Engineers have put sensors inside the existing Tyne Tunnel to make sure it doesn't move too much as a result of construction of the second tunnel nearby.
More than 70 sensors detect any movement in the tunnel, which is used by 38,000 vehicles a day. So far, there has been a little movement, but it is well within safety margins, say engineers.
When the new tunnel is completed, both tunnels will be open to trucks. The new tunnel will be much straighter than the original - which curves at one end - and is not as deep.
In future, northbound traffic will use the old tunnel, while southbound traffic will use the new one, which is due to open in December 2010.
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