HS2 | BTS


completed over the top of the TBM assembly area so the only way in or out is now the ARLT. The TBM drives do not end in a traditional


reception chamber. Instead, they will finish inside an SCL chamber where the machines will be broken apart and disposed of through the ARLT. It was explained that to achieve the level of train


service required, there needs to be two Up Lines into Euston station for the single Down Line. This bifurcation needs to take place below ground due to alignment constraints, requiring the construction of the Euston Caverns.


Euston Caverns The Down Line will finish just north of the Euston Cavern Shaft in an SCL reception chamber. This SCL tunnel will then be extended up to the open cut box outside Euston station. A large cavern will be driven from the shaft to facilitate the split of the Up


Line into two SCL tunnels (Up Line East and Up Line West). There will be three caverns of reducing size (15m, 13m and 11m) and the TBM will be driven into the smallest one at the north end. Both TBMs will be dismantled in the SCL tunnels and returned through the ARLT. To minimise ground movement, the outer lining


of the largest cavern will be driven as series of interconnecting ‘stacked drifts’, SCL tunnels circa 2.5m ID driven from the crosscut adit. These are subsequently backfilled with concrete to form a large pipe-arch. When the arch is complete, the cavern is excavated in traditional heading bench and invert methodology. This will be a critical piece of construction due to


the shallow cover to the West Coast mainline above, and the delicate Network Rail retaining wall to the side. Construction of the cavern is due to commence around 2027.


Isometric view of tunnelling in Euston Approaches


TBM drive volume loss by chainage and geology


March 2026 | 21


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