Rock weathering and failure of the "Tomba della Sirena" in the Etruscan necropolis of Sovana (Italy)

Sovana, in the southern part of ancient Etruria (Tuscany, Central Italy) represents a centre of high historical and artistic values. Its Etruscan necropolis is particularly important, even compared to other centres of ancient Etruria, in view of the fact that all the major kinds of funerary architecture of the Tyrrhenian region are present: one of the best examples of Etruscan funerary work from the Hellenic period is represented by the “Tomba della Sirena”, a tomb dating from the III century B.C. The Tomb is a large niche type monument shaped like an arch hewn from a single rock mass, and on its façade there is a carving of a double tailed mermaid, symbolizing the Sea Goddess and the Otherworld. A block of about 2 m3 detached from the tomb façade in May 1999; the event determined a great emphasis on the problems of its restoring and its conservation. In order to examine the whole geological framework of the failure, a series of analyses, tests and studies have been carried out. First of all, the rock has been petrographically and geochemically characterised: based on the field and laboratory observations, the pyroclastic rock forming the “Tomba della Sirena” can be considered as belonging to the “Sovana Formation” in the Vulsinian complex (Roman Comagmatic Province). Afterwards, geomechanical investigations (inventory of the rock discontinuities, sampling and laboratory testing and back analysis) have been undertaken in order to understand the failure mechanisms and the preparatory and triggering factors. The pre-existing joint network and the intense weathering and loosening of the tuffs have been detected as the main causes of the failure. The multidisciplinary approach proved to be useful: in fact, besides giving a summary of the causes of the event, it helps to formulate convenient measures aimed to the restoration of the monument.