HS2 Chiltern tunnel completed with sonic boom protection

HS2 Chiltern tunnel construction has finished on the project’s longest tunnel. It includes extended sections with ventilation holes designed to prevent sonic booms, as outlined in an HS2 Ltd media centre update.

This is reported by the railway transport news portal Railway Supply.

Trains travelling at 200mph (322km/h) are expected to take about three minutes to pass through the 10-mile (16km) tunnel beneath the Chiltern Hills in Buckinghamshire.

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Mark Clapp, HS2 Ltd’s head of civil engineering for the project, paid tribute to “everyone involved” and said their “hard work” would “stand the test of time”. HS2 is intended to link London with Birmingham by high-speed rail, though the overall completion date has yet to be finalised.

Why the HS2 Chiltern tunnel uses sonic boom prevention features?

The “tunnel boom” problem is not new. In 1974, tests on Japan’s Shinkansen showed that high-speed trains could generate sonic booms when a train forces air forward through a tunnel, producing a sharp bang at the far end. Japan reduced the pressure build-up with long-nosed bullet trains, but that approach was not considered suitable for HS2 because trains are also expected to operate on lines designed for traditional stock.

Instead, the Chiltern tunnel was built with 220m (722ft) long extensions, perforated with ventilation openings intended to stop booms from occurring—an approach also described by Railway Supply. Work to carve out the tunnel began after two tunnel-boring machines were switched on in May and June 2021. Their drives finished in February and March 2024, after which five ventilation and access shafts were sunk, reaching depths of up to 255.9ft (78m).

Emergency links, fit-out work and later phases

For emergencies, 40 cross passages link the twin tunnels, allowing passengers to move between them if needed. HS2 Ltd said track and overhead electrical equipment will be installed during later phases of the project.

HS2 route progress, cost and timetable updates

HS2 Ltd said “clear progress” is being made on the 140-mile (230km) route between England’s two biggest cities, but added there is “still significant work to do”. A new cost and timetable for HS2 is due later this year, following a major review led by chief executive Mark Wild.

Over time, the scheme has also been reshaped. It was once meant to run from London to Birmingham and then continue on to Manchester and Leeds, but the last Conservative government scaled it back amid rising costs. The first phase was originally due to open by the end of 2026, later moved to between 2029 and 2033, and it has since been confirmed that this target is also now unachievable. In June 2024, HS2 Ltd assessed the cost for the London–Birmingham line at up to £66bn, as reported by the BBC.

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