Monitoring and failure mechanism interpretation of an unstable slope in Southern Switzerland based on terrestrial laser scanner Pedrazzini Andrea, Antonio Abellan & Michel Jaboyedoff Institute of Geomatics and Risk Analysis, Univ. of Lausanne, Switzerland Thierry Oppikofer Norwegian Geological Survey, Trondheim, Norway ABSTRACT We present the application of terrestrial laser scanning (TLS) for the monitoring and characterization of an active landslide area in Val Canaria (Ticino, Southern Swiss Alps). At catchment scale, the study area is affected by a large Deep Seated Gravitational Slope Deformation (DSGSD) area presenting, in the lower boundary, several retrogressive landslides active since the 1990s. Due to its frequent landslide events this area was periodically monitored by TLS since 2006. Periodic acquisitions provided new information on 3D displacements at the bottom of slope and the detection of centimetre to decimetre level scale changes (e.g. rockfall and pre-failure deformations). In October 2009, a major slope collapse occured at the bottom of the most unstable area. Based on the comparison between TLS data before and after the collapse, we carried out a detailed failure mechanism analysis and volume calculation. RÉSUMÉ Dans cette étude, l’utilisation du scanner laser terrestre pour l’étude des mouvements d’un versant rocheux actif est présentée. Le site d’étude situé en Val Canaria (partie Sud des Alpes Suisses) est caractérisé par la présence d’une zone de tassement de grand ampleur qui présente à son pied plusieurs zones de glissement régressifs particulièrement actives. Un suivi régulier par scanner laser terrestre, effectué depuis 2006, a permis l’analyse très détaillé des mouvements en trois dimensions de l’activité de chute de blocs. En octobre 2009, un important éboulement de 360’000 m 3 est survenu dans la partie inférieure du versant. Les données laser acquises avant et après l’éboulement ont permi une analyse détaillée du mécanisme de rupture et du volume mobilisé. 1 INTRODUCTION Rockfalls and rockslides represent a major hazard in alpine valleys, especially along transportation corridors and populated areas (Remondo et al., 2008). Nowadays, almost all the alpine countries dispose of scientific studies to characterize rockfall hazard at least in areas where a potential conflict between human activities is present (Lateltin 1997; Raezo et al., 2002). Nevertheless, few studies investigated the indirect risk posed of landslide dam obstructing rivers and lakes in alpine catchment that could leads to downstream flooding (Bonnard, 2011; Eisbacher and Clague, 1984). In this study, the characteristics of the Val Canaria in terms of type and distribution of slope instabilities will be presented. Firstly, the lithological and tectonic characteristics, influencing the location and type of movement will be analyzed. Secondly, a more detailed study on a potential large collapse located directly above the Canaria River will be presented. This area represents a potential hazard to the village and the highway situated at the bottom of Val Canaria: Large rock collapse may create landslide dams in the narrow valley that could fail catastrophically and create an important downstream flooding. In this area, periodic Terrestrial Laser Scanning (TLS) acquisitions were carried out since 2006 aiming to characterize the instability in terms of structures and potential pre-failure displacements. In October 2009, an important rock collapse occurred on the monitored area. TLS datasets acquired the day before and the days after the event allowed the detection of pre-failure movements and the computation of the collapsed volume. Based on this information, an interpretation of main predisposing and triggering factors leading to the failure is proposed. The complex geological setting of Val Canaria represents also an interesting study area, where the type and the distribution of slope movements are clearly linked to tectonic and lithological characteristics. Detailed study of the relationship between lithology, tectonics and slope movements could also improve the understanding in term potential hazard posed by the different type of instabilities. 2 STUDY AREA The study area is located in the Gotthard area, near the village of Airolo (southern Switzerland). Val Canaria is a lateral NE-SW valley (Figure 1). This 18 km 2 -large catchment presents elevations ranging from 1100 m to 2700 m with mean annual rainfalls of about 1400 mm. The area is historically well known for its susceptibility to slope instabilities since more than 150 years. According to Baruffini and Ambrosi (2010), an important rock collapse occurred in the southern part of the valley in 1846. Furthermore, a landslide located in the north- western flank of the valley was reactivated in spring 1992, after a rapid snow melting. This event created a temporarily landslide dam obstructing the Canaria River (Seno and Thüring, 2006).