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Tyne tunnel's difficult ground conditions to be investigated

A DETAILED site investigation for the new Tyne tunnel, for what are thought to be very difcult ground conditions, is due to start this month, a project manager told GE.

'Our two initial SIs to build up the reference design for the public inquiry showed there is a lot of clay and made ground in the area as well as a lot of ssured rock, ' said project manager for client Tyne & Wear Passenger Transport Authority Paul Fenwick.

A consortium led by French contractor Bouygues has been named as preferred bidder to build and operate the £250M tunnel under a 30-year concession for the project that will involve building a 360m immersed tube tunnel.

'[Bouygues] now needs to make a more detailed investigation to get the additional information it needs for the detailed design, ' said Fenwick.

He added that Bouygues lead designer High-Point Rendell, along with the other shortlisted bidder Balfour Beatty, had developed tunnelling proposals at tender stage that differed from the reference design.

But Fenwick could not reveal details before the final contract is signed in August because it is commercially sensitive.

'The two bidders applied the reference design in slightly different ways to bring in some innovation, ' said Fenwick.

'We are happy with the financial model they have shown us but recognise things that need to be sorted out.'

If the deal with the consortium is signed in August it is hoped the project will start on site in February 2008 and open in summer 2011.

The contract will also include refurbishment of the existing Tyne tunnel, built in 1967, that rates as poor for safety because it does not have a 'longitudinal escape cell'.

Building the new Tyne tunnel

Bouygues must construct a 2.6km two-lane road tunnel comprising two cut and cover sections north and south of the Tyne with a central 360m long immersed tube section.

In the scheme designed by Tyne & Wear Passenger Transport Authority's advisers Arup and Royal Haskoning, three 100m and one 60m long sections will be constructed in a dry dock on the north bank of the Tyne. The concrete sections are rectangular and will measure 14m wide by 9m deep with 1.5m thick walls.

When the first two sections are complete, the dry dock will be ooded allowing them to be oated out. The process will be repeated for the second pair of sections.

The sections will then be pumped full of water and sunk into a pre-dredged channel, allowing them to be placed in a controlled manner.

They will then be connected and the tunnel de-watered.

Dredged material will be used to cover the tunnel with rock armour which will protect it from trailing ship anchors.

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