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HYPOTESIS ABOUT THE ANCIENT CONSTRUCTIVE AND ASSEMBLING
TECHNIQUES
Building a
stone bridge, more then 400 years ago, was really a feat and most probably the
highest difficulties of the time were the following:
§
structural
and architectural design was not determined trough a knowledge of the
theoretical principles related to stability and resistance but trough the
knowledge of the causes of collapses and effects of loads: therefore
anything new would have meant something unforeseeable;
§
the
building of a wooden false work, (centering),
that could bear the huge load of the stone blocks with limited settlings
has always been one of the most remarkable issues of large ancient stone
structures;
§
transporting
and lifting heavy stone blocks was a technological issue that was strictly
linked to the times of performing the works which had to be controlled to
avoid collapsing risks related to the season variations of the river water
level.
Unlikely, no
historical document is yet available about the tale of the bridge yard,
with organisation notes, constructive methods, difficulties found:
therefore, what above mentioned, is mainly an
hypothesis based on similar cases.
But what
follows is more scientifically proved by the architectural analysis held on
the ancient bridge surveys.
The centering was most probably a wooden one; nothing is
known about the exact configuration of it, apart from the fact that some
recurrent discontinuities have been found in the arch intrados, which may have
been caused by different sectors of the arch false work, (centering - scaffolding); these sectors may have been
defined by the length of the master beams used for the structure.
was quite
irregular towards the top, most probably due to the mentioned centering anomalous settlings. This increasing
inaccuracy has been recovered wholly in the three top rows at the key stone
level, where assembling has been performed regardless of the previously
surveyed criteria, and even a variation of cm 11 of the intrados size has
been checked in only one row.
Most probably
the arch top rows of voussoirs are the prove that the two teams of workers were not in
contact and not co-ordinated in the carrying-on of their works. It may be
possible, also, that something was going wrong with the centering
and that final rows have been quickly assembled to stop the gradual
settling of the vault. We shouldn't forget that the vault was about 145 m3
of stone which weighted almost 300 tons over a wooden temporary structure.
TECHNICAL DESCRIPTION OF PROCEDURES FOR AMENDMENT OF THE
WALL FROM THE LEVEL OF ABUTMENT BOTTOM TO THE LEVEL OF CORNICE
INTRODUCTION
The assumptions of work performance
on amending the wall from the level of abutment bottom to the level of
cornice, which are the subject of this project, are:
·
quality performed works on amending the wall beneath the
level of abutment bottom
·
excavation and archaeological works inside the abutments
of the bridge until the level of injected mass appearance (assumption is
that injected mass appears on the cornice level on both sides of the
bridge)
Works on amending the wall in
mentioned segment are composed of amendments of abutments (injecting the
wall through joints, replacement of damaged stones on the wall surface and
re-pointing), and injecting the wall.
Stone walls reparation
approximately 1.0 m into zone of injection works
already done in the section of foundation.
Amendment of wall
Amendment of wall is done by
injection and placing the constructive reinforcement into drills. The main
purpose of injection procedure is filling the possible smaller cavities and
cracks on joints between the layers.
Previously done injections in the
mentioned wall in section of foundation have shown small consumption of
mixture (respectively small penetration into the wall) in between the weak
and strong connected layers of conglomerate. Only in unconnected layers of
conglomerate the consumption of mixture was somewhat larger. Appearance of
unconnected
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It is most
likely that the ancient centering was not strong
enough to bear the loads of the stone blocks without settlings: this may be
proved by the fact that, by comparing north and south elevation, it is
possible to note that next to the springers the
rows were much more regular and well built, with little variations of
levels. While, proceeding to the key stones, irregularities increase: rows
are proceeding not parallel, levels are changing
from north to south in a range of about 10 centimetres. Presumably, the
more the centering was loaded, the more it
underwent to unforeseen settlings that were more remarkable by the south
side, as it can be observed with the numerical analysis of the
co-ordinates. To recover the settlings, it is possible that wooden wedges
were used, of which there are traces in many spots of the intrados surface
of the vault, (some steps between adjacent voussoirs may be surveyed even in the assembled blocks
recovered from the river). The use of wooden wedges was probably performed
with very small ones next to the springers in
order to adjust the voussoirs and to match their
joints and was performed with bigger ones towards the top, to correct the
geometry following the planned design shape.
Even if arch
stones were of remarkably different dimensions, the preparing and
assembling procedures were not randomly performed,and what is even more important to stress, the
work over the centering was most likely to be
quite limited to the assembling: in other words most of the work like stone
cut and carvings of slots were performed off-site. The above statement may
be proved by the following observations:
§
positioning
of voussoir joints was, as in ordinary masonry
works, accurately shifted to guarantee an efficient interconnection of the
stone blocks;
§
positioning
of cramps and dowels, and related slots and channels, required the exact
knowledge of the stone dimensions and of their joints positions of adjacent
rows of voussoirs to avoid interference between
metal strengthening devices and joints;
§
stone voussoirs were quite
variable in the vault, but the ones belonging to the same row were of very
close transversal base dimensions: average variation range cm 0.5-2 in a
length of almost 4 metres, which is an accuracy quite higher than the
average followed for all the other parameters.
From the above
observations it is possible to deduce that, at the time, they were using
different stone voussoirs for the bridge vault of
different size and shapes, (due to the natural availability of the quarry,
where it seems that some natural weakness veins compelled to limit the
dimensions), but each row of the vault was accurately pre-selected and
picked from a temporary deposit of rough blocks. Each row was composed of voussoirs of very close intrados dimensions, and was
prepared next to the preceding one in regard of the joint positioning and
of the metal elements positioning. Dowels were previously assembled
off-site and related slots were prepared. This procedure was most likely to
be performed on groups of rows and not on couples to avoid that the
assembling could be stopped by the lack of prepared arch rows. Over the centering it wouldn't have been possible to manage all
the vault requirements unless an efficient communication
of dimensions were performed between the working teams over the centering and the working team off-site.
Despite the
accuracy concerning the transversal thickness of arch rows was accurately
performed, the raising of the rows
THE WALL
The wall in the mentioned zone of
this project, according to geotechnical research works, is composed of
layers of firmly connected conglomerate alternately with layers of weakly
connected conglomerate, on both banks. Layers are approximately horizontal
with irregular changes of layer thickness.
That kind of structure is also
seen on upstream and downstream parts of the bank.
AMENDMENT OF THE ABUTMENTS ON THE
MENTIONED LEVEL
Amendment of the abutments is
done by injecting the wall through joints, replacing damaged stones on the
wall surface, re-pointing, as well as re-building on the surface where the
stones are missing. The purpose of injecting is better solidity of the
wall, decreasing the amount of voids in the wall, and accomplishing better
touch with the wall. Minimizing the voids in the wall is necessary for
making the wall less permeable in order to stop the leaking of grouting
mixture when injecting behind the wall.
The works will be done according to following schedule:
·
Cleaning the joints.
·
Replacement of the damaged parts of the stone
·
Re-building on the places where the stone is missing
·
Closing the joints with lime mortar that has the colour
and content according to demands of the conservator.
·
After consolidation of mortar in joints, drilling of
injecting drills θ 25 mm needs to be done. 5 drills on 1 m2 of the wall
surface are foreseen. Injecting the wall will be done only from one side so
the length of the drill is foreseen in the wall thickness. According to
researches, the wall thickness of the mentioned segment is 60 80 cm.
·
Injecting is done with factory-made mortar Calx Romana under the
pressure of 1 bar.
·
Control of performed work is done after completed
consolidation of grouting mixture. Two procedures are suggested: either
ulterior injection of a specific zone where additional filling of grouting
mixture can not be more then 10% of previously determined quantity that was
injected, either by disclosure of few square meters of the wall surface in
order to visually determine penetration of grouting mixture.
Reinforcement is placed
constructively for strengthening cracks between approximately horizontal
layers (drills are vertical on layers).
Injecting will be done with
downward method. Approximately one drill on each 1.5 x 1.5 m of surface is
foreseen, but the real schedule of drills will be adjusted according to the
existing situation of the walls within the surface of abutments. The
amendment includes the zone of 6.0 m from the bridge foot, which is
characterized as a zone of possible influence of the bridge load. Schedule of drills on the right bank shown
in supplements is conditional because the archaeological researches and
excavations to the level of cornice are not done yet. Some drills are done slantingly with the purpose of penetration into the
wall beneath the walls of larger thickness.
On the right bank vertical drill
6.5 m length is foreseen, and on the left bank 8.5 m drill. Length is
determined as a distance from cornice level to the depth of penetration
conglomerate (breca)
layers is not expected in the mentioned zone, according to geotechnical
researches.
Control of performed work
Control of performed work on
amending the abutments will be done with method of ulterior injecting or
with disclosure of injected part of the wall.
Conducting control is foreseen on
three places on each abutment wall.
The control of performed work on
improving the wall will be done by taking out cylindrical testing samples
by ulterior drillings. Vertical drilling on three places on each bank is
foreseen. The locations of performing control works will be determined by
supervising engineer.
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