Tunnelling shield
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Diagram of the tunnelling shield used to construct the Thames Tunnel |
A
tunnelling shield is a protective structure used in the excavation of
tunnels 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 or steel. In effect, the shield serves as a temporary support structure for the tunnel while it is being excavated.
The first successful tunnelling shield was developed by Sir
Marc Isambard Brunel to excavate the
Thames Tunnel 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 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, 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 in the course of the construction of the
Tower Subway under the River
Thames in central
London 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 for the construction of the
City & South London Railway (today part of
London Underground's
Northern Line) in
1884. To this day, most tunnelling shields are still loosely based on the Greathead shield.
In early shield tunneling, the shield functioned as a way to protect manual 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 Underground were built in this way. The shield also divided the workface into overlapping portions that each worker could comfortably dig at.
See main article: tunnel boring machineA 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 (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 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.
The tunnel lining is the wall of the tunnel. It usually consists of precast concrete segments which form rings.
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.
*
Herrenknecht, TBM manufacturer
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A picture of manual tunneling using a Greathead shield
