[322/03] Francke - Ingenia

INFRASTRUCTURE
ingenia
26
Figure 4: Aerial view of tunnel section being towed to site
The immersed tunnel was
constructed from twenty prefabricated
concrete tunnel elements. The elements
were cast at a purpose-built factory 12
km north of the tunnel site. Each
element was towed to the tunnel site by
four tug boats and lowered into the
tunnel trench with enormous precision,
using GPS satellite navigation.
The fact that casting took place
under cover allowed for efficient and
smooth production, regardless of the
weather. As a positive side effect the
number of work-related accidents was
subsequently reduced.
Each tunnel element is 176 metres
long, 49 metres wide, approximately 9
metres high and weighs around 57,000
tonnes. Each element was cast in eight
sections at the tunnel factory each of
which was cast in one 30 hour cycle.
Each section was cast against the
previous section and the element was
then gradually pushed out of the
casting hall and on to a ramp, in what
was, in effect, a giant lock system.
When all eight sections were cast and
one complete 176 metre element was
ready, a sliding gate was closed behind
the element, and the basin was flooded
until the element floated.
The element could then be pulled
into the deep end of the basin to await
being towed to the tunnel site. After the
water inside the lock had reached sea
level allowing the element to be towed
out, the next two elements would be
cast. Two parallel production lines
allowed for maximum output and
efficiency.
The towing of the elements
demanded great skill and precision. The
57,000 tonne element was almost
completely immersed during towing
and the changing currents in Øresund
had to be calculated in great detail
before the towing-out. This operation
took place in the Drogden Channel in
Western Øresund, which is an
international shipping route used by
some 40,000 ships per year. Early in
the construction phase a radar station
was established to monitor shipping
and guide vessels through the work
areas thus avoiding any major
accidents. During operations only a
handful of collisions and minor incidents
took place involving work vessels and
shipping, causing material damage only.
Not once was a tunnel element
damaged due to a collision. However,
on 4 August 1998, the accident that
everybody was dreading took place: a
tunnel element was flooded while being
positioned in the tunnel trench. Before
tow-out the tunnel tubes in the element
were sealed off with steel bulkheads.
Due to a number of mistakes one
bulkhead was not properly secured and
succumbed to the water pressure. In a
matter of seconds all five tunnel tubes
in the large concrete element filled with
water, and in accordance with
procedures the element was
immediately lowered into the trench,
but several metres out of position. It
took Øresund Tunnel Contractors eight
weeks to inspect, repair and eventually
move the tunnel element into its correct
position.
During this period the lowering gear
and platforms were attached to the
flooded element so the contractor was
unable to save time simply by building
the tunnel from both sides at the same
time. However the tunnel factory
continued to produce the tunnel
elements and once the flooded tunnel
element had been repaired and placed
in the correct position, the tow-out and
construction of the rest of the tunnel
elements was carried out at high
speed. As a result, despite this major
setback, the tunnel was constructed
ahead of schedule.
The Bridge
For the bridge section, which connects
the artificial island with the Swedish
coast at Lernacken, a cable-stay
design was chosen for the high bridge
due to the weight of the combined
railway and motorway link. The entire
bridge is 7.8 km long of which the high
bridge accounts for 1,092 metres. The