Chiltern tunnel

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

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

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 m (49 ft) per day; it is anicipated that it will be complete in March 2024.[3][4][5] As of 26 January 2022, the first TBM had excavated at distance of 2,892 m (1.80 miles).[6]

History[]

The construction of the High Speed 2 (HS2) railway is to involve numerous major civil engineering works along its intended route.[7] 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 102 km (63 miles) of tunnels that are to be bored to accommodate the line, around 16 km (9.9 miles) shall be bored for the Chiltern tunnel alone.[7]

The design of the Chiltern 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 km (1.6 miles) 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 (30 acres) of woodland such as Farthings Wood.[8][9] 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.[10]

Design[]

The basic configuration of the Chiltern tunnel shall consist of a pair of parallel bores, which are to be excavated by a pair of tunnel boring machines (TBMs).[7] Each TBM weigh around 2,000 tonnes, have a length of 170 m (560 ft), 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.[7]

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 km (9.97 miles) long, with the northern portal near South Heath. At their deepest point, they will be 90 m (300 ft) below ground level; the internal diameter of each bore will be 9.1 m (30 ft).[11] 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.[12] 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.[13]

Construction[]

In July 2020, work was completed on a 17 m (56 feet) high headwall at the southern end, at

 WikiMiniAtlas
51°36′42″N 0°31′27″W / 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.[7][10]

During September 2020, final preparations for the arrival of the first of the two TBMs were made.[14][15] 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.[16] A total of 56,000 precisely-engineered sections shall be required to complete each tunnel.[7]

Excavation commenced in May 2021, with completion estimated around the end of 2024.[5][17] 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 m (49 ft) 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.[7] During June 2021, the second TBM commenced boring as well.[18]

The chalk that has been excavated during the boring of the tunnels is to be used to create a nature reserve covering 127 hectares (310 acres) in the vicinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to 3,000,000 cubic metres (110,000,000 cu ft) of chalk slurry were provisioned onsite prior to excavation commencing.[10][19][20] Approval for this scheme was issued in May 2021.[21]

Progress[]

As of 26 January 2022, TBM Florence, the first machine to be launched in May 2021 had progressed a distance of 2,892 m (1.80 miles) from the south portal, with a remaining distance of 13,168 m (8.182 miles) to go.[6] It was located part way between the Buckinghamshire villages of Chalfont St Peter and Chalfont St Giles.[6]

External links[]

References[]

  1. ^ "Bouygues Travaux Publics - Project - HS2, High-Speed Rail Line". www.bouygues-tp.com. Retrieved 12 August 2021.
  2. ^ "Chiltern Tunnel". High Speed 2. Retrieved 4 February 2022.
  3. ^ "HS2 Corporate Plan 2021–2024" (PDF). Archived (PDF) from the original on 21 December 2021.
  4. ^ "Sir Jonathan Thompson appointed to the HS2 Ltd Board". GOV.UK. Retrieved 21 April 2021.
  5. ^ a b "HS2 high-speed rail project begins underground tunnel". BBC News. 13 May 2021. Archived from the original on 12 May 2021. Retrieved 13 May 2021.
  6. ^ a b c "HS2: In your area map". HS2 Ltd. Retrieved 4 February 2022.
  7. ^ a b c d e f g Horgan, Rob (14 June 2021). "The reality of working on HS2's biggest construction site". newcivilengineer.com.
  8. ^ "HS2 confirms Chilterns tunnel extension". Infrastructure Intelligence. Retrieved 19 March 2021.
  9. ^ A., Evans (28 May 2015). "High Speed Rail in the Chilterns Part 2: Chiltern Long Tunnel Proposal" (PDF). gov.uk.
  10. ^ a b c "Colne Valley Viaduct – HS2's largest bridge". railengineer.co.uk. 7 June 2021.
  11. ^ "The HS2 Tunnel Boring Machines (TBMs)". High Speed 2. Retrieved 20 March 2021.
  12. ^ "HS2's Chiltern tunnel headhouse vent shaft gains planning approval". Global Railway Review. Retrieved 19 March 2021.
  13. ^ Bamford, Thomas (5 February 2021). "HS2 reveals Chilterns 'barn design' vent shaft for Chalfont St Giles". bucksherald.co.uk.
  14. ^ "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.
  15. ^ "Giant machines for HS2's Chilterns tunnels unveiled". BBC News. 11 August 2020.
  16. ^ "HS2 Casts First of 112,000 Chiltern Tunnel Wall Segments". Railway-News. 19 March 2021. Retrieved 19 March 2021.
  17. ^ "Work begins to create a tunnel through the Chilterns for new HS2 rail line". itv.com. 13 May 2021.
  18. ^ Kajastie, Nia (1 July 2021). "Second HS2 tunnelling machine starts digging under the Chilterns". geplus.co.uk.
  19. ^ "HS2 to 'rewild' 127 hectares around its 10-mile Chilterns tunnel". The Guardian. 3 March 2021. Retrieved 19 March 2021.
  20. ^ Horgan, Rob (27 April 2021). "How HS2 plans to reuse chalk slurry from Chilterns tunnel to 'rewild' the Colne Valley". newcivilengineer.com.
  21. ^ "Green light for HS2's ambitious Chiltern tunnel chalk grassland project". railpro.co.uk. 21 June 2021.
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