290420838

Fig. 8 - Cross-sections of Orvieto Hill (For legenda see Fig.3)

To conclude, the most important stability problems of the Orvieto cliff, and, in generał of all rigid rock slabs on clay slopes, are closely related to the deformation and failure phenomena that occur in the underlying clays.

With reference to the human time scalę, the ratę of the natural evolution of hillside configuration can be consider-ed to be a slow one; however, it must be borne in mind that man's activity may significantly increase the ratę of

Fig. 10 - Portions of overhanging rock these changes.

Regarding Orvieto there is a generał feeling that the ratę of the changes in slope configuration has hastened over the last decades, but unfortunately well-founded data are totally lacking, and thus it is impossible for the time being to relate such changes directly to anthropic factors.

However, there are no doubts whatever that one of the main factors influencing the ratę of slope evolution is the changes in the hydrogeological situation brought about by greater availability of water for the towns that have ex-panded considerably as well as by the growth in needs of the citizens; indeed, there ensues an increase in the quan-

Fig. 9 - "Disarrangement" of the tuffaceous mass

Fig.ll - Broken and displaced check-dam; water infiltrat-ing into the clay formation

845