HS2 Ticks Another Box: Northolt Tunnel Safety Passages Break Through

HS2 has reached another key milestone beneath London, with engineers completing all 34 cross passages along the Northolt Tunnel—marking significant progress on one of the project’s longest underground sections.

The 8.4-mile tunnel, which runs between West Ruislip and Victoria Road near Old Oak Common, is a major part of the high-speed line into the capital. Built using four tunnel boring machines and reaching depths of up to 35 metres, excavation of the main tunnel was completed in June 2025.

Safety at the core

Cross passages are a critical safety feature, linking the northbound and southbound tunnels and providing a safe evacuation route in the event of an emergency. Their completion brings the project a step closer to finishing the core civil engineering works.

With this phase now complete, teams will move on to installing reinforced concrete secondary collars, forming the permanent openings for cross-passage doors. These will later house fire-rated sliding doors, designed to withstand extreme pressure and fire conditions.

Engineering beneath the surface

Construction of the passages used a sprayed concrete lining (SCL) technique, with teams carefully excavating between the twin tunnel bores—sometimes just six metres apart—using mini-excavators and reinforcing each stage as they progressed.

In more challenging ground conditions, particularly in water-bearing soils, engineers deployed a ground freezing method. This involved creating a frozen barrier around the excavation area to stabilise the ground before work could continue.

What comes next?

Structural work on the Northolt Tunnel—one of the longest on the 140-mile HS2 route between London and the West Midlands—is expected to be completed later this year. Attention will then turn to installing railway systems, including track and overhead power.

The work is being delivered by the Skanska Costain STRABAG (SCS) joint venture, responsible for the main civil engineering works between Hillingdon and Camden.

Pushing forward

HS2 Ltd says the milestone reflects strong progress across the programme, although significant work remains across the wider route. A broader project reset is currently underway to ensure the scheme is delivered as efficiently and cost-effectively as possible.

For now, deep beneath west London, another piece of the HS2 puzzle has clicked into place—quietly but critically shaping the railway of the future.