# Tunnel

A tunnel is a more or less horizontal underground passage excavated beneath the surface or, in the case of cut-and-cover tunnels, built in an open trench and then backfilled. The process of making such a passage is called tunnelling; a passage built by trenching down from the surface, constructing the arch, and refilling looks identical when finished but is correctly termed a cut-and-cover tunnel.<sup>[1](https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica%2C_Ninth_Edition/Tunnelling)</sup><sup> • </sup><sup>[29](https://www.fhwa.dot.gov/bridge/inspection/tunnel/tomie/hif15005.pdf)</sup> Tunnels carry roads, railways, water, sewage, cables, and mines, and they range from hand-excavated ancient aqueducts to machine-bored tubes tens of kilometers long.<sup>[2](https://www.arup.com/en-us/services/tunnel-design-and-ventilation/)</sup>

| Key fact | Detail |
|---|---|
| Definition | A more or less horizontal underground passage; a cut-and-cover tunnel is constructed in an open trench and then covered with backfill<sup>[1](https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica%2C_Ninth_Edition/Tunnelling)</sup><sup> • </sup><sup>[29](https://www.fhwa.dot.gov/bridge/inspection/tunnel/tomie/hif15005.pdf)</sup> |
| Broad categories | Mining, public works (water, sewage, gas), and transportation; geologically, soft-ground, rock, and underwater tunnels<sup>[3](https://science.howstuffworks.com/engineering/structural/tunnel3.htm)</sup> |
| Oldest known deep tunnel | Tunnel of Eupalinos on Samos, completed 520 BC, 3,400 ft long, excavated from both ends without vertical shafts; the two headings met with an elevation difference of only 2 ft<sup>[4](https://www.asce.org/about-civil-engineering/history-and-heritage/historic-landmarks/tunnel-of-eupalinos)</sup> |
| First shield tunnel | Marc Brunel patented the tunnelling shield in 1818 and used it on the Thames Tunnel, the first under a river, opened in 1843<sup>[5](https://ice.org.uk/what-is-civil-engineering/infrastructure-projects/tunnel-boring-machines)</sup> |
| Longest undersea tunnel | The Channel Tunnel, opened May 1994: 50.45 km long, 37.9 km under the sea, on average 45 m below the sea bed, excavated by 11 tunnel boring machines<sup>[6](https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/the-channel-tunnel)</sup> |
| Design fires | Heat release rates adopted for road tunnel design in different countries vary from 20 to 300 MW<sup>[7](https://www.piarc.org/en/order-library/26484-en-Design%2520fire%2520characteristics%2520for%2520road%2520tunnels)</sup> |
| US inspection regime | The National Tunnel Inspection Standards were published on July 13, 2015 and became effective on August 12, 2015<sup>[8](https://www.fhwa.dot.gov/bridge/ntis/151027.pdf)</sup> |

## Terminology

The vocabulary of tunnelling distinguishes the passage from the way it is made. A small tunnel driven first and later enlarged is a "heading"; in mining, small tunnels are called "galleries", "driftways", and "adits". A vertical underground passage is a shaft: sinking it from the surface is "sinking", while working it upward from an existing heading or gallery is called a "rising".<sup>[1](https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica%2C_Ninth_Edition/Tunnelling)</sup> Shafts sunk at intervals and headings driven from their bottoms remain the standard way of opening multiple working faces on a long tunnel, a method the 19th-century Britannica already noted as analogous to the swallow-holes that rainwater excavates in limestone districts.<sup>[1](https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica%2C_Ninth_Edition/Tunnelling)</sup>

## Types and uses

Tunnels fall into three broad categories by purpose: mining, public works, and transportation. Mine tunnels give workers and equipment access to ore deposits and are built to lower standards of permanence than tunnels designed for continuous public use. Public works tunnels carry water, sewage, or gas over long distances, and the earliest tunnels served exactly this purpose, moving water to and sewage away from densely populated regions. Transportation tunnels carry roads and railways.<sup>[3](https://science.howstuffworks.com/engineering/structural/tunnel3.htm)</sup> Geologically, engineers distinguish soft-ground tunnels, rock tunnels, and underwater tunnels, because the ground type governs both the excavation method and the support needed.<sup>[9](https://pdhonline.com/courses/c973/c973content.pdf)</sup>

A further design choice is between a single large-diameter bore and multiple smaller bores. The idea is old: in 1919 [George Washington Goethals](https://www.edgepedia.org/george-washington-goethals) proposed a shield-driven vehicular tunnel of about 12.8 m diameter under the [Hudson River](https://www.edgepedia.org/hudson-river) with two traffic decks and dedicated space for ventilation, emergency egress, and utilities. The proposal was not pursued, and the twin-bore [Holland Tunnel](https://www.edgepedia.org/holland-tunnel) was completed instead in 1927.<sup>[10](https://about.ita-aites.org/publications/wg-publications/2611/guideline-single-vs-twin-bore-tunnels-volume-1)</sup>

## History

The [Tunnel of Eupalinos](https://www.edgepedia.org/tunnel-of-eupalinos) on the Greek island of Samos, built under the tyrant Polycrates to bring water from the spring at Agiades through the 900-ft Mount Castro, was excavated by hand from both ends simultaneously and described by [Herodotus](https://www.edgepedia.org/herodotus) around 440 BC. A 2-ft-wide aqueduct channel cut into the bedrock beside the level tunnel floor carried the water on a 0.36% slope, and the pipes delivered water to the city for some 1,200 years. The tunnel was declared a UNESCO World Cultural Heritage Site in 1992 and an International Historic Civil Engineering Landmark by ASCE in 2017.<sup>[4](https://www.asce.org/about-civil-engineering/history-and-heritage/historic-landmarks/tunnel-of-eupalinos)</sup>

**The shield era.** Attempts to tunnel under the Thames had failed since 1799, and engineers concluded the task was impossible until Marc Brunel, working with his son Isambard, patented in 1818 a device for "forming drifts and tunnels underground": an iron framework of cells, each the height of a man, in which miners dug at the face a few inches at a time before the shield was advanced. The [Thames Tunnel](https://www.edgepedia.org/thames-tunnel) opened in 1843 after 18 years of work and was called the 8th Wonder of the World; within three months a million people, half the population of London, had visited it.<sup>[5](https://ice.org.uk/what-is-civil-engineering/infrastructure-projects/tunnel-boring-machines)</sup> [James Henry Greathead](https://www.edgepedia.org/james-henry-greathead) and William Henry Barlow developed the first round shield in 1868, and the first mechanical tunnelling machine followed in 1876.<sup>[5](https://ice.org.uk/what-is-civil-engineering/infrastructure-projects/tunnel-boring-machines)</sup>

**Railways and the alpine crossings.** The first railway tunnel was the Terre-Noir tunnel on the Roanne-Andrezieux line in France, 1,477 m long. The second half of the 19th century brought the trans-alpine tunnels: Mount Cenis at 12.6 km, Saint Gotthard at 15.2 km, and Simplon at 19.7 km, built with the modest tools of the time and regarded as a decisive leap in tunnelling engineering.<sup>[11](https://tunnel.ita-aites.org/en/cases-histories/history/xix-century)</sup> In Britain, the Tidenham Tunnel on the Wye Valley Railway (1874 to 1876) showed the period's method and economics: Ingersoll rock drills powered by compressed air raised the advance to six feet per day, allowing the roughly 1 km heading to be completed in just under 20 months for a line cost of a little over £318,000.<sup>[12](https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/tidenham-denhill-tunnel)</sup>

**Machine boring.** Tunnel boring machines had been envisioned for over a century without success, partly because early machines tried to grind rock rather than excavate it. The introduction of the disk cutter in the late 1960s changed this: the cutter causes rock to fail in shear, forming chips measured in tens of cubic inches, and much of the credit belongs to The Robbins Co. Advance rates rose from feet per day to tens or even hundreds of feet per day.<sup>[13](https://www.fhwa.dot.gov/bridge/Tunnel/pubs/nhi09010/tunnel_manual.pdf)</sup> Modern TBMs remain large cylinders with rotating cutterheads; on London's Crossrail project, eight 1,000-tonne machines, each 150 m long, dug 42 km of tunnel, and one TBM advanced 72 m in a single day.<sup>[5](https://ice.org.uk/what-is-civil-engineering/infrastructure-projects/tunnel-boring-machines)</sup>

## Construction methods

The principal methods for constructing a tunnel are well established in road tunnel practice:<sup>[14](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)</sup>

- **Cut and cover.** An open trench is excavated, the tunnel is built at design elevation, and the trench is backfilled with compacted material. Variations use piles and lagging, tieback anchors, or slurry walls.<sup>[14](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)</sup> HS2's Wendover "green tunnel" in [Buckinghamshire](https://www.edgepedia.org/buckinghamshire) is a current example: a 1.4 km cut-and-cover structure assembled from 2,740 precast concrete segments in an "M" shape, then buried under excavated earth to blend the railway into the countryside and cut noise for nearby residents.<sup>[15](https://mediacentre.hs2.org.uk/news/progress-for-hs2s-wendover-green-tunnel)</sup>
- **Shield-driven excavation.** A shield is pushed into soft ground ahead; material inside the shield is removed and a lining is built before the shield advances further.<sup>[14](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)</sup>
- **Bored (TBM) excavation.** A mechanical TBM excavates the full tunnel cross section at once with cutting tools suited to soft ground or rock, supporting the adjacent soil until temporary and permanent linings are installed.<sup>[14](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)</sup> HS2's Northolt Tunnel, a twin bore 8.4 miles (13.5 km) long between Old Oak Common and West Ruislip, was completed by this method, with emergency and maintenance walkways still being fitted afterward.<sup>[16](https://www.hs2.org.uk/work-items/update-on-northolt-tunnel-construction-in-hillingdon-and-ealing/)</sup>
- **Drill and blast.** In rock where a TBM is not used, the rock is drilled, blasted, and removed by conventional techniques.<sup>[14](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)</sup>
- **Immersed tube and jacked box** methods round out the standard catalog of road tunnel construction techniques.<sup>[17](https://rosap.ntl.bts.gov/view/dot/50019)</sup>

For small-diameter sewers and utilities, microtunneling and pipe jacking install the product pipe behind a TBM or microtunnel boring machine, with the pipe serving as both the construction lining and the permanent conduit, generally without man entry.<sup>[18](https://www.fhwa.dot.gov/utilities/utilitycuts/manappc.cfm)</sup>

## Groundwater and settlement

Water is one of the defining difficulties of tunnelling. Excessive water ingress causes groundwater drawdown, and drawdown causes surface settlement that can damage structures above the alignment. In one documented case of conventional tunnelling, measured settlements showed a considerably larger affected zone than in cases without drawdown, and about 65% of the total drawdown had occurred before the tunnel heading even arrived at the monitoring section, leading the authors to conclude that pre-grouting is of paramount importance in highly permeable ground.<sup>[19](https://www.sciencedirect.com/science/article/abs/pii/S0886779812000211)</sup> The scale of the risk can be large: during construction of the 15 km Uma Oya headrace tunnel in Sri Lanka with a double-shield TBM, unexpected groundwater inflows exceeding 290 L/s above background triggered aquifer drawdown, surface settlements up to 40 cm, and the drying of more than 340 domestic wells across a 15 km area.<sup>[20](https://artikeldatenbank.ernst-und-sohn.de/artikel/geot-2025-6-impact-of-water-ingress-and-tunnelling-operations-53037)</sup> Design responses include undrained (sealed) tunnel structures and pre-excavation permeation grouting, as used on the Waterview Connection in Auckland, where twin 14.5 m diameter TBM tunnels linked by 16 mined cross passages were designed as undrained structures to prevent drawdown beneath residential land.<sup>[21](https://www.nzgs.org/libraries/nzgs20_mccarrison/)</sup>

## Operation: ventilation, fire, and inspection

Road tunnel ventilation serves three purposes: diluting pollutants inside the tunnel, protecting air quality outside the portals, and controlling smoke in a fire.<sup>[22](https://tunnelsmanual.piarc.org/en/strategy-and-general-design-ventilation-concepts/ventilation-principles)</sup> The main system types are longitudinal ventilation, in which jet fans push air along the tunnel axis, and semi-transverse or fully transverse ventilation, which distributes fresh air from a separate duct and can extract smoke through an exhaust duct; transverse systems cost more to build but can create a smoke-free space near the road surface and are mainly applied in bidirectional tunnels or tunnels with congestion risk.<sup>[23](https://tunnelsmanual.piarc.org/en/strategy-and-general-design-ventilation-concepts/design-and-dimensioning)</sup> Because vehicle pollutant emissions have fallen continuously, smoke control in case of fire has increasingly determined ventilation equipment.<sup>[24](https://www.piarc.org/en/order-library/3854-en-Fire%20and%20Smoke%20Control%20in%20Road%20Tunnels.htm)</sup> The design fire, expressed as a heat release rate in MW as a function of time, sizes the ventilation and other equipment; adopted values vary from 20 to 300 MW depending on the traffic allowed, and no single universal design fire can be specified.<sup>[7](https://www.piarc.org/en/order-library/26484-en-Design%2520fire%2520characteristics%2520for%2520road%2520tunnels)</sup> Lessons from the Mont-Blanc, Tauern, and Saint-Gothard tunnel fires shaped modern safety concepts for fire resistance, emergency exits, escape routes, and automatic fire detection and suppression.<sup>[25](https://www.piarc.org/en/order-library/5425-en-Systems%20and%20equipment%20for%20fire%20and%20smoke%20control%20in%20road%20tunnels)</sup>

In the United States, tunnels are now managed as inspected assets. The National Tunnel Inspection Standards, published in July 2015, apply to structures defined as highway tunnels on all public roads, and are supported by the Tunnel Operations, Maintenance, Inspection, and [Evaluation](https://www.edgepedia.org/evaluation) (TOMIE) Manual and the Specifications for the National Tunnel Inventory; the FHWA notes that tunnels not adequately maintained usually require more costly and extensive repairs.<sup>[8](https://www.fhwa.dot.gov/bridge/ntis/151027.pdf)</sup><sup> • </sup><sup>[26](https://rosap.ntl.bts.gov/view/dot/42887)</sup>

## Costs and trade-offs

Tunnels carry higher capital costs and additional maintenance compared with bridges, viaducts, and depressed roadways, which is why significantly fewer road tunnels have been built in the United States than other highway structures. Cost depends on the effort and risk of the work: subsurface conditions raise cost and uncertainty, ground improvement can mitigate subsurface risks but is itself expensive, and highly urbanized settings add cost through congested space, proximity of buildings that could be damaged, and dense utilities.<sup>[27](https://www.fhwa.dot.gov/bridge/tunnel/)</sup> Against this, tunnels keep the land above the corridor available for development or recreation, and replacing a viaduct with a tunnel or capping a depressed roadway can create linear parks, raise nearby property values, and remove noise and emissions from the surface.<sup>[27](https://www.fhwa.dot.gov/bridge/tunnel/)</sup> The Channel Tunnel illustrates the transport payoff: before it opened in May 1994, London to Paris took around six or seven hours by rail and ferry; trains now do the journey in 2.5 hours.<sup>[6](https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/the-channel-tunnel)</sup> Urban tunnelling can also carry non-engineering costs, as Thessaloniki's metro showed when tunnels had to be bored deeper than planned, adding cost and delay, to preserve more than 300,000 archaeological finds now displayed in its stations.<sup>[28](https://apnews.com/article/7d3354fc6c08824825cd666fa708acb9)</sup>

## References

1. [Tunnelling, Encyclopædia Britannica Ninth Edition, Wikisource](https://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica%2C_Ninth_Edition/Tunnelling)
2. [Tunnel design, Arup](https://www.arup.com/en-us/services/tunnel-design-and-ventilation/)
3. [Types of Tunnels, HowStuffWorks](https://science.howstuffworks.com/engineering/structural/tunnel3.htm)
4. [Tunnel of Eupalinos, ASCE Historic Civil Engineering Landmarks](https://www.asce.org/about-civil-engineering/history-and-heritage/historic-landmarks/tunnel-of-eupalinos)
5. [Tunnel Boring Machines, Institution of Civil Engineers](https://ice.org.uk/what-is-civil-engineering/infrastructure-projects/tunnel-boring-machines)
6. [How Was The Channel Tunnel Built?, Institution of Civil Engineers](https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/the-channel-tunnel)
7. [Design fire characteristics for road tunnels, PIARC Report 2017R01EN](https://www.piarc.org/en/order-library/26484-en-Design%2520fire%2520characteristics%2520for%2520road%2520tunnels)
8. [Guidance on Structures Subject to the National Tunnel Inspection Standards, FHWA](https://www.fhwa.dot.gov/bridge/ntis/151027.pdf)
9. [Tunnel Basics, PDH Online](https://pdhonline.com/courses/c973/c973content.pdf)
10. [Guideline: Single vs. Twin-Bore Tunnels, Volume 1, ITA-AITES](https://about.ita-aites.org/publications/wg-publications/2611/guideline-single-vs-twin-bore-tunnels-volume-1)
11. [XIX Century, About Tunnelling, ITA-AITES](https://tunnel.ita-aites.org/en/cases-histories/history/xix-century)
12. [Tidenham (Denhill) Tunnel, Institution of Civil Engineers](https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/tidenham-denhill-tunnel)
13. [Technical Manual for Design and Construction of Road Tunnels, FHWA-NHI-09-010](https://www.fhwa.dot.gov/bridge/Tunnel/pubs/nhi09010/tunnel_manual.pdf)
14. [Highway and Rail Transit Tunnel Inspection Manual, FHWA](https://www.fhwa.dot.gov/bridge/tunnel/inspectman.pdf)
15. [Progress for HS2's Wendover green tunnel, HS2](https://mediacentre.hs2.org.uk/news/progress-for-hs2s-wendover-green-tunnel)
16. [Update on works inside Northolt Tunnel, HS2](https://www.hs2.org.uk/work-items/update-on-northolt-tunnel-construction-in-hillingdon-and-ealing/)
17. [Technical Manual for Design and Construction of Road Tunnels, Civil Elements, FHWA-NHI-10-034](https://rosap.ntl.bts.gov/view/dot/50019)
18. [Appendix C. Sample Trenchless Technology Specifications, FHWA](https://www.fhwa.dot.gov/utilities/utilitycuts/manappc.cfm)
19. [Tunnelling-induced ground settlements in a groundwater drawdown environment, Tunnelling and Underground Space Technology](https://www.sciencedirect.com/science/article/abs/pii/S0886779812000211)
20. [Impact of water ingress and tunnelling operations, Uma Oya case study, Geotechnik](https://artikeldatenbank.ernst-und-sohn.de/artikel/geot-2025-6-impact-of-water-ingress-and-tunnelling-operations-53037)
21. [Pre-excavation grouting for groundwater ingress control in tunnelling, Auckland, New Zealand Geotechnical Society](https://www.nzgs.org/libraries/nzgs20_mccarrison/)
22. [Ventilation principles, PIARC Road Tunnels Manual](https://tunnelsmanual.piarc.org/en/strategy-and-general-design-ventilation-concepts/ventilation-principles)
23. [Design and dimensioning, PIARC Road Tunnels Manual](https://tunnelsmanual.piarc.org/en/strategy-and-general-design-ventilation-concepts/design-and-dimensioning)
24. [Fire and Smoke Control in Road Tunnels, PIARC Report 05.05.B](https://www.piarc.org/en/order-library/3854-en-Fire%20and%20Smoke%20Control%20in%20Road%20Tunnels.htm)
25. [Systems and equipment for fire and smoke control in road tunnels, PIARC Report 05.16.B](https://www.piarc.org/en/order-library/5425-en-Systems%20and%20equipment%20for%20fire%20and%20smoke%20control%20in%20road%20tunnels)
26. [Tunnel Operations, Maintenance, Inspection, and Evaluation (TOMIE) Manual, FHWA-HIF-15-005](https://rosap.ntl.bts.gov/view/dot/42887)
27. [Tunnels, FHWA Bridges & Structures](https://www.fhwa.dot.gov/bridge/tunnel/)
28. [Ancient meets modern as a new subway in Greece showcases archaeological treasures, AP News](https://apnews.com/article/7d3354fc6c08824825cd666fa708acb9)
29. [fhwa.dot.gov](https://www.fhwa.dot.gov/bridge/inspection/tunnel/tomie/hif15005.pdf)

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*Topic: Encyclopedia › Technology and the built world › Architecture, buildings, and civil works › Civil and water works › Tunnels › Tunnels*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license