Dear Sir
The letter from Sir Alan Muir Wood (T&TI, March 2003) raises a number of interesting issues. He contrasts the methods used to construct the Heathrow Cargo Tunnel in the 1960’s with those currently being used on the Heathrow Airside Road Tunnel.
The comparison is valid as both projects represent the “state of the art” for their respective generations. Both tunnels are located under sensitive runways and taxiways, constructed in the London Clay with only minimal cover to the overlying water bearing sands and gravels. It is interesting to compare the key statistics for both schemes. The Heathrow Cargo Tunnel is a single bi-directional two-lane tunnel with an external diameter of 10.99m and a bored length of 625m. The Airside Road Tunnel comprises twin uni-directional single lane tunnels with an external diameter of 9.16m and a bored length of 1300m per tunnel.
Following an extensive site investigation and a detailed review of the available technologies, it was decided to procure the Herrenknecht dual mode TBM currently being used. During normal operation a nominal 0.5 bar of air pressure is applied to ensure face stability, thus obviating the need to employ individual face rams. The full EPB capability is available in the event of meeting pockets of sand and gravel. The success of the system is illustrated by the minimal ground settlement being recorded, typically less than 9mm.
By mechanising the tunnelling the labour requirements have been significantly reduced and progress has also dramatically improved. An average weekly advance on the first drive was 78m after full set up (peak 139m per week) as compared with 18m per week (peak 31m/wk) on the Cargo Tunnel.
Having achieved all of these improvements there is the question of cost. Even having made due allowance for inflation, is a comparison meaningful when not only the product is so different but so also is society’s attitude to risk? Undoubtedly tunnelling methods have evolved over the past thirty years to reflect our societal demands.
We are also keen to point out that the Designer has continued to have an active role during the construction phase to ensure that the design objectives are met.
Yours faithfully
Ian Fugeman
Head of T5 Rail & Tunnels, BAA
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