The paper describes the results of laboratory and in situ investigations carried out on weak pyroclastic materials which could be encountered during engineering activities in volcanic regions. The research was focused on the historical town of Orvieto, representing a typical geotechnical situation which is widespread in Central Italy. The in situ investigations include cross-hole and SASW measurements performed in adjacent areas. The laboratory tests provided static and dynamic properties which were compared to the in situ data, in order to reconstruct the geotechnical model of a representative area of the town.
The products of the volcanic apparata located on the western margin of the Italian peninsula covered a large region north of Rome. The soft pyroclastic rocks forming most of the volcanic plateau allowed the erosion to carve out a typical landscape characterized by mesas and hinselbergs delimited by subvertical cliffs, where, on top, historical towns rise. At Orvieto, repeated instabilities on the tuffaceous cliffs and in the underground cavities excavated in the pyroclastic materials have conditioned the urban development and have threatened historical buildings, which were also damaged during strong earthquakes. At the end of 70's, a research on the long-term stability mechanisms of the hill was undertaken (Manfredini et al. 1980, Lembo-Fazio et al. 1984, Tommasi et al. 1996). In particular this paper will focus on the geotechnical characterization of the pozzolana, which is weakest lithotype of the Orvieto slab. The investigation programme included in situ dynamic measurements and laboratory dynamic and static tests.
Orvieto extends over the top of an isolated rock slab, overlying a stiff clay substratum. The slab, delimited by subvertical cliffs up to 60 m high, is carved in the upper unit of the "Orvieto Tuff" formation, which consists of a lithic facies (tuff) and of a weakly lithified facies (pozzolana). The genesis of the pyroclastic formation was ascribed by Nappi et al. (1994) to a gaseous-rich dry flow of the Vulsini Volcano, whilst Faraone & Stoppa (1988) recognized different flows. The lower lithic tuff derived from a first wet flow; a second eruption produced a dry flow and a further condense-rich flow, which formed respectively the pozzolana and the upper lithic tuff. Observations on the cliff and in underground cavities, as well as borehole data, indicate that the passage from the lithic tuff to the pozzolana is quite irregular and often gradual. This could suggest that the two facies were also differentiated by other singenetic or secondary processes (e.g. zeolitization; Manfredini et al. 1980).
The zone where investigations were carried out is located at the southern margin of the slab. The pozzolana can be subdivided into a lower competent layer and in a softer upper layer, which weakens at the top. At the bottom of the pozzolana, large pumices are abundant and progressively decrease proceeding upwards; they are stronger than the matrix and have a high relief on the cliff face.
