Thames Tunnel's twin tubes are the shortest bored tunnels on the CTRL, but were the most programme-critical work in Section Two until Rail Link Engineering decided to order two tunnel boring machine (TBM) heads rather than one.
That moved overall criticality to the complex work at St Pancras.
Starting in May next year, the first machine at Swanscombe has to drive 2.5km under the river from the south bank to north. Its back up train has then to be withdrawn and rapidly hitched to the second TBM so it can excavate the other tube at an average speed of 85m a week.
Both tunnels need track formation and walkway concrete in place ready for handover to the M&E installation contractors on 27 June 2003, just over 12 months after the tunnelling start date. Damages of £38,000 a day are payable if the deadline is missed.
Tunnelling technology has moved on since the Dartford road tunnels were driven in the 1960s and 70s. The full face slurry pressurised TBMs being built by Herrenknecht for contractor Hochtief/Murphy JV should easily deal with the 4.5bar water pressure expected in the pair of CTRL running tunnels, which will be comparable in scale to the two road tunnels.
But flints are a major concern.
Between 15% and 25% of the chalk face will be flints which have a strength of up to 700MPa and will create devastatingly abrasive shards of material when they are crushed inside the head of a TBM.
A special site task force has been set up to explore the problem. Pumping tests are under way to investigate the best way of protecting the pumps and pipelines which will be needed to remove the spoil from the face and convey it for disposal in a nearby quarry.
The flints will be separated out and a proportion of the chalk slurry processed and returned to pressurise the face. Settlement control under the river is less important than on other parts of Section Two but there are some interesting utilities for the JV to watch out for - including a 60bar pressurised chalk slurry main which carries material from a quarry on the north bank to a cement plant on the south bank of the river.
Hochtief/Murphy JV project director Andy Tauschinger is planning to make the precast concrete lining on site: 'Prices in the UK were too high and the cost of bringing in cheaper lining from the Continent was excessive, ' he says.
In all respects, except the angle of the taper wedge, the lining will be the same design as that to be used on the London Tunnels. The gaskets and hydrophilic seals will also be identical under the Thames to those used in the Thanet Sands below North London.
Cross passage excavation and construction of the sump in 4.5bar plus saturated chalk will be challenging but for the moment Taushinger dismisses these as 'a little bit in the future.
'We will see what the ground is like in the region of the cross passages when deciding what ground treatment is necessary.'
In fact the start up of the tunnelling work itself is still somewhat remote.
There is a huge amount of construction to be dealt with first, especially on the south bank at Swanscombe Marshes where the deepest 80m length of the approach cut has to be in place and complete ready to receive the TBMs. Herrenknecht originally reckoned that 100m of space was needed but the JV insisted the back up train was made more compact to cut back time spent on essential pretunnelling construction work.
Overall, the 700m long southern approach and 350m ramp on the Thurrock bank absorb two thirds of the contract value, with around 150,000m 3of concrete and 18,000t of reinforcement being required for them.
To gain some time the JV negotiated special early access at Swanscombe from early June so that 40,000m 3of hard fill could be placed in the 2.5m thick platform needed to support the piling and diaphragm walling equipment on the marsh. Keller is putting in a mat of vibro compacted concrete piles and the pressure is on to complete detailed design of the diaphragm walls so subcontractor AmecSpie can start getting the first panels in the ground during early August.
Contract 320 Thames Tunnel Thurrock to Swanscombe Hochtief/Murphy Joint Venture 3.55km Target price £128M
Key Features: Very tight time constraints. Twin machine driven bores cut through flint-rich saturated chalk under the Thames.
Each is 2.5km long and 8.15m external diameter lined with 350mm thick segmental precast, fibre reinforced, concrete segments. Herrenknecht slurry tunnel boring machines will be used, starting in May 2002.
Previous highway tunnels near the site at Dartford driven using compressed air and open faces in 1960s and 1970s were hugely problematical. But on the CTRL two thirds of the contract value relates to constructing the diaphragm walled and piled, retained cut approach structures. They will run down through soft marsh and are 350m long on the north bank and 700m long on the south, from where the tunnels will be bored.