HS2 has revealed that workers have completed the construction of 34 cross passages in the Northolt Tunnel in London.

Cross passages are a safety provision in tunnels, connecting the northbound and southbound tunnels and so enabling passengers to cross to the adjacent tunnel in an emergency situation, and allowing maintenance workers easier access between the two tunnels.

The Northolt Tunnel runs for a distance of 8.4 miles between West Ruislip Victoria Road in Ealing, west London, emerging just outside HS2’s Old Oak Common station.

It is the second-longest tunnel on the 140-mile route that will carry high-speed trains between London and Birmingham. The tunnel is 35 metres below ground at its deepest.

It was built using four tunnel boring machines (TBMs), with the work of excavating the tunnels completed in June last year.

Now that engineers have finished the cross passages, their next task is to install reinforced concrete secondary collars. These will form permanent openings for the cross passages’ doors, and will allow engineers to install permanent fire-rated sliding doors.

HS2 expects that all structural works on the tunnel will be finished later this year. The next step will then be to prepare the tunnel for engineering workers to install the rail systems that the high-speed trains need in order to travel through the tunnel. These will include overhead power lines and tracks.

Engineers built the connections using a spray concrete lining (SCL) mining technique. This involved workers cutting through the ground between the two tunnel bores. The two tunnel bores are between six and twenty metres apart.

Workers also used mini-excavators, with the earth that they cut being supported with SCL. When the lined tunnel is finished, they will install a waterproof membrane and then a secondary concrete lining.

Construction of eleven of the 34 cross passages involved workers using a ground freezing technique, in which they insert freeze pipes through special tunnel segments to create a freeze wall which is two metres thick.

This stabilises the ground and enables excavation to be carried out. It was necessary to use this technique because This method was used because water bearing soils with a high-water table are present in the ground in the western section of the tunnel.

HS2 has contracted Skanska Costain STRABAG joint venture (SCS JV) to carry out work on its London Tunnel. The contractor is responsible for the main works civils construction of HS2 between Hillingdon and Camden. It is this section of the high-speed line that carries HS2 into Euston.

The doors for all HS2’s tunnels are being built by Booth Industries in Bolton, mostly using steel made in the UK. The doors have to withstand constant 14Kpa pressure cycles, and to provide two-hour Integrity and two-hour fire insulation. This is the first time that this has been done, and HS2 describes it as “setting a new benchmark within the industry”.

Although finishing the Northolt Tunnel’s cross passages shows that the building of the HS2 route is making progress, a large amount of work remains to be done on its 140-mile route between London and Birmingham.

“The progress made to construct the Northolt Tunnel demonstrates the commitment of our teams to deliver this vital infrastructure efficiently and safely.

“Completing the construction of the cross passages is an important step towards finishing the civil engineering on the tunnel which will allow us to ultimately begin rail systems work including laying track.”

Malcolm Codling, Project Client for HS2 Ltd