while the arch was temporarily propped and the boots replaced.
Replacing the boots involved crawling into the wrought iron structures and, using a hammer and bolster, breaking out mass concrete placed several decades earlier by British Rail to try and halt corrosion.
This change in working method cost CORBER four months, says MacAdam. It could have had major consequences.
'We had to get that gable done by July 2006, when the MML switched across from the interim platforms on the east of the station to its permanent home on the west, because passengers walk under it to get to and from the trains.
'We worked closely with the designers, the construction guys, the client, English Heritage and everybody that had anything to do with the station. We spent a lot of management time making sure everyone was on the same page and clawed back all four months.'
Time has been at a premium throughout the project and careful sequencing the only way to make sure it is finished in time for completion in 2007.
'We were totally dominated by the commitment to build the Thameslink box for Network Rail, ' explains Gardner.
The box lies beneath the new western extension to St Pancras and had to be constructed before work on the extension could start. Installation of the 400m long box's 24m deep, 1.2m diameter contiguous bored pile walls was approached like a jigsaw puzzle, with discrete sections added as parts of the site became available, and roads and utilities diverted repeatedly to free up land.
Piling took place either side of the live railway tunnels - Thameslink services were suspended only for excavation of the box itself, over a 35 week possession.
It was the Thameslink box schedule that drove the decision to temporarily divert MML services into the eastern part of the station extension - dubbed the eastern interim station: RLE had to nd a way to get the MML trains out of the Victorian shed so that work could start on the international platforms and all the internal works.
'Logically we would have built MML's new home rst, with everything else starting afterwards, ' says Gardner. 'But because of the Thameslink commitment we had to devise the more complex sequence of work to make it all possible.
'Only when we had finished the box and stabilised half of the train shed could we build MML's final home. It's made the whole thing fantastically interesting, but vastly more complicated than it might have been.'
Despite careful planning, construction of the west deck extension could have gone well behind programme when the contractor found considerably more human remains than anticipated while excavating the site of the former St Pancras graveyard.
'Managing a job like this, the key thing is to identify activities on the critical path and do what you can to de-risk them, ' explains MacAdam. 'This was a big obstacle, so we moved it off site [by removing the remains en-masse] so we could build the extension.' The discovery still set the programme back by over two months, but this was clawed back by a rethink of the way the Thameslink box was built. 'We went from cut and cover tunnelling to top down construction, ' says MacAdam.
'That gave us a couple of advantages: we could start building the west deck before we had nished the box and also work in a way that was less intrusive for the local community.' As the end of 2006 approaches, the west deck extension has been completed, the east deck is on schedule to be finished mid-2007, platforms are in place for Eurostar trains in the Barlow train shed, and mechanical, electrical and plumbing fit-out is well under way.
In the undercroft, blockwork walls have started to go up for the retail and catering units and to create the international departures and arrivals halls.