How would the Tunnel be built?

To the north and south of the River Tyne, the tunnel would be constructed using a technique known as 'cut and cover'. This is illustrated below.

Under the river, the tunnel would be constructed by the 'immersed tube' technique, which would connect with the cut and cover tunnels at the river banks.

The immersed tube tunnel sections would probably be constructed of concrete in a specially built dry dock in Howdon Basin. Four tunnel sections, each approximately 90 metres long, are likely to be required for the immersed tube.

A trench would be excavated into the river bed using appropriate conventional dredging
techniques. The sections would be floated out into the river and carefully positioned into the trench one at a time. Each section would be joined to its neighbouring unit, and each end of the tube could then be connected to the cut and cover tunnels north and south of the river.

The cut and cover construction would involve excavations of approximately 20 metres deep in North Tyneside and 30 metres deep in South Tyneside.

Immersed Tube
Tunnel Construction
(Illustrations of larger tunnels being
constructed elsewhere)

The tunnel units are constructed in a dry dock

The work is completed in a dry area

Temporary watertight bulkheads are installed and the dry dock is flooded

At the tunnel site, a trench is dredged

The units are floated out to their final destination

The tunnel units are sunk into position within
the trench, joined together, buried and water
is pumped out

Cut and Cover Tunnel Construction

The ground is cleared before construction commences.
Diaphragm walls or piles are sunk into the ground from
the surface. These retain the soil when excavation commences

Excavation between the walls commences. As the trench
becomes deeper, props are installed to support the
structure, and prevent ground movement

At the bottom of the excavation, the construction of the
tunnel can commence

The tunnel construction is completed and the trench is
backfilled with material previously excavated.The area is
now ready for restoration