Northolt Tunnel

{{Use British English|date=July 2021}} {{Use dmy dates|date=July 2021}} {{Infobox tunnel | name = Northolt Tunnel | image = HS2_London_route.png | image_size = | alt = | caption = HS2 route running west from London Euston, through Old Oak Common towards Ruislip | official_name = | other_name = | line = | location = | coordinates = {{coord|51.536|-0.319|display=inline,title}} | os_grid_ref = | status = Construction complete, fitting out | system = [[High Speed 2]] | crosses = West London | start = [[Old Oak Common railway station]] | end = [[Ruislip]] | stations = | startwork = | opened =  | closed =  | rebuilt = | reopened =  | owner = | operator = | traffic = High-speed passenger trains | engineer = | construction =6 October 2022 - 26 June 2025 | length = {{convert|8.4|mi|km}} | linelength = | tracklength = | notrack = 2 [[single track (rail)|single-track]] tunnels | gauge = {{Track gauge|Standard gauge}} | el = {{25 kV 50 Hz AC}} | speed ={{convert|320|kph|round=5|abbr=on}} | hielevation = | lowelevation = | height = | width = West - {{convert|8.8|m|ft}}<br />East - {{convert|8.1|m|ft}} | shipping_clearance_above = | grade =−0.59% to +2.50%<ref>{{Cite web |date=9 August 2016 |title=HS2 Phase One: updated plan and profile maps for London Metropolitan 2016 |url=https://www.gov.uk/government/publications/hs2-plan-and-profile-maps-post-house-of-commons-select-committee-2016-london-metropolitan |access-date=2026-01-22 |website=GOV.UK |language=en}}</ref> | map = | extra = | cross_passages=34}}

'''Northolt Tunnel''' is a [[High-speed rail|high-speed]] [[Tunnel|railway tunnel]] in [[Greater London]], [[England]], that will carry the [[High Speed 2]] (HS2) railway line under the West London suburbs. The twin-bore tunnels were excavated from October 2022 until June 2025 and run for {{convert|8.4|mi|km}} between [[Old Oak Common railway station|Old Oak Common]] and [[Ruislip]].

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 Regional Park|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{{nbsp}}billion had been awarded to a [[joint venture]] company, [[Skanska]] [[Costain Group|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 {{convert|102|km|abbr=in}} of tunnels are to be bored along various parts of its route in order to accommodate it.<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> During early 2013, it was confirmed that this section of HS2 would be constructed in a tunnel, rather than above ground.<ref>{{cite web |url = https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/310043/gov-res-northolt-bromford.pdf |title = The Government response to the design refinement consultation: Decisions and Safeguarding Directions for Northolt and Bromford |publisher = gov.uk |date = October 2013}}</ref> 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.<ref>{{cite web |date=24 October 2013 |title=HS2 tunnel confirmed for Ealing and Northolt |url=https://www.bbc.co.uk/news/uk-england-london-24657125 |access-date=3 July 2021 |publisher=BBC News}}</ref><ref>{{Cite web |last1=Watts |first1=Joe |last2=Beard |first2=Matthew |date=25 April 2013 |title=HS2 tunnel extended under west London to cut chaos |url=https://www.standard.co.uk/news/transport/hs2-tunnel-extended-under-west-london-to-cut-chaos-8584820.html |access-date=3 July 2021 |website=Evening Standard |language=en}}</ref>

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".<ref>{{cite web |url = https://www.ft.com/content/5ede29e8-291b-11e8-b27e-cc62a39d57a0 |title = HS2 builders find ancient sub-tropical coastline in west London |first = Rianna |last = Croxford |publisher = Financial Times |date = 16 March 2018}}</ref> During early 2018, a layer of black clay was discovered, which has been dubbed the "Ruislip Bed"; this material dates back 56{{nbsp}}million{{nbsp}}years and was formed from densely wooded marshes at the coast of a sub-tropical sea.<ref>{{Cite web |date=16 March 2018 |title=Workers discover ancient coastline in West London |url=https://www.heritagedaily.com/2018/03/workers-discover-ancient-coastline-in-west-london/118611 |access-date=3 July 2021 |website=HeritageDaily |language=en}}</ref>

During the early-to-mid 2010s, the possibility of extending the tunnel to traverse the [[Colne Valley Regional Park|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.<ref>{{Cite web |date=27 February 2015 |title=Proposal for Northern Extension of Northolt Tunnel SIFT Report |url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/410306/C222-ATK-DS-REP-020-000034_P06_with_drawings.pdf |access-date=3 July 2020 |website=High Speed 2}}</ref>

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.<ref name = "ian 2020">{{Cite web |date=15 April 2020 |title=HS2's contracts awarded – includes £3.3 billion London tunnels |url=https://www.ianvisits.co.uk/blog/2020/04/15/hs2s-contracts-warded-includes-3-3-billion-london-tunnels/ |access-date=3 July 2021 |website=IanVisits |language=en-GB}}</ref> In October 2020, it was revealed that [[Herrenknecht]] would be supplying the two [[tunnel boring machine]]s (TBMs) that would be used in the boring of the western section.<ref name=":0">{{cite web |date=12 October 2020 |title=Order goes in for Northolt tunnels TBMs |url=https://www.theconstructionindex.co.uk/news/view/order-goes-in-for-northolt-tunnels-tbms |access-date=3 July 2021 |publisher=The Construction Index}}</ref>

== Design == The tunnel has five shafts and associated headhouses at Victoria Road, Westgate, Green Park Way, Mandeville Road, and [[South Ruislip]], providing ventilation and emergency access. The tunnel reaches a maximum depth of {{convert|35|m|ft}}, while the distance between the two tunnel bores ranges from {{convert|6|m|ft}} to {{convert|20|m|ft}}.<ref name=":1" /><ref name=":5">{{Cite web |date=27 April 2026 |title=Cross passage construction complete on HS2’s Northolt Tunnel in London |url=https://mediacentre.hs2.org.uk/news/cross-passage-construction-complete-on-hs2s-northolt-tunnel-in-london |access-date=2026-05-03 |website=HS2 News and Information |language=english}}</ref>

The tunnel bores are connected by 34 cross-passages.<ref name=":1" /><ref>{{Cite web |date=2020-09-22 |title=Design of shafts and tunnels to decouple the construction programme |url=https://learninglegacy.hs2.org.uk/document/design-of-shafts-and-tunnels-to-decouple-construction/ |access-date=2025-06-27 |website=HS2 Learning Legacy |language=en-GB}}</ref>

== Construction == [[File:Bridge crane at HS2 West Ruislip tunnel portal - geograph.org.uk - 7806085.jpg|thumb|Tunnel portal, West Ruislip]] The tunnel was bored in two sections; the western section is {{convert|5|mi|km}} long and the eastern section is {{convert|3.4|mi|km}}<ref name=":0" /> Of these, {{convert|13.7|km|abbr=in}} was designed as a twin bored tunnel, which was excavated using a total of four [[earth pressure balance]] TBMs. Following the completion of boring, all four TBMs were retrieved at the point where the two sections meet; a site which later became the Green Park Way [[ventilation shaft]].<ref name=":0" /><ref name = "reconnect 2021">{{cite web |url = https://www.londonreconnections.com/2021/hs2-prepares-for-21km-tunnels-under-london-tunneltalk/ |title = HS2 Prepares for 21km tunnels under London (Tunneltalk) |publisher = londonreconnections.com |first = Mike |last = Long Branch |date = 22 February 2021}}</ref>

The Green Park Way shaft has both a main shaft {{convert|20|m|ft|abbr=on}} wide and a smaller satellite shaft around {{convert|15|m|ft|abbr=on}} in diameter, rising to {{convert|20|m|ft|abbr=on}} at the base, avoiding a National Rail track {{convert|2.7|m|ft|abbr=on}} away. They are both around {{convert|45|m|ft|abbr=on}} deep. During construction, excavation involved London Clay at the surface, then granular [[Harwich Formation]], [[Lambeth Group]] with sand [[Lens (geology)|lenses]], [[Upnor Formation]], and chalk. It passed through 3 intermediate [[Aquifer|aquifers]], with water pressure rising to 3-4 bars in places. This was countered with grout injections and dewatering wells. Due to these conditions, steel reception chambers were needed for the TBMs, filled with [[foam concrete]], allowing for the application of sealant before depressurisation to prevent water ingress.<ref>{{Cite web |last=Menteth |first=Thames |date=2024-12-04 |title=Shaft sinking: challenges at HS2's Green Park Way vent shafts site |url=https://www.geplus.co.uk/features/shaft-sinking-challenges-at-hs2s-green-park-way-vent-shafts-site-04-12-2024/ |url-access=subscription |access-date=2025-06-27 |website=Ground Engineering |language=en}}</ref><ref>{{Cite web |last=High |first=Richard |date=2024-12-31 |title=HS2's TBM Sushila completes Northolt Tunnel milestone |url=https://www.concreteconnect.co.uk/news/hs2s-tbm-sushila-completes-northolt-tunnel-milestone |access-date=2025-06-27 |website=Concrete Connect |language=en}}</ref>

Tunnel segments for the western section were made by Strabag in [[Hartlepool]], while those for the eastern section were by Pacadar UK on the [[Isle of Grain]], [[Kent]], and were transported to the construction site by rail.<ref name=":4" /><ref>{{Cite web |date=2024-09-03 |title=HS2 tunnel segment casting complete at Isle of Grain factory |url=https://mediacentre.hs2.org.uk/news/hs2-tunnel-segment-casting-complete-at-isle-of-grain-factory |access-date=2025-06-27 |website=HS2 |language=english}}</ref>

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 railway station|Old Oak Common station]] itself.<ref>{{Cite magazine |last=Kanaris |first=Sotiris |date=2023-08-25 |title='Caterpillar' shaft saves carbon and space on HS2's west London site |url=https://www.newcivilengineer.com/innovative-thinking/caterpillar-shaft-saves-carbon-and-space-on-hs2s-west-london-site-25-08-2023/ |access-date=2023-09-27 |magazine=New Civil Engineer |language=en}}</ref> This crossover box was also where the two TBMs which bored the eastern end of the tunnel were launched from.

The first two TBMs, with a cutter-head diameter of {{convert|9.82|m|ft|abbr=on}}, were designed to bore through [[Lambeth Group]] rock, [[chalk]] and [[London Clay]], and assemble 4,220 tunnel segment rings each.<ref name=":1">{{Cite web |title=Northolt Tunnel |url=https://www.hs2.org.uk/building-hs2/tunnels/tunnel-drives/northolt-tunnel/ |access-date= |website=[[HS2]] |language=en-GB}}</ref><ref name=":3">{{Cite web |title=High Speed Two (HS2) |url=https://www.herrenknecht.com/en/references/referencesdetail/high-speed-2/ |access-date= |website=[[Herrenknecht]]}}</ref> They are 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;<ref>{{Cite web|last=Menteth|first=Thames|date=2021-12-20|title=HS2: First London TBMs arrive at site for 2022 launch|url=https://www.geplus.co.uk/news/hs2-first-london-tbms-arrive-at-site-for-2022-launch-20-12-2021/|access-date=2021-12-23|website=Ground Engineering (GE)|language=en}}</ref> with TBM ''Sushila'' launching on 6 October 2022 <ref>{{Cite web |last=Mansfield |first=Ian |date=6 October 2022 |title=HS2 starts digging 26 miles of tunnel under London |url=https://www.ianvisits.co.uk/articles/hs2-starts-digging-26-miles-of-tunnel-under-london-57926/ |access-date=6 October 2022 |website=IanVisits}}</ref> and Caroline following suit on 27 October.<ref>{{Cite web |date=27 October 2022 |title=HS2 construction in London gathers pace as second tunnelling machine sets off underneath the capital |url=https://mediacentre.hs2.org.uk/news/hs2-construction-in-london-gathers-pace-as-second-tunnelling-machine-sets-off-underneath-the-capital |access-date=13 November 2022 |website=High Speed 2}}</ref>

The second pair of TBMs named ''Emily'' (after midwife and former Ealing mayor Emily Sophie Taylor) and ''Anne'' (after education reformer and philanthropist [[Lady Byron|Lady Anne Byron]]), arrived at the Victoria Road Crossover Box for assembly and launch in late October 2023.<ref>{{Cite web |last=Mansfield |first=Ian |date=2023-10-24 |title=Emily and Anne get ready to dig HS2's Northolt Tunnel under west London |url=https://www.ianvisits.co.uk/articles/emily-and-anne-get-ready-to-dig-hs2s-northolt-tunnel-under-west-london-66781/ |access-date=2023-10-25 |website=ianVisits |language=en-GB}}</ref> These TBMs, with a cutter-head diameter of {{convert|9.08|m|ft|abbr=on}}, were designed to bore through [[London Clay]].<ref name=":1" /><ref name=":3" /> ''Emily'' launched on 25 February 2024,<ref>{{Cite web |title=HS2 launches third giant tunnelling machine under capital building the Northolt Tunnel |url=https://mediacentre.hs2.org.uk/news/hs2-launches-third-giant-tunnelling-machine-under-capital-building-the-northolt-tunnel |access-date=2024-02-25 |website=HS2 News and Information |language=english}}</ref> with Anne following suit in April.<ref>{{Cite magazine |last=Hakimian |first=Rob |date=2024-04-08 |title=HS2 {{!}} Fourth TBM launched to bore 13.5km Northolt tunnel under London |url=https://www.newcivilengineer.com/latest/hs2-fourth-tbm-launched-to-bore-13-5km-northolt-tunnel-under-london-08-04-2024/ |access-date=2024-04-08 |magazine=New Civil Engineer |language=en}}</ref>

The tunnels were all completed by 26 June 2025.<ref name=":4">{{cite web |date=27 June 2025 |title=Breakthrough! HS2 completes excavation of huge 8.4 mile tunnel under the capital |url=https://mediacentre.hs2.org.uk/news/breakthrough-hs2-completes-excavation-of-huge-8-4-mile-tunnel-under-the-capital |website=HS2}}</ref><ref>{{Cite magazine |last=Kanaris |first=Sotiris |date=2023-06-27 |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/ |access-date=2023-10-06 |magazine=New Civil Engineer |language=en}}</ref><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 news |date=2022-10-06 |title='HS2 drill switch-on is milestone for the ambitious project' |language=en-GB |work=BBC News |url=https://www.bbc.com/news/uk-england-london-63163398 |access-date=2022-10-06}}</ref><ref>{{Cite magazine |last=Hakimian |first=Rob |date=2023-10-24 |title=HS2 {{!}} TBMs lowered into Ealing shaft ahead of next London tunnelling launch |url=https://www.newcivilengineer.com/latest/hs2-tbms-lowered-into-ealing-shaft-ahead-of-next-london-tunnelling-launch-24-10-2023/ |access-date=2023-10-25 |magazine=New Civil Engineer |language=en}}</ref> The TBMs, each weighing approximately 2,050 tonnes, excavated 4.16 million tonnes of London Clay, advancing at a peak of {{convert|38|m|ft|abbr=on}} a day. The TBMs, excavating 4,160,000 tonnes of London Clay, installed 94,233 segments, weighing 7 tonnes each, to form 14,300 tunnel rings in total.<ref name=":4" /> Excavated material was removed using a conveyor system to HS2's London Logistics Hub at [[Willesden Logistics Hub|Willesden Euro Terminal]] for reuse in other areas.<ref>{{Cite web |date=2021-11-01 |title=HS2 inaugurates Willesden Euroterminal logistics hub |url=https://www.modernrailways.com/article/hs2-inaugurates-willesden-euroterminal-logistics-hub |access-date=2025-06-27 |website=Modern Railways |language=en}}</ref>

Construction of the 34 cross passages was completed by April 2026, with 11 constructed using a [[ground freezing]] technique due to water bearing soils paired with a high [[water table]] in the western section.<ref name=":5" />

== Progress == {| class="wikitable" |+ TBM progress |- ! Name ! Section ! Launch location ! Launch date ! Destination ! Arrival date ! Distance travelled |- | '''Sushila''' | rowspan=2 | West | rowspan=2 | West Ruislip Portal | {{Date | 6 Oct 2022}} | rowspan=4 | Green Park Way Ventilation Shaft, Greenford | {{Date | 19 December 2024}} | {{convert|8|km|mi|2|abbr=on}} |- | '''Caroline''' | {{Date | 27 Oct 2022}} | {{Date | 3 April 2025}} | {{convert|8|km|mi|2|abbr=on}} |- | '''Emily''' | rowspan=2 | East | rowspan=2 | Victoria Road | {{Date | 25 Feb 2024}} | 6 June 2025 | {{convert|5.5|km|mi|2|abbr=on}} |- | '''Anne''' | {{Date |9 Apr 2024}} |26 June 2025 | {{convert|5.5|km|mi|2|abbr=on}} |}

Source: HS2 Ltd.<ref name="progmap">{{cite web |title=HS2: In your area map |url=https://www.hs2.org.uk/in-your-area/in-your-area-map/#13/51.5327/-0.3112/filter=hs2-stations,hs2-network,tbm |access-date=10 May 2024 |publisher=HS2 Ltd}}</ref><ref>{{cite web | title=Significant progress in excavating 8.4-mile-long HS2 tunnel under London | url=https://www.railadvent.co.uk/2025/06/significant-progress-in-excavating-8-4-mile-long-hs2-tunnel-under-london.html }}</ref>

== References == {{reflist}}

[[Category:Railway tunnels in London]] [[Category:Tunnels on High Speed 2]]