As soon as it surfaces from its tunnels under the Thames, the high-speed line becomes airborne in order to clear the twin hurdles of the A282/M25 Dartford Crossing approach viaduct and the far taller approach to the QEII Bridge. Standing side by side there is just enough space between the top of the Dartford Crossing approach structure and the soft of the QEII bridge to slip in a third structure - the Thurrock viaduct.
Rail Link Engineering (RLE) technical director Mike Glover describes the nely tuned manoeuvre as 'threading the needle'.
Thurrock viaduct forms a part of the leg from the Thames to Dagenham. Its 11.4km are packed with incident. Immediately north of the river the line rises through a retained cutting sunk in spongy marsh, mere metres from an oil tank farm.
After crossing the 1km long viaduct, the line follows the route of the London Tilbury & Southend Railway before launching across the Aveley viaduct. Next follow 7km of what Glover refers to as 'piled machine base' - essentially a ground-hugging viaduct - needed to cross Wennington and Rainham Marshes.
Rainham Creek viaduct brings the new line onto land owned by car firm Ford at Dagenham, where, Glover says, 'they wanted us out of their way as fast as possible, so they expedited our work', which included the replacement of two road-over-rail bridges.
Glover says that no part of this section was straightforward, but the most dramatic challenge facing RLE and joint venture contractor Morgan Vinci was the push-launched Thurrock viaduct.
RLE design liaison manager Martin Kirk played a major role, project managing the design. He recalls: 'Morgan Vinci had just completed the Medway viaduct on section one, which was also push-launched. They were able to bring their expertise from Medway and, incidentally, were also able to reuse the launching nose.' Thurrock viaduct is founded on three to four bored, reinforced concrete piles per pier, measuring 1.5m to 2m in diameter and installed into chalk that starts at depths of between 2m and 12m.
Piers of 5m wide and 2.9m thick, and rising to a maximum height of 10m, support a 3.3m deep, 5m wide pre-stressed concrete box spine, topped by a 12.2m wide deck.
Installing piles and erecting the piers brought construction hard up against existing structures.
'Pier 13 was tricky, ' says Kirk, 'because it was located next to an existing railway and close to an old bridge belonging to [quarry products rm] Civil & Marine/Hanson Aggregates.' Clearance to the bridge was less than 2m so careful monitoring was needed while the piles were installed. 'And although the nearby railway was not greatly used, temporary closures were necessary because of its close proximity, ' says Kirk.
Where the viaduct crosses the Dartford Tunnel's northern approaches, things were even tighter. Pier 16 sits between two viaduct decks - the Left Link viaduct and the main Essex viaduct - with less than 300mm clearance from each.
Ground stability was a concern and 'grouting was used all around that pier before piling took place so that they didn't risk undermining the foundations of the roads at either side', Kirk says. The piers were constructed using conventional piling machines and cranes.
The decision to push-launch the deck from a casting yard at Dartford was not taken until the contractor had been brought on board. 'It was an exciting turn, as the tender was not initially a launch proposal, ' Kirk explains.
'We started off looking at using a gantry system to support precast concrete segments, then post-tensioning the structure.
Push-launching had been rejected in the early design stages as the alignment made it difcult.' Morgan Vinci resurrected the push-launch idea to solve the problem of how to get the deck under the approach to the QEII and other obstacles, such as high-voltage power lines.
There is a cosy 5.7m clearance between Thurrock viaduct and the Dartford Crossing approach structure, and a just workable 750mm clearance between the CTRL's overhead power lines and the soft of the QEII approach viaduct.
Even without the physical challenge of getting a gantry under the QEII viaduct, pushlaunching posed a far lower safety risk when it came to crossing the A282/M25.
Accordingly the deck was cast in its 45m long sections, one behind the other, on formwork set up just behind the west abutment. As it was jacked forward, low-friction launch pads were continuously fed through as the deck slid over the pier heads, keeping bending loads to an acceptable minimum.
Pushes happened on a fortnightly cycle, with the complete push sequence, from start to nish, taking a year.
The deck is vertically curved on a constant 18.5km diameter radius over most of its length.
Over the end spans, however, the geometry changes, with a vertical straight alignment introduced. 'The deck had been pushed straight out of the casting yard, which was set up to produce a constant geometry. We were able to achieve the change by adjusting the formwork and continuing to push, meaning there was no break in continuity, ' says Glover.
The line barely touches the ground before rising again onto the 675m long, steel girder/ concrete composite Aveley viaduct - built off line and slid into place. From there it plunges into Wennington Marsh.
Kirk says: 'Running a railway line through bog - the worst land possible - has only one advantage. No one wants to live there so free land is available.' Of the 11.4km length of the section, some 7km had to be put onto a piled base. 'The water table was virtually at ground level so simple tasks, such as lifting out a pile casing, became a difcult problem because you risked losing the concrete into the soft ground, ' says Kirk.
Glover says that the need for the heavily engineered base was closely examined during scheme design: 'The Royal Society for the Protection of Birds have long-term commitment to increase water levels across the marsh so we couldn't rely on the hydrography. We had a lot of experience of building embankments and surcharging them on section one, but the evidence here was that simple earth bunds would keep subsiding.
'We'll have trains running at 230km/h in this area and at those speeds undulations or variability in the stiffness of the track bed would lead to oscillation of the trains. That increases track maintenance and creates train suspension problems. We decided that sitting the track on top of the marsh wasn't wise, resulting in construction of the longest piled viaduct in the UK.
'Of course, the structural engineers wanted a bridge.
Actually, all we needed was a piled machine base to take live loads and deal with vibration.'
Extensive eological investigation showed ballast and chalk underlying the marsh across most of the route. Driven 600mm square-section precast concrete piles were installed at close, 3m to 4m centres to reduce bearing stresses in the ballast. They worked in all but a few locations, where bored piles were needed to give higher skin friction - a total of 7,000 piles were used.
Leaving the marshland behind it, the railway rises over the 454m long Rainham Creek viaduct - like Aveley, a composite structure - grazing the Ford motor complex at Dagenham.
Work was largely completed on the contract last year, but completion of railway systems installation and testing is still under way in preparation for the arrival of Eurostar next year.