HS2 has confirmed that the two enormous 2,000 tonne tunnelling machines digging the high speed rail project's tunnels under the Chilterns have completed the first stage, up to the ventilation shaft at Chalfont St Peter.

The major achievement means that a combined total of over 3.6 miles has now been dug by the two machines – named Florence and Cecilia – since they launched from the southern end of the tunnels last summer.

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The 78m deep shaft at Chalfont St Peter is the first of five that will provide ventilation and emergency access to the ten-mile-long twin tunnels – which are the longest on the project. Once complete, the shaft will be covered by a headhouse designed to resemble local farm buildings.

Designed specifically for the geology of the Chilterns, each TBM is a 170m long self-contained underground factory, digging the tunnel, lining it with concrete wall segments and grouting them into place as it moves forward.

The TBMs named 'Florence' and 'Cecilia' by local school children launched in May and June last year and are expected to break out at the north portal in around two and a half years.

These first two TBMs on the HS2 project are operated by, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.