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Chiltern tunnel

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The Chiltern Tunnel is a high-speed railway tunnel currently under construction in Buckinghamshire and Hertfordshire, England, and will upon completion carry the High Speed 2 (HS2) railway line under the Chiltern Hills. The twin-bore tunnels, which are 16.04 km (9.97 miles) long,[1] will be the longest on the HS2 line.[2] Each tunnel will also have additional 220 m (720 ft) entry and 135 m (443 ft) exit perforated concrete portals to reduce sudden changes in air pressure and subsequent noise.[3]

Chiltern Tunnel
Overview
LineHigh Speed 2
LocationUnited Kingdom (Buckinghamshire, Hertfordshire)
Coordinates51°42′34″N 0°41′35″W / 51.7095°N 0.6931°W / 51.7095; -0.6931 (north portal)
OS grid referenceTQ022913
StatusUnder construction
CrossesChiltern Hills
StartWest Hyde, Hertfordshire
EndSouth Heath, Buckinghamshire
Operation
Work begun13 May 2021
Constructed2020–present
Opens2029 (scheduled)
TrafficHigh-speed passenger trains
CharacterTwin-tube
Technical
Length16.04 km (9.97 miles)
No. of tracks2
Track gauge1,435 mm (4 ft 8+1⁄2 in) standard gauge
Electrified25 kV 50 Hz AC
Operating speed320 km/h (200 mph)
Width9.1 metres (30 ft)
Cross passages40
Chiltern Tunnel
Legend
 HS2
to Birmingham Interchange
 South Heath
(north portal)
  B485 
Chesham Road intervention shaft
London-Aylesbury line
Little Missenden shaft
 A413 
 A404 
Amersham shaft
 A355 
Chalfont St Giles shaft
 A413 
Chalfont St Peter shaft
 M25 
Buckinghamshire-Hertfordshire border
West Hyde
(south portal)
 HS2
to Old Oak Common

A contract for the tunnel's construction was awarded in 2017; preparatory work commenced during the following year. In May 2021, it was announced that excavation had commenced. The boring process, which was largely performed by a pair of tunnel boring machines (TBMs), advanced at an average speed of 15 m (49 ft) per day; both TBMs completed their drives by March 2024.[4]

History

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The construction of the High Speed 2 (HS2) railway involves numerous major civil engineering works along its intended route, with construction periodical New Civil Engineer describing the Chiltern Hills as "HS2’s biggest and potentially most challenging site".[5] Of the 102 km (63 miles) of tunnels that are to be bored to accommodate the line, around 16 km (9.9 miles) will be bored for the Chiltern Tunnel alone.[5]

The design of the 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.[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] The two tunnel boring machines were supplied by Herrenknecht[2] and were delivered to the site in December 2020.[9]

Design

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The basic configuration of the tunnel consists of a pair of parallel bores, which are to be excavated by a pair of tunnel boring machines (TBMs).[5] Each TBM weighs around 2,000 tonnes, has a length of 170 m (560 ft), and has 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 will 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 will 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 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).[10] The completed tunnel will have a total of forty cross passages linking the two separate bores and five 18 m (59 ft) diameter shafts used for both ventilation and emergency access purposes between 35 m (115 ft) and 65 m (213 ft) deep.[11] These are located at Chalfont St Peter,[12] Chalfont St Giles,[13][14] Amersham,[15][16] Little Missenden[17][18] and Chesham Road[19] - the latter of which is an intervention shaft for emergencies. These vents are all to be fitted with headhouses, produced through engagement with both the Chilterns AONB Review Group and Buckinghamshire Council. The surface elements of the St Giles shaft have been intentionally disguised as agricultural buildings.[20] The initial design of the Amersham vent shaft headhouse was modified after being criticised due to its design being reportedly out of keeping with the location,[21][22] and after several iterations the design is at its final approval stage.[23]

The tunnel will have porous portals (a form of tunnel entrance hood) that extends for up to 220 metres (720 ft) beyond the end of the bored section in the hillside - this is to dissipate the pressure wave built up by trains at line speed, thus reducing the adverse effects of audible "sonic boom" in line with environmental commitments.[3][24][25]

Construction

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In July 2020, work was completed on a 17 m (56 feet) high headwall at the southern end, at 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 will 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,[26][27] and both arrived on site in December that year.[9] 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.[28] A total of 56,000 precisely-engineered sections were required to complete each tunnel.[5]

Excavation commenced on 13 May 2021,[29][30] with completion estimated around March 2024.[31][32] During the following month, it was reported that the first TBM had been advancing at a typical speed of 15 m (49 ft) per day. Each TBM was operated by a crew of 17 working in 12-hour shifts, while surface logistics and support for the boring work was provided by over 100 people on the surface.[5] During June 2021, the second TBM commenced boring as well.[33][34] The two TBMs were named Florence (after Victorian social reformer and nurse Florence Nightingale) and Cecilia (after astrophysicist Cecilia Payne-Gaposchkin) following a national vote on names suggested by local schools.[35][36]

The first stage of the drive (from the south portal to the Chalfont St Peter ventilation shaft) was completed in March 2022,[37] and the first of the cross passages between the main running tunnels were completed in August that year.[38] The Chalfont St Giles ventilation shaft was reached in October 2022,[39] Amersham in March 2023,[40] Little Missenden in August,[41] and Chesham Road in December the same year.[42]

The chalk 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.[8][43][44][45] Approval for this scheme was issued in May 2021.[46]

One of the tunnel bores was temporarily evacuated in May 2022 following a fire on a personnel transportation vehicle. No personnel were injured and the tunnel itself was undamaged after the fire was brought under control.[47]

Tunnelling work was unaffected by the formation of a sinkhole, estimated to be 5 m (16 feet) deep and 6 m (20 feet) wide, near Shardeloes Lake, Amersham in May 2023,[48] although an investigation was launched by the Environment Agency into the causes of the ground collapse.[49] The resulting inquiry found that the local water supply was not contaminated, and the sinkhole was filled with 165 cubic metres (5,800 cu ft) of chalk in September 2023.[50] Further work to ensure the hole is filled was completed in early 2024.[51] Another sinkhole formed in November 2023 near Hedgemoor Wood, Hyde Heath,[52] and two more followed in February 2024 adjacent to Frith Hill in South Heath,[51] although tunnelling remained unaffected in both cases.

By November 2023, nineteen of the thirty-eight mined cross passages had been completed.[53]

Florence's progress was hampered significantly during the last few weeks of her drive by a geology change, as the final 140m to the north portal replaced the usual chalk with a more clay-like substance, causing several blockages in the slurry pipes.[54] Despite these late challenges, overall the drive was reported to be smooth, with Align JV tunnelling lead Didier Jacques reporting that during 2023 the TBM team achieved an “incredible” average of 20m a day. The record progress for one day was 42m and the record for a week was 226m. In one calendar month of October 2023, 850m of progress was made.[54]

TBM Florence broke through at the north portal in South Heath on 27 February 2024,[55][56][57] with Cecilia following suit on 22 March.[4][58][59]

References

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  1. ^ "Bouygues Travaux Publics - Project - HS2, High-Speed Rail Line". www.bouygues-tp.com. Retrieved 12 August 2021.
  2. ^ a b "Chiltern Tunnel". High Speed 2. Retrieved 4 February 2022.
  3. ^ a b "HS2 reveals noise-cancelling Chiltern Tunnel north portal design". HS2 News and Information. Retrieved 3 June 2023.
  4. ^ a b Hakimian, Rob (22 March 2024). "HS2's 16km Chilterns tunnelling completed with second TBM breakthrough". New Civil Engineer. Retrieved 23 March 2024.
  5. ^ a b c d e f g 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. ^ a b c "Colne Valley Viaduct – HS2's largest bridge". railengineer.co.uk. 7 June 2021.
  9. ^ a b "Work on longest tunnels set to begin". Modern Railways. 17 March 2021. Retrieved 16 July 2022.
  10. ^ "The HS2 Tunnel Boring Machines (TBMs)". High Speed 2. Archived from the original on 3 March 2021. Retrieved 20 March 2021.
  11. ^ "Design of Chiltern Tunnel and ventilation shaft interfaces". HS2 Learning Legacy. 17 September 2020. Retrieved 14 September 2023.
  12. ^ "Chalfont St Peter ventilation shaft and headhouse". High Speed 2. 6 October 2021. Retrieved 5 July 2022.
  13. ^ "Chalfont St Giles ventilation shaft and headhouse". High Speed 2. 15 September 2021. Retrieved 5 July 2022.
  14. ^ "HS2's Chiltern tunnel headhouse vent shaft gains planning approval". Global Railway Review. Retrieved 19 March 2021.
  15. ^ "Amersham Headhouse and Ventilation Shaft". HS2. Retrieved 22 September 2023.
  16. ^ "HS2: Images show new design for ventilation shaft for tunnel". BBC News. 6 July 2022. Retrieved 6 July 2022.
  17. ^ "Little Missenden Headhouse and Ventilation Shaft". HS2. Retrieved 22 September 2023.
  18. ^ Evans, A (28 May 2015). "High Speed Rail in the Chilterns Part 1: General Long Tunnel Requirements" (PDF). HS2 Ltd. Retrieved 5 July 2022.
  19. ^ "Chesham Road Intervention Shaft". HS2. Retrieved 22 September 2023.
  20. ^ Bamford, Thomas (5 February 2021). "HS2 reveals Chilterns 'barn design' vent shaft for Chalfont St Giles". bucksherald.co.uk.
  21. ^ "HS2 reveals designs for tunnel vent shaft - in the middle of busy junction". 24 August 2020.
  22. ^ "Council warns Amersham HS2 vent shaft plans are 'totally unacceptable'". 22 May 2021.
  23. ^ "HS2 reveals updated designs for Amersham vent shaft headhouse".
  24. ^ "Aerodynamic design of HS2 tunnels". HS2 Learning Legacy. 24 February 2021. Retrieved 14 September 2023.
  25. ^ "Design of Chiltern Tunnel South Portal". HS2 Learning Legacy. 17 September 2020. Retrieved 14 September 2023.
  26. ^ "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.
  27. ^ "Giant machines for HS2's Chilterns tunnels unveiled". BBC News. 11 August 2020.
  28. ^ "HS2 Casts First of 112,000 Chiltern Tunnel Wall Segments". Railway-News. 19 March 2021. Retrieved 19 March 2021.
  29. ^ "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.
  30. ^ "Work begins to create a tunnel through the Chilterns for new HS2 rail line". itv.com. 13 May 2021.
  31. ^ Hakimian, Rob (15 July 2022). "HS2 timeline of major events to 2025 revealed". New Civil Engineer. Retrieved 16 July 2022.
  32. ^ "HS2 Corporate Plan 2021–2024" (PDF). Archived (PDF) from the original on 21 December 2021.
  33. ^ Kajastie, Nia (1 July 2021). "Second HS2 tunnelling machine starts digging under the Chilterns". geplus.co.uk.
  34. ^ "Momentum builds as HS2 launches second giant tunnelling machine". HS2 News and Information. Retrieved 22 March 2024.
  35. ^ "HS2 tunnelling machines set to leave factory with the names Florence and Cecilia topping public vote". HS2 News and Information. Retrieved 20 September 2023.
  36. ^ "Naming the HS2 Tunnel Boring Machines". HS2. Retrieved 20 September 2023.
  37. ^ "HS2 tunnelling machines reach Chalfont St Peter ventilation shaft". High Speed 2. 14 March 2022. Retrieved 17 August 2022.
  38. ^ "HS2 completes first tunnel cross passages". RailUK. 22 August 2022. Retrieved 22 August 2022.
  39. ^ "HS2 Chiltern tunnel - 'Florence' and 'Cecilia' reach 4 mile point at Chalfont St Giles". RailUK. 10 October 2022. Retrieved 11 October 2022.
  40. ^ "HS2 tunnel progress as 'Florence' and 'Cecilia' pass Amersham". HS2. 2 March 2023. Retrieved 15 August 2023.
  41. ^ "Tunnel progress as HS2's 'Florence' and 'Cecilia' reach Little Missenden". HS2. 8 August 2023. Retrieved 15 August 2023.
  42. ^ Moore, Catherine (19 December 2023). "HS2's Chiltern tunnelling reaches 90% completion". New Civil Engineer. Retrieved 20 December 2023.
  43. ^ "HS2 to 'rewild' 127 hectares around its 10-mile Chilterns tunnel". The Guardian. 3 March 2021. Retrieved 19 March 2021.
  44. ^ Horgan, Rob (27 April 2021). "How HS2 plans to reuse chalk slurry from Chilterns tunnel to 'rewild' the Colne Valley". newcivilengineer.com.
  45. ^ "HS2 celebrates milestone in Chiltern grassland transformation". RailUK. 8 September 2022. Retrieved 11 September 2022.
  46. ^ "Green light for HS2's ambitious Chiltern tunnel chalk grassland project". railpro.co.uk. 21 June 2021.
  47. ^ "HS2 tunnel fire: Workers shelter in digger near Maple Cross". BBC News. 11 May 2022. Retrieved 19 September 2023.
  48. ^ "HS2: Large hole opens above rail tunnel near Little Missenden". BBC News. 14 May 2023. Retrieved 6 October 2023.
  49. ^ Kanaris, Sotiris (27 June 2023). "Future of Tunnelling: Four High Speed 2 twin bores progressing well despite challenges". New Civil Engineer. Retrieved 14 September 2023.
  50. ^ "HS2 Chiltern tunnel sinkhole did not pollute water, says inquiry". BBC News. 6 October 2023. Retrieved 6 October 2023.
  51. ^ a b Johnson, Thomas (14 February 2024). "HS2 | Two more sinkholes form on Chilterns tunnelling route". New Civil Engineer. Retrieved 14 February 2024.
  52. ^ Kajastie, Nia (15 November 2023). "HS2: Another sinkhole appears above Chiltern tunnels in Buckinghamshire". Ground Engineering. Retrieved 21 November 2023.
  53. ^ "HS2 - Half of Chiltern tunnel cross passage mining complete". RailUK. 22 November 2023. Retrieved 22 November 2023.
  54. ^ a b Hakimian, Rob (28 February 2024). "HS2 Chilterns TBM overcame difficult geology and clogged slurry pipes to reach finish line". New Civil Engineer. Retrieved 29 February 2024.
  55. ^ "Historic breakthrough for HS2's longest tunnel". HS2 News and Information. Retrieved 27 February 2024.
  56. ^ "HS2 tunnelling machine completes journey under Chiltern Hills". BBC News. 27 February 2024. Retrieved 27 February 2024.
  57. ^ Hakimian, Rob (27 February 2024). "HS2 celebrates completion of first 16km drive for Chilterns tunnel". New Civil Engineer. Retrieved 27 February 2024.
  58. ^ "HS2 tunnelling machine completes journey in Buckinghamshire". BBC News. 22 March 2024. Retrieved 23 March 2024.
  59. ^ Hakimian, Rob (22 March 2024). "HS2's 16km Chilterns tunnelling completed with second TBM breakthrough". New Civil Engineer. Retrieved 23 March 2024.
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