HS2’s tunnelling machines reach Chalfont St Peter ventilation shaft

The two giant tunnel boring machines that set off just inside the M25 last summer heading north out of London have completed the first stage of their tunnelling, having reached the ventilation shaft close to Amersham.

This 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.

The 78m deep shaft, about halfway between Amersham and 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. A crew of 17 people on board each machine keep them running, working in shifts and supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Each of the separate northbound and southbound tunnels will require 56,000 fibre-reinforced concrete wall segments – which are all being made in purpose build factories on site at the south portal, located just inside the M25. During their first 3.6 miles, Florence and Cecilia have combined installed more than 20,000 separate segments, each weighing around 8.5 tonnes.

The two tunnel boring machines dug tunnels along either side of the deep vertical shaft, and once they have passed further along the route, at the bottom of the shaft, the concrete tunnel lining will be broken into from within the shaft to create a link between the two.

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 90 hectares of wildlife-rich chalk grassland habitats. Chalk grassland used to be widespread across the hills of southeast England and are considered a habitat of international conservation significance with just 700ha left across the Chilterns.

In total there will be ten TBMs building the HS2 railway – working to create 64 miles of tunnel between London and the West Midlands including major tunnels on the approach to London and Birmingham. Three machines have been launched so far.