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Atunnel boring machine (TBM), also known as a "mole" or a "worm",[not verified in body] is a machine used to excavatetunnels. TBMs are an alternative todrilling and blasting methods and "hand mining", allowing more rapid excavation through hard rock, wet or dry soil, orsand (although each requires specialized TBM technologies).[not verified in body] TBM-bored tunnel cross-sections extend up to 17.6 meters (58 ft) (through June 2023).[1] TBM tunnels are typically circular in cross-section, but may also be square or rectangular or U- or horseshoe-shaped.[2][3][full citation needed][4][better source needed] Much narrower tunnels are typically bored usingtrenchless construction methods orhorizontal directional drilling rather than by TBMs.[not verified in body]
TBMs limit disturbance to the surrounding ground and produce a smooth tunnel wall, which reduces the cost of lining the tunnel and allows for tunneling in urban areas.[not verified in body] Large TBMs are expensive and challenging to construct and transport, fixed costs which become less significant for longer tunnels.[not verified in body] Tunneling speeds generally decline as tunnel size increases,[need quotation to verify] but tunneling speeds using TBMs have nevertheless increased over time.[citation needed] TBM speeds excavating through rock can, in the 21st century, reach over 700 meters per week, while soil tunneling machines can exceed 200 meters per week.[5]
Thefirst successful tunnelling shield was developed by SirMarc Isambard Brunel to excavate theThames 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.[6]
The first boring machine reported to have been built wasHenri Maus'Mountain Slicer.[7][8][page needed][9][10] Commissioned by theKing of Sardinia in 1845 to dig theFréjus Rail Tunnel between France and Italy through theAlps, Maus had it built in 1846 in an arms factory nearTurin. 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. TheRevolutions of 1848 affected the funding, and the tunnel was not completed until 10 years later, by using less innovative and less expensive methods such aspneumatic drills.[11]
In the United States, the first boring machine to have been built was used in 1853 during the construction of theHoosac Tunnel in northwest Massachusetts.[12] Made of cast iron, it was known asWilson's Patented Stone-Cutting Machine, after inventor Charles Wilson.[13] It drilled 3 meters (10 ft) 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).[14] Wilson's machine anticipated modern TBMs in the sense that it employed cutting discs, like those of adisc harrow, which were attached to the rotating head of the machine.[15][16][17] In contrast to traditional chiseling or drilling and blasting, this innovative method of removing rock relied on simple metal wheels to apply a transient high pressure that fractured the rock.[citation needed]
In 1853, the American Ebenezer Talbot also patented a TBM that employed Wilson's cutting discs, although they were mounted on rotating arms, which in turn were mounted on a rotating plate.[18] In the 1870s, John D. Brunton of England built a machine employing cutting discs that were mounted eccentrically on rotating plates, which in turn were mounted eccentrically on a rotating plate, so that the cutting discs would travel over almost all of the rock face that was to be removed.[19][20]
The first TBM that tunneled a substantial distance was invented in 1863 and improved in 1875 by British Army officer MajorFrederick Edward Blackett Beaumont (1833–1895); Beaumont's machine was further improved in 1880 by British Army officer Major Thomas English (1843–1935).[21][22][23][24][25] In 1875, the French National Assembly approved the construction of a tunnel under theEnglish Channel and theBritish Parliament supported a trial run using English's TBM. Its cutting head consisted of a conical drill bit behind which were a pair of opposing arms on which were mounted cutting discs. From June 1882 to March 1883, the machine tunneled, through chalk, a total of 1,840 m (6,036 ft).[10] A French engineer,Alexandre Lavalley, who was also aSuez Canal contractor, used a similar machine to drill 1,669 m (5,476 ft) fromSangatte on the French side.[26] However, despite this success, the cross-Channel tunnel project was abandoned in 1883 after the British military raised fears that the tunnel might be used as an invasion route.[10][27] Nevertheless, in 1883, this TBM was used to bore a railway ventilation tunnel — 2 m (7 ft) in diameter and 2.06 km (6,750 ft) long — betweenBirkenhead andLiverpool, England, through sandstone under theMersey River.[28]
TheHudson River Tunnel was constructed from 1889 to 1904 using a Greathead shield TBM. The project used air compressed to 2.4 bar (35 psi) to reduce cave-ins. However, there were many workers that died via cave-in or decompression sickness.[29][30][5]
During the late 19th and early 20th century, inventors continued to design, build, and test TBMs for tunnels for railroads, subways, sewers, water supplies, etc. TBMs employing rotating arrays of drills or hammers were patented.[31] TBMs that resembled gianthole saws were proposed.[32] Other TBMs consisted of a rotating drum with metal tines on its outer surface,[33] or a rotating circular plate covered with teeth,[34] or revolving belts covered with metal teeth.[35] However, these TBMs proved expensive, cumbersome, and unable to excavate hard rock; interest in TBMs therefore declined. Nevertheless, TBM development continued in potash and coal mines, where the rock was softer.[36]
A TBM with a bore diameter of 14.4 m (47 ft 3 in) was manufactured by The Robbins Company for Canada'sNiagara Tunnel Project. The machine was used to bore a hydroelectric tunnel beneathNiagara Falls. The machine was named "Big Becky" in reference to the SirAdam Beck hydroelectric dams to which it tunnelled to provide an additional hydroelectric tunnel.
The TBM known asBertha, reportedly the largestearth pressure balance machine and second largest TBM in general (as of June 2023), has a bore diameter of 17.45 meters (57.3 ft), and was produced byHitachi Zosen Corporation in 2013.[1][37][better source needed] It was delivered toSeattle,Washington, for itsHighway 99 tunnel project.[38][full citation needed] The machine began operating in July 2013, but stalled in December 2013 and required substantial repairs that halted the machine until January 2016.[39] Bertha completed boring the tunnel on April 4, 2017.[40][full citation needed]
Two TBMs supplied after the 2013 acquisition of Germany's Aker Wirth (Aker Solutions) TBM and shaft-boring technology by China Railway Tunnelling Equipment (CRTE), now CREG (China Railway Engineering Equipment Group)-Germany,[41][42][better source needed] CREG-Wirth units with boring diameter of 6.67 m (21.9 ft), were used to bore two tunnels forKuala Lumpur,Malaysia's Metro system.[42][better source needed] The medium excavated was water "saturated sandy mudstone, schistose mudstone, highly weathered mudstone as well as alluvium".[42][better source needed] By the company's commercial description, its products achieved an advance rate of "more than 345 meters [1,130 feet] per month".[42][better source needed]
Reportedly the largesthard rock machine and fourth largest TBM overall (as of June 2023), a machine known asMartina, was built byHerrenknecht AG.[1] Its excavation diameter is 15.62 m (51.2 ft), and total length 130 m (430 ft); excavation area of 192 m2 (2,070 sq ft), and thrust value 39,485 t,[clarification needed] total weight 4,500 tons, and total installed capacity 18 MW.[1] Its yearly energy consumption was about 62 GWh.[1] Martina was used by the Italian Toto Group construction company (Toto S.p.A Costruczioni General) to bore a 2.4 km tunnel of the Variante di Valico project near Florence, Italy, in 2013. This project created the Sparvo gallery of the Italian Motorway Pass A1 ("Variante di Valico A1"), near Florence.[citation needed] As of this date,[when?] Martina was still owned and operated by the Toto Group.[citation needed]
Herrenknecht also built the world's largest-diameterslurry TBM and as of June 2023, perGuinness World Records, also the largest TBM overall; called the "Qin Liangyu" or Mixshield S-880, it has an excavation diameter of 17.63 meters (57.8 ft).[1] Owned and operated by a subsidiary of the French construction company Bouygues (Dragages Hong Kong), it was used to bore the Chek Lap Kok to Tuen Mun road tunnel, undersea, toHong Kong, China, clearing the first section of the tunnel at the large diameter, then being converted to 14 m, and working alongside 3 other TBMs (including another Herrenknect borer) to complete the tunnels, 30 m undersea, in 2019.[1]
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TBMs typically consist of a rotating cutting wheel in front, called a cutter head, followed by a main bearing, a thrust system, a system to remove excavated material (muck), and support mechanisms. Machines vary with site geology, amount of ground water present, and other factors.
Rock boring machines differ from earth boring machines in the way they cut the tunnel, the way they provide traction to support the boring activity, and in the way they support the newly formed tunnels walls.
Shielded TBMs are typically used to excavate tunnels in soil. They erect concrete segments behind the TBM to support the tunnel walls.[43][page needed]
The machine stabilizes itself in the tunnel with hydraulic cylinders that press against the shield, allowing the TBM to apply pressure at the tunnel face.
Main Beam machines do not install concrete segments behind the cutter head. Instead, the rock is held up using ground support methods such as ring beams, rock bolts,shotcrete, steel straps, ring steel and wire mesh.[43][page needed]
Depending on the stability of the local geology, the newly formed walls of the tunnel often need to be supported immediately after being dug to avoid collapse, before any permanent support or lining has been constructed. Many TBMs are equipped with one or more cylindricalshields following behind the cutter head to support the walls until permanent tunnel support is constructed further along the machine. The stability of the walls also influences the method by which the TBM anchors itself in place so that it can apply force to the cutting head. This in turn determines whether the machine can bore and advance simultaneously, or whether these are done in alternating modes.
Gripper TBMs are used in rock tunnels. They forgo the use of a shield and instead push directly against the unreinforced sides of the tunnel.[5]
Machines such as a Wirth machine can be moved only while ungripped. Other machines can move continuously. At the end of a Wirth boring cycle, legs drop to the ground, the grippers are retracted, and the machine advances. The grippers then reengage and the rear legs lift for the next cycle.
A single-shield TBM has a single cylindrical shield after the cutting head. A permanent concrete lining is constructed immediately after the shield, and the TBM pushes off the lining to apply force to the cutter head. Because this pushing cannot be done while a next ring of lining is being constructed, the single-shield TBM operates in alternating cutting and lining modes.
Double Shield (or telescopic shield) TBMs have a leading shield that advances with the cutting head and a trailing shield that acts as a gripper. The two shields can move axially relative to each other (i.e., telescopically) over a limited distance. The gripper shield anchors the TBM so that pressure can be applied to the cutter head while simultaneously the concrete lining is being constructed.
In hard rock with minimal ground water, the area around the cutter head of a TBM can be unpressurized, as the exposed rock face can support itself. In weaker soil, or when there is significant ground water, pressure must be applied to the face of the tunnel to prevent collapse and/or the infiltration of ground water into the machine.
Earth pressure balance (EPB) machines are used in soft ground with less than 7 bar (100 psi) of pressure. It usesmuck to maintain pressure at the tunnel face. The muck (orspoil) is admitted into the TBM via ascrew conveyor. By adjusting the rate of extraction of muck and the advance rate of the TBM, the pressure at the face of the TBM can be controlled without the use ofslurry. Additives such asbentonite, polymers and foam can be injected ahead of the face to stabilize the ground. Such additives can separately be injected in the cutter head and extraction screw to ensure that the muck is sufficiently cohesive to maintain pressure and restrict water flow.
Like some other TBM types, EPB's use thrust cylinders to advance by pushing against concrete segments. The cutter head uses a combination oftungsten carbide cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters.
EPB has allowed soft, wet, or unstable ground to be tunneled with a speed and safety not previously possible. TheChannel Tunnel, theThames Water Ring Main, sections of theLondon Underground, and most newmetro tunnels completed in the last 20 years worldwide were excavated using this method. EPB has historically competed with the slurry shield method (see below), where the slurry is used to stabilize the tunnel face and transport spoil to the surface. EPB TBMs are mostly used in finer ground (such as clay) while slurry TBMs are mostly used for coarser ground (such as gravel).[44]
Slurry shield machines can be used in soft ground with high water pressure or where granular ground conditions (sands and gravels) do not allow a plug to form in the screw. The cutter head is filled with pressurised slurry, typically made ofbentonite clay that applies hydrostatic pressure to the face. The slurry mixes with the muck before it is pumped to a slurry separation plant, usually outside the tunnel.
Slurry separation plants use multi-stage filtration systems that separate spoil from slurry to allow reuse. The degree to which slurry can be 'cleaned' depends on the relative particle sizes of the muck. Slurry TBMs are not suitable for silts and clays as the particle sizes of the spoil are less than that of the bentonite. In this case, water is removed from the slurry leaving a clay cake, which may be polluted.
Acaisson system is sometimes placed at the cutting head to allow workers to operate the machine,[45][46] although air pressure may reach elevated levels in the caisson, requiring workers to be medically cleared as "fit to dive" and able to operate pressure locks.[45][46]
Open face soft ground TBMs rely on the excavated ground to briefly stand without support. They are suitable for use in ground with a strength of up to about 10 MPa (1,500 psi) with low water inflows. They can bore tunnels with cross-section in excess of 10 m (30 ft). A backactor arm or cutter head bore to within 150 mm (6 in) of the edge of the shield. After a boring cycle, the shield is jacked forward to begin a new cycle. Ground support is provided by precast concrete, or occasionallyspheroidal graphite iron (SGI) segments that are bolted or supported until a support ring has been added. The final segment, called the key, is wedge-shaped, and expands the ring until it is tight against the ground.
TBMs range diameter from 1 to 17 meters (3 to 56 ft). Micro tunnel shield TBMs are used to construct small tunnels, and is a smaller equivalent to a generaltunnelling shield and generally bore tunnels of 1 to 1.5 meters (3.3 to 4.9 ft), too small for operators to walk in.
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Behind all types of tunnel boring machines, in the finished part of the tunnel, are trailing support decks known as the backup system, whose mechanisms can include conveyors or other systems for muck removal; slurrypipelines (if applicable); control rooms; electrical, dust-removal and ventilation systems; and mechanisms for transport of pre-cast segments.
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Urban tunnelling has the special requirement that the surface remain undisturbed, and that groundsubsidence be avoided. The normal method of doing this in soft ground is to maintain soil pressures during and after construction.
TBMs with positive face control, such as earth pressure balance (EPB) and slurry shield (SS), are used in such situations. Both types (EPB and SS) are capable of reducing the risk of surface subsidence and voids if ground conditions are well documented. When tunnelling in urban environments, other tunnels, existing utility lines and deep foundations must be considered, and the project must accommodate measures to mitigate any detrimental effects to other infrastructure.[citation needed]
