GTE-01 General contributions to engineering geology and geotechnics

Rockfalls in a tuff slab riddled with manmade caves: integrated techniques for landslide hazard assessment

Riccardo Fanti, University of Firenze (Italy)
Paolo Canuti, University of Firenze (Italy)
Nicola Casagli, University of Firenze (Italy)
Giovanni Gigli, University of Firenze (Italy)
Luca Lombardi, University of Firenze (Italy)

Pitigliano (Southern Tuscany, Italy) is an interesting case-history for the analysis of interactions between geotechnical features, anthropic factors and geological hazards. The town is placed on a flat slab, bounded by steep cliffs, and it represented an ideal location for defence purposes. Pleistocene tuffs with variable geotechnical features outcrop in the area, and the town cliff is composed of a 30 m thick plateau of the strongest material. The town has very ancient origins (Etruscan and pre-Etruscan), and nowadays it shows the complex result of the superimposition of architectural stiles and influence, with a prevalence of medieval and renaissance features.
The present landscape of Pitigliano is the result of a continuous evolution of the cliff, affected by slope instability problems, such as rock-falls, toppling and associated debris slides, which destroyed buildings and reduced the extension of the urban area. In fact, the tuff plateau is deeply affected by physical degradation and erosion processes an this, coupled with the presence of joint systems, led the slopes to be prone to landslides, thus threatening the heritage conservation. However, the described situation is very frequent in this area and some other towns are affected by the same problems: the additional element of interest in Pitigliano is determined by the presence of an extensive network of chambers and passages excavated probably starting from Medieval times. The network consists mainly of tunnels ranging in length from a few meters to over a hundred meters, with a rectangular cross-section one-two meters wide. This complex network (never completely surveyed in the past) is an important structural element and causes a general weakening of the rock mass.
Starting from the survey of tunnels, an extensive geomechanical survey of the cliff has been carried out, in order to obtain a complete set of information about the rock mass.
In this paper, the result of this wide-range study is presented. Different methods and techniques have been used. The survey of tunnels and chambers (that led to the identification of about one hundred different underground paths) has been realized with speleological techniques; the laser scanning survey allowed the reconstruction of the digital elevation model of the cliff and the integration of structural data in impervious areas; characterization and classifications of the rock mass have been performed through traditional methods; for the localization of the most critical sectors of the cliff kinematical analyses have been carried out; finally an innovative wireless monitoring system of some critical cracks has been implemented.
The results of these different analyses represent an interesting case history and a complete data set for the realization of consolidation works that are being carried out.