Page last updated at 10:19 GMT, Tuesday, 1 September 2009 11:19 UK

Sensors monitor tunnel movement

Tyne Tunnel - freefoto

Engineers have installed sensors inside the Tyne Tunnel to monitor any movement caused by work to construct a second river crossing.

More than 70 reflective prisms detect any shift in the structure, which is used by 38,000 vehicles a day.

On the north approach, workers need to excavate to just 3m (10ft) above the old tunnel, while ensuring a reduction in pressure does not cause it to rise.

So far it has moved 2mm - well within safety margins, contractor TT2 said.

The original tunnel passes beneath the riverbed, while the new crossing is to be made of several concrete tubes which will be lowered into a dredged trench.

Only at the northern approach - at East Howdon - will the new tunnel cross over the existing one.

Excavation is 80% to 90% complete

A project spokesman said: "Using sophisticated geotechnical analysis techniques, the contractor has been able to predict theoretical movement for the existing tunnel within this particular area of the project on the north side.

"Their findings suggest that during construction of the second vehicle tunnel at this point, the existing tunnel which may move upwards by around 11-12mm [just under half an inch].

"This level is well within the safe limits of the existing tunnel construction."

Work on the project began in October, and so far about 80% to 90% of excavation, for the approach routes at either end, has been completed.

Dredging of the river is set to start later this year, and the first of the tunnel units will be lowered in place around Christmas.

The new crossing is due to open in February 2011, when the existing tunnel will close for 10 months to allow modernisation work to be carried out.

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