Civil Engineering Reference
In-Depth Information
the prestress has been cut. If saw cutting is planned, it is advantageous to
have the tendons grouped to enable the cutting process to be as quick and
efficient as possible. Tendons are located towards the inside face of the tun-
nel structure to enable easier cutting and to ensure they are on the inside
of the segment joint waterstops. The cutting sequence should be designed
to release the prestress as evenly as possible along the length of the tunnel.
Care should be taken to avoid any blocking of the cutting recesses with
temporary equipment, such as ballast tanks, that would prevent cutting
being carried out, and the cutting operation needs to be timed with other
finishing works to allow free progressive access through the tunnel.
There is a practical limit to the anchorage size if it is to be located at
the end face of the tunnel element by the immersion joint. The available
depth typically limits anchors to something around a 19-strand tendon.
This enables the anchor to be located with reasonable clearance from the
inside face of the tunnel. This is also important for ensuring the standard
spiral bursting reinforcement can be fitted without infringing on the cover
depth to the inside face. This is not always possible and it is fairly common
for the spiral reinforcement to be replaced with orthogonal bars in the form
of closed links. This creates congestion in the main reinforcement mats,
however, and needs careful detailing.
The location of the anchors and the general tendon alignment will be
dictated by other features in the tunnel. At the end of the tunnel element,
there can be little space available as there are many cast-in items, such as
bulkhead support brackets and anchorages for marine equipment such as
bollards.
The grouting of the prestress cables is conventional, although the dis-
tances can be quite long for immersed tunnel elements, possibly up to
180 m, as seen on the Øresund and Busan Tunnels. Regular grout vents are
needed, but with some pre-testing of grout mixes to ensure complete filling
is achieved, the grouting is relatively straightforward. Well-sealed coupling
sleeves for the ductwork are needed at the segment joints to prevent grout
leakage into the segment joint. Pre-testing with water is common to check
the watertightness of the ductwork.
Special features
Two special features that are common in most immersed tunnels are the
drainage sump and technical rooms. These have particular structural
design and detailing considerations.
Drainage sumps
To collect the surface water or spilled liquids in the tunnel, a drainage
sump is required in the low point of the tunnel. For longer tunnels, there
