3141
The preservation of Agrigento Cathedral
La conservation de la cathédral
e d’Agrigente
Valore C., Ziccarelli M.
DICAM - Dept. of Civil, Environmental, Aerospace and Materials Engineering
–
Università di Palermo
–
Italy
ABSTRACT: The Agrigento Cathedral was built about nine centuries ago on the edge of a steep slope made up of alternating inclined
layers of soft calcarenites and fine-grained soils; its behaviour has never been satisfactory especially as far as the North aisle is
concerned. Many corrective measures have been attempted during its lifespan and also a few years ago. They have been proved
invariably to be unsuccessful. Recent geotechnical investigations, referred to in the paper, permitted to find out that the main cause of
the Cathedral distress is a sliding mechanism involving the upper zone of the slope underlying the North aisle; consequently the
essential requirement for the preservation of the Cathedral is the stabilisation of the slope.
RÉSUMÉ: Le C
athédrale d’Agrigente
a été élevée voici environ neuf siècles sur le bord d'une pente raide, composée d'une alternance
de couches inclinées de calcarénites tendres et de sols à grain fin. Le comportement de la Cathédrale n'a jamais été satisfaisant en
particulier dans sa nef Nord. De nombreux remèdes ont été essayés au cours de sa vie et jusqu'à il y a quelques années. Ils se sont
révélés toujours sans succès. De récentes investigations géotechniques, décrites dans le papier, ont permis de découvrir que la
principale cause de la désordre de la Cathédrale est un mécanisme de glissement impliquant la zone supérieure de la pente située sous
la nef Nord. En conséquence, la condition essentielle pour la préservation de la Cathédrale est la stabilisation de la pente.
KEYWORDS: Agrigento Cathedral; preservation; slope stability; clay; calcarenite.
1 INTRODUCTION
The Girgenti (now Agrigento, Sicily) gothic Cathedral was
built in the XI century on the top of Agrigento Hill right on
the edge of a steep slope about 40m high and inclined from 40
up to 50° towards North, Fig.1. The Cathedral has been
profoundly changed during the XIV, XVI and XVII centuries
as a consequence of structural damages caused by differential
settlements of the foundations as well as by the horizontal
thrust exerted on the northern masonry wall by the barrel vault
which once covered the northern aisle. Other damages have
been caused by earthquakes. Baroque superfetations were
introduced during the XV century and were partially removed
at the beginning of the XX century. A massive still unfinished
bell tower, adjoining the church on the western side, was built
in the XV century. The plan of the Cathedral is shown in
Fig.1.
The building was originally smaller and located in the
area of the present apse (East side); later extension towards
the West required the construction of a rather high fill in order
to keep unaltered the floor elevation. The present elevation of
the nave floor is 327.54 m a.m.s.l.
The behaviour of the Cathedral has never been
satisfactory: it has undergone settlements, tilting, distorsions,
partial collapses. Many interventions, including partial
demolitions and reconstructions, have been carried out in the
last century. The most notable remedial measure was the
underpinning of the columns and the northern wall by means
of Fondedile “pali radice” (root piles) in 1976
-1980 (Lizzi,
1993). Almost all interventions as yet carried out have been
unsuccessful, and sometimes even self-defeating.
As a matter of fact, the fissuration pattern of the Cathedral
observed after each intervention was a replica of the previous
ones. The settlements and the horizontal outward movements
of the North aisle have never stopped. In 2010, just a few
years after the completion of the last stabilising works, the
Cathedral has to be closed. Recently, systematic in situ
geotechnical investigations and monitoring of ground
movements as well as a precise survey of the Cathedral by the
laser-scanner technique have been started. The main results of
these investigations and an outline of the causes of the
malfunctioning of the Cathedral are reported in the paper.
2 GEOTECHNICAL INVESTIGATIONS AND FIELD
INSTRUMENTATION
The Cathedral foundation soils have been explored on
many occasions, namely:
in 1904, when an exploratory shaft 24m deep was
excavated inside the Cathedral nearby its North -West
corner; the shaft was filled with mass concrete
intermingled with irregular calcarenite blocks;
in 1976-80, when many vertical borings about 15m
deep, were drilled inside and along the perimeter of
the Cathedral, on the occasion of underpinning works
by means of Fondedile root piles;
in 2005: 8 vertical borings (marked with an asterix in
Fig. 1) located on the area of the Cathedral parvis and
around the ex Diocesan museum were drilled; 2 open
standpipe piezometers and 5 inclinometer tubes were
installed in the boreholes.
Inclinometric
measurements have been carried out in 2005-2006
and subsequently restarted in June 2011 with a new
initial reading;
in 2008: 20 vertical borings located inside and around
the Cathedral were drilled (S12-S29 in Fig. 1); max
depth: 62m; 3 Casagrande, 4 open standpipe
piezometers and 6 inclinometer tubes were installed
in the boreholes. Inclinometric measurements started
only in June 2011;
in 2012: 30 vertical and 6 inclined borings (S201-
S227 in Fig.1) were drilled inside and around the
