{{Short description|Tunnel under construction in England}} {{Use British English|date=May 2021}} {{Use dmy dates|date=May 2021}} {{Infobox tunnel | name = Chiltern Tunnel | image = | image_size = | alt = | caption = | official_name = | other_name = | line =[[High Speed 2]] | location =[[United Kingdom]] ([[Buckinghamshire]], [[Hertfordshire]]) | coordinates ={{coord|51.7095|-0.6931}} (north portal) {{ubl|{{coord|51.6116|-0.5241|display=inline,title}} (south portal)}} | os_grid_ref =TQ022913 | status = Construction complete, fitting out | system = | crosses = [[Chiltern Hills]] | start = [[West Hyde]], [[Hertfordshire]] | end = [[South Heath]], [[Buckinghamshire]] | stations = | startwork =13 May 2021 (boring) | opened = <!-- or |opens for future dates --> | closed = <!-- or |closes for future dates --> | rebuilt = | reopened = <!-- or |reopens for future dates --> | owner = | operator = | traffic = High-speed passenger trains | engineer = | length = {{convert|16.04|km|mi|abbr=in}} | linelength = | tracklength = | notrack = 2 [[single track (rail)|single-track]] tunnels | gauge = {{Track gauge|sg|al=on|allk=on}} | el = {{25 kV 50 Hz AC}} | speed ={{convert|320|kph|round=5|abbr=on}} | hielevation = | height = | width = {{convert|9.1|m|ft}} | depth_below_water = | shipping_clearance_above = | grade = −0.835% to +2.500%<ref>{{Cite web |date=9 August 2016 |title=HS2 Phase One: updated plan and profile maps for Country South 2016 |url=https://www.gov.uk/government/publications/hs2-plan-and-profile-maps-post-house-of-commons-select-committee-2016-country-south |access-date=2026-01-22 |website=GOV.UK |language=en}}</ref> | extra ={{routemap |title =Chiltern Tunnel |map = exCONTg~~ ~~[[High Speed 2|HS2]]~~to [[Birmingham Interchange]] extSTRa~~ ~~[[South Heath]]~~(north portal) extSKRZ-G2~~{{UK road|B485}} extDST~~Chesham Road intervention shaft CONTgq\xtKRZ\CONTfq~~ ~~ [[London-Aylesbury line]] extDST~~[[Little Missenden]] shaft extSKRZ-G~~{{UK road|A413}} extSKRZ-G2~~{{UK road|A404}} extDST~~[[Amersham]] shaft extSKRZ-G~~{{UK road|A355}} extDST~~[[Chalfont St Giles]] shaft extSKRZ-G~~{{UK road|A413}} extDST~~ [[Chalfont St Peter]] shaft extSKRZ-B~~{{UK road|M25}} extSTR+GRZq~~[[Buckinghamshire]]-[[Hertfordshire]] border extSTRe~~ ~~[[West Hyde]]~~(south portal) exCONTf~~ ~~[[High Speed 2|HS2]]~~to [[Old Oak Common railway station|Old Oak Common]] }} |construction=2020–2026|cross_passages=38}} The '''Chiltern Tunnel''' is a [[High-speed rail|high-speed]] [[Tunnel|railway tunnel]] in [[Buckinghamshire]] and [[Hertfordshire]], England, which will carry the [[High Speed 2]] (HS2) railway line under the [[Chiltern Hills]]. The twin-bore tunnels, which are {{convert|16.04|km|mi|abbr=in}} long, are the longest on the HS2 line.<ref name=":3">{{Cite web|title=Chiltern Tunnel|url=https://www.hs2.org.uk/building-hs2/tunnels/tunnel-drives/chiltern-tunnel/|access-date=2022-02-04|website=High Speed 2|language=en-GB}}</ref><ref name=":0">{{Cite web|title=Bouygues Travaux Publics - Project - HS2, High-Speed Rail Line|url=https://www.bouygues-tp.com/en/projects/hs2-high-speed-rail-line|access-date=2021-08-12|website=www.bouygues-tp.com}}</ref> Each tunnel also has additional {{convert|220|m|abbr=on}} entry and {{convert|135|m|abbr=on}} exit perforated concrete portals to reduce sudden changes in air pressure and subsequent noise.<ref name=":7">{{Cite web |title=HS2 reveals noise-cancelling Chiltern Tunnel north portal design |url=https://mediacentre.hs2.org.uk/news/hs2-reveals-noise-cancelling-chiltern-tunnel-north-portal-design |access-date=2023-06-03 |website=HS2 News and Information |language=english}}</ref>

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 machine]]s (TBMs), advanced at an average speed of {{convert|15|m|abbr=on}} per day; both TBMs completed their drives by March 2024.<ref name=":13">{{Cite magazine |last=Hakimian |first=Rob |date=2024-03-22 |title=HS2's 16km Chilterns tunnelling completed with second TBM breakthrough |url=https://www.newcivilengineer.com/latest/hs2s-16km-chilterns-tunnelling-completed-with-second-tbm-breakthrough-22-03-2024/ |access-date=2024-03-23 |magazine=New Civil Engineer |language=en}}</ref> Civil engineering work on the tunnel was completed by January 2026.

==History== 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".<ref name = "civil biggest2021"/> Of the {{convert|102|km|abbr=in}} of tunnels that are to be bored to accommodate the line, around {{convert|16|km|abbr=in}} were bored for the Chiltern Tunnel alone.<ref name = "civil biggest2021"/>

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 {{convert|2.6|km|mi|abbr=in}} north; this revision has allowed for the replacement of a separate planned [[cut-and-cover]] tunnel, as well as avoiding the destruction of around {{convert|12|ha}} of woodland such as [[Farthings Wood]].<ref>{{Cite web |title=HS2 confirms Chilterns tunnel extension |url=http://www.infrastructure-intelligence.com/article/aug-2015/hs2-confirms-chilterns-tunnel-extension |access-date=19 March 2021 |website=Infrastructure Intelligence |language=en}}</ref><ref>{{cite web |url = https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/436541/6._C222-ATK-TN-REP-020-000012_P03.pdf |title = High Speed Rail in the Chilterns Part 2: Chiltern Long Tunnel Proposal |publisher = gov.uk |date = 28 May 2015 |first = Evans |last = A. }}</ref> During 2017, Contract C1 (Central 1), which covers the 21.6km section of the line that the Chiltern Tunnel comes within, was awarded to the ''Align JV'' [[joint venture]], comprising [[Bouygues Travaux Publics]], [[Sir Robert McAlpine]] and [[VolkerFitzpatrick]].<ref name = "rail engineer"/> The two [[tunnel boring machine]]s were supplied by [[Herrenknecht]]<ref name=":3" /> and were delivered to the site in December 2020.<ref name=":4">{{Cite web |date=17 March 2021 |title=Work on longest tunnels set to begin |url=https://www.modernrailways.com/article/boring-chiltern-tunnels |access-date=16 July 2022 |website=Modern Railways}}</ref>

==Design== The basic configuration of the tunnel consists of a pair of parallel bores, which are to be excavated by a pair of [[tunnel boring machine]]s (TBMs).<ref name = "civil biggest2021"/> Each TBM weighs around 2,000{{nbsp}}tonnes, has a length of {{convert|170|m|abbr=on}}, 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 had 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|M25]] motorway, only one TBM was active at a time while near to this key trunk route.<ref name = "civil biggest2021">{{cite web |url = https://www.newcivilengineer.com/latest/the-reality-of-working-on-hs2s-biggest-construction-site-14-06-2021/ |title = The reality of working on HS2's biggest construction site |publisher = newcivilengineer.com |first = Rob |last = Horgan |date = 14 June 2021}}</ref>

The southern entrance to the tunnels is aligned with the nearby [[Colne Valley Viaduct]] and is near the M25. The pair of tunnels are {{convert|16.04|km|mi|abbr=in}} long, with the northern portal near [[South Heath]]. At their deepest point, they are {{convert|90|m|abbr=on}} below ground level; the internal diameter of each bore is {{convert|9.1|m|abbr=on}}.<ref>{{Cite web|title=The HS2 Tunnel Boring Machines (TBMs)|url=https://www.hs2.org.uk/building-hs2/phase-one-london-west-midlands/tunnels/tbmvoting/|access-date=2021-03-20|website=High Speed 2|language=en-GB|archive-date=3 March 2021|archive-url=https://web.archive.org/web/20210303102053/https://www.hs2.org.uk/building-hs2/phase-one-london-west-midlands/tunnels/tbmvoting/|url-status=dead}}</ref>

The tunnel has a total of 38 cross passages, between {{convert|15|m|abbr=on}} and {{convert|20|m|abbr=on}} long, spaced {{convert|500|m|abbr=on}} apart, and 5 [[Adit|adits]] linking the two separate bores.<ref name=":3" /><ref>{{Cite web |date=2025-08-13 |title=Chiltern Tunnel construction completed on HS2 |url=https://www.concreteconnect.co.uk/news/chiltern-tunnel-construction-completed-on-hs2 |access-date=2025-08-20 |website=Concrete Connect |language=en}}</ref><ref>{{Cite web |last=Morby |first=Aaron |date=2022-08-23 |title=Expert miners start HS2 tunnel emergency cross passages |url=https://www.constructionenquirer.com/2022/08/23/expert-miners-start-hs2-tunnel-emergency-cross-passages/ |access-date=2025-08-20 |website=Construction Enquirer |language=en-GB}}</ref> The tunnel has five {{convert|18|m|abbr=on}} diameter shafts used for both ventilation and emergency access purposes between {{convert|35|m|abbr=on}} and {{convert|65|m|abbr=on}} deep.<ref>{{Cite web |date=2020-09-17 |title=Design of Chiltern Tunnel and ventilation shaft interfaces |url=https://learninglegacy.hs2.org.uk/document/design-of-chiltern-tunnel-and-ventilation-shaft-interfaces/ |access-date=2023-09-14 |website=HS2 Learning Legacy |language=en-GB}}</ref> These are located at [[Chalfont St Peter]],<ref>{{Cite web |date=6 October 2021 |title=Chalfont St Peter ventilation shaft and headhouse |url=https://www.hs2.org.uk/in-your-area/local-community-webpages/hs2-in-buckinghamshire-and-oxfordshire/chalfont-st-peter-ventilation-shaft-and-headhouse/ |access-date=5 July 2022 |website=High Speed 2}}</ref> [[Chalfont St Giles]],<ref>{{Cite web |date=15 September 2021 |title=Chalfont St Giles ventilation shaft and headhouse |url=https://www.hs2.org.uk/in-your-area/local-community-webpages/hs2-in-buckinghamshire-and-oxfordshire/chalfont-st-giles-ventilation-shaft-and-headhouse/ |access-date=5 July 2022 |website=High Speed 2}}</ref><ref>{{Cite web |title=HS2's Chiltern tunnel headhouse vent shaft gains planning approval |url=https://www.globalrailwayreview.com/news/117053/hs2-chiltern-headhouse-vent/ |access-date=19 March 2021 |website=Global Railway Review |language=en}}</ref> [[Amersham]],<ref>{{Cite web |title=Amersham Headhouse and Ventilation Shaft |url=https://www.hs2.org.uk/building-hs2/tunnels/headhouses-and-ventilation-shafts/amersham-vent-shaft-headhouse/ |access-date=2023-09-22 |website=HS2 |language=en-GB}}</ref><ref name=":1">{{Cite news |date=6 July 2022 |title=HS2: Images show new design for ventilation shaft for tunnel |work=BBC News |url=https://www.bbc.co.uk/news/uk-england-beds-bucks-herts-62071800 |access-date=6 July 2022}}</ref> [[Little Missenden]]<ref>{{Cite web |title=Little Missenden Headhouse and Ventilation Shaft |url=https://www.hs2.org.uk/building-hs2/tunnels/headhouses-and-ventilation-shafts/little-missenden-ventilation-shaft-and-headhouse/ |access-date=2023-09-22 |website=HS2 |language=en-GB}}</ref><ref>{{Cite web |last=Evans |first=A |date=28 May 2015 |title=High Speed Rail in the Chilterns Part 1: General Long Tunnel Requirements |url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/436540/5._C222-ATK-TN-REP-020-000013_P02.pdf |access-date=5 July 2022 |website=HS2 Ltd.}}</ref> and Chesham Road<ref>{{Cite web |title=Chesham Road Intervention Shaft |url=https://www.hs2.org.uk/building-hs2/tunnels/headhouses-and-ventilation-shafts/chesham-road-intervention-shaft/ |access-date=2023-09-22 |website=HS2 |language=en-GB}}</ref> - 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.<ref>{{cite web |url = https://www.bucksherald.co.uk/business/hs2-reveals-chilterns-barn-design-vent-shaft-chalfont-st-giles-3125142 |title = HS2 reveals Chilterns 'barn design' vent shaft for Chalfont St Giles |publisher = bucksherald.co.uk |date = 5 February 2021 |first = Thomas |last = Bamford}}</ref> 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.<ref>{{cite web | url=https://www.buckinghamshirelive.com/news/buckinghamshire-news/hs2-amerhsm-vent-shaft-plans-5445363 | title=Council warns Amersham HS2 vent shaft plans are 'totally unacceptable' | date=22 May 2021 }}</ref><ref>{{cite web |title=HS2 reveals updated designs for Amersham vent shaft headhouse |url=https://mediacentre.hs2.org.uk/news/hs2-reveals-updated-designs-for-amersham-vent-shaft-headhouse}}</ref> Each tunnel has an emergency walkway that would allow passengers to evacuate and then cross into the other tunnel to be rescued on a passenger train.<ref name=":3" />

The tunnel has porous portals (a form of tunnel entrance hood) that extend for up to {{convert|220|m}} 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 [[Piston effect#Tunnel boom|audible "sonic boom"]] in line with environmental commitments.<ref name=":7" /><ref>{{Cite web |date=2021-02-24 |title=Aerodynamic design of HS2 tunnels |url=https://learninglegacy.hs2.org.uk/document/aerodynamic-design-of-hs2-tunnels/ |access-date=2023-09-14 |website=HS2 Learning Legacy |language=en-GB}}</ref><ref>{{Cite web |date=2020-09-17 |title=Design of Chiltern Tunnel South Portal |url=https://learninglegacy.hs2.org.uk/document/design-of-chiltern-tunnel-south-portal/ |access-date=2023-09-14 |website=HS2 Learning Legacy |language=en-GB}}</ref>

==Construction== In July 2020, work was completed on a {{cvt|17|m}} high headwall at the southern end, at {{Coord|51.6117|-0.5242|type:landmark_region:GB_dim:300|display=inline|name=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 was constructed on site; this site was also used to support the construction of the nearby [[Colne Valley Viaduct]] as well.<ref name="civil biggest2021" /><ref name="rail engineer">{{cite web |url = https://www.railengineer.co.uk/colne-valley-viaduct-hs2s-largest-bridge/ |title = Colne Valley Viaduct – HS2's largest bridge |publisher = railengineer.co.uk |date = 7 June 2021}}</ref>

During September 2020, final preparations for the arrival of the first of the two TBMs were made and both arrived on site in December that year. Names for the two machines were suggested by children from Meadow High School in Hillingdon and [[Chalfonts Community College]], and ''Cecilia'' and ''Florence'' were chosen by a public vote. The names commemorate Buckinghamshire-born astronomer [[Cecilia Payne-Gaposchkin]] and [[Florence Nightingale]].<ref>{{cite web |title=Progress at Chiltern tunnel site as HS2 prepares for arrival of first tunnelling machines |url=https://mediacentre.hs2.org.uk/news/progress-at-chiltern-tunnel-site-as-hs2-prepares-for-arrival-of-first-tunnelling-machines |url-status=live |archive-url=https://web.archive.org/web/20200916223348/https://mediacentre.hs2.org.uk/news/progress-at-chiltern-tunnel-site-as-hs2-prepares-for-arrival-of-first-tunnelling-machines |archive-date=16 September 2020 |access-date=30 July 2020}}</ref><ref>{{cite web |url = https://www.bbc.co.uk/news/uk-england-beds-bucks-herts-53726387 |title = Giant machines for HS2's Chilterns tunnels unveiled |publisher = BBC News |date = 11 August 2020}}</ref><ref name=":4" /> During March 2021, the first section of [[fibre-reinforced concrete]] tunnel lining was cast; this was produced onsite at the Align JV's purpose-built facility near the southern portal, which was capable of manufacturing up to 49 segments at a time.<ref>{{Cite web |date=19 March 2021 |title=HS2 Casts First of 112,000 Chiltern Tunnel Wall Segments |url=https://railway-news.com/hs2-casts-first-of-112000-chiltern-tunnel-wall-segments/ |access-date=19 March 2021 |website=Railway-News}}</ref><ref>{{Cite web |last=Horgan |first=Rob |date=2021-06-14 |title=The reality of working on HS2’s biggest construction site |url=https://www.newcivilengineer.com/latest/the-reality-of-working-on-hs2s-biggest-construction-site-14-06-2021/ |access-date=2026-01-23 |website=New Civil Engineer |language=en}}</ref>

Excavation commenced on 13 May 2021,<ref name="tunnellingstart">{{cite web |date=13 May 2021 |title=HS2 high-speed rail project begins underground tunnel |url=https://www.bbc.co.uk/news/av/uk-57093884 |url-status=live |archive-url=https://web.archive.org/web/20210512232434/https://www.bbc.co.uk/news/av/uk-57093884 |archive-date=12 May 2021 |access-date=13 May 2021 |website=BBC News}}</ref><ref>{{cite web |url = https://www.itv.com/news/meridian/2021-05-13/work-begins-to-create-a-tunnel-through-the-chilterns-for-new-hs2-rail-line |title = Work begins to create a tunnel through the Chilterns for new HS2 rail line |publisher = itv.com |date = 13 May 2021}}</ref> with completion estimated around March 2024.<ref name=":2">{{Cite magazine |last=Hakimian |first=Rob |date=15 July 2022 |title=HS2 timeline of major events to 2025 revealed |url=https://www.newcivilengineer.com/latest/hs2-timeline-of-major-events-to-2025-revealed-15-07-2022/ |access-date=16 July 2022 |magazine=New Civil Engineer}}</ref><ref>{{Cite web|title=HS2 Corporate Plan 2021–2024|url=https://assets.hs2.org.uk/wp-content/uploads/2021/07/HS2_CorporatePlan_2021-2024-Accessibility_FINAL.pdf|url-status=live|archive-url=https://web.archive.org/web/20211221135213/https://assets.hs2.org.uk/wp-content/uploads/2021/07/HS2_CorporatePlan_2021-2024-Accessibility_FINAL.pdf |archive-date=21 December 2021 }}</ref> 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.<ref name = "civil biggest2021"/> During June 2021, the second TBM commenced boring.<ref>{{cite web |url = https://www.geplus.co.uk/news/second-hs2-tunnelling-machine-starts-digging-under-the-chilterns-01-07-2021/ |title = Second HS2 tunnelling machine starts digging under the Chilterns |publisher = geplus.co.uk |date = 1 July 2021 |first = Nia |last = Kajastie}}</ref><ref>{{Cite web |title=Momentum builds as HS2 launches second giant tunnelling machine |url=https://mediacentre.hs2.org.uk/news/momentum-builds-as-hs2-launches-second-giant-tunnelling-machine |access-date=2024-03-22 |website=HS2 News and Information |language=english}}</ref> The first stage of the drive (from the south portal to the Chalfont St Peter ventilation shaft) was completed in March 2022,<ref>{{Cite web |date=14 March 2022 |title=HS2 tunnelling machines reach Chalfont St Peter ventilation shaft |url=https://mediacentre,hs2.org.uk/news/hs2-tunneling-machines-reach-chalfont-st-peter-ventilation-shaft |access-date=17 August 2022 |website=High Speed 2}}</ref> and the first of the cross passages between the main running tunnels were completed in August that year.<ref>{{Cite web |date=22 August 2022 |title=HS2 completes first tunnel cross passages |url=https://railuk.com/rail-projects/hs2/hs2-completes-first-tunnel-cross-passages/ |access-date=22 August 2022 |website=RailUK}}</ref> The Chalfont St Giles ventilation shaft was reached in October 2022,<ref name=":5">{{Cite web |date=10 October 2022 |title=HS2 Chiltern tunnel - 'Florence' and 'Cecilia' reach 4 mile point at Chalfont St Giles |url=https://railuk.com/rail-projects/hs2/hs2-chiltern-tunnel-florence-and-cecilia-reach-4-mile-point-at-chalfont-st-giles/ |access-date=11 October 2022 |website=RailUK}}</ref> Amersham in March 2023,<ref>{{Cite web |date=2 March 2023 |title=HS2 tunnel progress as 'Florence' and 'Cecilia' pass Amersham |url=https://mediacentre.hs2.org.uk/news/hs2-tunnel-progress-as-florence-and-cecilia-pass-amersham |access-date=15 August 2023 |website=HS2}}</ref> Little Missenden in August,<ref>{{Cite web |date=8 August 2023 |title=Tunnel progress as HS2's 'Florence' and 'Cecilia' reach Little Missenden |url=https://mediacentre.hs2.org.uk/news/tunnel-progress-as-hs2s-florence-and-cecilia-reach-little-missenden |access-date=15 August 2023 |website=HS2}}</ref> and Chesham Road in December of the same year.<ref>{{Cite magazine |last=Moore |first=Catherine |date=2023-12-19 |title=HS2's Chiltern tunnelling reaches 90% completion |url=https://www.newcivilengineer.com/latest/hs2s-chiltern-tunnelling-now-90-complete-19-12-2023/ |access-date=2023-12-20 |magazine=New Civil Engineer |language=en}}</ref>

The chalk excavated during the boring of the tunnels is to be used to create a [[nature reserve]] covering {{convert|127|ha}} in the vicinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to {{convert|3000000|m3|abbr=off}} of chalk slurry were provisioned onsite prior to excavation commencing.<ref name = "rail engineer"/><ref>{{Cite web |date=3 March 2021 |title=HS2 to 'rewild' 127 hectares around its 10-mile Chilterns tunnel |url=http://www.theguardian.com/uk-news/2021/mar/03/hs2-to-rewild-127-hectares-around-its-10-mile-chilterns-tunnel |access-date=19 March 2021 |website=The Guardian |language=en}}</ref><ref>{{cite web |url = https://www.newcivilengineer.com/latest/how-hs2-plans-to-reuse-chalk-slurry-from-chilterns-tunnel-to-rewild-the-colne-valley-27-04-2021/?tkn=1 |title = How HS2 plans to reuse chalk slurry from Chilterns tunnel to 'rewild' the Colne Valley |publisher = newcivilengineer.com |date = 27 April 2021 |first = Rob |last = Horgan}}</ref><ref>{{Cite web |date=8 September 2022 |title=HS2 celebrates milestone in Chiltern grassland transformation |url=https://railuk.com/industry-news/hs2-celebrates-milestone-in-chiltern-grassland-transformation/ |access-date=11 September 2022 |website=RailUK}}</ref> Approval for this scheme was issued in May 2021.<ref>{{cite web |url = https://www.railpro.co.uk/news/green-light-for-hs2s-ambitious-chiltern-tunnel-chalk-grassland-project |title = Green light for HS2's ambitious Chiltern tunnel chalk grassland project |publisher = railpro.co.uk |date = 21 June 2021}}</ref>

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.<ref>{{Cite news |date=2022-05-11 |title=HS2 tunnel fire: Workers shelter in digger near Maple Cross |language=en-GB |work=BBC News |url=https://www.bbc.com/news/uk-england-beds-bucks-herts-61409778 |access-date=2023-09-19}}</ref>

Tunnelling work was unaffected by the formation of a sinkhole, estimated to be {{convert|5|m|ft|abbr=on}} deep and {{convert|6|m|ft|abbr=on}} wide, near Shardeloes Lake, Amersham in May 2023,<ref>{{Cite news |date=2023-05-14 |title=HS2: Large hole opens above rail tunnel near Little Missenden |language=en-GB |work=BBC News |url=https://www.bbc.com/news/uk-england-beds-bucks-herts-65590981 |access-date=2023-10-06}}</ref> although an investigation was launched by the [[Environment Agency]] into the causes of the ground collapse.<ref name=":6">{{Cite magazine |last=Kanaris |first=Sotiris |date=27 June 2023 |title=Future of Tunnelling: Four High Speed 2 twin bores progressing well despite challenges |url=https://www.newcivilengineer.com/the-future-of/future-of-tunnelling-four-high-speed-2-twin-bores-move-ahead-despite-challenges-27-06-2023/#:~:text=Chiltern%20Tunnel,-Align%20JV%20–%20comprising&text=Two%20TBMs%20were%20launched%20at,varies%20from%2010m%20to%2090m |access-date=14 September 2023 |magazine=New Civil Engineer}}</ref> The resulting inquiry found that the local water supply was not contaminated, and the sinkhole was filled with {{convert|165|m3|cuft|abbr=off}} of chalk in September 2023.<ref name=":8">{{Cite news |date=2023-10-06 |title=HS2 Chiltern tunnel sinkhole did not pollute water, says inquiry |language=en-GB |work=BBC News |url=https://www.bbc.com/news/uk-england-beds-bucks-herts-67030442 |access-date=2023-10-06}}</ref> Further work to ensure the hole is filled was completed in early 2024.<ref name=":9">{{Cite magazine |last=Johnson |first=Thomas |date=2024-02-14 |title=HS2 {{!}} Two more sinkholes form on Chilterns tunnelling route |url=https://www.newcivilengineer.com/latest/hs2-two-more-sinkholes-form-on-chilterns-tunnelling-route-14-02-2024/ |access-date=2024-02-14 |magazine=New Civil Engineer |language=en}}</ref> Another sinkhole formed in November 2023 near Hedgemoor Wood, [[Hyde Heath]],<ref>{{Cite web |last=Kajastie |first=Nia |date=2023-11-15 |title=HS2: Another sinkhole appears above Chiltern tunnels in Buckinghamshire |url=https://www.geplus.co.uk/news/hs2-another-sinkhole-appears-above-chiltern-tunnels-in-buckinghamshire-15-11-2023/ |access-date=2023-11-21 |website=Ground Engineering |language=en}}</ref> and two more followed in February 2024 adjacent to Frith Hill in South Heath,<ref name=":9" /> although tunnelling remained unaffected in both cases.

The cross-passages were excavated with a remote-controlled excavator and supported by a [[Shotcrete|sprayed concrete]] lining, before a waterproof membrane and secondary concrete lining were applied.<ref name=":3" />

''Florence''<nowiki/>'s progress was hampered significantly during the last few weeks of its 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.<ref name=":12">{{Cite magazine |last=Hakimian |first=Rob |date=2024-02-28 |title=HS2 Chilterns TBM overcame difficult geology and clogged slurry pipes to reach finish line |url=https://www.newcivilengineer.com/latest/hs2-chilterns-tbm-overcame-difficult-geology-and-clogged-slurry-pipes-to-reach-finish-line-28-02-2024/ |access-date=2024-02-29 |magazine=New Civil Engineer |language=en}}</ref> 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.<ref name=":12" />

TBM ''Florence'' broke through at the north portal in South Heath on 27 February 2024,<ref name=":10">{{Cite web |title=Historic breakthrough for HS2's longest tunnel |url=https://mediacentre.hs2.org.uk/news/breakthrough-for-hs2s-longest-tunnel |access-date=2024-02-27 |website=HS2 News and Information |language=english}}</ref><ref>{{Cite web |date=2024-02-27 |title=HS2 tunnelling machine completes journey under Chiltern Hills |url=https://www.bbc.com/news/articles/cjqjqgxyzjwo |access-date=2024-02-27 |website=BBC News |language=en-GB}}</ref><ref name=":11">{{Cite magazine |last=Hakimian |first=Rob |date=2024-02-27 |title=HS2 celebrates completion of first 16km drive for Chilterns tunnel |url=https://www.newcivilengineer.com/latest/hs2-celebrates-completion-of-first-16km-drive-for-chilterns-tunnel-27-02-2024/ |access-date=2024-02-27 |magazine=New Civil Engineer |language=en}}</ref> with ''Cecilia'' following suit on 22 March.<ref name=":13" /><ref>{{Cite news |date=2024-03-22 |title=HS2 tunnelling machine completes journey in Buckinghamshire |url=https://www.bbc.com/news/uk-england-beds-bucks-herts-68626625 |access-date=2024-03-23 |work=BBC News |language=en-GB}}</ref><ref>{{Cite magazine |last=Hakimian |first=Rob |date=2024-03-22 |title=HS2's 16km Chilterns tunnelling completed with second TBM breakthrough |url=https://www.newcivilengineer.com/latest/hs2s-16km-chilterns-tunnelling-completed-with-second-tbm-breakthrough-22-03-2024/ |access-date=2024-03-23 |magazine=New Civil Engineer |language=en}}</ref> 112,350 concrete segments were used to line both tunnels, forming 16,050 rings.<ref>{{Cite web |date=20 December 2023 |title=Align JV reach milestone with the casting of the final segment for new tunnels |url=https://www.volkerfitzpatrick.co.uk/en/news/align-jv-reach-milestone-casting-final-segment-new-tunnels |access-date=2026-01-23 |website=VolkerFitzpatrick |language=en}}</ref>

On 19 January 2026, HS2 announced the completion of civil engineering works for the tunnel.<ref>{{Cite web |date=2026-01-19 |title=HS2 completes civil engineering works on longest tunnel project |url=https://mediacentre.hs2.org.uk/news/hs2-completes-civil-engineering-works-on-longest-tunnel-project |access-date=2026-01-22 |website=HS2 |language=}}</ref>

==References== {{reflist}}

== External links == * [https://www.hs2.org.uk/building-hs2/tunnels/tunnel-drives/chiltern-tunnel/ HS2 - Chiltern Tunnel]

{{High Speed 2 articles}}

[[Category:Tunnels on High Speed 2]] [[Category:Tunnels in Buckinghamshire]] [[Category:Buildings and structures under construction in the United Kingdom]]