Engineers working on Contract 310 had better pack their wellington boots before starting construction of the 11.4km route next year.
Waterproof footwear will be essential as a third of the route passes directly over the Inner Thames Marshes at Rainham and Wennington in east London.
Such poor ground conditions make tunnelling impractical.
Instead, Rail Link Engineering (RLE) engineers decided to use a piled slab to carry the Eurostar trains across the bog, acting like a ground level viaduct.
The 7km long slab will start where the track rises out of the London tunnels near Dagenham and end just past the eastern end of the marshes at Aveley.
However, it will set the winning contractor - due to be announced in September - a number of challenges, explains RLE area project engineer Steve Dyson. 'There are no public roads (into the marshes), ' he explains. 'In fact, one contractor is considering bringing materials in by boat.'
Moving plant across the bog itself would result in the equipment simply sinking into the marsh, explains Dyson. 'Instead contractors will have to work along the trace, as if it is a tunnel.'
Contractors will have to start at the ends of the piled slab and work inwards. The structure itself will be laid over 7,000 piles, which will be either driven or bored into the gravels to depths varying from 5m to 15m. The precise choice of pile is still dependent on the results of tests carried out last year (NCE 14 December 2000).
'Most of the viaduct is slightly above ground, ' says RLE package manager Anil Patel. This is to cope with potential water level rises in the marsh or flooding after the track is complete.
Control of water levels in the marsh during construction is also important. As a specially designated breeding ground for marsh birds, the area is carefully managed under a long lease by the Royal Society for the Protection of Birds.
Changes in water level can affect the delicate ecology of the area, some of which is also a Site of Special Scientific Interest. As a result, the winning contractor will have to devise a controlled and localised dewatering scheme to minimise the impact of construction.
But the contractor will also have to deal with the man-made hazards. Once owned by the Ministry of Defence, the land may be scattered with small bore ammunitions and even the occasional bomb from the Second World War.
Building the track's slab foundation is not the only challenge facing the eventual winning project team on Contract 310. The route also passes over roads, railways, rivers and industrial lands requiring construction of three viaducts and several road diversions.
Also critical to completion of the project is the 1,200m viaduct at the eastern, Thurrock, end of the contract. The largest of three viaducts along the route, it negotiates London's main traffic artery - the M25 motorway.
The viaduct lifts the track above the northbound exit road from the Dartford tunnel, but below the north approach of the giant Queen Elizabeth II cablestayed bridge. This will leave a minuscule 1m headroom between the train's overhead power supply and the bridge.
Work will continue under the close eye of the Highways Agency and operate in strictly policed work windows. The viaduct must also be completed in time to allow the rolling fit-out works, starting from the beginning of Section Two in Ebbsfleet, to continue along the trace.
'The viaduct will be built using precast segmental construction, ' says Dyson, adding that the 45m span segments will be supported at a maximum 20m height by piers resting on piles bored in the deep alluvium.
A particularly tricky operation will be to fit one of the piers between the Dartford tunnel exit road bridge and slip road. In addition, once the track is complete careful consideration will have to be given to protecting Eurostar trains from road traffic and vice versa.
Drivers on the M25 will also be protected from the possibility of train derailment by concrete derailment containment walls either side of the tracks.
'Eurostar has a very low centre of gravity, however, ' says Dyson, pointing out that this makes it very unlikely that a train would topple onto the motorway.
C310 West Thames - currently out to tender Key features: Track runs at ground level over marshland; site access is restricted by poor road access, the water table is high; a viaduct takes trains over the M25 motorway and below the Queen Elizabeth II bridge.