Before casting the key segment it was necessary
to block one cantilever to the other in order to avoid
differential movements between both sides due
to thermal effects on stays and arch that could crack
the concrete. For this purpose 4 struts were disposed
anchoring both cantilevers. Once the arch was closed,
the temporary stays were no longer necessary
so the cable stays were loosened and disassembled in
order to begin the construction of the 8 piers over
the arch.
While the piers over the arch were growing,
the overhead gantries were assembled again and
were moved forward to the main piers in order to
start the construction of the deck over the arch. Once
the piers were finished, the construction of the first
span per each side could start. It was necessary to
resolve the clashes between the two MSS in
the construction of the three central spans
of the deck. As mentioned, the central span is
anchored to the arch in the central 17 metres,
achieving the final configuration of the fixed point
of the viaduct, and able to transmit all
the longitudinal forces of the deck, through the arch
to its foundation. In this stage it was necessary to
release the provisional fix point in the main piers.
3.2.
ARCH AND RETAINING FOUNDATIONS
3.2.1. Arch Foundations
The foundations of the arch are also foundations to
the main piers (P6 & P15) and were achieved
with shallow foundations over bedrock, which is slate,
with concrete volumes between 6,000 and 7,000 m3.
The main support plane of the foundations is
orthogonal to the arch axis in its beginning and
the upper side is horizontal in order to support
the main pier, which is also the tallest. Before starting
the construction of the foundation and after
the excavation was done, a systematic ground
treatment needed to be made, in order to fill the gap
between the schistosity planes of the slate, which are
usually filled with clay. These gaps filled with clay
could result in movements in the structure when all
the loads were applied, so they had to be filled
with an appropriate material.
Due to the foundations being of such a large size,
the pouring of the piece had to be divided in several
phases, adjusting approximately the volume of each
phase to 1,000 m3 and assuring the shear capacity
of the layers between concrete surfaces.
Fig. 3. Concrete performance phases of the arch
foundations
3.2.2. Retaining Foundations
The retaining foundations are the two adjacent
foundations to the arch foundations. The main
purpose of these foundations, besides of being
the foundation of a pier, is to anchor the stays
to the bedrock, avoiding movements in the structure
during the construction of the cantilever. As well as
the arch foundations, the ground had to be treated
with cement injections under pressure, because
of the huge effort during construction that
the foundation was going to support in order to avoid
movements in the structure.
For this purpose 60 ground anchors of 12Ф0.6” were
made in each one of the four retaining foundations,
with a fix part grouted to the ground of a length
of 16 m, and a free length between 6 and 10 m
depending on the anchor. These anchors were
stressed to 2,000 KN. The drilling was made after
the implementation of the foundations, by means
of several pipes embedded in the concrete with
the accurate angle and position. On the other hand,
the stays were anchored to the foundation by means
of Macalloy bars (high tensile alloy steel bars) housed
previously in the foundation.
3.3.
MAIN PIERS
For the construction of the piers of the viaduct,
a climbing modular formwork specially designed for
the variable geometry of the piers of Almonte viaduct
has been used.
4/2016