Tunnelling shield
Encyclopedia
Tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...
s through soil that is too soft or fluid to remain stable during the time it takes to line the tunnel with a support structure of concrete, cast iron or steel. In effect, the shield serves as a temporary support structure for the tunnel while it is being excavated.
History
The first successful tunnelling shield was developed by Sir Marc Isambard BrunelMarc Isambard Brunel
Sir Marc Isambard Brunel, FRS FRSE was a French-born engineer who settled in England. He preferred the name Isambard, but is generally known to history as Marc to avoid confusion with his more famous son Isambard Kingdom Brunel...
, and patented by him and Lord Cochrane
Thomas Cochrane, 10th Earl of Dundonald
Admiral Thomas Cochrane, 10th Earl of Dundonald, 1st Marquess of Maranhão, GCB, ODM , styled Lord Cochrane between 1778 and 1831, was a senior British naval flag officer and radical politician....
in January 1818. Brunel and his son Isambard Kingdom Brunel
Isambard Kingdom Brunel
Isambard Kingdom Brunel, FRS , was a British civil engineer who built bridges and dockyards including the construction of the first major British railway, the Great Western Railway; a series of steamships, including the first propeller-driven transatlantic steamship; and numerous important bridges...
used it to excavate
Excavate
The excavates are a major kingdom of unicellular eukaryotes, often known as Excavata. The phylogenetic category Excavata, proposed by Cavalier-Smith in 2002, contains a variety of free-living and symbiotic forms, and also includes some important parasites of humans.-Characteristics:Many excavates...
the Thames Tunnel
Thames Tunnel
The Thames Tunnel is an underwater tunnel, built beneath the River Thames in London, United Kingdom, connecting Rotherhithe and Wapping. It measures 35 feet wide by 20 feet high and is 1,300 feet long, running at a depth of 75 feet below the river's surface...
, beginning in 1825 (though the tunnel would not be opened until 1843). Brunel is said to have been inspired in his design by the shell of the shipworm
Shipworm
Shipworms are not worms at all, but rather a group of unusual saltwater clams with very small shells, notorious for boring into wooden structures that are immersed in sea water, such as piers, docks and wooden ships...
Teredo navalis, a mollusc whose efficiency at boring through submerged timber he observed while working in a shipyard. The shield was built by Maudslay, Sons & Field
Henry Maudslay
Henry Maudslay was a British machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool technology.-Early life:...
, of Lambeth, London, who also built the steam pumps for de-watering the tunnel.
Brunel's original design was substantially improved by Peter W. Barlow
Peter W. Barlow
Peter William Barlow was an English civil engineer.He was born at Woolwich, the son of an engineer and mathematician, professor Peter Barlow of the Royal Military Academy, Woolwich...
in the course of the construction of the Tower Subway
Tower Subway
The Tower Subway is a tunnel, dug in 1869, beneath the River Thames in central London, close to the Tower of London. Its alignment runs between Tower Hill on the north side of the river and Vine Lane to the south...
under the River Thames in central London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
in 1870. Probably the most crucial innovation of Barlow's design was that it had a circular cross-section (unlike Brunel's, which was of rectangular cross-section), which at once made it simpler in construction and better able to support the weight of the surrounding soil.
The Barlow design was enlarged and further improved by James Henry Greathead
James Henry Greathead
James Henry Greathead was an engineer renowned for his work on the London Underground railway.-Early life:Greathead was born in Grahamstown, South Africa; of English descent, Greathead's grandfather had emigrated to South Africa in 1820. He was educated at St Andrew's College, Grahamstown, and the...
for the construction of the City & South London Railway
City & South London Railway
The City and South London Railway was the first deep-level underground "tube" railway in the world, and the first major railway to use electric traction...
(today part of London Underground
London Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
's Northern Line
Northern Line
The Northern line is a London Underground line. It is coloured black on the Tube map.For most of its length it is a deep-level tube line. The line carries 206,734,000 passengers per year. This is the highest number of any line on the London Underground system, but the Northern line is unique in...
) in 1884. To this day, most tunnelling shields are still loosely based on the Greathead shield.
Manual shield tunneling
In early shield tunneling, 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 London UndergroundLondon Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
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.At the front end of the shield a rotating cutting wheel is located. Behind the cutting wheel there is a chamber where, depending on the type of the TBM, the excavated soil is either mixed with slurry
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
(so-called slurry TBM) or left as-is (earth pressure balance or EPB shield). The choice for a certain type of TBM depends on the soil conditions. Systems for removal of the soil (or the soil mixed with slurry) are also present.
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 1.5-2 meters), a new tunnel ring is built using the erector. The erector is a rotating system which picks up precast concrete
Precast concrete
By producing precast concrete in a controlled environment , the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete...
segments and places them in the desired position.
Behind the shield, inside the finished part of the tunnel, several support mechanisms which are part of the TBM can be found: 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 London UndergroundLondon Underground
The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
tunnels, while steel liners were sometimes used elsewhere.
Shields in Japan
In Japan there are several innovative approaches to shield tunneling, e.g. the Double-O-Tube or DOT-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.External links
- Herrenknecht, TBM manufacturer
- A picture of manual tunneling using a Greathead shield