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CASSAS LANDSLIDE (Italy, Western Alps)

1. Description of the site

The "Cassas" landslide extends on the right hydrographic slope of the Susa Valley (near Turin, Piedmont).

This slope is located in correspondence of the contact between Briançonnais Domain, represented by Ambin Massif, and Piedmont Domain, represented by "calcschists with ophiolites". The second complex, mainly outcropping on the right slope, is mainly represented by carbonate schists, and subordinately by ophiolites, phyillites and quartzitic-micaceous intercalations. The slope morphostructural characteristics of the "Cassas" landslide sector are connected to deep-seated gravitational slope deformations phenomena and present different evolutive stages, which have obliterated and deformed previous evidences of glacial morphology.

Two different zones of accumulation can be recognised, adjacent and partially overlapping. The main scarp, characterised by an arcuate pattern, is modelled in very fractured carbonate schists. The affected slope has an extent of 1.8 km in length (total area about 0.5 km²), a total height of 900 m (from 1900 m to 1000 m of the talweg) and a supposed total volume of the mobilised mass involved of 20÷30x10⁶ m³ (more active sector supposed volume not less than 10x10⁶ m³).

2. Hazard analysis

First historical data concerning instability processes in this area are referred to the May 1728 flood event, which heavily affected all the Susa Valley. At the end of the 19th century the general instability of this area caused a lot of problems to the constructions of railway tunnels between Chiomonte and Salbertrand. Afterwards, in the first half of the 50’s of the 20th century, a sequence of localised movements were recorded and then, during the June 1957 flood event, a paroxysmal phase interested the western part of the slope, determining the actual morphological pattern. Recent monitoring techniques recorded a movement rate of 100/120 mm/y in the youngest part of accumulation. During the 80’s and 90’s of the last century several studies (related to historical analyses, field works and deep investigations) were carried out.

In order to preserve some important infrastructures (international highway and railway) present at the bottom of the investigated slope a monitoring system, a rock-fall earth barrier and a series of fences have been installed.

In order to understand the evolution of the instability phenomena it is necessary a calibration of geological-structural data and the subsequent realisation of a geological model. Thus, it is relevant to link the different datasets referring to a proper local tectonic and/or cinematic model and to construct geologic profiles and 3D models.

3. Elements at risk

Near the gravitative phenomenon of Cassas are the highway A32 (Frejus tunnel), the service station "Gran Bosco" and the international railway line Torino-Modane; with the Monte Bianco tunnel closure and the 2006 winter Olympic Games that will take place in this alpine sector, these infrastructures will hold a strategic role. The evolution of the gravitative process, slow but continuous, can produce accelerations in the western sector of the Cassas landslide with potential involvement of the flood plain and the infrastructures.

Therefore, it is extremely important to deepen the knowledge concerning this area instability in order to define all the risk scenarios, also considering the economic and social aspects.