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In most mountainous regions, landslides are a major natural hazard endangering inhabitants and infrastructure. For landslide hazard assessment, efficient techniques are needed in order to characterize, analyze and monitor slope instabilities. Many landslide monitoring techniques provide information on only a few selected points. On the opposite, area-based techniques offer data on the whole landslide surface. Terrestrial Laser Scanning (TLS) produces high-resolution point clouds of the topography which enable to detect slope movements.
In June 2006, a 2 million m3 Jurassic limestone spur on the eastern flank of the Eiger mountain started to fail. Two steep, 250 meters long open cracks parallel to the valley flank form the back-crack of the rockslide and a middle fracture that separates the instability into a front and a rear part. The fast slope movements caused high rockfall activity and a 170'000 m3 collapse of the northern flank on 13 July 2006. The rockslide is linked to the debuttressing of the spur after the retreat of the Lower Grindelwald glacier during the last century.
Classical monitoring techniques (hand measurements, total station) did not succeed due to very high displacement rates and frequent rockfalls. Reflectorless and contactless TLS monitoring initiated on 11 July 2006. Sequential scans acquired in the following days, weeks and months revealed the displacements of the whole instability. Based on TLS data the rockslide can be divided into blocks with different displacement directions and/or velocities.
During summer 2006 the rear block slid mainly downwards along the steep back-crack with velocities up to 65 cm/day. The front block moved with up to 22 cm/day on the 35° steep basal failure surface that is buried by remnants of dead ice and the scree deposits. The velocities decreased due to lateral break-up and dismantling of the rear block (5 cm/day and 2 cm/day during winter 2006/2007 for the rear and front blocks, respectively). Overall displacements vectors over one year are 52.5 m towards 026/76 (trend/plunge) and 19.5 towards 041/35 for the rear and front block, respectively. Structural information on major discontinuity sets determined using the TLS point clouds were used to establish an instability mechanism that explains the measured slope displacements. This model shows that the rear block acted as an active wedge splitting apart the front block from the stable rock mass and pushing it passively along the basal failure surface.
The comparisons of the sequential TLS point clouds acquired before the 13 July 2006 collapse revealed higher displacement rates (up to 125 cm/day) in the initiation area of the collapse. This finding opens new perspectives in the prediction of mass wasting events by identifying areas with higher velocities. This study demonstrates the use of precise TLS data in landslide monitoring and the possibilities of failure prediction and deduction of landslide mechanism.
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