Field investigations and monitoring as tools for modelling the Rossena castle landslide (northern Appennines - Italy)

During the night of February 28th 2004 a landslide involved the village of Rossena (Northern Apennines, Italy), built at the base of a crag shaped in a basalt mass and wrapped in highly deformed formation of clay and shale with blocks. The failure damaged some houses, roads and fields but, fortunately, the medieval Rossena Castle, lying on the crag, was not involved at all. The goal of the study was to attain a technical and geological model of the slope in order to generate a landslide risk zonation, for regularity and development planning, so that the most correct action plans could be proposed. A detailed geological and geomorphological survey allowed for distinguishing the different gravitative landform of this area. It was very helpful to plan direct and indirect investigation, including borehole drillings, samplings, seismic (tomography) and electrical surveys. A monitoring system was built up immediately after the event (3 wire extensometers and 1 inclinometer), then progressively substituted by a more complete one (2 tiltmeters, 2 jointmeters, 4 inclinometers, 2 incremental extensometers and 2 piezometers). The phenomenon can be divided in different parts. The central sector of the slope is interested by compound slides, likely affecting the bedrock, and can be considered, at present, the ‘engine’ of the whole instability framework. Indeed, as a consequence, in the upper portion of the slope the huge blocks in which the outer part of the crag is disjointed experienced vertical displacements and, locally, topplings. Finally, the lowest sector is affected by slow movements, probably connected to bedrock creep or rock flow, while the toe, really at the foot of the slope, by shallow landslides. This instability framework is the result of a complex evolution, starting almost more than 9,000 years ago, as testified from a radiocarbon dating. In more recent time (XIX century) , the Rossena landslide was also triggered by an earthquake, that induced the partially breaking up of the crag, causing rock falls and cracks in the ground.