Civil Engineering Reference
In-Depth Information
Whichever system is used, the approach ramps may need to be designed
for different conditions through the construction period. When the
approach ramps are dewatered to enable construction of the permanent
structure, there will be no uplift from the groundwater, so the structures
will be inclined to settle. If the underlying ground is soft material, the foun-
dation will need to be designed to prevent unacceptable levels of settlement
occurring before the dewatering is switched off and the uplift pressures
become active. Therefore, it is common for piled foundations to the ramp
structures to be designed for a short-term compression load regime and a
long-term tension load regime.
An innovative solution was used for the Jack Lynch Tunnel in Cork, Ireland,
in the mid-1990s, where the contractor chose to float in one of the approach
structures in the same manner as an immersed tunnel element. There was
no cut and cover tunnel, just a floated U-shaped ramp element, which tran-
sitioned directly to the immersed tunnel. This had particular advantages for
this site as it avoided the need for extensive temporary earthworks and the
need to install a potentially difficult temporary groundwater cutoff solution
that was high risk and not guaranteed to be successful because of the under-
lying voided limestone. This has not been done elsewhere, and it is really
a contingency solution where risks are otherwise high. It is unlikely to be
adopted as a routine construction method for immersed tunnels.
There were a great many design and construction challenges that the
design and build contractor had to consider. Because of the function the
ramp serves, it is a naturally tapering structure and therefore would not float
in the same way as a conventional tunnel element. In addition, it had no roof
to mount equipment on, so it was a floating trough structure, referred to as a
boat unit. Although unusual, this was overcome and the contractor devised
a method to float, transport, and immerse the boat unit. This included
some temporary wall extensions, to ensure there was a freeboard achieved,
and additional ballast boxes externally to the main U-shaped structure, to
enable sufficient weight to be achieved after immersion such that the mini-
mum factors of safety against uplift were met. As the boat element was to be
immersed to the highest point on the tunnel alignment, the immersion oper-
ation was carefully timed to utilize high tides and enable the element to be
placed. The boat was immersed against a small section of in situ ramp. The
normal immersion philosophy was followed, and a rubber gasket was used
to achieve a watertight seal between the boat unit and the slab. Because of
the low water pressures, a softer natural rubber gasket was used in place of
the usual Gina SBR rubber gasket. Once placed and ballasted, the first tun-
nel element could then be immersed against the boat unit in the normal way.
The design of the boat structure required some specific measures. In par-
ticular, because the U-box was quite wide and subject to uplift water pres-
sures, the base slab would tend to hog, causing the structure to flex and
open out. To prevent this, the base slab required a high level of stiffness
