BTS | HS2
Q&A
Following their well received presentation to the joint meeting of BTS and BGA, in October 2025, at the ICE in London, the speakers - Martyn Noak and Mark Lemmon - took questions from the packed audience.
Q: Noemi Barrington, Project Manager SCS: Can you share a few elements of innovation you have implemented across HS2 on the tunnelling? A: HS2 has an innovation team that manages and shares innovation ideas across the programme. In tunnelling, we have done research into reduced carbon sprayed concrete lining (SCL), although that wasn’t progressed. We did, however, install low carbon concrete as temporary works in some of our sites. We have gone diesel-free at some construction sites, i.e., Canterbury Works in London Borough of Brent. EKFB have also been looking at innovation in the Green Tunnels, including efficiencies in installation.
Q: How do you manage uncertainty on a project like HS2? A: While clients are sometimes seen as risk-averse, i.e., using new products or the approach to the way things are done, HS2 has a pool of Subject Matter Experts who can review new ideas that maybe depart from Standards or Specifications for consideration.
Q: Peter Townsend, Mott MacDonald: How have the HS2 GBRs performed on various packages against Contractor’s claims? Have you found any unexpected ground conditions not covered by the GBR? A: The HS2 GBRs are part of the current live contract documents and were jointly drafted during the Integrated Project Team phase, by the Contractors and HS2. These GBRs are currently in use in the contracts to assess compensation events raised by the Contractors. The GBRs have been used to assess some compensation
events. There have been ground conditions encountered, raised by Contractors, that are not in the GBR but in these circumstances the contract has mechanisms to assess those claims by reviewing physical conditions information available to the Contractors at contract award, such as, for example, the Ground Investigation data and publicly available information.
Q: Phillipa Halton and James Lawrence, Geological Weathering: Why do you think the weathering grade in the Mercia Mudstone and Chalk was so highly varied? A: The variation could be for a number of factors. In both the Mercia Mudstone and the Chalk Group the grading was taken from logging of borehole samples and there is a possibility the variation was due to spacing of boreholes, sample quality, and/or logging interpretation. The main reason for variation in geology such as in the Mercia Mudstone and Chalk Group is the presence of jointing, where percolation of water leads to weathering and erosion of the rock near the joints. This is supported by the visibly deep weathering under the River Misbourne in the Chalk.
Q: Charles Allen: Could the panel give an update on the status of the cost and programme of the HS2 Civil Engineering Works? A: The HS2 Project is currently going through a reset on cost and programme, the conclusion will be announced to the DfT and Government at the end of 2025.
Q: Phil Quelch, Jacobs: If you were doing it again, as a Client, what would you do differently? A: One aspect that could be approached differently would be the Ground Investigation. The way it was set out by HS2 was for an initial phase of GI be undertaken by HS2, which was spaced at least to the minimum requirements of Eurocode 7. The plan was for the Contractors to come onboard and supplement that GI with their own GI, focusing on the areas of highest risk. We could have done more GI upfront, which might have helped with cost certainty, however, over the 200km+ alignment this would have been a significant cost, before the Project had Royal Assent.
Q: One of the Problems with London Clay reuse has been in embankments. How have you managed the known risk of moisture content issues and subsidence? A: London Clay is not being used for any embankments on HS2. London Clay tunnelling spoil from Euston Tunnels and Northolt Tunnels will be placed sustainably but not in any engineering structures.
Q: Rosa Diaz, Mott MacDonald: Considering this project is the first time Variable Density (VD) TBM machines have been used in the UK, do you think they were the right choice? How do you think they performed? A: They were used effectively in the Chiltern Tunnels, as shown in the geology presentation, particularly dealing with the high proportion of flint. Also, on the first 1km of the Chiltern Tunnels, under the M25, it was effective at minimising impact on third party assets. There is less evidence that the VD machine was utilised effectively at Bromford and Long Itchington Wood, but we are waiting for BBV to report on that via papers, etc.
Q: HS2 length in tunnels – What percentage of the journey to Birmingham is in tunnels? A: 20%, so about 10-12minutes.
Q: What are your considerations for changes as a Client? A: As a Client, HS2 is now tending to resist change as the knock-on effect to other aspects of the work, i.e., follow-on contracts, leads to significant costs. Proposed change is compared to Specification and Standards, reviewed by Subject Matter Experts. Acceptable change has to be safety critical or demonstrate good value.
22 | March 2026
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