Civil Engineering Reference
In-Depth Information
occasionally used, this is the most common, and therefore, we will use this
name throughout the discussion on joints. The Gina gasket is formed by a
composite of two parts: a lower stiffness nose and a higher stiffness body. In
the early immersion stages, when the elements are pulled together, the soft
nose makes contact with the end face of the opposite tunnel element and
provides the initial seal that allows the joint space to be dewatered. The ends
of the elements are fitted with bulkheads, which make the elements water-
tight and are set back a little from the end of the tunnel element. During the
initial pulling together, water is trapped in the space between the bulkheads
of the two tunnel elements. When this water is removed, the pressure in
the space between the bulkheads reduces, but the full water pressure is still
exerted on the other end of the tunnel element. This out-of-balance hydro-
static force causes both the tunnel element to move closer to the previously
immersed element and the Gina gasket to compress further. The compres-
sion load in the gasket is now taken by the stiffer main body of the Gina.
The finishing works to the joint include the installation of a second seal,
an Omega seal, on the inside of the Gina gasket and the construction of
shear connectors to ensure that the tunnel elements cannot displace hori-
zontally or vertically relative to each other. There may also be specific fea-
tures to control the future movement of the tunnel under loading, such as
during seismic events.
Concrete tunnels have the typical arrangement of Gina and Omega seals,
shown in Figure 10.2. This has been adopted as a robust and safe solution
since the 1960s and has only changed marginally over the years.
Steel tunnels are also now adopting this combination of sealing gaskets,
but historically have featured a pair of small rubber gaskets to form a tem-
porary seal during the immersion process, and then, a welded connection
across the joints. This is shown in Figure 10.3. These do not have the same
flexibility, but have ductility, which has proved successful in allowing some
movement to occur without leakage arising within the tunnel. Some recent
tunnels in Japan have used different solutions because of their seismic load-
ing. These are discussed later.
Design of gaskets
The Gina and Omega seals are often referred to as the primary and second-
ary seals. Some use this terminology simply to indicate which is installed
first and which second. However, some projects have used these terms to
designate which seal provides the primary barrier to watertightness and
which is secondary.
Historically, the initial seal—the Gina (or an equivalent)—was a tem-
porary measure only. The secondary seal would be the permanent seal,
which would be designed for the desired life of the tunnel. In this context,
the Omega seal is occasionally referred to as the primary seal as it is the
