HS2 announces launch of first TBM

13 May 2021


Tunnelling can finally begin on Phase One of the UK’s high-speed rail line following HS2’s announcement regarding the launch of the first of 10 TBMs that will dig a total 64 miles (102km) of tunnels.

The 170m-long machine (Florence) is set to be launched from HS2’s south portal site next to the M25 motorway in Buckinghamshire, England. For the next three years, it will work 24/7 at up to 90m below ground, excavating a 10-mile (16km) tunnel through the chalk and flint geology of the Chiltern Hills.

The 10.26m-diameter Herrenknecht Variable Density machine is the largest TBM ever to be used on a UK rail project. It will create a tunnel with an internal diameter of 9.1m, installing steel-fibre-reinforced concrete segments as it advances at an expected 15m/day.

In June 2021, an identical machine (Cecilia) will start to excavate the second tunnel, also from the south portal. Both TBMs were transported in pieces over multiple shipments to the UK from Germany. They will be operated by HS2 main contractor Align JV – a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Tunnelling through the Chilterns could face a variable geology. Herrenknecht’s Variable Density TBMs are able to switch between EPB and slurry modes by maintaining permanent control of face pressure without the need for interventions in the excavation chamber.

Lining both tunnels will require 112,000 concrete segments, each at 2m wide. They are being produced in a purpose-made facility at the south portal, which is also the project headquarters.

When construction of the tunnels is completed, the segment factory and the rest of the south portal site will be cleared. The area will be landscaped with the chalk excavated from the tunnels to create high-quality grassland and other wildlife habitats.