Safeguarding historic towns on hilltops threatened by land sliding: the case of San Fratello in Sicily Camillo Airò Farulla,* Fabio Cafiso,** Federico Calvi,*** Marco Rosone* Summary The paper presents the case history of San Fratello, a small town in the Nebrodi Mountains in Sicily rich in history and having very peculiar environmental and cultural characteristics. Both slopes of the narrow ridge where San Fratello lies were affected in 1754, 1922, 1986 and 2010 by very large landslides. The landslide of February 2010 is largely the reactivation of that of 1754. It destroyed many infrastructures and seriously damaged more than 200 buildings and caused the evacuation of 25% of the population. Emergen- cy actions to block regression of uphill movements and secure the buildings and their inhabitants as quickly as possible, were carried out. The sliding slope down to the town was left to its natural evolution. The paper presents the results of the geomorphological and geotechnical analyses recently carried out to understand the whole landslide reactivation mechanisms. It is felt necessary to define an advanced geotechnical model of the landslide for quantitative evaluation of hazard and rational landslide risk management in the area. 1. Introduction tens of metres within a few hours or days, causing serious damage to the towns and the infrastructures In Italy many towns of historical and artistic im- involved. The causes that determine reactivation or portance were placed on the tops of hills for defence “sudden” acceleration of the displacements can be of or climatic reasons, and are usually difficult to ac- various origins such as, for example, earthquakes, cess [CROCE, 1985]. Many of them are now affected changes in climatic conditions or natural actions de- by very extensive landslides, due mainly to the mor- riving from long-term tectonic evolution. phological evolution of slopes and changes in climat- However, alongside the triggering factors, also ic and environmental conditions. the role of predisposing factors, according to Ter- From this point of view many relevant cases can zaghi’s approach to the analysis of landsliding pro- be found in Southern Italy and Sicily, where soils are cesses are very important [TERZAGHI, 1950]. In t he very heterogeneous and intensely tectonized. These case of landslides involving stiff and highly tec- soils, known as “structurally complex”, in Italy have tonized clays, the predisposing factors are intense been the object of experimental research aiming fissuring and inclusion of fractured rock blocks. In- at the study of their mechanical characteristics for tense fissuring negatively affects the resistance of evaluation of the stability and the behaviour of the clays [PICARELLI et al., 2000 and 2003; VITONE et al., Civil Engineering works interacting with them [AGI, 2011] and increases their permeability when discon- 1997; AGI, 1979]. tinuities open up because of unloading. Slopes, where intensely tectonized clay forma- The inclusions of extensive blocks and layers of tions outcrop, are often affected by very extensive more or less fractured rocks can strongly affect seep- landslides (surfaces of one or more km2) with medi- age in the slope and the intensity of pore water pres- um to high sliding surface depth. They usually move sure. In this connection, when, due to accumulated at a slow or very slow rate or are quiescent. Such land- rainfall, the system of rock discontinuities becomes slides, nevertheless, can be reactivated and/or accel- saturated, the inclusions themselves act as aquifers, erate considerably, accumulating displacements of causing high pore water pressures at great depth. These characteristics create the premises for the onset of land sliding processes that determine the * Department of Civil, Environmental, Aerospace, Materials breakup of huge bodies of tectonized clays along Engineering (DICAM), University of Palermo, Italy deep surfaces. ** Civil Engineer - PRO-GEO Progettazione Geotecnica, Palermo, Italy In the study of landslides in clays with a complex *** Geologist – Sicilian Regional Department of the structure in Southern Italy, in the last few years sig- Environment, Soil Defense Service nificant progress has been made with the develop- RIVISTA ITALIANA DI GEOTECNICA 1/2015 8 AIRÒ FARULLA - CAFISO - CALVI - ROSONE ment of research regarding characterization of the ecute the stabilization works. Emergency safeguard- factors which determine landslides and the mecha- ing measures were taken soon after the landslide oc- nisms controlling their phenomenological manifes- curred in order to block uphill regression of move- tations [PICARELLI et al., 2000]. The results obtained ments and secure the buildings and their inhabitants emphasise the complexity of the stress-strain process- as swiftly as possible. es developed inside these slopes because of the varia- The paper also reports the results of accurate geo- bility of the mechanical behaviour of the various lith- morphologic surveys recently carried out in order to ological types and because of the complex tectonic achieve a comprehensive layout of movement evolu- history, which has determined their present mor- tion in the entire slope involved in the February, 14th phology. Nevertheless, it is possible to recognize and 2010 landslide. The need for new studies, investiga- quantify some of the main characteristics of the me- tions and monitoring programmes to be implement- chanical processes involved [COTECCHIA et al., 2014], ed in order to undertake rational risk management through the application of purposely devised meth- of the San Fratello territory is discussed. odologies and with the help of different specialistic competences. The results of these analyses prove very impor- 2. History and physical environment of San tant for the fundamental knowledge that they can Fratello give and for the prospects that they open up in the field of applications for quantitative evaluation of San Fratello is a small town in the Nebrodi landslide hazard and rational management of asso- Mountains on the northern coast of Sicily, about ciated risk. 700 metres above sea level. It is situated on the top Some of these problems are discussed with ref- of a narrow saddle in a commanding position for erence to the case of the historic town of San Fratel- the control of communication between Palermo lo in Sicily, seriously damaged by a large landslide and Messina (Fig. 1). For this reason, it was inhab- that occurred on February 14th, 2010 and caused the ited during the Greek and Roman ages, when it was collapse of numerous buildings. After a short pres- named Apollonia. Destroyed during the Arab domi- entation of the history of San Fratello and the physi- nation of Sicily, it was rebuilt in the 11th century by a cal characteristics of its site, the paper illustrates the group of soldiers from the North of Italy and France results of the analyses carried out to design and ex- who conquered Sicily. They were led by the Norman Fig. 1 – View of San Fratello. Fig. 1 – Veduta di San Fratello. RIVISTA ITALIANA DI GEOTECNICA SAFEGUARDING HISTORIC TOWNS ON HILLTOPS THREATENED BY LAND SLIDING: THE CASE OF SAN FRATELLO IN SICILY 9 Count Roger. The Normans built a fortress at San coming back again. Because of their origin and tra- Fratello, whose defence was entrusted to Longobard ditions, the inhabitants of San Fratello speak a dia- and French mercenary troops. lect of Gallic origin, for which they were considered Between the 11th and 12th Century near the for- foreigners by the Sicilian people and named “the tress a village developed for the settlement of the French”. They testify the passage of several domina- families of garrison soldiers. In that time a wonder- tions and civilizations in Sicily. ful church was built in Byzantine style on the top of At present, San Fratello is the only village in a Monte Vecchio, very near to the archaeological site very wide territory that extends from the coast of the where the ruins of the Greek-Roman town were dis- Tyrrhenian sea to the Alcantara river valley in front covered. The Middle Age town-planning survives al- of Mount Etna and includes the Nebrodi mountains. most intact in the old centre of San Fratello charac- It is in the middle of large wood and it is the cen- terized by picturesque paths of narrow streets and tre of an environmental and ecological system that archways. In the 16th and 17th Centuries San Fratel- could be susceptible to important developments. In lo was enriched with notable monuments. Among fact one of the principal economic activities of San them the elegant cathedral, of which a wall, part of Fratello is the breeding of free grazing horses, cat- the floor and some columns still survive after the tle, oxen, sheep, goats and swine which supply meat devastating landslide of 1922. San Fratello is famous and cheese. in Sicily for its traditions and cultural heritage. One In particular, San Fratello is the principal centre of the most singular and characteristic of these is for the breed of a horse, just named “of San Fratel- the popular annual representation during the Holy lo”, which the tradition believes to go back to the Week. In the three days from Wednesday to Friday, Normans or, going further back in the time, to the hundreds of people go through the town streets and Siculi the ancient inhabitants of this part of Sicily. squares wearing gaudy coloured and arabesque pat- The economy of the village and surrounding territo- terned dresses. With their heads and faces covered ry is underdeveloped and suffers of an endemic lack by grotesque masks, they make fun of the passers-by of financial support. However, if suitably organized and sound characteristic trumpets, playing tradition- and financially supported it could became the nucle- al music. Then on the Holy Friday afternoon in a us of a new economy based on the protection of the blank silence they follow the religious procession environment and the development of cultural tour- behind a precious 16th century statue of the Cruci- ism and breeding of selected animals.