Civil Engineering Reference
In-Depth Information
chlorides through the concrete cover zone, and hence predict the onset of
corrosion. This provides a tool for planning future maintenance activities and
budgets. Similarly on tunnels with external steel membranes, it is common to
install a corrosion monitoring system to check the corrosion rate of the exter-
nal steelwork. This can be done by reference electrodes that can be monitored
from inside the tunnel or retractable coupons that again can be pulled in
through the tunnel wall for inspection. Such items should be checked annu-
ally, at least initially until long-term corrosion rates have been established.
Water leakage is a commonly perceived risk to durability. In an immersion
joint, it is unlikely that both the Gina and the Omega seals will leak. This
would have been identified during the initial construction phase. However,
if they did leak, the water would be evident inside the tunnel or would be
noticed as an additional water flow in the tunnel drainage system. Vent
tubes are installed between the Omega and the Gina seals, and these can
be inspected during tunnel cleaning to check that no water is building up
between the two seals. Inspection pipes can also be used to check if water
is building up under the ballast concrete. An array of such inspection pipes
was installed in the Bjørvika Tunnel for this purpose. In addition, inspection
chambers were installed at each side of the immersion joints to identify any
water inflow arising over the length of each tunnel element. Water is inter-
cepted and a sump provided so that the water can be evacuated and the refill
time measured to arrive at an inflow rate. This can be seen in Figure 16.1.
Segment joints should be checked for leakage during tunnel cleaning
operations, although if they do leak it will be apparent to users of the
Inspection pipe for void
above infill
Inspection pipe for joint
leakage
Inspection pipe for
tunnel element leakage
Pipe from tunnel
element
Sump to measure inflow
rate
Immersion
joint
Figure 16.1
Inspection facilities for immersion joint leakage.
