Northolt tunnel

The Northolt Tunnel is a high-speed railway tunnel currently under construction in Greater London, England, and will upon completion carry the High Speed 2 (HS2) railway line under the West London suburbs. The twin-bore tunnels will run for 8.4 miles (13.5 km) between Old Oak Common and Ruislip. As of August 2023, the first tunnel boring machine has completed 35% of its bore length.

Northolt tunnel
HS2 route running west from London Euston, through Old Oak Common towards Ruislip
Overview
Coordinates51.536°N 0.319°W / 51.536; -0.319
StatusUnder construction
SystemHigh Speed 2
CrossesWest London
StartOld Oak Common railway station
EndRuislip
Operation
Work begun6 October 2022
TrafficRail
Technical
Length8.4 miles (13.5 km)
No. of tracks2
Track gauge1,435 mm (4 ft 8+1⁄2 in)
Electrified25 kV 50 Hz AC
Operating speed320 km/h (200 mph)
WidthWest - 8.8 metres (29 ft)
East - 8.1 metres (27 ft)

Plans for the construction of a tunnel were first unveiled in early 2013; it was hailed as being less disruptive and quicker to construct than a surface-based alignment. An option for the tunnel's extension along the Colne Valley to replace the proposed Colne Valley Viaduct was studied but rejected as unnecessarily costly during 2015. Extensive ground surveys along the intended route were conducted during the 2010s in advance of construction work. During April 2020, it was announced that a contract worth approximately £3.3 billion had been awarded to a joint venture company, Skanska Costain Strabag, for the tunnel's construction.

History

The construction of the High Speed 2 (HS2) railway is to involve numerous major civil engineering works along its intended route; it has been anticipated that around 102 km (63 miles) of tunnels are to be bored along various parts of its route in order to accommodate it.[1] During early 2013, it was confirmed that this section of HS2 would be constructed in a tunnel, rather than above ground.[2] At the time, HS2 Ltd, the delivery company behind the line, stated that the use of a tunnel was the optimum solution, reducing the disruption caused by the railway's construction upon the local community while also accelerating the timetable for that portion of the route.[3][4]

Prior to any construction activity commencing, numerous surveys and ground investigations were conducted along the intended route; the effort was described by Steve Reynolds, HS2’s ground investigation programme manager, as "the largest ground investigation programme that the UK has ever seen".[5] During early 2018, a layer of black clay was discovered, which has been dubbed the "Ruislip Bed"; this material dates back 56 million years and was formed from densely wooded marshes at the coast of a sub-tropical sea.[6]

During the early-to-mid 2010s, the possibility of extending the tunnel to traverse the Colne Valley, as an alternative to the proposed Colne Valley Viaduct, was examined in detail. However, this option was formally dismissed in a report released in February 2015, primarily due to the increased costs and construction time that such a change would predicted to involve.[7]

During April 2020, it was announced that a contract had been awarded to a joint venture company, Skanska Costain Strabag, for the tunnel's construction.[8] In October 2020, it was revealed that Herrenknecht would be supplying the two tunnel boring machines (TBMs) that will be used in the boring of the western section.[9]

Construction

The tunnel is being bored in two sections; the western section will be 5 miles (8.0 km) long and the eastern section will be 3.4 miles (5.5 km)[9] Of these, 13.7 km (8.5 miles) has been designed as a twin bored tunnel, which will be excavated using a total of four TBMs. Following the completion of boring, all four TBMs are to be retrieved at the point where the two sections meet; this site will then become the Green Park Way ventilation shaft.[9][10]

The bored section of the tunnel ends at the Victoria Road Crossover Box, the first "caterpillar shaft" in the UK. This is an open air shaft where trains can switch running lines before entering the tunnel, connected by a 360m long sprayed concrete lined tunnel to Old Oak Common station itself.[11] This crossover box will also be where the two TBMs boring the eastern end of the tunnel will be launched from.

In advance of the tunnel's boring, a smaller 853m long logistics tunnel is being bored using a separate smaller TBM that will facilitate access between the primary construction compound at Atlas Road and Old Oak Common Station, the launch site for the Northolt East and Euston TBMs, through which construction materials such as the precast concrete lining segments will be brought into the site and spoil removed.[10] Boring on the Atlas Road Logistics Tunnel began with the launch of TBM Lydia (formerly TBM Ellie on the Crossrail project before being refurbished for HS2 by Herrenknecht)[12] on April 12, 2023,[13] and is expected to be complete by November the same year.[14]

The first two TBMs, named Caroline (after astronomer Caroline Herschel) and Sushila (after local school teacher Sushila Hirani, Lead for STEM at Greenford High School), arrived at the West Ruislip launch site to dig the western section of the tunnel in December 2021;[15] with TBM Sushila launching on 6 October 2022 [16] and Caroline following suit on 27 October.[17] The second pair of TBMs, named Emily (after midwife and former Ealing mayor Emily Sophie Taylor) and Anne (after education reformer and philanthropist Lady Anne Byron), arrived at the Victoria Road Crossover Box for assembly and launch in late October 2023.[18] Both are set to be launched in early 2024,[19] and the tunnel is set to be complete by early 2025.[20][21][22]

Progress

As of 6 October 2023, TBM Sushila, the first machine to be launched from the West Ruislip portal, had progressed a distance of 2.915 km (1.811 miles) from the north portal – 37% of the length of its drive.[23] TBM Caroline, the second machine to be launched, had completed 2.528 km (1.571 miles) – 32% of the length of its drive.[23]

References
  1. Horgan, Rob (14 June 2021). "The reality of working on HS2's biggest construction site". newcivilengineer.com.
  2. "The Government response to the design refinement consultation: Decisions and Safeguarding Directions for Northolt and Bromford" (PDF). gov.uk. October 2013.
  3. "HS2 tunnel confirmed for Ealing and Northolt". BBC News. 24 October 2013. Retrieved 3 July 2021.
  4. Watts, Joe; Beard, Matthew (25 April 2013). "HS2 tunnel extended under west London to cut chaos". Evening Standard. Retrieved 3 July 2021.
  5. Croxford, Rianna (16 March 2018). "HS2 builders find ancient sub-tropical coastline in west London". Financial Times.
  6. "Workers discover ancient coastline in West London". HeritageDaily. 16 March 2018. Retrieved 3 July 2021.
  7. "Proposal for Northern Extension of Northolt Tunnel SIFT Report" (PDF). High Speed 2. 27 February 2015. Retrieved 3 July 2020.
  8. "HS2's contracts awarded – includes £3.3 billion London tunnels". IanVisits. 15 April 2020. Retrieved 3 July 2021.
  9. "Order goes in for Northolt tunnels TBMs". The Construction Index. 12 October 2020. Retrieved 3 July 2021.
  10. Long Branch, Mike (22 February 2021). "HS2 Prepares for 21km tunnels under London (Tunneltalk)". londonreconnections.com.
  11. Kanaris, Sotiris (25 August 2023). "'Caterpillar' shaft saves carbon and space on HS2's west London site". New Civil Engineer. Retrieved 27 September 2023.
  12. "Crossrail TBM upcycled for HS2 logistics tunnel drive". Construction Enquirer. Retrieved 17 September 2023.
  13. May, Tiana (13 April 2023). "HS2 Launches Third Tunnel Boring Machine in London". Railway-News. Retrieved 17 September 2023.
  14. "Atlas Road Logistics Tunnel". HS2. Retrieved 17 September 2023.
  15. Menteth, Thames (20 December 2021). "HS2: First London TBMs arrive at site for 2022 launch". Ground Engineering (GE). Retrieved 23 December 2021.
  16. Mansfield, Ian (6 October 2022). "HS2 starts digging 26 miles of tunnel under London". IanVisits. Retrieved 6 October 2022.
  17. "HS2 construction in London gathers pace as second tunnelling machine sets off underneath the capital". High Speed 2. 27 October 2022. Retrieved 13 November 2022.
  18. by (24 October 2023). "Emily and Anne get ready to dig HS2's Northolt Tunnel under west London". ianVisits. Retrieved 25 October 2023.
  19. Kanaris, Sotiris (27 June 2023). "Future of Tunnelling | Four High Speed 2 twin bores progressing well despite challenges". New Civil Engineer. Retrieved 6 October 2023.
  20. Hakimian, Rob (15 July 2022). "HS2 timeline of major events to 2025 revealed". New Civil Engineer. Retrieved 16 July 2022.
  21. "'HS2 drill switch-on is milestone for the ambitious project'". BBC News. 6 October 2022. Retrieved 6 October 2022.
  22. Hakimian, Rob (24 October 2023). "HS2 | TBMs lowered into Ealing shaft ahead of next London tunnelling launch". New Civil Engineer. Retrieved 25 October 2023.
  23. "HS2: In your area map". HS2 Ltd. Retrieved 6 October 2023.


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