HS2 TBMs complete first stage in Chilterns

The major milestone means the two machines – named Florence and Cecilia – have dug a combined total of more than 5.8km since they were launched from the southern end of the tunnels last summer.

The 78m-deep shaft at Chalfont St Peter is the first of five that will provide ventilation and emergency access to the 16km-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.

Each of the Herrenknecht TBMs – the first variable density TBMs to be used in the UK and the largest ever used for railway tunnelling in the UK – is 170m long and has a 10.26m diameter. They also contain their own grout batching plants. The TBMs are operated by Align – a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Each of the separate northbound and southbound tunnels will require 56,000 fibre-reinforced concrete wall segments, which are all being made in purpose-built factories on site at the south portal, located just inside the M25 motorway. During their first 5.8km, Florence and Cecilia combined have installed more than 20,000 separate segments, each weighing around 8.5 tonnes.

Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels and used for landscaping on the south portal site. Once construction is complete, this will help create around 90ha of chalk grassland habitats.

In total there will be 10 TBMs on the HS2 project to create 103km of tunnels between London and the West Midlands, including major tunnels on the approach to London and Birmingham. Three machines have been launched so far; in addition to Florence and Cecilia, Dorothy was launched in December last year to dig the 1.6km tunnel under Long Itchington Wood in Warwickshire.