Tunnel boring machine
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A tunnel boring machine (TBM) also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 metres to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.
Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional "hand mining" in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less. This is because tunneling with TBMs is much more efficient and results in shortened completion times.
Herrenknecht AG built the world's largest diameter hard rock TBM: "Martina" (excavation diameter of 15.62 m, total length 130 m; excavation area of 192 square m, thrust value 39,485 t, total weight 4,500 tons, total installed capacity 18 MW; yearly energy consumption about 62,000,000 kWh) is owned and operated by the Italian construction company Toto S.p.A. Costruzioni Generali (Toto Group) for the Sparvo gallery of the Italian Motorway Pass A1 ("Variante di Valico A1"), near Florence.
The former largest diameter hard rock TBM, at 14.4 m, was manufactured by The Robbins Company for Canada's Niagara Tunnel Project. The machine was used to bore a hydroelectric tunnel beneath Niagara Falls. The machine was named "Big Becky" in reference to the Sir Adam Beck hydroelectric dams to which it is tunnelling to provide an additional hydroelectric tunnel.
In 2013, Hitachi Zosen Corporation delivered an earth pressure balance TBM to Seattle, United States for the Highway 99 tunnel project. The bore is 17.45 metres (57 ft 3 in). As of 2015 the tunneling machine is stalled after hitting an unknown object and overheating. 
The first successful tunnelling shield was developed by Sir Marc Isambard Brunel to excavate the Thames Tunnel in 1825. However, this was only the invention of the shield concept and did not involve the construction of a complete tunnel boring machine, the digging still having to be accomplished by the then standard excavation methods.
The first boring machine reported to have been built was Henri-Joseph Maus' Mountain Slicer. Commissioned by the King of Sardinia in 1845 to dig the Fréjus Rail Tunnel between France and Italy through the Alps, Maus had it built in 1846 in an arms factory near Turin. It consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel. The Revolutions of 1848 affected the funding, and the tunnel was not completed until 10 years later, by using less innovative and less expensive methods such as pneumatic drills.
In the United States, the first boring machine to have been built was used in 1853 during the construction of the Hoosac Tunnel. Made of cast iron, it was known as Wilson's Patented Stone-Cutting Machine, after inventor Charles Wilson. It drilled 10 feet into the rock before breaking down. The tunnel was eventually completed more than 20 years later, and as with the Fréjus Rail Tunnel, by using less ambitious methods. The first successful use was on the Oahe Dam in 1952 by James S Robbins.
Modern TBMs typically consist of the rotating cutting wheel, called a cutter head, followed by a main bearing, a thrust system and trailing support mechanisms. The type of machine used depends on the particular geology of the project, the amount of ground water present and other factors.
Hard rock TBMs
In hard rock, either shielded or open-type TBMs can be used. All types of hard rock TBMs excavate rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the rock in front of the machine, called the tunnel face. The excavated rock, known as muck, is transferred through openings in the cutter head to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.
Open-type TBMs have no shield, leaving the area behind the cutter head open for rock support. To advance, the machine uses a gripper system that pushes against the side walls of the tunnel. Not all machines can be continuously steered while gripper shoes push on the side-walls, as in the case of a Wirth machine which will only steer while ungripped. The machine will then push forward off the grippers gaining thrust. At the end of a stroke, the rear legs of the machine are lowered, the grippers and propel cylinders are retracted. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring begins again. The open-type, or Main Beam, TBM does not install concrete segments behind it as other machines do. Instead, the rock is held up using ground support methods such as ring beams, rock bolts, shotcrete, steel straps, Ring steel and wire mesh.
In fractured rock, shielded hard rock TBMs can be used, which erect concrete segments to support unstable tunnel walls behind the machine. Double Shield TBMs have two modes; in stable ground they can grip against the tunnel walls to advance. In unstable, fractured ground, the thrust is shifted to thrust cylinders that push off against the tunnel segments behind the machine. This keeps the significant thrust forces from impacting fragile tunnel walls. Single Shield TBMs operate in the same way, but are used only in fractured ground, as they can only push off against the concrete segments.
Soft ground TBMs
In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines (EPB), Slurry Shield (SS) and open-face type. Both types of closed machines operate like Single Shield TBMs, using thrust cylinders to advance forward by pushing off against concrete segments. Earth Pressure Balance Machines are used in soft ground with less than 7 bar of pressure. The cutter head does not use disc cutters only, but instead a combination of tungsten carbide cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters. The EPB gets its name because it is uses the excavated material to balance the pressure at the tunnel face. Pressure is maintained in the cutterhead by controlling the rate of extraction of spoil through the Archimedes screw and the advance rate. Additives such as bentonite, polymers and foam can be injected ahead of the face to increase the stability of the ground. Additives can also be injected in the cutterhead/extraction screw to ensure that the spoil remains sufficiently cohesive to form a plug in the Archimedes screw to maintain pressure in the cutterhead and restrict water flowing through.
In soft ground with very high water pressure or where ground conditions are granular (sands and gravels) so much so that a plug could not be formed in the Archimedes screw, Slurry Shield TBMs are needed. The cutterhead is filled with pressurised slurry which applies hydrostatic pressure to the excavation face. The slurry also acts as a transport medium by mixing with the excavated material before being pumped out of the cutterhead back to a slurry separation plant, usually outside of the tunnel. Slurry separation plants are a multi-stage filtration systems, which remove particles of spoil from the slurry so that it may be reused in the construction process. The limit to which slurry can be 'cleaned' depends on the particle size of the excavated material. For this reason, slurry TBMs are not suitable for silts and clays as the particle sizes of the spoil are less than that of the bentonite clay from which the slurry is made. In this case, the slurry is separated into water, which can be recycled and a clay cake, which is pressed from the water.
Open face TBMs in soft ground rely on the fact that the face of the ground being excavated will stand up with no support for a short period of time - this makes them suitable for use in rock types with a strength of up to 10MPa or so, and with low water inflows. Face sizes in excess of 10 metres can be excavated in this manner. The face is excavated using a backactor arm or cutter head to within 150mm of the edge of the shield. The shield is jacked forwards and cutters on the front of the shield cut the remaining ground to the same circular shape. Ground support is provided by use of precast concrete, or occasionally SGI (Spheroidal Graphite Iron), segments that are bolted or supported until a full ring of support has been erected. A final segment, called the key, is wedge-shaped, and expands the ring until it is tight against the circular cut of the ground left behind by cutters on the TBM shield. Many variations of this type of TBM exist.
While the use of TBMs relieves the need for large numbers of workers at high pressures, a caisson system is sometimes formed at the cutting head for slurry shield TBMs. Workers entering this space for inspection, maintenance and repair need to be medically cleared as "fit to dive" and trained in the operation of the locks.
Behind all types of tunnel boring machines, inside the finished part of the tunnel, are trailing support decks known as the back-up system. Support mechanisms located on the back-up can include: conveyors or other systems for muck removal, slurry pipelines if applicable, control rooms, electrical systems, dust removal, ventilation and mechanisms for transport of pre-cast segments.
Urban tunnelling and near surface tunnelling
Urban tunnelling has the special requirement that the ground surface be undisturbed. This means that ground subsidence must be avoided. The normal method of doing this in soft ground is to maintain the soil pressures during and after the tunnel construction. There is some difficulty in doing this, particularly in varied strata (e.g., boring through a region where the upper portion of the tunnel face is wet sand and the lower portion is hard rock).
TBMs with positive face control, such as EPB and SS, are used in such situations. Both types (EPB and SS) are capable of reducing the risk of surface subsidence and voids if operated properly and if the ground conditions are well documented.
When tunnelling in urban environments, other tunnels, existing utility lines and deep foundations need to be addressed in the early planning stages. The project must accommodate measures to mitigate any detrimental effects to other infrastructure.
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Notes and references
- Alaskan Way Viaduct - Home
- Shield Tunneling Machines
- Bertha's Stall has politicians stuck for answers.
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- Hapgood, Fred, "The Underground Cutting Edge: The innovators who made digging tunnels high-tech",Invention & Technology Vol.20, #2, Fall 2004
- Bernhard Maidl, Leonhard Schmid, Willy Ritz, Martin Herrenknecht (2008). Hardrock Tunnel Boring Machines. Ernst & Sohn. p. 1. ISBN 978-3-433-01676-3.
- Smith, Gary. "FINDING AID FOR THE HOOSAC TUNNEL COLLECTION at the NORTH ADAMS PUBLIC LIBRARY". Hooac Tunnel Historical Notes. North Adams Public Library. Retrieved 14 July 2011.
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- Green, Amanda. "Just Keep Digging: A Brief History of Tunnels". Popular Mechanics. Retrieved 21 January 2014.
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- Walters, D. "Sydney Airport Link Rail Tunnel Project, Des Walters: Under Pressure Underground". Descend Underwater Training Centre. Retrieved 2008-10-08.
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- Bilger, Burkhard, "The Long Dig: Getting through the Swiss Alps the hard way", The New Yorker, September 15, 2008
- Foley, Amanda, "Life on the Cutting Edge: Dick Robbins", "Tunnels & Tunnelling International", May 2009, www.tunnelsonline.info
- Stack, Barbara, "Encyclopaedia of Tunnelling, Mining, and Drilling Equipment", Copyright 1995 by Barbara Stack
- Barton, Nick. 'TBM tunnelling in jointed and faulted rock'. Balkema,Rotterdam, 173p., 2000.
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