THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 102

inside the tunnel boring machine in a stretching bond until a ring was
formed. Each ring has an internal diameter of 10.10 metres and is 2 metres
wide (in the axial direction). The joint between two rings is called the ring
joint. Due to the difference in the position of the hinged expansion joints the
rings situated next to each other will deform differently. These deformation
differences are limited by mutually interlinking the rings by means of a con-
crete 'dowel-recess' joint ( Nocke-Topf joint). This connection is loaded on
radial forces. In order to guarantee that the tunnel is watertight, a rubber seal
is fitted round the segments. Due to the segments being pressed against
each other, the seal also becomes compressed and the water pressure, with
a maximum of 6.5 bar due to the depth, can be withstood.

Design calculation values
The bored tunnel was constructed in both sand and clay layers. This meant
that the design of the lining had to take into account any rises in ground-
water in the sand layers as a result of the tide. The following table gives the
values used in the design calculations for the parameters of the various soil
layers. GZ stands for 'glauconitic sand' and BK stands for 'Boom clay'.

Fig. 9.2
Calculation values of
parameters for design
calculations

(kN/m 3 ) Layer Type c u (kPa) c (kPa) K 0 E (MPa)

( ° )

Z1
GZ1
BK1
BK2
GZ2

Sand
Sand

19 32.5 0.5

0.5-0.8
0.6-0.8
0.6-0.8
0.5-0.8

40
80
40
40
100

20 10 30

Clay
Clay

20 150 20 22.5

20 100 10-20 27.5

Sand 20 15 10 30

It was especially the horizontal soil pressure coefficient (K 0 ) and the modu-
lus of elasticity (E) that were important for the ring calculations. The horizon-
tal soil pressure coefficient gives the relationship between the vertical and
horizontal soil pressures. If the vertical soil pressure is much higher compared
to the horizontal one, the tunnel tube will deform in an oval shape. As a result
of this, the soil above and below the tunnel tube relieves; the soil pressure
becomes active and therefore decreases.The soil at the sides however is under
pressure and increases due to passive behaviour.The differences between the
soil pressures around the tunnel ring decreases as a consequence, which
results in a reduction of the bending moment. A higher horizontal soil pressure
coefficient is better for the moment distribution curve in the rings because the
soil load on the ring is more evenly distributed.
The stiffness of the soil can be expressed in the modulus of elasticity (Eoed).
Compared with other bored tunnels in the Netherlands, the soil in which the
Westerschelde Tunnel was bored, with a minimum Eoed of 40 MPa, was not
really considered to be soft.
It was assumed in the design of the segments that the salt content of the
water would be comparable to that of sea water. This resulted in a specific
gravity of the water with a maximum of 10.3 kN/m 3 .

Requirements set on the design of the lining
Requirements were of course set on the design of the lining. So the calcula-
tion had to be carried out according to the technology available at that time
and various types of load had to be taken into account. In addition to the
structural behaviour due to soil and water pressures, the possibility of colli-
sions, fire and explosions and the possibility of a ship sinking next to or
above the tunnel tubes were also taken into account.These loads play a role
during the phase when the tunnel is in use. Yet, perhaps even more import-
ant, are the loads on the segments that occur when constructing the tunnel