Question to the Department for Transport:

To ask the Secretary of State for Transport, with reference to the HS2 six-monthly report to Parliament published in May 2026, what lessons learned and innovative construction methods were deployed in the completion of the north portal structure at the Chiltern Tunnel; and which other transport or infrastructure projects have subsequently adopted, or are expected to benefit from, those lessons and methods.

The Chiltern Tunnel North and South Portals have been specially engineered with perforated concrete hoods to prevent the possibility of sudden changes in air pressure and a resulting ‘sonic boom’ caused by trains entering and exiting the 10-mile-long tunnel at high speeds. Delivered by our construction partner, Align, the North Portal was built several months faster than the South Portal, drawing on lessons learnt from the South Portal. The formwork for the walls of the North Portal was set up in panels on rails, the steel reinforcement was assembled on the ground, lifted and placed in the preset formwork ready for concrete to be poured, and the walls on both sides of the Portal were advanced concurrently. The formwork for the curved roof was similarly set up on rails, steel reinforcement pre-assembled on the ground and the roof panels constructed sequenced with the advancing wall construction. The use of rails significantly reduced the number of shutter lifts required, reducing the risk of down time as a result of high wind speeds.