THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 108

moments in the ring will increase and the coupling forces in the dowels will
decrease. In addition, this gives the most realistic estimate of the compres-
sive forces in the concrete in the concrete hinge.
In order to support the input parameters chosen - stiffness and location of
tie-bars, failure of tie-bars, stiffness of the rotation hinge, longitudinal joint
and bed - a parameter study was carried out, in which different values were
entered for the variables referred to.
Bedding (interaction with the ground)
A relatively slim structure as a bored tunnel has a special interaction with
the surrounding soil: on the one hand, the soil puts a load on the tunnel and
on the other hand the soil contributes to the strength and stiffness of the
structure (bed). It is very important that when the tunnel rings come to be
outside the shield because of the progress of the tunnel boring machine,
they are directly supported in their surroundings. For this purpose, the tail
void, which is the space between the outside of the shield and outside of the
lining, is filled with grout under pressure during the boring process.
If, once outside the shield, a tunnel ring deforms, the ring encounters a react-
ive force from the surrounding ground (and grout).The design calculations,
in accordance with the Duddeck model, assume a support in the radial direc-
tion. In the assumed representation, notwithstanding the degree of soil cov-
ering, this bed was not applied at a vertex angle of 90°, except in the load
cases where vertical ovalization of the tunnel ring occurred. In those cases
the spring stiffness of the soil across the vertex angle was reduced to 50%,
whereby it was checked if the weight of the soil cover above the tunnel was
sufficient to take up the reaction forces.
Stiffness of the tie-bars
The interaction between the two rings processed in the model by means of
so-called tie-bars representing the concrete dowels ( Nocke-Topf joint) of the
segment. The stiffness of these tie-bars can be very decisive for the distri-
bution of forces in the rings. High stiffness results in more interaction
(higher coupling forces) and lower ring forces.
A parameter study carried out also shows that the location and the number
of couplings have an effect on the distribution of forces. On the basis of this
parameter study the location and the number of tie-bars were chosen as
such, that the calculated coupling forces, based on experiences (and later
verified by shearing tests) of the physically possible dimensions of the
concrete dowel-recess could be included.
Loads
A large number of load factors - and combinations of these - were taken
into consideration when carrying out the calculations. A distinction was
made between loads which are the result of constructing the tunnel (con-
struction stage loads) and loads which (could) play a role during the stage
of usage.
Overall safety factors were used for the combinations of load factors
instead of partial factors, especially due to the high degree of complexity.
The magnitude of the overall factor depends on the degree of probability of
a particular combination occurring.
Load due to dead weight, soil and water pressures
The basic loads for all combinations are the result of the dead weight of the
ring (based on segment thickness excluding grout) and the water and grain