81
Honour Lectures /
Conférences honorifiques
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Mesozoic formation (Fig. 25). The lines of larger V
p
gradient
obtained from the seismic reflection investigation (Fig. 26)
show the main units of the stratigraphic section and the probable
traces of the discontinuity surfaces (Brogi et al. 2003).
A likely hypothesis is that the sinking of a dihedral mass
between two convergent fault planes is made possible by a
horizontal displacement rate of the downhill zone too small to
be topographically detectable. While it is evident that the faults,
along whose surface the clay shear strength has a residual value,
and the sets of minor discontinuities have a critical influence on
the equilibrium conditions of the slope, their geometric
characteristics and the cleft water pressures (Calabresi and
Manfredini 1973, Sciotti and Calabresi 2004) have not yet been
sufficiently defined to get a convincing explanation of the
phenomenon.
The project of a deeper geostructural and geotechnical
research has been recently submitted to the study committee
recently charged of carrying out an updated analysis of the
Cathedral conditions, but its implementation has been delayed
by economic problems. However the fundamental question still
remains: assuming that the above explanation be correct, could
a geotechnical measure, such as a decrease of the piezometric
head, be designed to slow down the movement?
a)
b)
Figure 25. Seismic refraction tomography. a) The section trace;
b) P-wave velocity contours.
Figure 26. Wave P velocity gradients from the seismic reflection
measurements. 1. Remoulded superficial soil; 2. Limestone and
Sandstone (Pliocene); 3. Over-consolidated marly clays (Pliocene); 4, 5
carbonate-siliceous formations “Tuscan Series” (Mesozoic);
6 Anhydrite (Trias).
Consolidation measures of the Cathedral of an entirely different
approach, aimed at supporting the apse area with new reinforced
concrete structures hinged to the ground uphill from the fault,
are repeatedly being submitted by groups with different
opinions. Some structural solutions were presented and
discussed at a special conference (Mascardi 1992, Migliacci
1992), where however the concept of protecting the monument
and its history also from a material point of view, without
modifying its original design with inappropriate changes,
largely prevailed. Luckily the rates of subsidence and rotation of
the apse are very small and leave time for geotechnical
engineers to look for a possible soft solution. There is a hope
that they may win the challenge as it has happened for the
Tower of Pisa.
CONCLUSIONS
Geotechnics may offer a significant contribution to the
knowledge of ancient designs and construction techniques and
to the interpretation of the causes of instability. The effects of
deformations in foundation soils that occurred in ancient times,
or that are difficult to trace back to any specific cause, can often
be observed in ancient buildings. An ancient building or historic
site is interesting in and of itself for geotechnical engineers,
since it constitutes a monitoring instrument of the long term
behaviour of the soil that influences them.
The progress of geotechnical engineering and of the
specialized technologies offer the means to perform complex
and efficient interventions to save monuments, historic
buildings and old towns. However too often in the last decades
the new opportunities offered by this progress and the
cooperation of geotechnical engineers has been utilized
inappropriately by applying new deep foundations and
structural modifications, that overcome the ancient building
distress in a simplistic way, that ignores the history of the object
of the intervention, from its initial building to our time, and the
witness value of the technical solutions adopted by our
predecessors and of their expertise.
The great challenge is how to save monuments and historic
buildings together with the physical token of their conception,
their original construction techniques and their historic
modifications, that are tangible witnesses of the history of
mankind.
The problems posed by slow, continuous settlements induced
by deep seated deformations, which require long, in-depth and
expensive investigations, are among the most difficult to be
understood and explained. However the geotechnical engineers
should feel themselves engaged in exploiting their knowledge of
soil mechanics and applied geology to look for a way, if it
exists, to save monuments and historic sites by removing the
cause of distress and avoiding heavy structural interventions
that distort their substantial characters.
The cooperation of architects, historians, archaeologists,
structural and geotechnical engineers is the necessary
precondition for a respectful attitude towards conservation
problems.
In this context the geotechnical engineers have also the
opportunity of actively contributing to the knowledge of the
history of architecture and engineering, by following the
unforgettable example and the footsteps of our great colleague
Jean Kerisel.
ACKNOWLEDGEMENTS
The Author is very grateful to his colleagues of the Pienza
Scientific Committee Antonio Lazzarotto and Silvia Briccoli
Bati for their continuous, friendly collaboration and for their
help in gathering the relative documentation. The very precious
help of Dr. Manuela Cecconi in preparing the paper is gratefully
acknowledged.
