Investigating the stability conditions of a high historical value cliff

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Gigli, Giovanni۱; Bertolini, Giovanni۲; Casagli, Nicola۱; Lombardi, Luca۱; Mugnai, Francesco۱
Holding Date 08 September 2008

Canossa is a very small village located in Emilia Romagna Region (Reggio Emilia), famous as the site where Holy Roman Emperor Henry IV did penance in 1077, in order to reverse his excommunication by Pope Gregory VI. The castle, built on a 45m high cliff, was in fact the dwelling of Matilde Countess, the principal Italian supporter of Pope Gregory VII during the Investiture Controversy.
The geological setting of the study area consists of the superimposition of a stiff and brittle arenaceous plate belonging to the Arenarie of Bismantova Formation, on a marly layer of the Antognola Formation, which, in turn, overlies a thick chaotic unit of sedimentary origin, called the Canossa Melange. This type of geological sequence is also known in literature as rock slab - soft substratum system.
The mechanical characteristics of the materials in the study area are, therefore, very different in terms of strength, stiffness and brittleness; this leads to typical geomorfological processes such as flow phenomena on the underlying ductile units and brittle ruptures involving the overlying rock masses. These cracks are often linked to huge sub-vertical joints, that isolate large blocks.
Associated with the slow - long term evolution of the rock slab - soft substratum system we often find small scale rapid instability phenomena, that may lead to dangerous rock falls. In fact, the rock mass slopes are very steep, sometimes overhanging, and there are several elements at risk over and around the cliff. With the aim of finding the most suitable risk reduction measures, we therefore considered both of the following topics:
• overall mechanism of instability involving the whole rock slab – soft substratum system, which is responsible for the damage to the castle ruins;
• rockfalls from the steep cliff, that create conditions of high risk for the provincial road, some dwellings and the recreational footpath winding around the cliff.
In order to assess which of the cliff slopes were more prone to rockfalls, we carried out a kinematical analysis, integrated with an inventory of fallen blocks. Based on these analyses, several numerical rockfall simulations were performed, in order to evaluate the trajectories and associated energy of falling blocks. The numerical modelling allowed the reconstruction of the overall deformational processes of the Canossa rock slab - soft substratum system.
The digital elevation model (DEM) employed in the analyses was obtained from a Laser Scanning survey.
The Laser data were also used to integrate the geostructural data from impervious areas and to determine the deformational field of the main cliff slopes by comparing point clouds acquired at different times.