Civil Engineering Reference
In-Depth Information
Figure 14.16 Primary support beams at sides of tunnel with pulling jack, center.
(Photo cour-
tesy of NRA/DirectRoute.)
Given the low density of the sand foundation, it is generally better to keep
the temporary supports active until the sand foundation is placed under the
subsequent tunnel element. This minimizes the risk of differential settlement
occurring due to short-term increased loading at one end of the element.
The ram and jack arrangement is shown in Figure 14.17. The ram is
housed within a watertight sleeve, which is cast into the concrete. The
components are carefully machined to ensure a tight fit and the sleeve/
ram interface features O-ring seals designed to ensure watertightness under
high water pressure. The sleeve is anchored back into the concrete to resist
the acting water pressures.
The temporary support assemblies should ideally be located as close to
the walls of the tunnel element as possible to enable loads to be transferred
directly into the walls of the structure. A number of tunnels have achieved this
by housing the assembly within a recess in the external walls (Figure 14.18).
If this is not possible, the jack can be housed within a frame that is tied into
the base slab of the tunnel. In this instance, the design of the tunnel base slab
needs to account for punching shear effects. Consideration has been given to
placing them in toes on the outside of the element. This simplifies the internal
construction, but their operation is more complicated as it requires activation
by divers, and this option has so far not been pursued.
The assembly normally features a packing piece between the ram and
the jack to enable sufficient extension of the ram beneath the base slab to
achieve the required height above the trench in which the foundation layer
will be placed. Once the temporary supports are released, the jack and
