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07 December 2005

Mott MacDonald implements Heathrow Express tunnel monitoring system at Heathrow Airport

Mott MacDonald has designed a tunnel monitoring system to measure movements in the Heathrow Express (HEX) Central Terminal Area tunnels, above which Laing O’Rourke is constructing a new multi-storey car park for BAA. The £47 million project forms part of a redevelopment and extension of Terminal 3.

The project involves the construction of a new multi-storey car park and demolition of the existing Terminal 3 short-stay car park. The new car park is located directly above the HEX tunnels and requires deep piled foundations. A challenge was at locations where the large 1.8m diameter piles had to be bored to depths of around 50m in close proximity (min 2.3m) to the station platform tunnels. To achieve this, the piles were cased to a depth beneath tunnel invert level.

Mott MacDonald was initially approached to assess the impact of constructing the car park above the existing HEX tunnels. This assessment looked at ground movements, noise and vibration. Following this the company was appointed to design, implement and manage a tunnel monitoring system to safe-guard HEX assets within a defined zone of influence during the construction of the new car park. The monitoring system measures movement of key tunnel structures and systems in real time, using total station theodolites and prisms.

Accelerometers installed directly onto the tunnel lining also continuously measured vibration due to the piling works, which were successfully completed in October by Expanded Piling Ltd, a Laing O’Rourke Group company, with no operational impact on the CTA or the Heathrow Express railway. The monitoring system is linked to an Emergency Preparedness Plan which defines specific trigger levels for movements, noise and vibration and identifies a sequence of actions in the event that a trigger level is exceeded.

Construction of the car park will be completed by mid 2006.
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