Workers tunnelled in poor conditions working in a compressed air environment which affected the health of many of them in later life.

The New Tyne Crossing will be built using a different technique, known as immersion. Financial modelling has shown this method to be less expensive than a bored tunnel.

Immersion involves dredging a trench across the river and placing pre-fabricated sections in it.

These would then be covered and protected from any possible impact from above.

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

A trench would be excavated into the river bed. The sealed 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 will then be connected to the cut and cover tunnels north and south of the river.

Work in the river will be restricted to winter months when fish numbers are at their natural lowest.

Immersed tube tunnelling was pioneered in Holland and has been used to construct the Conwy Tunnel in Wales, the Medway Tunnel in England and the Oresund crossing between Denmark and Sweden which was opened in June 2000.  

The tunnel units are constructed in 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 there final destination	The tunnel units are sunk into positions within the trench, joined together, buried and water is pumped out.