Civil Engineering Reference
In-Depth Information
3. Check that the minimum compression—taking into account the
opening of the joint due to tolerances, realignment, temperature, and
shrinkage—does not result in a compression in the flat portion of
the force-compression curve. This would be indicative of the gasket
relying on the soft nose for sealing, rather than on the body of the
gasket.
4. From the minimum compression value, deduct an equivalent com-
pression that corresponds to the relaxation of the rubber material
over its life (typically 45%-50% of its elasticity).
5. For the corresponding sealing pressure, check it exceeds the external
water pressure by the required safety factor, with all effects taken into
account.
It should be noted that once the immersion process is complete, the
geometry of the joint gap is more or less fixed. Subsequent movements can
occur due to settlement, shrinkage, and temperature effects, but the creep/
relaxation behavior of the rubber materials will not cause a change in the
joint gap, but simply a change in the sealing pressure.
The season during which immersion takes place can have an effect on
the subsequent positive and negative temperature range that should be con-
sidered when assessing the sealing behavior and this should be taken into
account in design. This is covered in more detail in Chapter 9.
For the condition during immersion where the tunnel element is first
drawn up to the preceding tunnel element, the sealing behavior under low
compression forces must be considered. At this stage, the joint has not been
dewatered, and so, the sealing pressure is applied by winching or by a pull-
ing jack. The force-compression characteristics of the seal at these low
compression forces can be used to determine the winch cable loads or the
size of pulling jack required to achieve the initial seal.
The Omega seal is fixed in place shortly after the immersion process. A
suitable seal should be selected to accommodate the predicted movement
range at the immersion joint over the life of the tunnel. Movements along
the axis of the tunnel will occur due to temperature change, shrinkage,
and creep, resulting in the opening or closing of the immersion joint and
the corresponding stretching or compressing of the Omega. In addition,
some opening or closing may occur due to settlement effects. A degree of
shear movement at the immersion joint may occur before shear keys are
introduced and the relative positions of adjacent tunnel elements are fixed.
The selected seal must be able to deal with both the elongation and shear
movements combined.
To provide an effective seal, the Omega must be compressed against
the steel end frame. The flange of the Omega is clamped to the end frame
by a bar arrangement and the clamping bolt is torqued until a pressure
is reached that gives a factor of safety of 2.5 against the acting external
