Civil Engineering Reference
In-Depth Information
Table 11.1
Watertightness criteria
Case
Description
In situ and prefabricated concrete
structures, including immersed tube
tunnel
Free of all visible leakage, seepage, and damp
patches
Diaphragm walls
Secant piles
Leakage shall be restricted to damp patches on
the concrete face and at construction joints.
Jetting of water will not be acceptable. Prior to
construction of an inner lining, total inflow
over a given area of structure shall not exceed
0.12 L/m
2
per day overall and 0.23 L/day on any
individual square meter.
Probably not, as this means different things to different people. It is there-
fore common to try to describe this further. Table 11.1 gives a typical word-
ing that the authors would recommend to define watertightness.
It is appropriate to define more relaxed criteria for the construction of
approach structures using diaphragms walls or secant piles. This type of
construction carries an inherently higher risk of leakage due to the con-
struction techniques involved. It is usual for an additional inner concrete
lining to be constructed to achieve the higher level of watertightness inside
the tunnel and to provide appropriate drainage paths.
Acceptable rates of leakage are often quoted for bored tunnel construc-
tion in terms of liters/day ingress of water per square meter of tunnel surface
area. Typical values used are in the order of 100 L/day/m
2
. For a bored tun-
nel with a segmental lining with many thousands of meters of sealing gas-
ket, this is appropriate. Leakage is not detrimental to the tunnel segments
forming the lining and there simply needs to be a limit that corresponds to
the drainage sump and pumping capacity. This type of criteria is meaning-
less for an immersed tunnel and should not be used. The ratio of joint length
to surface area of tunnel will be in the order of 100 times smaller.
The nature of the joints is also quite different between bored and immersed
tunnels. A leak through an immersed tunnel joint suggests a significant
defect, compared to occasional variations in the effectiveness of the bored
tunnel gasket joints that accumulate to the total leakage inflow. Leaks at
immersion joints can have a direct impact on the durability because of the
cast-in steelwork components of the joint. Leaks at segment joints would
suggest incomplete concreting and post-grouting of the groutable water-
stops, and the reinforcement local to the joint may, therefore, suffer cor-
rosion. Leaks at welds in steel shells would be similarly detrimental to the
durability of the shell and the internal concrete structure. Therefore, given
the greater risk to the structure, and the inappropriateness of the bored
tunnel leakage criteria, no leaks are tolerated in immersed tunnel structures
and watertightness means that the ingress of water is prevented entirely.
