The landscape of Tuscia, i.e. the area extending between Latium, Toscana and Umbria, features wide volcanic plateaux overlying clayey-sandy-conglomeratic sedimentary deposits. The volcanic deposits identified in this area originate from different stages of eruption of three volcanic districts: Cimino (Sabatini, 1912; Nicoletti, 1969; Micheluccini et alii, 1971; Sollevanti, 1983); Vico (Locardi, 1965; Mattias & Ventriglia, 1970; Borghetti et alii, 1981; Sollevanti, 1983; Fornaseri, 1985; Bertagnini & Sbrana, 1986; Laurenzi & Villa, 1987; Perini et alii, 1997; Barberi et alii, 1994); and Sabatini (Alvarez, 1973; Corda et alii, 1978; Fornaseri, 1985; Cioni et alii, 1993; De Rita et alii, 1993; Campobasso et alii, 1994; Karner & Renne, 1998; Karner et alii, 2001). The sedimentary deposits, of lower Pliocene to Quaternary age, were emplaced into the Chiani-Tiber river basin (Barberi et alii, 1994; Mancini et alii, 2001; Girotti & Mancini, 2003). Numerous villages and towns of high historical-scenic value, such as Orte, Bassano in Teverina, Bomarzo, Chia, Faleria, Civita Castellana, Castel S. Elia and Calcata--which are the focus of this paper--are situated on the above plateaux. The morphological evolution of the investigated area makes it prone to slope instability processes (Manfredini et alii, 1980; Lembo Fazio & Ribacchi, 1991). This paper describes the findings from a geological-engineering study of the above towns, supported by facies analysis of the deposits ascribable to eruptive units. Based on the above findings, the paper discusses the impact of the factors bearing on the morphological evolution, thus on the local slope instabilities. Particular emphasis was placed on the geomechanical characterisation of the jointed rock masses lying at the top of the local hills and on the construction of geological-engineering sections representing the observed instability conditions. The latter sections made it possible to also assess the influence of geological and geometrical factors affecting the tuff plateau/sedimentary bedrock interactions, such as thickness, size and isolation from nearby portions of the rock mass. The assumptions on the influence of the specific factors identified, adopted in the reference geological-engineering models, were validated by stress-strain analyses conducted via FEM and FDM. The analyses, which relied on a critical interpretation of the reference geological-engineering models, enabled assessment of the role that the various susceptibility factors play in instability processes. In summary, the results from the study infer that jointing conditions have a crucial impact on the evolution of the cliffs of Calcata, Castel S. Elia and Civita Castellana, whereas at Faleria, Orte, Bassano in Teverina, Bomarzo and Chia, the dominant controlling factor is the stress/strain interaction induced by deformability contrasts. It is worth emphasising that, in the latter case, a significant role is also played by the thickness/width ratio of the tuff plate, as well as by its isolation or connection to the nearby main plateau. The above-mentioned morphometric conditions are involved in two main styles of stress evolution: i) stress release, i.e. lateral stress relaxation of the tuffaceous plates, due to the cutting of the cliffs by streams and to their subsequent evolution as a result of gravity; or ii) stress relief, i.e. stress relaxation of the overall tuff plate, due to deformations which are roughly parallel to its planar extension and resulting from rigidity contrasts with respect to the underlying bedrock, having a marked elasto-plastic behaviour. In the morphological evolution of the investigated area, the above two stress responses may be ascribed, in time and space, to stages of a single evolutionary process. Through this process, the initially fairly regular margins of the tuff plates must have been gradually dismembered by channelised waters and by gravity, leaving relict reliefs of small size superimposed on the sedimentary bedrock.

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