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National Grid's new cable tunnel below the Thames sought out a special kind of chalk, Gareth Beazant reports from Swanscombe.

The Channel Tunnel Rail Link's Thames Tunnel has rightfully been grabbing the headlines in what is a busy time for the UK tunnelling sector. It has certainly stolen the limelight from another tunnel below the Thames, just a couple of kilometres away, which ordinarily would have been big news in the tunnelling world.

Soldiering away, without the paparazzi in tow, the National Grid has quietly built a 2.4km long, 3m diameter cable tunnel under the river at Thurrock.

National Grid has three crossings of the Thames in east London: a cable tunnel in Barking; an overhead line from West Thurrock to Gravesend; and cables under the road deck of the west tunnel on the M25/A282 Dartford River Crossing (DRC).

The DRC cables form part of a circuit from Littlebrook substation in Kent to Barking substation north of the Thames in Essex. These cables were in line for replacement and National Grid initially looked into running more cables through the Dartford road tunnel.

But, says National Grid project manager Jim Street, 'The main problem with the road tunnel is you can't access the cable when you want because people use the roads all the time.' This ruled out the DRC option and the company looked at putting in an alternative.

Consultant Mott MacDonald carried out the feasibility study. It considered various options including a second overhead line over the Thames, a submarine cable and directional drilling.

Mainly due to cost, risk and environmental factors, Mott's went for a tunnelled solution.

Fortunately land was available near the Littlebrook substation to sink a 9m diameter launch shaft and establish a route running beside the M25 crossing, which minimised the length of both tunnel and cable.

The resulting proposal for a 2.4km tunnel has capacity for future upgrading of the system and the advantage of allowing existing cables to be removed from the DRC. At 3m in diameter it has easy access for maintenance and might even provide the ideal film location for the Italian Job sequel.

The outline design contract was awarded to Babtie in 2001 and Amec subsequently won the contract to drive the new tunnel, along with decommissioning the old DRC cables.

Babtie commissioned site investigations from Soil Mechanics between June and August 2001.

These established ground conditions of made ground, alluvial sands, soft peaty clay alluvium, river gravels and upper chalk at 15m to 20m below ground level.

Concern over flint bands prompted the company to draft in Professor Rory Mortimore from Brighton University to carry out a stratigraphical survey. He identified the preferred tunnelling horizon at 35m to be within Lewes Nodular Chalk beds where flints were unlikely to be a problem for the Lovat TBM.

This proved to be the case and construction of the £16M tunnel was completed earlier this year.

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