In recent years, the village of Colsano in the Venosta Valley has witnessed the recurring phenomenon of rock mass collapse. Large blocks from past landslides are still visible on the edge of the valley. The investigation has brought to light the presence of a strongly disintegrated rocky ridge, within a larger and more complex area, strongly disturbed by the presence of a Deep-Seated Gravitational Deformation of the Slope (DSGSD) still studied and examined in depth. The analysis of the kinematisms, in addition to the geological and geomechanical investigations conducted on site, used extremely detailed morphological surveys conducted with the Laser Scanner technique. The same technique, combined with the topographic measurement of fixed points, has made possible to monitor and detect the slow movement of a large rock mass, of which “Kastenknott” it is the most unstable element. The study of collapse scenarios led to the decision to proceed with its controlled partial demolition in agreement with the provincial technicians and the municipal administration. This decision has been taken after using modelling software considering the hypothesis of an event occurring in an uncontrolled and unexpected way. Due to the extreme proximity of zip-line cables, the rock volume was reinforced with a cortical system, placed by experienced rock climbers, with the purpose to minimise propagation distance of the blocks during the explosion. Specialised operators took care of the control and stringing of the zip-line ropes, moving them further away from the rocky pillar. Small-diameter mine holes limited the invasivity of drilling on a precariously balanced mass of rock. The technicians’ knowledge and experience in choosing detonation systems and distribution and depth of the holes provided a further guarantee of minimal invasiveness on the slope. The local volunteer fire-fighters, the Alpine Rescue and police-men ensured a safety cordon downstream the rockfall risky area, also managing the evacuation of the inhabitants outside the pre-established safety perimeter. The controlled demolition was also an opportunity to carry out a Back Analysis study, through it which was possible to relate the evidence found on the slope with the results of the 3D modelling of the rockfall, considering them fully comparable.

Keywords: slopestability, geomechanics, blasting, Sudtirol.