A tunnelling shield is a protective structure used during the excavation of large, man-made tunnels. When excavating through ground that is soft, liquid, or otherwise unstable, there is a potential

health and safety Occupational safety and health (OSH), also commonly referred to as occupational health and safety (OHS), occupational health, or occupational safety, is a multidisciplinary field concerned with the safety, health, and welfare of people at wo ...

hazard to workers and the project itself from falling materials or a cave-in. A tunnelling 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 may be made up of, depending on the period, bricks, concrete, cast iron, or steel. Although modern shields are commonly 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 load-bearing top surface. The structure protected the men from cave-ins as they laboured within it, digging the tunnel out in front of the shield.

History

The first successful rectangular tunnelling shield was developed by Marc Isambard Brunel and patented by him and Lord Cochrane in January 1818. Brunel and his son

Isambard Kingdom Brunel Isambard Kingdom Brunel (; 9 April 1806 – 15 September 1859) was a British civil engineer who is considered "one of the most ingenious and prolific figures in engineering history," "one of the 19th-century engineering giants," and "on ...

used it to excavate the Thames Tunnel beginning in 1825 (though the tunnel was not opened until 1843).Becket, Derrick (1980). ''Brunel's Britain''. Newton Abbot: David & Charles. . Chapter 10: "Tunnels". Brunel is said to have been inspired in his design by the shell of the

shipworm The shipworms are marine bivalve molluscs in the family Teredinidae: a group of saltwater clams with long, soft, naked bodies. They are notorious for boring into (and commonly eventually destroying) wood that is immersed in sea water, including ...

, a mollusc whose efficiency at boring through submerged timber he observed while working in a shipyard. The shield was built by

Maudslay, Sons & Field Maudslay, Sons and Field was an engineering company based in Lambeth, London. History The company was founded by Henry Maudslay as Henry Maudslay and Company in 1798 and was later reorganised into Maudslay, Sons and Field in 1833 after his sons ...

of Lambeth, London, who also built the steam pumps for de-watering the tunnel. In 1840,

Alfred Ely Beach Alfred Ely Beach (September 1, 1826 – January 1, 1896) was an American inventor, publisher, and patent lawyer, born in Springfield, Massachusetts. He is most known for his design of New York City's earliest subway predecessor, the Beach Pneu ...

, editor of ''Scientific American'' journal, was the first to suggest a circular design would be superior to Brunel's rectangular design. In 1868 Beach built a circular shield - a picture of which was printed in a New York news article about his pneumatic tunnel system idea. The design was based upon Brunel's shield lattice and screw-jacked forwards as the face advanced manually. In 1864

Peter W. Barlow Peter William Barlow (1 February 1809 – 19 May 1885) was an English civil engineer, particularly associated with railways, bridges (he designed the first Lambeth Bridge, a crossing of the River Thames in London), the design of tunnels and the ...

patented a design that had a circular cross-section. Theoretically this made the shield easier to build and better able to support surrounding soil; theoretically, because no shield was ever built using this design. The 1864 Barlow patent was further improved and given a provisional patent in 1868 but never ratified as Barlow died a short time afterwards. Brunel's original design was substantially improved by

James Henry Greathead James Henry Greathead (6 August 1844 – 21 October 1896) was a mechanical and civil engineer renowned for his work on the London Underground railways, Winchester Cathedral, and Liverpool overhead railway, as well as being one of the earliest p ...

who was granted three patents for different shield designs. Additionally, he invented the concept of sprayed concrete grout to stabilise earthworks with shot concrete, a gritting pan that hydraulically injected reinforcing grout between the constructed lining and the tunnel wall. Greathead was the first to ever use a cylindrical tunnelling shield, which he did in the course of the construction of the Tower Subway under the River

Thames The River Thames ( ), known alternatively in parts as the River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the second-longest in the United Kingdom, after the R ...

in central

London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...

in 1870. The Greathead shield was in diameter. Greathead also used one in the construction of the City and South London Railway (today part of

London Underground The London Underground (also known simply as the Underground or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in England. The ...

Northern line The Northern line is a London Underground line that runs from North London to South London. It is printed in black on the Tube map. The Northern line is unique on the Underground network in having two different routes through central London, t ...

) in 1884, with tunnels in diameter. His shield was also used in the driving of the diameter running tunnels for the

Waterloo & City Railway The Waterloo & City line, colloquially known as The Drain, is a London Underground shuttle line that runs between Waterloo and Bank with no intermediate stops. Its primary traffic consists of commuters from south-west London, Surrey and Ham ...

which opened in 1898. The station tunnels at the City station (now known as Bank) were the largest diameter tunnelling shields in the world at the time, measuring . An original Greathead shield used in the excavation of the deep London Underground lines remains in place in disused tunnels beneath

Moorgate station Moorgate is a central London railway terminus and connected London Underground station on Moorgate in the City of London. Main line railway services for Hertford, Welwyn Garden City, Stevenage and Letchworth are operated by Great Northe ...

. Most tunnelling shields are still loosely based on Greathead's design.John C Gillham, ''The Waterloo & City Railway'', The Oakwood Press, Usk, 2001,

Manual shield tunnelling

In early shield tunnelling, the shield functioned as a way to protect labourers who performed the digging and moved the shield forward, progressively replacing it with pre-built sections of tunnel wall. The early deep tunnels for the

were built in this way. The shield divided the workface into overlapping portions that each worker could excavate.

Modern tunnel boring machines

A tunnel boring machine (TBM) consists of a shield (a large metal cylinder) and trailing support mechanisms. A rotating cutting wheel is located at the front end of the shield. Behind the cutting wheel there is a chamber where the excavated soil is either mixed with

slurry A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal p ...

(so-called slurry TBM) or left as-is (earth pressure balance or EPB shield), depending on the type of the TBM. The choice of TBM type depends on the soil conditions. Systems are also present for removal of the soil (or the soil mixed with slurry). Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward. Once a certain distance has been excavated (roughly ), a new tunnel ring is built using the erector. The erector is a rotating system which picks up

precast concrete Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples include precast b ...

segments and places them in the desired position. Several support mechanisms can be found behind the shield, inside the finished part of the tunnel, which are part of the TBM: dirt removal, slurry pipelines if applicable, control rooms, rails for transport of the precast segments, etc.

Lining

The tunnel lining is the wall of the tunnel. It usually consists of precast concrete segments which form rings. Cast iron linings were traditionally used in the

tunnels, while steel liners were sometimes used elsewhere. The concept of using precast moulded lining sections is not new and was first patented in 1874 by

Shields in Japan

In Japan there are several innovative approaches to shield tunnelling, e.g. the Double-O-Tube or DOT-tunnel.
Double-O tube Tunnel

This tunnel looks like two overlapping circles. There are also shields with computerized arms which can be used to dig a tunnel in virtually any shape.

References

External links

A picture of manual tunneling using a Greathead shield
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