Civil Engineering Reference
In-Depth Information
Outlet
Inlet
Cooling pipe layout in outer
wall of tunnel above joint
with base slab
Finite element mesh for time-
step analysis of temperature
development
Stress contour plot
generated to assess risk
of cracking
Figure 3.2
Cooling arrangement and stress analysis.
A detailed thermal analysis of the section was carried out to identify the
stress contours in the cross section. Figure 3.2 shows some typical analysis
output with the arrangement of cooling water pipes in the wall, the finite
element model used, and the resulting stresses. The darker areas indicate the
highest longitudinal tensile stresses in the concrete and hence the areas most
likely to crack. The figure shows no dark areas close to the construction
joint between the base slab and the wall and hence the effectiveness of the
cooling pipes. Note that there are some dark areas toward the top of the sec-
tion that may require further curing measures if there is a risk of cracking.
Through the application of these construction techniques, it became pos-
sible to cast the cross section for an immersed tunnel without any cracks.
The concrete itself became the watertight barrier and there was no need for
an external membrane. However, that was not the end of the development,
because elements of 25 m are not very practical for immersed tunnels.
A complete tunnel element was built up of several such segments, for
example, five 25 m long segments, to form a typical 125 m long tunnel
element. The segments are match cast against each other with the joints
between segments being discontinuous and debonded. These segments are
then clamped together with longitudinal prestress so that they behave as
a single homogeneous tunnel element for transporting and placing. The
prestressing cables were regarded as temporary and were later cut at each
of the joints between the segments after the element had been placed. The
result is a reinforced concrete tunnel with regularly spaced articulation
joints and hence very little longitudinal bending. Cracking due to shrinkage
and early thermal effects has been eliminated, so no external waterproofing
membrane is required. Articulation occurs not only at the joints between
the elements but also at the segment joints. Figure 3.3 illustrates the seg-
mental arrangement of the Øresund Tunnel, where each 175 m long tunnel
element was made up of eight 22 m long segments.
