77
Honour Lectures /
Conférences honorifiques
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
activities stopped in 1981 the cracks and fissures continued to
widen.
The origin of the distress remains unclear so that further
geotechnical investigations and more extended studies are
necessary. The safety of the fissured masonry structures –
arches and vaults – is currently ensured by provisional and
removable props, but while it is increasingly difficult to obtain
public economic support to carry out research into the causes of
the on-going phenomena, the proposals of consolidating the
masonry walls of the basilica by means of important structural
measures are bound to increase.
The Pienza Cathedral
The Pienza Cathedral (Fig. 14) is perhaps less famous than the
Tower of Pisa, but it is just as problematic and intriguing.
Figure 14. Cathedral of Pienza and Piccolomini Palace from the square.
Perhaps there is no other monument that, in its lifetime, has
been subjected to so many consolidation and strengthening
measures as the Pienza Cathedral, because of the very slow, but
continuous settlements of the foundation soil underneath its
apse (Forlani Conti 1986).
Figure 15. The Piccolomini Palace and the Pienza Cathedral apse seen
from the rock scarp downhill.
In 1459 Enea Silvio Piccolomini, newly elected pope with the
name of Pius II, decided to raise the status of his birth town
with the construction of a Cathedral and some noble palaces.
Works for the construction of the new cathedral started in 1459
and were completed in only three years. In order to make sure
that the cathedral would be of appropriate proportions without
restricting the size of the main square, situated symbolically
between the Cathedral and the City Hall, the architect, Bernardo
Rossellino extended its layout beyond the walls of the village
and the sandstone scarp that delimits it towards the Orcia Valley
(Fig. 17): the apse, with its underlying crypt, had its foundations
downhill from the scarp where the level of the ground is about
15 m lower (Fig. 15).
The construction of the apse ran into considerable and
unexpected difficulties. In his memoirs the Pope wrote that the
foundation plane rested on rock masses crossed by large fissures
and that large arches were built across them to support the
foundations. Some fissures appeared in the walls of the apse
before the completion of the construction works, but Rossellino
attributed them to the setting of the mortar (Piccolomini 2008).
The church was inaugurated on 29 August 1462. New
cracks appeared soon after between the nave and the apse and in
the underlying crypt. Since then, for five centuries, there has
been an uninterrupted succession of instability phenomena and
consolidation works under the foundations; drifts and deep
drainage wells have been driven, reinforcement buttresses have
been built to uphold the apse, repairs and restructuring measures
have been adopted for the side walls, the crypt under the apse,
the vaults and the roof (Di Pasquale 1992).
All these measures were made necessary by the constant
lowering of the apse foundation downhill from the rock scarp:
there is proof that between 1520 and 1530 the floor of the apse
was al-ready lower than that of the nave by about 27
centimetres. A sudden settlement of about 0.3 m of the soil
downhill from the scarp occurred on the night of 26 November
1545 and caused the partial collapse of the apse and of the bell
tower. The event, described in the memoirs of a citizen of
Pienza is defined
Terrae motus
(literally a movement of the
earth), but there are doubts about it being an earthquake or a
sudden slope instability phenomenon, perhaps triggered by a
seismic quake. At present the overall difference in level of the
apse with respect to the nave is about one metre, as it can be
seen from the relative displacement of the cornice in Figure 16.
Figure 16. The cornice displacement shows the apse settlement.
Figure 17. Planimetric position of the Cathedral.
