4.2Tunnel Engineering

Geotechnical investigation

A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.

Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.

Other key geotechnical factors

Stand-up time is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time.
Groundwater control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out. A very effective but expensive technology is ground freezing, using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built.
Tunnel cross-sectional shape is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of stress at the corners.

Choice of tunnels versus bridges

For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel.

Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in favor of a tunnel. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.

The 1934 Queensway Tunnel under the River Mersey at Liverpool was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 Kingsway Tunnel under the Mersey. In Hampton Roads, Virginia, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent US Navy vessels from leaving Naval Station Norfolk.

Water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City; the Queens-Midtown Tunnel between Manhattan and the borough of Queens on Long Island; the Detroit-Windsor Tunnel between Michigan and Ontario; and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia; the 1934 River Mersey road Queensway Tunnel; the Western Scheldt Tunnel, Zeeland, Netherlands; and the North Shore Connector tunnel in Pittsburgh, Pennsylvania.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the 51.5-kilometre or 32.0-mile Channel Tunnel), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in Virginia.

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in Italy in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.

Project planning and cost estimates

Government funds are often required for the creation of tunnels. When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds.

The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Increased taxes to finance a large project may cause opposition.

Construction

Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the groundwater conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and the shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally, a tube can be sunk into a body of water, which is called an immersed tunnel.

Cut-and-cover

Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Two basic forms of cut-and-cover tunneling are available:

Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.

Top-down method: Side support walls and capping beams are constructed from ground level by such methods as slurry walling or contiguous bored piling. Only a shallow excavation is needed to construct the tunnel roof using precast beams or in situ concrete sitting on the walls. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services, and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.

Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.

Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation. This contrasts with many traditional stations on London Underground, where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.

A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century.

Boring machines

Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as a quick and cost-effective alternative to laying surface rails and roads. Expensive compulsory purchase of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size – the difficulty of transporting the large TBM to the site of tunnel construction, or (alternatively) the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed.

There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the bentonite slurry, and earth-pressure balance machines have pressurized compartments at the front end, allowing them to be used in difficult conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurized compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred over the older method of tunnelling in compressed air, with an airlock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like deep-sea divers.

Sprayed concrete techniques

The New Austrian Tunnelling method (NATM)—also referred to as the Sequential Excavation Method (SEM)—was developed in the 1960s. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on geotechnical measurements, an optimal cross section is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as shotcrete. Other support measures can include steel arches, rock bolts, and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibers being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation.

Safety and security

Owing to the enclosed space of a tunnel, fires can have very serious effects on users. The main dangers are gas and smoke production, with even low concentrations of carbon monoxide being highly toxic. Fires killed 11 people in the Gotthard tunnel fire of 2001 for example, all of the victims succumbing to smoke and gas inhalation. Over 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning was the main cause of death. In the Caldecott Tunnel fire of 1982, the majority of fatalities were caused by toxic smoke, rather than by the initial crash.

Motor vehicle tunnels usually require ventilation shafts and powered fans to remove toxic exhaust gases during routine operation.

Rail tunnels usually require fewer air changes per hour, but still may require forced-air ventilation. Both types of tunnels often have provisions to increase ventilation under emergency conditions, such as a fire. Although there is a risk of increasing the rate of combustion through increased airflow, the primary focus is on providing breathable air to persons trapped in the tunnel, as well as firefighters.

Aerodynamic pressure wave produced by high speed trains entering a tunnel reflects at its open ends and changes sign (compression wave-front changes to rarefaction wave-front and vice versa); When two wave-front of the same sign meets the train, significant and rapid air pressure may cause aural discomfort to passengers and crew. When high-speed trains exit tunnels, a loud "Tunnel boom" may occur, which can disturb residents near the mouth of the tunnel, and it is exacerbated in mountain valleys where the sound can echo.

When there is a parallel, separate tunnel available, airtight but unlocked emergency doors are usually provided which allow trapped personnel to escape from a smoke-filled tunnel to the parallel tube.

Larger, heavily used tunnels, such as the Big Dig tunnel in Boston, Massachusetts, may have a dedicated 24-hour manned operations center which monitors and reports on traffic conditions, and responds to emergencies. Video surveillance equipment is often used, and real-time pictures of traffic conditions for some highways may be viewable by the general public via the Internet.

Database of seismic damage to underground structures using 217 case histories shows the following general observations can be made regarding the seismic performance of underground structures:

①Underground structures suffer appreciably less damage than surface structures.
②Reported damage decreases with increasing over burden depth. Deep tunnels seem to be safer and less vulnerable to earthquake shaking than are shallow tunnels.
③Underground facilities constructed in soils can be expected to suffer more damage compared to openings constructed in competent rock.
④Lined and grouted tunnels are safer than unlined tunnels in rock. Shaking damage can be reduced by stabilizing the ground around the tunnel and by improving the contact between the lining and the surrounding ground through grouting.
⑤Tunnels are more stable under a symmetric load, which improves ground-lining interaction. Improving the tunnel lining by placing thicker and stiffer sections without stabilizing surrounding poor ground may result in excess seismic forces in the lining. Backfilling with non-cyclically mobile material and rock-stabilizing measures may improve the safety and stability of shallow tunnels.
⑥Damage may be related to peak ground acceleration and velocity based on the magnitude and epicentral distance of the affected earthquake.
⑦Duration of strong-motion shaking during earthquakes is of utmost importance because it may cause fatigue failure and therefore, large deformations.
⑧High frequency motions may explain the local spalling of rock or concrete along planes of weakness. These frequencies, which rapidly attenuate with distance, may be expected mainly at small distances from the causative fault.
⑨Ground motion may be amplified upon incidence with a tunnel if wavelengths are between one and four times the tunnel diameter.
⑩Damage at and near tunnel portals may be significant due to slope instability.

Earthquakes are one of nature's most formidable threats. A magnitude 6.7 earthquake shook the San Fernando valley in Los Angeles in 1994. The earthquake caused extensive damage to various structures including buildings, freeway overpasses and road systems throughout the area. The National Center for Environmental Information estimates total damages to be 40 billion dollars. According to an article issued by Steve Hymon of TheSource – Transportation News and Views, there was no serious damage sustained by the LA subway system. Metro, the owner of the LA subway system, issued a statement through their engineering staff about the design and consideration that goes into a tunnel system. Engineers and architects perform extensive analysis as to how hard they expect earthquakes to hit that area. All of this goes into the overall design and flexibility of the tunnel.

Tunnel Construction Explained

video from The B1M

https://www.youtube.com/watch?v=qvkytMLBKFc

Marvels of engineering: 

railway tunnels

video from Interesting Engineering

https://www.youtube.com/watch?v=Je-4h3V5PMg&list=PLew_Btt-OXp6pd_hnqxT4X_z_k_OwSXg4&index=2

Quiz

Topic Discussion

 

References

Sutcliffe, Harry (2004). "Tunnel Boring Machines". In Bickel, John O.; Kuesel, Thomas R.; King, Elwyn H. (eds.). Tunnel Engineering Handbook (2nd ed.). Kluwer Academic Publishers. p. 210. ISBN 978-1-4613-8053-5.

 Powers, P.J. (2007). Construction dewatering and groundwater control. Hoboken, NJ: John Wiley & Sons Inc.

 United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.

 "Capital Projects Funds". Cord.edu. Archived from the original on 17 December 2011. Retrieved 19 April 2013.

 Chan, Sewell (3 August 2005). "$100 Million for a Tunnel. What Tunnel?". The New York Times.

"Encouraging U.S. Infrastructure Investment – Council on Foreign Relations". Cfr.org. Retrieved 19 April 2013.

Ellis 2015, p. 118.

 "Tunnels & Tunnelling International". Tunnelsonline.info. Archived from the original on 16 March 2012. Retrieved 19 April 2013.

 "The Groene Hart Tunnel". Hslzuid.nl. Archived from the original on 25 September 2009. Retrieved 19 April 2013.

Johnson, Kirk (4 December 2012). "Engineering Projects Will Transform Seattle, All Along the Waterfront". The New York Times.

"Tunnelling". tunnellersmemorial.com. Archived from the original on 23 August 2010. Retrieved 20 June 2010.

"Understanding the New Austrian Tunnel Method (NATM)". Tunnel Business Magazine. Benjamin Media. 5 December 2018. Retrieved 27 December 2018.

"Mushrooms and Tours | Li-Sun Exotic Mushrooms". www.li-sunexoticmushrooms.com.au.

BISCOE, EMMA (15 May 2014). "Mittagong mushroom business will close down". Illawarra Mercury.

 "Archived copy". Archived from the original on 4 March 2016. Retrieved 2 April 2009. 

The Railway Magazine August 2015, p12.

"Report on Redeveloping Railway Tunnels". www.tunnel-online.info.

"San Francisco-Oakland Bay Bridge". Bay Area Toll Authority. 4 November 2015.

"Eurasia Tunnel Project" (PDF). Unicredit – Yapı Merkezi, SK EC Joint Venture. Archived from the original (PDF) on 20 January 2016. Retrieved 13 April 2014.

"Istanbul's mega-project: World's first three-level tunnel to be built under the Bosporus – Daily Sabah". 19 February 2017. Archived from the original on 19 February 2017.

"Shanghai to Dig Double-deck Tunnel". 15 May 2006. Archived from the original on 15 May 2006.

 "Maastricht's new highway tunnel: a role model for Europe". DW.COM.

Unique stacked tunnel under Maastricht (archived) – Imtech Traffic & Infra

"To Break Ground For 63rd St., East River Tunnel". New York Leader-Observer. 20 November 1969. p. 8. Retrieved 29 July 2016 – via Fultonhistory.com.

"Governor Rockefeller and Mayor Lindsay Attend 'Holing Through' of 63d St. Tunnel". The New York Times. 11 October 1972. ISSN 0362-4331. Retrieved 3 February 2018.

Lorch, Donatella (29 October 1989). "The 'Subway to Nowhere' Now Goes Somewhere". The New York Times. Retrieved 20 October 2011.

MTA East Side Access program

https://en.wikipedia.org/wiki/Tunnel