Chilterns tunnel

The Chilterns tunnel will carry the High Speed 2 (HS2) railway line under the Chiltern Hills in England. The 16.04 kilometres (9.97 mi) long[1] twin-bore tunnels will be the longest on the HS2 line.[2]

Chilterns tunnel
Overview
StatusUnder construction
SystemHigh Speed 2
CrossesChiltern Hills
StartWest Hyde
EndSouth Heath
Operation
TrafficRail
Technical
Length16.04 kilometres (9.97 mi)
No. of tracks2
Track gauge1,435 mm (4 ft 8+1⁄2 in)
Electrified25 kV 50 Hz AC
Width9.1 metres (30 ft)

A contract for the tunnel's construction, amongst other elements of the railway, was awarded to the Align JV joint venture in 2017; preparatory work commenced during the following year, which was largely centred around a temporary facility near the tunnel's southern portal. During May 2021, it was announced that excavation commenced at the site. The boring process, which is to be largely performed by a pair of tunnel boring machines (TBMs), is advancing at an average speed of 15 metres per day; it has been anticipated that it shall take roughly three and a half years to complete.[3][4]

History

The construction of the High Speed 2 (HS2) railway is to involve numerous major civil engineering works along its intended route.[5] Construction periodical New Civil Engineer has described the Chiltern Hills as "HS2’s biggest and potentially most challenging site". It has been anticipated that, of the 102km of tunnels that are to be bored to accommodate the line, around 16km shall be bored for the Chilterns tunnel alone.[5]

The design of the Chilterns tunnel has been subject to changes during the planning stages of the project. During August 2015, it was announced that the tunnels would be extended 2.6 kilometres (1.6 mi) north; this revision has allowed for the replacement of a separate planned cut-and-cover tunnel, as well as avoiding the destruction of around 12 hectares of woodland.[6][7] During 2017, Contract C1 (Central 1), which covers the 21.6km section of the line that the Chiltern tunnels comes within, was awarded to the Align JV joint venture, comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.[8]

Design

The basic configuration of the Chilterns tunnel shall consist of a pair of parallel bores, which are to be excavated by a pair of tunnel boring machines (TBMs).[5] Each TBM weigh around 2,000 tonnes, have a length of 170m, and have been specially customised to suit the local geology, which primarily consists of chalk and flint. Staff are conveyed between the surface and the TBM using people carriers, which shall have up to a one hour transit time towards the later years of the boring. While both TBMs are intended to be operational simultaneously throughout the majority of the work, due to sensitivities surrounding the M25 motorway, only one TBM shall be active at a time while within close proximity to this key trunk route.[5]

The southern entrance to the tunnels is aligned with the nearby Colne Valley Viaduct and is near the M25. The pair of tunnels will be 16.04 kilometres (9.97 mi) long, with the northern portal near South Heath. At their deepest point, they will be 90m below ground level; the internal diameter of each bore will be 9.1 metres (30 ft).[9] The completed tunnel shall have a total of five shafts, which shall be used for both ventilation and emergency access purposes; one shaft is located in Amersham, while another is in Chalfont St Giles.[10] These vents have often been sympathetically designed to blend in with their surrounding, produced through engagement with both the Chilterns AONB Review Group and Buckinghamshire Council; one example is the surface elements of the St Giles shaft having been intentionally disguised as agricultural buildings.[11]

Construction

As early as 2018, protestors had taken up positions in the Colne Valley to voice their objections to the project.[12] In April 2019, twelve Extinction Rebellion protesters began treesitting in order to prevent HS2 Limited chopping down trees as part of preparatory works at Harvil Road, near to the proposed site of the tunnel.[13] There had been a protest camp next to the road since October 2017, which HS2 began evicting in January 2020.[14]

In July 2020, work was completed on a 17 metres (56 ft) high headwall at the southern end, at 51.6117°N 0.5242°W / 51.6117; -0.5242 (HS2 Chiltern Tunnel Portal Southern Headwall), close to the M25 motorway. To accommodate workers during the construction phase, a temporary facility containing accommodation and various amenities has been constructed on site; this site shall also be used to support the construction of the nearby Colne Valley Viaduct as well.[5][8]

During September 2020, final preparations for the arrival of the first of the two TBMs were made.[15][16] During March 2021, the first section of fibre-reinforced concrete tunnel lining was cast; this is produced onsite at a purpose-built facility near to the southern portal, which is capable of manufacturing up to 49 sections at a time.[17] A total of 56,000 precisely-engineered sections shall be required to complete each tunnel.[5]

In May 2021, it was announced that the excavation process had begun, it has been forecast that boring shall continue until some time around the end of 2024.[4][18] During the following month, it was reported that the first TBM, which was being operated around the clock, had been advancing at a typical speed of 15 metres per day. Each TBM is operated by a crew of 17 working in 12 hour shifts, while surface logistics and support for the boring work is provided by over 100 people on the surface.[5] During June 2021, the second TBM commenced boring as well.[19]

The chalk that has been excavated during the boring of the tunnels is to be used to create a nature reserve covering some 127 hectares in the vacinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to three million cubic meters of chalk slurry were provisioned onsite prior to excavation commencing.[8][20][21] Approval for this scheme was issued in May 2021.[22]

References
  1. "Bouygues Travaux Publics - Project - HS2, High-Speed Rail Line". www.bouygues-tp.com. Retrieved 12 August 2021.
  2. "Buckinghamshire tunnels". High Speed 2. Retrieved 29 April 2021.
  3. "Sir Jonathan Thompson appointed to the HS2 Ltd Board". GOV.UK. Retrieved 21 April 2021.
  4. "HS2 high-speed rail project begins underground tunnel". BBC News. 13 May 2021. Retrieved 13 May 2021.
  5. Horgan, Rob (14 June 2021). "The reality of working on HS2's biggest construction site". newcivilengineer.com.
  6. "HS2 confirms Chilterns tunnel extension". Infrastructure Intelligence. Retrieved 19 March 2021.
  7. A., Evans (28 May 2015). "High Speed Rail in the Chilterns Part 2: Chiltern Long Tunnel Proposal" (PDF). gov.uk.
  8. "Colne Valley Viaduct – HS2's largest bridge". railengineer.co.uk. 7 June 2021.
  9. "The HS2 Tunnel Boring Machines (TBMs)". High Speed 2. Retrieved 20 March 2021.
  10. "HS2's Chiltern tunnel headhouse vent shaft gains planning approval". Global Railway Review. Retrieved 19 March 2021.
  11. Bamford, Thomas (5 February 2021). "HS2 reveals Chilterns 'barn design' vent shaft for Chalfont St Giles". bucksherald.co.uk.
  12. Topham, Gwyn (8 December 2020). "Veteran activist Swampy among protesters in HS2 site standoff with police". theguardian.com.
  13. Snaith, Emma (27 April 2019). "Extinction Rebellion activists camp out in trees to protest HS2". The Independent. Retrieved 29 July 2020.
  14. Taylor, Diane (7 January 2020). "HS2 begins evicting activists from protest site after two years". The Guardian. Archived from the original on 13 June 2020. Retrieved 29 July 2020.
  15. "Progress at Chiltern tunnel site as HS2 prepares for arrival of first tunnelling machines". Archived from the original on 16 September 2020. Retrieved 30 July 2020.
  16. "Giant machines for HS2's Chilterns tunnels unveiled". BBC News. 11 August 2020.
  17. "HS2 Casts First of 112,000 Chiltern Tunnel Wall Segments". Railway-News. 19 March 2021. Retrieved 19 March 2021.
  18. "Work begins to create a tunnel through the Chilterns for new HS2 rail line". itv.com. 13 May 2021.
  19. Kajastie, Nia (1 July 2021). "Second HS2 tunnelling machine starts digging under the Chilterns". geplus.co.uk.
  20. "HS2 to 'rewild' 127 hectares around its 10-mile Chilterns tunnel". The Guardian. 3 March 2021. Retrieved 19 March 2021.
  21. Horgan, Rob (27 April 2021). "How HS2 plans to reuse chalk slurry from Chilterns tunnel to 'rewild' the Colne Valley". newcivilengineer.com.
  22. "Green light for HS2's ambitious Chiltern tunnel chalk grassland project". railpro.co.uk. 21 June 2021.
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