A tunnel shield or tunnel shield is a protective structure used during the excavation of large tunnels. When excavating in soft, waterlogged or unstable ground, there is a potential danger to the safety of workers and the project itself due to falling materials or a collapse. In these cases, a tunnel shield can be used as a temporary support structure. It is usually in place for the short term, from when the tunnel section is excavated until it can be lined with a permanent support structure. The permanent structure can be composed, depending on the period, of bricks, reinforced concrete, cast iron or steel. Although modern shields are usually cylindrical, the first "shield", designed by Marc Isambard Brunel, was actually a large, rectangular, scaffold-like iron structure, with three levels and twelve sections per level, with a solid top platform to support the load of the terrain. The structure protected the men from collapses as they worked inside it, excavating the tunnel in front of the shield.
The concept of the classic shield has been transferred to modern tunnel boring machines, whose heads perform the function of the primitive shields, automating it and minimizing the danger of collapse of the gallery being excavated.
History
Marc Isambard Brunel developed the first successful rectangular tunneling shield and patented it with Lord Cochrane in January 1818. Brunel and his son Isambard Kingdom Brunel used it to excavate the Thames Tunnel from 1825 (although the tunnel was not opened until 1843).[1] Brunel is said to have been inspired by the shell of the teredo navalis, a mollusk whose efficiency to drill submerged wood he observed while working in a shipyard.[1] The shield was built by Maudslay, Sons & Field of Lambeth, London, who also built the steam pumps necessary to evacuate water infiltrated into the tunnel.
In 1840, Alfred Ely Beach, editor of Scientific American magazine, was the first to suggest that a circular design would be superior to Brunel's rectangular design. In 1868, Beach constructed a circular shield, the image of which was published in a New York news article about his idea for the pneumatic tunnel system. The design was based on Brunel's cell shield and was screwed forward as the face was advanced manually.
In 1864 Peter W. Barlow patented a design that had a circular cross section. Theoretically, this made the shield easier to construct and better able to support the surrounding soil; theoretically, because a shield with this design was never built. Barlow's patent of 1864 was further improved and he was granted a provisional patent in 1868, but it was never ratified because Barlow died shortly afterwards.
Simple shield tunnel boring machine
Introduction
A tunnel shield or tunnel shield is a protective structure used during the excavation of large tunnels. When excavating in soft, waterlogged or unstable ground, there is a potential danger to the safety of workers and the project itself due to falling materials or a collapse. In these cases, a tunnel shield can be used as a temporary support structure. It is usually in place for the short term, from when the tunnel section is excavated until it can be lined with a permanent support structure. The permanent structure can be composed, depending on the period, of bricks, reinforced concrete, cast iron or steel. Although modern shields are usually cylindrical, the first "shield", designed by Marc Isambard Brunel, was actually a large, rectangular, scaffold-like iron structure, with three levels and twelve sections per level, with a solid top platform to support the load of the terrain. The structure protected the men from collapses as they worked inside it, excavating the tunnel in front of the shield.
The concept of the classic shield has been transferred to modern tunnel boring machines, whose heads perform the function of the primitive shields, automating it and minimizing the danger of collapse of the gallery being excavated.
History
Marc Isambard Brunel developed the first successful rectangular tunneling shield and patented it with Lord Cochrane in January 1818. Brunel and his son Isambard Kingdom Brunel used it to excavate the Thames Tunnel from 1825 (although the tunnel was not opened until 1843).[1] Brunel is said to have been inspired by the shell of the teredo navalis, a mollusk whose efficiency to drill submerged wood he observed while working in a shipyard.[1] The shield was built by Maudslay, Sons & Field of Lambeth, London, who also built the steam pumps necessary to evacuate water infiltrated into the tunnel.
In 1840, Alfred Ely Beach, editor of Scientific American magazine, was the first to suggest that a circular design would be superior to Brunel's rectangular design. In 1868, Beach constructed a circular shield, the image of which was published in a New York news article about his idea for the pneumatic tunnel system. The design was based on Brunel's cell shield and was screwed forward as the face was advanced manually.
Brunel's original design was substantially improved by James Henry Greathead, who was granted three patents for different shield designs. Additionally, he invented the concept of shotcrete grout to stabilize shotcrete earthworks, using a gravel-lined tray through which reinforcing grout was hydraulically injected between the constructed lining and the tunnel wall.
Greathead was the first to use a cylindrical tunnel shield, which it did during the construction of the Tower Subway under the River Thames in central London in 1870. Greathead's shield was 7 ft 3 in (2.2 m) in diameter.
Greathead also used one in the construction of the City & South London Railway (today part of the Northern Line of the London Underground) in 1884, with tunnels 10 ft 2 in (3.1 m) in diameter. His shield was also used in the execution of the 12 ft 1+3/4 in (3.7 m) diameter tunnels for the Waterloo & City Railway which opened in 1898. The shields used in the City station gallery (now known as Bank) enabled the construction of the largest diameter tunnels in the world at the time, measuring 23 ft (7 m).
An original Greathead shield used in the excavation of the deep London Underground lines remains in place in disused tunnels beneath Moorgate Station.[2].
Most tunnel shields are still loosely based on Greathead's design.[3].
Manual tunnel shields
In the first tunnels built with a shield, it functioned as a way to protect the workers carrying out the excavation. To advance the gallery, they moved the shield forward, progressively replacing it with pre-constructed sections of the tunnel wall. The first deep tunnels of the London Underground were built in this way. The shield divided the work surface into overlapping parts that each worker could excavate by hand individually.
Modern tunnel boring machines
A tunnel boring machine (also known as TBM) consists of a shield (a large metal cylinder) and support mechanisms for pulling.
A rotating cutting wheel is located at the front end of the shield. Behind the cutting wheel is a chamber where the excavated soil is mixed with mud (so-called slurry TBM) or left as is (earth pressure balanced or EPB shield), depending on the type of TBM. The choice of the type of tunnel boring machine depends on the ground conditions. There are also systems for the elimination of excavated soil (or its mixture with sludge).
Behind the chamber is a set of hydraulic jacks resting on the finished part of the tunnel that are used to push the TBM forward. Once a certain distance has been excavated (approximately 1.5 to 2 meters), a new tunnel ring is constructed using the erector. The erector is a rotating system that collects the precast concrete segments and places them in the desired position, where they are screwed together and to the previous ring.
Behind the shield, inside the finished part of the tunnel, you can find several support mechanisms that are part of the tunnel boring machine: hydraulic jacks to advance the machine, formwork removal system, sludge circulation pipe if applicable, control rooms, tracks for transporting the prefabricated support segments, conveyor belts for the extraction of the excavated material or the ventilation system.
Coating
The tunnel lining is the wall of the tunnel. It usually consists of precast concrete segments that form rings. Cast iron linings were traditionally used in London Underground tunnels, while steel linings were sometimes used elsewhere. The concept of using prefabricated molded siding sections is not new and was first patented in 1874 by James Henry Greathead.[4].
Shields in Japan
In Japan there are several innovative approaches to the execution of tunnels, such as the Double-O-Tube or DOT-tunnel, whose section is composed of two overlapping circles. There are also shields with computerized arms that can be used to dig a tunnel in virtually any shape.[5].
References
[1] ↑ a b Becket, Derrick (1980). Brunel's Britain. Newton Abbot: David & Charles. ISBN 0-7153-7973-9. Chapter 10: "Tunnels".
[3] ↑ John C Gillham, The Waterloo & City Railway, The Oakwood Press, Usk, 2001, ISBN 0 85361 525 X.
[4] ↑ Roger P. Roess, Gene Sansone (2012). The Wheels That Drove New York: A History of the New York City Transit System. Springer Science & Business Media. pp. 38 de 432. ISBN 9783642304842. Consultado el 31 de octubre de 2022.: https://books.google.es/books?id=qfZ0VxuLoc0C&pg=PA38#v=onepage&q&f=false
In 1864 Peter W. Barlow patented a design that had a circular cross section. Theoretically, this made the shield easier to construct and better able to support the surrounding soil; theoretically, because a shield with this design was never built. Barlow's patent of 1864 was further improved and he was granted a provisional patent in 1868, but it was never ratified because Barlow died shortly afterwards.
Brunel's original design was substantially improved by James Henry Greathead, who was granted three patents for different shield designs. Additionally, he invented the concept of shotcrete grout to stabilize shotcrete earthworks, using a gravel-lined tray through which reinforcing grout was hydraulically injected between the constructed lining and the tunnel wall.
Greathead was the first to use a cylindrical tunnel shield, which it did during the construction of the Tower Subway under the River Thames in central London in 1870. Greathead's shield was 7 ft 3 in (2.2 m) in diameter.
Greathead also used one in the construction of the City & South London Railway (today part of the Northern Line of the London Underground) in 1884, with tunnels 10 ft 2 in (3.1 m) in diameter. His shield was also used in the execution of the 12 ft 1+3/4 in (3.7 m) diameter tunnels for the Waterloo & City Railway which opened in 1898. The shields used in the City station gallery (now known as Bank) enabled the construction of the largest diameter tunnels in the world at the time, measuring 23 ft (7 m).
An original Greathead shield used in the excavation of the deep London Underground lines remains in place in disused tunnels beneath Moorgate Station.[2].
Most tunnel shields are still loosely based on Greathead's design.[3].
Manual tunnel shields
In the first tunnels built with a shield, it functioned as a way to protect the workers carrying out the excavation. To advance the gallery, they moved the shield forward, progressively replacing it with pre-constructed sections of the tunnel wall. The first deep tunnels of the London Underground were built in this way. The shield divided the work surface into overlapping parts that each worker could excavate by hand individually.
Modern tunnel boring machines
A tunnel boring machine (also known as TBM) consists of a shield (a large metal cylinder) and support mechanisms for pulling.
A rotating cutting wheel is located at the front end of the shield. Behind the cutting wheel is a chamber where the excavated soil is mixed with mud (so-called slurry TBM) or left as is (earth pressure balanced or EPB shield), depending on the type of TBM. The choice of the type of tunnel boring machine depends on the ground conditions. There are also systems for the elimination of excavated soil (or its mixture with sludge).
Behind the chamber is a set of hydraulic jacks resting on the finished part of the tunnel that are used to push the TBM forward. Once a certain distance has been excavated (approximately 1.5 to 2 meters), a new tunnel ring is constructed using the erector. The erector is a rotating system that collects the precast concrete segments and places them in the desired position, where they are screwed together and to the previous ring.
Behind the shield, inside the finished part of the tunnel, you can find several support mechanisms that are part of the tunnel boring machine: hydraulic jacks to advance the machine, formwork removal system, sludge circulation pipe if applicable, control rooms, tracks for transporting the prefabricated support segments, conveyor belts for the extraction of the excavated material or the ventilation system.
Coating
The tunnel lining is the wall of the tunnel. It usually consists of precast concrete segments that form rings. Cast iron linings were traditionally used in London Underground tunnels, while steel linings were sometimes used elsewhere. The concept of using prefabricated molded siding sections is not new and was first patented in 1874 by James Henry Greathead.[4].
Shields in Japan
In Japan there are several innovative approaches to the execution of tunnels, such as the Double-O-Tube or DOT-tunnel, whose section is composed of two overlapping circles. There are also shields with computerized arms that can be used to dig a tunnel in virtually any shape.[5].
References
[1] ↑ a b Becket, Derrick (1980). Brunel's Britain. Newton Abbot: David & Charles. ISBN 0-7153-7973-9. Chapter 10: "Tunnels".
[3] ↑ John C Gillham, The Waterloo & City Railway, The Oakwood Press, Usk, 2001, ISBN 0 85361 525 X.
[4] ↑ Roger P. Roess, Gene Sansone (2012). The Wheels That Drove New York: A History of the New York City Transit System. Springer Science & Business Media. pp. 38 de 432. ISBN 9783642304842. Consultado el 31 de octubre de 2022.: https://books.google.es/books?id=qfZ0VxuLoc0C&pg=PA38#v=onepage&q&f=false