HS2’s longest tunnel completed

Construction has finished on HS2's longest tunnel, a 10-mile route under the Chiltern Hills featuring special extensions designed to prevent sonic booms from high-speed trains.

Construction has finished on HS2’s longest tunnel, a 10-mile route under the Chiltern Hills in Buckinghamshire, the BBC reports.

The tunnel includes special extensions designed to prevent sonic booms.

Trains travelling at 200mph will take just three minutes to pass through the Chiltern tunnel. The design incorporates 220-metre extensions perforated with ventilation holes to stop loud bangs caused when high-speed trains force air through tunnels.

Mark Clapp, HS2 Ltd’s head of civil engineering, said the work would “stand the test of time”.

Special design for UK conditions

Japan’s bullet trains use long noses to reduce pressure build-up, but that solution wasn’t suitable for HS2 as trains will also operate on traditional rail lines. Instead, the tunnel was built with ventilated extensions.

Two boring machines started carving the tunnel in May and June 2021, finishing in early 2024.

Five ventilation and access shafts were then sunk, reaching depths of up to 78 metres. Forty passages link the two tunnels for emergency evacuations.

Tracks and overhead electrical equipment will be installed in later phases.

Costs and delays continue

HS2 Ltd said “clear progress” was being made on the 140-mile route between London and Birmingham, but “significant work” remains. A new cost and timetable is due later this year.

The project was scaled back by the previous Conservative government due to rising costs. The first phase, once due to open by 2026, was pushed to 2029-2033 and that target is now also unachievable.

In June 2024, HS2 Ltd assessed the cost for the line between London and Birmingham at up to £66 billion.