75
Honour Lectures /
Conférences honorifiques
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
A detailed history of the tower and the nearby Cathedral
(Labate 2009), their original design and subsequent
modifications, was obtained from the study of many archive
documents and was checked against the comparison of the
material and stylistic characteristics of the various masonry
levels of both buildings. In addition, on the basis of
archeological escavations made in 1913 (Sandonnini, 1983) and
more recent investigations (Labate, 2009), it was possible to
identify the position of the late medieval cathedral, the pre-
Lanfranco cathedral and the actual Lanfranco cathedral (Fig. 8).
Figure 9. Vertical sections of Ghirlandina tower: from the left, view
towards West, view towards North, view towards South, view towards
East (Lancellotta 2013).
Since the foundation soil has “memory” of the previous
loading history, this detailed reconstruction was the key to
explain the differential settlements, suffered by the cathedral
and in particular the tilt of its apse towards East and not only
towards the Ghirlandina tower.
Additional borings allowed to identify a detailed profile of
the soil upper layer and to find the remains of the ancient
Roman road Via Aemilia at a depth of about 7 m. By comparing
the different elevations of its pavement below the tower and
outside, it became possible to deduce the settlements of the
tower and the compressibility of its foundation soil. In order to
explore the stability equilibrium of the leaning tower (Cheney et
al. 1991, Di Tommaso et al. 2012) the inverted pendulum model
has been adopted. Its parameters were derived from the soil
investigations and from an experimental identification analysis
of the tower dynamic behaviour in the presence of ambient
vibration. The model parameters were chosen according to the
time histories of the tower vibration, collected by means of a set
of accelerometers at different heights; then a thorough analysis
of soil-structure interaction was carried out in order to get a
reliable estimate of the rotational stiffness and of the dynamic
response of the tower foundation. The results gave reason for
the good performance of the tower during the past seismic
events and showed that there is no need for underpinning
interventions. Furthermore it appeared that if the tower had been
underpinned on micropiles, following the dogmatic trend of 20-
30 years ago, the decrease of the fundamental period of the
structure would have increased its seismic vulnerability.
In this connection another emblematic and famous case is
the Cathedral of Mexico City (Ovando-Shelley et al. 1997,
Tamez et al. 1997, Santoyo and Ovando-Shelley 2000, Ovando-
Shelley and Santoyo 2001). Historic information made it
possible to identify the origin of the differential settlements of
the foundation soil, part of which had been consolidated by pre-
Columbian works, and to design sub-excavation and soil
consolidation measures to offset the differential settlements
(Fig. 10). On the other hand, the studies on foundations have
contributed to a thorough understanding of the historic events of
the Cathedral and of the surrounding area.
Figure 10. Underexcavation at the Cathedral of Mexico City (Santoyo
and Ovando-Shelley, 2000).
5 CRITICAL CASES
There is a long list of monumental buildings that, owing to the
slow or very slow displacements in the foundation planes, suffer
progressive instability. In these cases a conflict sets in between
the purely technological approach (aimed at reinstating the
safety of the monument with structural interventions which,
while ensuring that the external aspects are preserved, modify
the original structural design), and a softer approach, on the
other hand, that begins with a study of the phenomena
underlying the instability and makes a long and perhaps
uneventful search of the causes that need to be removed to stop
the instability and if possible save the monument without
substantial alterations so as to respect its historic integrity. It is
worth recalling that the search for the causes is always a time-
consuming exercise that is often much more expensive than
ordinary, obvious structural and geotechnical engineering
interventions. A systematic study of the saving projects carried
out in Italy until 1995, including buildings of different kinds
(Table 1), has shown that pure underpinning by micropiles was
the largely predominant type of measure (Fig. 11) which in
many cases was probably unnecessary or unsuited.
