HS2 | BTS
rock, whilst Grade III & IV are more weathered. Discontinuities in the structured mudstone give
the rock a high bulk permeability, which has caused some historic difficulties in tunnelling with high water ingress. Also, the low plasticity of the mudstone means there is a risk of rapid softening upon wetting, which is a particular risk in TBM tunnelling at the cutterhead and in the muck away system.
Challenges The Mercia Mudstone grading varied both vertically and horizontally along the Bromford Tunnel. BBV reported challenges with the variability and
plasticity of the mudstone. In particular, they cited that the ‘sticky’ material in the TBM cutterhead: ● caused blockages in the slurry network; ● required continual adjustment of the TBM Operating parameters; and,
● requiring regular stopping and cleaning.
TBM PERFORMANCE DATA The first TBM launched in May 2021 and HS2 has been gathering data ever since. Total HS2 tunnelling was compared to other London tunnelling projects, and it was noted that HS2 has bored more tunnelling than the Jubilee Line Extension, Tideway, London Power Tunnels Phase 1 and 2, and Crossrail. HS2 has now driven nearly 75km of TBM tunnel,
plus about another 2km of SCL-lined tunnel which equates to about 80% of all the Phase 1 tunnelling. The HS2 team developed a TBM tracker that
enabled the tunnelling rates for all drives to be tabulated. There are many factors that influence TBM rates so it was noted that a direct comparison between each drive shouldn’t necessarily be made but it provides interesting data nonetheless. The rates provided are rolling averages and it
was noted that each drive was not far away from achieving the programmed progress rates they set out to achieve.
March 2026 | 19
Similarly, TBM utilisation was presented with the
same caveat that there are many different influences on when a TBM is available or not. A graph was presented that showed a range of TBM availability between just over 30% and just over 80%. The HS2 tunnels team has also tracked volume
loss. Generally speaking, the values are well below the contract value of 1%, with the highest values being for the Northolt Tunnel East drive (semi-open mode). The volume loss values for the Chiltern and Bromford tunnels, in Chalk and Mudstone, were generally too small to measure except for a small spike towards the north end of the Chilterns.
FUTURE TUNNELLING Euston tunnels The Euston tunnels will be launched in Q1 2026 from the stub tunnels at OOC. These tunnels are 7.55m-diameter and 7.3km-long with 18 CPs. Both EPBM machines are now nearly fully
assembled in the stub tunnels, which has been a challenge for the SCS team with access through the station box having to be coordinated with TBM and gantry delivery and availability. The roof is now
Total HS2 tunnelling
Below left: Photo from head interventions showing clay
Below right: Photo from head interventions mudstone
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