All 34 cross passages have been mined under London, marking another milestone for the HS2 project
HS2’s Northolt Tunnel now has all of its cross passages mined, connecting the northbound and southbound tunnels.
The cross passages are a safety feature for passengers in case of emergency, allowing them to cross from one tunnel to the other.
The tunnel reaches 35 metres underground in places
Running for 8.4 miles – the second longest tunnel on the whole route – underground between West Ruislip to Victoria Road in Ealing, near Old Oak Common station, the tunnel was made using four tunnel boring machines (TBMs) and excavation finished in June last year.
Now that the cross passages have been completed, reinforced concrete secondary collars will be installed to make the openings permanent and allow for fire-rated sliding doors to be installed.
All structural works on the Northolt Tunnel are expected to be completed by the end of the year, and it will then be ready for the installation of the rail systems.
Malcolm Codling, project client for HS2 Ltd, said: “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.”
The cross paths were created using a SCL mining technique
Spray concrete lining (SCL) was used to create the connections where engineers cut through the ground between the tunnels, covering a distance anywhere between 6-20 metres. The digging is done by mini-excavators, and a concrete lining is then sprayed onto the cut earth. When the tunnel is complete and fully sprayed, a waterproof membrane is installed, and another concrete lining is placed over this.
Some of the passages were built using a ground freezing technique to freeze water in soil, preventing water clogging. Freeze pipes were inserted in tunnel segments, creating a freeze wall of 2 metres around the tunnel, stabilising it for excavation.
Dave Hannon, tunnels & routeway director from SCS JV said: “The completion of all 34 cross passages along the Northolt Tunnel marks another important step forward for the programme, demonstrating consistent progress across multiple work fronts and the coordinated effort of our teams working deep beneath London.
“With this phase complete, we can progress on the next stage of works on the tunnel, with safety and productivity at the forefront.”
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