The fifth tunnel boring machine (TBM) to be put to work on London’s HS2 tunnel project has been launched from a site in West Ruislip in west London.

The 140-m (460-ft) long, 2 kt (2,200 st) Herrenknecht TBM - named Caroline –will bore 8 km (5 miles) toward Greenpark Way.

New Civil Engineer (NCE) reported that Caroline will be operated by Skanska Costain Strabag JV (SCS JV), HS2’s main works civils contractor constructing the HS2 tunnels in London.

HS2 civils delivery director Michael Lyons told NCE that the ground conditions at this site are quite different to the other HS2 tunneling locations but that the team was able to draw from experience from other major tunneling projects in London, notably the Crossrail project.

Planning to alter certain methods when driving the TBMs through the West Ruislip site started back in March 2018 when a prehistoric clay coastline was discovered by HS2’s ground investigations team.

They named the sub-tropical coastline, dating back 56 million years, the “Ruislip Bed” which consists of a large layer of black clay roughly 33 m (108 ft) below the surface. Geological experts believe the bed was formed from densely-wooded marshes very close to a sub-tropical sea.

As well as the mixture of materials that make up the geological composition, there is a lot of water content in the area. How the TBM is used will be affected by changes in the composition of the ground along the tunnel drive, for example if the amount of moisture, chalk or clay reduces or increases.

The SCS team is also aiming to reuse the spoil excavated by the drive to aid the project and rebuild West Ruislip golf course, which was located on the TBM launch site before HS2 set up. Spoil from the tunnels at West Ruislip will also be used for sustainable placement to the north, creating embankments for the new railway, as well as new environmental habitats.

All of the TBMs operated by SCS JV are earth pressure balance machines. Caroline and Sushila are slightly different to other TBMs as they both have automated systems to place the concrete rings behind the cutter head. Machinery to aid placement has been used for many years but previous TBMs have called for manual input to position the rings. The automation idea came from Align JV, which is boring the Chiltern tunnels.
TBM Sushila is now 67 m (220 ft) into its dig but both machines have a long way to go. They will drive 24/7 for 22 months, only excluding Christmas Day. Named after 18th century astronomer Caroline Herschel, TBM Caroline will be operated by a crew of 15 people, working in shifts. An additional 25 people will directly support each tunnel drive on the surface.

Separately, two other TBMs will set off towards Greenpark Way in Greenford from HS2’s Victoria Road site in Acton in 2023 to build a further 5.4 km (3.4 mile) twin-bore tunnel. Combined, the quartet of TBMs will build 13.5 km (8.4 mile) of twin bored tunnels between West Ruislip and the new high speed rail super hub station at Old Oak Common.