TBMs have an average speed (which depends on the soil ground geology) of 10 to 30m per day
for tunnels of 3 to 15m diameter.
In most cases, the tunnel inner walls are made with precast-elements called segments, those segments are put together in rings. Rings have several roles:
» They provide the tunnel with resistance against external
pressure. The area between the bored rock-face and
the segments is filled with concrete injection.
» The segments compose the tunnel inner lining..
» They offer axial support for the moving part of the boring
machine and maintain its progress.
» In some cases, rings support the building of a rail
craneway on which the boring machine progresses,
this is done by laying a special bottom segment
called invert.
The segment laying system is located in the rear part of the tunnel boring machine.
After the boring of a distance equivalent to the width of a ring, the cutting shield is stopped and a new ring placed.
During the drilling phase, the segments that make the ring are moved from the backup train to the transport table
on which they are placed in their respective assembly order.
The universal ring
In most cases, the universal segment ring is made of several segments of the same size and of one smaller segment at the end : The key-stone closing the ring. The universal-ring sides are not made parallel but tend to form an angle that allows the tunnel to bend
according to the relative angular position of each segment.
For the curved parts the rings are placed by lining up the key, for straight parts the rings are switched from upward key to downward key.
The relative positioning of keys can be played upon to modify the curved radius.
The segment and its mould
The segment geometry is rather complex, it requires great accuracy as well as perfect flatness.
As a matter of fact, the ring and its segments must be able to fit perfectly in the tunnel.
For a several meter-long diameter, the ring accuracy tolerance is a few millimetres.
This brings the segment tolerance down to one tenth of a millimetre.
The segment polishing will determine the final lining of the tunnel.
The contractors are more and more demanding on this particular point.
The quality of the mould will determine the accuracy and finishing of the segment.
Optimal flatness of the bearing areas is vital to avoid excessive pressure during the
drilling phases of the tunnel boring machine.
Segment fastening
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In order to achieve curved parts in tunnels, there are several ring types: right rings, left rings and universal rings.
Despite their more complex design, the latter is increasingly used for economical reasons,
as only one type of ring is used, regardless the tunnels' curve to be realised.
Tunnel bore made out of segment rings
The ring and its moulds
The ring is usually built up with 6 to 11 segments, according to the geometry and tunnel constraints.
The smaller key is always present, so the number of segments is counted namely:
5+1, 6+1, 7+1, 8+1, 9+1.
Mark 1, 2, 3, 4 : Standard segments
Mark 6 et 7 : counter-key segment
Mark 8 : key segment.
Segments on each side of the key are named counter-key. To make a complete ring, according to the geometry,
several different moulds are needed: The standard segment, the counter-keys, and the key itself.
The set of moulds needed to build up the ring is called a ring of segment moulds.
The segment also has its own specific functions and devices. Thus, the segments are put together
by a special fastening system. The segment must therefore have built-in holes for screws to fit in, as well as
devices to allow its handling during its installation in the tunnel.
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