ALASTAIR SMITH, CHAIR, BRITISH TUNNELLING SOCIETY | GUEST EDITORIAL


The second great change is consenting. The Planning Act 2008 and the Development


Consent Order (DCO) regime were designed to accelerate nationally significant infrastructure. For tunnels, I would argue the regime has quietly inverted the logic of good underground engineering. It is now not unusual for a major scheme’s consent application to run to hundreds of thousands of pages across thousands of documents, and to absorb hundreds of millions of pounds before permission is granted — sums that can approach the cost of building a substantial tunnel outright.


...DECISIONS HAVE MIGRATED


AWAY FROM THE ENGINEERS WITH THE TUNNELLING EXPERTISE TO HOLD THEM.


Norway, by way of contrast, built the 24.5km-long Lærdal


Tunnel — still the world’s longest road tunnel — for NOK 1.08bn, equivalent to some £160m at today’s prices.2 The DCO front-loads certainty: promoters


must commit to alignments, limits of deviation, construction methods and mitigation in exhaustive detail, well before a contractor with a tunnelling method in mind is anywhere near the table, and at a level of design maturity that better ground understanding will inevitably force to change. We are required, in effect, to fix the design of the tunnel too early, and then to administer reality through change mechanisms when it arrives. Every tunneller knows that is precisely backwards. Put the two shifts together and the pattern behind


our modern overruns comes into focus. It is not that we have become worse engineers — our machines, our understanding of ground behaviour and our risk management are incomparably better than what we had on the Channel Tunnel. It is that decisions have migrated away from the engineers with the tunnelling expertise to hold them. Risk is allocated by commercial and legal teams before the engineering is mature; consent fixes method before construction; and, when the ground does what the ground does, too often nobody at the table holds both the knowledge and the authority to respond quickly. Delay on our major projects today is more often a function of contractual and consenting process than of the ground itself. None of this is inevitable, and


the encouraging part of the story is that we already have the counter- examples — major projects, not modest ones.


September 2026 | 11


The Thames Tideway Tunnel, 25km beneath


London, was delivered broadly to budget by a structure built to answer precisely these failings. A dedicated delivery company, Bazalgette Tunnel Ltd, was created to own and finance the works, giving the programme the single accountable mind our fragmented models so often lack. Its three main works contracts were run as an alliance in which, as those involved put it, parent-company identity was “left at the door,” bound by a painshare/gainshare regime that made each party’s interest the interest of the whole. Contractors were engaged early, and — crucially for a tunnel — government acted as insurer of last resort for the genuinely uninsurable events, so the tail risk no contractor can sensibly price was held where it belonged rather than fudged into a lump sum everyone privately knew could not hold.3 Silvertown tells a complementary story: a


Design-Build-Finance-Operate-Maintain (DBFOM) concession, in which the RiverLinx consortium carried construction and long-term performance risk, opened in April 2025.4


Because the financiers


had equity at stake and decades of operating risk to protect, they wanted disciplined engineering at the table from the outset — and made sure they got it. Two very different structures; one common lesson.


Where the client is a capable and committed partner, where a single mind is accountable for the whole, and where risk is placed with intent rather than transferred wholesale to whoever signs last, the ground ceases to


...BUT WE ALREADY HAVE THE COUNTER-EXAMPLES — MAJOR PROJECTS, NOT MODEST ONES


Silvertown road tunnel, which runs under the Thames in London, is among key recent successful projects in terms of time and cost performance.


Image credit: TfL/Luca Marino.


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