THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 105

47% was assumed - it was designed to withstand a water pressure of 13 bar.
Watertightness cannot be shown in a calculation and therefore it is common
to prove the seals in a test set-up. Generally the section is only tested in a steel
mould. However, for the watertightness it is also of great importance that the
concrete around the rubber seal remains intact. This was also investigated in
the testing of the seals which were fitted in the lining of the Westerschelde
Tunnel: the test was carried out in a concrete set-up with the aim of testing
both the seal and by testing the possible shearing behaviour ('spalling') of the
concrete 'behind' the seal. During the tests, the seal was tested for water-
tightness with the minimum pressure stress present.This minimum pressure
stress occurs at a maximum joint opening and a maximum shift between
2 lining elements. The testing of spalling occurred at a maximum pressure
stress, thus at a minimum joint opening and without shifting.
Ring joint
The tunnel rings are interlinked by a ' Nocke-Topf ' joint (concrete dowel-
recess) joint. On the side of the segments on which the tunnel boring
machine pushes with the aid of jacks, two recesses are located each at a 1 ⁄ 4
distance from the ends in the segment circumference. The concrete dowels
are located on the other side of the segment, which is placed against the ear-
lier completed tunnel ring, at the same positions on the circumference. This
concrete dowel-recess joint increases the stiffness of the structure and is
intended to reduce the differential displacements between the tunnel rings
by taking up the radial forces. If the rings move in relation to each other
(greatly exaggerated this would be 'escaping' from the tube shape) the trans-
fer of forces which is associated with this, occurs through the concrete
dowels and recesses. A ring that deforms gets support from the adjacent
rings by means of the coupling and therefore the mutual deformations
can remain limited. This is important because too large a movement of the
rings in respect of each other has an adverse effect on the watertightness
of the lining; the rubber seals would no longer butt up against each other
properly.
Further optimization of the geometry of the concrete dowel-recess joint has
resulted in the concrete dowel being located deeply into the recess.The sur-
faces of the recess have a differentiated gradient with the result that when the
segment is in the correct position, the concrete dowels are located right down
into the deepest part of the recess.This detailing has a positive effect on the
distribution of forces in the recess, because the force is applied so deeply
that the reinforcement contributes (indirectly) to the shearing capacity.
Longitudinal joints
The longitudinal joints form the connections between the segments within
a ring.These joints are designed as a concrete hinge with a certain rotational
capacity. The longitudinal joint of the keystone is an exception to this and
has a 'tongue and groove' joint to affix this segment during the building-in
and grouting. Due to the parallel joint surfaces of the keystone, this could
otherwise shift as a result of the major radial grout forces if no normal forces
have developed as yet.
Bolt connections
The segments were pushed against each other by the jacks which pushed
the TBM forwards during the boring so that the seals in the ring joints were
compressed. In the first place this occurred in the longitudinal joints by the
erector force and in the final phase, by the ring compressive force caused by
the external load on the ring. In order to absorb any relief of pressure on the
joints, during the construction phase, temporary bolt connections were
fitted in the joints on each segment: 4 on the ring joint side and 2 on the
longitudinal joint side.