The £140M Thames Tunnel carries the Channel Tunnel Rail Link (CTRL) from Swanscombe Marshes on the south side of the Thames to emerge at West Thurrock, Essex on the north side. The construction contract was awarded to a joint venture of Hochtief and J Murphy in January 2001.
The crossing comprises twin, 2.5km long, 7.2m internal diameter bored tunnels and 300m long cut and cover approaches followed by 150m of retained cut. On the south side, there is an additional 300m open cut section built through chalk.
Site geology generally consists of 10m of very soft alluvial clays and peats underlain by highly permeable Thames Gravels which are over Upper Chalk with flints, belonging to the Seaford and Upper Lewes Chalk units.
Groundwater levels in both the gravels and chalk lie close to the surface and are tidal close to the Thames. At the crossing point, the river channel is 1,200m wide and 18m deep. At its deepest point the tunnel, located within fissured chalk, will be subjected to hydrostatic water pressures of up to 4.5bar.
As previous major crossings of the Thames have proved, a number of major geotechnical challenges had to be overcome.
Ground engineering involves 25,000m 2of diaphragm walling and 500 reinforced concrete piles.
The diaphragm walls are 1.2m thick and extend down to depths of up to 32m (10m into the chalk) and were excavated using cable or hydraulic grabs. Both bottom up and top down construction techniques were used to build the cut and cover structures.
Dewatering of the Thames Gravels and the chalk was needed to allow excavation of the approach structures, with over 50 pumping wells installed inside the boxes on either side of the river. These had to be continuously pumped over a period of two years, with maximum outflow pumped over the two years of construction 580 litre/s on the Swanscombe side and 140 litre/s on the Thurrock side.
To mitigate against possible settlement of a nearby petrochemical tank farm on the Thurrock side, a recharge system was installed and the diaphragm walls further deepened into the chalk.
All the dewatering equipment has to remain in place until roof structures are complete and the backfill in place. Extensive propping, instrumentation and monitoring was also undertaken.
Highly abrasive flints within the chalk meant special attention was paid to the design of the TBM.
The gravel and chalk slurry from the tunnel excavations was pumped and treated to create an engineered material to infill an old chalk quarry close to the site.
This was the first time that a major chalk tunnel in the UK was driven using a mixshield slurry TBM and large scale trials of both the machine and proposed treatment plant were necessary.
Several innovative techniques were used during the contract, including the use of glass-fibre reinforcement in the diaphragm wall at the tunnel breakout, which allowed the TBM to cut with ease into a pressurised chamber mounted on the diaphragm wall.
Both tunnels are now complete.
Project team Client: Union Railways North Clients representative: Rail Link Engineering Main contractor: HochtiefMurphy Joint Venture Main subcontractors:
Amec-Spie Dewatering: WJ Groundwater Earthworks: Land and Water Ground treatment: Keller Ground Engineering