Fig. 2. Construction process of the arch
3.
CONSTRUCTION PROCESS
3.1.
GENERAL DESCRIPTION
OF THE CONSTRUCTION PROCESS
Initially all the efforts of the construction team were
focussed on how to get a good access to the springing
points of the arch and also to the foundations
of every pier of the approach viaducts. To that end,
two haul roads were designed throughout
the hillsides of both banks of the river, limiting
the minimum radius and the maximum slope allowing
the access to the springing points of the arch for huge
trucks. The implementation of these access roads
involved a challenging labour of earthworks
and cutting slopes supports.
Once the access roads were finished, the foundations
of the piers and the arch could start.
All of the foundations have been achieved
with shallow foundations on bedrock. In order
to carry out the excavation of the foundations, slope
supports have been necessary. Once the foundations
were built the implementation of the piers could
begin. Built in 40 MPa characteristic resistance
concrete, they were executed in lifts of 5.0 m long
using climbing formwork. The steel reinforcement
is assembled on the ground and lifted with cranes.
The deck of the approaching spans started once
the piers were finished. South and north spans were
built almost simultaneously with two Overhead
Movable Scaffolding Systems (Overhead MSS). When
the last span on each side were finished (the one
supported by the main pier and the prior), the MSS
went back and waited until the arch were finished,
because they were liable for the execution of the deck
over the arch.
In parallel, the arch started to grow on each side.
First of all, segments 1 & 2 were built by means
of a conventional falsework of high loading capacity
(due to the huge depth and weight of these
segments), working perpendicular to the axle
of the arch in the beginning. Once the segments
1 & 2 were built, the traveller formworks were
assembled and anchored to these segments in order
to begin the construction of the arch by means
of the successive cantilever construction method,
temporarily supported by cable stays. The arch is
divided into 32 segments each side, with approximate
length of 6.70 m each segment and the key central
segment. This construction process is typical of cable
stayed bridges. The 8 firs t families of stay cables were
anchored to the main concrete piers while
the 18 remaining families were anchored
to a temporary steel tower specifically erected over
the deck, just above the main piers, in order to get
the necessary height for an optimum attack angle
of the stays. For the purpose of avoiding longitudinal
movements in the temporary tower due
to rheological and thermal effects on the deck that
could have had damaging effects on the execution
of the cantilever, a fixed point between deck and
main piers was materialised. From segment
3 to segment 15, a couple of form travellers were
used in each side to build the arch. Once both form
travellers reach the single section part of the arch
they change their configuration and turn into a single
form traveller to finish the arch.
When the 32 segments per side were built, and
the form travellers of one side were disassembled,
only closure of the arch was left in what we call key
segment. This segment was built by means
of the formwork travellers of the North side,
anchored at the end of the cantilever of both sides.
4/2016