THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 104

of the segment (where the jacks push against the segments) is smaller than
at the back (where the segment is placed up against the already completed
tunnel rings).This difference in length between the front and back is necessary
to be able to put the wedge-shaped keystone in position.
The segments, which weigh approximately 10,000 kg, are 2 metres wide in
the longitudinal direction with a circumference length of about 4.5 metres.
In order to be able to make bends during boring and to give the tunnel the
required alignment, the segments are conically shaped by varying the width
of the ring around the circumference by 45 mm. Left and right-hand rings
were used to be able to place the keystones in the top half of the tunnel ring
as far as possible.

Fig. 9.4
Diagrammatic
representation of a
tunnel ring

C

K

B

A1

A5

A2

A4

A3

Fig. 9.5
Diagrammatic
representation of
conical rings

Seal
A single rubber seal is fitted round the whole circumference of the seg-
ments. Similarly to the concrete and the reinforcement used in the lining,
this seal also has to fulfil the set requirement of a life span of 100 years and
this means that it must be able to guarantee watertightness under a pres-
sure of 6.5 bar for this whole period. Since the material with which the seal
was made slackens over time - during the life span a total relaxation of