First stage of excavating the HS2 Chiltern tunnels completed
Two Tunnel Boring Machines (TBMs) excavating HS2’s longest tunnel have completed the first stage. The first section from the Chiltern tunnel’s south portal to the first ventilation shaft are finished.
Since their launch last summer, TBMs Florence and Cecilia have travelled a combined distance of 5.7 kilometres. In the end, the Chiltern tunnel will be a 16-kilometre-long twin tunnel, the longest on the HS2 project. The TBM’s have now reached the Chalfont St Peter vent shaft, a 78-metre deep shaft which is the first of five that will provide ventilation and emergency access to the tunnel.
Designed specifically for the geology of the Chilterns, each TBM is a 170-metre long self-contained ‘underground factory’. It digs the tunnel, lines it with concrete wall segments and grouts them into place as it moves forward.
In total there will be ten TBMs on the HS2 project to create 64 miles of tunnel between London and the West Midlands. This includes large tunnels on the approach to London and Birmingham. Three machines have been launched so far.
The Chiltern tunnel will take HS2 underneath the hills, which is to safeguard the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation of the new railway, says HS2 Project Client David Emms.
17 people on board the TBM
The two TBM’s are expected to break out at the north portal of the Chiltern tunnels 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. A crew of 17 people are on board each machine to keep them running. They are supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.
During their first tunnelling, TBM’s Florence and Cecilia have together installed more than 20,000 separate segments, each weighing around 8.5 tonnes. Each of the separate northbound and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete wall segments. These are made in build factories on site at the south portal.
Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels. This will be used for landscaping on the south portal site. Once the construction is complete, this will help create around 90 hectares of chalk grassland habitats. Chalk grassland used to be widespread across the hills of south east England and are considered habitat of international conservation significance with just 700ha left across the Chilterns.