Civil Engineering Reference
In-Depth Information
Fill over
toes
Figure 9.6
Fill over toes can improve FoS against uplift.
Tunnel element roof slab
Bulkhead
Perimeter used for
calculation of tunnel
element volume and
uplift due to water
displacement
Weight of water acting
within perimeter of tunnel
structure
Tunnel element base slab
Figure 9.7
Water in bulkhead space.
These loads are quite significant and must be included. They could be reduced
to a weight per linear meter of tunnel element, but it is probably easier to deal
with real loads and the whole element. In addition, the tunnel cross section
may not be uniform along the length of an element, with features such as
recesses, openings, or voids that could affect the overall weight. The behavior
of a tunnel element can be quite sensitive and it would be easy to miscalculate
the factors of safety or the freeboard while floating if all the loads and varia-
tions are not taken into account. Another important load to remember is the
weight of water that is within the perimeter of the tunnel at the bulkheads.
The bulkheads are often set a short distance in from the end of the tunnel
element and it is important not to assume there is air right up to the end of the
elements; instead, the appropriate weight of water within the tunnel perim-
eter should be applied at the ends of the element (Figure 9.7).
