Euston tunnel

To bring the HS2 high speed line into London, it was decided that a series of twin-bore tunnels underneath the city would be the most practical approach.[1] These will have a combined total of 26 miles (42 km), roughly equivalent in length to those built for the Crossrail programme. Responsibility for the construction of the section between central London and the M25 has been assigned to HS2's main works contractor, Skanska Costain STRABAG JV. In total, it has been envisaged that ten tunnel boring machines (TBMs) will be used to bore the 64 miles (103 km) of tunnelling along the HS2 route between the West Midlands and London.[1][2]

During 2018, the disused Euston Downside Carriage Maintenance Depot, close to Euston railway station, was demolished in order to clear space for the future construction work; the tunnel portal of the future Euston tunnel is to be present at this location.[3]

During 2019, a legal challenge to the design of Euston tunnel was launched by a local resident, alleging failures to address safety concerns;[4] in June 2020, the High Court dismissed the challenge as "impossible to accept".[5]

During October 2020, HS2 Ltd signed a contract with the manufacturing group Herrenknecht to build and supply the two TBMs for Euston tunnel.[1] These are to be custom-designed to suit the local geology, which ground surveys indicated to consist largely of clay and chalk, with which the tunnels are to be bored through. Each TBM is 140 m (460 ft) in length, headed by a 9.84 m (32.3 ft) diameter cutter head, weighing approximately 2,050 tonnes.[1] Delivery of the two TBMs has been scheduled to take place sometime in late 2021.[1]

In summer 2022, initial boring is expected to take place; this phase of the work is expected to take roughly two years to complete. Boring is set to proceed on a non-stop working pattern, only shutting down temporarily on Christmas and for bank holidays. During the excavation phase, it has been projected that 1,200,000 cubic metres (42,000,000 cu ft) of spoil (excavated material), weighing 2.46 million tonnes, will be removed by the TBMs.[6][1] Between August 2021 and December 2033, a portion of London Zoo's car park is to be temporarily borrowed for use by construction vehicles while Euston Tunnel is being built.[7]

Euston tunnel will be bored using tunnel boring machines launched from the western entrance, near Old Oak Common railway station.[1] The tunnel will be 4.5 miles (7.2 km) long, with an outer diameter of 9.5 m (31 ft). At its deepest point, the tunnel will run 50 m (160 ft) below ground.[1]

The pre-cast concrete sections lining the tunnel will be manufactured by Pacadar UK at their factory on the Isle of Grain, Kent.[6] These sections, each weighing seven tonnes, will be transported to site by rail. In excess of 160,000 cubic metres (5,700,000 cu ft) of concrete has been anticipated to be used to produce these sections.[6] The use of 3D printing technology will reportedly reduce the amount of concrete used in Euston tunnel in comparison to traditional methods.[8]

One of the few above-surface elements of the tunnel will be a three-storey building near Euston railway station, adjacent to the existing West Coast Main Line, which will accommodate a 12 m (39 ft) diameter ventilation shaft from the surface to the twin tunnels below; this structure will also facilitate emergency access to the tunnel along with supporting electrical plant rooms. The top of this building will be covered by a green roof, stone-paved courtyard and have an entrance from Park Village East.[9][7]