The expansion of Heathrow Airport has once again brought the spotlight onto one of the most complex tunnelling proposals ever considered in the UK: the realignment of the M25 motorway through a new underground section beneath a third runway. At the core of the £49 billion programme is a 3,500-metre northwest runway that would cut directly across the M25 between junctions 14 and 15.

Heathrow’s preferred solution involves constructing a new section of motorway offline, diverting traffic into a cut-and-cover or mined tunnel, and then switching flows onto the new alignment in a single overnight operation. The airport claims this would minimise disruption on a motorway that currently carries more than 200,000 vehicles per day. From a tunnelling perspective, the project presents several unprecedented challenges. The motorway carries exceptionally high axle loads, requiring the tunnel lining to meet stringent durability and settlement performance criteria. Highways England has already expressed concerns about long-term structural resilience, particularly under the dynamic loads of heavy goods vehicles and the additional stresses introduced by the proximity of aircraft operations above.

The choice of construction methodology remains open to debate. A cut-and-cover solution would require significant excavation alongside live traffic, with complex phasing to ensure continuous motorway capacity during construction. Alternatively, a bored tunnel option would need to overcome issues of limited depth, soft ground geology typical of the Thames Valley, and the presence of water-bearing strata. Both approaches carry risks related to ground movement, waterproofing, and the protection of existing utilities and airport infrastructure.

The scheme’s cost is estimated at £21 billion for the runway and motorway works, forming a major portion of the overall £49 billion expansion. This includes extensive land acquisition and the demolition of up to 750 residential properties. For tunnelling contractors, the scale of the motorway diversion and its interface with airport development represents both a technical challenge and an opportunity to deploy advanced construction methodologies in one of the UK’s highest-profile projects.

While Heathrow projects a swift overnight switchover of the M25 to the new alignment, many in the engineering community remain sceptical. Complex temporary works, safety management under live traffic conditions, and the need to maintain continuous resilience of the UK’s busiest motorway raise questions about the feasibility of such an accelerated programme.

More realistic scenarios point to staged commissioning with extensive testing before live traffic adoption. An alternative expansion concept, promoted by the Arora Group, proposes a shorter 2,800-metre runway that avoids tunnelling under the motorway altogether.

This £25 billion scheme has been positioned as simpler and less disruptive, but it lacks Heathrow’s institutional support. If pursued, the M25 tunnel would represent a flagship project for the UK tunnelling sector, demanding innovations in design, lining materials, waterproofing, and construction sequencing under extremely constrained conditions. For tunnelling professionals, it exemplifies the growing role of underground infrastructure in enabling major surface-level developments and the importance of engineering solutions that balance ambition with practicality.