THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 103

tubes: the construction loads, in which particularly the maximum occurring
jacking forces and/or grout pressures can be normative.
Furthermore, the requirement of a life span of 100 years was set on the tun-
nel as a whole - and therefore also on the segments from which the lining
is constructed - and the tunnel wall has to be able to withstand a hydrocar-
bon fire with a maximum temperature of 1,350 °C for a period of two hours
(RWS curve).
Evidently the design had to fulfil (geometric) requirements which arose from
the boring process.

Geometry of the segments

The choice of the dimensions of the segment is determined by a variety of
interrelated factors.
In general, the width of a segment (in the longitudinal direction) is chosen to
be as wide as possible with the aim of optimising the speed at which the tun-
nel boring machine advances.The width is determined by factors including:
- the maximum possible length of the jacks;
- the space available for transporting and placing the segments;
- the maximum weight that can be lifted by the erector.

The length (along the circumference) of a segment is determined by the
number of segments that a complete ring is subdivided into.This is affected
by the following factors:
- the space available for transporting and placing the segments and the
capacity of the erector;
- the structural behaviour in the elements.

The thickness of the segments is determined by the following aspects:
- the structural behaviour in the segments in relation to the chosen length
and width;
- the magnitude of the jacking forces that the TBM exerts on the segments
and the degree in which this is distributed by the jacking shoe;
- the detailing of the joints in connection with the sealing and the transfer
of forces between the segments;
- the way in which the segments are interlinked.

In the case of the Westerschelde Tunnel, a tunnel ring consists of a total of
7 segments and a wedge-shaped keystone, all of which are 0.45 metres thick.
Five of the 7 segments all have the same dimensions. These segments are
called the A stones.The other 2 segments (B and C stones) complete the ring,
together with the closing stone (the keystone) - which is placed between the
B and C stones.The B and C stones are each other's mirror image and are dif-
ferent from the A stones because the length of the circumference at the front

Fig. 9.3
The geometry of a
segment (A-stone)

Recesses

Ring joint

Recesses

Nut for
sealing section

(Temporary)
bolt
connection

Dowels

Kaubit/triplex
plates

Longitudinal joint

Dowels