TUNNELLING WORK at Heathrow Airport has led to settlement of up to 6.5mm on the Piccadilly Line tunnel.
The settlement is being closely monitored by London Underground (LUL) engineers, who will close the tunnel if the situation worsens.
Work has begun on an access tunnel from the central terminal area to the western side of the airfield, near to where the proposed Terminal 5 will be located.
A spokesman for BAA said the tunnel was a standalone project to provide vehicle access for new aircraft stands, and was being built regardless of Terminal 5 .
The new tunnel passes across the top of the Underground tunnel and is being constructed by cut and cover.
The Piccadilly Line tunnel is made from precast concrete rings, which give a degree of flexibility.
Bored concrete tension piles, up to 1m in diameter, have been sunk around the new tunnel by contractor Miller Group. These are to prevent ground heave in the clay before excavation works start.
But monitoring by engineers from BAA and LUL has shown that the Underground tunnel has settled. A spokesman for LUL said that it had an effective monitoring system in place and engineers on site. Should anything 'untoward' happen, it was ready to 'swing into action', she added. This could include placing speed restrictions on the line or even closure.